\RCH1EF
Proceedings
P1984-7
Seventh
VOL.3
Ship Control Systems
Symposium
24
27 September 1984
Bath, United Kingdom
Volume 3
Lab.
v.
Scheepsbouwkun4,
Technische Horschooi
Delft
PUBLICATION INFORMATION
These papers were printed just as received from the authors In order to ensure their availability for the Symposium.
Statements and opinions contained therein are those of the authors and are not to be construed as official or reflecting the views of the United Kingdom Ministry
of Defence.
Authors have given permission on behalf of themselves and/or their employers
for the United Kingdom Ministry of Defence to publish their paper(s)
in the Proceedings of the Seventh Ship Control Systems Symposium. No materialcontained within these Proceedings may be reproduced without the prior
permission of the publishers.
Requests for further information regarding the Proceedings and the Symposium should be addressed to'
B BLOCK, MINISTRY OF DEFENCE (PE), FOXHILL, BATH, BA1 5AB, UK: ATTENTION: ADMC.
Volume 3
Contents
Warship Electrical Power System Controls: 1
Mr S L Blackburne. Mr B J Saunders. Vosper Thornycroft (UK) Ltd, UK
Propulsion Control with Microcomputers for Ships with DC 23
Propulsion Motors:
Mr H W Sachse. Siemens AG. Federal Republic of Germany
Microprocessor Control of a Shaft Driven Generator Based on an 39 Induction Machine Project G ICE N;
Dr L Garcia-Tabares Rodriguez. Dr A Lopez Piiieiro.
Mr J Bueno Perez, Prof R Faure Benito. Escuela Tecnica Superior de Ingenieros Navales, Spain
Real Time Ship Power System Simulation; 61 Mr B W Scott, Mr G P Williamson. Mr J P Mabey.
Vosper Thornycroft (UK) Ltd Controls. UK
Simulation of a Marine Gas Turbine Power Plant; 89
Mr J E Cooling, Loughborough University of Technology, UK
Control System Simulation Studies for the Single Role Mine Hunter: 107 Mr C J Bruce RAE (West Drayton). UK
Fundamentals of Automatic Harbour Manoeuvring: 123 Prof T Koyama, University of Tokyo, Japan
Integrated Attitude Estimation and Tracking; 147
Prof Ir R P Offereins, Twente University of Technology. Netherlands
A Digital Control System for Ship Manoeuvring In Ports and 163 Waterways:
Dr C G K:ellstrOm. MarIntekniska Institutet SSPA, Sweden
A Real-Time Ship Performance Monitoring System; 181
Dr R E Reid, Erskine Systems Control Inc, USA
Automatic Data Capture in Warships; 201
Mr D Roberts. Admiralty Research Establishment (Portsdown), UK
Simultaneous Computer-Assisted Data Aquisition and 209
Evaluation During Warship Sea-Trials:
Mr W Otto, Blohm & Voss AG, Federal Republic of Germany
Mariner Surface Ship System Identification; 217
Mr T Moran, Miss A B Wemple, David Taylor Naval Ship
Research & Development Center, USA, Mr W Smith, ORI INC. USA
The Ljung Innovations Filter used for Identification of see vol 5
Nonlinear Ship Speed Dynamics:
Prof M Blanke. L B Sorenson. Servolaboratorlet, Denmark
Volume 3
Contents
Control Related Problems in Digital Data Bus Systems; 315
Dr J R Ware. Mr N R Seidle, ORI Inc, USA
Interconnection Between Digital Data Highways: 329 Mr D VaHer, Admiralty Research EstablIsment (Portsdown), UK
Digital Data Transmission in Warships: 339
WARSHIP ELECTRICAL POWER SYSTEM CONTROLS
by S L Blackburne and B J Saunders Vosper Thornycroft (UK) Limited
ABSTRACT
The control of electrical power systems on warships is of major importance since virtually all equipment and systems depend on elec-tricity for their operation, and these systems are required to work following action damage.
This paper outlines the functions that form part of the control of a modern warship power system and then discusses the vulnerability considerations that have to be taken into account, such as the con-figuration of the power system controls. The selection of control hardware is influenced by system configuration and the nature of the interfaces between various parts of the control system.
The paper concludes with two contrasting electrical power con-trol systems which have been designed to satisfy substantially different operational requirements.
INTRODUCTION
Virtually every system and equipment on a modern warship depends on electrical power for its continued operation. The electrical power system is arguably the most important service on the vessel and users expect power to be available at all times and under all
con-ditions. Hence the aim of the power system controls is to maintain
power supplies at all times, even following action damage.
It is the importance of maintaining equipment supplies and the extent of the likely hazards to be considered which forms the prin-cipal difference between merchant marine and naval power systems.
This leads to the typical power system arrangement shown in Figure 1. The power system consists of two sub-systems either one of which can support the minimum essential load of the ship, ie pumps propulsion and weapons, with the important loads being dual fed from both switchboards. Further, some facility for providing emergency power will exist, either from the switchboards or directly from the generators.
The advancements in control and data transmission has a snowball effect on the demands of the control system design. Whether to reduce ship price, manpower and required operator skill or to
increase facilities and flexibility, there is a necessity to exploit advanced technology in warships.
However, there is inevitably a need for a navy to be self-sufficient in the maintenance and repair of its warships and systems.
I
NON ESSENTIAL LOADS SWITCHBOARD DIESEL GENERATOR DIESEL GENERATOR ESSENTIAL LOADS SWITCHBOARD DIESEL GENERATOR DIESEL GENERATOR NON ESSENTIAL LOADSLimitations in the ability to support high technology solutions
should therefore not be overlooked. Whilst recognising these factors, this paper will restrict discussion to the practical problems of applying microprocessors to control electrical power systems with aims of reducing cost and manpower and increasing performance and
facilities.
CONTROL FUNCTIONS
The fundamental aim of electrical power system control is to keep the output voltage and frequency as near constant as possible. This simple requirement is complicated by the protection require-ments of the system.
It is difficult if not impossible to separate control, protec-tion and surveillance either conceptually or in hardware except perhaps for segregating non-essential surveillance. The term control will be used to embrace all of these functions.
Basic power system control comprises control of; Generator Plant.
Switchgear.
Operating configuration. System Management
To set the scene, the above points will be expanded upon. Generator Plant
Diesel, gas turbine and steam turbine generators have been used on previous Royal Navy vessels, although diesel units are now more
popular. The associated alternators are usually of the self-excited
brushless type. The generators are normally dependent on their switchboards to supply power, but emergency distribution facilities may be provided at the set via a simple fused socket.
