Troubleshooting a 4-20mA Accelerometer System

Accelerometers with integral temperature sensing RTD vs Semi-Conductor Sensors
8th March 2021

Troubleshooting a 4-20mA Accelerometer System

Fig.1

Fig.1 The first thing to check is that the wiring is carried out and in accordance with the calibration sheet provided with the sensor. Check polarity of the voltage power supply.

Fig.2

Fig.2 The next thing to check is that the accelerometer is being powered correctly. Check the cable datasheet to see which two pins to be checking the voltage across. The voltage should be somewhere between 15 and 30 volts. Many systems would typically be 24volts. If the accelerometer has an integral cable then a point along the signal path needs to be checked. The closest point to the accelerometer should be sought to check the voltage. This ensures that a check for discontinuity in the wiring is taken with as much of the signal path being checked as possible.

Fig.3

Fig.3 If the accelerometer is seeing the correct voltage then the next check is to see if the accelerometer is generating a current that is at least 4 mA . To do this the multimeter needs to be in series with the accelerometer. The circuit wiring needs to be disconnected at some point along the signal circuit and the meter inserted. If the machine on which the accelerometer is mounted is off you would expect 4mA. (No vibration condition). If the machine is running and vibration is being generated you would expect to see anywhere from 4 to 20 mA depending on the level of machine vibration. If you have 15-30V of power and you are seeing less than 4mA then the accelerometer is defective. If you are seeing greater than 20mA then the vibration levels are saturating the sensor or the sensor is defective.

Fig.4

Fig.4 If at least 4mA of signal is being generated then the accelerometer is most likely functioning correctly. If the accelerometer is showing more than 20mA and the machine is running then the machine vibration might be saturating the accelerometer and a lower sensitivity sensor is required. To check for this disconnect the sensor and repeatedly hit it with a light object. This should cause the mA current to increase. The Hansford Sensors 4-20mA sensors have a 5 second averaging circuit so ensure the light tapping of the sensor is repeated for at least 10 seconds to register above 4mA. If the accelerometer remains at 20mA or above despite being disconnected from the vibration then the accelerometer is likely defective.

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