low speed control
low speed control
(OP)
I am using AC brushless servo motor in my motion control system. From the motor's spec, the speed can goes from 0 to 4000RPM. I am wondering how low speed of the motor I can get in my system. If I want 0.01RPM for the motor, what pre-requisite is needed, resolution of encoder, mechanical accuracy, control tunning?
How high encoder resolution is needed at least for this low speed?
Any opinion is very appreciated.
Thanks
How high encoder resolution is needed at least for this low speed?
Any opinion is very appreciated.
Thanks





RE: low speed control
Do you have a application or just wondering?
Barry1961
RE: low speed control
RE: low speed control
Other limits must be considered. If the control has a 16 bit DAC (15 bit resolution +/-), the speed resolution for a 4000 RPM top speed would be 4000/2^15 = 0.122 RPM.
Tell us more about what you want to do and we can make better recomendations.
RE: low speed control
I am using a AC brushless servo motor to control pump's injection speed. Sometime we need it run at very low speed.
The motor we are using has resolver with 1 speed and 500ppr. The driver converts it to quadrature incremental encoder signal, so the controller can see 4096ppr.
sreid, which top speed are you refer to, from spec or real max speed in the application?
The motor's speed at bus voltage is 4000rpm, but use it up to 2000rpm. And the resolution of encoder feedback is 12bits on driver, which is Baldor SD26M and controller, which is NI's PCI7342. From your post, it is absolutely not possible to get 0.1rpm for my system. The limits is from the resolution of the encoder feedback. Am I right?
Do I describe my situation clear enough?
Thanks.
RE: low speed control
The above is not an absolute but points to the problems that will be encountered.
RE: low speed control
Do you really need to go all the way up to 4000 RPM? I guess not.
First; define your speed range - say 0,01 - 10 RPM (which I think is adequate in an injection pump application, correct me if I have misunderstood your requirements).
Second; select drive technology - I think that a stepper would suit you better than an expensive servo motor with encoder or resolver.
Third; calculate what gear-box you need between motor and pump. You will probably need to gear down something like 100:1, which implies a worm gear or a two-stage gear.
Four; enjoy sucessful operation.
RE: low speed control
Thanks for your points.
First, the stepper motor is not suitable for my application. The pump has speed control and pressure control function.
Actually, we used a 1:200 gear box between motor and pump. But sometime we still need to run the motor at about 0.1rpm. Highest speed is about 2000rpm.
RE: low speed control
RE: low speed control
If the gearbox is connected the other way around then the output of the gearbox at 0.1 motor RPM would be 0.0005 RPM which makes no sense to me but then again I don't know what your application is.
Your other option would be a larger gear ratio.
RE: low speed control
The gearbox is a speed reducer.
The PID loop time of the controller is 62.5us. If the lowest speed is 0.1rpm, what encoder resulotion should be used for this application.
I am trying to find a way from controller side if it is possible because the gearbox is there already and we have some space limit.
Now I am using 1 speed resolver and 1024ppr, 12bits resolution of encoder signal on amplifier. If I change the resolver to 4 speed and the amplifier with 4096ppr, 16bits resolution encoder, how about the cost?
Thanks.
Thanks.
RE: low speed control
Why you don't increase a resolver resolution by Baldor driver? Most of them have such opportunity - up to 4096 PPR maybe reached.
RE: low speed control
RE: low speed control