Knowledge Base:  
Tech Note 122: Double Integration
Last Updated: 05/10/2017

Double IntegrationFrequently we get requests for a "Displacement Output" using an Accelerometer. This is a fairly common request for Fans where the Operators are used to seeing displacement in mils or microns. Signal integration converts acceleration (g's) to velocity (in/sec of mm/sec) or velocity (in/sec or mm/sec) to displacement (mils or um). Converting Acceleration to Displacement is called "Double Integration" as the signal is integrated twice and substantial amplifier gain is required.

Due to the low level signals produced by accelerometers at the lower Fan RPM speeds, the use of Velocity Sensors such as the CMCP793V is recommended. Velocity Sensors complete the first level of integration internal to the accelerometer before noise is introduced into the system by wiring which is then compounded by the gain required for integration. We then recommend the second level of integration occur in STI's CMCP700S-CV Case Vibration Transmitter.

As an example, on a 900 RPM Fan where 3 mils (76 um) output is expected, an Accelerometer will have an output in g's RMS of 2.4 millivolts and a Velocity Sensor will have an output more than 4 times higher or 10 millivolts. The signal will also be more noise free as the first level of integration was internal to the accelerometer/velocity sensor. The CMCP793V Velocity Sensor is also available with a 500 mv/in/sec output (20x higher) which we highly recommend when noise or long cable lengths are expected.

Velocity Sensors are available in both Top Exit, Side Exit and Integral Cable just like our standard Accelerometers. Give them a try when you are faced with a difficult installation! You are also welcome to use our "STI's Vibration Calculator" to calculate your own results!

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