Start/Stop Control. As on merchant systems, diesel starting and stopping sequences have progressed from manual operation of lubrica-tion priming controls and air controls with gauge panels for sur-veillance, to fully automatic press to start systems, with annunciated 2 stage warnings and instrumentation. Diesel starting by air or electrical starting is possible. The latter offers the concept of a self-contained module having its own charger and battery, indepen-dent of the vessels' compressed air system.
Speed Control. Electronic governors offer little over modern hydraulic types when a prime mover of reasonable performance is to be controlled, hence modern warships will continue to use the latter. Since both the inputs and outputs of the governor interface with the prime mover, it is normally mounted on the engine.
Voltage Control. Automatic voltage regulators have increased in reliability as experience with power electronics and new
semi-conductor devices has been developed. Historically hand field regu-lators have been used during paralleling for the purposes of generator changeover, but AVRs are now suitable for continuous parallel
opera-tion. AVRs have usually been mounted in the switchboard since it is
switch-board and because the switchswitch-board provides less onerous ambient operating conditions. As with prime mover controls it would be equally at home on the generator set.
Switchgear
The switchgear on warships, as on merchant ships, has changed little in recent years with Air Circuit Breakers being used for the heavier currents (over 500A) and Moulded Case Circuit Breakers for lower currents. Both breaker types provide for direct manual opera-tion with remote control via solenoids and/or motor-wound charged
springs. The main change in breaker design is the trend to use
electronic modules to provide overcurrent protection, rather than the traditional thermal and electromechanical trips. Switchboard auto-mation is limited and usually covers only breaker synchronisation
prior to generator paralleling and breaker closure.
Synchronisation. This is traditionally carried out manually at the switchboard using a combination of three lamps and instrument synchroscopes. To guard against operator error an electronic check synchroniser is often provided. The current trend is to provide fully automatic synchronisation and closure of breakers particularly on parallel operated systems.
Operating Configuration
Historically the main generators in warships have been split operated with each generator feeding its own set of loads. The current trend is for warships to operate with paralleled generators with the total ship's load shared between them.
By reducing the throw-over load that the remaining generators must absorb following a generator failure, parallel operation has the potential advantage of improving the normal load factor of the
diesel. This can result in longer periods between diesel overhauls
leading to lower operating costs.
However, parallel run power systems are prone to cascade failure and, ultimately, total loss of power unless discriminating protection is
provided.
Parallel Protection. Whilst generators were only paralleled during generator changeover; simple voltage, frequency and reverse power protection was adequate. For continuous parallel operation there is the risk that failure of one generator set will cause all connected sets to fail. It is therefore necessary to discriminate between failed and healthy sets.
Fuel failures are normally underfuel faults, in which case generator reverse power protection is adequate. Overfuel faults are extremely rare and there is little point in providing special
pro-tection.
Excitation failures are less easy to discriminate, but fortuna-tely are usually less frequent than fuel failures. Excitation pro-tection involves analysing the nature of the excitation current to the generator to determine if it is reasonable. This function should be incorporated into the AVR but in practice is rarely provided.
Short Circuit Protection. Short circuit discrimination is achieved by grading the overcurrent responses of the breakers such that those supplying loads trip faster and at a lower current than the generator supply breakers, with the bus section and switchboard interconnector breakers somewhere inbetween. This provides good reliable discrimination unless a short occurs on the generator side of its supply breaker, in this case differential (unit) protection must be employed around the generator supply cable. Since warship power systems are not usually operated with a power system ring; the general use of differential current schemes can be avoided, with con-sequential savings in reliability and cost through reduced protection
hardware.
Preferential Load Tripping. In addition to discriminating pro-tection it is necessary to control the load on the remaining healthy set, or sets, to prevent overload. Preferential load tripping would normally be employed; although it may be possible to regulate certain large loads, such as electrical propulsion or air conditioning, if this is acceptable in the vessel's particular operating mode.
Load Sharing. To make use of the generator operating range, it is necessary to maintain close load sharing between sets. In order to maintain the independence of generator sets whilst in parallel, both real and reactive VA must be shared using a drooping frequency and voltage characteristic on each governor and AVE. In many instan-ces this droop is acceptable.
There is advantage in having a droopless voltage control scheme where other volt drop considerations are critical. This may be the case on large vessels where distribution cables have to be longer or where large loads force design acceptance of larger than normal
voltage transients.
Isochronous governing schemes have the advantage that they remove the warm-up drift of hydraulic governors which if uncorrected would lead to substantial real power imbalance.
The use of automatic load sharing schemes have the additional advantage that they can also detect fuel and excitation faults, since if a generating set refuses to take its fair share of the total
system load then it is most probably faulty! System Management
Traditionally power system management - the control of how many sets are connected at any time and which loads are supplied - has been carried out by ships personnel. Warship design will eventually follow commercial practice and will implement automatic start-up and connection, disconnection and stopping to meet load demand, respond to generator failure, or to equalise generator set running hours.
Indeed the Royal Navy Type 23 Frigate will have automatic start and connection sequencing and the Vosper Thornycroft MK 18 Frigate will have all of these facilities.
SYSTEM DESIGN
So far the control functions have been discussed and it would be possible to build a system by isolating the most cost effective hardware for each individual function; whether microprocessor,
conventional electronic or relay logic. But such an approach would incur considerable penalties in packaging, ensuring compatibility between control hardware, ease of maintenance and general support
logistics.
When designing the control system it is important to bear in mind the potential of such state of the art products as
micro-processors, serial data links and visual display terminals. The aim should be to produce a properly integrated system with all the elements fully compatible both physically and electrically with each
other.
It is only by doing this that the designer can ensure that he is not compromising the full potential benefits of the new technology by his conservatism with individual equipments. It will be the balance of cost and facilities which will steer the final design to a compromise which does have the benefits of both.
Control system design and philosophy is part of the overall power system philosophy. Clearly the control system must be compa-tible with the availability of the plant reflecting its reliability and vulnerability. These parameters will dictate the interfaces, equipment location and reversionary modes of the system.
VULNERABILITY OF CONTROLS Autonomous Plant Operation
When designing a power system to be less vulnerable to the effects of action damage and control logic failure, the first line of defence is to select power generation plant which is capable of operation (albeit at increased risk or reduced performance), without its control system being operational.
This dictates the use of control interfaces which are only active when it is necessary to change the state of the plant. For example, a signal should only be necessary to initiate the starting
of a diesel or closing of a breaker, and not to keep the diesel running or the breaker closed. As suggested earlier, the governors and AVRs associated with the generators should have matched droop characteristics to provide an inherent degree of load sharing when operated in parallel.
The added bonus of using passive control interfaces is that any control system performs a largely supervisory role and can be taken off-line at any time without affecting the present state of the power
system. The implication for designers of warship power systems is
that they must liaise with their control system suppliers when selecting and specifying the power system plant.
