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3 years 7 months ago - 3 years 7 months ago #29531982 by fnick2812
slow pwm output was created by fnick2812
Recently, I make a test to see how long it takes for Roboteq to output PWM from maximum value to minimum value. The experiment setup is as follow:
_using the SDC2160 roboteq to control the motor SDA18-263 which we bought from this website www.servocity.com/html/560_lbs__thrust_l...to.html#.Vx9DwjB95hE
_the oscilloscope is connected to Motor+ and ground to Motor-
_I configure roborun as it is indicated in the attached configuration file
_finally i go to the RUN tab, set the command at first to 1000 then -1000 and try to capture the PWM transition. You can see the screen capture of the transition attached with this post.

From what i observe, the roboteq takes more than 1 second to swing the motor pwm from 100% duty cycle Positive to 100% duty negative. It is a very long duration for motor controller. Do you think that if it is normal?
We want it drive a lot faster than that. What should we suppose to change in the software. Or is it the limitation of the board itself?

ps: sorry. i cannot attach the files so i put them on my google drive
drive.google.com/open?id=0B2u2v0w71fy_b3BXN0RhSzV4SXM
drive.google.com/open?id=0B2u2v0w71fy_b1dDQUg5b2Rtek0

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3 years 7 months ago #29531983 by blake
Replied by blake on topic slow pwm output
I'm confused as to where you are getting the 1 second from? Based on the image of the oscilloscope reading you attached there appears to be an almost instantaneous duty cycle change as there should be.

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3 years 7 months ago #29531984 by blake
Replied by blake on topic slow pwm output
After further investigation and testing of your configuration file I think I see the problem. First let me state that the duty cycle change of the controller is nearly instantaneous, in my testing I was able to get it to 5-10ms. However, that change is going to be affected by the controller's configuration that you set.

Whether you are in closed loop or open loop mode, the change is first affected by how you have your motor output max speed, acceleration and deceleration configured. For example in my specific test I had my controller in open loop and set the output max speed at 1000rpm, and accel/decel at 50000rpm/s respectively. This resulted in a duty cycle change at ~5ms.

When in closed loop not only is the above listed settings going to affect your duty cycle change but the PID gain settings will as well.

So to conclude to increase your duty cycle change time you need to alter your configuration until you achieve the results you want. There is no cut and dry answer to this as these settings will differ with every motor setup and will always require experimentation.
The following user(s) said Thank You: fnick2812

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3 years 7 months ago - 3 years 7 months ago #29531985 by fnick2812
Replied by fnick2812 on topic slow pwm output
That is interesting. I have never fully understood how the acceleration of roboteq works. I did the test again, still in OpenLoopMode, but with maxSp(10) and acceleration(50000). The results shows 256 ms. It is noted that the time i measure is from 100% positive voltage to 100% negative voltage, not just the instance of which the pwmOutput starts to change. You can see it is shown in the picture of the link below (duration between the 2 vertical cursor)
drive.google.com/open?id=0B2u2v0w71fy_OHhiV3dlZ1lsdUU

However, I expect it should be in the 10ms domain. Can we make it accelerates faster than that ? What other parameter i should try? (because it runs in Open Loop, so all the sensor feedback parameter, and PID parameter should be irrelevant already) ? I doubt whether it is the firmware limitation or the board model limitation. If it is the board model limitation, we can try to use the bigger model, say MDC2460, shouldn't we?

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3 years 7 months ago #29531986 by blake
Replied by blake on topic slow pwm output
I would recommend resetting your configuration to defaults and starting making the adjustments fresh. From the configuration file you sent yesterday I saw a lot of changes were made. Your max speed should be set to the RPM value that is compliant with your motor. You should find this on your motor's datasheet. If it is a geared motor you'll need to determine if the max RPM value given is for the output shaft or the input shaft. Then adjust the acceleration/deceleration until you reach the results you are looking for, both should be adjusted to the same value.

All of the details about these thing's I've mentioned are in our in our User Manual . Please refer to page 77 for info about accel/decel for example. Finally, we do not suggest that you alter the default PWM Frequency setting as this can result in inefficiency of the switching operation (see page 74 manual).

If your motor is capable you should not need a bigger model of controller. As I mentioned yesterday I was able to achieve <10ms switch time with the same controller. It will just take some tuning of your configuration parameters to achieve that. Such tuning is motor dependent.

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3 years 7 months ago #29532000 by fnick2812
Replied by fnick2812 on topic slow pwm output
I have tested both using smaller motor and using the mdcxxx controller. The slow PWM swing response, in my case, is affected by the motor inductance.

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