stepper driver misbehaviour

[killabyte]

hi guys, recently ive been dealing with l298 stepper driver and using a l297 to control it through a 8 bits MCU in order to develop a small movement control system. the motor consumption is 1,2 A. power supply is 12vdc

i found two problems when testing:

1. when inserted an amperimeter (multimeter) to check the current, the stepper motor delivered more torque than if i dont use the mmeter to make the measurement.

i supposed that was because bad filtering at the supply. then i filtered using a choke and capacitors. i got the same result. unfortunately at this moment i dont have a scope to view the signal. i think it is not the supply. maybe the amperimeter is generating spikes but im not sure.

2. the l298 rises the current consumption but its temperature is just warm; it rises about 0,1 A per second. i always turn off circuit when near to 2 A sems to be a unnatural runaway. the L298 is not defective, nor the motor.

any tips?

regard

killa

 

[cbarn24050]

There isn't much that can go wrong with this pair of devices, try increasing the current sence resistors to 1 ohm, make sure your ammeter is in the vcc line (pin4) on the 298. Are you sure your getting more tourque or does it just sound like it? You really need a scope if you going to build stepper drives.

 

[killabyte]

im totally sure there is more torque, the ammeter is well connected.

regards

killa

 

[cbarn24050]

How do you know?

 

[killabyte]

just by trying to stop it with fingers and using the multimeter the motor dont miss a step.

when i dont use the amperimeter and try to stop motor it losses steps.

the thing is that i can see that im not getting plenty of power from the l298. im trying to get the best power allowable.

anyway ill keep checking with a well filtered supply.

regards.

killa

 

[zappedagain]

How long are your wires to the motor? You might have a lot of inductance that is disturbing the pulses to the motor and causing you to lose torque. Putting the ammeter in series breaks up the inductance (and adds a bit of series resistance) and may give you better performance. Like cbarn said, you'll only see this with an oscilloscope. You can try adding some capacitors across each phase AT the motor to see if that helps; a nice quiet signal at the driver doesn't help if inductance is causing lots of ringing at the motor end of the wire. Some series resistance at the driver might help too.

 

[cbarn24050]

Are you getting correct operation at standstill? You should find it very hard to move the motor with your fingers on the shaft at 1amp. Step it 1 step and check you still have correct operation. Once you get past the correct standstill torque you can step it slowly, less than 100 steps/sec, and check you still have full torque.

 

[killabyte]

zappedagain may be correct, ill add some series resistance and put a snubber.

cbarn: at 1A it is good torque but it depends on the mode used to drve the motor, the 2 phase mode gives the highest torque but other modes provides weak power and the shaft is easily stopped with fingers.

now im creating the ramp to accelerate and decelerate the motor, but i can see 2 possibilities:

1. controlling the frequency in the clock

2.controlling the vref in l297 which controls the current delivered to the wires.

with pwm from my mcu i can easily create a dc ramp to accelerate the motor. but not sure if it is correct.orif should be in the clock.

regards


killabyte