System Configuration
After having arranged that the plant to be controlled is capable of autonomous operation, the next matter for consideration is the actual configuration of the power system itself. The integrity of
the complete system may be compromised if the control system is more vulnerable than the power system, and the design would not achieve its primary objectives if the controls were considerably less vul-nerable than the power system plant.
The key point is to consider the distribution and physical separation of the generators and switchboards and to locate the controllers adjacent to them. This minimises the length of any feed-back and monitoring cabling between the plant to be controlled and the controller(s).
Failure Mode Considerations
The benefits of having plant capable of autonomous operation are lost if the failure modes of the control system are not constrained. It is pointless to have a generator which can run without its con-troller if when the concon-troller fails it always puts out a stop
signal! This means that the likely failure modes of the controller
hardware have to be considered early, and features designed so that the ways in which the hardware fails can be predicted. Additionally, if software based controllers are used then the correct operation of the software must be monitored by additional hardware and software which forces the controller to a safe state if software failures are detected.
Distributed Control
The foregoing consideration dictates the use of a distributed control system rather than a centralised arrangement. The minimum
requirement, since warships normally have at least two switchboards, is one controller responsible for the equipment associated with each switchboard. This arrangement also prevents total loss of power system controls in the event of a controller failure. The number of additional controllers employed depends on the degree of controller integrity required, the number of operator control positions and the costs of interfacing between the various controllers.
The necessity of using distributed control systems precludes the use of most existing commercial equipment since they tend to utilise centralised controllers.
Control Positions
The use of reversionary control positions is a very important consideration in the design of warship electrical power control
systems.
Under peacetime cruising conditions the operation of the power
system is controlled from the ship control centre or machinery
con-trol room, so it is necessary to provide the ability to reconfigure the power system and to monitor its operation at this position. However, if this was the only control position, then relatively minor damage to the ship could render control of the entire power system difficult if not impossible.
Consequently it is usual practice to site reversionary control positions at each switchboard. These reversionary control positions can monitor the operation of the complete power system, but to save costs can usually only control the equipment directly associated with the local switchboard or sub-system.
The final reversionary mode of operation for emergency or mainte-nance use is usually one control facility adjacent to each generator. This control position becomes essential if facilities exist for
taking power fro. Lu, nerator bypassing the switchboard itself. The provision of these various control positions can be made easier if the man-machine interface at a control position is functio-nally separated from the actual controllers themselves. This means that the controllers can be sited close to the equipment they face with providing good integrity and that the man-machine inter-faces (MMI) can be sited at positions convenient for the operator. This splitting of the MMI from the controller itself can prove expensive in ships cabling, if rigorous control is not kept over the extent of the interface, such that only information and control facilities essential for an operator at a control position are
provided. However once this conceptual step is made, then it becomes
a relatively simple matter to duplicate the MMI at different control
positions. This makes it possible for an operator to maintain control
of the power system even when one MMI is unavailable. Control Transfers
Multiple control positions means that provision for the transfer of control authority between the control positions has to be made. It is essential that control can be transferred between control positions without affecting the present operating state of the power system, and this is made simple by the use of the passive interfaces which are necessary for autonomous operation of the power system
plant.
Where alternative control positions are reversionary positions providing limited control nearer to a controller or the plant, then it is normal practice to select the control authority from the lower reversionary position.
Where the alternative control position is a duplicate position then the transfer of control authority would be made by the con-troller in response to a request from an MMI. Where there is a need
to maintain a command priority scheme this would be achieved by
limiting the requests possible from a particular MMI. Man Machine Interfaces
There are 4 types of MMI found on existing warship power systems: Basic instrumentation provided as part of a generator or circuit breaker.
The generator watchkeeping panel. The switchboard.
One or more remote main control panels.
On larger vessels the main controls for generator starting, stopping and synchronisation and electrical instrumentation are mounted on remote panels. On smaller vessels the electrical controls are on the switchboards.
Of particular note is the use of system mimic diagrams depicting the main system bus. The diagram operates on a dark board principle
with rotary switches, engraved with part of the bus, represent circuit breakers. These 'discrepancy switches' control their res-pective circuit breaker and illuminate when their position is not in sympathy with the breaker state. A typical mimic control panel is
shown in Figure 2.
With microprocessor based control logic and serial data signal transmission, there are advantages in replacing conventional instru-mentation, switches and indicators with visual display terminals. These offer the potential for:
Highly versatile interactive mimic displays.
High density surveillance on a small panel by simple display paging.
Enhancing the cost benefits of serial data transmission by minimising the demultiplexing required by conventional instrumentation.
Displaying operator guidance or particular course of action following a fault for diagnosis, maintenance or
repair.
Potentially costing less than a conventional control and instrumentation panel.
One such page for a visual display terminal together with its operators panel is shown in Fig 3.
COMMUNICATION INTERFACES
Having decided on the configuration of the control system the various interfaces have to be given careful consideration, these can be grouped into three main areas:
transducers and/or actuators to controller. controller to controller.
controller to man-machine interface.
The main decision to be made for each interface is whether to use some form of multiplexed serial data link or to stick with the traditional dedicated discrete parallel wiring.
Discrete Wiring Versus Serial Data Links
Discrete wiring has the advantage of being a reliable low tech-nology solution which provides the least delay in the transmission of information. The penalty lies in the weight and installation cost of wiring up a large number of individual cores.
Serial data link schemes have the potential for dramatically reducing control cabling costs and weight, and may make the utilisa-tion of plug and socket connectors more cost effective. It is impor-tant when implementing such schemes that the installation advantage is not lost by the increased number of connections required to inter-face with the multiplexing and demultiplexing equipments. This can
Fig 2. CONVENTIONAL MIMIC CONTROL PANEL.
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DELETE ENTER PAGE PAO PAGE BACK SELECT PAGE PAGE CONTROL MASTER ACCEPT MASTER RESET ALARMS START ON FAIL START ON LOAD Comm AUTHORITY
START STOP 7 8 EMERG CY STOP 02 . GENERATOR 4 5 6 EMEROCT STOP 153 OPEN CLOSE 1 2 3 ENMG CY STOP MCR IN CONTROL MESSAGES - -0 451 I 1 600 I 112 63 V CI 22 03 SELECT AUTO SELECT MANUAL SELECT AUTO SELECT MAN. AFT SWBO ROOM OCR ENCL MOUE *00 300 -61 62 63
be overcome by integrating these units into t,e generator set module, the switchboard and the various control panel .
Against serial data transmission is the limitation in sur-veillance update rates due to the inherent delay of serial data
transmission when carrying multiple signals, and the dependence on electronics for correct operation.
