High frequency chopping/pwm locks up servo. Why?

[cncorbust.com]

I'm playing around with a dc motor and a microcontroller.

I find that high-frequency chopping locks up the motor, ie, it does not move at all when I control the speed of a dc motor by modulating its power supply connection. This is despite the fact that the on/off ratio remains constant.

Why ?

I would have thought that a higher chopping frequency would improve smoothness, but no...

Thanks for helping

Thomas
cncorbust.com


The Setup:

Atmel microcontroller >> ULN2003A >> IRFZ44N mosfet (pdf) >> Motor

Power Supply is a 12V 8A PC PSU.
I use a dismantled battery power drill dc motor (12 Volt).

At a 10% on, 90% off ratio, the motor moves ok if the chopping frequency is set to, say, 1ms on, 9ms off, but it locks up when I set chopping to 0.01ms on, 0.09ms off. All I hear is a high-pitched sound coming from the motor.

One guess is that it may relate to the PSU being stabilised, or that I may not fully saturate the mosfet in its on-state, though it remains nice and cool.

[Pandinus]

I think you might be overlooking the obvious... motors are inductive, are you using any kind of kind of capacitive voltage stabilization ???

[cncorbust.com]

Pandinus: I use a regulated power supply (old PC power supply). I did not add any capacitors, if that's what you mean, assuming that the psu takes care of changing current demand. I may be completely wrong, as I'm new to this.

[Pandinus]

Electronics 101, an inductor is a low pass filter... and stores energy by way of a magnetic field. The PWM Chopper you are creating is an extremely high frequency wave form... lots of math but a square wave is just a bunch of sine waves added up with their harmonic frequencies approaching infinity. that is why the motor is locking up when you increase the frequency of the chopper, without a capacitive element to the motor supply the magnetic field that would turn the motor does not have enough time to build before it collapses again. Essentially you are trying to drive the DC motor with an AC signal source of sorts. If you add a capacitive element to the equation it will store some of the energy and the motor will see more of a DC supply. That is how most switching power supplies work... turn on the current until the output voltage rises to a preset level, turn of the current until it falls to low and repeat. They use an inductor and a capacitor to store/convert the high frequency energy to a more stable waveform(DC) for the load to use as it is needed.

[Pandinus]

Just thought to add the reason its working at a lower frequency is the motor has enough "on time" to start turning and store some energy in kinematic form and carry it through the "off time."


They used to simulate physical systems with analogue electronics this way... a capacitor could store energy like the inertia of an object, a resistor would change how long it takes to charge the cap (just like a smaller physical force would take longer to get the object moving) a drain resistor to ground would discharge the cap... friction slowing the object down... ETC...