Motor drive error question
frank freedman
Posts: 1,983
Motor/drive experts:
I have a question regarding a rather complex belt driven rotor system that is giving me fits; very intermittent . It is a pretty high mass ~800lbs balanced rotor driven by a large 3 phase PLC. It runs slow (<15RPM max) then slowing when moving to specific positions. The belt is new, steel or fiber reinforced V belt. When it is moving the last few degrees to position, the system logs very many changes of direction entries until it finally settles out. Balance is well within tolerances. The belt tension is set by measuring the gap between the tensioner arm and the mount and is where it was when the old belt was in place. The gap does indicate the tension may be lower than called for, so I plan to add tension to the belt to see if this resolves the problem.
Is it possible the belt tension is such that when moving into position, the new belt has enough play/springiness/something that when the PLC drives, the belt has to "catch up" and then the rotor will slightly overshoot, PLC says drive back (change of direction) drive forward etc. and the rotor exhibits a slowly decaying oscillation. It appears to resemble the behavior of an under-damped PI control system. So, this is really part question, part reasonableness check on my logic with this issue. Any ideas would be great, thanks to all.
Comments
Hi,
it is difficult to say much from the amount of information you give about this system. (Photo, sensors, sensor position, controller type, did it work fine before changing the belt?...)
But yes, the oscillation might get better, if you add some tension onto the belt, if the tension is so low now, that there is play or low stiffness.
Christof
@"frank freedman"
I assume that the "large 3-phase PLC" is the motor driver?
What is the feedback device and where is it mounted; on the motor or on whatever the belt is driving?
What's the recent history?
Craig
Yes, such belts should be tight. Tight enough to pluck a note from them. The belt slipping in the pulley could indeed be a reason for oscillation (assuming there is feedback).
In more complex motion controllers, one can configure what is called a positioning window. Which is basically a tolerance range for what will be accepted as having reached the final setpoint position.
Example: If you're measuring in degrees, say you want 60 degrees, but anywhere between 58 and 62 will be okay. The positioning window will be 2 degrees. There's also a time aspect, so be within 2 degrees plus or minus, for 2 seconds. (Or whatever) and consider this movement completed.
Without having a tolerance range, the feedback loop is trying to hit a perfect number it can never reach. Called by different names but I call it "hunting".
There will always be oscillation, but there are tricks to deal with it. Overtightened belts waste energy and are prone to snapping.
If this isn't part of the system and it's programmable, consider writing in a tolerance positioning window.
This sounds to me like the feedback is coming from the load, as opposed to the motor shaft. If something needs to catch up, there is lost motion (aka: backlash)
Working blind here but if the belt is good, is a pulley slipping on the shaft?
That is way too much "hunting". At worst, with a correctly-tuned PID, one would only be able to feel some oscillation.
PI has never worked for me. Always PD until the motor comes to a steady-state and then apply the integral.
Craig
Disclaimer: I'm not a motor control expert, but I've bent enough pulleys and smoked enough belts to know when to call one.
That being said, does your application meet both the following conditions:
1). The pulley diameter is smaller than ~15x the width (inner/outer) of the belt, and
2). Belt used is not a "notched" (or "cogged") V-belt
Where I am going with this: a V-belt that is being forced around a tight radius pulley will chatter unless the belt has the "cogging" notches cut into it. Its just physics, and there isn't any effective way to prevent it except to either increase the pulley diameter and/or use a cogged belt. See this link for a better discussion of belt properties: https://www.altramotion.com/en/newsroom/2014/08/understanding-trade-offs-when-selecting-belted-drive-systems
Note also in multiple-belt (parallel belt) applications, you want to make sure all of the belts get replaced at the same time, with the same part number, and ideally from the same batch, or you can have similar problems. (I chased this for months on a feedlot auger system until a grizzled old farmer pointed-out the now-obvious need to replace all three belts at the same time).
Some updates. Tonight, I tried to up the tension, but due to the method of mounting the tensioner, I could not increase the tension. The motor is a 3Phs driven by an emmerson PLC. The sensor is a resolver system that generates positional values as well as the Q0, Q90, and index. that is mounted in the rotating ring. The belt is a 6 groove belt with the ring. This works on 5 other systems using this , so something has changed further in the system than I can see. Likely a drive parameter I can not get to. It is being escalated to the factory national service. Really curious as to what it will turn out to be. Will let you know what happens. This one is either a real head scratcher or a real dumba$$ overlook. Thanks for all the replies, they pretty much validate what looks to be happening.
Shaft is keyed, no slippage on the drive motor pulley. The system generates about 30K pulses around the ring with the target position being 90 degrees. It will sometimes hunt between 79 and 91 degrees, though the resolution is down in the 1/100th of a degree.
Typically, on an Emerson (Control Techniques) driver, there is an LED display and a few buttons. Any of the gazillion parameters can be accessed and modified but it's a horrendous list. I just recently tackled similar.
I have CTsoft and all Unidrive documentation (wasn't easy to find). Better-off hooking up a laptop to one of the functioning systems, sucking-out the parameters and compare to the problem system. Some of them have an SD card slot also.
Craig