Posted On March 26, 2018 By In Ewave, Sensors and Wireless And 348 Views

Choosing The Proper Accelerometer For Measurement Success And Sanity

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Choosing The Proper Accelerometer For
Measurement Success And Sanity

from Sensors | Online

Selecting the correct accelerometer technology for a measurement application is vitally important for obtaining accurate results. Your measurement results are only as good as your accelerometer selection criteria. In this article, we look at the different types of accelerometers available on the market, and the considerations around each type.

 
BASIC ACCELEROMETER TYPES

In broad terms, accelerometers are classified into two categories: AC-RESPONSE and DC-RESPONSE.

  • An AC-response accelerometer, as the name implies, is a sensor which has an output that is ac coupled. An AC coupled sensor can thus not be used to measure static acceleration such as gravity and constant centrifugal acceleration. It is only suitable for measuring dynamic events.
  • A DC-response accelerometer, on the other hand, is DC coupled, and can respond down to zero Hertz. It therefore can be used to measure static, as well as dynamic acceleration. However, measuring static acceleration is not the only reason a DC-response accelerometer should be selected.
    Let’s break down the core capabilities of each type:

Let’s break down the core capabilities of each type:

AC ACCELEROMETERS

The most common AC-response accelerometers use piezoelectric elements for their sensing mechanism. Under acceleration, the seismic mass of the accelerometer causes the piezoelectric element to “displace” a charge, producing an electrical output proportional to acceleration.

Electrically, the piezoelectric elements look like a source capacitor with a finite internal resistance, typically in the order of 109 ohms. This forms the RC time constant which defines the high-pass characteristics of the device. For this reason, piezoelectric accelerometers cannot be used to measure static events. Piezoelectric elements can be natural or man-made. They come with varying degrees of transduction efficiency and linearity characteristics.

PE accelerometers incorporate piezoelectric crystals such as quartz or tourmaline or more often with ferroelectric ceramic materials. These accelerometers are self-charging devices, meaning that they create an electric signal when a force is applied. No power supply is required however an external charge amplifier is required to convert the high impedance output to a usable low impedance voltage signal. Due to the high impedance output of PE accelerometers noise treated cables are required.

Two types of piezoelectric accelerometer are available on the market: CHARGE OUTPUT TYPE and VOLTAGE OUTPUT TYPE.

 
CHARGE MODE PIEZOELECTRIC

Most piezoelectric sensors are based on Lead Zirconate Titanate ceramics (PZT) which offer wide temperature range, broad dynamic range, and wide bandwidth (usable to >20 kHz). When housed in a hermetic, welded metal case, a charge mode accelerometer can be considered one of the most durable sensors because of its ability to tolerate hostile environmental conditions. Due to its high impedance characteristics, a charge mode sensor must be used with a low-noise shielded cable (typically in a coaxial configuration) to minimize triboelectric noise. A charge amplifier is generally used to interface with charge mode accelerometers and convert the output to a usable low impedance voltage signal. With a modern charge amplifier, the broad dynamic range (>120 dB) of the charge mode sensors can be easily realized.

For extreme temperature applications, charge mode accelerometers have also been made using Quartz or Tourmaline to extend the operating range from -200°C to +640°C and beyond. They are especially suitable for use in vibration measurements at temperature extremes, such as in turbine engine monitoring.

VOLTAGE MODE PIEZOELECTRIC

The other type of piezoelectric accelerometer provides voltage output instead of charge. This is also the most common type of accelerometer used today in the test & measurement market.

Voltage mode piezoelectric accelerometers are essentially a charge mode accelerometer with on-board integral electronics that will convert the charge output into a low impedance voltage output. Voltage mode sensors typically come in either 3-wire configuration (Signal, Ground, Power) or two-wire configuration (Power/Signal, Ground). The two-wire mode is also known as Integral Electronics Piezo-Electric (IEPE) accelerometers. IEPE (also known as ICP1) is the most popular choice due to its convenient coaxial two-wire configuration in which the ac signal is super-imposed on the dc power line.

Many modern accelerometer power supplies and amplifiers provide the IEPE (ICP1) input option which allows a direct interface to IEPE accelerometers. If the IEPE power option is not available, a signal conditioner/power supply with constant current power is required to interface with this type of device. The three-wire mode device requires a separate dc power supply line for proper operation.

Unlike a charge mode device that only contains ceramic sensing element(s), voltage mode device includes a microelectronic circuit which limits the operating temperature of the device to the maximum operating temperature of the electronics, usually tops at +125°C. Some designs push the limit to +175°C, but they come with compromises elsewhere in the performance envelope.

Due to the exceptionally wide dynamic range in piezoelectric ceramic elements, charge mode accelerometers are most flexible in terms of scalability because the system full scale range can be adjusted from the remote charge amplifier at the user’s command. Voltage mode devices, on the other hand, have their full-scale range pre-determined by the internal amplifier at the factory and cannot be altered.


Piezoelectric accelerometers are available in very small footprint. They are therefore suited for dynamic measurements in lightweight structures.

DC-ACCELEROMETERS
Two popular sensing technologies are used in making DC-accelerometers: VARIABLE CAPACITIVE and PIEZORESISTIVE.
 