Serial Data Link Implementations
The merits of the various serial data link multiplexing proto-cols does not concern this paper since it has little effect on the
structure of the control system. Of greater interest to power system controls is the serial data link bus architecture and trans-mission medium since both these affect control system integrity.
Potentially fibre optic transmission offers high interference immunity but in practice twisted conductor tranmission can offer sufficient immunity to interference in the warship environment. In
costandrepairability terms the twisted pair is cheaper and requires no additional skills over basic electrical practice.
Serial data link architecture offers a choice between point to point, local area network, or common carrier approaches covering a complete ship. For a simple system a switchboard based controller could, for example, communicate via separate point to point serial data links with its diesel generator sets. Alternatively a 'common data bus' could be provided, onto which all equipments, generator sets, switchboards, control panels are connected. The latter scheme can indeed be expanded into a shipwide bus which any ship system can
use.
With common carrier buses their immunity to damage can be questionable even when duplicated. For example, short circuits on all the buses entering a single damaged equipment may defeat even duplicated common bus schemes. Another concern is the difficulty in unconditionally guaranteeing separation of control data to the different control units.
Consequently there will be a tendency to prove serial trans-mission in point to point schemes on the present generation of
war-ship designs before moving on to LANS or common carrier systems. Transducer and Actuator Interfaces
Least freedom of choice exists for the transducer/actuator to controller interface. With current technology each transducer must be hardwired back to its controller or multiplexing unit, and in many cases the transducer signal must be conditioned before it is in
a form suitable for use by a controller. Hopefully in the next few years transducers which are more directly compatible with modern digital electronic controllers will become available.
However some choice does exist and it is important to select transducers which permit the controller to detect if they or the associated wiring has failed. This means using complementary open and closed digital signals for breaker states, and suitable analogue transducers whose failure results in an unrealistic signal given the operating state of the plant.
A particularly critical signal is the speed sensing associated with diesel generator stop/start sequencing. Failure to distinguish between loss of the speed sensing signal and the diesel being stopped can lead to serious damage being done to the diesel auxiliaries.
As far as actuators are concerned these should normally be of the bang-bang type, ie one signal to start or increase, another to stop or decrease and no signal to remain in the present state. These
signals are a natural consequence of achieving autonomous plant operation. If a proportional analogue signal is essential, then it should be of the 4-20mA or frequency modulation type and the actuator itself must detect that the signal is in the valid control range. Inter-Controller Communications
The controller to controller communication interfaces could be either data linked or hardwired. Duplicated data links are normally used for general information transfer and to reduce cabling costs. Hardwired parallel interfaces being reserved for any fast response, performance critical, communications such as preferential
load-shedding or zone differential current protection.
In this application serial data links have the further advan-tage that electrical isolation between controllers can be incorporated at little or no cost, eliminating earth loop problems and preventing a major electrical fault from cascading through the control system. Controller to MMI Interface
The nature of controller to MMI communication interfaces depend on the complexity of the interface and locality of the controller
and MMI. If the interface is small, or the controller adjacent to
the MMI, then a hardwired interface is usually employed on grounds of cost and simplicity.
As the distance between controller and MMI increases and the amount of information to be transferred increases, then a serial data
link is adopted. At the MMI end this information is demultiplexed onto either a conventional lamp, meter and switch mimic panel or dis-played on a visual display unit of some type.
The trend is towards the increasing use of 'soft
(non-dedicated)displayseven at the plant local control level, so that serial data links are being employed even when the MMI is directly adjacent to the controller.
CONTROL HARDWARE SELECTION
When choosing hardware to implement the control system both its hardware and communication interfaces need to be considered. The
usual reaction these days is to use microprocessors without thinking but this can be both impractical and undesirable for power system
control.
In this section the term microprocessor is used to refer to a general purpose microprocessor system using standard hardware inter-facing onto a common bus. It is assumed that the microprocessor operates under some multi-tasking executive and is programmed in a high level language.
Microprocessors which perform a single function, within the confines of a PCB and programmed largely in assembler, have many of the characteristics of random logic designs and are treated as being dedicated hardware.
The Benefits of Microprocessors
Microprocessor have the advantage of flexibility and can inter-face easily with various serial data link schemes. These features make them particularly suitable for those aspects of control which are dependent on the power system configuration. A particular feature of microprocessors is their ability to correlate data and using models of the power system, to provide powerful self diagnos-tics at little additional cost. Such fault finding techniques beyond the normal input data checking include:
applying the triangle of real, reactive and apparent power to all critical control data.
checking that the input data does not conflict with any physical constraints within the power system, ie breaker interlocks.
checking that the generated and consumed power and currents match within the system.
Indeed a controller with such inbuilt features can be considered to be a rudimentary form of expert system.
The Problems with Microprocessors
Interfacing. The task of interfacing the power system to a general purpose microprocessor system for single functions can prove more complex and costly than performing the same function in dedi-cated hardware. A good example here is that of an automatic synchroniser, in that the electronics needed to provide a self-contained unit is no more complex than that necessary to convert ac voltage, frequency and phase into several digital input channels for a microprocessor to read.
Response Times. A more fundamental problem is that the current 16-bit microprocessors, when operating under a multi-tasking
executive, can lack the necessary processing power to respond to the power system in a time compatible with the requirements of
certain control and protection functions. Two functions which can be particularly troublesome are overload protection for the diesels and differential current protection on the generator supply cables,
since these functions may require a response time of around 150
milli-seconds. The hardware delays associated with measurement and breaker
operation can easily be 130 milliseconds, so the processor has less than 20 milliseconds, including any latency, to detect the fault and make a response - a stiff order on top of the other control tasks if interrupt facilities are not available.
Fault Analysis. In addition, the current state of software writing is such that it is difficult, if not impossible, to predict all failure modes and their likely frequency of occurrence. While this situation may be acceptable for some control functions, it is unacceptable for critical protection and performance aspects, if
sufficient operator confidence is to exist in the control system to allow unattended operation.
Practical Implementations
The current solution to the hardware selection question is use a hybrid implementation. In general the critical functions are performed in dedicated hardware, which can be subjected to thorough analysis and testing. Microprocessors being used for slow control loops and providing overall system management.
Table 1 summarises the implementation options, which are
expanded below.
Generator Plant. The control and protection functions associa-ted with generator plant control are very suitable for microprocessor implementation. The response required is relatively slow sec)
and the essential protection, such as diesel overspeed, is usually backed up by direct acting mechanical systems on-plant. Secondary surveillance may be incorporated into the microprocessor if it forms part of a wider data gathering net.
Switchgear. The majority of the control and protection functions required are implemented in dedicated hardware either within the breakers or in add-on items such as synchronisers. Micro-processor implemented functions are limited to general surveillance and acting as a multiplexing unit for remote control positions.
