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NBS TECHNICAL NOTE 855

U.S. DEPARTMENT OF COMMERCE/ National Bureau of Standards
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act of Congress March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology and facilitate their effective application for public benefit. To this end, the Bureau conducts

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An Experimental Technique for the

Evaluation of Thermal Transient Effects

on Piezoelectric Accelerometers

-=^

-£. e. c V\ <V c_ <\\ ^o"V«\ - "5s ~>S

Carol F. Vezzetti and

Paul S. Lederer

Institute for Applied Technology

National Bureau of Standards Washington, D.C. 20234

Sponsored by

Lawrence Livermore Laboratory

University of California Livermore, California 94550

and

The National Bureau of Standards Washington, D.C. 20234

U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

^£ NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director

Issued January 1975
Library of Congress Catalog Card Number: 74-600188

National Bureau of Standards Technical Note 855

Nat. Bur. Stand. (U.S.), Tech. Note 855, 48 pages (Jan. 1975) CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE WASHINGTON: 1975

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 (Order by SD Catalog No. C13.46:855).

Price $1.05
FOREWORD

The work described was performed as a task within the InterAgency

Transducer Project. This is a continuing project for the development

of calibration and evaluation techniques for electromechanical transducers, and is supported by a number of government agencies. This

task was funded by the U.S. Atomic Energy Commission through the

Lawrence Livermore Laboratory of the University of California under

order number 9231609.

Paul S. Lederer

Assistant Chief/Transducers

Instrumentation Applications Section
CONTENTS

AN EXPERIMENTAL METHOD FOR THE EVALUATION

OF THERMAL-TRANSIENT EFFECTS ON PIEZOELECTRIC ACCELEROMETERS

1

Background 1 1.1 Needs for Thermal -Transient Test Method 1 1.1.1 Use of Piezoelectric Accel erometers 1 1.1.2 Pyroelectricity and Differential Expansion of

Components 2 1.1.3 Consequences of Unpredicabili ty of Thermal

Transient Effects 2 1.1.4 Test Method Requirements 3 1.2 ANSI Method: Transient Temperature Effects on Piezoelectric Transducers 3 ".2.1 Description 3 1.2.2 Shortcomings 4

2. Development of New Thermal -Transient Test Method 5 2.1 Test Method and Test Conditions 5 2.1.1 Brief Description of Method 5 2.1.2 Description of Experimental Apparatus 5 2.1.3 Selection of Test Conditions 7 2.1.3.1 Vibration Frequency 7 2.1.3.2 Vibration Amplitude ..... 8

2.1.3.3 Radiation Power Level 8 2.1.3.4 Exposure Time 9 2.1.4 Sources of Error 10

2.1.4.1. Vibration Amplitude 10

2.1.4.2 Output of Test Accelerometer 1 2.1.4.3 Radiant Power Level 1 2.1.4.4 Error Summary 1 2.2 Signal-Conditioning Considerations 1 2.2.1 Charge Amplifier 1 2.2.2 Voltage Amplifiers 12

2.2.3 Use of Electrical Filters 13

3. Thermal -Transient Tests of Fourteen Accel erometers 13

3.1 Test Results Using One-Second Exposure 13

3.1.1 Tests with Charge Amplifier 15

3.1.2 Tests with Voltage Amplifier (Electrometer) ... 15

3.2 Test Results Using Fifteen-Second Exposure 15

3.3 Discussion of Test Results 16

3.3.1 Top and Side Exposure 16

3.3.2 System Time Constants and the Effects of

High-Pass Filters 17

3.3.3 Relation of Test Results to Accelerometer

Features 19

4. Conclusions 20

5. Recommendations 21

6. References 21

iv
AN EXPERIMENTAL METHOD FOR THE EVALUATION

OF THERMAL-TRANSIENT EFFECTS ON PIEZOELECTRIC ACCELEROMETERS

Carol F. Vezzetti and Paul S. Lederer

A simple, inexpensive method was developed for determining the effects

of thermal transients on the zero output .and sensitivity of piezoelectric accelerometers. Thermal transient stimuli are generated by an incandescent lamp and can be made to heat the top or side of the test

accelerometer. Fourteen commercial accelerometers were tested using

this technique. Zero shifts with magnitudes as high as 640 gn * were

observed. Zero shifts of up to 2% of full scale resulted from 1-s

duration transients, and up to 7% of full scale from 15-s transients.

These results were obtained at a radiation power density of 1.8 W/cm2

. No changes of accelerometer sensitivity exceeding experimental uncer- tainties were noted as a result of the thermal transients used.

Key words: accelerometer; performance characteristics; piezoelectric;

test method; thermal radiation; thermal transient; zero shift.

1 . Background

1.1 Needs for Thermal-Transient Test Method

1.1.1 Use of Piezoelectric Accelerometers

A piezoelectric accelerometer consists of a base structure, a loading

mass, and a piezoelectric crystalline sensing element located between

the mass and the base. Vibrational or shock acceleration applied to the accelerometer induces mechanical strains in the sensing element

which generate electric charges in proportion to the applied acceleration. The sensing element may be a natural crystal, like quartz, or one of many proprietary piezoelectric materials.

Piezoelectric accelerometers are used extensively for measuring shock

and vibrational motion in structures and components. These measurements may be used to demonstrate the ability of structures to withstand acceleration environments without damage, and to verify that

components will operate with specified performance during and

following exposure to shock or vibration conditions.

Accelerometers used for the measurement of blast-induced shock and

vibration are also subjected to thermal -transient stimuli from the

blast. Certain properties of piezoelectric accelerometers can

g = approximately 9.8 m/s 2
cause them to generate spurious output signals in response to such

thermal transients, leading to significant measurement errors.

1.1.2 Pyroelectricity and Differential Expansion of

Components

Many piezoelectric crystalline materials are also pyroelectric [1],[2]*,

that is, a change of temperature causes a change in the polarization

charges in the material.

Pyroelectric output signals can result from a uniform or non-uniform

distribution of thermal energy within the material. In addition,

mechanical strain within the piezoelectric element, resulting from

differential thermal expansion of the components of an accelerometer

subjected to thermal transients, may generate spurious output signals.

In view of the transient nature of the thermal radiation stimulus

experienced in blast conditions, non-uniform heating of the accelerometer so exposed is probable. The resultant output signal will thus

include pyroelectrically generated charges and charges produced by

changes in the mechanical loading of the crystal resulting from

differential expansion of accelerometer components.

1.1.3 Consequences of the Unpredictability of Thermal - Transient Effects

Piezoelectric accelerometers are made in a large variety of models

based on different design principals and constructed of various

sensing-element, base, and case materials. The size of accelerometers

varies as do connector locations. Furthermore, these accelerometers

may be mounted in different ways on a variety of structures made of

a variety of materials. The combination of these factors renders

difficult and time-consuming the task of predicting analytically the

effects of non-uniform heating, such as produced by a thermal transient,

on the performance characteristics of a particular accelerometer

structure.

Moreover, it is difficult to characterize the thermal radiation stimuli

produced by blasts. The character of the blast (nuclear or non-nuclear)

and its intensity, pulse shape and duration, spectral intensity, the

location of the radiation measuring instruments, and the performance

characteristics and calibration of these instruments may contribute to the uncertainty in any measured values of blast-induced thermaltransient radiation stimuli.

These limitations to an analytical prediction of the effects of thermal

transients on the performance characteristics of piezoelectric accelerometers demonstrate a need for an experimental method of assessing

these effects..

