NBSIR 74-597

Piezoelectric Accelerometer

Low-Frequency Response liy

Signal Insertion Methods

Robert S. Koyanagi, James D. Pollard

Vibration Section, Mechanics Division

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

May 1975

Final Report

Prepared for Department of Defense

Calibration Coordination Group

Subgroup for Shock and Vibration

NBSIR 74-597

PIEZOELECTRIC ACCELEROMETER

LOW-FREQUENCY RESPONSE BY

SIGNAL INSERTION METHODS

Robert S. Koyanagi, James D. Pollard

Vibration Section, Mechanics Division

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

May 1975

Final Report

Prepared

Department of Defense

Calibration Coordination Group

Subgroup for Shock and Vibration

U.S. DEPARTMENT OF COMMERCE, Rogers C.B. Morton. Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts. Director

TABLE OF CONTENTS

Page

Forward 1

Abstract 2

1. Introduction 3

2. Signal Insertion Method . 3

3. Mechanical Vibration Tests 3

4. Voltage Measuring Systems 4

3. Discussions of Results 5

5.1 Accelerometers Tested .... 5

5.2 Plotted Data 5

6. Conclusions * 7

7. Acknowledgements 8

8. References

.

9

LIST OF TABLES

Table 1 Accelerometer Systems Tested 5

Table 2 Results of Tests with a Charge Amplifier as a Signal

Conditioner 6

Table 3 Results of Tests with a Voltage Amplifier as a Signal

Conditioner 7 UST OF FIGURES

Figure 1 Signal Insertion Schematic . ,

Figure 2 Acceleration and System Outputs versus Frequency 21

Figure 3 Accelerometer ERK14 with Charge Amplifier 22

Figure 4 Accelerometer ERK18 with Charge Amplified . 13

Figure 5 Accelerometer ERK19 with Charge Amplifier 14

Figure 6 Accelerometer C05D7 with Charge Amplifier 15

Figure 7 Accelerometer C05D1 with Charge Amplifier ........ 16

Figure 8 Accelerometer COM28 with Charge Amplifier I7

Figure 9 Accelerometer C0151 with Charge Amplifier 18
Figure 10

Figure 11

Figure 12

Figure 13

Figure 14

Figure 15

Figure 16

Figure 17

Figure 18

Figure 19

Figure 20

Acceleroineter

Accelerometer

Accelerometer

Accelerometer

Accelerometer

Accelerometer

Accelerometer

Accelerometer

Accelerometer

Accelerometer

Accelerometer

C07C5 with

C07E0 with

C0AC8 with

C05D7 with

C05D1 with

COM2 8 with

C0151 with

C07C5 with

C07EO with

C0AC8 with

C0253 with

Charge An^jlifier

Charge Amplifier

Charge Amplifier

Voltage Amplifier

Voltage Amplifier

Voltage Amplifier

Voltage Amplifier

Voltage Amplifier

Voltage Amplifier

Voltage Amplifier

Voltage Amplifier

19

20

21

22

23

24

25

26

27

28

29
POREWARD

The evaluation of the piezoelectric accelerometers discussed in this

report was sponsored by the Department of Defense Calibration Coordination

Group (DOD/CCG) , Subgroup for Shock and Vibration, consisting of: the

Metrology Engineering Center, Bureau of Naval Weapons Representative,

Pomona, California 91766; the Aerospace Guidance and Metrology Center,

Newark Air Force Station, Newark, Ohio 43055; and the Metrology Calibration

Center, Redstone Arsenal, Alabama 35809. The Atomic Energy Commission was represented by an observer from Sandia Laboratories, Albuquerque, New

Mexico 87115.

The DOD/CCG project number assigned was CCG 7A-93 and work was performed

under NBS cost center 2130427.

1
PIEZOELECTRIC ACCELEROMETER LOW- FREQUENCY RESPONSE

BY SIGNAL INSERTION METHODS

by

Robert S. Koyanagi

James D. Pollard

ABSTRACT

The purpose of this study was to compare the frequency response of

selected piezoelectric accelerometers using a signal insertion method to the

response using traditional mechanical vibration tests. Signal insertion

methods included "voltage inserticHi" and "charge insertion" techniques.

