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[DPRG] PWM Frequency and torque

Subject: [DPRG] PWM Frequency and torque
From: Chris Jang cjang at ix.netcom.com
Date: Mon Feb 26 23:29:02 CST 2007

Hello,

I think that there is something wrong here. (I am not an expert in this area.)

I have driven both 18 volt Panasonic gear motors and small Mabuchis at 2000 Hz without problems. They tended to stall out around 30% duty cycle. Note that the stall point exhibits hysteresis. It might take more to get the armature started. Why 2000 Hz? Just because it happened to work out given the clock/oscillator frequency on the microcontroller I was using. From what I read, this frequency was reasonable as something in the low kilohertz range. I wasn't too concerned with optimal PWM, just something that worked and was stable.

Everything I've read advocates much higher PWM frequencies than 25 Hz!

My guess is that one or both of the duty cycle or voltage is decreasing with PWM frequency. That could be a firmware bug from the microcontroller or a limitation of the H-bridge circuit. I've always used large discretes for driving motors. So the transistors have a comfortable safety margin over what the motors draw.

If you have an oscilloscope, I'd use that to see what the PWM signal really looks like. Check the microcontroller output. Then check the voltage across the motor terminals. If the motor is so big that it scares you to connect the scope probes to it (flyback voltage), then get a small Mabuchi or similar tiny motor and drive this. It should be obvious what is going on.

Chris


-----Original Message-----
>From: Vimal Shankar <vimalsshankar at yahoo.com>
>Sent: Feb 26, 2007 10:42 PM
>To: dprglist at dprg.org
>Subject: [DPRG] PWM Frequency and torque
>
>Ok
>
>Here's the deal. All that email made my head spin and
>I decided to do what always worked out best for me.
>
>I turned to my home lab.
>
>I sat down with my HBridge (based on L298N), a
>function generator and connected the motors.
>
>I measured the torque of the motors using my hands.
>
>The motors were connected to 5cm diameter wheels with
>rubber grip.
>
>The motors seemed to give the highest torque at 25Hz,
>but when I made it stall, I could literally feel the
>motor pulsing...
>
>As I increased the freq, I felt gradual decrease in
>torque and after 250Hz the motors torque was
>decreasing way below the torque which I wanted my bot
>to run over my carpet floor.
>
>The funniest thing is - at 2Khz and more, my motors
>became nice loudspeakers.
>
>Got distracted and programmed a PIC to play "London
>Bridge is falling down". Used 2khz to 10Kz to do that.
>(Why do people use peizo buzzers when they have motors
>on their bot ? Maybe the amplitude is lower than the
>buzzers ?)
>
>So I decided to run my motors at 250HZ...
>
>Or should I try 25Hz ?
>
>Now the probelm is figuring the best way to make my
>20Mhz PIC16F877A give out 200Hz pulses at 10% to 98% %
>duty cycle...
>
>Timer ? CCP ? Bit Bang ?
>
>But seriously - anyone here ran their DC Geared motors
>at that low a freq ?
>
>PS : Locked antiphase is something I would not worry
>about now. It requires good HBridges (instantaneous
>swithicng is important). Plus, the software has to
>keep track of the direction and adjust the phase
>accordingly. If I used signed magnitude, I get double
>the resolution, simple driving logic...
>BTW - what is "sign-magnitude, synchronous
>rectification". I googled. I got phd papers :(
>
>
> 
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