An updated design of a disposable wave buoy

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TECHNISCHE HOGESCHOOL DELFT

AFDELING DER MARITIEME TECHNIEK

LABORATORIUM VOOR SCHEEPSHYDROMECHANICA

AN UPDATED DESIGN OF A DISPOSABLE

WAVE BUOY

M. Buitenhek and J. Ooms

Rapportno. : 463

May 1978

Deift University of Technology

Ship Hydromechariics Laboratory Mekeiweg 2

2628CD DELFT

The Netherlands Phone 01 5 -786882

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)

Summary.

Introduction.

Requirements for the design.

Functional description.

Stabilization of the vertical position.

Blockdiagrarri of the wave buoy.

5.1. The accelerometer. 5.2. The carrier oscillator. 5.3. The differential amplifier. 5.4. The demodulator.

5.5. The voltage controlled oscillator (V.C.O.)

5.6. The transmitter and the antenna. 5.7. The stabilized power supply. 5.8. The batteries.

Calibration and the sensitivity of the wave buoy.

The composition of the buoy.

The wire reel and the stabilization weight.

The launching.

io, Data reduction.

Results.

Design data.

References.

Appendix A : Wave buoy circuit description.

Appendix B : Directions for use.

CONTENTS Page 2 2 3 4 4 5 5 5 5 5 7 7 7 8 8 8 9 li 12 13 24

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O)

Summary.

A description is given of a simple wave height measuring buoy which is in-expensive enough to be disposed of after it has been used once to measure the

sea state in the environment of a coiiiinercially run ship.

Introduction.

A wave heigth measuring buoy has been developed by the Shiphydromechanics La-boratory of the Delft University of Technology for the measurement of seawaves. The buoy supplies continuously data of the vertical accelerations to which the

floating body is subjected.

The vertical position of the body (and the accelerometer) is maintained by means

of a stiff arm with a length of I meter and a thin steel wire with a length

between 2 and 40 meters and a weight of 100 N at the end.

The length requirement depends on the expected significant wave period. During the launching, the steel wire is wound upon a cardboard case and will be rolled

out when the buoy touches the surface of the sea.

The frequency modulated measuring signal is transmitted by radio to a receiver

on board of a ship. The principal design consideration has been to find a compromise between good technical specifications and relatively low costs

because in most cases the buoy cannot be recovered after launching from merchant ships.

The wave buoys have been used in experiments on board of the S.S. tAtlti

Crown" (ref. 1) and,recently,HNMS Tydeman.

Both analog and digital data-reduction methods can be applied to compute histogrammes, covariance functions and power-density spectra of wave heígths

and wave periods by means of double integration and a square bandpass filter.

Requirements of the design.

Bcause the recovery of a measuring device from the sea is _{a time consuming and} expensive job, the buoy is designed and constructed as a disposable one.

The buoys are intended to be launched from ordinary merchant ships by the crew,

without interfering with the ships schedule. The following requirements must be met:

- reliable information of the vertical accelerations - reliable operation

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- shockproof

- easy to handle by one person

- inexpensive.

Functional description.

The absence of any fixed reference necessitates the use of a floating body from which the information about the wave heigth and the wave period must be obtained from the vertical acceleration-signal by means of double integration.

Proceeding from a linear accelerometerwhich is connected to the vertical axis of the buoy, care must be taken that this axis is being stabilized in the

cor-rect dicor-rection.

Stabilization of the vertical position.

A stipulation for the correct data reduction of the acceleration signal is, that the pick-up keeps the vertical position regardless of the shape of the wave. In connection with the single use of the buoy, expensive stabilization prin-ciples are left out of consideration. In figure 1 a schematic diagram is given, on the basis of which the operation of the wave buoy can be illustrated.

for: 12 » L FL1 L1Gsinß

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e

A thin wire, to which a weight G is connected at the end is, via a short stiff

construction, connected to the floating body.

A disturbing moment M, caused by the moving sea surface, will turn the buoy

around a horizontal axis.

The erecting moment is brought about by the product of the length L1 of the stiff tail and the force F1 which is the perpendicular resolution of the

tension in the wire.

