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(Lri^s--cTioiN)DEPARTMENT OFMINERALS AND ENERGY

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BUREAU OF MINERAL RESOURCES,GEOLOGY AND GEOPHYSICS

1 975/141

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MODIFICATION OF THE PRESS■EWING LONCPERIOD

SEISMOMETER TO COMPENSATE FOR THE EFFECTS

OF TEMPERATURE CHANCES

by

P.J. HILLMAN

The information contained in this report has been obtained by the Department of Minerals and Energy

as part of the policy of the Australian Government to assist in the exploration and development ofmineral resources. It may not be published in any form or used in a company prospectus or statementwithout the permission in writing of the Director. Bureau of Mineral Resources, Geology and Geophysics.

BMRRecord

1975/141c.3

1975/141

MODIFICATION OF THE PRESS-EWING LONG-PERIOD

SEISMCVETER TO COMPENSATE FOR THE EFFECTS

OF TEMPT:NATURE CHANGES

by

P.J. HILLMAN

CONTENTS

Summary

1. INTRODUCTIOV

2. OUTLINE OF OVERALL SERVO—SYSTEM

3. DETAILED DESCRIPTION OF SYSTEM

Seismometer drift compensator

Assembly

Initial adjustments and tests

Adjustments in use

PLATES

Plate 1. Modified seismometer (external)

Plate 2. Modified seismometer (internal)

Plate 3. Seismometer drift compensator

FIGURES

Fig. 1. Block diagram

Fig. 2. Servo amplifier

Fig. 3. Current regulator

Fig. 4. D.C. converter

Fig. 5. Regulator boardFig. 6. Chassis wiring

Fig. 7. Interconnecting cable

Fig. 8. Seismic recording system chassis wiring

Page

1

2

2

344

SIMINIARY

A system has been designed for use with a Press—Ewing vertical

long—period seismometer, Whioh will compensate for the mechanical drift

of the boom due to temperature changes of a few degrees Celsius. This

record describes the mechanical modifications made to the seismometer

and gives design details of the controlling servo—amplifier.

With this system the seismometer can be used, unattended, in

most field conditions in Australia and PG; without the system, use is

limited to an observatory vault or similar environment.

The record gives instructions for the assembly, adjustment,and testing of the system.

1. INTRODUCTIOL

The Press—Ewing seismometer boom moves upward into the top

mechanical stop when the temperature rises a few degrees Celsius and

down into the bottom stop during a similar fall in temperature. When

the boom is on or near a stop either the seismometer becomes completely

inoperative or the records obtained are distorted because the boom

bounces on the stop instead of oscillating freely.

These seismometers have been employed at unmanned remote sites

where the diurnal temperature change is several degrees Celsius and

consequently useful records were obtainable only during a small portion

of each day. To extend this time to the whole day a servo—system has

been developed which does not alter the seismometer response to oscillations

with periods of less than 1000 seconds. The system keeps the boom closeoto its central position throughout temperature changes of — 11 C, provided

the rate of change of temperature is less than 1 °C per hour, a requirementachieved by surrounding the seismometer with a styrene foam cover 10 cm

thick.

Field use of the modified seismometer

Three seismometers have been modified and three compensator

modules plus a spare set of circuit cards have been built. At time of

writing, one seismometer is in use in the field at the Cooney observatory

tunnel near Armidale, V.S.W. and is operating satisfactorily.

2. OUTLINE OF OVERALL MVO—SYSTEM

Figure 1 is a block diagram of the system. A focused light

beam is reflected from a small mirror, mounted on the end of the seismometer

boom, onto a horizontally split photocell. The output of the photocell

depends on the boom's position and varies between plus 25 milliamps and

minus 25 milliamps. This current determines the input to an integrating

operational amplifier which has the seismometer calibration coil as its

output load. The calibration coil is located in the annular radial magnetic

field between the poles of a permanent magnet and is mechanically attached

to the seismometer boom.

2

When the boom moves from the central position the output current

from the servo—amplifier slowly increases until the force between the

calibration coil and the magnet is sufficient to recentralize the boom.

The boom will then remain centred until the rate of change of temperature

exceeds 1 °C per hour when the deflection will still be small or until theamplifier reaches its maximum output voltage of ± 12 V. In the second

case the amplifier must be switched off for a few minutes to allow the

feedback capacitors to disoharge; the boom is then rebentred mechanically,

and the amplifier switched on again.

3. DETAILED DESCRIPTION OF SYSTEM

The perspex window on the end of the seismometer pressure case

was removed and replaced by the light—source/photocell assembly, a

commercial item purchased from Askania, Germany.

A hole was cut in the top of the seismometer pressure case,

above the end of the boom, and the perspex window was fitted over it.

This allows visual observation of the boom position with the case fitted.

Two small dowels were fitted into the base plate at diagonally

opposite corners and corresponding holes drilled in the pressure case.

This ensures that the alignment of the light—source/photocell assembly

is maintained when the case is removed and replaced. (Plate 1)

A small front—silvered mirror was cemented with Araldite to

the front surface of the limit stop screw to reflect the light beam back

onto the photocell. A second mirror was mounted on the inside surface of

the front of the pressure case and angled so that a reflection of the

limit stop screw, the zero pointer, and the scale on the limit stop plate

was visible through the top window (Plate 2).

Seismometer drift compensator (Plate 3)

The seismometer drift compensator unit is designed to fit into

the seismic recording system Chassis (Drawing No SRI) 1) and will normally

be used in this chassis. It can be used separately, but no outside cover

is available and the components are exposed and unprotected.

