Motion control resources

Software & Algorithms. Motion control, machine learning, planning, filtering, drivers, health monitors, etc..
MC_Newbie
Posts: 3
Joined: Thu Jan 12, 2017 6:40 pm

Motion control resources

Postby MC_Newbie » Thu Jan 12, 2017 6:53 pm

Hello,
I have been working with motion controller and analog servo drives lately until i discover Elmo, their powerful products, Ethercat and all what goes with.
I would like to improve my knowledge about motion control so I'm looking for any recommendation for good websites, technical notes, book, etc...
Of course this website is one of them :)
For example, I still don't understand exactly what bandwidth represent? how is it related to speed?
By discovering Elmo Application Studio, I realize that i need some knowledge like how to interpret bode plots?
So let's share :p
Thank you.

RobotsforRoboticists
Posts: 9
Joined: Fri Jul 08, 2016 6:55 pm

Re: Motion control resources

Postby RobotsforRoboticists » Fri Jan 13, 2017 3:15 am

Hi
I dont really know of many good resources for this. The Galil learning center is pretty good. There are a bunch of online videos http://www.galilmc.com/learn

When people talk about controls I think of it as two levels:
1. Spinning a motor. Basic position control, velocity control, human tuned PID loops etc..
2. Proper control theory for optimizing a controller. Things more advanced than a PID loop. Such as looking at frequency response based on loading. Generating Bode plots, root locus, looking at poles, complex math, etc..

I am working on a post going into detail on the first level. In practice the second level is utilized a lot less. For the second part there are some control system classes online and is what is in a typical undergrad control systems class.

For your specific question about bandwidth and speed. The bandwidth is just the operating speed range of the motor. For example if a motor can spin at 200RPM in either direction. The speed bandwidth can be specified as +/-200RPM. For motors most people just say speed. If you are talking about a filter in the controller, than bandwidth refers to the part of signal that is being passed through to be used.

Hope that helps.

MC_Newbie
Posts: 3
Joined: Thu Jan 12, 2017 6:40 pm

Re: Motion control resources

Postby MC_Newbie » Thu Jan 19, 2017 3:32 pm

Hi

Thank you for your reply,
I didn't know about the Galil learning center, i started watching some videos, it's quite interesting.
Are you familiar with Elmo products?
If you look at the image attached, what can you tell about the system?
it says the velocity bandwidth is 118.9 Hz, does it mean high frequencies won't follow correctly?
How is the frequency related to speed? i mean how can i identify the speed (in RPM or count/sec) at which i won't get good results...
and the curves at the end, is that some kind of resonance?
I know about bode diagrams but not as advanced or with a practical example like tuning a motor
If you have any book or online resources about control theory, please let me know.

Thank you in advance for your help.
Attachments
Elmo_BodePlot.JPG
Velocity closed loop
Elmo_BodePlot.JPG (208.2 KiB) Viewed 353 times

RobotsforRoboticists
Posts: 9
Joined: Fri Jul 08, 2016 6:55 pm

Re: Motion control resources

Postby RobotsforRoboticists » Mon Jan 23, 2017 7:56 pm

Hi
I have used a bunch of Elmo controllers and in general I am happy with them. However I still use the original Application Studio and have not had a chance to work with the new Application Studio II.

In this context my prior response is not what you were looking for.

The frequency of a system can be hard to visulaize by just watching the motors motion. The frequency of the system is when you convert the time domain signal (ie. you look at the motors motion over time), and convert it into the frequency domain using a Fourier transform. Once you convert the signal into the frequency domain we can use Bode plots.

In general the bode diagram is showing the phase and gain (magnitude) from your input control signal until it reaches the output (command) in the frequency domain.
- Gain is the shift in value between the input signal and resultant command. If you had a system with no gain and no loses, then you would have a straight horizontal line with 0dB of gain. You will usually see the gain drop off (decrease) on the right side of the magnitude plot.
The larger the frequency until the drop shows a system that will be stable in more system conditions. Random bumps and curves in the main curve are often signs of instability.

- Bandwidth is the area from the top of the curve until the magnitude degrades by -3dB. So in your case it is around 100Hz (actually 118.9Hz based on the box in the lower right of your image). So the bandwidth of your system is around 100Hz. Controls past 100Hz will by sluggish or unresponsive.

- Phase describes the time shift between when the input signal and the output command. A phase of 360 is for a single cycle. So if you have a 1KHz command signal each 360 degrees in the chart would represent 1/1000 of a second (converting frequency to time) or 1milisecond. You can see in your phase diagram that once you exceed the bandwidth it will take longer for the desired input signal to be transferred to the output signal.

In the image I also see you have a low pass filter applied. Filtering (low-pass, notch, etc..) are often done to remove a resonance. So if you spin a motor and you see at a specific frequency things start to shake violently, you can add a filter so that gain is reduced at that frequency. In general you should tune a motor with no filters, and only add the filters after.

You can verify that you selected good gains by looking at the output waveform when a step command is applied (in the time domain); this is sometimes referred to as oscilloscope mode. If there is a lot of initial ringing (constant changing) or overshoot in the beginning of motion, your gains are probably to high. If the initial command is slowly reaching the desired output you might need to increase your gains. http://robotsforroboticists.com/pid-control/

This was a good question. I will probably turn it into a post.

RobotsforRoboticists
Posts: 9
Joined: Fri Jul 08, 2016 6:55 pm

Re: Motion control resources

Postby RobotsforRoboticists » Thu Jan 26, 2017 2:40 pm


MC_Newbie
Posts: 3
Joined: Thu Jan 12, 2017 6:40 pm

Re: Motion control resources

Postby MC_Newbie » Thu Feb 02, 2017 9:50 am

Hello,

Thx a lot for you post and your answer.
So from what I understood, the goal is to achieve higher bandwidth but by keeping the system stable (Gain Margin and Phase Margin).
When you say Controls past Bandwidth will be sluggish or unresponsive, in a practical way what does it mean? does it mean there will be a speed if you past it the response will be sluggish...?


- Another question relative to the picture below : What information can we get from a Velocity Plant? does it tell the maximum bandwidth of the system?
also in the pic, the green plant has a phase starting from -358 degree...what does it mean?

Thank you in advance for your answer, and I hope those questions would be helpful for other people.

Regards,
Attachments
Velocity_Plant.JPG
velocity Plant, weird phase
Velocity_Plant.JPG (185.25 KiB) Viewed 310 times


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