Subject:
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Re: To step or not to step
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Newsgroups:
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lugnet.robotics.handyboard
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Date:
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Sat, 7 Nov 1998 16:49:55 GMT
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Original-From:
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FTHOMPSON9@AOL.spamlessCOM
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Viewed:
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2009 times
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Russell ,
Having that encoder will help you a lot. One of the biggest problems
with using steppers is that they will stall when you get to higher speeds.
What ever program you use to drive the steppers should have a part that asks
"has it made the last step?" before issuing the next step.
I have a small line following robot using steppers that runs very slowly
since it does not have this feedback. Your program will need to generate the
stepper timing, and if you do not use a "stepper driver chip" you will also
need to generate the stepper pattern. These are not hard, but they will eat
into you program time budget a good bit. I'd count on 30%-50% of your
software time just running the two steppers with feedback. All the timer
features of the 6811 will help this a bit, but it will still call for a good
amount of attention. The feedback might ease the limits on ramping that you
need to do. If the encoders are an overriding part of your program you could
change your speed from 0 to max, and let the encoders do the ramping for you.
The only problem there is insuring the motors run at the same speed.
If you use an external stepper driver chip, you will not need to generate
the step pattern. If you are just using a simple current amplifier (I don't
think the Handy motor driver chips will give you enough current), you will
need to generate the steps in your program. It is not hard to do the steps in
the program, you just have to do it. I don't think your code will be too
different either way. I would guess that you would set your timers to issure
a software interrupt when it is time for the next step. So your code would
look something like:
TimeForNextStep:
Did this motor take the last step (read encoder)
Did the other motor take its step (if going straight)
Issue next step (if external stepper driver this is just a pulse on a pin,
otherwise you "look up" the next step from a step table in code space)
return from interrupt
Even with encoders you will need the timer to slow the motors down. Your
software will need to ramp the speed down for stopping if you want the
position of the robot to be accurately known. Steppers slow down faster than
they speed up.
Hope this helps
Later pherd
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