Motor Control - 3 Position Start-Stop

Sounds like you could use a servo motor from a radio controlled plane or car. They respond to pulsed signal. By feeding pulses of varying widths to the servo, the servo can be made to turn to a certain position and stop. These pulses fall in the range of 1mS to 2mS with 1mS causing the servo to move all the way in one direction and 2mS moving it to the other limit. Widths in between will make move the servo to a proportionate location.

All you have to do is be able to generate 3 different pulse widths and switch between them. If you search for 555 and "servo tester" using Google, you will find sample circuits to drive servo motors. Here is one:

If you want to "use a PIC" instead, just say so. ;-)

Hi Anthony,

Thanks for your quick reply! I did some research from the information that you sent me and had a few questions. These are mostly due to my lack of knowledge with electronic components.

I was able to find most of the parts, but couldn't find any 390nF or

100nF capactiors. I was able to find 390uF and 100uF, but I assume there is a difference between the two. Also, the diagram specs a "2K7" resistor and potentiometer. Is this short for something? I couldn't find anything with those labels.

Is there a really big "amazon.com" of electronic components that I don't know about? Where do you usually get your components?

Also, I wouldn't be opposed to using a PIC, but I wouldn't know where to start. Are there any great benefits to using a PIC?

Thanks so much for your help!

-Alok

one:

alokw wrote:

use a stepping motor, other wise, you'll need to use something like a little DC motor that reverse it's direction via a little switching controller that monitors a set of photo detectors so that it knows where it is. of course, the motor will have to run slow so not to do swing loads which would cause oscillation.. You could also look into using a Gray code decoder that can connect to the shaft. this reports position how ever, you then would need to build a decoder in something like a pic or avr chip. from my stand point that would be simple. from yours i don't know? Even using the stepping motor, you still need ways to detect the number of pulses. etc.. i did how ever see one trick doing something like that. they drilled a hole in the shaft with 3 photo detectors on one side mounted in a concave pattern. the emitter on the other side of the hole. what it did was move the shaft slowly and when the emitter was passing through the hole, different detectors would pick up the intensity to stop or slow down the motor and even attempt to reverse it. I suppose you could put a set up like that on there. or you could simply use micro switches on a cam that is attached to the shaft one lobe on it. you could simply push the button long enough to leave one switch and thus the switch would then close keeping the motor running until it hit the next switch. I suppose you could use a 555 timer to do that.

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one:

Ah, that reminds me. you could use a Voltage Comparator and have a continuos potentiometer on the shaft. You just need three different voltage levels to select from. the output of the comparator would drive the motor one way or the other to balance the position of the disc. Using 2 comparators in a bridge mode to drive the motor. actually, 2 op-amps driving a dc bridge to position the motor will help. Just position the pot so that the cross over point is is between one of your selections.

--
"I\'m never wrong, once i thought i was, but was mistaken"
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radio shack sells an assortment package of "nF" valued caps (if you can find a radioshack that still sells components), and if you check some place like mouser.com or digikey.com, they should have what you need also. (or somplace equivalent in your part of the world :) Check around and see of you can find an electronics surplus store in your area.

2K7 is short for 2.7 K or 2700 ohms

a PIC or AVR microcontroller can do the pulse generation for you, and there is sample code available for controlling both R/C servos and stepper motors. The disadvantage is that you need to learn how to write code for the chips because of your custom application, and that you need a "programmer" module to load the compiled code into the chip, on top of the hardware that will actually hook the controller to the servo or stepper.

The R/C servo approach has the advantage over the stepper in that you dont have to set up an "index" sensor to identify the "home" position -- thats already built into the servo.

If you go with the 555 timer with switched resistors to control the pulse width for an R/C servo, thats pretty much going to be the simplest solution over all.

Note there are 2 basic types of R/C servos out there -- "Standard" and "Continuous Rotation" -- you want the standard type -- CR servos are basically motors where the servo pulse controls the speed and direction, not the absolute position.

Hello, please don't top post.

Yes there is a difference of several orders of magnitude. :-) 390nF is equivalent to .39uF, it's nano-Farads vs. micro-Farads, three decimal places. The 2k7 is another way of saying 2.7k Ohms of resistance. It's supposed to remove the confusion, ha ha. ;-)

Also, I wouldn't be opposed to using a PIC, but I wouldn't know where

is a good resource for that, but you may want to stick with the basics a little longer first. As to the benefits, they are the difference between putting a computer to work on a problem vs. doing it some other way. IOW, they can be astounding or mundane, it depends upon the task being performed.

DISCLAIMER: They do require an investment in time in order to learn how to use them, but this is mostly a one time issue. You also need special equipment (though it's not that terribly expensive) to be able to flash (store) your programs into them. If you already know how to program, things will be much easier to learn. /DISCLAMER

Perhaps a good way to get started in microcontrollers is to get something like a "BASIC Stamp" kit. You won't need any additional special equipment to program them and you do that using BASIC. They have nice tutorials to help beginners. You will find tons of information on that here:

Can it always rotate in the same direction?

The speed of rotation would also be useful.

Surely you must have some old floppy disc, CD, VCR or whatever Drive collecting dust somewhere. Cannibalize the motor and associated circuitry to do your little project.

I think I'd do it with relays and microswitches.

Make a cam or bump on the disk, and mount three microswitches so that the cam will hit a switch at each desired position.

When you press button 1, relay 1 operates, and latches itself through microswitch 1 and applies power to the motor. When the disk is turned so that the cam hits microswitch 1, that switch opens, and unlatches the relay, stopping the motor. Depending on disk speed, inertia, and friction, a brake may be required to prevent the motor from coasting too far when the power is removed.

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