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brazierp

Syncing two motors

6 posts in this topic

I'm just about to do something which I've never done before and thought I would get your collective advice, tips and warnings. We have a simple conveyor belt with two drive drums. To help reduce slippage we have decided to try and drive both drums. Both drums are driven by identical motors with encoders on the drum shaft. Motors are controlled by regular variable frequency drives speed controlled by 4-20ma from a SLC500. The rate encoders are separate encoders into HSCE cards. We don't think this needs to be very accurate and to start I am thinking of just sending the same signal to both drives and see what happens but I would like to be able to keep control of the belt tension by running one drive a little faster than the other. In this case I am guessing I will take the "master" drive and set it at my required speed, look at the rate encoder and make the "slave" drive keep at say 98% of it's speed. So is there a simple way to look at the master rate and set the slave speed signal depending on the rate observation from the slave drive? The actual belt position is being monitored by a separate surface encoder. I hope that's clear. Pete.

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Use a single drive rated for the combined horsepower of both motors. Install an overload device [motor protective switch] between the drive output and each motor. Wire the alarm output of each overload device to the overload/alarm input of the drive. Control speed with a single 4-20mA PLC output. Monitor position of one encoder with the HSCE. If the application is reversing then maybe use an encoder at each end. Monitor the encoder at the driving end depending on conveyor direction. My 2 Cents

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If you want tensioning, you probably don't want to run both motors with the same drive, as the only way to control tension would be to use mechanical gearing. The HSC card should give you rate information that you can scale to become your 4-20mA output to the slave drive. However, you might get better control by just taking your master speed command, decreasing slightly for tension and sending it to the slave. There are a lot of factors, but between encoder resolution, HSC computation delays, PLC scan time, digital to analog conversion (PLC side), and analog to digital conversion (drive side), your slave command will always be slightly behind what it truly needs to be. If your master is consistent enough, or the application slow enough, you might not notice it, but it will be there. I recommend keeping it simple unless you don't get the performance you want. Probably not appropriate to your application, but if you truly want tension control, make the tensioning motor a servo and set it up for tension control. Then simply run the other motor with a drive and the servo will take care of the rest. You will want to have some sort of auto/manual switch on the servo that is tied to a belt break sensor so that it doesn't take off trying to maintain tension when there is nothing on it.

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Thanks guys. Yes, I need two drives as I want tension control and the hardware is already in place so servo's are out of the question right now. I'm going to keep it simple and just scale the rate input to the current output and see what happens. It's a $7,000 belt so I need to be a little careful!! Peter.

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This is how we load share on our conveyors - Granted, ours are 1-2 miles long, but load sharing should be similar I would say. Basically, our conveyors come in 2 or 4 motor setups, with either case having two drive rollers. The belt is tensioned hydraulically. The master VFD receives a speed reference through some fashion (either 4-20 mA, MODBUS or Ethernet/IP depending on the age/VFD). Older setups are sent a single speed reference, and the master drive ramps up according to the acceleration rate parameter in the VFD. Newer drives that use ControlLogix have an S-curve block in the PLC program that sends a varying speed reference to the master drive to ramp it up, and the acceleration rate parameter in the drive is minimized. The slave drive(s) are sent a torque reference, which is just the torque feedback from the master drive. If your drive rollers are set up such that there can be little to no slippage between drive rollers (gearbox or a big, tight conveyor in our case) the torque reference is fine, but if the drive rollers are free to vary slightly in speed, the drives should just share a speed reference, scaled if the drive ratios are different. Usually, at least if you are sending a torque reference to the slave drive(s), the only other parameter modifications necessary are to reduce the Kp and Ki values for the speed feedback loop of the master drive, or else the torque reference will vary too rapidly.

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Thanks Amish. I don't have the ability to measure torque on these drives so I am limited to looking at and setting speeds. It's an interesting approach though. I'm a couple of weeks away from having to do this for real. I'll let you all know how it goes.

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