The signal is inserted in series with the electrical low-side of the

accelerometer by means of a suitable series resistance. Commercially

available insertion devices were used. Confidence in the use of insertion

methods is increased where there is agreement between the results from

insertion tests and mechanically excited tests.

UNITS OF MEASURE

SI units were used throughout this report with one exception. To

facilitate communication with the Intended readers "g" was used for the

acceleration of gravity. The SI»units can be obtained by use of the

relationship: 1 g = 9.80665 m/s .

KEY WORDS

Accelerometer, calibration, vibration, low frequency, signal insertion.
1. INTRODUCTION

The report deacribes tlie investigation of signal insertion methods of

calibrating piezoelectric accelerometers at low frequencies. This method

may be particularly useful to laboratories which do not have the facilities

to mechanically produce a suitable, sinusoidal low-frequency waveform. For

purposes of comparison, each accelerometer was calibrated mechanically on

an electrodynamic exciter [1] and by insertion techniques [2, 3, 4].

Tests were conducted from 1.5 to 50 Hz. A total of 11 accelerometers

of eight different models were tested. Where possible, each accelerometer

was tested with a voltage amplifier and a charge amplifier used as signal

conditioners. ^

2. SIGNAL INSERTION METHOD

Figure 1 illustrates the signal insertion technique. The calibration

voltage is inserted in series with the accelerometer on the ground side.

This is facilitated by use of commercially available insertion devices [5, 6].

The accelerometer must be electrically isolated from ground.

The frequency response of the accelerometer-amplifier system is obtained

by measuring the voltage output of the amplifier (V )

, and the calibration

voltage (V ) . The variation in the ratio ^q^^^^. ^^^^ frequency is the

frequency §isponse of the system.

°

3. MECHANICAL VIBRATION TESTS

NBS electrodynamic Vibration Standard LF-1 was used to calibrate the

accelerometer-amplifier systems mechanically. Sensitivity factors were

determined by comparing the voltage output of the amplifier used with the

accelerometer with the output of the Standard.

Varying degrees of difficulty were encountered in measuring the voltage

output of the accelerometer-amplifier systems at very low frequencies. Vibration Standard LF-1 has a maximum double amplitude of 4.45 cm [1]. At low

frequencies this limits the maximum attainable acceleration. For example,

at 2 hertz the maximum acceleration is 0.36g peak. These low acceleration

levels coupled with the low sensitivity of some of the accelerometers result

in very low signals and poor signal-to-noise ratios.

Figure 2 shows the level of mechanical test acceleration versus frequency.

The ordinates on the right side of the graph gives the voltage output at

these acceleration levels for various system sensitivities. The intersection

of this curve with the 7 mV output point of the various systems gives the

approximate lowest frequency at which reliable voltage measurements were made

(column 2 of tables 2 and 3).

[1] Numerals in brackets refer to references found at the end of this paper.

3
4. VOLTAGE MEASURING SYSTEMS

Low-frequency voltage measurement is a difficult problem. In these tests,

the quantities sought were voltage ratios and therefore, some of the errors tend to cancel. The following voltage measuring systems were used: \

1. The voltages were measured with two true rms differential voltmeters, or

2. '^he voltages were fed directly into two voltage-to-frequency converter?;

the ratios of the converter frequencies were measured on a counter and are proportional to the voltage ratios.

A principal problem of this study is the result of limitations of voltage

measurements at low frequencies and low amplitudes . Voltmeters, in general,

do not provide accurate measurements at frequencies below about 5-10 Hz, and

at 1.5 to 5 Hz, their uncertainties are often large. It was a principal goal

of this study to compare signal insertion results to mechanically excited

results. Because of the limitations discussed in Section 3 above, (i.e. low

voltage signal amplitudes due to low acceleration amplitudes) it was also

necessary to perform the signal insertion tests at nominally the same low

signal amplitudes. Otherwise, ranging errors in the voltmeters could be as large as the effects being studied.

Voltmeter errors due to roll-off at low-frequencies could be taken into

account by using the same voltmeter on the same range when measuring by both

the insertion and mechanically excited methods. By keeping the voltmeter range

the same, only differences were being sought between the two methods, even

though voltmeter roll-off with decreasing frequency might have been present.