For L2» L1 applies, that the anglea approaches zero. Therefore, the erecting moment may be written as:

F L =

_L1Gsin

The larger the angle of rotation 3 , the larger the erecting moment will be.

The above conclusion is based on a static situation.

The dynamic behaviour is kept out of consideration. With the decision of the length of the wire, allowance must be made for the wave dimensions.

In the measurements reported presently, the following lengths of wire have been

used with good results:

During these masurements it has been established that by means of this tail stabilization the vertical position will be kept within a deviation of + or -5

degrees.

5. Block diagram of the wave buoy.

In figure 2 the block diagram of the wave buoy is shown. It consists of the

following parts: carrier oscil-lator. regula-ted po-wer supply. batte-ries. synchronisation signal

Fig.2 Block diagram of wave buoy. >

on the IJsselmeer : 2.5 m

on the North Sea : 12 m

on the Atlantic Ocean : 40 m

signal flow. antenna accele-rometer dif phase- sensiti-v/F con verter trans-mitter. and ampi. ve demo- and

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- the accelerometer and bridge circuit

- the carrier oscillator which powers the accelerometer

- the differential amplifier

- the phasesensitive demodulator - the voltage to frequency converter - the transmitter and antenna

- the stabilised power supply

- the batteries

5.1. The acceleration sensor.

The accelerometer must meet the following specifications:

- frequency response : O - 5 Hz

- hysteresis _{< 0,2%} F.S.

- measuring range ± IO rn/sec2

- overload range 20 times

- inexpensive

In the present design an acceleration sensor has been chosen which is equiped

with an inductive pick-up. Its particulars are:

manufacturer : Schaevitz EM Ltd.

type number : A411 - 0001

range : ± 3 g. (± 29.4 m/sec?)

natural frequency approx. 30 - 40 Hz

hysteresis and nonlinearity max. 0.5% FSO

At first glance, the last specification may seem to be rather high in view of the requirements as listed before. However, the hysteresis is just a small part of it and sufficiently low for our purposes.

5.2. The carrier oscillator.

The consequence of the use of an inductive sensor is the necessity of a carrier oscillator, for the excitation of the coil, and a phase-sensitive demodulator.

In the oscillator the 8038 waveform generator of Intersil is used, which

is capable of producing sine, square and triangular wave forms.

The sine waveform is preferable to avoid harmonics of the base frequency in other parts of the electronic circuits. The chosen frequency of the carrier

oscillator is approximately 6000 Hz.

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'w

Ò

5.3. The differential amplifier.

Because the output voltage of the accelerometer is rather small, a differential

amplifier is needed to amplify the acceleration signal. It has to be a differential

amplifier while the accelerometer is made a part of a Wheatstone bridge.

5.4. The demodulator.

The phase-sensitive demodulator is used to eliminate the before mentioned

6000 Hz oscillator signal.

of the remaining signal,the amplitude andpolarity are proportional with those of

the appearing acceleration..

The demodulator is composed of an analog switch, controlled by a comparator and

an op.amp.

5.5. The voltage to frequency converter.

Because the signal strength at the receiver side fluctuates during the transmission by radio of the acceleration signal,(whích may influence the

amplitude of the signal) the information is converted into a frequency deviation of a low frequency tone that is transmitted with the transmitter. The converter is built up with a sign changer, an integrator and an analog switch, controlled by a smith-trigger. To meet certain requirements of the Dutch Radio Service of the P.T.T., the converter is followed by a low-pass filter, to assure that the bandwidth of the transmitter signal confirms with the regulations that apply.

5.6. Thetransmitterand the antenna.

With the help of a crystal controlled AN-transmitter (± 27 MHz) the frequency modulated acceleration signal is transmitted by means of a X14 antenna.

5.7. The stabilised power supply.

Because the voltage of the batteries is not constant, the power is stabilised to increase the stability of the measuring part. The transmitter, however, uses the unregulated voltage to get the highest efficiency of the batteries.

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I

p

V

4/

Fig.3 Composite drawing of wave buoy.

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5.8. The batteries.

The batteries must meet the following requirements;

- sufficient power, also at low temperatures

- robust

- good connection with the electrodes, even after launching of the buoy.

