stepper rotates in reverse direction at start up

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Emb3_BP

Joined: 24 Jun 2009
Posts: 4

24 Jun 2009 13:59 reversing direction of stepper motor

I am using L297 and L298 combination to drive a 12v, 0.5 amp stepper motor.
At start up there is a small jerk in reverse direction say about one or 2 step and then the motor moves in the desired direction, this leads to position in accuracies.
This does not happen regularly, but intermittently.
What could be the source of the problem. I have attached my ckt pls take a look and let me know if there is any hardware problem.

Motor speed is very slow = 25 rpm without any motion profile. The clock is given by the µc (Rabbit3000)
Thanks in advance

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FvM

Joined: 22 Jan 2008
Posts: 5160
Helped: 767
Location: Bochum, Germany

24 Jun 2009 14:12 why step motor rotates reverse

Enabling the drive at an arbitrary motor position will result in a "jump" to the position according to initial state of the step counter. Because your circuit doesn't involve a reset circuit, the initial step counter state is also undefined. Thus the observed behaviour is basically normal operation.

I can't imagine however, how you want to achieve position accuracy, without any specified initialization technique.

Emb3_BP

Joined: 24 Jun 2009
Posts: 4

25 Jun 2009 13:38 stepper motor reverse direction

thanks a lot... resetting has resolved the start up problem that was being observed,
now the same problem (motor moves in reverse direction for 1 or 2 steps) is observed when the motor stops.

I am using the motor in half step mode (400 steps for 1 revolution).
Now I am resetting L297 every time i start the motor. At 25 rpm speed (without motion profile algorithm) the motor starts and stops as desired, i.e if i give 400 steps the motor starts and stops at the same location every time.
But at higher speed say at 70 rpm the motor halts with a jerk in the reverse direction.
The same is observed for slow speed of about 30 rpm, the only difference being that it does not occur every time.

1. Do i need to implement motion profile algorithm. (my speed requirement is 70 rpm max)

any suggestions would be greatly appreciated.

FvM

Joined: 22 Jan 2008
Posts: 5160
Helped: 767
Location: Bochum, Germany

25 Jun 2009 14:03 how to reverse stepping motor direction

How do you stop the motor? You should stop the clock and hold enable for some time. Generally, depending on the load, when releasing enable, the motor may move to a slightly different position. In some applications, it may be necessary to keep it powered, possibly at a lower current.

Irregular movement also happens when operating the motor above start/stopp frequency without a speed ramp. But I assume that you are still far below, as long as your application doesn't involve a high inertia moment.

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Emb3_BP

Joined: 24 Jun 2009
Posts: 4

25 Jun 2009 15:22 l297 stepper motor moves on enable disable

thanks a lot.. disabling the stepper motor (pulling the enable line low) after some delay has done the trick for me.

I have set the Vref at 1.1V, sense resistor is 2.2ohms (1/4 watt) for motor current of 0.5amp and supply of 15V to the motor, but when i measured the voltage across the sense resistor on the scope it showed me a max value of 1.8v which means more current is being pumped into the motor.
Motor resistance/phase is 16.48ohms

I tried reducing the Vref to 0.5V, but the Vsense was still 1.8V.
what could be the possible cause of the problem do i need to use a higher watt resistors, although the reisitors are not getting very hot when the motor is running.

FvM

Joined: 22 Jan 2008
Posts: 5160
Helped: 767
Location: Bochum, Germany

25 Jun 2009 22:58 stepper motor reverse leads

1.8V seems to indicate, that the PWM action is disbaled resulting in 100 % duty cycle, but I don't know why. It surely hasn't to do with resistor power rating. You can check the /INH respectively enable lines, they should reflect the PWM action normally.

Fortunately your motor seems to be able to bear full supply voltage and respective current.