We recently had a set of questions from one of our customers regarding the selection of accelerometers for vibration testing. Accelerometers are sensors that measure acceleration in the time domain during a shaker test.
QUESTION 1
How do I select the optimal accelerometer and sensitivity depending on the required test conditions? For example, most tests are 3G peak sine dwell and a 50G, 11mS classical shock pulse. Which type of sensitivity accelerometer would be recommended for each of these test profiles?
Answer
There are many accelerometers on the market today. Their specifications vary based on the maximum acceleration, frequency range, test environment, and more.
For most test environments, we recommend using an IEPE accelerometer with TEDS. An IEPE accelerometer powered by a constant current provides a clean signal and removes many of the noise sources that are common with other types of accelerometers.
A transducer electronic data sheet (TEDS) chip contains sensitivity and other details, which reduces the possibility of operator error.
Accelerometer Sensitivity
If the sensitivity of an accelerometer is entered incorrectly, it could possibly cause significant damage to the shaker. The TEDS chip on an accelerometer holds the important information required for a test (calibration date, manufacturer, sensitivity, output range, Model Number, etc.). TEDS ensures that the sensitivity of a connected accelerometer is entered properly.
The most common sensitivities for accelerometers in vibration testing applications are 100mV/G and 10mV/G. Common IEPE accelerometers can output a ±5V excitation signal. Therefore, the range of a 100mV/G accelerometer with a ±5V output will be 50Gpk (5Vpk / 100mV = 50Gpk) and a 10mV/G accelerometer will be 500Gpk (5Vpk / 10mV = 500Gpk). These accelerometers can generate a signal greater than ±5V; however, beyond that point, the response is no longer linear, and the overall measurement uncertainty increases significantly.
For the test profiles in Question 1, a typical IEPE/TEDS 100 mV/G would be an ideal accelerometer for the low amplitude (3Gpk) sine dwell test, but would not be the best choice for the shock test as defined (50Gpk). A 10mV/G accelerometer would make for a much better choice. Most laboratories have both 10mV/G and 100mV/G accelerometers for the appropriate applications.
QUESTION 2
Is there a rule of thumb concerning the maximum accelerometer cable length?
Answer
Simply put, the shorter, the better. The cables in a vibration testing system are one of the most easily damaged components and are common sources of noise problems.
Cables get strewn across the floor, scraped, and kinked, and the shielding begins to break down. As it breaks down, the radiant electrical noise of the environment is induced into the cable and measured by the vibration controller. This noise is environmental; therefore, the controller cannot control it. This applies to the drive output cable between the controller and the amplifier and the accelerometer cables between the accelerometer and controller. Eventually, the ground/shield bond established by the cable will fail, and any signal generated or measured will have no real reference. This failure can cause damage to other components in the system.
This is one reason why Vibration Research uses a 1000 Base-T Ethernet connection between the controller and the PC. It allows the technicians to install the controller close to the amplifier and shaker while the operator station can be in a control room up to 300 feet (100m) away. The shorter cables (drive and accelerometer) are more easily maintained, more cost-effective when they need to be replaced, and remove a lot of confusion when determining which accelerometer is connected to a particular channel.
Max Cable Length
The maximum cable length depends on the type of accelerometer and cable. For instance, if you are using charge mode accelerometers, the distance between the accelerometer and the charge barrel/amplifier needs to be short. These accelerometers output very low voltage levels, and any noise that the cable absorbs from the environment can cause variation in the signal.
For IEPE accelerometers, the wire that carries the voltage supplying power to the accelerometer also carries the signal. The signal has much more power and is less subject to variation. As the cable length increases, the inherent impedance and capacitance will change. Depending on the cable type and the output of the IEPE power supply (2mA vs. 5mA), the maximum length can be up to several hundred feet for a well-insulated and low capacitance straight cable. Joining multiple cables, running cables near high voltage wires, and other factors will affect the signal, which may cause issues.
Accelerometer Calibration
Calibration accurately determines an accelerometer’s sensitivity at various frequencies of interest. A calibration schedule helps labs detect incorrect sensor sensitivity and discover sensor defects so they can have confidence in their results. VR’s Accelerometer Calibration software performs automatic and accurate calibration and verification of IEPE accelerometers and charge-mode piezoelectric accelerometers/transducers in your lab.