- leak proof - good shelf life

A few buoys have been equiped with rechargeable Nicad batteries. These buoys are intended to be recovered after use and to be used many times. They can be

retharged via a waterproof connector mounted on the buoy.

The batteries used presently are of the type R 20 HD manufactured by Philips, while the rechargeable buoys use NCC 400 Nicad batteries made by Berec.

Calibration and sensitivity of the wave buoy.

With each separate buoy a static calibration is executed in a simple way, by turning the accelerometer or the complete buoy over a known angle. The sensi-tivity (Hz per m/sec2) is determined from the values at O and 90° with regard to the vertical.

The composition of the buoy.

In figure 3 the composite drawing is shown of the complete wave buoy. Referring

to this figure we can distinguish the following parts; antenna

copper earthstring

transmitter housing

housing for electronic circuits

batteries accelerometer stiff 3-legg tail

iron wire with length of 2.5 to 40 m

cardboard case

k) short cotton wire

1) stabilisation weight

To facilitate transport the buoy can be disassembled in more manageable parts which can be reassembled on the spot in a simple way.

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Ò

The wire reel and stabilization weight.

The wire reel and stabilization weight have to meet the following requirements:

- inexpensive

- simple to assemble when the buoy is being prepared prior to launching - not vulnerable during transport

- not vulnerable during launching especially while dropping from a large height

- reliable

It has been found that the functions of the wire reel and the stabilization

weight have to be disconnected.

Earlier in the development, the wire had been wound on the stabilization weight, which had the shape of a spool. It was found, however, that the force in the wire became too high at the moment when the buoy touched the water.

This extreme force had been caused by the high deceleration of the floating

body and the rotation inertia of the stabilization weight.

With the present construction a twined steel wire (diam. I iiaii) is wound on a

cardboard reel, after which the wire is fastened with a layer of paper tape. After touching the water surface the weight is not stopped by the wire which

freely unwinds from the reel but primarily has been braked by the resistance of

the weight moving through the water.

The launching.

The tail buoy, wire reel and stabilization weight have been fastened to each

other with the help of snapshackles.

The transmitter is set into action by removing a small magnet on the outside of the buoy. After the complete system has been launched, the wire can freely unwind and after a short time, the stabilization weight will be in position at

the desired depth.

During full scale measurements (ref. 1) launchings have been performed succes-fully from a platform, situated ± 15 m above the sea surface, while the

ship-speed was ± 24 knots.

lo. Data reduction.

The received signal consists of the frequency modulated acceleration signal. Dependent on the question in which way the measuring data have to be offered,

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a

selection has to be made of the different reduction methods.

Independent of the used method, the following remarks have to be made:

- owing to the not optimum stabilization of the wave buoy, although the rolling

motion remains small according to observations, a horizontal acceleration

corn-ponent is introduced causing a rectifying effect, which makes it impossible

to treat the signals from D.C. (direct current).

- because the acceleration signal has to be integrated twice to obtain the information about the wave height, allowance has to be made for the drift of the acceleometer inside the buoy and at the receiving side for the drift of the demodulator and both integrators.

- because during the experiments other conversations by radio can take place,

receiving conditions have to be taken into account.

This demonstrates the necessity of the use of band filters.At the receiving side it proved to be advantageous to use a Phase Locked Loop (PLL) demodulator to convert the frequency modulated information signal into a voltage proportional with acceleration. This type of demodulator can be given a small bandwidth

( approximately O Hz), such, that noise and other unwanted components are

rejected to a very large degree.

Sonexamples of reduction methods are:

- on-line reduction with the help of a "Frequency to voltage" converter, band

filters and integrators

Input of the signal in a TReal_TimeTT correlator and a Spectrum Display produces

the covariance function and the energy density spectrum of the wave height.

- off-line reduction with the help of an instrumentation tape recorder.

By counting during equal time intervals the frequency modulated acceleration signal, the information is obtained about the amplitude and the frequency of

the original measuring signal.

The autocovariance function and the energy density spectrum can be obtained from this information both with a digital computer and a hybrid computer

(ref. 2).

