Awide variety of accelerometers has been developed for measuring vibration, shock, and inertial motion. To reduce risk and ensure that meaningful and accurate data is collected, you must understand the environments in which the accelerometer will operate as well as the performance attributes and limitations. In addition, recent technological advances now make it possible for accelerometers to have integrated memory and digital outputs that significantly decrease installation time while improving measurement accuracy.
An accelerometer senses motion and produces an electrical output proportional to the magnitude and frequency of the input. There are several types of accelerometers—piezoelectric, piezoresistive, variable-capacitance, and servo devices—each differing in performance, power requirements, and signal-conditioning characteristics.
The piezoelectric (PE) accelerometer uses a simple spring-mass principle in which a force is generated that relates to amplitude and frequency. This force is applied to the PE element, which develops an electrical charge proportional to the mechanical motion.
Because a PE device generates a charge output when actively stressed, it does not require an external power supply for operation. The PE accelerometer is treated as a charge generator, and a charge amplifier is used for signal conditioning.
Quartz is the best-known type of PE element but has a low charge sensitivity coefficient. More often, ferroelectric materials such as lead-zirconate-titanate (PZT) are used. Through artificial polarization, this material exhibits a charge sensitivity coefficient approximately 50 times higher than that of quartz and is useful for measurements to 260°C.
Ferroelectric materials such as bismuth titanate and lithium niobate handle temperatures to 530°C and 650°C, respectively. Tourmaline, a natural crystal material, accommodates operation to 700°C.
Different configurations of PE elements are used in accelerometers for specific applications. The single-ended compression type is optimum for low-level measurements because of the high sensitivity that can be achieved by stacking multiple crystals and connecting them in series (See Figure 1 in the May 2001 issue of Evaluation Engineering).
The shear design allows construction of small, lightweight sensors suitable for monitoring acceleration of small components (See Figure 2 in the May 2001 issue of Evaluation Engineering). A key advantage of the shear design is the isolation of the sensing element from the base, which provides excellent protection from base strain and temperature transients.
PE accelerometers frequently are used where an extremely rugged device is required. Piezoresistive (PR) devices can measure a wide range of temperatures, from cryogenics to the extreme heat environments of gas-turbine engines or nuclear reactors. When the operating temperature (typically <125°C) permits, signal-conditioning circuitry can be integrated into the accelerometer. This eliminates the need for low noise cabling, allowing longer cable lengths to be used without affecting the output of the accelerometer signal.
Gain also can be optimized so you can substitute a smaller accelerometer for a given application. The upper end of the frequency response can be tailored with electronic filtering to match the expected measurement range and suppress natural mechanical resonances. The low-frequency response typically is set at 1 Hz for PE accelerometers and can be pushed close to DC for some designs.
Traditionally, a discrete strain gauge accelerometer is mechanically attached to a cantilever beam, then electrically connected in a Wheatstone-bridge configuration. This produces an electrical signal proportional to motion.
More recent designs of the PR strain gauge accelerometer consist of a rugged monolithic assembly with solid-state silicon resistors that change resistance in proportion to the applied mechanical stress. The PR unit is several orders of magnitude more sensitive than the conventional strain gauge sensor.
The microfabricated monolithic sensor has a diffused silicon strain gauge and the mechanical components of a strain gauge in a common silicon chip. This device exhibits a high sensitivity with an excellent signal-to-noise ratio and a typical temperature range of -20°C to 120°C.
The PR accelerometer features a DC response that makes it useful for measuring long-duration pulses such as those experienced in automotive crash test studies and munitions blast testing. Since this accelerometer uses an external source of excitation, output impedance is low. In many applications, preamplification of the output is not necessary.
The variable-capacitance (VC) accelerometer has a sensor element typically manufactured using silicon bulk micromachining techniques (See Figure 3 in the May 2001 issue of Evaluation Engineering). The sensor element is sandwiched between a lid and a base and electrostatically bonded to form a parallel-plate capacitor.
This accelerometer features DC response, stable damping to give good frequency coverage, and rugged construction. Integral electronics with DC excitation provide a high-level, low-impedance output signal stable from -20°C to 120°C. This sensor is designed for low-g measurement and suitable for trajectory monitoring, structural evaluation, flutter testing, automotive suspension, and brake testing.
A servo accelerometer (SA) is an extremely high-accuracy device that offers DC response and is used for measurements of a few milli-gs. It measures the electrical energy required to balance inertial forces caused by acceleration of a mass.
The traditional servo accelerometer uses electromagnetic effects to provide the balancing force. The electrical current required to reach a stable condition is the measurement result. However, such an arrangement has a soft, pendulous suspension for the mass, and the assembly is relatively fragile.
A small micromachined servo accelerometer (MSA) developed by Endevco retains the inherent measurement sensitivity of the traditional technology, yet has shock ratings of several thousand gs. In this device, the relative motion of a microscopic pendulum mass is detected by the variation of its capacitive coupling to the housing. The plates are energized to restore the pendulum structure to its original position, and the magnitude of this restoring voltage is amplified and output as data. A typical temperature range is -55°C to 105°C.