So long as the voltmeter error remained the S£ime between the two methods, this source of error ideally subtracts to zero in the difference between the methods.

In addition to the kinds of problems discussed above in making voltage

measurements, it was discovered that the true rms differential voltmeters

exhibited a strange increase in uncertainty when certain con±)inations of

voltage and frequency were being used. For example, the voltmeter would

exhibit a rather normal and somewhat predictable roll-off at low frequencies

when the voltage amplitude was maintained at reasonably high levels (I.e. , in the tens of millivolts or greater). However, when the voltage amplitude

reached about 7 mV or less, the roll-off occurred at a very much greater rate;

this happening only at low frequencies. At higher frequencies, this effect

does not occur and low level voltages less than 7 mV could be measured.

When using voltage-to-frequency converters to measure low frequency

voltages, any dc levels present in the signal can create errors In their

readings. Some of the signal conditioners (i.e. , voltage or charge amplifiers)

used in this study did produce small dc voltage offsets. The zero stability

of the balancing circuits would not remain sufficiently stable to permit

meaningful measurements to be made. Furthermore, slowly changing voltage

levels from such causes as the pyroelectric effect In some of the accelerometers

would cause large and unmanageable errors.

4
The inability to measure low -level, low -frequency voltages limited the

lowest frequencies at which meaningful tests could be performed. These are shown and discussed in section 6 of this report.

5. DISCUSSION OF RESULTS

5.1 iAccelerometers Tested

Some of the accelerometers tested were not compatible with the voltage

or charge amplifiers used. Table 1 indicates which systems were tested and

gives the nominal system sensitivity for the accelerometer and amplifier

combination used.

Table 1. Accelerometer Systems Tested. Values in Parenthesis are Nominal

Sensitivities in Units of mV/g.

* Accelerometer Code

Design

Type Charge Amplifier

Voltage

Amplifier

Fig. No. Fig. No.

"ERKIA" Compression X (10) 3

ERK18 Cocp resaion V (10) 4

_ERK19_ Comp ression X (10) 5 rC05D7| Compression X (1400) 6 X (40) 13

[c05DlJ Compression X (1500) 7 X (40) 14

COM2 8 Comp ression X (110) 8 X (20) 15

C0151 Compression X (lAO) 9 X (10) 16

C07C5 Comp ression X (290) 10 X (150) 17

C07E0 Comp ression X (390) 11 X (200) 18

C04C8 Shear X (130) 12 X (40) 19

C0253 Shear - —I X (15) 20

*Bracketed codes indicate accelerometers of the same make and model.

5.2 Plotted Data

Plots were made to facilitate the comparison of the results of the signal

insertion method and the mechanical vibration testing. Each plot shows the

frequency response in percent referenced to the 10 Hz point. The first set

of curves present the results using a charge anq>lifier as a signal conditioner.

The second set presents the results using a voltage amplifier.

Tables 2 and 3 emphasize the more pertinent results obtained from the

data presented in the plots. The columns for both tables present the following

5
Column 2 gives the lowest frequency at which reliable voltage measurements

were made.

Column 3 gives the lowest frequency at which the insertion and mechanical

methods checked each other to within one percent.

Column 4 indicates which method tends to yield a higher accelerometer

sensitivity at lower frequencies. A "VI" indicates that the signal

insertion tends to be higher; a "MV" indicates that mechanical vibration

tends to be higher; and a "C" indicates that the two methods check each

other to within the accuracies of the tests. Only data above the low

frequency cut off of column 2 were considered in making these judgements.

Other data with unreasonable deviations from the trends of the majority

of data were not considered.

Table 2. Results of Tests with a Charge Amplifier as a Signal Conditioner

Accelerometer

Code

Lowest Reliable

Frequency, Hz

Lowest Frequency

Check, Hz Higher Sensitivity

ERKIA 5 5 C

ERK18 5 5 C

j ERK19 4 4 VI

} C05D7 1.5* 4 MV

j C05D1 1.5* 3 MV

COM2 8 1.5* 4 VI

C0151 4* 4 VI

C07C5 2* 2 C

C07E0 1.5 4 MV

C04C8

i

1.5 1.5 C

*Excessive dc voltage prohibited use of voltage-to-frequency converters;

measurements use rms differential voltmeters only.