11. Results.

The first experiments have been carried out to test the operation of the wave buoy and to compare the results with those of the fixed wave height platform

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E 3 (t)

3

2.0 k r io r

lo

-/s'

2'

- --- triton 13CO... 13O hr

tatbuoy 14°.... 14' hr 1-o t i :3.0 20 20 3.0 0

u e 1lsec

Fiq.4 Comparison between he wave spectrum b F.5 Cornp3rison between the wac spectrum by tho tixed patform and he t-3Tbuoy. the waverider aid the aiIbuoy.

80 :.0 2.0 t-tailbuoy H,3 3.70 ni --- wayrider H3.4ßm 0.5 1.0 1.5 _j -.-. r sec H 705 m o o.s 1.0 .5 O

w ----'Isc

w

Fg.6 Measured wave speotrum during Fig7 Maured wave spectrum during

fuU cote rrieasuremens. tu;! sc&e measurements.

:- triton I

i'

i 15°_ 15° hr 02 -0.1 3.0 H13 4.08 .n

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The reference records of the Triton measured during the same time as the records of the wave buoy appeared not to be useful.

Therefore a comparison has been made with records taken before and after the

experiments with the wave buoy.

A second comparison has been made with a wave rider. The wave rider is a wave height measuring buoy made by "Datawell" which is equiped with an internally stabilized pick-up. Because in the wave rider the acceleration signal is con-verted into a wave height signal before transmission, the data of both buoys have been computed in two totally different ways. The results of the comparison

are shown in figure 5.

During full scale measurements on board of the container ship "Atlantic Crown", sailing from Le Havre to New York [i) , and recently, on board of HNMS Tydeman,

E3] the wave buoys have been used under real conditions.

Although every launched buoy worked perfectly the transmitting range appeared too small for a long term record on board of a fast ship. Therefore, for that kind of records the transmitting power of the newest wave buoys has been

in-creased to 1.5 W.

In figure 6 and 7 two wave height spectra have been given of records near New

Foundland.

12. Design data.

Supply voltage : 22.5 V

Center frequency of modulation signal : 1688 Hz

Sensitivity : J350 Hz/g

Action radius : 15-35 km

Working life time : 8 h

Transmitted power : 1.5 W

Transmitter frequency : 27.7 MHz

Buoy diameter : 0.43 m

Length of the antenna : 1.50 m

Wire length : 2-40 m

Weight of the buoy : loo N

Stabilization weight : 100 N

- depends also of the receiver, reciever antenna and position of receiver

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I)

References.

Ei] Beukelman W. and Buitenhek, M.:

"Full scale measurements and predicted seakeeping nerformance of the

Containership "Atlantic Crown",

Netherlands Ship Research Centre TNO, Report 185 5, November 1973.

L21 Pasveer, F.J.:

"Speed-corrected demodulation and on-line processing of data from

analog tape",

Delft University of Technology Computation Centre, Proceedings of

7th AICA Congress, Praag, 1973.

L3] In preparation: (Report no.494)

"Full scale measurements and predicted seakeeping performance of

the HNNS Tydeman".

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Appendix A.

Wave buoy circuit description.

A. I . Introduction.

In this appendix we shall discuss the hardware implementation of the electronic

circuits of the wave buoy. The complete schematic diagrams of figure AI (low frequency part) and figure A2 (transmitter) can be broken down to the blocks as shown in figure A3. In the following, the how's and why's of each block shall

be described.

The power supply.

Power supply regulation is accomplished with a simple, low cost, three terminal voltage regulator (see figure A4). It delivers an output which lies in the

range of 14.4 to 15.6 V with a typical value of 15 V. Line regulation over

the power supply input voltage range of 18 - 24 V is 144 mV worst case and typical 5.3 mV. Due to the ratiometric design of the buoy circuits the cali-bration of the buoy does not change for even very large variations of the

regu-lated voltage. In practice, the circuits still work perfectly with voltages down

to approximately 12 V.