*Figures in brackets indicate literature references listed in section 6.

2
1.1.4 Test Method Requirements

An experimental method capable of realistic simulation of the blastinduced thermal -transient radiation environment to which the transducer

may be exposed during measurement, while highly desirable, could be

difficult and expensive to achieve because of the variety of possible

blast parameters, the large amount of energy involved, and the problems

involved in attempts to scale accurately blast-generated environments.

Accordingly, in agreement with the sponsoring agency, the task of

developing a suitable thermal-transient radiation test method for

screening accelerometers was provided the following guidelines:

1. The method is to be capable of identifying accelerometers that are least sensitive to reasonably short-duration thermal transients;

2. The method need not provide accurate simulation of blast-produced

radiation environments;

3. The method is to be inexpensive to implement; and,

4. The method is to be simple to perform.

A thermal-transient test method does exist, but it is based on immersion and involves heat transfer by conduction only.

1.2 ANSI Method: Transient Temperature Effects on Piezoelectric

Transducers

1.2.1 Description

In 1969, the American National Standards Institute published ANSI

S2. 11-1969, "American National Standard for the Selection of Calibrations and Tests for Electrical Transducers Used for Measuring Shock

and Vibration"[3]

.

Section 8.6.2 of this standard deals with "Transient Temperature

Effects on Piezoelectric Transducers." It is quoted below:

"Pyroelectric outputs are generated in all piezoelectric transducers subjected to transient temperatures. The magnitude of

the pyroelectric output depends upon the crystal material and

the design of the transducer. Usually the predominant frequency

of the pyroelectric output is significantly less than 1 Hz. Therefore, most of the pyroelectric output from the transducer

is filtered out due to the low frequency characteristics of the

amplifier. Accordingly, the pyroelectric output is dependent

on the rate of the temperature change and on the characteristics

of the amplifier as well as the characteristics of the transducer.

"The pyroelectric test is performed using the type of amplifier

normally used with the transducer. The transducer is attached

to an aluminum block and both are quickly immersed in an ice

bath or other liquid bath at a temperature 50°F [10°C] different
from room temperature. The block mass should be approximately

ten times the mass of the transducer. The maximum amplifier

output and the time from the start of the transient at which

this maximum output is reached are measured on a dc oscilloscope.

If the output reverses within the first two seconds and reaches

a peak of opposite polarity, the magnitude and time of this

peak are recorded also. For an accelerometer, the sensitivity

is expressed in equivalent g/°? lg n /°C] by dividing the maximum

amplifier output by the product of the difference of the bath

temperature and room temperature and the accelerometer sensitivity determined in 7.2. For a force transducer, the units

are equivalent lbs/°F [M/°C]. The amplifier type is specified

for which the pyroelectric sensitivity is applicable.

"For special applications using amplifiers having significantly

different low frequency characteristic, the pyroelectric test is

performed with the specific amplifier of intended use. Also,

for applications in which the transient temperature rate is much

different than that described by the above conditions, the test

is performed by simulating the specific temperature environment."

1.2.2 Shortcomings of ANSI Method

The ANSI method described above is not satisfactory for evaluating

effects of radiant-energy transients because it relies entirely on conductive heat transfer. Also, this method provides for the thermal

input to be applied uniformly to all accelerometer surfaces except

the base. In blast measurement situations, the radiant energy may

impinge on the top or the side of the instrument; this condition is

likely to induce differential expansion of various accelerometer

components.

Further, a liquid-immersion method used to simulate the effects of

short thermal transients is likely to result in splashing with

consequent spurious acceleration signals. Finally, the assessment

of a thermal -transient effect in terms of external temperature

differentials may be misleading. Experience suggests that it is the radiant energy of the thermal transient absorbed by the accelerometer which most affects accelerometer performance. This energy

input depends on the emissivity and conductivity of the components

involved, factors not readily determined. However, if it is feasible

to control or measure the radiant energy or radiant power level of

the stimulus, repea table test conditions may be achieved and verified,

Based on the considerations developed in this background section, a new, simple, and repeatable test method was developed for determining

the effects of reproducible radiant-energy thermal transients on the

performance of piezoelectric accelerometers.
2. Development of New Thermal -Transient Test Method

2.1 Test Method and Testing Conditions

2.1.1 Brief Description of Method

The method developed involves subjecting the test accelerometer to vibrational acceleration at the same time it is exposed to the thermal - transient stimulus created by a beam of radiant energy from a highintensity incandescent lamp. A second accelerometer, shielded from

the thermal stimulus but subjected to the same vibrational acceleration, measures the acceleration amplitude which both transducers

experience. The thermal radiation can be made to impinge on whatever

surface of the test accelerometer would be exposed during actual use

in a blast situation, and there is little difficulty in simulating

thermal transients as short as tens of milliseconds. The range of

conditions can be extended beyond that used in this study by means

of the use of sources of greater intensity and faster shutters.

The method is based on one previously developed which uses a beam of

infrared radiant energy from a continuous-wave laser for investigating

the effects of thermal transients on the performance characteristics

of flush-diaphragm pressure transducers [4].

The high-intensity incandescent lamp selected is capable of a radiantenergy output somewhat less than that of the laser used previously,

but its cost is substantially less than the cost of the laser.

2.1.2 Description of Experimental Apparatus

Figures 1 and 2 identify the major components of the experimental

apparatus. Figure 1 shows the arrangement of components for tests

with the accelerometer side exposed to the thermal radiation and also

shows the photographic shutter used for most tests. Figure 2 shows

the test arrangement for exposing the top of the accelerometer.

Both figures show a covered, aluminum, cup-like structure fastened to the amature of an electromagnetic vibration exciter. The reference

accelerometer is mounted on the bottom of the cup, with its cable

passing through a hole in the side wall of the cup. The accelerometer

under test is mounted on the removable cover plate of the cup. The

vibration exciter can be mounted so that the direction of vibration

is aligned with the beam of incident radiation for irradiating the

top surface of the test accelerometer, or perpendicular to the beam

for irradiating the side of the test accelerometer. The metal framework surrounding the vibration exciter supports thermal shielding

materials, the source of radiant heat energy, and a radiation sensor.

The thermal shield is a square plate of thermal insulating material

about 20 cm x 20 cm supported by the framework to be about 5 cm above

the cover plate on which the test accelerometer is mounted. A
circular hole in the shield, about 8 cm in diameter, is concentric

with the test accelerometer axis (figure 2). A sliding shutter of the same insulating material (not shown in figure 2) that normally covers

this hole is withdrawn to permit the radiation to pass through the hole, or a commercial photographic shutter (as shown in figure 1) is centered

over the hole. The shutter that has been used is self-cocking, opens

to a clear diameter of 2.5 cm, and has speeds from 0.01 s to 1.0 s and

bulb and time settings. It is necessary to protect the photographic

shutter leaves from the thermal radiation by covering the shutter with

an aluminum plate. Just before making an exposure, the plate is removed; it is replaced immediately afterwards.

A secondary radiation shield (shown in figure 2) is used to protect

the cable of the reference accelerometer from the thermal radiation

in tests in which the accelerometer top is exposed. The shield is

not used in tests in which the accelerometer side is exposed as in

this test configuration the cable exits from the side of the cup

opposite the incoming radiation.

