Praise (and a quick fix) for our Fat Cat Motorsports work horse: the MTS Roehrig 2VS shock dynamometer

For a couple weeks in late October and early November, I was noticing some odd behavior from our previously rock-solid Roehrig Engineering 2VS (2 horsepower) shock dynamometer. The velocities were not hitting ‘5.0’ or ‘10.0’ in/sec but were off a few tenths. That was the first sign of strangeness. Then in the ‘Show Live’ screen we weren’t always able to get the damper to stop at true bottom dead center (BDC). It would be visibly off, or would just keep cycling and time out. Something certainly was amiss.

Working with MTS Lexington (who now owns the Roehrig technology apparently) technical support was quite helpful. The engineer was able to get on our computer via their TeamViewer software, verify the correct software and hardware settings, and use a few monitor screens for troubleshooting. He noted that the Displacement signal was oscillating quite a lot and that was a problem. The stroke measurement, which the engineer said should remain at 2.0 inches as initially calibrated, was reading 1.8, or 1.6, or even lower. Not good. Ideas about changing data card, the power supply, or the motor were proposed and I was getting close to ordering a kit (only being charged for what would ultimately need to fix the problem) but my intuition said to keep looking. Our machine had been working so well for so long, maybe it really was something simple and easy!

With the help of my our shock tech Tyler, we poked at cables and started to realize vibration in the machine affected the converted ADC signal being displayed on screen. Further tracing back led to a loose connector (secured via a grounding screw) on the position sensor.

This screw secures the connector for the position sensor being used to track motor position. If that loosen ups, you’ll get jumpy displacement signal readings which cause the machine to not measure the correct stroke, and not find bottom dead center (BDC) consistently. Tightening it every few years needs to be part of your preventative maintenance (PM) procedure!

Securing that screw, which snugs the connector’s electrical connections solidly to the sensor housing, solved the problem! Wow! How can it possibly get any better than this?!

The displacement signal was now rock solid, within a couple ADC counts as we were told should be the case, the software could find BDC consistently, and the velocities were being correctly displayed as we’d previously seen. This was a rather unusual but not unexpected problem as with lots of vibration over time, certain types of connectors can indeed loosen. I would suggest anyone with the same model or similar connector types to snug up those grounding screws every couple years, or if the software has issues consistently measuring BDC and you see similar unexplained jittery displacement sensor values in the ‘Hardware -> Monitor’ screen with ‘integer’ selected at top left of the screen.

We’re now back to building dampers and using our Roehrig 2VS dyno up to 22 in/sec which helps us consistently and accurately see subtle behaviors in our new ‘Ultimate KBO rally-style’ damper designs.

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