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By Peter Eitnier, Senior Application Engineer
Industrial piezoelectric accelerometers are typically voltage mode devices, meaning they produce an Alternating Current (AC) output voltage relative to the vibration signal being measured. This vibration signal voltage is subject to all the limits of AC signals and is affected by the capacitance in AC circuits.
Long cable runs create a capacitive load on the output of the vibration sensor’s amplifier. Amplifiers can sink fairly large amounts of current but cannot provide an indefinite amount of current for driving capacitance. When the current required to drive the cable is insufficient, the signal becomes distorted, and harmonics are generated. This could lead to vibration signals being misinterpreted as having strong harmonic components when they, in fact, do not.
When cable capacitance increases, either the constant current value must increase, or the maximum usable frequency must decrease to keep the equation in balance and avoid signal distortion.
Example: A vibration practitioner uses an accelerometer with a 100 mV/g sensitivity; measures vibration at up to 50 g levels (50g x 100 mV/g = 5 Volts); powers the accelerometer with a 4 mA constant-current source; and wants to measure signals at 10,000 Hz. How long could the cable be?
Starting at the 10,000 Hz location on the x-axis, move up the y-axis to the 4 mA line. At that point, the user could run up to 300 feet of cable and make reliable vibration measurements.
Wilcoxon’s downloadable Excel Accelerometer Maximum Cable Length Calculator will help you perform these calculations for your vibration monitoring installation.
For more information, the equations behind the calculator, and cabling installation techniques for accelerometers, read Wilcoxon’s Tech Note 22: Vibration sensor wiring and cabling. It delves into powering requirements, high temperature applications, eliminating EMI, avoiding ground loops, cable types, cable anchoring and more.
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