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Has anyone developed software and/or hardware for controlling the absolute position of a stepper motor with a potentiometer? It has to be a stepper, not a servo. I need the stepper to follow the movement of a potentiometer shaft around a 180 degree axis. I work in PicBasic Pro, so that would be my preference, but any help will be very appreciated. |
Stepper motor control with PIC
Started by ●February 8, 2005
Reply by ●February 8, 20052005-02-08
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http://www.piclist.com/io/stepper/linistep has source code and
schematics for the stepper control part.
piclist.com has gobbs of code for A2D. Your
challenge will be to marry the two.
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James.
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Reply by ●February 9, 20052005-02-09
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--- In , "Roy Wheaton" <roy_wheaton@y...> wrote: > > Has anyone developed software and/or hardware for controlling the > absolute position of a stepper motor with a potentiometer? It has to > be a stepper, not a servo. I need the stepper to follow the movement > of a potentiometer shaft around a 180 degree axis. I work in PicBasic > Pro, so that would be my preference, but any help will be very > appreciated. I know I'm gonna get yelled at, but I'll say it anyway....You can't! You need some way to sense the position to get the movement to follow the pot. Now, you can use the pot to move the stepper relative to where you thought you told it to go previously. But there'll be initialization issues, maximum step rate issues, missed step issues, etc. While it's certailnly possible to buy very linear pots, don't make the mistake of thinking any old pot will be sufficiently linear. Or that two samples of the same pot will have the exact same curve. You can't set the pot very accurately anyway. Maybe you can use some visual feedback from the motion system instead of some dial on the pot. If you're trying to get two mechanical systems to track, you've got a lot of pot issues... All I'm sayin's that you got a lot more issues than just generating the quadrature step signals. Somewhere I've got some pic code in PBP that uses lookup () and generates steps to get linearly from one point to another in 3D space. If you're clever, you can get all 3 sets of up/down/none direction sequences out of a single lookup table and just bang on the step command till you run out of steps. But there's no feedback to guarantee that the stepper actually went there. mike |
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Reply by ●February 9, 20052005-02-09
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--- In , "gettingalongwouldbenice" <gettingalongwouldbenice@y...> wrote: > > --- In , "Roy Wheaton" <roy_wheaton@y...> wrote: > > > > Has anyone developed software and/or hardware for controlling the > > absolute position of a stepper motor with a potentiometer? It has to > > be a stepper, not a servo. I need the stepper to follow the movement > > of a potentiometer shaft around a 180 degree axis. I work in PicBasic > > Pro, so that would be my preference, but any help will be very > > appreciated. > > I know I'm gonna get yelled at, but I'll say it anyway....You can't! > You need some way to sense the position to get the movement to follow > the pot. > > Now, you can use the pot to move the stepper relative to where you > thought you told it to go previously. But there'll be initialization > issues, maximum step rate issues, missed step issues, etc. > While it's certailnly possible to buy very linear pots, don't make > the mistake of thinking any old pot will be sufficiently linear. > Or that two samples of the same pot will have the exact same curve. > You can't set the pot very accurately anyway. Maybe you can use some > visual feedback from the motion system instead of some dial on the pot. > If you're trying to get two mechanical systems to track, you've got > a lot of pot issues... > > All I'm sayin's that you got a lot more issues than just generating > the quadrature step signals. Somewhere I've got some pic code > in PBP that uses lookup () and generates steps to get linearly from one > point to another in 3D space. If you're clever, you can get all 3 > sets of up/down/none direction sequences out of a single lookup table > and just bang on the step command till you run out of steps. But > there's no feedback to guarantee that the stepper actually went there. > mike To get it right, you probably need to have an encoder on the stepper motor. There are a lot of ways to do this including just coupling an absolute encoder (kind of like a pot but with more wires) to the shaft. If should be an absolute encoder so you can tell the exact position of the stepper. Depending on resolution you can make a simple Gray scale encoder but it would take 8 bits to get down to 1.8 degree resolution. With an absolute encoder the process is easy. Convert the pot voltage to some kind of position and step the motor until the feedback matches. If you want to use an incremental encoder then you need to start from a known position. If the process allows, you could drive the stepper to an end point switch (or optical) and then keep track from there. But, if you miss a step the result will be wrong forever. Maybe periodic recalibration against the end point? It all depends on the application. |
Reply by ●February 11, 20052005-02-11
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--- In , "rtstofer" <rstofer@p...> wrote: > > --- In , "gettingalongwouldbenice" > <gettingalongwouldbenice@y...> wrote: > > > > --- In , "Roy Wheaton" <roy_wheaton@y...> > wrote: > > > > > > Has anyone developed software and/or hardware for controlling > the > > > absolute position of a stepper motor with a potentiometer? It > has to > > > be a stepper, not a servo. I need the stepper to follow the > movement > > > of a potentiometer shaft around a 180 degree axis. I work in > PicBasic > > > Pro, so that would be my preference, but any help will be very > > > appreciated. > > > > I know I'm gonna get yelled at, but I'll say it anyway....You > can't! > > You need some way to sense the position to get the movement to > follow > > the pot. > > > > Now, you can use the pot to move the stepper relative to where you > > thought you told it to go previously. But there'll be > initialization > > issues, maximum step rate issues, missed step issues, etc. > > While it's certailnly possible to buy very linear pots, don't make > > the mistake of thinking any old pot will be sufficiently linear. > > Or that two samples of the same pot will have the exact same curve. > > You can't set the pot very accurately anyway. Maybe you can use > some > > visual feedback from the motion system instead of some dial on the > pot. > > If you're trying to