A digital recording system for model tests in irregular waves

RCHEF

Davidson Laboratory Stevens Institute of Technology

Hoboken, New Jersey

y. ScIuepsbouwkunde

Tec!sche HogschooI

NfiNU. 550

August 1959

A DIGITAL RECORDING SYSTEM FOR MODEL TESTS IN IRREGULAR WAVES

by

PAUL SPENS

FOR PRESENTATION AT TWELFFH MEETING OF AMERICAN TOWING TANK CONFERENCE

SESSION ON SEAGOING QUALITIES AT UNIVERSITY OF CALIFORNIA

BERKELEY, CALIFORNIA AUGUST 1959

A DIGITAL RECORDING SYSTEM FOR MODEL TESTS IN IRREGULAR WAVES

by

Paul G. Spens

FOR PRESENTATION AT TWELFTH MEETING OF AMERICAN TOWING TANK CONFERENCE

SESSION ON SEAGOING QUALITIES

AT UNIVERSITY OF CAL!FORNIA. BERKELEY. CALIFORNIA

Note No. 550 Approved by

August 1959 Edward V. Lewis

SUMMARY

This note describes a recording system of moderate cost, designed to record data in digital form on punched paper tape during tank tests of ship models in irregular seas.

Part I deals with the general characte-ristics of the system, and its application In tank tests.

Part II contains a somewhat detailed description of the working of the equipment, and it is intended primarily for those concerned with the construction or use of this or similar apparatus.

-1-TABLE OF CONTENTS

Introduction

General Description

...

Operation and Checking Application Technical Description DataProcessing Page i 3 5 6 7 lO

References.

., ...il

INTRODUCTION

For some time past shíp model tests in irregular tank seas have formed a part of the research program at the Davidson Laboratory. In

early tests the energy spectra of wave and ship responses were determined, and response amplitude operators were deduced. (See Lewis" 2) _More recently, co- and quadrature spectra have also been computed, and it has been found possible to deduce the phase relationships between wave and ship

motions. (See Dalzell and Yamanouchi3)

The various spectra and other results are calculated on an electronic digital computer. It was formerly necessary to read values of wave height and other quantities observed at intervals of 0.1 to 0. 3 seconds from photo-graphic galvanometer records. A manually operated key punch was then used to enter this information on punched cards for input to the computer. Since sorne 500 data points for each variable are used in computing the spectra, the amount of work needed to prepare the data for the computer was very considerable. To reduce the labor and time involved, the digital recording system described in this paper has been developed. This equip-ment enables data to be recorded during tests in digital form on punched paper tape. The tape can be converted mechanically to punched cards suitable for input to an IBM 650 computer, on which the spectra and other results can readily be computed. It is also possible that arrangements may be made to feed information directly from the tape into the computer without the use of cards.

There are, of course, many systems in existence for recording data in digital form. Very high speeds of operation are possible, but such equip-ment is usually elaborate and costly. Many other systems exist whose

speed of ope ration is too low for the present purpose. However, the system described herein provides the speed and other capabilities required for re-cording the motions of ship models in irregular seas. It also has consider-able flexibility and may be found useful for a number of other purposes. The principal elements of the system, namely the digitizer and tape punch, are standard manufactured item s, to which only slight modifications have

been made. _{Many of the other components are also standard products, and}

only one electronic unit , of simple and non-critical design, was specially constructed. The total cost of material purchased for the system amounted

to about $5, 000.

It is of some interest to note that a similar system, using the same type of digitizer and tape punch, has been developed independently by

H. G. Farmer at Woods Hole Oceanographic Institute for use in wave measure-ments at sea. In connection with this work, R. G. Stevens at New York Uni-versity is woicing on improvements in the means of transferring information from the tape to an IBM 650 computer, and has written computer programs for calculation of spectra and other results. The availability of these

-2-programs has saved much work which would otherwise have had to be under-taken in connection with the equipment developed at the Davidson Laboratory.

As an alternative to the use of digital techniques in ship model studies, analog systems for recording and analysis are also being developed and used. The relative merits of the two methods are open to considerable discussion. An analog system recording on magnetic tape certainly lends itself very well

to the determination of energy spectra by filtering techniques, and this is probably the most rapid method of producing energy spectra and response operators. However, the production of co- and quadrature spectra, and hence phase relationships, by similar methods presents considerably greater diffi-cultíes. Equipment for this purpose, suitable for application to ship model tests, does not appear to have been fully developed as yet.

