A Guide to Accelerometer Specifications
A spec-by-spec guide helps explain accelerometer characteristics.
By Bradley J. Thompson, Contributing Technical Editor
Long ago, Shake, Rattle and Roll found a home in rock n roll music. But shakes, rattles, and rolls are always out of place in safe and stable motor vehicles. If youve added all three to your vehicles hit parade, youll need accelerometers to measure them.
To help you choose the best accelerometers for your applications, weve prepared sheet musicspecifications for a representative accelerometershown at right. Lets go through the spec sheet item by item.
Accelerometer type. Four accelerometer technologies compete to cover most applications. Piezoelectric accelerometers produce an electrical charge in response to mechanical stress, while piezoresistives alter resistance in response to stress. These devices dont offer static-load (or DC) response but are suitable for most general-purpose and high-shock measurements.
Capacitive accelerometers offer true static-load response. They work well for vehicular-suspension testing, which requires mathematical integration to obtain component velocity and position data.
Microelectromechanical system (MEMS) sensors feature micromachined silicon mechanical components and integrated support electronics. Early versions used resistive elements. Todays technology favors capacitive sensors to create truly integrated and very small accelerometers, typically packaged in surface-mount ICs. Besides instrumentation applications, tens of millions of MEMS accelerometers see service in automotive air-bag actuators.
Amplitude range. Select an accelerometer by matching its output for expected acceleration levels. Dont crowd the full-scale specifications. Allow a margin for unexpectedly large accelerations. Using only the lower 20% of an accelerometers response range will ensure ample margins for unpredicted overloads.
Shock limits. The shock limit generally explains how many gs of shock an accelerometer can withstand. Shock duration and impulse shape play a role, with narrow impulses causing more damage than broad-duration, relatively slowly changing shock pulses.
Suspension tests impose unexpectedly high shock loads. If you choose an accelerometer for high-resolution, low-g measurements, then an unplanned encounter with a pothole could produce a 2000-g shock. That shock can damage an accelerometer, so make sure your sensor is resistant to shocks of 2000 g and higher.
Protect your accelerometer investment. Never drop an accelerometera 2-in. fall onto a benchtop can easily generate a 2000-g spike, while a longer plunge onto concrete can produce more than 20,000 g. Store an unused accelerator in its padded shipping carton, and only remove it for immediate, carton-to-UUT travel and installation.
Commodity-grade accelerometers for air-bag actuators that simply make a go/no-go decision sell for approximately $5 each in large quantities. In contrast, instrumentation-grade accelerometers cost from $100 to $1000 eacha strong incentive to protect these pricey components from damage or destruction.
Temperature range. The maximum ambient temperature the accelerometer will encounter must not exceed the sensors operating temperature range (212°F, 100°C). Inside a closed vehicle, solar heat-soak temperatures can exceed 1408F (60°C). Inside the engine compartment, a hot exhaust manifold or catalytic converter can drive temperatures over 100°C.
For hot spots, charge-mode accelerometers can operate at temperatures up to 254°C if you place your signal-processing electronics in a cooler area. Packaging methods and a piezoelectric elements Curie-point temperature define temperature limits.
Packaging. Depending on its location, an accelerometer may get splashed with (or immersed in) hot or cold hostile fluids ranging from fresh-water or saltwater, oil, and hydraulic fluids to engine coolants and battery acids or bases. Many accelerometers feature waterproof hermetic seals and titanium, stainless steel, or plastic packaging. So, choose appropriately for the tests chemical environment. Chemical exposure can also damage cables and connectors. Avoid using delicate laboratory-rated accelerometers in a field-measurement environment.
You also need to select an accelerometer based on its size relative to the component under test. For example, attaching a relatively massive accelerometer to a sheet-metal panel will alter the panels mass load and its vibration frequency. An accelerometer that approximates a weightless point with respect to the UUTs mass and dimensions will maximize accuracy. As a rule of thumb, select an accelerometer whose mass and mounting dimensions dont exceed 1% of those of the UUT.
Base strain sensitivity. If you use a stud-mounted accelerometer and tighten it against a concave surface, you strain the accelerometers base and alter its sensitivity. Avoid overtightening, and consider welding flat mounting pads onto a UUTs uneven surfaces. Ensure that an added mounting pad will not significantly alter mechanical properties (for example, stiffness) of the UUT.
Transverse sensitivity. To achieve specified accuracy, make sure the accelerometers preferred axis aligns precisely with the direction of the acceleration youre measuring. A small offset misalignment introduces a lateral-acceleration error vector into the straight-line measurement. Alignment methods vary with UUT configuration and accelerometer design and accessibilityconsult the accelerometers manufacturer for suggested methods.
