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Re: AID: Motor Questions
From: Jeff Mann jefman
utcc.utoronto.ca
Date: Sat, 16 Nov 2002 19:25:14 -0800
: Sent via the Art Interface Device mailing list: aid@interaccess.org : Use your "Reply All" to reply to the list, "Reply" for private response Tim Moody wrote: > I'm working on a PC parallel port interface to drive some 5V lights and > some motors. The logic chips work fine as do the drivers for the > lights (L293) L293's are push-pull, H-bridge type drivers, designed for eg. turning DC motors in either direction. Using them to run lights should work ok, but there are cheaper solutions, the ULN2803 transistor array chip or discrete transistors for example. On the other hand, the L293's have a thermal shutdown feature that would have been handy when I blew up the main controller board for Istvan's machines the day before the show... > but I'm having problems with the drivers for the motors (L298), > which have been unreliable (either I blew them or they were bad). L298's are also H-bridge drivers. For unipolar steppers, again a cheaper solution is to use four transistors. It's possible to drive a unipolar stepper with an H-bridge, but in this case you should leave the centre taps unconnected and treat the motor as a bipolar. Since this will halve the current, you may want to increase the voltage - but not necessarily double it, since the motors are more efficient when used in this configuration. Whenever you're driving inductive loads (motors, relays, solenoids) make sure you have adequate protection diodes. Some drivers (ULN2803) have these built-in, others (transistors, L298's) don't - check the data sheets. > 1) I know that some of you at least are interested in PWM which I assume > is for driving non-stepper motors. I'm interested in comments on the > trade-offs between steppers and non. They are completely different animals. Steppers are good at low speeds and incremental discrete positioning steps, and have holding power (torque) even when at a stand-still. They require a microcontroller, computer, or dedicated controller chip to provide the required pulse sequences to step them. Their maximum speed is relatively slow. Regular DC motors like to turn really fast, and are generally more powerful. You can slow them down by reducing the voltage or by "chopping" the current (PWM). But at slow speeds they have little or no holding torque. Also, they don't have discrete steps; given a pulse of current you can't be sure how far they will turn. Servo motor systems combine the precise positioning and holding torque of steppers with the increased power of DC motors. However, they require relatively sophisticated sensor/feedback circuitry. "Hobby" servos used in radio-controlled (R/C) models are very nice because they include this circuitry at a low price (~$20) due to economies of scale. But they are quite small and are limited to about 180 degrees of motion - they can't turn continuously. Industrial servos use (expensive) optical shaft encoders and controller chips. However, sometimes you can find the motors (with encoders) at surplus dealers, and there is code available to use $15 PIC chips as controllers. > 2) I found the steppers (5V 8.5W unipolar) at Active Surplus. Is there > a better source for motors? Is there a better place than heaven? Active is one of the best places in the world to find surplus steppers (but buy your chips and components from Supremetronics a few doors down). I sure wish there was an Active here in Amsterdam! But you can also try Above All on Bloor, across from Honest Ed's, or scrounge small ones from disk drives and printers you find on the road. cu later <Jeff : messages saved at http://www.interaccess.org/aid/list : unsubscribe/help requests to mailto:Majordomo@interaccess.org