i have already posted one message before but to no reply...please help me out...
i m rotating a 4-phase 12 volt stepper motor using the printer port of the PC and ULN2003 driver..but i want to increase the torque so that it can drive a wheel..
what should i do? will changing the mode to half step mode increase the torque??

please reply

 

First, I assume you actually mean "2-phase", which means four drive transistors in unipolar mode. Either one or both phases may be energized, with half-step being one phase on, two phases on, one phase on, etc in alternation. A "standard" stepper is 2-phase, 50-tooth, moving 1.8 degrees per full step, 200 full steps per revolution.

You have not stated if your issue is one of low-speed or high-speed torque. Generally, steppers have a fairly constant torque response in the low-speed realm outside of resonant frequencies (that's another issue entirely). Manufacturers typically will present their data showing full-step mode, although full-step is generally not a very desirable way to operate a motor. Full-step with both phases on gives the best torque for a given type of drive, but it is sensitive to resonance and quite "clunky." You aren't going to get more torque using half-step mode, but half-step is preferable for getting smoother operation, though microstepping is even better. These two strategies also help avoid resonance-induced stalling due to their varying torque.

There are a couple of ways to get more low-speed torque outside of changing motors entirely. Of course, you can use a gearhead or pulley system to gain mechanical advantage, at the cost, of course, of requiring greater motor speed. This is eventually self-defeating because steppers are less efficient in the high-speed realm. You can get a 1.414 factor increase in torque by using bipolar drive, but the ULN2003 isn't going to cut it. Outside of going to a manufacturer-supplied drive unit, you will need to implement an H-bridge and be aware of certain pitfalls. If your motor has eight leads, it can be driven in parallel bipolar mode, which gives you the lowest inductance and best high-speed performance. If it has six leads, you are restricted to series bipolar, which actually has poorer high-speed performance (though better torque) than your present unipolar drive. If it has five leads, forget it -- you are restricted to your present unipolar drive.

If all this is new to you, check out Oriental Motors' web site (no I am not an employee or stock owner, just have used one of their motors). They give a good comparison between the various types of motor drive capabilities and the differences in voltage, current, torque, etc. Compumotor and Superior Electric (a division of Danaher Motion) also have info (I have used their motors also).

Good luck to you.

 

Hi,
I am trying a "one-phase" on scheme with a standard 2 phase 8 lead 1.8 step angle motor. I am successful in achieving correct operation in "two-phase" on scheme but with the same connections, I am facing trouble with one phase on scheme. Motor moves two steps forward and moves back 2 steps thus not performing the rotation operation. What might be the problem? I would appreciate your guidelines.
Thanks,
SB

 

Maybe your talking too fast to the stepper motor. It has latency and can only move up to a set speed. That's what it looks like. If you want to use high torque u need to use a DC motor (servo). But some may disagree on this.