Frequency range. Specification sheets often omit important data such as flat-frequency response range. For example, an accelerometer specification offering . . . flat response to 5 kHz (3 dB) describes a 30% high-end response lossthats hardly flat.
To clarify any confusion, use a calibrated shaker table to search for an accelerometers resonances and to explore its frequency and phase responses.
As with amplitude response, avoid crowding the frequency-response limits. An undamped accelerometers amplitude vs. frequency response may peak sharply near its upper end due to internal components mechanical-resonance frequency. For accurate measurements, use only the lower 20% of the accelerometers response curve.
For a damped accelerometer, ensure accurate response by plotting the devices phase shift vs. frequency. Consider the accelerometers response as flat below its 908 phase-shift frequency.
Sensitivity. After you select an accelerometer
that can survive predicted worst-case shock limits, compute the sensors output
voltage. At a sensitivity of 10 mV/g, for example, an accelerometer that encounters a
100-g shock will produce a 1-V peak signal. Make sure that your data-
Resolution. A minimum-resolution specification of 0.02 g, which corresponds to a voltage of 200 mV, establishes a lower sensitivity limit analogous to a noise floor.
Amplitude linearity. Next, account for any
nonlinearity error, which at 0.9% of full scale (500 g) corresponds to 4.5 g. If
youre using 20% of full scale range, youd expect a
Temperature coefficient of sensitivity. A comprehensive accelerometer spec sheet includes a value for temperature coefficient of sensitivity. Yet, other parameters that arent characterized on the spec sheet may also change as a function of temperature.
If you configure a test vehicle in a warm building and drive it outdoors into a Minnesota winter, or expose a crash-test vehicle to hundreds of kilowatts of illumination, consider thermal transients and allow time for stabilization. When in doubt, thoroughly characterize your accelerometers at multiple temperatures between high and low extremes.
Discharge time constant. For nonstatic (non-DC) accelerometers, discharge time constant defines the lower-frequency corner (or 3-dB departure from midband amplitude) of the accelerometers response. One time constant after application of a step-function change in acceleration, the sensors output will fall to 63.7% of its initial value.
Risetime. Typically specified as a 10% to 90%
value, risetime defines an accelerometers response to a step-function transient
(that is, when the sensors output will reach 90% of
Excitation voltage. This value specifies the
allowable voltage range you can use with an accelerometer. While manufacturers specify a
range of excitation voltage or current, use a well-regulated, well-filtered power supply
to nail down one more potentially variable factor affecting
Dont overlook installation-related error sources. Mechanical compliance (stiffness) of its interconnecting cables can alter an accelerometers response. Cable defects can render a sensor useless. For example, high-compliance silicone rubber-insulated cables mounted beneath a vehicles chassis may fail quickly due to chemicals and abrasions, while Teflon-insulated cables survive in style. If you use piezoelectric accelerometers, be sure to select cables built with special lubricants between insulation layers to reduce errors due to triboelectric effects.
Accelerometers equipped with connectors simplify installation and wire routing, but the junction between accelerometer and cable connectors typically requires sealing to prevent moisture contamination. Rigidly attaching an accelerometer to its UUT via a thin layer of adhesive minimizes attenuation of high frequencies of interest, while a thicker adhesive layer or double-sided adhesive tape dampens high-frequency response. A thin layer of grease between an accelerometers case and the UUT can improve high-frequency coupling.
If you use an accelerometer as a control element in a closed-loop test system, permanently mount the accelerometer on the UUT. If such an accelero meter becomes disconnected from the UUT, it will drive the system to its full uncontrolled output.
Before you perform actual tests, use dress rehearsals and make exploratory measurements at excitation levels below those that may damage the UUT. Thoroughly debug the procedure and data-acquisition processes before you commit the UUT to whats often an expensive one-shot measurement.
If you are an electronically oriented test engineer who is unfamiliar with mechanical terminology, this step-by-step tour of an accelerometers specification sheet should help to ease you through the selection process. You will also find it helpful to compare several vendors specification sheets for allegedly identical products. Note any outstanding or missing parameters, and dont be reluctant to question vendors.
Do you still feel challenged? Fortunately, most accelerometer
vendors publish extensive application notes that supplement spec sheets and are worth
reading for their technical content. And never hesitate to request advice from an
accelerometer manufacturers applications-
Brad Thompson has been writing for Test & Measurement World since 1986. Currently, he serves as a Contributing Technical Editor and works as an independent electronics consultant and writer. E-mail: email@example.com.
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