Therefore, it is not necessary to adjust the power supply or using a better regulator. Also, because of the low current requirements (typically 22 mA) of

the low frequency part of the electronics there is no need for a heatsink. On the board, an operational amplifier, preceeded by an attenuator (R27 and R28), generates a 7.5 V voltage which tracks the 15 V voltage. C14 and C15 decouple this (7.5 V) voltage while R30 is added to keep the operational amplifier stable with such a large capacitive load. To prevent h.f. signals to enter the

elec-tronics, a 71-filter, consisting of a feed-through capacitor, L1, C10 and C11

is added to the input of the voltage regulator. Decoupling at the output of this

regulator is done by C12 and C13.

Carrier oscillator.

The carrier oscillator consists of a single chip waveform generator that

delivers the sine wave we need. Because the sine wave output may not be

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18_24V 2n OV r-lo

cl

2 Accelerometer type A411_000l Rl R2 15k 15k

rT

6T5,

7 ₈₀₃₈ ll o 82k

-I

R5

IopL

F-20k Tp i ii 32 14 2

î

12 _R7 -t-R4 R&0 10k 4k?

I

R8 20 03 02 P 10

,,

rp2 201i C c3

E10

i= Rbalance P2= c_balance in c5 in

'-4

k 10k R27 9 20k1 P28 20k

T

P12 5ik 3

10

I 10k

H

10 R38 L2 2n2 o Ri3 709 b

47

_c7 4

Hopi

R29

rn

_10k 324 8

1

to trans-mitten Printed circuit 053 Tp3 -1 I R16 P3 2k P14 R15 10k 2k Tp 4 R30

oj

51

c u

31tJ3

Fig. Al Schematic diagram of buoy electronics (low frequency part)

16k Sensitivity Tp 9 4 13 +75V

:

R3l F0î1 l5Ji

L

il 14 ---- 2 --?.5 _RiS -Oli 513 R19

p'

Center 2Th frequency i R32

cisl

l2n7 324 6 4 14 150 R17 10 k o 10 9

LJ

R33 62k 2 C9 Opi

IF-R21 150 k 324 P20 P22 39k 11k S-. 8 12 3139 b H 7 I ciz 4 R34 10k R35 1 20k

loi

TplO R363k3 Tp 5 R23 20k

-r,>-R250 70k R24 lo k Tp ii

R371

2k2J

324 P26 20 k C19 33n P39 o cl8 6k8 5n

6T

Tp 6

;ø

IP 12 12n

[I2»

Tp7 _+15V C12 c13 8 7815 3 2p2 c20

I

-H

I metal film

1/4W 1%

(

F-- =carbon film

i/w

5%

,z&_ =chassis ground

1k

= feed- through capacitor

---

=ceramiC feed-through

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+18-24V R40 22k C21 input o-'47 R41 2k7 0V R42 2k7 Ti IJE2955 R45 470 2 R47 4n7 2k2 R4 X-ta i 1 IL3 2k2 I IOji5 R4 820 120

Fig. A2 Schematic diagram of transmitter.

carrier oscil-lator. a c c e 1 e --__-.-rometer and bridge regula-ted po-'. wer supply dif ampi. Li 2u2 2n2 7815 + - o__,-Y_Y-v-5 clOcil+ 18-24V

- =

-

I

3n3}u31

i

R29 -I

jumper selects transmitter power(.2 or 1.5W)

Fig. M lieguiated power supply.

-C2447p

T3

T

2N3904 C2 3 i50 phase-s e n phase-s i t i ve demo-dulator i50p synchronisation signal

Fig. A3 Block diagram of the wave buoy.

C271 4n7 L4 5 2N3 p 4 5 I LS 4p7 C28 L6 signal flow. antenna v/F con trans-verter mitter. and filter. , Tp7 ol5V out

113 I0fup8 0V out

Tp9 p, C14 + fC15

3u31

[ûu1 2p2 L7 2p2 C29 to antenna ,I1 N444 detector output C30 4n7

-.-to copper earths tring R27 9 10k -R31 324 i .5V out 4- 51 io- _Op batte-ries. > 20k io R28 20k

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an operational amplifier is added to the circuit (see figure A5). 16 -I Jj2

-4

cl I

_'R3 82k R4 :i:

jiOk

C 3 ou P5 20k 324 14 1

Fig. A5 Carrier oscillator.