This is a consequence of the fast response required (down to 40 milliseconds) and the need to provide functional independence
between the protection of different breakers.
Operating Configurations. Functions such as preferential load-shedding and diesel reverse power protection can be implemented within a microprocessor but integrity considerations may bias the decision towards dedicated hardware. Load sharing covering both real and reactive power is much more convenient to implement in a general purpose microprocessor since the speed of response required (1-10 seconds) is slow and use can be made of the system responses to diagnose faults.
The need to implement control of demanded load is very dependent on the requirements of individual systems and so the flexibility of any microprocessor is very useful here.
The one function which is not suitable for implementation within a microprocessor is that of differential current protection. This
is a consequence of the awkward interfacing which requires a large (up to 10 times) overload capability and a fast speed of response.
System Management. System management covering such functions as responding to generator set failures, matching generating capacity to the power demand, equalising the running hours between sets, and sequencing load connection are best implemented within a micro-processor. The requirements of and for these functions vary between systems and again the flexibility of a microprocessor is the deter-mining factor, since the speed of response required is generally
Table 1. Practical Control Hardware Selection FUNCTION DEDICATED HARDWARE IMPLEMENTATION GENERAL PURPOSE MICROPROCESSOR COMMENTS GENERATOR PLANT 1) Protection, eg Overspeed. 2) Control, eg Start Stop Sequencing. 3) Surveillance, eg Exhaust Temp Monitoring. SWITCHGEAR 1) Protection, eg Overcurrent. 2) Control, eg Synchronisation. 3) Surveillance, eg Voltage Frequency etc. OPERATING CONFIGURATION 1) Differential Current protection. 2) Load Shedding. 3) Reverse Power protection. 4) Loadsharing. 5) Control of Demanded Load SYSTEM MANAGEMENT 1) Connected Capacity Control. 2) Responses to Generating Set Failures. 3) Equalisation of Generator Set Running Hours. 4) Load Sequencing. V V V V V V V V V V V V V V V V V V
i
VOne backs up the
other.
Depends on display facilities required.
Very fast
response required. Failure could cause
blackouts. Fast response. )Depends on system )integrity and )performance )requirements. ) ) ) ) )The requirements )for these functions )are very dependent )on the individual
)power system. ) ) ) ) )
Future Trends. As the performance of microprocessors increase and the demonstratable quality of software improves, then more functions will become incorporated within the ubiquitous micro-processor. Loadshedding and reverse power protection are currently on the margin with breaker synchronisation being the next function under attack.
TYPICAL SYSTEMS
To illustrate the application of the points made above two typical but contrasting power control systems will be described. Royal Navy Type 23 Frigate
The British Royal Navy Type 23 Frigate currently under design and construction is a 3500 tonne ship employing electrical
propul-sion. Figure 4 shows the main 600V generation part of the power
system.
System Structure. The system has two sub-systems each con-sisting of two diesel generators feeding a switchboard. The electri-cal power is distributed to two thyristor stacks, which drive the electric propulsion motors, and to two motor generators which pro-vide 440V general services power. The size of the system can be judged in that each switchboard contains a mix of 40 air circuits and moulded case breakers.
The system has controllers adjacent to each diesel generator and the two switchboards. Control positions are provided at each
generator for maintenance use, at the two switchboards for rever-sionary use and in the Snip Control Centre (SCC) for use under cruising conditions. The controllers are housed in the same enclo-sures as the Man Machine Interfaces.
The MMIs in the Ship Control Centre and in the two switchboard rooms use conventional instrumentation and switches arranged in a mimic format. The diesel generators local MMIs again use meters,
lamps and switches, but also incorporate a two row by 40 character display to provide access to secondary surveillance data.
The two switchboard controller communicate with each other, and with the demultiplexing electronics associated with the SCC MMI via duplicated serial data links. All other interfaces within the
control system are hardwired.
Controller Functions. The diesel generator controllers are responsible for the start/stop sequencing of the generating sets and providing running protection and standby services. In addition, the controller performs health monitoring or secondary surveillance on the engine and passes the information collected down a data link to a
central data collection unit. These functions are performed within a microprocessor.
The switchbaord controllers are responsible for synchronisation, general electrical surveillance, real power sharing, loadshedding and limiting the current demand and power regeneration of the electric propulsion motors. The controllers also act as multiplexing/de-multiplexing units for the data links to the SCC and pass secondary surveillance data via a data link to the central data collection unit.
MMI & CONTROLS
Li
PLANT CONTROLLER 7 MAN MACHINE INTERFACE SWITCHBOARD DIESEL GENERATOR DIESEL GENERATOR SEC mvu HMI CONTROLSCONTROLS POWER CABLES
\
(HI & CONTROLS DISCRETE WIRED SIGNALS CONTROLLERz
MAN MACHINE INTERFACE SWITCHBOARD DIESEL GENERATOR MMI & CONTROLS -> SERIAL DATA Ai\ DIESEL GENERATORThe above functions are all microprocessor implemented with the exception of synchronisation. The differential current and reverse power protection of the diesel is also implemented in dedicated
hard-ware.
The electronics associated with the SCC MMI are again micro-processor based but perform no control functions other than multi-plexing/demultiplexing signals onto and off the data links.
System Features. A consequence of this system structure and one of its principal features is that the electronics associated with the switchboards perform a general supervisory function which a man can take over should the electronics fail. This is an important consideration for a reversionary control position under action
con-ditions.
Particular attention has been paid to ensuring that the micro-processors detect as many hardware and software failures as prac-tical, and that it fails to a state which does not inhibit manual
control. This is done to realise the full potential of the
rever-sionary control.
Conventional MMIs were chosen to provide some degree of familiar-ity for the operators in a system which is substantially different in structure and operating practice to other ships of the fleet.
The extensive use of microprocessors and the integration of control, multiplexing and secondary surveillance functions reduce the quantity of electronic hardware required, and makes the use of serial data links very cost effective. In this particular case they eliminated some fifty six 24-core cables and a substantial amount of isolation electronics between the controllers.
Problem Areas. The large thyristor stacks used to control the electric propulsion motors cause substantial distortion of the power system waveforms with multiple cross-overs and significant power in the high frequency harmonics. This can send conventional transducers and instrumentation haywire and special precautions have to be taken. These include second order filtering of frequency inputs and the use of wide bandwidth (5kHz) transducers for power, voltage and current measurement.
Apart from the difficulties caused by the power system distortion, the main practical problems revolve around ensuring that the soft-ware runs fast enough for particularly the switchboard controllers. Vosper Thornycroft (UK) Limited, MK 18 Frigate
The Vesper Thornycroft MK 18 light frigate is of 1850 tonnes and designed to provide significant capability with reduced manpower. Figure 5 shows the main power generator system.