6
Table 3. Results of Tests with Voltage Amplifier as a Signal Conditioner.

Accelerometer

Code

Lowest Reliable Frequency

Hz

Insert Frequency Check

Hz

Higher

Sensitivity

C05D7 2* 2 C

C05D1 3* 3 C

COM2 8 3 4 VI

C0151 2 5 VI

C07C5 2** 8** VI

C07E0 2** 8** VI

C04C8 2** 5** VI

002 5 3 4 4 c

*Excessive dc voltage prohibited use of voltage-to-frequency converters;

measurements use rms differential voltmeters only.

**Large roll-off in sensitivity occurred.

6. CONCLUSIONS

Some of the accelerometers tested are not suitable for low-frequency

vibration measurements. Two requirements for low -frequency accelerometers

are low sensitivity to case strain and high sensitivity to acceleration.

Discontinuity or irratic points in the frequency response curves may be

due to case strain or low signal-to-noise ratios. These problems are especially prevalent below 4 or 5 Hz.

The low acceleration sensitivities (approximately 10 mV/g) of accelerometers

ERK18 , ERK19 and C0151 limited their lowest calibration frequency to

approximately 5 Hz. It should be noted that signal insertion methods could

be employed when the voltage or charge insertion amplitudes would be maintained

at some relatively high level, say 100 mV (or equivalent charge) or so. In

this instance, good signaljto-noise ratios could be maintained and the noise

would be no major problem. However, direct cooqiarison with mechanically

excited results could not be realistically made, as discussed in section 4 above.

Difficulties were encountered in attempting to calibrate acceleroneter C0253

with a charge amplifier. This system has low acceleration sensitivity (approximately

8.5 mV/g) and is very temperature sensitive.

It should be emphasized that the primary objective of this project was

to test the feasibility of the signal insertion method to calibrate various

models of accelerometers.

In separate tests where signad levels were maintained well above the noise

levels using signal insertion methods, the frequency response appears flat to

within the ability to measure the low frequency voltages. This is in general

agreement with results reported by other experimenters.

7
In general, there is reasonable agreement between the two methods.

At lower frequencies, in the instances where the two methods did not check

each other, the voltage insertion method tends to be higher than the2

mechanical vibration method; especially for voltage sensing systems. The

only instances where the mechanical vibration method tended to be higher

were when accelerometers C05D7 , C05D1 and C07EO were tested with a charge

amplifier. It was noticed that these accelerometers are all of similar

construction and have the same type of piezoelectric material.

If it is assumed that the results of the mechanical method represent

the "right" response, correction factors might be useful for some of the

models of accelerometers tested. However, caution must be exercised by

placing a large enough uncertainty on signal insertion measurements. A

three percent estimated uncertainty down to the frequencies where a one percent check was made (column 3 in tables 2 and 3) seems reasonable.

Additional conservatism is aecessary due to the small number of accelerometers

tested.

7. ACKNOWLEDGEMENTS

John D. Ramboz of NBS, and Michael J. Luwe and Dale W. Rockwell of

MEC Pomona contributed guidance and shared their expertise on these

measurement methods.

This effect was also noted by test results reported by private correspondence

with D. E. Upton, and later by M. J. Luwe of the Metrology Engineering Center

in Pomona Calif. See references 7 and 8.

8
8. REFERENCES

1. Koyanagi, R. S., "Development of a Low Frequency Vibration Calibration

System," Nat. Bur. Stds Report 10 529, Jan. 1971.

2. Rhodes, J. E. "Piezoelectric Transducer Calibration Simulation Method

Using Series Voltage Insertion," Environmental Quarterly, April, 1962.

(Reprinted by the Endevco Corporation as publication TP 216).

3. Pennington, D. , Piezoeleatria Acoelerometer* Manual ^ Endevco Corporation,

San Juan Capistrono, Calif., 1965; pp. 71-72.

4. Keast , D. N. , Measurements in Meahaniaal DynamioBy McGraw-Hill, New

York, 1967; pp. 133-134.

