Logig level phot interrupter and other position switching thoughts.

[fastline]

I am working on a device that requires a motor to stop a small load every 15* in rotation, fairly accurately. On a hunch, I bough a very small gear reduction DC motor and determined that the gear reduction will indeed give enough resistance to position the load accurate enough for my needs.

That means I still need an accurate way to tell the micro controller when to stop. I am honestly VERY new to MCs and this will be my first MC project. I just want SIMPLE, but I am also trying to stay away from a mechanical switch that might fail.

I already have a rotating device mounted to the motor so my thought was to add something to the device to pass through the interrupter. I have never used one and not sure if this is the right way to go. I was going to just add bolts or something that could pass in the center of the sensor but not sure if that will work right and there is some risk of the bolts striking the sensor and ruining it.

I also noticed the reflective interrupters and not sure if you just add a mirror and they have an LED and sensor? Not sure if that could work here or not.

So my questions might mostly pertain to understanding the "logic level" interrupter as well as ideas on stopping my load now that I might have a solution for the motor.

 

[dr pepper]

Just having a quick look with google, digikey do a photoreflective infra red photointerrupter, with a logic output you could connect stright to a uC, all you need is a piece of reflective tape on your wheel, or even just a blob of white paint.
The best kind of 'box to use for this application would be a worm drive, as when stopped the load cannot turn the motor, but I spose it depends on your application.
You wont get really good accuracy unless you implement a full servo, but as you said you want simple so try what you have.
If you use a fet you could implement a brake function, use the fet to put an effective short accross the motor once you have turned itv off, this will stop the motor from running on as much.

 

[Roff]

Can you use a stepper motor?

 

[fastline]

Thanks. I actually added a similar load mass to the motor for testing and because I am only operating it at maybe 20 rpm, it stops almost instantly. I was concerned about coasting in which which what you propose would work well with a small, very low value resistor. However, it seems to work so far so I was just going to use a FET to turn it on/off.

I am curious about the reflective photo unit. would it likely require a black body and highly reflective contrast or..... IIRC, this is how optical encoders operate anyway?? IE, highly accurate and repeatable? I was thinking about anodizing the part black and then adding something reflective at each position.

 

[fastline]

Ron, I have tried to avoid the stepper because I still need position verification and steppers also are a little more complicated to drive. I figured I would try the simple DC motor first.

 

[KeepItSimpleStupid]

Mmm!

If you can use a AC synchronous motor, they stop fairly quickly. I used one to advance film for a shutter without feedback. I just had to modify the time as the take up reel changed size. The film kept the glass from being deposited on.

In another DC gearmotor model gantry setup, I had the same issue. You can get the DC motors to stop very quickly by shorting out the winding. For a single direction, that means an SPDT relay wired so that the motor is shorted when the relay is closed and connected to V when the coil is activated.

You will need both a fast way to stop and a sensor. In one case I used a sliding contact and in the other microswitches. In another application, a filter wheel for a monochometer, I used detents along the wheel for a rolller microswitch to stop at with an AC synchonous motor. So you just rig the switch to be open in the detent. To move the motor, you momentarily short out the switch. The software had access to a "motor moving" signal. To sense the position, I used two hall effect switches. There were 4 positions.

 

[ronv]

I have used little gear motors for various projects and I don't see why you couldn't use one for your needs since the gear ratio is very high at 20 RPM so it will stop pretty fast and you probably couldn't turn it with your fingers.


**broken link removed**

The better the contrast the better for the reflector - maybe a mat finish and a silver spot,

 

[dr pepper]

You can use a little trick with steppers to give you positional accuracy without feedback, allthough it'll only work if you dont want the motor to rotate more than 360 degree's, which it sounds like you dont.
In software on powerup you drive the motor for a litttle over a turn, and the motor has a hard stop at a position where its not going to be in during normal use, when you drive the motor for a full turn in software you know that the motor will hit the hardstop and stall, so you know the position of the motor, then its just a matter of counting pulses, so long as the motor doesnt slip.