Simcenter Testlab: Working with Charge Accelerometers
There are different types of accelerometers for measuring vibration that can be used with a Simcenter Testlab and Simcenter SCADAS system:
ICP Powered Accelerometers: Sensor generates a voltage proportional to vibration. Integrated Circuit Piezoelectric (ICP) means that the sensor has some micro electronics so it can be powered over a standard coaxial cable. Measures dynamic vibrations only. ICP is a trademark name. Sometimes IEPE is used place of ICP. IEPE is short for Integrated Electronic Piezo Electric.
Charge Accelerometers: Sensor generates a voltage proportional to vibration but has no microelectronics. This lack of microelectronics allows charge accelerometers to be used to measure vibration in higher temperature applications. For example, some charge accelerometer could be used up to 1200 F (648 C) whereas a typical ICP accelerometer might be limited to 325 F (163 C).
DC Accelerometers: Measures both dynamic (alternating) and static (constant) vibrations. For example, a DC accelerometer would read a constant level of one “g” of vibration when resting where an ICP accelerometer would read zero.
This article concentrates on how to use charge accelerometers with a Simcenter Testlab system.
Index: 1. Simcenter SCADAS Hardware 2. Charge Cable Wiring 3. Phase of ICP versus Charge Accelerometers 4. Triboelectric Effect 5. Simcenter Testlab Software 6. Using Charge Accelerometers with ICP
1. SCADAS Hardware
To measure charge accelerometers, the appropriate Simcenter SCADAS hardware is required. With a SCADAS Mobile VC8 card (Figure 1), it is possible to measure charge accelerometers, ICP accelerometers, or voltage inputs.
Figure 1: SCADAS Mobile VC8 Charge Card
The "C" in VC8 stands for Charge. Other Simcenter SCADAS cards. like the V8 and V24 cards, only measure ICP and Voltage inputs.
Note that the cable connectors of ICP and charge accelerometers are different. Charge accelerometers require a tight electric connection. Typically a threaded microdot, or 10-32, connection is used as shown in Figure 2.
Figure 2: Charge accelerometers typically use a threaded microdot connection (upper right) while ICP accelerometers typically use a BNC connection (lower left).
A BNC (Bayonet Neill–Concelman) cable connector is typically used for ICP accelerometers or voltage signals.
3. Phase of ICP versus Charge Accelerometers
If mixing ICP and charge accelerometers on the same test, the phase of the measured signals will 180 degrees out of phase. This means that if an ICP and charge accelerometer measured the same vibration, the amplitudes would be the same, but inverted in direction.
This would be important if doing an Operational Deflection Shape (ODS) analysis. To compensate, the direction field could be switched. For example, instead of selecting +Z, the direction -Z could be used instead as needed.
4. Triboelectric Effect
When using long cables in a high vibration environment, it is important that they are tied down properly to minimize their movement. Some transducers, like charge accelerometers, can generate local changes in capacitance due to cable movement alone. This might show as large spikes in the vibration data that are not real. This is called the triboelectric effect but is also sometimes referred to as “cable whip”.
To avoid this problem, tying down the cable in a way that it cannot move excessively is helpful. This is illustrated in Figure 3.
Figure 3: Top – Cable is tied to reduce motion, Bottom – Cable vibrates freely increasing the chance of spurious vibration readings.
Not only should the motion of the cable be minimized, it is also important to eliminate relative motion between the cable and accelerometer.
5. Simcenter Testlab Software
Simcenter Testlab can be setup to measure charge accelerometers, assuming the appropriate SCADAS hardware is available, as described in this section.
Charge based accelerometers have a different calibration basis than ICP accelerometers:
ICP Calibration Values: An ICP accelerometers output voltage, so a calibration values are expressed in mV/g. An example calibration value is 100 mV/g.
Charge Calibration Values: A charge accelerometer outputs charge proportional to vibration, expressed in picoCoulombs per g. A typical calibration value is expressed in pC/g. For example, 31.6 pC/g.
With a VC8 card installed in the SCADAS, in Simcenter Testlab Channel Setup worksheet, use the pulldown under "Input Mode" to select “Charge” from the list (Figure 4).
Figure 4: To measure with a charge accelerometer, set the “Input Mode” to “Charge” in the channel setup menu.
The “Input Mode” is selectable on a per channel basis. For example, a charge accelerometer can be used on the first channel of the card, and a ICP accelerometer could be used on the second channel.
6. Using Charge Accelerometer with ICP
It also possible to use a charge accelerometer with Simcenter SCADAS hardware that has only Voltage/ICP by using an external converter. PCB makes an external inline converter that allows a charge accelerometer to be powered using ICP signal conditioning (Figure 5).
Figure 5: PCB ICP to Charge conditioner. Model number is 422E51.
This might be useful if only V8 or V24 SCADAS cards are available.