A 600-W quartz-bromine incandescent lamp mounted in a small aluminum

reflector supplies the radiant energy. Energy output can be adjusted

by means of a variable autotransformer fed from the laboratory a-c

line. The lamp is mounted directly above the hole in the shield to irradiate the accelerometer under test. The lamp may also be swung

laterally to position it directly above the absorption head of the

radiation monitor (not shown in figures 1 and 2, but indicated in

figure 3) to permit measurement of the radiant power output of the

lamp.

The radiation emitted by the lamp is measured by a commercial broadband

power meter described by the manufacturer as having a flat response over wavelengths of from 0.3 pm to 30 ym, and full-scale power readings of

1, 3, 10, 30, and 100 W. The meter consists of an absorption head and

a control and read-out unit. The absorption head, which is mounted on the support framework, contains a coated metal disc. This disc absorbs

the incident radiation over the specified range of wavelengths. The

periphery of the disc is in intimate thermal contact with a finned,

convection-cooled, heat sink in the absorption head, and the resultant

radial heat flow from disc to sink is sensed by a concentric array of

thermocouples connected to form a thermopile and mounted on the back

surface of the disc. The voltage generated by the thermopile is

amplified by the control unit and displayed on a meter in watts of

radiant power.

To measure the radiant power, the sensing disc of the radiometer head

is placed at the same distance from the radiation source as the test

accelerometer top (or side) surface during thermal transient exposure.

This distance, about 19 cm, is measured with a ruler to within an estimated ±2%.
The remainder of the apparatus consists of signal-conditioning and

display devices for the accelerometer output signals, and a power

amplifier and audio-frequency oscillator to drive the electromagnetic

vibration exciter. A block diagram of the apparatus is shown in

figure 3. A discussion of the signal -conditioning devices and their

influence on test results is given in section 2.2

2.1.3 Selection of Test Conditions

The thermal -transient test method described in section 2.1.1 exposes

the accelerometer under test to a transient of given duration and of

given thermal radiant power, at the same time as it is subjected

to vibrational acceleration. The applied acceleration, in turn, is of

a given amplitude and at a given frequency. Initial experimental

development of the new test method was concerned with the determinations of recommended values for each of the above test conditions.

Considerations and experiments leading to the selection of particular

test conditions will be described in the following sections.

2.1.3.1 Vibration Frequency

Selection of the vibration frequency at which the tests were to be run was based on the results of the experiment outlined below and some additional considerations derived from experience in evaluating

accelerometer performance. Figure 4 shows the experimental results

when a representative test accelerometer-charge amplifier system was subjected to a 1-W thermal transient at four vibration frequencies:

20, 50, 100, and 500 Hz. The observed differences in the responses

at these four frequencies of about 5% are within the limits of

experimental uncertainty. Since all test accelerometers have very

high natural frequencies, their response should be the same over

the range of frequencies of from 20 Hz to 500 Hz. Thus, any

frequency in this range could presumably be selected as the test

frequency.

Frequency limitations at the low end are dictated by the fact that

the majority of commercial piezoelectric accelerometer-signal

conditioner systems show varying amounts of roll -off at frequencies

below about 20 Hz. In addition, the lower frequency limit of

electronic voltmeters, including digital types, is typically about

50 Hz. These factors suggest that the test frequency be 50 Hz or higher. In regard to an upper test-frequency limit, it can be

shown that the output of an accelerometer may be affected by the manner in which it is mounted and the material on which it is

mounted and the material on which it is mounted. These effects

are more pronounced at higher frequencies and, according to recent

work, not significant below about 3000 Hz [5].

A convenient choice of frequency above 50 Hz and below 3000 Hz is

100 Hz. Accelerometer manufacturers generally use 100 Hz as their

base, or reference frequency, when calibrating their accelerometers.
Further, 100 Hz provides adequate time resolution for examining the

response of accelerometers to transient phenomena as short as the

one-second test duration of the thermal-transient tests.

For the various reasons given above, 100 Hz was selected as the basic

test frequency.

2.1.3.2 Vibration Amplitude

Selection of the vibration amplitude to be used for the thermal

transient test was based on the results of experiment. The accelerometer system used for the vibration frequency experiment (section

2.1.3.1) was also used to assess the effects of vibration amplitude.

Experimental thermal-transient test data were obtained at 100 Hz and at ±2, ±4, ±6, ±8, ±10 g n

. A thermal power level of 1 W was

used. Plots of accelerometer zero shift versus time for the five

selected gn values are similar in form to the plots of figure 4. Differences in response at these acceleration levels of less than

5% are within estimated experimental uncertainty.

As important consideration in the selection of the test vibration

amplitude is the necessity of obtaining an adequate output from

the transducer under test. Even with a charge sensitivity as low

as 0.01 pC/gn (near the minimum offered in commercial accelerometers)

a ±10 g n stimulus may produce an output of 100 mV from a commercial

charge amplifier. This level of output is readily measured by a number of types of customary laboratory instruments, and this fact

contributes to the selection of ±10 gn as the test level. While a number of the earlier accelerometer evaluations were conducted at

a level of ±5 gn (10 gn peak-to-peak), later tests use ±10 g n (20 gn peak-to-peak). This acceleration produces a higher output level

(better signal-to-noise ratio) and is within the capability of most

commercially available vibration exciters.

2.1.3.3 Radiation Power Level

Selection of the test radiation power level is based on the guideline

reauirement that the method be able to screen accelerometers to identify those experiencing large zero shifts on exposure to thermal

transients.

Several factors were considered in the selection of the test radiation

power level. A lower limit is the level for which the radiant energy

is insufficient to produce any noticeable change in the output of a typical accelerometer. Upper limits are determined by cost-versuscapability constraints for the source of radiation, the shutter

mechanism, and the radiation monitoring equipment.

The low cost of a 600-W quartz-bomine lamp and its capability of

generating an adequate amount of radiant energy to produce a readily

measurable effect on typical accelerometer output led to selection

of this lamp as the radiant energy source. A broad-band radiometric
power meter was already available in the laboratory, as was a photographic shutter capable of withstanding the radiation without

malfunction. No effort was made to evaluate other sources of

radiation, meters, or shutters.

The radiant energy impinging on the accelerometer (and shutter) is

a function of lamp power and the distance from the lamp to the test

specimen. A distance of 19 cm was selected to allow adequate working

space between lamp and test specimen or shutter.

A series of tests was conducted with a variable-voltage transformer

controlling the lamp electrical power as shown in figure 3. Figure 5,

showing the effects of radiation power level on accelerometer zero shift, indicates a zero shift roughly proportional to power level over

the range of power levels used. In this test, radiant power levels of

G.5, 1.0, 1.5, 2.0, and 2.5 W were used as measured by the radiometer.

Based on the 1.9 cm-diameter of the radiometer absorbing disc,

corresponding values of radiation power density are calculated to be

0.18, 0.35, 0.53, 0.70 and 0.88 W/cm 2

, respectively. In this test,

a charge amplifier was used to amplify the accelerometer signal, while

the data shown in figure 6 for two other accelerometers tested at 5 W and 10 W were obtained with an electrometer amplifier. Again, the

magnitudes of the zero shifts are roughly proportional to power level

(at calculated power densities of 1.8 W/cm2 and 3.6 W/cm2

, respectively)

for each device during exposure. In all of these tests the thermal

stimulus lasted 15 s. A power level of 5 W (power density 1.8 W/cm2

) was chosen for the thermal -transient test method because it produced

an easily measurable effect on the output signal for a typical test

accelerometer without inducing excessive thermal stress in, or otherwise damaging, the shutter.

