EVOLUTION AND COMPARISON

OF ACCELEROMETER

TECHNOLOGIES

Patrick L. Walter

TCU Engineering and PCB Piezotronics

History of the Accelerometer • Reference: Proceedings of the 67th Shock and Vibration

Symposium, Monterey, CA (Pioneers and Historians), November 1996

Pioneers 1st Hand Stories (Circa 1996)

Walter Kistler

Kistler Instruments (by Walt Kistler) • Mr. Kistler worked for the Swiss Locomotive and Machine

Works (SLM), Winterthur Switzerland • Tasked to measure pressure inside the cylinder of diesel engines • Developed quartz piezoelectric pressure transducers during WWII • Awarded Swiss patent for 1st charge amplifier development (1950) • Moved to United States (1951) and was employed by Bell

Aerosystems, Buffalo NY • Concurrently he imported sensors from SLM for sale in the U.S.

• Founded Kistler Instrument Company with Walter Tanner (1954) • Bill Waytena and Bob Lally soon joined • Mr. Tanner left and Jim Lally joined

• Kistler Instrument Company incorporated (1957) • Accelerometer development followed (initially quartz based)

• Sold to Sunstrand and moved to Redmond, WA (1970) • Kistler Group in Winterthur (former SLM) bought Kistler U.S. and moved it

back to N.Y. (1980)

Early Kistler Piezoelectric Circuit Calibrator

Pioneers 1st Hand Stories (Circa 1996)

Abraham Dranetz

Gulton Corporation (1st U.S. Piezoelectric Accelerometer [Abe Dranetz])

• Inventor: “Abe Dranetz” (1950) • In the early 1940s Barium Titanate was being considered

as a replacement for mica capacitors due to its very high dielectric constant • Investigators soon noted this material had piezoelectric properties. • In 1948 Gulton was awarded a Navy contract to look at feasibility of

BaTiO3 for sonar transducers. • In 1949 a former German scientist working at Gulton suggested the

need for a lightweight accelerometer. • Model A403 BaTiO3 compression type accelerometer was

developed (1950). • Gulton went on to develop a broad line of accelerometers. • Became Gulton-Statham (post 1985). • Now part of AMETEK (no longer manufacture accelerometers).

Gulton Model A403 • 15 mV/G, 10 KHz resonant frequency, 600 G, 1500 pF

Pioneers 1st Hand Stories (Circa 1996)

Peter Stein

1st Commercial Strain Gage Accelerometer (by Prof. Peter Stein)

• 1st resistance based accelerometer designed by Burton McCollum and Orville Peters while working at U.S. National Bureau of Standards (published this work in 1923 and 1924) • Commercialized in the U.S, in 1927 through Southwark, later Baldwin-

Southwark, then BLH Electronics. • July 1936 Southwark Bulletin 132 listed 110 U. S. users and 12 others in 8

countries. • Price for 2-axis unit was 420$ (1932!)

• Louis Statham developed an unbonded strain gage accelerometer at Curtis Wright (1942) to monitor G-loads on pilots. • Founded Statham Instruments (1943).

• Purchased by Gould, Inc. (1983), then Gulton-Statham (see Gulton previous).

McCollum-Peters Accelerometer • 500 Hz mounted and 250 Hz unmounted resonance;

versions 0.1 to 100 G, carbon discs in compression in E frame, integrated in Wheatstone bridge

Statham Unbonded Strain Gage • Typical unbonded strain gage accelerometer

By those who followed (Circa 1996)

John Kubler

Culmination of Early Kistler History (John Kubler) • Kistler originated the concept of incorporating a two wire

integrated circuit (a FET) within a piezoelectric accelerometer. This was in the 1964 -1965 time frame and was patented (PIEZOTRON) in 1968. • Early developers were Mr. Verne Siegel and Mr. Bob Lally.