System Structure. The power system consists of three diesel generators feeding two switchboards which are connected in a ring. Emphasis was given during design to the use of standard switchgear modules, or building blocks so that expansion to four or more genera-tors would merely involve the addition of further modules. The con-trol system is designed to complement this modular approach.
17> NLICE CR FAALC E UNIT PLANT MMI
A
E
1
-L_
CONTROLLER SWITCHBOARD-"
A
_;
1 Al I I I - I W___.V
I CONTROLLER CONTROLLER SWITCHBOARD DIESEL GENERATOR LOCAL INTERFACE UNIT DIESEL GENERATOR DIESEL GENERATOR LOCAL INTERFACE UNIT(__1
DISCRETE WIREDPOWER CABLES
>
SERIAL DATACONTROLS
I=>SIGNALS
MMI I I MMI
A switchboard section is associated with each generator and comprises:
The generator supply breaker. Two main busbar coupler breakers.
A shore supply or emergency supply connection facility. A number of moulded case distribution breakers.
A switchboard section control unit.
In addition there is a controller associated with each generator. The separation of the man machine interfaces from the controllers has been used to great advantage with control terminals sited in the Enclosed Bridge, the Machinery Control Room and Aft Switchboard Room. Each terminal comprises a visual display unit and a simplified
dedicated keyboard.
All interfaces within the control system employ serial data links except those between the controllers and the equipment they control.
Control Functions. The controller adjacent to each generator uses a microprocessor to provide start/stop sequencing and basic monitoring and protection, and supports the data link to the associated switchboard controller.
The switchboard controllers provide synchronisation, power sharing, automatic control of the number of connected generators to satisfy the ship load, connection of replacement generators following failure of a connected set, and preferential tripping of selected ship services to reduce the system load. These automatic responses can be selected by the operator as required.
The switchboard controllers also support the data links between themselves and the visual display terminals which form the man machine interfaces. These terminals display a clear and simple mimic diagram of the main power system at all times with the remain-ing display area showremain-ing paged surveillance information. This is displayed using a combination of analogue bar charts and digital readouts and permits parameters out of range or generators out of balance with each other to be clearly seen.
As is the case with the Type 23 Frigate the switchboard control functions use an hybrid implementation. Dedicated hardware is used for automatic synchronisation and voltage and current protection with a microprocessor providing the overall automation.
System Features. The provision of a high level of automation and widely distributed control positions minimises manpower
require-ments. The use of 'soft mimic displays permits control of the
whole power system from all three main control positions without needing a large amount of space for the control panels.
Distribution of the control positions also permits the full automatic control of the system to be supervised even after action damage to large areas of the vessel. This complements the location of the generator sets which are distributed as widely as possible throughout the vessel.
Extensive use of serial data links eliminates the heavy cabling penalty normally associated with multiple control positions and, more
importantly, reduces system installation time.
Providing a controller adjacent to the generator enables fall-back to 'at plant' control if control cabling becomes damaged. It
also permits the generator set to be installed, run and tested as a single module. Overall the design uses the majority of the latest control techniques discussed in this paper and marks a significant advance in warship power system control.
CONCLUSIONS
Parallel operation of warship power systems potentially offers better power availability, but this requires additional control and protection over split operated systems.
Consideration of system vulnerability dictates the use of distributed control systems with electrical isolation between the control positions. This substantially complicates the implementation of the controls.
The use of microprocessors, serial data links and visual display units can simplify this task but will not inevitably reduce costs.
The cost is influenced considerably by the complexity of the control system required and in fact simple systems can be cheaper to imple-ment using dedicated hardware.
Forseeably there are few advancements to be made over the control system functions provided on a modern merchant vessel. These auto-matic functions will soon be commonplace on warships.
The attractiveness of alternative control positions, redundant and distributed control logic without cable or installation penalties may cause a growth in technology on warships over merchant systems in an area where there has been little advancement since the days of dc generation.
BIBLIOGRAPHY
A.J. Scott "Design of Electrical Systems for Warships" Trans I Mar E, Vol 95, Paper 43, 1983.
J.E.D. Kirby "Digital Data Transmission Systems for Warships" Proceedings of 7th Ship Control Symposium, 1984.
PROPULSION CONTROL WITH MICROCOMPUTERS FOR SHIPS WITH DC PROPULSION MOTORS
by Helmut W. Sachse Siemens AG ABSTRACT
Propulsion systems with battery-electric drives require rather complex systems for open and closed-loop control. The design of a possible propulsion system and the tasks of the control system are
described. Some of the associated problems and their solutions are shown in this report. Special emphasis is placed on the safety and reliability of the complete system. Some special points in connec-tion with the use of microprocessors are menconnec-tioned. Additionally test equipment is described with which a complete system check is possible and which reduces the commissioning time for the control equipment of a propulsion system.
1. SYSTEM STRUCTURE
A common requirement for special vessels is a propulsion system with a very wide range speed control, frequently up to 20:1. If
silent running is also a requirement, and periods of operation are
called for that are independent of air for combustion, the simplest solution is a battery-electric system with separately-excited DC
mo-tors.
In such a system the speed is determined by the battery voltage and the level of excitation current. Therefore, in order to maintain a set speed with a falling battery voltage, i.e. as the battery dis-charges more and more, the excitation current has to be reduced. This means, of course, that the armature current increases for the same power and the losses are also increased.
Conversely, the excitation current has to be increased when the battery is fully charged.
When the speed is reduced the excitation must be increased even more with the same voltage; once again the losses are increased. It is apparent, therefore, that the limits of the speed range are set mainly by
the range of battery voltage the maximum permitted losses
The range can be expanded by subdividing the battery capacity and the motor power. One possibility, comprising 4 batteries and 2
mo-tors, is shown in Fig. 1; the motors drive through a common propeller
shaft.
Figure 1. Propulsion Plant
Case 1: The batteries are connected in parallel and the motors in series (Fig. 2).
The armature voltage for the motors is 1/2 x U.
Figure 2. Speed Range II
Case 2: The batteries are connected in parallel and the motors also in parallel (Fig. 3).
The armature voltage for the motors is U.
Figure 3. Speed Range III
Case 3: The batteries are connected in series and the motors also in series (Fig. 4).
The armature voltage for the motors is 2 x U.
Case 4: The batteries are connected in series and the motors in
par-allel (Fig. 5).
The armature voltage for the motors is 4 x U.
Figure 5. Speed Range V
This arrangement gives 4 different modes, whereby the voltage is
doubled in each case from the preceding mode.
Case 5: Another possibility is to reduce the armature voltage further for the lower part of the range by using a series resistor or
some kind of final control element (Fig. 6).