This amplifier acts as both a buffer and an amplifier. For AC, its gain is set to three by means of R4 and R5. However, for DC, C3 keeps the gain at unity and therefore, the DC voltage at the output of the amplifier is equal to the DC voltage at the input. And because the input sine wave is riding on a DC level which is equal to V the DC voltage at the output of the amplifier and over

C3 is approximately 7.5 V.

R6 is a bleeder resistor which prevents excessive cross-over distortion of the operational amplifier when passing the 7.5V level.This cross-over distortion is

caused by the low quiescent current of the amplifier.

The frequency of the generator can be calculated with the formula

f = O.3/R1C1 (R1 = R2) and is, for the values given, approximately 6 kHz.

A.4. Accelerometer, bridge and differential amplifier.

The accelerometer is, as already mentioned in chapter 5.1., of the inductive type. For them, who are not yet familiar with this type of accelerometer we shall give a brief description. In principle, the accelerometer consists of a small seismic mass connected between two springs and surrounded by a damping

fluid (see figure A6).

flR6 L 4k7

0

springs cavity filled with damping fluid. seismic mass

Fig. A6 Principle of accelerometer.

coil with center tap . R2

TC2ul

8038 2 Tp 1, 2 Rl 15}c 4 J- Tp2

)

s

(a) (b)

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An acceleration acting on the seismic mass yields a force on it according to Newtons second law F m.a. This force changes the position of the mass. For

dynamic measurements, the response of the accelerometer is determined by the

equation of motion which states:

mx" + rx' + cx = F(t) =

ma(t)

(1)

However, for frequencies much lower than the natural frequency

.

irc

(with f0 =

;;

) the equation can be replaced by:

cx(t) = m aCt) (m = mass c = stiffness) (2)

which is a good approximation. The accelerometers that are used in the wave-buoy have a natural frequency of 35Hz which is, of course, more than sufficient for our application. From equation 2 we see that the deflection x(t) is

propor-tionally with aCt).

To get an electrical signal from the accelerometer to the outside world a coil with a centertap is wound around the bus that contains the seismic mass, springs and damping fluid. The induction of both parts of the coil depends on the posi-tion of the (metal) seismic mass. When we feed the coil with an AC voltage and the seismic mass is in its mid position the induction of both parts of the coil is equal and so is the voltage drop. In that case the output voltage at the centertap is half the supply voltage when measured with respect to the outer taps of the coil. When the position of the mass is changed, due to acceleration or gravity forces, the voltage at the centertap changes. The voltage difference

is proportional with the acceleration. So, to measure the wave accelerations we must measure the output voltage changes with respect to the output voltage

at rest.

Figure A7 shows us the circuits that surround the accelerometer.

R7 C4 in 20k C5 i Inn Rl i

io -

10k 51k Tp3 R13

-H

F-i rl il R14 10k

Accelerometer type A411-0001

Fig. A7 Transducer,bridge and differential amplifier.

51k

(20)

)

The accelerometer is made part of a Wheatstone bridge. A differential

amplifier, consisting of an operational amplifier and R0 and R13 amplifies the voltage difference at the output terminals of the bridge.

In the rest position of the accelerometer the amplifier AC output voltage is adjusted with P1 to the minimum value. It is not possible to adjust back to exactly zero volts. This is caused by the stray capacitances around and inside

the accelerometer which cause the output voltage (of the accelerometer) to be a few degrees phase shifted with respect to the input voltage. When subtracting

a voltage which is in phase with the input voltage of the accelerometer we shall never get zero volts. To overcome this problem another network consisting of

C4, C5, P2 and R1 was added. When the reactancy of C4 and C5 is large compared

with the resistance of P2 the current through C4, C5, and 2

is leading

approximately 900 with respect to the input voltage. So, the voltage over P2 also leads approximately 900. Now, it is possible to compensate the out of phase

component of the accelerometer output voltage with P2. Resistor R11 sets the adjustment range of P2 and minimises the interaction between adjustments of pl and P2. However, there is still a little interaction left and therefore, the adjustments of P1 and P2 must be repeated two or three times to get minimum

out-put voltage after the differential amplifier.

The resistor R14 is again a bleeder resistor as already described in section A3.

A.5. Phase sensitive demodulator.

The phase sensitive demodulator is shown in figure A8.

