Accelerometers - Pros & Cons
Voltage Output IEPE Accelerometers
This type of accelerometer features an integrated circuit within the Accelerometer converting the charge from the piezoelectric crystal before it leaves the sensor, this voltage can then be transmitted across a standard coaxial cable, as it is more stable and inherently immune to noise and interference from the cable.
These kinds of accelerometers have a maximum operating temperature of 125ºC, with some high temperature variants of these accelerometers, which can withstand temperatures of up to 185ºC. There is also a water cooled accelerometer that can operate on surface temperatures in excess of 900ºC.
IEPE Accelerometers can be connected to the measuring equipment, or a computer with minimal interfacing/instrumentation, and does not require the charge amplifier. It is important that the Accelerometer is powered by a stable, constant current supply to reduce noise on the signal generated by the Accelerometer.
The disadvantage of low impedance accelerometers is that the sensor has an internally fixed range and time constant, potentially limiting their uses in some applications.
|Operation at higher temperatures, up to 185°C for test and measurement accelerometers.||Requires more expertise to operate the system.|
|Direct connectivity to the majority of readout instruments, or vibration analysers.||Fixed sensitivity. Range and time constant are fixed within the sensor.|
|More reliable.||More expensive per unit|
|Signal can be driven down a longer cable without increased noise or loss of resolution.||Lower operating temperature than a charge accelerometer|
|Can be operated from low cost constant current sources.||Less robust|
|Integrated electronics can provide a higher output from a smaller mass|
|Use wide range of cable types|
|Less sensitive to dirt|
|Signal power often built into DAQ systems|
Charge Output Accelerometers
In this type of accelerometer, a charge will be emitted proportional to the force applied on the piezoelectric accelerometer. This charge will be transmitted directly down the cable and will require special low noise cable, with a charge amplifier on the receiving end. Charge accelerometers have high impedance and are particularly suited to high temperature applications (more than 185°C) where IEPE Accelerometers could not be used.
It is important to note that due to the high impedance nature of these sensors, they require a low noise of cable to be used when connecting the sensor to the measurement equipment. This is due to the cable used having an effect on the signal; movement on the cable can distort this signal. It is also important to note that cable length will have a significant effect on the signal from the sensor, and it is advisable to have a short cable between the accelerometer and charge amplifier. Low noise charge amplifiers, or amplifiers with integrated low pass filters can be used, meaning that the signal from the charge amplifier useable without any further filtering.
The main advantage of a charge system over an IEPE system is the ability of the accelerometer to operate at higher temperatures. Charge systems are generally more versatile due to the time constant, gain and normalisation all being controlled from the charge amplifier.
|Operation at higher temperatures, up to 250°C for test and measurement accelerometers, and 400°C for industrial accelerometers.||Noise from the cable affects results and reduces resolution – Must use Low Noise Cable.|
|More flexibility in adjusting the time constant, gain and normalisation via the charge amplifier.||More costly with the requirement for additional instrumentation, and special low noise cables.|
|Extended low frequency response.||High impedance nature of the signal makes it more vulnerable to noise. Ie, Sensitive to cable flex|
|Wider Dynamic Range||Requires more expertise to operate the system.|
|Robust||Sensitive to dirt on connectors|
|Lower cost per unit|