T
Figure 6. Speed Range I For example:
through the field windings (operating as series-wound machines) by using a variable series resistor
by using a motor-generator set by using a static converter
By using these methods it is possible to achieve the required turndown range of speed of approximately 20:1.
There are several alternative circuits for various modes which fall between the extremes represented by Case 4 and Case 5, but since they do not contribute in any way to the topic under discussion they will not be mentioned any more.
2. THE TASKS OF OPEN-LOOP AND CLOSED-LOOP CONTROL
A system of automatic control for a propulsion plant of the type which has just been outlined has the following principal tasks to perform:
operating the circuit-breakers according to the required speed, i.e. changing the voltage in steps
setting the excitation current for operating in the various modes and regulation of speed in the closed control loop.
There are also secondary tasks to perform, such as:
interrogation of enabling criteria for automatic control (e.g.
whether all voltages are available and all auxiliaries are running) supervision of command execution
supervision of the timing of switching operations
supervision of the automatic control system and switchgear control of the cooling system for the propulsion motors
Having dealt briefly with the fundamental aspects of propulsion systems with batteries and DC motors, the intention now is to concen-trate on the open-loop and closed-loop control of such systems.
In order to operate a plant of the type shown in Fig. 1 it is
necessary to open and close several circuit-breakers one after the
other. This switching has to be performed very rapidly when the
running mode is being changed otherwise there is a danger of the shaft speed and ship speed falling too much during the delay. Then,
if a faster running mode is being engaged, the motor speed is too low for the armature voltage and very high armature currents flow. The
result is almost like a short-circuit, the switchgear protection operates and trips the circuit again.
It is in any case important to control the armature currents during the switching. If it is required to attain maximum speed in a particular mode it will be necessary to reduce the excitation ac-cordingly. Assuming the speed remains constant when the armature voltage is disconnected, the back EMF of the motor will also remain constant. If, now, the next upward mode is selected, twice the arma-ture voltage is suddenly applied. It produces a high armature cur-rent, corresponding to the difference between back EMF and armature voltage, which can be regarded almost as short-circuit current.
The corresponding jump in torque places an enormous strain on the propeller, shaft, motor and ship, quite apart from the switchgear. In order to avoid this situation, the excitation has to be increased as quickly as possible during the switching delay until the back EMF and the anticipated armature voltage are equal.
The situation is made worse by the fact that the speed falls
during the switching delay. Therefore, although the switching mus'.; be performed as quickly as possible, it must not be before the ex-citation has had time to react. Obviously, choosing the optimum instant is a very important factor. Once this has been effected, the speed is established according to the armature voltage and ex-citation current. If a constant speed is then required, regardless of the state of charge of the battery, i.e. regardless of changes in armature voltage, the excitation current must be adjusted appro-priately, and a closed-loop control system is an obvious choice.
From the number of tasks to be performed and the precise co-ordi-nation required it will be apparent that, if manual operation is employed, the operators have to be trained very well indeed and an automatic control system provides a useful alternative.
3. THE PROBLEMS
If a propulsion system designed for a power of several megawatts is taken as a starting point, and for safety reasons the voltage must not be substantially above about 1000 V, it will be obvious that currents will be of the order of several thousand amperes.
Switching such high DC currents with a load containing an inductive component places a severe strain on the circuit-breakers, so efforts are made to limit the normal breaking current to around several hun-dred amperes.
On the other hand, the current must not be too low because the interrupting devices of the circuit-breakers cannot then operate properly, e.g. quench any arcs which may be drawn, and the current will continue to flow although the circuit-breakers are open. If
the next running mode were to be engaged under these conditions there would be a short-circuit. Suitable interrogation can, therefore, not only increase safety but also assist in optimizing the instant of
breaking. This also minimizes switchgear noise and switching
delay,
and increases the life of the switchgear.
Overall, control of the switchgear involves a large number of complex logic operations which are dependent on a series of "On" and "Off" command criteria. It can be represented in terms of "And" and "Or" logic with limit interrogation - boolean algebra.
The open-loop control can be assembled from suitable logic cir-cuits and this has already been done. However, such systems are
inflexible and difficult to modify. Also, this particular applica-tion requires a very large number of logic funcapplica-tions which can be equated to a high hardware cost.
A more modern and elegant solution is to use microcomputers. The remainder of this paper will concentrate on the various aspects of this application of microcomputers to open-loop and closed-loop
control.
As mentioned before, the open-loop control system is a logic pro-blem of several different switching states. Because of the architec-ture of most microcomputers, this presents no fundamental problems, although it would be desirable to work with individual signals, i.e.
individual bits. Usually, however, 8 or 16 bits are processed in parallel and this leads to unnecessary time wastage and programming
effort.
Closed-loop control of the speed in the various running modes can be executed with little power loss by regulating the exciting field.
There are three alternative possibilities for stepless control: a variable series resistor
a static-generator set a static converter Variable Series Resistor
This is a variable resistor connected in series with the field winding. It provides the simplest form of speed regulation and is still frequently used for emergency operation. The serious drawback of a resistor, however, is that it simply converts the unneeded power into heat, which is lost and thereby reduces the overall efficiency
of the system (Fig. 7).
Figure 7. Excitation circuit with variable resistor Motor-Generator Set
In this case there is a DC motor connected to the battery and driving One or two generators. The excitation current is controlled through the excitation of the generators. In order to make for easy main-tenance, the motor-generator set should have brushless AC genera:ors and rectifiers (Figs. 8 and 9). This means that each generator com-prises an exciter and primary machine. Then, as shown in Fig. 9, the combination of motor-generator set and propulsion motor contains at least 3 inductances which cannot be neglected. Hence this is
a critical structure for a closed-loop control system.
Figure 8. Excitation Circuit with MG Set - Block Diagram
Stator
Rotor
Stator
Figure 9. Excitation Circuit with MG Set
Nevertheless, the efficiency is better than when using a series resistor and there are not the same EMC problems to struggle with as with a static converter. The speed control system described later on employs a motor-generator set as the final control element for
the excitation current. Static Converter
From the point of view of the closed-loop control system a static converter (Fig. 10) can be regarded as almost the ideal final control
element.
However, in addition to the EMC problems mentioned earlier, there are also other difficulties. One is the measurement of the output
parameter - the excitation current. The magnitude of the current is determined by pulses of supply voltage and the variation of cur-rent then adjusts itself to the inductance of the field coil. The
amount of ripple content depends on the amount of magnetization. Closed-loop control requires the mean value of current to be measured. However, a microcomputer can only sample at intervals, processing each value obtained before sampling again.