18 -p3 R15 2k 18k 2k RO 10k 2

F-R12 10k

HL

-1 -- 4o- -I_I5_i HC7 o Oui 14 R17 2 10k

Fig. A8 Phase sensitive demodulator.

Tp5 C9 Oui R2 i

-HF--

_150k 324 13 R18 R20 240k 39k R19 150k 709 Tp P4 4 20k o

s

C8 Oui R16

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At the input, the modulated signal from the differential amplifier appears

and passes C8 which stops unwanted DC-components. With P3, which acts as a

regular "volume control", we can adjust the sensitivity of the demodulator. After R16 the signal is entering the analog switch. This switch is switched on and off

synchronously (see figure A9) with the carrier frequency by means of a fast

UTp2(t) UTp3(t) UTP4 (t) 1R17 (t) U (t) -Lp5 UT5(t)

Fig. A9 Timing diagram

operational amplifier which acts as a comparator. When the switch is off, no

current flows to the inverting input of the 324 operational amplifier. If

Output carrier oscillator.

Output dif

fe-rential am-fier. Solid line; acceleration in one di-rection. Dashed line; acceleration in opposite direction. Output comparator. Current through resistors Rl6 and R17. Output voltage of demodulator with C9 removed. Output voltage of demodulator with C9. /'_ \ ,---' I

\

¡\

K\J

I

\\/

-(j

\

'

\

I \__J

Ç)

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switched on1the current to the inverting input (summing point) is equal to

)

U'(t)/(R16 + Ron + R17) U'(t)/(R16 + R17). With C9 removed the signal

at the ampli fier output should look as shown in figure A9,Thisvoltage can be

thought of as consisting of a DC component and an AC ripple. Now, adding C9, the AC component is filtered away and only the DC component remains. In practice, of course, the "DC component" we mention here is a more or less slowly varying signal proportional with acceleration and only a real DC component when measuring gravity or a uniform acceleration. The output of the amplifier is send to the voltage to frequency converter (FM modulator).

Because the FM-modulator needs a DC voltage at its input to set the required center frequency R18, R19 and P4 has been added to the demodulator. P4 is

con-nected to a voltage which is 7.5 V negative with respect to the voltage at the

inverting input. Therefore, for the given values, the output is shifted

)

approximately 2.8 V positive which gives the desired center frequency. With P4 can adjust this frequency to the exact value (1688 Hz). This must be done

with minimal output voltage at the output of the differential amplifier (Tp 3).

A.6. Voltageto frequency converter.

e

As already mentioned before the voltage to frequency converter consists of a

sign changer, integrator, analog switch and a smith-trigger (see figure AlO).

R25 R22 11k

H

R24 R32 10k è

'?'

10 L.j9 R31 -- 15k 6

f-

II - 2ri7

Fig. 10 Voltage to frequency converter.

-1

F-R35 20k in _TplO R33 62k C16 10k 10k -r

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o

e

)

The sign changer is a regular inverting amplifier consisting of an operational amplifier and resistors R23, R25 and R26. An input voltage of X volt at the input gives an output voltage of -X volt.

Now, for a good understanding, it is necessary to consider the integrator and

analog switches together. Notice that the integrator has two inputs. One of them is connected to U. and the second to -U. coming from the sign inverter. When

in in

the switches are on, +11. is, via R and R , shorted to ground and no current

in 22 24

flows through R32 to the integrator operational amplifier summing point. How-ever, through the second input a current isflowing continuously from the

sum-ming point which means that the integrator output becomes more positive

propor-tionally with time.

After switching the switches off, current starts flowing to the summing point through R , R and R . Because the sum of these resistors is half the value

22 24 32

of R33, the current , now flowing through these resistors, is twice as large

as the current flowing from the summing point through R33. As a result, the net current flowing to the summing point is equal to the current flowing from vir-tuai ground when the switches are on and so the integrator output is becoming more negative with time.

At first glance, the use of two analog switches may be not particularly obvious,

but this is done, just as in the demodulator, for thermal reasons. Due to the

relatively large and temperature dependent on resistance of the switches, the

attenuation when on is not ideal. Switching two switches in parallel gives an improvement of a factor two for both attenuation and temperature stability. But adding resistor R24 gives an additional improvement of a factor five.

