lower speed control
lower speed control
(OP)
I am fine tune my motion control system right now and I encountered some issue which frustrated me.
I fine tune my system at 9,000psi (which is in 2 in" diameter area). At high speed, it worked fine; but at low speed, I always got overload error and current jump up and down untill reached the amplifier's peak current(15A).
Is it because the fine-tuning is not very good or because the amplifier's peak current is not high enough (my motor's peak current is 31A)? If it is because the peak current, how come it draw such big current at low speed?
Any opinion is very welcome.
Thanks,
Wendy
I fine tune my system at 9,000psi (which is in 2 in" diameter area). At high speed, it worked fine; but at low speed, I always got overload error and current jump up and down untill reached the amplifier's peak current(15A).
Is it because the fine-tuning is not very good or because the amplifier's peak current is not high enough (my motor's peak current is 31A)? If it is because the peak current, how come it draw such big current at low speed?
Any opinion is very welcome.
Thanks,
Wendy





RE: lower speed control
I am fine tune my motion control system right now and I encountered some issue which frustrated me.
I fine tune my system at 9,000psi (which is in 2 in" diameter area).
///Please, would you elaborate\\\
At high speed, it worked fine; but at low speed, I always got overload error and current jump up and down untill reached the amplifier's peak current(15A).
///Apparently, the motor shaft load increases with the decreasing RPM.\\\
Is it because the fine-tuning is not very good or because the amplifier's peak current is not high enough (my motor's peak current is 31A)?
///If the motor current increases with the RPM decreasing and the motor is rated for the RPM decrease, then the amplifier is not adequately rated.\\\
If it is because the peak current, how come it draw such big current at low speed?
///If the motor shaft load increases, then the motor draws the higher current.\\\
Any opinion is very welcome.
Thanks,
Wendy
RE: lower speed control
Thank you very much for your reply.
Actually, the load on the shaft is constant. I fine tune the system under constant load. I don't understand how come such big change in motor current. I am wondering what it means of "motor is rated for the RPM decrease"
Thanks again.
Wendy
RE: lower speed control
What kind of feedback is being used? If the feedback resolution is not high enough it can cause system instability at low speeds.
RE: lower speed control
Actually, the load on the shaft is constant. I fine tune the system under constant load. I don't understand how come such big change in motor current.
///The constant torque under lower speed causes the motor current increase, if the voltage is used to decrease the motor speed. The constant torque usually implies constant HPs\\\
I am wondering what it means of "motor is rated for the RPM decrease"
///There are various NEMA designs of squirrel-cage induction motors, namely, A,B,C,D, and E. D characteristics has torque increasing with decreasing speed. \\\
RE: lower speed control
Hi, EDanzer. Thank you very much for your helpful point.
I guess that probably the instability of my system is caused by low resolution. Now I have 1024counts/rev quadrature encoder, so the controller see it is 4096 counts/rev. And the looptime is 2ms. If the speed is 1rpm, for one loop, encoder counts changing is (4096/60)/500=0.136count/loop. So the controller can see the movement after sevral loops. What do you think about it? Is the resolution too low to use for my system?
To jbartos,
The torque of my motor just decrease a little bit with speed increasing. From the torque and speed curve, it can be seen the torque is almost constant regardless of the speed of the motor . Thanks
RE: lower speed control
What make of control, servo amp and servo motor are you using?
It is also possible that the other parts of the system are causing or amplifying the instability. Excessive clearance in any of the components can cause this, use a dial indicator on the positioned part and a bar to move the system with the motor shaft locked to check for clearance in the drive parts. If the slide way has excessive clearance or is tight you can get slip-stick at low movement speeds that increase motor torque requirements.
RE: lower speed control
RE: lower speed control
Have you looked at all the parameters for tuning the drive?
RE: lower speed control
I tuned the controller's parameter, including Proportional gain, velocity feedback gain, velocity feedforward gain, integral gain and integral range. I got amplifier's parameter from auto tune, including current proportional gain, current integral gain, speed proportional gain and speed integral gain.
Maybe it is tuning problem. But the things are after the fine tuning, if the motor run at short time, it is ok, the current jump up and down, but it is still whthin continuous range; if the motor run for several minites, the current keep growing untill overload. I am wondering if it is because the mechanisim of the system.
It is really frustated.
Thanks
RE: lower speed control
RE: lower speed control
RE: lower speed control
RE: lower speed control
RE: lower speed control
RE: lower speed control
it is closed-loop control. there are velocity feedback, velocity feedforward and intergration in the controller. And current loop and speed loop in the amplifier. I am wondering how come the current increasing seems time related.
RE: lower speed control
You may want to look, with an oscilliscope, at the voltage waveform out of the amplifier. Since the energy used on the load isn't changing, but the supplied energy is increasing, I would be suspicious of a high frequency hidden oscillation being the cause. Digital controls are commonly afflicted with this sort of problem. Good luck.
Doug Ford
RE: lower speed control
See regular undergraduate textbooks on controls.
Nothing or Differentiator (D element or filter) might cause some fuzziness in variable behavior.
Visit
http://cmosedu.com/cmos2/Figs_35.pdf
etc. for more info
RE: lower speed control
I run the system without load, it worked fine. So Is it still tuning problem?
Thanks.
RE: lower speed control
RE: lower speed control
RE: lower speed control
You will need to recalculate the load reflected to the motor including inertia of all components including the motor armature to determine actual starting torque for the motor at the programmed acceleration rate.
It may be possible to reduce the acceleration rate to make the existing motor work.
RE: lower speed control
RE: lower speed control
Have you had any luck with your problem?
RE: lower speed control
One thing has been found which is mechanical wear in the harmonic drive. Some parts were worn and there are some trash in it.
I hope it would be ok after this problem is fixed.
Thank you very much for your kindly help.
Thanks everyone.
RE: lower speed control
Barry1961
RE: lower speed control