Simcenter Testing Solutions Simcenter Testlab: Working with Charge Accelerometers

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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.
 
The "C" in VC8 stands for Charge.  Other Simcenter SCADAS cards. like the V8 and V24 cards, only measure ICP and Voltage inputs.

See the Knowledge article “SCADAS Mobile and SCADAS Recorder” for a complete list of card types available.

2.  Charge Cable Wiring 

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. 
 
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.
 
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). 
 
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).
This might be useful if only V8 or V24 SCADAS cards are available.

More information in the knowledge article: All About Accelerometers.


Questions?  Email nicholas.divincenzo@siemens.com or contact Siemens Support Center.

Simcenter SCADAS Related Links:

• Index of Testing Knowledge Articles
• Simcenter SCADAS
• Simcenter SCADAS Mobile and Recorder Hardware
• Simcenter SCADAS XS: Everything you need to know!
• Simcenter SCADAS Cable Guide
• All About Accelerometers
• High Channel Count Dynamic Data Acquisition
• What's in the box?  Getting started with Simcenter SCADAS
• Single Ended versus Differential Inputs
• AC versus DC Coupling
• Cool Triaxial Accelerometer Setup Tricks
• Simcenter Testlab Thermocouples
• How to use SCADAS T8 Thermocouple card?
• ICP versus IEPE versus Charge Accelerometers
• Ground Loops and How to Avoid Them
• Simcenter Testlab Data Acquisition Tips
• Cool Channel Setup tricks for Triaxial Accelerometers
• Long Cable Lengths and ICP Transducers
• Long Cable Lengths and Strain Gauges