When the switches are switched on and off periodically, the output voltage

of the integrator becomes a triangular signal. This switching is controlled by a smith-trigger which changes state when passing one of two well defined vol-tage levels from a particular direction. In our case, the smith-trigger

consists of a CA 3130 operational amplifier and R34 and R35. Suppose that the output of this amplifier is positive, i.e. approximately 15 V. Then the

resistors R34 and R5 act like an attenuator and bring the non-inverting input

to a voltage of +2.5 V with respect to the 7.5 V. The high output of the amplifier causes the analog switches to be on and so the integrator output

becomes more positive with time. When this voltage becomes positive with

res-pect to the non-inverting input of the CA 3130 the smith-trigger triggers. Its output becomes low ( 0V) and the reference level at the non-inverting input goes to -2.5 V(referred to 7.5 V), Now, the analog switches are at their off state and the integrator output is going to the negative direction until it crosses the new reference voltage and changes state again, and so on. The

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-CA 3130 was chosen primarily because its output swing comes very close to the power supply voltage (within approximately 20 mV). So, the reference levels of

the smith-trigger are very well defined and temperature sensitivity is negli-gible. Also, and just as important, the ratiometric design of the circuits

de-manded an amplifier that swings to its supply voltages as close as possible.

The frequency of the converter can be calculated by using the formula:

U. R34 + R35

in

f =

Îç .

R33.R34C16

Substituting the values as shown in figure AlO we get

U.

f = Ti:.a 8960

Thus, because the preceding circuits are designed ratiometric, which means that

if _UB changes U changes proportionally, the frequency is independent of power supply variations.

A.7. Low-pass filter.

To meet the requirements of the Dutch postal services, the voltage to frequency converter is followed by a low-pass filter. Its purpose is to limit the band-width of the converter signal. As shown in figure A11, it is a conventional

R39 R36 B L ci9jSn6 6k8 R37 3k3 2k2 lo C18J33n

_[

Fig. All Low-pass filter.

second order low-pass filter which needs no further explanation The cut-off frequency is set at approximately 3 kHz. Capacitor C20, coil L2 and the

feed-through capacitor form a n-filter to stop unwanted h.f. signals. R38 enhances

the effectiviness of this filter.

Tp 12 ,0' i38

iYp

L2 7 2n2 to 2u2 1k transmitter.

C2n3

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D

A.8. The transmitter.

The transmitter is a fairly simple design (figure A2). Transistor T3 with its

associated components forms the oscillator which drives the power transistor

T4 which, on its turn, drives the output. When the supply voltage of T3 and T4

varies, the output voltage changes accordingly. So, by modulating this supply

voltage the output signal of the transmitter is amplitude-modulated.

The modulator consists of T1 and T2 and associated circuitry . By changing a

jumper at the input of T2 the transmitted power can be set at 200 mW or 1.5 W. To obtain 100% modulation an input signal of approximately 4 V peak-peak is

re-quired. Antenna matching, and hence maximum transmitted power can be adjusted with the trimming capacitor at the output. A simple detector is added to

faci-litate this adjustment. Its output voltage is adjusted to a maximum with the

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Appendix B.

Wave buoy Mk 3.

Directions for use:

Preparing for launching.

1. Connect tail, antenna, cardboard case and stabilization weight.

* 2. Make sure that the protection cap of the charger connector is in place. Remove magnet.

Tune receiver for optimum reception and check for proper buoy working by

shaking the buoy up and down.

Drop buoy with two man. One keeping the buoy and the second the

stabiliza-tion weight. Throw the buoy away from the hull. After launching try to observe if buoy erects.

Reception and data recording.

During measurements with the buoy radio silence must be maintained. This is

necessary to prevent the receiver AGC (automatic gain control) of being acti-vated by unwanted h.f. signals which are pushed into the receiver by the on board transmitter. Even when the transmitter frequency is totally different from the buoy frequency this effect can show up. It results in strongly de-creased receiver sensitivity and so, fluctuating signal amplitude at the receiver output. In fact, the signal can even be lost completely.