2.1 .3.4 Exposure Time

As indicated in section 1.1.4, for a variety of reasons the realistic

simulation of blast-induced thermal -radiation environments is both

difficult and expensive. A previously developed thermal-transient

test procedure for pressure transducers involved transient stimuli

lasting 50 s and 300 s [3]. The guidelines described earlier suggested

that the test exposure duration be within an easily achievable range

of times; experiment suggested a specific time from that range.

In exploratory tests conducted prior to making the decision to adopt

5 W as the standard power level, tests were conducted in which a beam

of radiant energy was permitted to impinge on a representative test

accelerometer over a variety of time intervals ranging from 1 s to 45 s and at a radiation power level of 1 W. In each experiment the

path of the beam was normally blocked by a sliding shutter of thermally

insulating material. The shutter was quickly removed manually and

reinserted after the desired interval of time. Figure 7 shows the

test results for this accelerometer, whose zero shift does not increase

significantly for exposure times greater than 25 s. An exposure of
15 s produced a zero shift of about 75% of that attained after 25 s, but the choice of a 15-s exposure permitted more rapid testing since

the energy absorbed by the test instrument dissipated more rapidly

than with longer times. Tests with other accelerometers confirmed

that a 15-s exposure produced easily measurable zero shifts with a minimum test time. This exposure time was easy to achieve with the

manually operated sliding shutter. A variety of transducers was tested using the 15-s exposure and results are reported in section 3.;

Subsequently, it became apparent that constraints imposed by the

electrical time constants of the accelerometer-signal conditioner

made it desirable to shorten the exposure time (section 3.2.1).

Specifically, it was found that with short system time constants

the electrical charge produced in the accelerometer by the thermal

transient leaked off too rapidly for good measurements of zero

shift. On the other hand, use of long system time constants tended

to result in drifting output signals, making thermal -transient

induced signals difficult to interpret. Accordingly, an exposure

time of 1-s was selected, when practicable, to be combined with the

selection of short time constants.

A photographic shutter replaced the sliding shutter for tests with

thermal transients of 1-s duration. The majority of the test

results reported in section 3 were obtained with this exposure

time.

As indicated earlier, the sponsor agreed to the selection of specific

test parameter values, as described above, and with the determination

that the chosen values provided an adequate test method.

2.1.4 Sources of Error

2.1.4.1 Vibration Amplitude

The accuracy of calibration of the reference accelerometer at the

frequency of test is estimated by the manufacturer as being within

limits of uncertainty of ±2% of the reading up to 1 kHz (traceable

to NBS). Test and reference accelerometers are mounted on surfaces

parallel to each other within ±1 degree, and perpendicular to the

direction of vibration within the same tolerance. The estimated

uncertainty in the acceleration applied to the test accelerometer

that may be charged to angular misalignment is less than ±U.01%,

negligible compared to errors from other sources. The charge

amplifier used as signal conditioner for the reference accelerometer

showed a maximum deviation of -1.3% from nominal output values at

100 Hz. The digital voltmeter used to measure the output of the

reference accelerometer system has an estimated uncertainty of

±0.1%. Based on the individual uncertainties, the uncertainty of

the knowledge of the applied vibrational acceleration amplitude is

estimated to be within about -3.4%, +0.8% of the reading at test

values.

10
2.1.4.2 Output of Test Accelerometer

The output of the test accelerometer was amplified with the aid of a charge-sensitive amplifier or an electrometer amplifier and displayed

on the screen of an oscilloscope.

The charge-sensitive amplifier used showed a maximum deviation of

+2.0%, -2.4% from the nominal output values at 100 Hz, over the ranges

used. The gain of the electrometer amplifier at 100 Hz has an estimated uncertainty of ±2% of full scale. Deflection sensitivity of

the oscilloscope enters into the error considerations since the

thermally induced zero shift is scaled with reference to the displayed

vibration amplitude from the reference accelerometer-digital voltmeter

system. Hence the output of the test accelerometer is influenced by

amplifier uncertainties and the ability of the test operator to measure deflections on the oscilloscope screen, estimated to be ±3%

of the signal amplitude for a typical operator. The total uncertainty

of the test accelerometer output values is estimated at about ±5% of

the reading.

2.1.4.3 Radiant Power Level

The radiometer used is described by its manufacturer as having an accuracy of ±5%, of full scale. The area of irradiation (or the area resulting when the surface of the accelerometer exposed to the

radiation is projected onto a plane normal to the radiation beam

axis) was calculated from scale measurements and is estimated to have

an uncertainty of ±5%. Consequently, the value of the radiant power

density (W/cm2

) is estimated to be known to better than ±10% of full

scale.

The repeatability of radiant power-level measurements is limited by

the resolution of the meter scale, approximately ±1% of full scale.

2.1.4.4 Error Summary

In a typical measurement, the uncertainty in the knowledge of the

radiant power level is estimated at ±10%. The resultant zero shift

of a test accelerometer is estimated to be known within an uncertainty

of approximately ±5% (section 2.1.4.2) in the absence of time-constant

sources of error. These errors will be treated in more detail in

section 3.2.

2.2 Signal Conditioning Considerations

2.2.1 Charge-Sensitive Amplifiers

It may be useful to review briefly some of the signal conditioning

considerations pertaining to piezoelectric accelerometers. The

output of piezoelectric accelerometers can be amplified either by

charge or voltage amplifiers. The charge-sensitive amplifier (also

referred to simply as "charge amplifier") converts an input in the form

of a charge, from the piezoelectric transducer, to a voltage signal.

Most charge amplifiers consist of two sections: a charge converter and a voltage amplifier. The gain of the amplification section is frequently

adjustable so that the transducer can produce a conveniently scaled output.

11
In order to achieve stable operation, most charge amplifiers make use of a feedback resistor connected between the input and the output of

the amplifier section, and in parallel with the feedback capacitor.

The time constant of the system is then defined by the R-C time

constant of the feedback resistance-feedback capacitance combination.

Often several switch-selectable feedback resistors are provided allowing the operator to choose the desired time constant by switching the

proper resistor into the circuit. The feedback network which permits

selectable gain will also control the time constant. This arrangement

may cause measurement errors in the case of low-frequency phenomena.

The main advantage of the charge amplifier is that the system sensitivity is unaffected by the length of cable between the transducer and

the charge amplifier, although the noise level increases with an increase in cable length. With long cables -the charge amplifier is

particularly sensitive to spurious signals generated in the cable.

The effect of the charge-amplifier time constant on accelerometer

system output during a thermal-transient test is shown in figure 8. Both maximum output amplitudes and the time over which the output

remains elevated are controlled by the time constant.

Medium and long time-constant settings are yery difficult to use in

tests with most piezoelectric accelerometers because of the yery

large, random drifts in output caused by minute accelerometer

temperature changes, cable flexing, small vibrations, etc. Such

time constants are rarely used in field measurements. Measurements

with long time constants are generally used only for quasi-static

calibration of transducers with low internal leakage, such as quartz-crystal pressure transducers.

In view of these limitations, a useful experimental technique should

employ a short time-constant setting of the charge amplifier, unless,

because of the interaction between time constant and range setting, the

longest available time constant is less than the duration of the test

stimulus. It is important to realize the thermal-transient test data

obtained at short time constant settings will not be representative of

accelerometer system response measured with long time-constant settings

of the charge amplifier.

