On Thu, 2008-06-05 at 20:54 -0700, Kirk Wallace wrote: > On Thu, 2008-06-05 at 21:19 -0500, Jon Elson wrote: > > Kirk Wallace wrote: > > > A while back I played with a KBIC speed controller which demonstrated > > > fairly good torque at high and low speeds. I was curious about how a PWM > > > amp would do, so I set up a Pico amp to a parport PWM signal at about > > > 25kHz and was surprised to find that low end torque was much less. I > > > thought the PWM frequency might have had something to do with the torque > > > issue, so I made a similar setup with a 50kHz PWM signal from a Pico PWM > > > controller and got similar results. Then I got to thinking, the Pico amp > > > is a PWM to voltage converter. The motor torque will be the torque for > > > the voltage that the motor happens to be driven with at the time. I > > > thought the KBIC had feed-back during the zero crossing period ,but I > > > didn't realize how important it is. So, in order for the PWM amp to have > > > good torque and speed control, feedback needs to be set up. Does this > > > seem correct? > > > > > Yes, you have it exactly. My PWM servo amps are truly PWM => > > voltage output, with no compensation for the motor resistance or > > back-EMF. You could, of course, use some kind of feedback. If > > you have a spindle encoder, you could make HAL read the encoder > > velocity and add a correction to the PWM output. > > > > Jon > > What are the issues with choosing a PWM frequency? I would think that > efficiency would be one, where you would balance switching losses and > iron or copper losses? Evidence of this might be seen as higher motor or > amp temperatures? Lower frequencies might produce noise. I believe the > Pico PWM amp has a 25kHz to 100 kHz range? What would happen with lower > or higher frequencies?
The torque roll off at low speed that you described earlier is not a product of the PWM frequency because the PWM signal is not commanding torque, it is commanding velocity. The PWM signal you are looking at is simply a percent of full speed. I'm guessing that the KBIC drive sensed something about motor performance, perhaps back EMF and increased the power it sent to the motor based on the load that was applied to the motor. This info should be available in the manual for that drive. Jon's drive does not do that. It simply puts out power proportional to the PWM sent to it. It's a simple velocity command, not a torque command. What PWM frequency does is limit the number of velocities that can be commanded. If you want nearly constant torque, regardless of speed, you'll have to sense something to do with the motor's performance and feed that signal back to the amp as the KBIC seems to or to the system producing the PWM as Jon suggested. We are comparing two different things here and changing the max frequency of the PWM will not help a bit. Feedback, and where it is applied, is the key that unlocks the magic of servo systems. Rayh ------------------------------------------------------------------------- Check out the new SourceForge.net Marketplace. It's the best place to buy or sell services for just about anything Open Source. http://sourceforge.net/services/buy/index.php _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
