I'm a newbie in mciro-electronics, alrhough I have an electronics course, i've never worked with PICs before...
I'm trying to design an "actuator" alike circuit, wich would run a dc motor foward or backward, depending on it's position....Kinda like the CD-Rom draw does, or a garage door opener....
I'm guessing that to figure the current position out I would have to use some micro-switch or optic barrier along with one or two relays to invert the direction....
Anyone ever deone or came across something similar?....
Aside from the optical beam across a garage door, they might sense the motor current to make the reverse direction decision. Will your design stop at each extreme as a garage door or CD tray does or will it continuously reverse at the limit of travel?
It's done routinely. In fact, with a microprocessor, you shouldn't even need a reversing relay - just do it in software. Of course, you'll need drivers for "forward" and "reverse", but that can be done with relays, power transistors or FETs, H-bridge drivers, or whatever you want.
You might want to add "fail-safe" limit switches, such that if the uP doesn't respond to the "at the end" signal, and the motor continues to run, you have a switch that cuts power to the motor completely.
(i.e., two switches at each end, one to the micro, and one to cut power if the micro fails.)
"Like" a garage door opener or "a garage door opener - "
Do you need relative position, absolute position, variable speed, proportional speed control, how much over/undershoot is tolerable, what size horsepower, 360 rotation, etc. etc.
Describing the application goes a long way to helping make good decisions on how best to implement what you want to do.
This is a common problem and been solved lots of times, so yes, someone has done it.
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My project is to motorize a compartiment door activated by a single button like "open/close". Since DC motors are alot more inexpensive and I have afew of them around from slot-cars, Speed is not an issue, of course the more smooth the moviment the better, I've tryed to buil a "relay logic" circuit but got stuck in the midle and eventually dropped that concept...and turned to uP....I was planing to use PicBasic since I do some Basic related programing in Windows and WIndows Mobile enviroments....
The movements should be like this:
If on point A go to point B and stop If on point B go to point A and stop If in between go to point A
Position would be determine by either micro-switch or integrated IR barriers....I dont know waht sp+pecs to search in order to choose a uP so I was counting on Your input and experience...I figure I only need 5 I/O pins, and the code should be that big either....
A reversible motor can turn in either direction so you may want a double pole double throw switch and relays to implement the simplest solution. (I didn't automatically assume that there were A and B and the motor had to reverse to get there - an endless loop chain, or platter with 360 rotation could also stop at two points with - state your problem better and you have a better shot of getting a good answer)
For instance a ball valve without stops can be told to go to "open" or "closed" with 360 rotation and a simple shaded pole gear motor. Likewise other devices where the motor rotates in only one direction - I replaced a "voice valve" in an acoustic organ with a single gear motor that operated a slide valve - taking advantage of the mechanical advantage (low motor loading) at full open or full closed.
Well that would be a perfect relay logic application so that's what I would use.
What you want is simple (keep it simple). In the old days they called it "zero seeking."
For instance you turn on a motor and it revolves a platter (opens a valve, moves a chain, moves something) and it rotates until it reaches a switch that turns off the motor.
That is "zero seeking." in its basic form. Windshield wiper motors use it today so they stop at the bottom of the stroke when power is removed.
You have two positions you want to access and a default position to goto when it starts up in between stations. (presumably it would be at A or B most of the time unless something outside intervened).
You are probably familiar with those single pole double throw switches? (from electronics suppliers or hardware stores to control a light from two locations?)
Well, in its simplest iteration you would transfer power to one or the other limit switch and have it move the motor until that switch was "satisfied" (actuated - normally closed then opened by the motor or gearing) whereupon it would switch off power.
move switch to A motor goes to A then stops. Ditto B.
NO microprocessors works with AC or DC and no electronics.
Now you also want it to goto A when in between - the in between a power failure or something physically moved it despite the motors wanting to return it to the desired location???
The power failure scenario can be done with two relays. One comes on with applied power (assuming that's how it got between stations) and runs to A until it energizes a second relay that stays energized dropping the first relay out of the circuit and latching in that state.
If, for some reason you want it to do all that and you have over-riding clutches or can physically overpower the drive mechanism resulting in an in-between state you could use a PICAXE micro controller to do that for about $3 plus $10 in cabling to program it - then you could add delays, and have it make decisions based on time or other inputs - motor loading, hazard items, etc..
So I come back to my earlier statement; you should just tell us what you want to do, and that will determine the best way.
Don't jump into a programmable device as the first solution you try - they offer a lot of versatility and lots of options for later change -
But if you only want to do one thing - the simplest way is often the most reliable way and that could be the lowly relay. (no special power supplies, no power up reset hassles, less/no interfacing circuitry, better noise immunity, current sensing, etc.)
Relay logic rules! (until you want to get fancy)
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Default, Thank You very much for you input, with it I learned the "Zero Seeking" term...
What I want to acomplish is a movable faceplate for a car audio system that would Act when the power was feed to the circuit and then return when powered off.
Like this.
car key in On position go from A to B and Stop car key in OFF position go from B to A and stop
I havent yet startred the face plate movement design because I havent yet decided how far the movement should go, althought taht is not important to the circuit design...
There must be a million ways to skin this cat, but you can do it with a DPDT relay, a permanent-magnet motor, two limit switches and two diodes.
The motor and both limit switches are in series. Each limit switch has a diode across it, allowing the actuator to move away from that limit. The relay, driven by the ignition-switch "accessory" contact switches the polarity applied to the motor, driving the actuator toward one limit or the other until that limit switch opens. The motor, of course, has to be powered from some point that's always on. If nothing else, you can use the "battery" contact of the ignition switch and an inline fuse holder.
The simplest microcontrollers are so cheap that they can profitably replace relay logic in many cases, but in this case you need the relay anyway, to carry the motor load, and all the logic is done by the two diodes. I suppose you _could_ use a microcontroller and an H-bridge to drive the motor, but a small relay is probably cheaper, and comes pre-programmed.
I never expected this kind of foward help from a news group... You guys rock!!!
I think that I willgo with the relay option, as it is cheaper for me, since I don't even have a uP programer, and also attending to the caution notes posted....
As far as the "mechanical design, I'm thinking on using a "blind" like faceplate that rolls up and down...My only doubt is ... Waht will happen if the motion is stoped by exterior interaction, like a slider getting stuck' What will burn first? The motor, the relay, or the fuse?
Anyway all that rest me to do is experimenting and give you guys a big THANK YOU for all your input..
Get a schematic/wiring diagram of a car with retractable headlights, and see how they do it. They'd have these things at the local auto parts store or possibly the public library.
You could use a magnet and a reed switch. When the magnet comes need the reed switch if goes open or closed circuit.
Another option is to monitor the current to the motor. When the current goes high switch the motor off. Have you ever seen a garage door that closes so far stops because it hits something and then reverses. They can monitor the current to the motor to do that.
Or use an infra red transmitter and receiver.
Probably depends on how easy mechanically it is to do it.
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