By those who followed (Circa 1996)

Torben Licht

Bruel & Kjaer History (Torben Licht) • Founders Per Bruel and Viggo Kjaer (graduated U. Of

Denmark 1939) (Pioneers)

• B& K formed (1943) Naerum, Denmark • Developed and sold 1st commercial piezoelectric accelerometers

(1st used Rochelle salt: Potassium sodium tartrate, (NaKC4H4O6)) – replaced with ceramic elements in the early 1950s

• Focus has remained on sound and vibration measurements, calibration, and system solutions

B&K 4303 Assembly dated December 26, 1943

Early B&K Hand Held Accelerometer

By those who followed (Circa 1996)

Len Maier and Bob Whittier

Endevco History (by Len Maier, Bob Whittier, and Wm. F. Cox (early sales)) • Founded H. Dudley Wright (1947) in Pasadena, CA (moved to

San Juan, CA 1974) • Manufactured their 1st piezoelectric accelerometer 1951 • Early entrant into silicon sensor technology

• Solid state laboratory set up in Sunnyvale, CA (1961) • First silicon (PR) accelerometer manufactured (1962) • 1st U. S. Patent charge amplifier (1959) • 1200 degree F accelerometer (1981)

• Cox (re. Dudley Wright): “A portion of his energy and magic distill into a typical stint of his: Write a spec for something the market needs, peddle same, get orders, THEN turn on the troops to MFG same.” Mr. Wright retired 1964. • Early troops: “Ms, Dennis Winton, Wilson Bradley Jr., Wm P Simpson,

Wm F Cox, Eric G Laue, Robt Cother” • Multiple owners (Meggitt since 1992; included with them are

Wilcoxon and Vibrometer)

Endevco Founder: Harry Dudley Wright title = "Accelerometer", number = "2714672", author = "Dudley, Wright Harry, Laue, Eric G.", year = "1955", month = "August", url = "http://www.freepatentsonline.com/2714672.html"

By those who (closely) followed (Circa 1996)

Jim Lally

PCB History (by Jim Lally [Pioneer])

• Founders: Bob and Jim Lally (+3 others) (1967) • Kistler was bought by Sunstrand in 1968 and moved in 1970. PCB

split off of Kistler and stayed in NY state.

• PCB was initiated based on a technical focus on consumer acceptance of integrated circuit technology (ICP®). • Modal testing accelerated that focus.

• Worked early with U. of Cincinnati’s Structural Dynamic's Lab (early 1970s) to develop Modally Tuned hammers (IR 100 Award) and large channel count accelerometer arrays.

• Analyzer manufacturers designed (ICP®) front ends.

PCB Modal Hammer

Jim Lally, PCB Emeritus

February 8, 2005: Dick DeMichele Award at IMAC by Mrs. DeMichele

By those who followed (Circa 1996)

Ray Bouche and Bev Payne

Accelerometers had to be calibrated

• Dr. Ray Bouche, U. of Maryland graduate student, worked with S. Levy, NBS, to develop vibration reciprocity calibration (Journal of Research NBS 57, 227-243, (1956) RP2714. Moved to Endevco ~ 1960.

• Bev Payne: National Bureau of Standards (now NIST) since 1960. His career has encompassed work in accelerometer calibration methods and shaker design and calibration.

Other Early Companies • Columbia Research Labs: Founded 1955 by Victor Alibert

and his sister Olive. Dr. Victor Alibert joined in 1959. • Formidable early competitor (with Gulton and Endevco) in

piezoelectric accelerometers.

• Wilcoxon Research, Inc. Founded 1960 by Ken Wilcoxon, Al Sykes, and Fred Schloss. • Spun off from David Taylor Model Basin to build impedance heads

(Mr. Schloss received Patent 3,070,996 for self-driven Mechanical Impedance Head (piezoelectric force transducer, accelerometer, and exciter)). Early focus was submarine quieting.

• Moved into industrial market place and now part of Meggitt since 2004.

Newer Company: Dytran • Mr, Nick Change, Founder and also early entrant into

piezoelectric transducer business (1959 employed by Kistler, 1971 to PCB, 1978 back to Kistler (2nd time)); officer at both Kistler and PCB • 1980 Dytran founded in NY • 1982 Dytran moved to Chatsworth, CA.

Mr. Nick Change

Other Companies to Mention

Vibrometer (existing) Clevite (extinct) Massas (extinct, mentioned as early Endevco contributor)

Piezoelectric Transducer Manufacturers – Their Origins

1. Bruel and Kjaer 1942 2. Kistler Instruments (SLW) 1943 (Buffalo, NY) 1954 3. Gulton Manufacturing Company (approx) 1944 4. Endevco 1947 5. Columbia Research Laboratories 1955 6. Wilcoxon Research 1960 7. PCB Piezotronics 1967 8. Dytran 1980

Where have we evolved to thanks to these folks and others????

