unipolar steppers

[samarsingla]

hi
i have unipolar stepper motors from bigger floppy drives. they are rated at 12 volts and 200 ma. can i get higher torque from them in some way, other than increasing voltage? i mean is chopping possible? what exactly does it do?
thanx

 

[Nigel Goodwin]

hi
i have unipolar stepper motors from bigger floppy drives. they are rated at 12 volts and 200 ma. can i get higher torque from them in some way, other than increasing voltage? i mean is chopping possible? what exactly does it do?
thanx

The easiest way is to feed them off a higher voltage via wirewound current limiting resistors, this increases both speed and torque.

To avoid the resistors, but make it more complicated, you can use PWM to restrict the current rather than the resistors - the 'chopping' you mentioned.

 

[samarsingla]

how will resistors help? wouldnt the voltage difference across the motor be same?
thanx for your reply anyways.

 

[Nigel Goodwin]

how will resistors help? wouldnt the voltage difference across the motor be same?
thanx for your reply anyways.

Try searching for "stepper motor tutorial", you will find plenty of info about it - but essentially the higher voltage allows the full current to be reached MUCH faster.

 

[philba]

Chopping is a fairly straightforward concept. It works by sensing the current through the coils and turning off (chopping) the voltage when the current rises past a preset limit. You can construct your own with comparators and sense resistors. There are a number chopping unipolar stepper ICs out there. Allegro has several.

Using current limiting resistors can get expensive though a 200 mA stepper probably means something in the 2W range. Not super pricey. a cheap 10% 2 Watt will run about a dollar. You will need 2 and will have to calculate the value based on the coil resistance of your stepper and the driving voltage. If your motor was in the the 1A range, I'd say get a chopper as the resistors will cost more than the chopping IC.

 

[samarsingla]

my steppers are 160 mA per phase..
what are sense resistors? i could not get this part... please elaborate on that.
actually i am using batteries(12 ni metal hydride batteries) as my power source. the motors i am using are from the older floppy drives. the motors are already 12 volt(and they apply very meager torque at this voltage) and applying 15 volt to them wont make much difference. should i step up this voltage in some way? but that would lead to power wastage too.. please ask me more details if you need any but suggest me some solution that i could implement. i am tinkering myself too but maybe you all could tell something better. thanx for your quick responses till now.
thanx

 

[philba]

I think you are trying to get blood out of a stone. Those floppy motors are pretty weak. I'd say get a stronger motor. maybe a NEMA17 class motor.

a sense resistor is a very low value resistor. In series between ground and the load, it will develop a voltage drop that is proportional to the current. For example a 1 Ohm resistor with 1A through it will develop 1V (ohms law). 500mA through it will develop .5V and so on. You can use that to feed a comparator or an ADC.

 

[samarsingla]

any suggestions from where i can get some good motors, like some old equipment or something, we dont get gud motors here and have to import them online, which is quite a costl affair...

 

[Nigel Goodwin]

You don't give any clue as to what you're trying to do with them?, so how can we make suggestions?. As I said before though, you need to feed them from a much higher voltage (48V would be reasonable) with current limiting.

 

[samarsingla]

i am trying to build a micromouse. so i dont have much option of using power source of higher voltage..

 

[Nigel Goodwin]

i am trying to build a micromouse. so i dont have much option of using power source of higher voltage..

Don't use steppers then! - or use low voltage steppers, where 12V is high enough for decent current limiting.

I thought MicroMouse was essentially a race?, using steppers is the MicroMouse equivalent of starting on crutches for a marathon!.

 

[samarsingla]

any suggestions from where i can get some good motors, like some old equipment or something, we dont get gud motors here and have to import them online, which is quite a costly affair...

 

[Nigel Goodwin]

any suggestions from where i can get some good motors, like some old equipment or something, we dont get gud motors here and have to import them online, which is quite a costly affair...

5-1/4 inch floppy drives, printers, photo-copiers - those are usually the most useful.

 

[samarsingla]

but i already have them from 5-1/4 inch floppy drives. they are not powerful enough...

 

[samarsingla]

can you give me some indications on how to spot good steppers, as i might have to buy them from some repaire shop and checking wont be an option...

 

[Nigel Goodwin]

but i already have them from 5-1/4 inch floppy drives. they are not powerful enough...

Steppers aren't powerful, you NEED to feed them off a much higher voltage as already suggested - you could always use an inverter to genrate the higher voltage?, but it's adding more complexity.

For a start I would suggest trying adding a couple of series resistors and an external 48V supply for the steppers, and see if that gives you the extra power you need?.

There are hundreds of MicroMouse sites about, have you tried checking what they use?.

 

[agent420]

In my 1st venture into stepper-land, I too was initially confused with voltage ratings. It seems that no matter what voltage a stepper may be rated for, in almost every case they are run off a much higher supply to increase response speed and torque, but it is the current parameter that is most meaningful and the one that is controlled.

The sense resistors are simply very low value power resistors that are in series with the motor (usually at the connection right before ground) that allow measurement of the current flowing in the motor by measuring the small voltage difference that is developed between the resistor terminals. This sense voltage is usually fed back to the control device (like an L297) that uses the information to adjust it's pwm signal, thus controling the motor current.

fwiw you can get some small inexpensive steppers at places like Jameco, complete with documention.

Also, bipolar, though slightly more complex control hardware, is the preferred type because they produce torque more efficiently.

 

[samarsingla]

ok, i should drive them at high voltage... but can you tell me some regulator that can switch up from 15 volt to say 24 volts... how much power will it consume?

 

[samarsingla]

also can anyone please tell what vref on pin 15 does in l297. what does its value determine?

 

[Nigel Goodwin]

ok, i should drive them at high voltage... but can you tell me some regulator that can switch up from 15 volt to say 24 volts... how much power will it consume?

Do the maths, it's simple enough - assume 80% efficiency for your conversion (assuming you're doing it pretty well) - but aim for 48V rather than 24V.

But like I said, confirm it works OK first, by using an external supply. No point building a converter just to find it's still too weak!.