Accelerometer Selection - Points to Consider
This guide outlines the differences between the types of Accelerometer available.
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.
|Requires more expertise to operate the system.|
|Fixed sensitivity. Range and time constant are fixed within the sensor.|
|More expensive per unit|
|Lower operating temperature than a charge accelerometer|
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.
|Noise from the cable affects results and reduces resolution – Must use Low Noise Cable.|
|More costly with the requirement for additional instrumentation, and special low noise cables.|
|High impedance nature of the signal makes it more vulnerable to noise. Ie, Sensitive to cable flex|
|Requires more expertise to operate the system.|
|Sensitive to dirt on connectors|
Sensitivity and Resolution
Consider the measurement range you require. An IEPE accelerometer is limited by its voltage output, so the sensitivity you use needs to cover the whole range of your likely measurement.
The sensitivity of an accelerometer is given as a voltage/charge returned per unit of acceleration, (mV/g or pC/g). For applications measuring high-level vibrations a lower output accelerometer is recommended, and for applications measuring low vibrations a higher output is recommended. E.g An Accelerometer with an output of 10mV/g measuring an acceleration of 0.1g would return a voltage of 1mV. In this case a higher output accelerometer would be recommended in order to obtain a higher resolution measurement.
An Accelerometer with an output of 100mV/g measuring an acceleration of 500g would return a voltage of 50v. In this case a lower output accelerometer would be recommended in order to obtain a reading within the constraints of readout instrumentation.
When measuring the vibration of structure, adding an accelerometer will alter the vibration. To minimise this effect, the accelerometer should not weigh more than 10% of the overall weight of the structure to be tested. DJB Instruments manufactures Accelerometer with masses of just 0.19gm, suitable for the testing even PCB boards.
Ground Isolation and Mounting
For Accelerometers mounted on conductive surfaces, it is important to opt for a ground isolated variant to prevent erroneous data. This is caused by a difference in ground voltages between the accelerometer and the readout instrumentation, resulting in a ground loop. Please see our separate Mounting Guide.
Mass loading is the effect of adding mass to a test item which then changes the dynamic characteristics of that item. Click here for a great video on our Youtube page to show this phenomena, well worth a look.
Charge accels usually operate up to 260°C whilst standard IEPE accels operate up to 125°C. Make sure this matches your requirement. If you need a high temperature IEPE accelerometer, look at our world leading HT range which operate up to 185°C
Accelerometers can last for decades if treated properly, however regular calibration is important, things can change with age…I think we all know that feeling! See page 22 for more details.
The difference between Low noise and Non-Low noise cables…
It is important to use the correct cable when using accelerometers. A charge output accelerometer must be used with a LOW NOISE cable. The term low noise refers to the cables ability to reduce triboelectric noise (otherwise known as motion induced noise) rather than electrical noise. This reduction is accomplished by the inclusion of a graphite or silver wrap on the outer surface of the inner dielectric layer. If a non low noise cable is used movement of the cable will add an additional signal into the data causing errors in data analysis.
An IEPE accelerometer is much more forgiving and can be used with almost any type of cable. The more traditional coaxial cable is commonly used for these accelerometers. If using both Charge and IEPE accelerometers within your facility. It is best to standardise all cables to be low noise types to avoid potential errors in using the wrong cable.
For more details about cable selection and for a practical demonstration of triboelectric noise please visit our Youtube page.