Reduction Gearing & Backlash

[UKRobotics]

My research so far indicates that most stepper motors can be connected directly to the ball screw but servo motors need some sort of reduction gearing to make them useable.

what is the best way to approach this ?

Gears ? Belts? or some other method ? - I would have thought gears would add signigicant ammounts of backlash to the system, but the small toothed belts such as the the ones available from RS components wouldnt have the strength to transfer the ammount of torque my motors are capable of delivering.


What does everyone else do ?



Dominic

http://www.ukrobotics.com

[HuFlungDung]

Hi Dominic,

I don't think your conclusion about servos needing to drive through a reduction is correct, not torquewise anyways.

If you must use a gear reduction, there can be a problem with backlash if the encoders are mounted directly on the screw, while the servo acts through the gear train. What happens is that it becomes difficult to set the PID parameters for the servo because the feedback loop has to deal with this backlash. The motor needs to continually shift back and forth across the backlash amount in order to move the encoder to test the position. The motor will do this, but it is a form of oscillation and makes it quite difficult to find PID settings to give good performance, because the settling time is lengthy.

The best way around this is to simply make sure the encoder is mounted directly on the motor, then it will operate properly. However, the actual position of the mechanism becomes suspect. So for applications that really require the best of both worlds, you can use scale (or encoder) feedback off both the motor and the screw. I don't know much about how that is set up, but I know it is possible to do.

If your machine happens to be constructed in such a way that there is a constant load on the backlash affected axis in one direction, then the backlash can simply be factored into the controller software as a parameter to take into account upon every reversal of that axis. However, the mechanical load will take care of the motor "settling to position" problem. Then you could safely apply the encoder directly to the screw and not have a PID nightmare with the servo running through the gear train.

[NeoMoses]

Gear/Belt reduction is not always necessary. It usually is done to obtain reasonable rapid traverse speed. For example, let's say I happen to have motors that can spin at 5000 RPM. Running these direct drive on a 10 TPI leadscrew would give me 500 IPM as my maximum speed. For most of us, that would be nice, but is a bit excessive. Something like 100 IPM would be nice, because it would allow greater torque on the drive screw, thus greater cutting forces. In this case, going with a 5:1 reduction would get you to the 100 IPM rapid speed.

However, had you thought about this in the beginning, and bought 1000 RPM motors, they could be direct drive and achieve the same thing.

Another thing to think about when choosing drive systems is whether or not this machine will be used in manual mode. I chose the belt drive option so that I could still attach handles and use the machine in manual mode quickly and easily. Since you're designing it, it's all up to you. Good luck!