2.2.2 Voltage Amplifiers

Special-purpose, impedance-matching voltage amplifiers are frequently

used with piezoelectric transducers. The amplifiers have extremely

high input impedances to minimize the unavoidable effects of lowfrequency roll off due to a short time constant.

The voltage amplifier is perhaps easier and simpler to use than the

charge amplifier. Because of its simplicity, it is the lower in cost

of the two classes of signal-conditioning amplifiers. The voltage

amplifier also has a higher frequency-response capability than the

charge amplifier.

A disadvantage of the voltage amplifier is the fact that voltage

amplifiers with long time constants are difficult to obtain.

Further, when used with piezoelectric transducers the signal

12
available at the amplifier input terminals is inversely proportional

to transducer and cable capacitance, and therefore, in effect, to cable length.

On the other hand, the voltage amplifier does not suffer from one of

the basic constraints in the design of the charge amplifier, the

change in time constant with range. This feature makes it less

difficult to compare accelerometers of widely differing sensitivities

using voltage amplifiers.

The voltage amplifier has a time constant determined by the total

capacitance (transducer, cable, amplifier input) as seen by the amplifier, and the total resistance across the input. The electrometer

amplifier has a number of voltage ranges, as well as a control for

changing its input resistance settings of up to 10 lk n are in decade

ranges. The effects of input resistance on accelerometer zero-shift

were investigated and the results are shown in figures 9 and 10, for

two different accelerometers. The zero-shift observed is greater for

larger values of input resistance (and therefore longer instrument

time constants).

Changes in voltage range settings of the electrometer amplifier were

found to have no effect on the zero-shift observed. An electrometer

amplifier therefore makes it possible to compare the thermal -transient

responses of different accelerometers using the same electrical time

constant, through proper setting fo the input resistance control of

the electrometer.

2.2.3 Use of Electrical Filters

To reduce the problems of instrument drift, some users insert a simple,

high-pass filter in each piezoelectric accelerometer instrumentation

channel. The amplitude-frequency response characteristics of a typical

filter shows a drop of 5% at 2 Hz. To investigate the effects of such

a filter on the thermal -transient response of the accelerometer, tests were conducted with the filter inserted between the charge amplifier or electrometer amplifier and the oscilloscope. The filter used to provide

the desired response consists of a 0.24-yF capacitor and 1.0-Mfi resistor,

Since the input resistance of the vertical amplifier of the oscilloscope

used is 1.0 Mfi, the addition of a 24-pF series capacitor at the amplifier input is sufficient to provide the desired high-pass filtering.

The time-constant of this filter is 0.24 s. For tests in which this

filtering was not used, the output signal from the amplifier was directly coupled to the oscilloscope.

3. Thermal -Transient Tests on Fourteen Accelerometers

3.1 Test Results Using One-Second Exposure

Fourteen accelerometers, two each of seven different models, were

received from the sponsoring agency for testing. Five of the models

are piezoelectric ceramic types (three of shear design, and two of

13
single-ended compression design), one model contains a piezoelectric

quartz crystal, and the remaining model has a piezoresistive straingage sensor. Table 1 lists the characteristics of the accelerometers

as provided by their manufacturers. As indicated earlier, comparison

tests of thermal -transient effects on the performance of various

accelerometers could best be accomplished through the use of electrometer

amplifiers as there would be no gain-time constant constraints. Since

the types of accelerometers tested are frequently used in conjunction

with charge amplifiers, however, tests were conducted with charge

amplifiers as well

.

In the tests, each accelerometer was subjected to a radiant power

density of 1.8 W/cm2 for 1 s. The acceleration level applied during

the test was 20 gn peak-to-peak at 100 Hz. Four different circuit

configurations were used: (1) charge amplifier directly coupled to the oscilloscope; (2) charge amplifier capacitively coupled to the

oscilloscope; (3) electrometer directly coupled to the oscilloscope;

and (4) electrometer capacitively coupled to the oscilloscope. Highpass filtering due to capactive coupling is discussed in section 2.2.3.

Figures 11, 12, and 13 show results from the 1-s exposure tests.

Figure 13 shows the effect on accelerometer output of each of the

circuit configurations listed above. Tables 2 through 5 contain

tabulated test results.

Test results from transducer evaluations are generally expressed as percentages of the full-scale range of the transducer. Most transducers

are used for measurement of physical quantities with amplitudes near

the full-scale range. In these cases, transducer errors or deviations

expressed as percentages of full scale convey a useful measure of the

quality of the measurement. This may not always be true for piezoelectric accelerometers.

In the case of many piezoelectric accelerometers, particularly those

used for shock measurements, the full-scale design range of the

accelerometer is \/ery high. Among the accelerometers evaluated by the

thermal-transient test, the lowest full-scale range was ±2000 gn

; the

highest +100,000 gn

, -20,000 gn

. Measurements of very high acceleration levels may not really be ^ery meaningful. Errors may result

from attachment constraints, inhomogeneities in materials, lack of

adequate frequency response to reproduce correctly transient

accelerations, and other factors. The full-scale range of such an instrument is primarily chosen to give adequate margins of safety

against overloading. Actual acceleration amplitudes measured with

such accelerometers may not exceed one- tenth of their full-scale

range.

It appears then that reporting accelerometer characteristics only as

a percentage of a (possibly quite unrealistic) full-scale range may

ultimately lead to a misleadingly optimistic interpretation of

14
experimental measurements. For this reason, thermal -transient zero- shift values obtained in the tests are expressed in this report in

terms of the change in acceleration corresponding to the zero shift

(in units of g n

).

3.1.1 Tests with Charge Amplifier

Thermal -transient test results using the charge amplifier are compiled

in tables 2 and 3. Results of tests on four accelerometers having

their top surface exposed to a thermal transient are shown in figure 11 and figures 13-c and -d. The character of the response differs

considerably for different accelerometers, as it does for top and side

exposures. The test results compiled in table 2 were obtained with

the charge amplifier directly coupled to the oscilloscope used to display the thermal -transient response of the accelerometer. Table 3 contains the results from tests using capacitive coupling between the

charge amplifier and the oscilloscope, equivalent to the introduction

of a high-pass filter into the system.

With the charge amplifier directly coupled to the oscilloscope, the zero shifts observed ranged from +640 gn (B-l) to +2.3 gn (E-l) for

top exposure, and from +280 g n (B-l) to +1.4 g (E-l) for side exposure.

With the charge amplifier capacitively coupled to the oscilloscope, the zero shifts displayed were smaller because of the high-pass filtering.

The exposure values ranged from +80 g n (B-l) to -0.17 g n (C-2). Side

exposure values ranged from +22 g n (B-l) to -0.06 g„ (C-2). The effects

of the high-pass filter are discussed in more detail in section 3.3.2

below.

3.1.2 Tests with Voltage Amplifier (Electrometer)

Thermal -transient test results with the electrometer amplifier are compiled in tables 4 and 5. Figure 12 shows the thermal -transient

responses of two different accelerometers when top or side surfaces

are irradiated. In these cases the high-pass filter was used

(section 3.3.2). Figures 13-a and -b also show accelerometer responses

using the electrometer. In the case of the one-s tests the

electrometer input resistance was 10 10

fi.

With the electrometer amplifier directly coupled to the oscilloscope,

the zero shift observed ranged from +365 gn (B-l) to +2.6 g„ (E-2) for

the top exposure, and from +90 gn (B-l, B-2) to +1.5 g~ (E-l) for

side exposure. With capacitive coupling, the zero shifts observed

ranged from +53 g„ (B-l) to 0.17 g n (C-l) for top exposure, and from

+25 g n (B-l) to "-0.09 gn (C-l) for side exposure.

