Quote:
Originally Posted by Al Skierkiewicz
16kHz is still audible to students and I can tell you from experience that 15,734 Hz is annoying to anyone under 50 and some over 50. I question the choice of switching frequency vs. the inductance of the motor windings though. Testing will confirm this.
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The
flyer (pdf) I read said 20kHz. Am I mistaken?
As to claims of linearity, that depends on what you mean. The output may actually be very linear, but the system need not respond in a linear way.
My experience in automotive systems is that when you go from low PWM freq (100-400Hz) to higher PWM freq (10-20kHz), the motors do not respond the same, especially at low duty cycles.
For example, if, in order to get a system moving, we needed say 20% duty cycle at 100Hz, we would need to bump the duty cycle up to 30-40% at 10kHz PWM freq.
This was very puzzling but we eventually attributed this our mechanical system's time constant. At 100Hz, the mechanical system's time constant was such that the system had time to react during a single PWM pulse -- These "full on" pulses provided a kick to get the motors and gears turning. At 10kHz, the system can only react to the average not the individual pulses, so it required more on time to get things rolling.
It does not say in the flyer but I hope that the H-bridge is configurable in ways that are feedback loop friendly. What I mean is that you can drive motors via an H bridge in several ways.
- ON = 12V, OFF = Open Circuit
- ON = 12V, OFF = MOTOR LEADS SHORTED
- ON = 12V, OFF = -12V
#2 is very nice for feedback control of large arms for example.
#3 sounds very strange at first, but it can provide some very nice control features as well (thought it can be tough on the electronics) in this case
- "OFF" for the motor is accomplished by outputting 50% 12V & 50% -12V.
- "50% Forward" is accomplished by outputting 75% 12V & 25% -12V.
- "25% Reverse" is accomplished by outputting 38% 12V & 42% -12V.
As I said, it seems strange but it has some very nice behaviors. This type of control is called locked anti-phase motor control and I discuss this issue at length
here
Once they implement CAN, I hope that they also consider some other control friendly strategies:
- using current messurements to allow for velocity (and even position) control without the use of encoders or other sensors. I discuss this here: What Dr Joe Wants for Christmas... (Man, I love referencing myself ;-).
- implementing PID control onboard the speed controller (send it your gains and from then on just give it a desired position -- let IT figure out what the PWM outputs should be)
- Diagnostics, diagnostics, diagnostics. At a minimum teams will know if a speed controller has become unplugged -- I guess that feature alone would have changed the outcome of about 20% of the FIRST tournaments ever held. But open your mind a bit and you can imagine diagnostics that tell you if your motor is overheating, if your mechanism is binding up, if your wheel treads need to be replaced, etc. etc. The mind reels at the possibilities.
It is only August, but it is looking more and more like Christmas to me...
Joe J.