What are the principal accelerometer technologies today? • Piezoelectric (mature technology)

• Piezoresistive (PR), synonym (MEMS)

• Variable Capacitance (primarily also silicon based)

• Servo (also increasingly becoming silicon based)

Below left is a MEMS type accelerometer (PR) and below right is a piezoelectric

(PE) accelerometer. IEST-RP-DTE011.1: 5.1 (PE), 5.2 (PR)

Mechanical Flexure for a MEMS Accelerometer

Cut Away of a Shear-Mode Piezoelectric Accelerometer

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Piezoelectric Technology

Piezoelectricity: results from no center of charge symmetry

piezoelectric constant dij = q/F d33 = width expansion

i = electrical direction d31 = length expansion

j = direction applied force

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Piezoelectric Technology

Of the thirty-two crystal classes, twenty-one are non-centrosymmetric (not having a centre of symmetry), and of these, twenty exhibit direct piezoelectricity (the 21st is the cubic class 432). Ten of these are polar (i.e. spontaneously polarize), having a dipole in their unit cell, and exhibit pyroelectricity. If this dipole can be reversed by the application of an electric field, the material is said to be ferroelectric.

Crystal Classes:

Mechanical

Stress

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The Process (quartz) [1/2] axes identification

machining

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The Process (quartz) [2/2]

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Direct Piezoelectricity (quartz)

• Advantages

• Naturally piezoelectric

• Excellent long term stability/no aging

• Non-pyroelectric

• Works well in thermally active environments

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Direct Piezoelectricity (quartz)

• Low capacitance • High voltage sensitivity (V=Q/C)

• Low temperature coefficient • Little deviation over wide temperature range

• High stiffness • Extremely linear

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Direct Piezoelectricity (quartz)

• Limitations

• Non-piezoelectric above 1060 F (573 C)

• Low charge output (pC)

• Not as effective in low-noise, charge-amplified systems

as typical ceramic

• Pressure/force transducers use multiple plates for this

reason

• Limited Geometry

• Can only cut crystal in certain number of

ways and have it still remain piezoelectric

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Ferroelectric Ceramics • Man-made material • Lead-zirconate titanate, barium titanate, lead metaniobate,

bizmuth titanate, … • Polycrystalline structure

• Naturally isotropic (physical and electrical properties the same in all directions)

• Naturally non-piezoelectric • Becomes piezoelectric through high voltage

“poling”

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Ferroelectric Ceramics [1/4]

Powders mixed and screened

Compacted into pellet form

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Ferroelectric Ceramics [2/4]

Placed in high temperature oven, >

1200o C typical

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Ferroelectric Ceramics [3/4]

Precision grinding

Electroding (gold or nickle-silver)

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Ferroelectric Ceramics [4/4]

Poling at .2-10KV 200-400o F

+ -

After poling

Before poling

Impedance testing

+ -

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Ferroelectric Ceramics

• Advantages • High charge output (pC)

• Excellent for use with tests requiring high sensitivity and/or low-noise, charge-amplified systems

• Unlimited geometry • “Poling” occurs after shaping

• Better resolution and frequency response than comparable size quartz accelerometers – lower noise

• Miniaturization

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Ferroelectric Ceramics

• Limitations • Pyroelectric

• Unwanted output generated by a thermally active environment

• Larger temperature coefficient than quartz • Electrical properties are dependent on temperature

• Artificially Polarized • Exhibit temporary/permanent changes under large mechanical, thermal or electrical shock

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Upper Useful Operating Temperature for Piezoelectric and Ceramic Materials

• PZT (ferroelectric ceramic): 500 deg. F.

• Quartz (direct piezoelectric) (SiO2) 760 deg. F.

• Bismuth Titanate (ferroelectric ceramic) (Bi4Ti3O12) 900 deg. F.

• Gallium Orthophosphate (direct pieoelectric) (GaPO4) >1250 deg. F.

• LGT Langatate (lanthanum gallium tantalate)

(La3Ga5.5Ta0.5O14 LGT) (direct piezoelectric) 1112 deg. F.

• Tourmaline (direct piezoelectric/pyroelectric) 1112 deg. F.