Farmer and Stevens are using a convenient combination of the analog and digital systems in which data is initially recorded on magnetic tape. It

is later played back into the digitizer to produce a punched-tape digital record for analysis on a digital computer.

Adoption of the digital method of recording at the Davidson Laboratory was largely influenced by consideration that

the digital recording system could be used with already established digital techniques of analysis. New instru-mentation would therefore have to be provided only for

recording, and not also for analysis, and problems arisìng from a new type of analysis would be avoided.

the recorded data could be analyzed by any process for which a digital computer program could be written.

Thus considerable flexibility would be available to meet changing requirements in the future.

Experience with the digital recording equipment at the Davidson Labo-ratory has indicated that it does greatly reduce the labor involved in the analysis of model tests in irregular seas. It has thus become possible to

carry out a greater amount of testing than could be undertaken when the analy-sis had to be done by hand. A further advantage is that the computed spectra, response operators and phase relationships can all be obtained within a day or so after the performance of a test,

The cost of engineering and assembling this equipment has been shared by the Davidson Laboratory and the Bureau of Ships under Contract

GENERAL DESCRIPTION

The digital recording system is arranged to accept input signals in the form of voltages varying in direct proportion to the quantities being recorded, over the range ± 50 volts.

Six input channels are provided and a rotary commutator, driven from the tape-punch, connects each of these in turn to the digitizer. To

avoid confusion in subsequent examination of the tape it is desirable to use only five channels for data and to arrange that the sixth channel record a blank or other identifiable marking on the tape.

The signal voltages are converted to digital form by an electronic digitizer which is a standard instrument manufactured by Franklin

Electronics, slightly modified so that its output is in binary instead of

decimal form. The digital output of this instrument controls the tape-punch. The characteristics of the system as a whole are largely governed by the capabilities of the tape-punch. The most suitable punch available at moderate cost is the Teletype seven hole high-speed type, which operates at speeds up to 60 characters per second. Each character compri5es a

single row of holes, extending transversely across the tape, and can consist of any combination, or none, of the seven possible holes. The various holes correspond to the numbers:

1, 2, 4, 8, 16, 32 and 64.

Using this binary code a suitable combination of holes can represent any integer from i through 127, or -63 through +63.

As the system is required to record up to ten values per second on each of five channels, each value must be recorded by one tape-punch character. Since only integral values can be recorded, a "rounding error" is necessarily introduced. This error is less than ± 1. 0% of the maximum value which can be recorded, and is considered acceptable, especially as

rounding errors appear in spectral analysis as "white noise" and have only a small effect on the result.

The inconvenience of using a binary instead of a decimal code is justified by the fact that it enables information to be recorded at the fastest

rate possible with the chosen punch. With any other code it would be neces-sary either to accept a greater rounding error, or to use two tape-punch characters for each value with a corresponding reduction in the number of values recorded per second.

The form of the tape output may be seen in Fig. 1, where one

-4-five characters representing the signal values on each of the -4-five data chan-nels. lt should be noted that a group does not represent the simultaneous values of signals on all channels. The values recorded are all equally spaced in time, at intervals equal to one-sixth of the interval between consecutive readings on a given channel. It is not difficult to make allowances for this in the subsequent computations.

The tape-punch is driven by a 3600 rpm synchronous motor operating from the main ac supply. A chain and sprocket drive has been fitted and may be changed to provide different speeds of operation. At the highest

speed normally used, the punch operates 60 times per second, corresponding to a sampling interval of 0. 1 seconds on each channel. Sampling intervals

of 0. 15, 0. 2, 0. 25, and 0. 3 seconds are readily available by changing sprockets, and other intervals can be provided with little difficulty. If only two or three signal inputs have to be handled, shorter sampling intervals can b obtained by connecting each input to more than one position on the rotary commutator and a single input can be sampled up to 60 times per

OPERATION AND CHECKING

When a test is being made in the towing tank with the digital record-ing equipment, the older recordrecord-ing system is operated simultaneously to produce photographic records of galvanometer traces. A mark is made on this record at each revolution of the commutator in the digital recorder so

that the records may be coordinated. The record of the galvanometer traces is helpful in checking the operation of the test, since it is not pos-sible to see whether anything unusual has occurred by inspection of the

punched tape.

