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View Full Version : overdriving stepper...doubt!



shreek1123581321
11-24-2007, 01:55 AM
i used to drive steppers using l298, giving the op directly to the motor....it worked fine(....probably, the machine used to move,, but i always complained about high current and less torque)

but now i was reading some articles and saw ppl talking about overdriving, ie at high voltage controlling the current. some guy got an hbridge which chops current. some other guy uses resistor to control the current.

i am not clear with the situation.

my motors are from junk, so i dunno the specs, and i am driving them at 12v.is it harmful if the motors have specs of 5v? does it affect the performance of motor?

and what exactly does this chopping of current does? as in if we are giving 12v to the terminals of the motor, no matter from what hbridge, current is fixed, as the resistance is fixed...isnt that so?
and if we use a resistor, then is it not equivalent to just supplying 5v to the motor?

reference:
http://micromouse.cannock.ac.uk/motors/stepper_driving.htm

docel
11-24-2007, 08:47 AM
Hi Shreek!!!

Nice question. Its is also debated on in the Forum earlier.

1. Any DC motor will exhibit more torque at switch-on time. The torque is max then and reduces at rated maximum RPM.
This means Lo RPM high torque, Hi RPM Lo torque situation.
This is @ constant DC.

2. Now, if you switch the motor on for some time and during the high torque period switch it off .....and on again, two things will happen: the torque will be more and the RPM will be less. If the switching is continuous, you get a motor running at reduced speed and increased torque!!!
This is " Chopping" the DC current at regular intervals. This is nothing but PWM or Pulse Width Modulation , in the simpler sense.

3. Steppers are notorious for their Lo-Torque. They are just positional motors and NOT load bearing motors. They cannot carry their own weight.....

This can be improved by " chopping" the DC current , as with the DC motor to increase torque.

4. Since the chopping reduces motor RPM, a higher DC voltage is called for to get the "Maximum" out of the stepper. The maximum means, also, greater current than the specified ratings. This is OK since it is 'intermittent' duty and not 'continuous duty' cycle.
A 25% increase in the supply voltage is safe enough, and should be limited by the heat of the motor.

5. In a L297 driving a L298, you have this built in advantage. The other shortcut is to PWM the "enable" pin of the L29X device by a simple 555 oscillator or any other multivibrator.

But......Why Steppers :roll: :roll: :?:

shreek1123581321
11-24-2007, 09:02 PM
So it is the same funda as PWM in dc motors....

i am using stepper because i want to know the distance travelled by the machine...micromouse...

i am not using l297, i am directly giving steps from atmega to l298, and also in my circuit i have permanently enabled the l298... but i guess by writing 1 and 0 at high frequency at the inputs i can get pwm effect and should work fine...

shreek1123581321
11-24-2007, 09:23 PM
but what are the disadvantages of giving continuous 12v, other than high current drain? as in, in dc motors the torque goes on decresing, but in stepper the torque will remain the same, because the magnetic field effect stays the same...

docel
11-25-2007, 12:21 AM
1. Torque cannot be the same. Torque is dependent on the field, which is dependent on the current. Both of these should change when you switch the coils. If the switching is faster, then the Torque reduces due to insufficient increase in current and the subsequent field.

2. PWM-Switch the enable pin.

shreek1123581321
11-25-2007, 12:29 AM
for pwm, cant i give sequence like,

0001,0000,0001,0000,0001,...and so on at high frequency, say 20khz?

it will produce a pwm effect on the last coil, wont it?

...i have permanently enabled the enable pin in my printed pcb, you see, cant change it now...

xrayd
03-22-2008, 11:56 PM
Sorry to revive such an old thread but wanted to ask this... Where can i get DC geared motors small enough for a micromouse? I tried searching at lamington road(servo electroncs) but he didnt have them... any motors with inbuilt encoders would be awesome... is that hoping for too much???

-Sinan

rohan_bhatia
03-23-2008, 11:29 AM
hey shrek i have experienced same problem as yours. first thing u need to know is the motor rating, start by measuring the resistance/ phase of the stepper then if your motor is 12v it should ideally draw 12/resistance current. but to be precise if u have a battery supply, then measure current using that(battery can source almost any amount of current, the more u draw, the quicker they discharge). Now once u no the rating , u can decide on what current-voltage u want to give depending upon torque required.
Generally a 12v stepper runs good at 600mA to 1A /phase.
* also keep in mind that ur driver circuit is able to handle that much amount of current, i burned a lot of my uln2003 coz my stepper was sinking in 1 A of current.