get two mechanical systems to track, you've got > > a lot of pot issues... > > > > All I'm sayin's that you got a lot more issues than just generating > > the quadrature step signals. Somewhere I've got some pic code > > in PBP that uses lookup () and generates steps to get linearly > from one > > point to another in 3D space. If you're clever, you can get all 3 > > sets of up/down/none direction sequences out of a single lookup > table > > and just bang on the step command till you run out of steps. But > > there's no feedback to guarantee that the stepper actually went > there. > > mike > > To get it right, you probably need to have an encoder on the stepper > motor. There are a lot of ways to do this including just coupling > an absolute encoder (kind of like a pot but with more wires) to the > shaft. If should be an absolute encoder so you can tell the exact > position of the stepper. > > Depending on resolution you can make a simple Gray scale encoder but > it would take 8 bits to get down to 1.8 degree resolution. > > With an absolute encoder the process is easy. Convert the pot > voltage to some kind of position and step the motor until the > feedback matches. > > If you want to use an incremental encoder then you need to start > from a known position. If the process allows, you could drive the > stepper to an end point switch (or optical) and then keep track from > there. But, if you miss a step the result will be wrong forever. > Maybe periodic recalibration against the end point? > > It all depends on the application. He is right. I wouldnt use a pot because we want stepper motors to be free running and a pot isnt. The best is to use an encoder disc. The type depends on the accuracy you need. You can even use the entire assembly from a mouse which has a encoder disc and a light all assembled. And basically a stepper motor has to do homing in when you start your system. Even commercial systems advise this, to safeguard against controller problems and mechanical drift. So best is to add a limit switch and use that as your 0,0 reference.From there, the whole thing performs as a incremental encoding. Since the encoder can sense direction, you are have directional control as well. B |
Reply by ●February 12, 20052005-02-12
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--- In , "bg3009" <bg3009@y...> wrote: > > --- In , "rtstofer" <rstofer@p...> wrote: > > > > --- In , "gettingalongwouldbenice" > > <gettingalongwouldbenice@y...> wrote: > > > > > > --- In , "Roy Wheaton" <roy_wheaton@y...> > > wrote: > > > > > > > > Has anyone developed software and/or hardware for controlling > > the > > > > absolute position of a stepper motor with a potentiometer? It > > has to > > > > be a stepper, not a servo............. Wow! Thanks to each of you that took the time to respond. I have a much better idea as to how to proceed. I hope that I can return the favor some day! |
Reply by ●February 12, 20052005-02-12
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At 08:10 AM 2/9/2005, rtstofer wrote: >To get it right, you probably need to have an encoder on the stepper >motor. There are a lot of ways to do this including just coupling >an absolute encoder (kind of like a pot but with more wires) to the >shaft. If should be an absolute encoder so you can tell the exact >position of the stepper. The tried and true method used by the tens of thousands of keytag engraving machines scattered around the world is to just drive the stepper against a mechanical end stop during initialization. Kind of clunky (in my opinion) but why argue with sucess? So: you would drive the stepper hard against its end stop for more pulses than 1 full turn needs, then fall into your main loop. The main loop would read the pot value and compare that to a counter that represents the stepper position. You simply generate the 4 phase signals as needed (think of a 4 or 8 count phase counter) and increment or decrement both counters at the same time. Its harder to explain than to do. dwayne -- Dwayne Reid <> Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax Celebrating 20 years of Engineering Innovation (1984 - 2004) .-. .-. .-. .-. .-. .-. .-. .-. .-. .- `-' `-' `-' `-' `-' `-' `-' `-' `-' Do NOT send unsolicited commercial email to this email address. This message neither grants consent to receive unsolicited commercial email nor is intended to solicit commercial email. |
Reply by ●February 14, 20052005-02-14
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Hi Dwayne- In the prehistoric days of the Apple II when they developed the floppy disk drive for the Apple, the design engineers there decided that the flag stop on the Sugart drive indicating track zero was redundant and they stepped the drive out more than 40 tracks to establish the home track 0 position rather than being more "gentle" and stepping out until the track zero optocoupler indicated the head was at track zero. We, who also did floppy disk controllers, didn't think to highly of Apple's solution, but I guess in hindsight, they got away with it. I would certianly suggest that in order to give the associated mechanical hardware a break that the designer incorporate some sort of electrical signal to indicate limits. Simple optocouplers are often used as are switches. Best of luck on your project. Cheers, Rich S. --- In , Dwayne Reid <dwayner@p...> wrote: > At 08:10 AM 2/9/2005, rtstofer wrote: > > >To get it right, you probably need to have an encoder on the stepper > >motor. There are a lot of ways to do this including just coupling > >an absolute encoder (kind of like a pot but with more wires) to the > >shaft. If should be an absolute encoder so you can tell the exact > >position of the stepper. > > The tried and true method used by the tens of thousands of keytag engraving > machines scattered around the world is to just drive the stepper against a > mechanical end stop during initialization. Kind of clunky (in my opinion) > but why argue with sucess? > > So: you would drive the stepper hard against its end stop for more pulses > than 1 full turn needs, then fall into your main loop. The main loop would > read the pot value and compare that to a counter that represents the > stepper position. You simply generate the 4 phase signals as needed (think > of a 4 or 8 count phase counter) and increment or decrement both counters > at the same time. > > Its harder to explain than to do. > > dwayne > > -- > Dwayne Reid <dwayner@p...> > Trinity Electronics Systems Ltd Edmonton, AB, CANADA > (780) 489-3199 voice (780) 487-6397 fax > > Celebrating 20 years of Engineering Innovation (1984 - 2004) > .-. .-. .-. .-. .-. .-. .-. .-. .-. .- > `-' `-' `-' `-' `-' `-' `-' `-' `-' > Do NOT send unsolicited commercial email to this email address. > This message neither grants consent to receive unsolicited > commercial email nor is intended to solicit commercial email. |