Owing to the rounding error inherent in the digital system, it is desirable to keep the signal level as high as possible without exceeding the available range. _{The galvanometer record provides a means of} see-ing whether the range of the digital system has been exceeded accidentally. In this event a number of data points may be read from the galvonometer

record and used to correct the cards punched from the tape before they are inserted in the computer. This may avoid the necessity of repeating a test. The provision of an indicator to show if an overflow has occurred is under consideration.

It is extremely important to ensure that the digitizer and tape-punch are functioning correctly, since it is generally very difficult to detect random errors on the tape.

Furthermore, it is quite likely that

if errors occur they will be in the higher binary digits where they may

have a serious effect on the result. The only obvious method of checking

is to compare a large number of points with the galvanometer record, but such a procedure is extremely laborious. To overcome this difficulty arrangements have been made to apply a predetermined signal to _{one input} channel by means of a step-switch operated from a contact on the rotary commutator shaft. _{The signal varies in such a way that the digitized} out-put on this channel increases by one unit each time it is sampled until the full range has been covered. Then it returns to zero and starts to rise

again. _{The correctness of the signal on this channel can be checked on} the tape without excessive difficulty. Alternatively it may be checked at some stage in the computing process.

_{If correct, it also serves to}

number sequentially any cards produced from the tape. (This check signal has not been recorded on the section of tape illustrated in Fig. 1. It could

replace the blank separating the groups if no other spare channel was available.)

-6-APPLICAT ION

The digital recording system has been used for ship model tests of pitching and heaving motions in irregular head seas and rolling in irregular beam seas. In the former case the results obtained by several different experimental techniques were compared. This would have involved an al-most prohibitive amount of time and labor if the old method of reading data points from galvanometer records had been used. However, with the digital recording system the analysis was performed easily. The full capacity of the digital system was not used, since only three variables were recorded, namely wave height, and pitching and heaving motions of the model. The computer output gave the following results in tabular form:

values of each of the three variables, wave, pitch and heave, at the chosen interval (0. 2 seconds in this case),

energy spectra of wave, pitch and heave,

co- and quadrature spectra of wave and pitch, and of

wave and heave,

response amplitude operators, pitch/wave height, and heave/wave height both over a range of frequency,

phase relationships between wave and pitch and between wave and heave over a range of frequency.

A complication was introduced by the fact that the length of the tank limited the run of the model to some 20 seconds while the irregular wave program runs for about 90 seconds. Several runs were therefore made at different parts of the wave program so that its full duration was covered. Results from these several runs were averaged to obtain the final results. In this case the averaging was done by hand after the data were obtained from the computer, but a program is being written whereby the averaged results can be obtained on the computer.

To illustrate the information obtained, plots are shown in Fig. 2 of some of the results from these tests. Some points obtained by tests of the same model in regular waves are indicated for comparison.

TECHNICAL DESCRIPTION

As has been mentioned previously, the signal input requiredfor the digital system is a voltage varying in direct proportion to the quantity being measured. The signal range which corresponds to the full binary output is approximately +50 to -50 volts. Preamplifiers are provided as necessary to achieve this level, and signals using a carrier frequency are first amplified and then demodulated. For these purposes it has been found very convenient to use Philbrick type KZX computing amplifiers and

Sanders Associates' Model 2 demodulators. The use of such plug-in de-vices has been found to simplify considerably the construction of the

equip-ment.

Signals from the six input channels are sampled in turn by the Computer Instruments rotary commutator. The latter is driven mechani-cally from the tape-punch shaft by a sprocket chain drive giving a 6:1 speed

reduction. The signal from the commutator is connected to the digitizer driver. This unit incorporates two Philbrick computing amplifiers, types

K2X and K2B. Full feedback is provided so that the input and output volt-age changes are the same. However, the input impedance is high, while the output impedance is very low, and the KZB can provide output currents up to 30 milliamps. The purpose of the digitizer driver is to provide the power necessary to charge condenser C1 to the signal voltage during the short time available between one reading and the next. It is important that the digitizer driver bring the voltage on the condenser exactly to the level corresponding to the signal input, unaffected by the voltage remaining on the condenser from the previous channel. Tests with fixed voltages applied to the input channels have shown that the error arising from failure to satisfy this requirement perfectly is of the order of one quarter of the lowest binary digit recorded. After condenser C1 has been charged it is disconnected from the input by switch S1 which is operated

mechani-cally from the tape-punch shaft. The voltage on the condenser remains equal to the input signal at the instant when S1 opened.

