View Full Version : Need help with repeatable motor positioning be it a stepper or DC motor.

Volts and Bolts
02-22-2005, 03:08 AM
Here is the application:
I need to position a sensor for a LASER power meter in front of three different LASERS using a home made liner actuator, basically a piece of ¼-20 all-thread rod and plastic nut. It needs to move to· 4 locations, I’ll call them HOME, Pos 1, Pos 2, and Pos 3. Total movement of only about 2 inches, and none of the positions are equally spaced. It would be ok for the system to Home and then move to each location upon a button press, then return Home.
It would be preferred to use a button for each location and have the system drive to the location selected.
Repeatability needs to be good. All the mechanical stuff I can work out, the things I don’t know and need advice on are.
·Motor type:
·DC with opto sensors to report position.
Stepper with a means of counting steps to determine position.
Or some other way to do it…

Of course I’ll be using a BS2. And· I am new to the BS2 so be kind…
Thanks, Rob

02-22-2005, 04:00 AM
was there an option to drop a mirror in front of the beams and reflect them back to the sensor?!· you wouldn't have to get an specific portion of mirror in to the beam so long as the whole beam contacted the mirror and reflected.·
Use small 1"x1" mirror cuts (like you see on disco balls---no bee gees tunes please!).

Just another option!

As far as a mechanism to put the mirrors in place....could use solenoids...but I'd hate to risk wrecking the mirrors.· servos....just build in an adjustable backstop for aiming the reflected beam.· The servo would be told to move for X amount of time, at which point you'd be hitting your backstop.· You could use switches too.


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Don Buczynski
02-22-2005, 10:58 PM
Floppy disk drives work along the lines you are proposing. It might be possible to use an old 5 1/4 inch drive mechanism though I am not sure if the linear travel is enough for your requirements. This would eliminate gearing backlash issues that you would need to account for in the Basic Stamp code for your home made linear actuator. This can be done in the Basic Stamp code.

There is a simple stepper motor driver circuit in the Basic Stamp application notes. I used an external stepper motor control chip for my turntable project shown on my web page. Have a look at the code description and schematics for some additional ideas. I had to deal with backlash issues in the code.

Don Buczynski


Ray Steers
02-25-2005, 01:39 PM

Order the PSC module from parallax and get the continous rotation servos instead of the regular ones (you'll get 2 free when you order)

Now heres the tricky part:

You'll have to dissmantle the servos down to the pcb level and dissconnect the internal potentiometer. Measure the value of the pot they are using and then wire in a linear fader pot of the same value.

connect the fader to your positioning system and you will get precise feedback about it's location by querying the PSC. (it has a funcion made just for that)

be careful NOT to order an Audio (Log) Taper fader pot or the readings will be very odd and unusable. You'll need a linear taper one. (i.e. a 10k fader will read 5k with the wiper in the middle)

Read through all the PSC (parallax server controller) docs first.