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Comparative Sensitivities of Piezoelectric and Ceramic Materials

• PZT (ferroelectric ceramic): 250 pC/N

• Quartz (direct piezoelectric) (SiO2) 2.3 pC/N

• Bismuth Titanate (ferroelectric ceramic) (Bi4Ti3O12) 20 pC/N

• Gallium Orthophosphate (direct piezoelectric) (GaPO4) 4.5 pC/N

• LGT Langatate (lanthanum gallium tantalate)

(La3Ga5.5Ta0.5O14 LGT) (direct piezoelectric) 6.5 pC/N

• Tourmaline (direct piezoelectric/pyroelectric) 1.8 pC/N

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Some considerations in selecting piezoelectric technology • Capable of operating cryogenic to 1200 degrees F

• Frequency capabilities from ~ 0.25 Hz to 20 KHz

• WIDE dynamic range (milli-Gs to 10s of thousands of Gs)

• Available under 1 gram in size

• Integral electronics yield 0-5 volts signal

• Typical applications are vibration and short duration shock

monitoring

• Rugged!

Silicon Technology: (bulk piezoresistive (PR))

Silicon Technology (synonym: PR)

Es

gage factor = (dR/R)/( dL/L) = 1 + 2 + d

*d is large for doped silicon gages

PCB crash accelerometer

Accelerometer Flexure

PCB fluid damped accelerometer

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Silicon Technology (MEMS)

Chemically machined silicon with atomically diffused gage Completed MEMS flexures –

diffused and metallized

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• Resistive / Capacitive MEMS Manufacturing Process

Apply “photoresist” material to the silicon wafer in order to create desired (and often very complex) pattern for each sensor

Any area on the wafer which is uncovered is then processed. (ie: depositing conductive traces / bond pads, doping with boron to

create piezoresistive strain gauges, or etching away silicon material creating structures, etc…)

Rep

eat t

o cr

eate

ano

ther

“la

yer”

Remove “photoresist” material and inspect to insure process was completed properly

Bond multiple wafers together to create sensor

Mount MEMS element in suitable packaging

Silicon Technology (MEMS)

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• MicroElectroMechanical Systems created from silicon wafer

Sensors prior to dicing Individual sensor core Magnified structure Raw silicon wafer

How small are MEMS structures? Smaller than the eye of a fly!

Silicon Technology (MEMS) - size contrast

10μm

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Some considerations in selecting piezoresistive (MEMS) accelerometer technology • DC response (thermal zero stability must be considered)

• Typically not used outside -65 to + 250 deg F and require

compensation

• Typical signal levels ~200 mV FS

• Often recommended for severe mechanical shock

• Typical applications auto and defense industry

• Available in wide G ranges

• Favored in VERY large quantities (e.g. airbag deployment)

Variable Capacitance MEMS (IEST-RP-DTE011.1: 5.3 (VC)

Older Technology

MEMS VC and PR: Model 3741 2 – 200 G

Model 3710 tri-ax

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Some considerations in selecting variable capacitance technology • Typically provide good thermal specifications -65 to + 250

deg F.

• DC response capability

• Ranges +/- 2 to 200 Gs FS

• Large over range capability (e.g., 5,000Gs)

• Typically provide +/- 2 volts from unregulated DC power

• Typical applications: transportation, aircraft flutter

Servo Type IEST-RP-DTE011.1: 5.4 (Servo)

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New Pendulous Servo

• MEMS technology • Electrostatic servo mechanism • Electrostatic rebalancing • Higher shock survivability, smaller size • Very low zero drift • Bandwidth, dc to <1 kHz • Fragile with power supply off

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Pendulous Servo

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Some considerations in selecting servo technology • Most accurate and most expensive type of any accelerometer

• DC response to a few hundred Hz typical

• Generally FS range is less that 50 Gs

• Fragile: limited over range (MEMS is changing! Being used in

drilling and munitions.)

• Typically provide several volts output

• Typical applications: guidance (IMUs with gyros)

Final Statements and Conclusions: • New technologies are rapidly enabling accelerometer

applications. (e.g., wireless, small and ruggedized electronics)

• Piezoelectric accelerometers will remain a mature technology.

• MEMS will continue to integrate into the accelerometer landscape and consumer electronics will/are encouraging.

• Remember, we all walk on the back of the Pioneers!