Speed synchronization of 3 DC Motors


Thread Starter

Faisal Saleem

I am working on the sizing machine which have three DC motors.The centre motor is the master drive and the rest two are slave.The yarns are fed through one slave drive to the master and then form master, yarns are fed to the second slave drive which is the winding beam drive.The yarns are fed in such a way that the tension of yarns during the whole process should remain same.This can be achieved by synchronizing the 3 DC motors.I am working on 2 PI controllers in cascade mode. If anyone have any idea or practical implementation for this problem please reply. The 3 drives are equipped with 3 tacho motors.
1. I would suggest that you should use two control loops: one for the overall speed control (master drive) and one more (maybe two more - if on both sides of the master drive) for the tension control. You will not be able to control the tension by directly controlling the speed of the motors. Even the slightest difference of speed of the motors results in large tension variations because tension is a function of the rotor angles (not the speed) and, therefore, is an
integral of speed difference.
2. You better use a tension sensor (lever) for tension measurement. It also can be measured as a diference between rotation angles but it is less
3. When controlling the tension, note that the motor has an integrating property. Therefore, use very little of I-action in your PID control for that loop or don't use at all.
Wishing you every success,
Igor Boiko
([email protected])
Is the beam a center driven winder? It gets a little more complicated if it is. Can you give more detail on the line up?
The best solution would be to make all three position servos and command them in parallel. An alternative is to make the master drive a velocity drive, but monitor its position and use that as the command for the other 2 drives. If you take this approach, you will need to use velocity feed forward to ensure that they are all in synchronism at all speeds. If you try to use speed controls only in all three axes, they will diverge in position with time even with the slightest error. Therefore you will need to monitor position or tension and make corrections - you might as well have a position loop.

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Unclear exactly how your application works but could you try running your master motor in speed mode and the slaves in torque mode?

Bob Peterson

I have a certain amount of experience with this type of machine, having spent a lot of time getting them to work. A couple comments:

The beam motor (the takeup motor) really should be independant of the other two motors. Set it up as a constant tension center winder. Normally you will want a constant tension on the yarn (actually you probably want to let the tension drop off some as the beam diameter builds). Many drives can be configured as constant tension center winders, or you can try and do it yourself, with or without some feedback indicating what the actual tension is. If you try to make the speed of the takeup motor some ratio of the master motor, you will never get this to work because as the beam builds the diameter changes, so your ratio has to change as you build the beam.

You can do the center winder yourself by calculating the beam diameter to adjust your motor torque as the beam builds (you need more torque to get the same yarn tension as the beam builds), taking into account your beam inertia for acceleration and deceleration torque adjustments, and some kind of adjustment to the torque for static and dynamic friction in the system. This can get a bit involved so its probably a lot easier to just buy something that already does this. You can calculate the beam diameter from the ratio of the two tachometer signals. Using digital drives makes these things work a lot better though so consider upgrading the drives.

Normally we found that the yarn reacted best when the tension on the yarn between the two drive rolls was somewhat higher then the beam tension, so that the yarn relaxed some as it wound up on the beam. You also really need to look at reducing the yarn tension as it builds on the beam. We did this as a linear function of the beam diameter, usually 10-20% yarn tension reduction from an empty beam to a full beam.

The other thing you must keep in mind is that you will normally run the line speed as a function of the moisture content of the yarn. As the yarn gets damper you reduce the line speed, as it gets dryer you can increase it. The line speed is thus the output of the moisture control PID loop.

BTW, this was all for denim. It may not be applicable for synthetics.

Bob Peterson
One drive in speed and the others in torqe is the way to go. Try to use load cells as a tension feed back, and if you need to buy new
equipment go AC. Also did you say tach generators I hope you realy plan to use good pulse encoders.
> Unclear exactly how your application works but could you try running your master motor in speed mode and the slaves in torque mode?<

I agree with your suggestion. In fact, this is the only way for winder and unwinder applications where you need to keep the line tension constant.
What happened? Did someone make an adjustment or replaced a board? Did something change on the

You should consider calibrating your tension feedback element with this application. Dancer
roller or transducers fed back into your PID will be a problem if not working properly. You also
need to check the element that keeps track of the building roll size. Calculate what your result
should be and adjust accordingly.

Are the drives calibrated? Check power supplies around the PID, etc.

Have tachs on motors been inspected? Are they correctly coupled and not loose on line shaft?

PID: Verify the Prop gain is set correctly, you can trim down the Integral gain until the Prop
gain is correct. Then adjust the Integral gain until you are satified with the "reaction" of the
motors. Derivitave gain may not be in use and is normally trimmed down.

Hope this helps.