3.2 Test Results Using Fifteen-Second Exposure.

As indicated in section 3.1.3.4, initial experiments in the development of the thermal-transient test method suggested a 15-s exposure

as a reasonable test value. Experimental data were obtained at this

exposure duration for all accelerometers, using the electrometer

15
amplifier directly coupled to the oscilloscope. The results are compiled in table 6. Some accelerometer responses are shown in

figure 14. The input resistance for these tests was 10 9ft, and

the radiant energy density was 1.8 W/cm2

, applied to the top

surface only in these tests.

Each accelerometer model appears to have a distinctive pattern of zero shift, with slight variations between different accelerometers

of the same model. The maximum zero shifts ranged from +2.4 g n (E-2)

to +350 g n (B-l).

3.3 Discussion of Test Results

The test results indicated general patterns of behavior in thermal

transient environments for the types of accelerometers sampled;

however, a larger sampling of accelerometers than the limited one available for these tests would be required for definitive predictions

about the behavior of other accelerometers of these types.

During some of these tests, attempts were made to measure changes in

sensitivity in addition to zero shift. Small changes were observed

in some cases, but results were not consistent. It appears that a maximum shift in sensitivity of about 3% may be expected to occur after a 15-s exposure at 2.5 W (radiant energy density of 0.88 W/cm2

) and that somewhat smaller changes may be expected at the lower power

levels. The effects of such sensitivity changes during actual vibration measurements are negligible in comparison with the predicted

zero shifts observed during these tests.

3.3.1 Top and Side Exposure

Table 7 gives the exposed area in cm2 for each accelerometer type

tested, for both top and side exposures. These areas are the

projected areas which intercept the incoming thermal radiation and

are calculated from measurement of accelerometer dimensions. The

table also gives calculated values of radiant power intercepted by

the projected areas for a radiant power density of 1.8 W/cm2

.

The experimental results given in tables 2 and 4 (1-s exposure, use of charge amplifier or electrometer amplifier directly coupled to oscilloscope) show that accelerometer types B and D exhibit significantly greater zero shift when the accelerometer top is exposed than

when the side is exposed. Accelerometer type E shows a larger zero shift when its top surface is exposed to radiation, possibly because

its exposed top area is almost four times as large as the exposed side

area. For accelerometers B and D, the respective areas are almost

equal. Accelerometer types A and F show a larger zero shift when

their sides are exposed; the side areas of these types are considerably

larger than the top areas. In the case of the other accelerometer types

no clearly detectable differences were found between the effects of

exposing the two surfaces.

16
3.3.2 System Time Constants and the Effects of High-Pass

Filters

As described earlier, the accelerometers were tested in four different

circuit configurations: charge amplifier with direct and capacitive

(i.e., with high-pass filter at the input of the monitoring oscilloscope)

coupling and electrometer with direct and capacitive coupling. The

data from these tests, using a 1-s thermal radiation exposure, are

shown in tables 2 through 4. These tables also show the values of the

time constant used for each measurement. In the case of the charge

amplifier, the value of time constant given is that obtained from the

manufacturer's instruction manual, which tabulates time-constant values

for all combinations of range and time-constant switch settings.

The piezoelectric accelerometer can be assumed to be an instantly

responding source of charges with extremely high internal resistance.

When used with the electrometer, which is a voltage-sensing device,

the accelerometer can be assumed to be an instantly responding voltage

generator in series with a capacitance. This series capacitance is

the sum of the accelerometer and cable capacitances. The capacitanceresistance network formed by this series capacitance and the electrometer's input resistance acts as a high-pass filter with a specific

time constant, listed in the appropriate tables under the heading

"Electrometer Time Constant."

Since some laboratories use an additional R-C high-pass filter in all

piezoelectric accelerometer channels to reduce drift caused by the

generation of spurious charges, our tests also included measurements

using such a filter with a sinusoidal amplitude-frequency response

down 5% at 2 Hz and a corresponding time constant calculated to be

0.24 s.

A series of measurements of the equivalent time constant was made

using the various circuit configurations —charge amplifier with

direct and capacitive coupling and electrometer with direct and

capacitive coupling. A step voltage from a function generator was

fed into the charge amplifier's calibrating input or into the electrometer, and the resulting signal was displayed on the oscilloscope and

photographed. The time constant was estimated from the photograph as the time required for the signal to drop to 1 /e (36.8%) of its initial

value. Some of the time constants for the electrometer directly

coupled are too long to be determined in the above manner with the

equipment available.

The tests were made with the sensitivity, range, input resistance, and

time-constant settings that had been used with the various accelerometers,

The voltage of the function generator was set so that the output signal

displayed on the oscilloscope approximated the zero-shift observed for

the accelerometer being simulated.

17
Values for the time constant with capacitive coupling ranged from

0.20 s to 0.25 s with a scaling accuracy estimated as ±0.05 s. This

agrees well with the calculated value of 0.24 s. Time constants using

the charge amplifier with direct coupling measured 10.2 s for the

simulated type A accelerometer (calculated time constant 10 s);

6.5, 5.7, and 7.5 s for types B, C, and D respectively (calculated

time constant 5 s) and 3 s for types E and F (calculated time constants

2 s). Scaling accuracy for these measurements is estimated at ±0.5 s.

The data from the thermal -transient tests -were analyzed on the assumption that the thermal -transient input to the accelerometer and the

resulting output are ramp functions. The rise time of this ramp is

assumed equal to the time interval from the initiation of the transient

to the time of maximum output observed. This assumption is based on the appearance of the output signal (figures 11-14) and experimental

data taken at longer exposures (figures 7 and 8) which indicate that

the output signal appears proportionately greater at these exposures.

The 1-s thermal -transient stimulus cannot be considered either a step function or an impulse because shutter opening and closing times

are significant compared to 1 s. Assumption of an exponential rise time

would have increased mathematical complexity. The ramp function was chosen as the most practical representation of the thermal -transientgenerated output signal, and the results of data normalization (table 8)

appear to justify this choice.

A theoretical study was undertaken of the output resulting from the

application of a ramp function to a series combination of two R-C

high-pass filters. The filters were chosen to represent the capacitance

of the accelerometer and the input resistance of the electrometer,

followed by the coupling capacitance, and the input resistance of the

oscilloscope. Alternately, they can represent the charge-amplifier time

constant, followed by the coupling capacitance-oscilloscope input resistance time constant. The other two test conditions which do not use

the oscilloscope input high-pass filter were also studied, being

represented by a ramp function applied to a single R-C high-pass

filter. In the studies, parameters considered were the ratio (Q) of

ramp rise time (T) to filter time constant (RC), and the ratio (m) of

the time constants of the two filters (RC) l5 and (RC) 2 , when these were used.