Since the digitizer input range is zero to 100 volts while the signal range is t 50 volts, the negative side of the digitizer is biased approxi-mately 50 voltsnegative with respect to ground, This is possible because the digitizer circuit is isolated from the chassis. The required bias is conveniently obtained by connecting ground to a divider across the stabi-lized +300-volt supply in the digitizer. It is necessary also to isolate

from ground the circuits connected to the digitizer output. Some care is necessary to avoid leakage to ground on these circuits, lest the bias level be altered thereby.

Shortly after switch S1 opens a second switch, S2 , closes and

*Reference _{to Fig. i is likely to be helpful in reading this section.}

-7-initiates operation of the digitizer. The method of operation of this instru-ment is such that the digital output corresponds to the voltage at its input terminais a variable length of time, between zero and five milliseconds, after ₅ closes. However, throughout this period the digitizer input volt-age is equal to 50 volts plus the voltvolt-age on G1 , which is constant. Thus

the digital output indicates the signal level at the instant when S1 opens.

S1 and

manufacturer. They are operated by eccentrics on the punch shaft, and the timing of their operation is adjustable. Some trouble has been experienced in making these switches operate cleanly. In particular, erratic action of S2 at one time caused digitizer readings to be taken at the wrong point in the cycle. This difficulty has been overcome by adjusting the switches and increasing the contact pressure. However, in the event of further trouble more elaborate switching arrangements may be developed.

After switch S2 closes and initiates operation of the digitizer, the voltage across a condenser in this instrument commences to rise at a precisely controlled constant rate. When this ramp voltage is equal to the

voltage at the negative input terminal a gate opens. When the ramp voltage

is equal to the voltage on the positive input terminal the gate closes again. The interval for which the gate is open is therefore proportional to the voltage between the input terminals. When open the gate passes pulses from a 100 KC crystal-controlled oscillator to an electronic counter. The

count recorded is thus directly proportional to the voltage at the input ter-minals.

For normal use the digitizer is supplied with three decade counters so that an input voltage of 100 gives a count of 1000. For the present pur-pose the manufacturer has provided an interchangeable set of counters modified to operate on the binary scale. By altering the rate of rise of the

ramp voltage, a count of 512, i. e., 29, is made to correspond to an input of approximately 100 volts. The binary counter outputs correspond to

counts of

1, 2, 4, 8, 16, 32, 64, 128, 256, and 512 pulses.

Since the tape-punch character only has seven holes it can only record numbers from 1 to 127. Each digit on the binary tape-punch scale therefore corresponds to four pulses, and values are recorded on the binary scale

1, 2, 4, 8, 16, 32, 64.

Digitizer outputs corresponding to one quarter, one half and 128 on this scale are also available, but are not normally included in the recording. However, the availability of the complete range as illuminated numerals

-9-on the digitizer panel is c-9-onvenient for making adjustments. Furthermore, in order to record with the tape-punch any other series of seven consecu-tive binary outputs from the digitizer, it is only necessary to insert the plug connecting the digitizer and punch driver at a different position in its

socket. This possibility is useful when checking for correct operation

using test signals whose values are known., Then the higher binary digits are disregarded while the lower levels are examined to check the

correct-ness of the recording.

The digitizer output circuits are of high impedance. Their voltage levels are i-35 volts for binary O and +70 volts for binary 1. Each of the seven binary outputs is connected through suitable biassing and limit-ing circuits to the grid of a correspondlimit-ing 6V6 tube in the punch driver.

Each of these tubes has connected in its plate circuit the winding of a magnet which controls the punching of the hole in the tape corre spond-ing to a particular binary digit. If the digitizer count is binary i on a

particular channel, the punch magnet current is approximately 30 milliamps while for binary O the tube is cut off and passes very little current.

In order to limit operation of the punch magnets to the appropriate parts of the tape-punch cycle,the screen voltage to the 6V6 tubes is con-trolled by a third switch, S3 , which is also driven by the tape-punch

shaft.

The load on the unstabilized punch driver plate supply, and conse-quently its voltage, vary over a considerable range according to the number of punch magnets energized. The provision of a stabilized screen supply is necessary in order to keep the variation in the punch magnet currents within acceptable limits regardless of the number of punch magnets ener-gized simultaneously.

The tape-punch was received from the manufacturer with the motor directly coupled to the punch shaft. A counter shaft mounted in ball bear-ings has been fitted in the original position of the motor which now drives this shaft by chain and sprockets whose ratio can be changed. Switch S3

is a simple wiper type, mounted on this shaft, which also carries the sprocket driving the rotary commutator.