5. "Endevco Specification Sheet for Model 2944.1 T-Junction Calibrator,"

Endevco Corporation, May, 1968.

6. Insert Voltage Adapter, UA 0322, Bruel and Kjaer specification sheet

for Type 8305 Accelerometer Reference, p. 165, Master Catalog, 1971.

7. Private coiranunication , D. E. Upton of the Metrology Engineering Center,

Pomona, California; Jan. 20, 1972. Vibration tests from 1.5 to 30 Hz,

insertion tests from 0.5 to 15 Hz. High level voltage measurements at

approximately 1000 mV for insertion tests. Maximum displacement for

vibration tests was about 1 cm. Voltage insertion results were higher

than mechanical excited results.

8. Private communication, M. J. Luwe of the Metrology Engineering Center,

Pomona, California; May 6, 1974. Vibration tests from 5 to 200 Hz,

insertion tests from 1 Hz to 200 Hz. Insertion results were consistently

higher than those obtained by mechanical excited tests.

USCOMM-NBS-DC 9
Voltage

®

Charge

Accelerometer Accelerometer

Cable

cal

Voltage

Insertion

Device

Signal

Conditioning

An^llfier

Legend: e^^ = Accelerometer generated open circuit voltage

= Accelerometer generated charge

c = Accelerometer capacitance

. PC^ = Cable capacitance

C = Calibration voltage source capacitance

^ CSiX. R^^j^ = Voltage insertion resistor

V = Calibration voltage

V > cal

V^ = Amplifier output voltage

Fig. 1. Signal Insertion Schematic

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HBS-114A (REV. 7-73)

U.S. DEPT. OF COMM. 1 OlDLIUuKArnlv, UA 1 A SHEET 1

1. PL'fU.li ATION OR REPORT NO.

NBSIR 74-597

2. Gov't Accession

l\ o.

3. Recipient's Accession No.

4, TITLK AND SUBTITLE

Piezoelectric Acceleroneter Low-Frequency Respoase

by Signal Insertion Methods

S. Publication Date July 1975

6. Performing Organization Code

7. AUTHOR(S)

Robert S. Koyanagi. James D. Pollard

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAMF AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Pro)ect/Task/Work Unit No.

11. Contract /Grant No.

12. Sponsoring Organization Name and (-omple-tc Addres.s (Street, City, Stale, ZIP)

Metrology Engineering Center, Bureau of Naval Weapons

Representative, Pomona, California 91766; the Aerospace Guidan(

and Metrology Center, Newark Air Force Station, Newark, Ohio

43055; and the Metrology and Calibration Center, Redstone Arsei

Alabama, .35 809

13. Type of Report & Period Covered

Final Report

'A

14. Sponsoring Agency Code

lal.

15. SUPPLEMENTARY NOTE.S

CCG Project CCG 74-93

16. ABSTRACT (A 200-word or less (actual summary ot moat sifjpilicant mtomation. II document includes a signiUcant

bibliography or literature survey, mention it here.)

The purpose of this study was to compare the frequency response of selected

piezoelectric accelerometers using a signal insertion method to the response using

traditional mechanical vibration tests. Signal insertion methods included *'voltage

Insertion" and "charge insertion" techniques. The signal is inserted in series

with the electrical low-side of the accelerometer by means of a suitable series

resistance. Commercially available insertion devices were used. Confidence in

the use of inserticm methods is increased where there is agreement between the

results from insertion tests and mechanically excited tests.

17. KEY WORDS (six to twelve entries; alphabetical order, capitalize only the lirst letter ol Hie first key word unless a proper name; separated by semicolona)

Accelerometer; calibration; vibration', low frequency,' signal insertion.

18. AVAILABILITY [Xl] Unlimited 19. SECURITY CLASS

(THIS REPORT)

21. NO. OF PAGES

[^n For Official Distribution. Do Not Relea.se to NTIS

UNCL ASSIFIED 33

Order From Sup. of Doc, U.S. Government Printing Office Washington. D.C. 20402. SD Cat. No. CM20. SECURITY CLASS

(THIS PAGE)

22. Price

1

1 Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151 UNCLASSIFIED $3.75

USCOMM.DC 29042-P74