It can be shown for the case of the single high-pass filter that the

maximum output signal reached at time T is proportional to 1-e"^ . The signal decays exponentially after this. In the case of

Q two high-pass filters, the output signal at time T is proportional to

i (j^jjj-) (e-^-e"

1 ). With the aid of these relations, the data from the

1-s test on accelerometers A-l , B-l , C-l, E-l , and F-l were normalized

(for comparison purposes) to represent the actual transducer output

with an assumed infinite time constant. The results are given in

18
table 8 and show reasonable agreement for four of the six accelerometers

tested (B-l, D-l, E-l , and F-l). In the case of A-l, it should be noted

that the original test data showed a large, continuing, negative drift

for measurements in which no high-pass filter was used (precisely the

situation for which the filters are required in actual measurement

systems). The direction of the observed drift was consistent with the

large discrepancy observed. Figure 11-a shows this drift. The discrepancy in the data from accelerometer C-l was noticeable, but smaller than

that for A-l. The former type of accelerometer also showed some negative

drift, prior to the thermal -transient input. Figure 13-a shows this

drift for accelerometer C-2.

3.3.3 Relation of Test Results to Accelerometer Features

The zero shift exhibited by some of the accelerometers tested is substantial; but there is no measurably significant change in sensitivity, as evidenced by the nearly constant signal amplitude observed over each

measurement. Zero shift varied considerably both in amplitude and

direction among the different models of accelerometers. These variations are believed to result from differences of geometry (shape, size,

and placement of electrical connectors and fastening hardware), design

(shear or compression type, materials), and electrical characteristics

(capacitance, leakage resistance) of the accelerometers. The effects

of time constants of the ancillary electronics are discussed in 2.3.2.

Accelerometers B-l and B-2 show the largest radiation-induced zero shifts

(positive direction). Accelerometers E-l and E-2 also show a positive

zero shift (but of much smaller magnitude). These two are the only

types of transducer tested to exhibit positive shift; both are of

shear-type design, with the piezoelectric crystal serving as both sensor and seismic mass. Accelerometer type B has a massive metallic

center post closely coupled to a substantial outside case. The design

is such that any heat energy impinging on top or side would be expected

to diffuse rapidly into the round center post (to which the crystal is attached and with which it is concentric). Consequent lateral expansion

of the center post resulting in stress on the piezoelement appears likely.

Accelerometer type E has a much smaller center post than type B and

had massive case walls; furthermore, when the instrument is mounted

for measurements, the piezoelement is actually below the mounting surface. This design should provide good shielding from thermal radiation

consistent with the small zero shifts observed.

Accelerometer type D is also a shear design. The center post in this

design is less massive than in type B, and is pierced for the mounting

screw. Walls are moderately thick, and the connector block is massive.

A reasonable prediction of the behavior of this design when exposed to

a thermal transient is that a portion of the heat energy would be

transmitted to the center post resulting in expansion of the post and

consequent application of stress to the piezoelement. On this basis,

19
the zero shift for this instrument should be greater than that for type

B. There is no satisfactory explanation of the observed negative

direction of zero shift.

Accel erometer types A, C, F (single-ended compression types) all show

similar negative zero shifts, with F (having a quartz sensing element)

showing the smallest shift. In the single-ended compression design the

circular sensing-element disc is loaded with a circular plate mass

on top. Both elements are pierced with a center hole through which a threaded stud protrudes from the instrument base. A nut engages the

top of the stud and compresses mass and sensing element; a case surrounds

the elements and is attached to the base. This design should result in

small zero shifts being induced by thermal irradiation because of the

isolation between case and sensing element. The large zero shift

observed for type A has not been explained, but it should be noted

that these instruments showed a considerable negative drift even before

they were exposed to thermal radiation in the test. Type C also showed

a similar negative drift, but of smaller magnitude.

When irradiated for 15 seconds, accelerometer types D and G and one of

the type F accelerometers displayed a sharp reversal of zero-shift

direction upon closing of the shutter. Addition of the high-pass filter

to the signal conditioning circuit greatly reduced the zero shift in all cases (in come cases almost to zero). These observations show that

the thermal -transient zero shift in piezoelectric accelerometers is

essentially a low-frequency phenomenon, and that the resulting effects

can be controlled with selection of the proper high-pass filter.

Whether an accelerometer is affected more by irradiation impinging on

its side or on its top surface varies with the different accelerometer

models. Most of the accelerometers (five of the seven types) showed

greater zero shifts when the surface having the larger area was irradiated. But no definite correlation between surface area and zero- shift amplitude could be established.

4. Conclusions

Many users of piezoelectric accelerometers may be unaware of the

measurement errors that can result from the use of these accelerometers

in thermal-transient environments. Because it has been shown that the zero shift due to thermal transients may vary widely among the different

models of accelerometers and because there is no simple means of

predicting the zero shift, it is recommended that tests be performed to determine such shifts for accelerometers that are to operate under

conditions in which thermal -transients are present.

The thermal -transient test method reported here for piezoelectric accelerometers is simple, inexpensive, and well suited for such tests. Adjustability of the duration or intensity of the thermal transient is possible

over a wide range of values and adaptation of the system for irradiating

20
various accelerometer surfaces is possible. The use of radiant energy

to provide the thermal transient simulates the conditions of field use for blast measurements more closely than the previously used method of

ANSI S2. 11-1969 [3].

5. Recommendations

Recommended is an investigation of the adaptability of the test method

described for the testing of other types of transducers for thermaltransient effects. In particular, it it felt that this method would

be much less expensive than the laser method referenced earlier for

measuring thermal-transient effects in pressure transducers.

6. References

[1] Cady, W. G., Chapter 29-Pyroelectricity, Book, Piezoelectricity,

pp. 699-712 (McGraw-Hill Book Co. Inc., New York, NY, 1946

and Dover Publishers, Inc., New York, NY, 1964).

[2] Dranetz, A. I., and Orlacchio, A. W., Chapter 16-Piezoelectric

and Piezoresistive Pickups, Book, Shock and Vibration Handbook,

Ed. C. M. Harris and C. E. Crede, (McGraw-Hill Book Co. Inc.,

New York, NY, 1961).

[3] "American National Standard for the Selection and Calibrations

and Tests for Electrical Transducers Used for Measuring Shock

and Vibration", ANSI S2.11-1969 (American National Standards

Institute, New York, NY, 1969).

[4] Lederer, P. S., and Hilten, J. S., A Laser Technique for Investigating the Effects of Thermal Transients on Pressure Transducer

Performance Characteristics, NBS Tech. Note 723 (May 1972).

[5] Koyanagi, R. S., Pollard, J. D., and Ramboz, J. D., A Systematic

Study of Vibration Standards-Mounting Effects, NBSIR 73-291

(September 1973).

21
Table 1

Test Accel erometer Characteristics

(Supplied by Manufacturers)

Accel erometer

CaAc

&

pacitance

eel erom- eter

Cable)

SensitivityDesigr

Type

i Range

Natural

Frequency

Mounted

pF pC/^n

3x\* to peak kHz

A-l 2902 13.1 SEC** ±2,000 37

A-2 2886 13.1 SEC ±2,000 37

B-l 282 0.0286 Shear +100,000

-20,000

80

B-2 287 0.289 Shear +100,000

-20,000

80

C-l 2358 65.0 SEC ±10,000 29

C-2 2062 54.1 SEC ±10,000 29

D-l 563 0.161 Shear ±20,000 100

D-2 572 0.164 Shear ±20,000 100

E-l 253 0.127 Shear ±20,000 125

E-2 242 0.129 Shear ±20,000 125

F-l 239 0.331 SEC

(Quartz]

+100,000

1 -20,000

60

F-2 239 0.303 SEC +100,000 60

(Quartz) -20,000

G-l - - 5.84yV/#n Piezore- ±20,000 90

si stive

G-2 - - 5.84yV/#n Piezore- ±20,000 90

sistive

* 1.00 gx\ = approximately 9.8 m/s 2

, ** Single-Ended Compression

22
Table 2

Thermal -Transient Effects on Accel erometers

For One-Second Exposure Using

Charge Amplifier Directly Coupled to Oscilloscope

Maximum Zero Shift {gn )

Side

Irradiated

-39. **

-44. **

+280.

