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[DPRG] PWM Frequency and torque

Subject: [DPRG] PWM Frequency and torque
From: Vimal Shankar vimalsshankar at yahoo.com
Date: Mon Feb 26 21:42:55 CST 2007

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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