As shown in Fig. 11, free sampling produces a stochastically
Figure 11. Asynchronous Sampling
sensed actual value, with the result that it appears to fluctuate. If a closed-loop control system were incorporated, it would attempt to eliminate the apparent error, so the result would be a constantly fluctuating, unstable excitation current. In turn, this would cause fluctuation of the armature current and therefore the propeller
tor-que. Possible remedies are: smoothing or
synchronous sampling.
(Figs. 11 and 12)
Figure 12. Synchronous Sampling Smoothing
The usual method would be to smooth the measured value over se-veral periods and sampling points (at least 3 but preferably about
Synchronous Sampling
A true replica of the actual value at each measurement can be obtained by synchronizing the sampling points with the clocking
fre-quency.
However, synchronizing the sampling time with the clocking cycle of the converter presents a problem. The high clocking frequency that is desirable in order to obtain low ripple content increases the cost and complexity of measurement. The indisputable advantage, however, is that the correct value can always be obtained and so rapid events can be controlled. A slow-running motor with a power
in the megawatt range has a field circuit of high inductance which leads to excitation time constants of the order of several seconds.
lAt 'Amax
speed range V
speed range IV
speed range
speed range
II I Fmax FFigure 13. Armature Current Versus Excitation Current (Average Discharging Characteristic)
60 . I
In the armature circuit, on the other hand, the time constants are of the order to tens of milliseconds. Contrasting this with the relationship between the armature current and the excitation current (Fig. 13) shows how even a slight change in excitation cur-rent produces large changes in armature curcur-rent (approx. 1:300). This is particularly true in the case of low excitation currents when the exciting field has a high time constant and the motor al-ready has a tendency towards instability.
A simplified block diagram of the closed-loop speed control system is shown in Fig. 14. There are 3 control loops; the inner loop
Speed- IA* Exciting
current
controller Amplifier
Figure 14. Propulsion Plant, Closed-Loop Speed Control
controls the excitation current according to the armature current, which is controlled by the centre loop; the speed control is then superimposed upon it. It will be quite obvious that the excitation current governs the behaviour of the other controlled variables, i.e. the response of the machine to changes is related directly to the dynamic characteristics of the excitation circuit.
Despite the large excitation time constants, the control system is also expected to deal with rapid changes in armature current and to provide highly accurate speed control. Accordingly then, a fast-response field current control system is needed, but one consequence of that is that smoothing of the static converter current causes excessive delays.
Simulation calculations, verified by tests, have shown that time constants around several milliseconds are needed for the control
system. Therefore, digital control necessitates program run times
suitable for this requirement. In an actual system the cycle time would have to be less than 1 millisecond.
Summarizing it can be said that the main demand made in the con-trol system is one of speed. When a motor-generator set is used it introduces the difficulty of having to deal with several induc-tances in series, and when a static converter is used, synchronous sampling brings extra difficulties.
THE SOLUTION TO THE PROBLEM
According to the requirements for an automatic control system that have been mentioned so far, the following features are
necessi-ties:
single-bit processing
execution of tasks as in Section 2 control of switching times
operation of several control loops
IF* Armature current controller
high sampling rate high computing speed
All the tasks are real-time problems and there are no fundamen-tal difficulties for continuous analog control circuits, apart from the complexity of the tasks. However, if all of the tasks are to be performed by one processor, the limits of computing can be reached very quickly. The closed-loop control in particular requires a mul-titude of arithmetic operations which generally take up a relatively large amount of computing time. Even when complex floating-point
calculations can be avoided for such cases, at least one 16 bit x 8 bit operation is still needed several times per cycle.
8-bit processors which have to perform such operations purely by software take over 100 usec for a multiplication. With 8 to
10 multiplications, 1 millisecond is reached quite quickly, and that
is the cycle time of the proposed controller.
This estimate alone shows that only processors capable of hard-ware arithmetic operations can be considered. Hence an Intel 8031
processor was chosen for the application in question. With 8-bit architecture it offers single-bit processing and boolean algebra as well as multiplication and division. Arithmetic operations are executed in approx. 2 /usec.
The average execution time of commands is approximately 1.3 /us.
A 1 byte x 2 byte multiplication takes about 20 /usec. The 8031 processor can also perform extra functions which, in the case of 8-bit or 16-bit microprocessors, generally require additional peri-pheral units.
The computing section comprises the following components
(Fig. 15).
Coon er 3.16 bit
I/0
RAM
Figure 15. Digital controller
RAM
(ER PROM) E PROM
±lov DAC 12 bit E PROM SAC 8 bit 0 10V I/0 Internal Bu Cour, er 3 MOO Counter Port RAM
I/O Port DAC8 bit
an 8031 microprocessor several counters EPROMs
RAMs
digital/analog converters
analog/digital converters with multiplexers
clock
input/output ports
All the components, including a serial interface, are mounted
on 2 PBCs of 160 mm x 100 mm format. They perform all the proces-sing but are not able to communicate directly with the power system. As yet, there are no interface modules which can connect the elec-tronic equipment to process signal I/O units such as circuit-break-ers, excitation current controllcircuit-break-ers, etc. Isolating amplifier stages are provided instead, matched to the conditions in the power system.
In this particular case, for example, the circuit-breakers are oper-ated at 60 V and approximately 2 A. The output stages are short-cir-cuit-proof and are also monitored. For binary inputs there are iso-lating input assemblies. Analog values are either decoupled poten-tial-free or the circuit is made symmetrical, provided the potential
isolation is outside the computer.
Potential isolation is necessary in order to keep disturbances and equalizing currents, which can be picked up either inductively or capacitively from the power equipment, away from the computer sub-assemblies, and also so that differences in potential do not matter.
The speed is sensed by a multi-channel pulse transmitter which is driven
through
a toothed belt and delivers 5000 pulses per propel-ler revolution on each channel.The direction of rotation is sensed from the phase angle of the
pulses. The pulses themselves are fed to counter chips
which
arescanned periodically; this gives a speed accuracy and constancy of better than 0.5%.
One of the declared objectives was to avoid as far as possible the character of a computer in relation to its operation, i.e. both with operation and with fault-finding the only essential knowledge should be about the functioning of the propulsion plant itself, with only minimum knowledge being needed about the internal functioning of an electronic control system. For this reason the principal values from the operator's point of view are set with digital thumbwheel switches, and the system is such that only deviations from the base values determined during commissioning have to be entered. This also prevents the entering of completely nonsensical values which could cause a malfunction. A continuous supervisory program signals any faults on a numerical display; the nature of the fault can then be deciphered from a reference list. Basically, it would be possible to print out the fault messages in plain text but this facility has not been provided here.
5. REDUNDANCY CONCEPT AND SUPERVISION
The propulsion system of a ship is possibly the most important one on board and its proper functioning is the most important factor governing the safety of the ship. For this reason all principal