+150.

-4.0

-4.0

-S.0

-4.0

+1.4

+1.9

-16.

-7.0

-5.0

-5.5

* Piezoresistive transducer, no signal conditioner used

** Considerable negative drift before radiation exposure

Accel er- ometer

Charge Amplifier

Time Constant(s)

A-l 10

A

10

B-l 5 B-2 5 C-l 5 C-2 5 D-l 5 D-2 5

E-l 2 E-2 2

F-l 2 F-2 2 G-l *

G-2 *

Top

Irradiated

-25. **

-39. **

+640.

+340.

-4.6

-6.1! -31,

-24,

+2,,3 +2,,5

-8,,3

-3,,3

-2,,4

-4,,0

23
Table 3

Thermal -Transient Effects on Accel erometers

For One-Second Exposure Using

Charge Amplifier-Capacitively Coupled to Oscilloscope

Accel er- Charge Amplifier Maximum Zero Shift [g n ) ometer Time Constant(s)

A-l 10

A210

B-l 5 B-2 5 C-l 5 C-2 5 D-l 5 D-2 5 E-l 2 E-2 2 F-l 2 F-2 2 G-l *

G-2 *

Top

Irradiated

Side

Irradiated

-0.57 -3.0

-0.57 -2.9

+80. +22.

+57. +12.

-0.23 -0.15

-0.17 -0.06

-8.6 -1.5

-7.0 -1.1

+0.46 <0.1

+0.47 +0.14

-3.0 -4.2

-1.3 -2.5

-1.0 -2.0

-1.6 -1.8

Piezoresisti ve transducer, no signal conditioner used

24
Table 4

Thermal -Transient Effects on Accel erometers

For One-Second Exposure Using

Electrometer Amplifier Directly Coupled to Oscilloscope

AccelerometerElectrometer

Time Constant(s)

Maximum Ze

Top

Irradiated

ro Shift (g n )

Side

Irradiated

A-l 29. -37. * -60.

A-2 29. -37. * -48.

B-l 2.8 +365. +90.

B-2 2.9 +210. +90.

C-l 24. -10. -11.

C-2 21. -12. -9.0

D-l 5.6 -27. -5.0

D-2 5.7 -46. -4.3

E-l 2.5 +2.7 +1.5

E-2 2.4 +2.6 +2.5

F-l 2.4 -7.5 -2.6

F-2 2.4 -3.3 -7.6

Considerable negative drift before radiation exposure

25
Table 5

Thermal-Transient Effects on Accel erometers

For One-Second Exposure Using

Electrometer Amplifier Capacitively Coupled to Oscilloscope

Accel er- Electrometer

ometer Time Constant(s)

A-l 29.

A-2 29.

B-l 2.8

B-2 2.9

C-l 24.

C-2 21.

D-l 5.6

D-2 5.7

E-l 2.5

E-2 2.4

F-l 2.4

F-2 2.4

Maximum Zero Shift {g n )

Top

-0.54

-0.57

+53.

+30.

-0.17

-0.27

-7.6

-8.8

+0.50

-0.57

-2.6

-1.1

Side

-2.7

-2.9

+25.

+8.6

-0.09

-0.13

-1.4

-0.87

+0.20

+0.19

-8.0

-2.5

26
Table 6

Thermal -Transient Effects on Accel erometers

For Fifteen-Second Exposure Using

Electrometer Amplifier Directly Coupled to Oscilloscope

Accel erometer Electrometer

Time Constant(s)

Max

Sh

imum Zero

1ft (gr.„)

A-l 2.9 -130

A-2 2.9 -120

B-l 0.28 +350

B-2 0.29 +210

C-l 2.4 -39

C-2 2.1 -45

D-l 0.56 -160 ** +15

D-2 0.57 -180

+19

E-l 0.25 +2.6

E-2 0.24 +2.4

F-l 0.24 -4.2

F-2 0.24 -2.1

+3.3

G-l * -30

G-2 * -11

Time Interval

to Peak Output(s)

20

20

5

5 19

18

1 16

1 16

15

5

1

1 16

15

2

* Piezoresistive transducer, no signal conditioner used ** Positive and negative peaks observed

27
Table 7

Thermal Power Input Levels in Accelerometer Tests,

Power Density = 1.8 W/cm2

Accelerometer

Type

Exposed

(cnv

Top

Area

Side

Radiant Power I Accelerometer

Top

ncident on

(W)

Side

A 2.1 3.1 3.8 5.4

B 1.8 1.7 3.1 3.0

C 2.1 3.1 3.8 5.4

D 1.0 0.67 1.5 1.2

E 0.54 0.16 0.95 0.30

F 0.91 1.5 1.6 2.6

G 1.4 0.61 2.4 0.81

28
CM *3" OJ CO

CO Ol ^ r^ O r—

COLOOO

lo <jd co ^r + CO + "X>

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29
Figure 1. View of radiant heat lamp and shutter with test accelerometer side exposed to thermal transient.

30
MAIN RADIATION SHIELD

RADIATION

SHIELD

SUPPORT

Figure 2. Test and reference accelerometers on vibration

exciter.

31
HIGH INTENSITY

RADIOMETER

HEAD

READ OUT

UNIT

INCANDESCENT

LAMP

VARIABLE

VOLTAGE

TRANSFORMER

117 V,60 Hz

HE

S

s~\

( x^

1 l

CHANIC

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ACC.

i

< i CHARGE A C

REF. VOLTMETER

ACC.

.,

< i AMPLIFIER

VIBRATION

EXCITER

POWER

AMPLIFIER

AUDIO

OSCILLATOR

Figure 3. Block diagram of test equipment for thermal transient

test on piezoelectric accelerometers.

32
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43
NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS TN-8552. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

AN EXPERIMENTAL TECHNIQUE FOR THE EVALUATION OF THERMAL TRANSIENT EFFECTS ON PIEZOELECTRIC ACCELEROMETERS

5. Publication Date January 1975

6. Performing Organization Cod« 425.03

7. AUTHOR(S)

Carol F. Vezzetti and Paul S. Lederer

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS NATIONAL BUREAU OF STANDARDS DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 4253434

11. Contract/Grant No

AT(04-3)-911

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP) Lawrence Livermore Laboratory

University of California

Livermore, California 94550

13. Type of Report & Period Covered Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES Library of Congress Catalog Card Number: 74-600188

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant bibliography or literature survey, mention it here.) A simple, inexpensive method was developed for determining the effects of thermal

transients on the zero output and sensitivity of piezoelectric accelerometers.

Thermal transient stimuli are generated by an incandescent lamp and can be made to heat the top or side of the test accelerometer. Fourteen commercial accelerometers were tested using this technique. Zero shifts with magnitudes as high as 640 g*' were observed. Zero shifts up to 2% of full-scale resulted from one-second duration

transients, and up to 1% of full-scale from fifteen-second transients. These results were obtained at a radiation power density of 1.8 W/cm. No changes of accelerometer

sensitivity exceeding experimental uncertainties were noted as a result of the thermal transients used.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

' Accelerometer; performance characteristics; piezoelectric; test method; thermal

radiation; thermal transient; zero shift.

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