VARIABLE CAPACITIVE (VC)

Capacitive type sensors (based on the capacitance changes in the seismic mass under acceleration) are the most common technology used for accelerometers today. They are made popular by large commercial applications such as air-bag, inertial measurements and mobile devices. They employ Micro Electro-Mechanical Systems (MEMS) fabrication technology which brings economy of scale to high volume applications, hence lower manufacturing cost. But this class of low-price capacitive accelerometers typically suffers from poor signal to noise ratio and limited dynamic range.

On the other hand, bulk micro-machined silicon sensors featuring capacitive technology, offer far superior performance than the single wafer etched sensor elements used in large commercial applications. These MEMS sensors feature gas damping and an on-board ASIC for precise linearity and temperature correction. The bulk micro-machined sensors offer the most accurate temperature performance and long-term stability of DC MEMS sensors available on the market. They are typically available in 4-wire differential output (although some 3-wire options exist) and the electrical interface is straight forward, requiring only a stable DC voltage source for power.

Bandwidth of capacitive accelerometers is dependent on the measurement range selected but typically ranges from 100 Hz to >2 kHz. Capacitive sensors are also favored in the lower range of acceleration measurement. Maximum range is typically limited to less than 200g.

PIEZORESISTIVE (PR)

Piezoresistive is the other commonly used sensing technology for DC-response accelerometers. Instead of sensing the capacitance changes in the seismic mass (as in a variable capacitive sensors), a piezoresistive accelerometer produces resistance changes in the strain gages that are part of the accelerometer’s seismic system. Most engineers are familiar with strain gage and know how to interface with its output. The output of most piezoresistive designs is generally sensitive to temperature variation. It is therefore necessary to apply temperature compensation to its output internally or externally. Modern piezoresistive accelerometers incorporate an on-board ASIC for all forms of on board signal conditioning, as well as temperature compensation.

Bandwidth of piezoresistive accelerometers can reach upwards of 7 kHz. Many of the piezoresistive designs are either gas damped (MEMS types) or fluid damped (bonded strain gage type). Damping characteristics can be an important factor in choosing an accelerometer. In applications where the mechanical input may contain very high frequency input (or excite high frequency response), a damped accelerometer can prevent sensor ringing (resonance) and preserve or improve dynamic range. Because the piezoresistive sensor output is differential and purely resistive, signal to noise performance is generally outstanding; its dynamic range is limited only by the quality of the DC bridge amplifier. For very high g shock measurements, some piezoresistive designs can handle acceleration levels well above 20 kg.

Due to its broader bandwidth capability, piezoresistive type accelerometers are most suitable for impulse/impact measurements where frequency range and g level are typically high. Being a DC-response device, one can accurately derive from its acceleration output the desired velocity and displacement information without integration error. Piezoresistive accelerometers are commonly used in automotive safety testing, weapons testing, and higher shock range measurements.

SUMMARY

Each accelerometer sensing technology has its advantages and compromises. Before selecting, it’s important to understand the basic differences of the various types and the test requirements.

Charge mode piezoelectric design is the most durable accelerometer type due to its simple construction and robust material properties. For high temperature (>150°C) dynamic measurement applications, charge mode piezoelectric is an obvious choice; or in most cases, the only choice. With charge mode device, a low-noise coaxial cable should be used due to its high impedance output, and a remote charge amplifier (or an inline charge converter) to condition its charge output.

Voltage mode piezoelectric is the most popular type of accelerometer for dynamic measurements. It offers small size, broad bandwidth and a built-in charge converter which allows direct interface with many modern signal analyzers and data acquisition systems (those that offer integrated IEPE/ICP power source). Voltage mode piezoelectric is typically limited to <125°C applications, but it is no longer necessary to use a low-noise coaxial cable due to its low impedance output.

Capacitive design features critically damped to overdamped response which lends itself to low frequency measurements. Bulk micro-machined silicon MEMS capacitive accelerometers have superior long-term stability and outstanding temperature performance. Capacitive accelerometers have low impedance output and typically either ±2V or ±4V full scale output. Most designs require a regulated dc voltage for power.

Piezoresistive accelerometers are versatile in terms of their frequency and dynamic range capabilities. Piezoresistive accelerometers (without electronics) feature a Wheatstone bridge sensor configuration with a ±100 mV to ±200 mV full scale output. The amplified models (with built-in ASIC) feature low output impedances and ±2V to ±5V full scale output.

Both Capacitive and Piezoresistive accelerometers are good for static acceleration measurements and produce accurate velocity and displacement data.

References

1. ICP is a registered trademark of PCB Piezotronics.

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About the author
Bjorn Ryden is TE Connectivity’s Sr. Product Manager, Vibration and Force Sensors. Bjorn has over 20 years of experience in the sensor industry. He worked at AMETEK Aerospace for over eight years as a Senior Design Engineer where he designed critical sensors for the aerospace industry. Prior to joining TE Connectivity, he also worked as a Senior Applications Engineer and Product Manager at Meggitt Sensors. At TE Connectivity for the past 12 years, he has held roles of increasing responsibility within the Product Management team. Bjorn holds a B.S. in Mechanical Engineering from Worcester Polytechnic Institute in Massachusetts.

Bjorn Ryden
Sr. Product Manager, Vibration and Force Sensors
TE Connectivity

 
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