Some applications of the MSA include dynamic balancing of equipment, gear-vibration monitoring, automotive dynamics, automotive-suspension sensing, seismic measurement, and navigation-system development.
Accelerometer Performance Characteristics
To obtain acceleration data that is meaningful for your application, you need to understand the performance characteristics of the accelerometers under consideration. There are several types of accelerometers and many designs within each category. The most critical trade-offs relate to sensitivity, weight, and frequency response.
- Sensitivity—High sensitivity results in a high signal-to-noise ratio. Interfering electrostatic and electromagnetic noise will be less bothersome than with a low-sensitivity device. This may bring two disadvantages: greater weight and a lower resonant frequency.
- Mass Loading—Motion of the equipment being tested will be attenuated if the accelerometer’s dynamic mass approaches the dynamic mass of the structure on which it is mounted. Consequently, a lightweight sensor must be used for accurate evaluation of low-mass elements.
- Low-Frequency Response—With a PE accelerometer, the low-frequency cutoff often is set at 1 Hz to 5 Hz to reject any pyroelectric output. Some models, however, extend the cutoff to near DC. The PR, VC, and MSA accelerometers have DC response.
- High-Frequency Response—This is a function of the mechanical characteristics and the method used to attach the device. Most accelerometers exhibit an undamped single-degree-of-freedom response when securely mounted. Response is relatively flat to about 20% of the mounted resonant frequency. Correction factors can be derived for data obtained at higher frequencies. Electronic filtering can increase the flat response to 50% of the mounted resonant frequency.
- Transverse Sensitivity—The sensor must not produce any significant response when motion is applied in the lateral axes. Sensitivity to lateral motion can be held to <5% of the transverse sensitivity on a good device.
- Amplitude Linearity—PE accelerometers have a predictable nonlinearity that can be expressed as a percentage increase in sensitivity as the acceleration increases, such as 1%/500 g. The upper limit can be determined and expressed for each model. PR, VC, and MSA sensors are extremely linear and specified for the combination of nonlinearity, hysteresis, and nonrepeatability.
- Temperature Sensitivity—Accelerometer sensitivity varies with temperature. Many accelerometers are optimized for stable sensitivity over a wide temperature range.
- Transient Temperature Effects—PE accelerometers produce an output when temperature changes. This error is at a very low frequency and may not be detected. PR, VC, and MSA devices have no significant response to temperature changes.
- Strain Effects—The test specimen may flex, stretch, or bend at the point where the accelerometer is mounted, causing it to produce an erroneous output. Isolation can be improved by use of insulative mounting studs or adhesive mounting adapters. Shear accelerometers are much less sensitive to such errors than conventional compression types.
- Dirty Environments—Every component in the measurement chain must be kept clean and dry to achieve optimum performance. PE accelerometers require more care because they are very sensitive to external contamination due to their high output impedance.
The Next Step: Sensors With On-Board Memory
Commonly referred to as smart sensors, accelerometers with on-board memory provide an inherently improved signal-to-noise ratio. The key feature of these new sensors is conformance to the IEEE P1451.4 Transducer Electronic Data Sheet (TEDS) specification, which includes sensor-specific data.
The sensor with a permanent memory (ROM) stores the manufacturer’s identity, model, serial number, manufacturing date, and type. With a programmable memory (EEPROM), calibration data and date, sensitivity, unit reference frequency, sensor location, high-pass and low-pass cutoff frequencies, transfer function coefficients, and service history can be stored in the sensor.
The capability to access TEDS sensor information on demand has many merits. It provides sensor plug-and-play functionality, enables a seamless sensor/electronics interface, and simplifies the design of test systems for acquisition of accurate, reliable, and repeatable data.
The smart-system approach eliminates sensor lookup tables since all up-to-date information about the sensor is stored in the TEDS memory chip. There is no need to generate a separate database for sensor sensitivity obtained from the manufacturer’s calibration certificates. There are no cable-connection errors since TEDS does away with manual connection tracking.
Location identification is stored in TEDS, then exported as required. All information about the sensor is known to the system once the location has been entered into the TEDS chip. Also, transducer substitution is quick and easy since TEDS data contains all the required device parameters.
About the Author
Dave Olney is the marketing manager for industrial and commercial high-volume accelerometers at Endevco. He has a B.S.M.E. from Cal Poly and an M.B.A. from the University of California.
Bruce Swanson, the marketing manager for test instrumentation, is responsible for smart sensor and wireless products as well as new business development at Endevco. He has a B.S.M.E. from Texas Tech University and an M.B.A. from the University of California.
Bob Arkell is the marketing communications manager at Endevco. He earned a master’s degree in test and measurement from the University of Saskatchewan.
Endevco, 30700 Rancho Viejo Rd., San Juan Capistrano, CA 92675, 949-493-8181.
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