ELECTRICAL CONNECTION
ICP-IEPE integrated electronic
Caution:This guide should be read carefully before installation. Download
INTRODUCTION
Compatible instrument
Most vibrations analysis instruments now incorporates a current source. In this case, just connect the sensor to the instrument and check the current source is enable (software or hardware configurable). Pay attention to the value of the current indicated as it is usually fixed at 4mA. This value will limit your cable length for a defined frequency range (See graph below).
GENERAL FEATURES
These sources are used when
- your processing instrument is not compatible with ICP type
- you need more than 4mA (long cable and high frequency measurement)
- you need gain on your output
- you need a velocity / displacement output
- you need antialiasing filter before your DAQ card
- you need a low current for high temperature application
- you need a battery powered source as there is no power available
Some constant current source provide one or more features described above. Check manufacturer’s datasheet for exact specifications.
Schematic
For a maximum imunity to EMI/RFI, the wiring diagram of the measurement chain should follow the rules define in the figure below.For example, if you have a sensor’s case grounded and a line powered constant current source, then you should connect the shield to the instrumentation ground through a 0.01uF capacitor.This will draw high frequency signal through the ground while avoiding low frequency current in the shield/ inner case.
| (1) Case | (2) Electronics | (3) Shield | (4) Armor | ||||
|---|---|---|---|---|---|---|---|
| Grounded | Isolated | Battery Powered | Line Powered | Direct | Capa 0.01uF/200V | Connected | Open |
| X | X | X | X | X | |||
| X | X | X | X | X | |||
| X | X | X | X | X | |||
| X | X | X | X | X | |||
(1) Case Isolated:
isolated stud Epoxy (not conductive) mounting Isolated Mounted surface
Driving long line
Driving long cables may effect frequency response and introduce distorsion when an insufficient current is available. The maximum frequency that can be transmitted over a given cable length is a function of the cable capacitance,
the peak signal voltage and the current available from your electronic conditioner.
Note that the current available for the cable equals the current from the signal conditioner minus the current for powering the sensor integral electronic.
Note that the current available for the cable equals the current from the signal conditioner minus the current for powering the sensor integral electronic.
To simplify the nomograpgh we have assumed a cable capacitance of 90pF/m and a maximum peak voltage of 8 Volts. In this condition we have
The nomographs below provide a simple, graphical method to define your constant current source taking into consideration that - the frequency determined by the
The nomographs below provide a simple, graphical method to define your constant current source taking into consideration that - the frequency determined by the
monograph should be 1.5 to 2 times greater than the maximum frequecy of interest. (for safety margin) - The current should be minimum to avoid creating heats in the sensor. This point is very important when using the sensor at
already elevated temperature or near it’s maximum continous temperature sepecification. Supply excessive current that will not be use by the cable will lead to failure in the electronic and can drastically reduce the MTBF.
Example:You need to transmit a 10 kHz signal at a distance of 100 meters. The signal will use the complete dynamic range and the capacitance of your cable is 90 pF/m. For safety margin we choose f=15kHz maximum,
therefore I=15*100/221=6.9 mA. If the consumption of the circuit is 4 mA then you need a constant current source of 6.9+4=10.9mA.
The figure below uses a passive RC filter to remove the DC.
TOUBLESHOOTING
ICP accelerometer
Troubleshooting ICP accelerometer is usually achieved by measuring the BIAS with a voltmeter. When properly connected and powered, the sensor bias should be within +- 2 VDC of its nominal value (usually 12 VDC). If it differs, it may indicate a cable, wiring, conditioner or sensor problem. The timewaveform, power spectrum are also useful tool to detect an installations problems.
Measurement chain overview
Measurement chain overview
All ICP accelerometers have a preset DC bias voltage. Ideally, the bias is halfway between ground and supply voltage. It defines the available voltage swing of the signal. If a voltage source is lower than 18 Volts, the amplitude range will be lowered accordingly. The AC signal which represents the vibration signal is superimposed on a DC bias
voltage. The DC bias voltage must be removed so that the AC vibration signal oscillates positive and negative about 0 Volt. This technique help amplifies the AC signal only. The signal can now be connected to the processing electronic for AC measurement calculations (RMS, AVG, PK-PK, ...).
The figure below uses a passive RC filter to remove the DC.
For example one can choose R=500 kOhms and C=22uF (35 VDC mini) which gives a frequency cutoff frequency of 0.01 Hz.
The Bias is a useful value to check the integrity (open or intermittent contact) of the connection between the senor’s electronic and the conditionner. Unfortunately, the bias will not indicate if there is a short or open circuit betwwen the piezoelectric crystal and the sensor’s electronic.
