Hi all,

I am having difficulty getting meaningful data from a PCB 208C model dynamic force sensor. I am trying to acquire the maximum force applied during a dynamic event. This device is an ICP device, so we are using an SCXI 1531 module to excite the sensor. I have some issues with the data values we are seeing and I am not sure of the source of the problem. We have calibration data from the OEM which states the sensor returns about 10.33mV per pound-force (lbf). My first problem is that this sensor is a DYNAMIC force sensor that only measures CHANGES in applied force. If you apply a static load, you get a zero reading. This leads me to believe the output from the sensor is actually in V/lbf/sec. I have integrated the input signal then applied the calibration value to determine the maximum force applied during the dynamic event in lbf. The results are REALLY low. The range of the sensor (from the calibration data) is 0-5V (0-500lbf approx). I have applied a known force on the sensor using a force gauge and examined the signal in MAX test panels and simple DAQ assisstant Vi's. Applying a force of around 150lbf we see values of 0.3V max which equates to around 30lbf according to the cal data.

Not coming from an electronics background, I am unsure whether there are issues with AC coupling for this particular sensor or if there is anything particularly unusual about this combination of sensor and SCXI 1531 module. If anyone has used this type of sensor successfully, I would appreciate any advice.

Cheers & Beers


This post has been edited by Phil Duncan: Jul 9 2008, 04:34 AM

Taking a quick look at the data sheet for the sensor, it appears to go down to 0 Hz, meaning you can measure a static load, however there is attenuation of -5% at 0.0003 Hz (effectively 0 Hz). You mentioned AC coupling... that will take out any static/DC signal you have; switching the settings to DC coupled will electrically allow you to measure a static load. I'm not familiar with the SCXI module to help you there.

Hope that helps.

Tim

Tim's got it with the AC coupling there. If you want to see the static load as a DC offset, the SCXI-1531 is not the correct module. You will have to step up and get a 4472 or 4474. You can configure the 447x in AC or DC coupled mode.

The sensor does output in units of mV/lbf and not a rate of change in force. It's just that when you are AC coupled, you only see a voltage swing corresponding to the amount of change in force.

Thanks for the responses. I agree the 4472 is a better solution for the client unfortunately they are using an SCXI system that has been there for years and they are hesitant to pay for a replacement system, particularly when the 1531 module is the second SCXI module they have tried to use (initially they were trying to use a 1520 which does not incorporate IEPE excitation).

My only concern is that the local distributor for PCB informed that these sensors only measure a change in applied force as Dan mentioned. He stated they would not measure a static load. So even if we went with the 4472 option and applied DC coupling we may not be any better off. Which leads me back to another aspect of the original problem. The acquired signal levels are very low and applying the calibration factors from the OEM does not help.

I am waiting to hear from a senior technical guru from PCB about the sensor, just to confirm we are using it correctly.

Thanks again for your help

Cheers & Beers