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Scribbler Stepper Motors — Parallax Forums

Scribbler Stepper Motors

st33dst33d Posts: 4
edited 2006-03-10 02:00 in Robotics
I'm planning on connecting my Scribbler to some unipolar stepper motors (second one down I think). I bought my motors from an english company called Active Robots. They have a control circuit that will allow me to operate a stepper straight from the PIC. It will require 4 I/O lines. That's 8 lines in total to replace the DC motors with steppers.

I know the hacker port has at least 3 I/O lines I can use, and there's 4 taken up by the DC motors, though I'm not sure if those are the power lines coming out of the plugs on the board, or whether they come from the I/O pins on the Basic stamp. So there might only be 2 lines for the DCs.

Has anyone got any suggestions on how I can rerout control? I'm quite happy to ditch a few sensors. If I'm going to run into a power issue I would love to know about it. I'm very new to this whole re-soldering gig (although I have just successfully extended one of the LEDs on long wires so it can draw on photographic paper - green LEDs will leave a mark on photographic paper - saves on building a circuit for a solenoid to lift and drop a pen eh?).

Comments

  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2006-03-08 16:32
    Well, it seems that you will have to by-pass a left and a right motor controller. Each uses one line of logic. There is also a stall line.
    You would definitely have to re-write code to drive the steppers and use two other inputs from the hacker lines UNLESS.....

    Why not see if you can locate a couple of stepper controller chips to do the same thing as the motor logic chip does now?
    They do exist and may make it a better project. You could program an SX-28 to do it and use its internal oscillator. Can you program in SX/B?

    If you have the BasicStamp driving the Stepper Motors, it will likely be two busy with that task to do much else. I suspect that is the reason that the Scribbler doesn't have hobby servo motors. Also, you might find that stepper motors are a bit more power hungry that the motors you have. You could find that you need to replace batteries more often.

    Finally, be sure that you have enough power for the steppers. Measure their resistance. Much under 30 ohms and you may need to use FETs to turn them on and off as they will burn up logic. At the very least you wll need a UNL2803 to drive them.

    I just spent a bit of time removing a gear from a stepper. I used a hacksaw and small file.· First I cut a spiral slot nearly all the way through the gear, then I chipped away at it, and finally I used a tringular file to get as close as I dare to the shaft. Finally I was able to pull it off with pliers.·

    If you want to connect wheels, take a look at modle airplane propeller collets as the attach to small precise shafts and have a nut and thread or screw arrangement to attach the prop.· Just use a wheel in place of the prop.

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    "When all think alike, no one is thinking very much.' - Walter Lippmann (1889-1974)

    ······································································ Warm regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan

    Post Edited (Kramer) : 3/8/2006 4:36:42 PM GMT
  • st33dst33d Posts: 4
    edited 2006-03-08 17:21
    Looking at a data sheet of the ULN2803, it looks like a setup of transistors and diodes - which I've read up is what you need to run a stepper. I think the control boards I'm looking at do that job - I'll email them and post here if they do. The data sheet says they work straight off the pins. I need no brainer solutions really - I'm not hugely clever and need to get this project glued and running damned quickly. I don't mind splashing out £40 for a couple of controllers - especially when this project will give me something of lasting value -> a turtle.

    I'm personally only interested in the Scribbler controlling the motors - with development for control of a solenoid at a much later date. I've already written a program in PBASIC to control the motors from a program in Java on the PC. It's not running like rocket science - but it does what it says on the tin. The only problem is the crappy DC motors introducing more than enough approximation into the equation. The Scribbler needs to know only enough to take orders from the PC - the real magic will work from the GUI on the PC.

    The SX-28 looks like too much research for me to get my head around. I've no idea what it is I'm looking at.

    I have aluminium wheels bought from the same people. I also have ballbearing wheels for balancing the fore and aft of the new mount for the circuit board.

    I'm looking at the circuit diagrams for the Scribbler and I'm not sure what goes on with the motor controller. I take there's a chip running the DC motors? Should I be stealing lines from that?
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2006-03-09 06:41
    Well if you are removing their motors, the motor controller are redundant, aren't they?

    Since they have merely one line of input, it may be formated in a RS-232 Rx Serial format. You would have to remove that software output and replace it with your own.

    So, you must understand the software code that drives the existing motors in order to know where to cut and fit.

    I must say I wonder why you are so ambitious about hacking the Scribbler's motors.
    You could more easily do all this by starting from scratch with another chassis and the BasicStamp. A bit more cost, but substantially less risk and more clarity. You eliminate the need to reverse engineer and overcome existing systems [noparse][[/noparse]both require more thought and frustration].

    As I said before, if you use the ULN2803; the BasicStamp will be quite busy with pulsing the motors. The robot may have to stop moving at times to do other chores. If you have a 'stepper motor controller', movements may be on-going while other tasks are occuring.

    I have a wonderful robot designed on a Frisbee for it's overall cover and using a convienently round stainless steel trivet [noparse][[/noparse]found it in a local discount store] to support motors , batteries, sensors, and microcomputer. Since it looks remarkably like the Scribbler, you might consider the same kind of configuration.

    Most people have the greatest difficulty when mounting wheels to the stepper motors. Either you have to remove the existing gear or you can simply 'gob it up' with epoxy and glue your wheels in place. I used 'hot glue' in the manner and was quite pleased as I was later able to make repairs. And in the end, I could re-deploy the stepper motors into another project with little effort.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    "When all think alike, no one is thinking very much.' - Walter Lippmann (1889-1974)

    ······································································ Warm regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan
  • gibbmangibbman Posts: 98
    edited 2006-03-10 02:00
    An interesting note. The March issue of Circuit Cellar has an article by Jeff Bachiochi on hacking a Scribbler. Who' have thunk it...it does have a discussion of using the hacker port to add some ins and outs to the Scribbler.
    Jim

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    In the end, it seems that it's all about getting the LEDs to blink....
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