Motor control problem

[chris414]

I have a relatively simple set up to control the speed of a 24V DC wheelchair motor:

- my microprocessor (PIC18F2550) is generating a PWM signal at 1Khz
- the PWM is fed to a high side driver (IR2113)
- the high side driver controls an H-bridge made with mosfets (SFP30N06)

The weird thing is that if I hook up my ammeter to measure the current through the motor / h-bridge, everything works perfectly. However as soon as I replace my ammeter with a standard fuse or just a piece of wire, the motor turns on for half a second and then turns off. The power to my microprocessor is still on, but the microprocessor seems to have shutdown or something since the PWM signal stops showing on my oscilloscope.

I have checked the PIC18F2550 datasheet and it says something about an autoshutdown on the PWM module for overcurrent detection, however I can't see how this applies since I have not enabled that feature nor is there any means for the microprocessor to know the current flowing through the motor.

Has anyone experienced a similar problem? Could the resistance of the ammeter have anything to do with the circuit working when the ammeter is connected?

 

[mbarazeen]

technically it should work when you replace the ammeter by a wire / fuse. do you hold the terminals touching your hand when you connect the meter? check your power supply to have good DC if not it may make the processor to malfunction and make it to shutdown if the feature is built in.

try a small roll of coil wound on a ferrite bed (choke) and a capacitor. connect the choke in series and the capacitor (accross)after the choke. it will give quit good ripple free source.

 

[crutschow]

Can you view one side of the motor voltage with an oscilloscope both with and without the ammeter to see if there's any difference?

It may be that the ammeter resistance is somehow stabilizing the circuit. You might try adding a small resistance approximately equal to the ammeter resistance (just measure the ammeter resistance with another multimeter).