As noted in a previous blog post, the most common transducer (sensor) used in the vibration industry is an accelerometer. Accelerometers are available in many competing models with widely varied specifications. A Piezoelectric (PE) accelerometer is the most commonly used accelerometer in vibration testing.
Common Types of Accelerometers:
Three common types of accelerometers are:
Piezoelectric (PE); Piezoresistive (PR); and Capacitive.
Piezoelectric (PE):
- A mass, within the accelerometer, is free to push against a crystal. When the mass moves and pushes against a crystal, an accumulation of electric charge results.
- A PE accelerometer is used for high temperature and high frequency situations and when mounting issues are simple. It is the most commonly used accelerometer.
Piezoresistive (PR):
- A mass, within the accelerometer, is attached to a variable resistor. When the mass moves the resistance changes, and consequently, the current.
- A PR accelerometer is most useful in sudden or extreme vibrations.
Capacitive:
- In a capacitive accelerometer a mass moves which is attached to one plate of a capacitor. The movement of the plate changes the capacitance of the capacitor.
- Most widely used for very low frequency applications.
When to use a Piezoelectric (PE):
PE accelerometers are superior to other accelerometers at high frequency (<3 Hz to 30 kHz) and high temperature situations (260oC). Since many vibration tests require high frequency testing it is important to use the PE accelerometers. Some vibration tests are also performed at high temperatures. PE accelerometers perform well under these conditions also.
PE accelerometers are well suited for vibration and shock testing. They can be purchased with a wide range of sensitivities and come in a variety of sizes and weights.
Why is sensitivity important
The sensitivity of an accelerometer largely determines the range of acceleration that can be read with the device. A 100mV/G accelerometer is typically limited to 50G measurements. Measurements about 50G will be inaccurate and clipped. A 10mV/G accelerometer will have lower voltage outputs. For very low signals the measurement can be lost to the noise floor of the controller input. When using a quality controller with a low input noise floor 10mV/G will give good results from very low signals out to 500G typically. This can be important when testing as the device under test often has high resonances which measure higher the 50G.
When to use a high temperature PE:
Accelerometers perform well under certain temperature ranges. PR accelerometers operate well at normal ambient temperatures. IEPE accelerometers can be used to a max temperature of about 175oC. Regular PE accelerometers can be used to approximately 260oC and the high temperature PE accelerometers work up to 650oC.
When test conditions will exceed 250oC, such as may occur in a particular thermal cycling Environmental Stress Screening (ESS) test, it would be advisable to use a high temperature PE accelerometer.
Why use an IEPE Piezoelectric Accelerometer?
A particular type of piezoelectric accelerometer is the Integrated Electronic Piezoelectric (IEPE). An IEPE is a piezoelectric accelerometer that contains built-in electronics to boost the signal. This is beneficial because the cabling is where much of the noise affects the signal. Boosting the signal after the cabling is a huge advantage to minimize the effects of noise.
Why support TEDS?
Many accelerometers are equipped with TEDS (Transducer Electronic Data Sheets). Embedded into the memory of the IEPE accelerometer is the calibration data and characteristics of the accelerometer, such as, its calibrated sensitivity, calibration date, serial number, manufacturer, and calibrated frequency range.
TEDS is a very efficient way to maintain record of important information regarding the accelerometer. Rather than a testing facility needing to make paper copies of accelerometer sensitivity and calibration data available to each technician, all of that important and pertinent information is stored electronically on the PE. This also permits the rapid controller to verify and read calibration directly from the accelerometer, preventing transcription, cabling, and data entry errors. TEDS is a very helpful and worthwhile tool for test engineers.