The punch shaft is driven continuously by the synchronous motor. A system of eccentrics and links operated from the shaft advances the tape and punches the holes. However, these linkages are so arranged that

they only perform their functions when appropriate magnets are energized. Feeding of the tape is controlled by the feed magnet which may be ener-gized by a switch under the control of the operator, Holes are only punched in the tape when the appropriate punch magnets are energized

through the punch driver. Punching may be started and stopped by the operator through a switch which cuts off the screen voltage to the 6V6 tubes in the punch driver. When this switch is on, the punch driver is controlled by the digitizer output, and holes are punched in the tape corresponding to the digitized values of the input signals.

DATA PROCESSING

The tape output from the digital recording system may be converted

to IBM cards on the IBM Type 46 tape-to-card punch. Normally this ma-chine handles five or eight-hole tapes punched in IBM or telegraph code. As a check on the correct punching of the tape these codes are so arranged that every character consists of an odd number of holes. The tape-to-card punch is designed to stop on any character having an even number of holes. A small modification to the machine in order to disconnect this parity

check is necessary before tape from the digital recorder can be handled. It is also necessary to provide a simple guide for the tape where it enters the reader, since the width of the seven-hole tape is different from that of the tapes for which the machine is designed.

Some ingenuity is required in setting up the tape-to-card punch to handle binary tapes. The machine incorporates ten "selectors" which

perform similar functions to relays. One of these selectors may be arranged to correspond to each of the seven possible holes in a tape character. When the machine reads a character the selectors corresponding to punched-holes are transferred, while those corresponding to holes not punched remain in their normal position. The machine next proceeds to punch holes in a num-ber of consecutive columns of the card. The punch in each column is

governed by the position of a particular selector. For a selector not trans-ferred, an "8" is punched corresponding to binary O, while for a transferred selector a "9" is punched, corresponding to binary 1. Alter all the binary digits of the character have been transferred to the card in this way the next

tape character is read and the selectors are re-set. A single card may

con-veniently contain the data corresponding to a group of five or six tape-punch character s.

The IBM 650 computer has the ability to test each card column for the presence of an "8' or "9". It is therefore easy to write a short program which will read cards containing the data in binary form and punch out decimal values of the data. A printed list of the decimal values can be prepared on the listing machine if it is desired to inspect the data. The cards containing the data in decimal form may be used as input for an existing computer program

known as Statisan II. This program calculates the energy spectra and co-and quadrature spectra for any pair of the data channels. Output cards from Statisan II may be used as input for another program, Statisan III, which computes response operators, phase angles, coherencies and other functions derived from the spectra.

It is hoped that the process of computation described above may soon be shortened by making the binary-to-decimal conversion part of the Statisan II program, and by conveying the information electrically from the tape-to-card punch to the computer without the use of cards,

REFERENCES

1. Lewis, E. V,: "Ship Speeds in Irregular Waves', Trans. SNAME,

1955.

2, _{Lewis, E. V.,, and Numata, E.: "Ship Model Tests in Regular and} Irregular Seas", ETT Report 567, September 1956.

3. Daizell, J. F., and Yamanouchi, Y.: "Analysis of Model Test Results in Irregular Head Seas to Determine Motion Amplitudes and Phase Relationships to Waves", Paper presented at Second Summer Seminar on Ship Behavior at Sea, Stevens Institute of

6 INPUT CHANNELS -o -o DIGITIZER DRIVER ROTARY COMMUTATOR SPROCKET CHAIN DRIVE 6 : I REDUCTION K2X K28 8uF ANALOG DIGITAL

INPUT -OUTPUT- O-lOO V

10K

T-DIGITIZER\

SYNCHRONOUS

MOTOR

PUNCH DRIVER FIGURE

1 MECHANICALLY OPERATED SWITCHES 2 S3 +1 315v 150v -STABILIZED OK IO K lOOK 5M SCHEMATIC DIAGRAM OF DIGITAL RECORDING SYSTEM 7 SIMILAR CIRCUITS CHANNEL NUMBERS DECIMAL VA LUES SPROCKET VARIABLE TAPE

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RESULTS OF TESTS OF DESTROYER MODEL IN REGULAR a IRREGULAR WAVES

- RESULTS FROM SPECTRA

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AMPLITUDE RESPONSE PHASE ANGLE

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