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IM4K4 Motor Resonance Crashing a Turbo

TMO had a strange problem back in April: whenever they would move IM4K4 in or out, it would vibrate the stand violently to the point that it would cause a nearby turbo pump to crash, which was extremely disruptive to endstation operations.

TMO

• Granite 3 (all components are checked) is ready to go for dream mirror check out experiment(starting from 8/7): IP2(no chamber)-Granite 3 Controls I

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 Scientists will run dream mirror check out experiment and test all PMPS and veto groups at the same time

Stepper motors are known for having some problems with resonance and high vibrations. Typically in industry you would pick servo motors if extremely smooth motion was a requirement. This isn't a requirement here, but "do not shake the stand when moving" is absolutely a reasonable ask.

All of the PPM imager/power meter combination units were tuned based on a single instance before it was installed onto the beamline. As such, the parameters were optimized for a lab bench in the temporary clean room in B750 rather than for the beamline as-installed. With the rush to close out the installation of the L2SI components, it looks like this had never been revisited.

In general, we expect every instance of a motor to have slightly different tuning needs based on the precise specifications by which the assembly was installed. This won't always be worked on to completion because often there won't be any problems, and because the most important specification for the majority of our motors is position accuracy and precision of the end position, along with motion reliability, which is not covered by this vibration case.

It was observed that the PPM units have higher vibrations than one would expect in the FEE, RIX, and TMO, culminating in this turbo pump incident. So, we took some time to do the following for every installed/active PPM:

1. Make the position correction loop more gentle. The PLC software that governs the in-transit motion was very aggressively conforming the stepper's movement profile to the "optimal" profile, leading to overcorrection and some slight increase in the vibrations. They were also tuned for an expected runtime velocity of 65 mm/s, when in practice we were running them at 5 mm/s.
2. Pick a runtime speed for the PPM that minimized vibrations from resonances. It turns out that all of the PPMs have strong resonances with their stands in the 5 mm/s - 8 mm/s movement speed ranges. This is why most of the PPMs could perform OK if they were slowed down. What isn't obvious if you aren't aware of the resonances is that you can also make the PPM perform better by speeding it up. Most of the PPMs had a "best-case" (minimum vibration) speed at around 12 mm/s, though some performed best as high as 15 mm/s or as low as 11 mm/s.

After these adjustments, the PPMs are silky smooth, quiet, and reach their destinations reasonably quickly.

MEC SPL Upgrade

Peregrine McGehee 

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