The compensator unit chassis (Dwg No ZSA 5 Fig. 6) containssix plug—in cards:

Servo—Amplifier (2 cards) Dwg No ZSA 1 — Figure 2

Current Regulator (1 card) Dwg ro ZSA 2 — Figure 3

D.C. Convertor (1 card) Dwg No ZSA 3 - Figure 4

Regulator Board (2 cards) Dwg.No ZSA 4 - Figure 5

The D.C. Convertor card operates from a +12 volt D.C. input on

pins 1 and 18 and provides two D.C. outputs: +20 V between pins 26 and

29 and —20 V between pins 28 and 27. These two voltages are the inputs

to the first regulator card, which contains all the regulator circuits

with the exception of the two output transistors Q1 and Q4. The output

transistors (BDY60s) are mounted on an aluminium heatsink.which also has

a third BDY60, a zener diode 1ZC1275, and a 4.7K resistor mounted on it.

These three components are in circuit if the compensator is operated from

24 volts D.C. to reduce the supply to 12 volts and provide some stabilization.

The outputs from the regulator Garde are +15 V D.C. and —15 V D.C. which

are connected to the Servo—Amplifier card on pins 14 and 15 respectively.

The servo—amplifier (Figure 2) consists of an Analog Devices

low—drift operational amplifier P501A which feeds the connected bases of

a BFY50 transistor and an AY6102 transistor. These transistors are

emitter—coupled, and the two feedback polystyrene capacitors are connected

to this commoned output so that the total amplifier drift is minimized.

These commoned emitters supply the control current to the seismometer

calibration coil via a 68—ohm current—limiting resistor (The voltage

between this point and ground is displayed on a 15-0-15 volt meter

mounted on the ohassis front panel). There is a second output to the

calibration coil from the collector of a second BFY50. This provides

a step function, for calibration of the seismometer, when the "CALIBRATE"

switch on the chassis front panel is operated.

The current regulator card operates from 12 volts D.C. and supplies

a constant current to the 6—volt lamp in the light—source/photocell assembly.

The current can be varied by a IK Trimpot, and its value is shown on a

0-500 milliammeter. Both are mounted on the chassis front panel. The

value of this aurrent determines the gain of the servo—system.

Assembly

After the seismometer has been assembled in the usual manner, the

4

boom centred, and the cover clamped, connect the multi—core cable supplied

between the seismometer and the seismic recording chassis. The connexions

to the seismometer are shown in Drawing Vo 3RD-1 Shtla (Fig. 7) and mustbe followed exactly. However, the connexions to seismometer output coil

may be reversed if necessary to obtain the correct polarity to the TAK5

seismometer amplifier: Connect seismic recording chassis to a 12 voltD.C. supply.

Initial adlystments and tests

1. After switching power on, adjust lamp current to 270milliamps with the 1K Trimpot.

2. Disconnect lead from Terminal 1 at rear of seismometer.

3. Disconnect light green lead from +ve terminal on photocelland connect Ave meter, or similar meter, on 50 microamprange between lead and terminal.

4. Cheek that boom is central and adjust if necessary.

5. Adjust light—source/photocell assembly to obtain zerocurrent on the Avometer using the four adjusting Allenscrews in the mounting. The six mounting screws will haveto be slackened slightly during this adjustment.

6. Cheek that boom is still oentral. If not, repeat 4 and 5.

7. Reconnect lead to Terminal 1 and displace boom upwardbetween one and two scale divisions using the vernieradjustment. Note the reading on the Avometer and also theambient temperature. Wait between five and ten minutesand then cheek that the Avometer reading is smaller, thatthe 15-0-15 voltmeter reads between zero and one volt, andthat the ambient temperature is unchanged.

8. Switch amplifier off for a few minutes to dischargecapacitors. Displace boom downward between one and twodivisions. Switch amplifier on, and note the reading onthe Avometer (reverse of reading in 7) and the ambienttemperature. After five to ten minutes check thatAvometer reading is smaller, that the 15-0-15 voltmeterreads between zero and minus one volt, and that theambient temperature is constant.

9. If both these tests give correct results the system isserviceable. If either 7 or 8 is incorrect, the servo—system is faulty and must be repaired.

Adjustment in use

The only adjustment needed is mechanical recentring of the boom

when the reading on the 15-0-15 voltmeter reaches either +11 volts or —11

volts. The amplifier must be switched off and time allowed for the voltage

to return to zero before the boom is recentred.

. l ~ m t m c r llTNlT PUT

WBrnl)rlAL

PLATE 1

/-, L I M I T STOP PLATE A N D SCALE

PLATE 2

f SERVO AMPLIFIER BOARD 7

//I--.--. CURRENT REO

TOR BOARDS

CONVERTER

,

D.C.Converter

VoltageRegulator

ServoAmplifier

CurrehtRegulator

,

±15 voltsD.C.

Splitphotocell

+12 volts D.C. ,„

Mirror

Boom

Magnet

Calibrationcoil

Control current

± 20 voltsD.C.

+12 voltsD.C.

1=11 NM NM ME MI MI I= NMI IMMI NM 1=11 INN NE 1=1 =I =I 1=1 11=1 MI NM

cb

Seismometer^

Compensatorkr)

SEISMOMETER DRIFT COMPENSATORBLOCK DIAGRAM

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TOLERANCE EXCEPT WHEREOTHERWISE STATED :L.-

COMMONWEALTH OF AUSTRALIA — COPYRIGHT ACT 1912-1950

This drawing is the property of the Commonwealth of Australia (Dept. of National Development).The information contained herein is for official use only and must not be divulged to any unauthorisedpersons, nor shall it be retained, copied or used, in whole or in part, unless the written authority of theDept, of National Development is first obtained.

MACHINED FINISHES EXCEPT WHEREOTHERWISE INDICATED TO BE NOT LESSTHAN CLA-b3 VALUE.

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