Subject:
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Re: Simultaneous Control of Multiple Servos
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Newsgroups:
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lugnet.robotics.handyboard
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Date:
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Fri, 17 Jul 1998 05:15:23 GMT
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Original-From:
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(michaelj@wt)stopspammers(.net)
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Viewed:
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1104 times
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You are correct to assume that all legs should be in sync with each other
and that all the legs on the ground should be moving at the same rate in the
same direction to move straight (sometimes you don't want to move
straight!). BUT now think of the speed of the processor and how many
different pins can be turned on and off in a VERY short amount time. The
servo pulse for each motor is about 50 hz but much of that time is spent
waiting for the time to end the pulse and then waiting for the delay to end.
Interlacing the signal control allows you to compress the time spent handing
the signal generation because the processor can start and stop the pulse on
several different motors in a much shorter time frame. This would make it
appear as if the motor pulses were being done in parallel when in fact the
signals were handled sequentially. See my crude example:
S = start pulse
T = terminate pulse
- = active pulse
. = inter-pulse delay
S--------T..................................................................
..................
.S----T.....................................................................
...................
..S----------T..............................................................
..................
...S-----T..................................................................
..................
....S--------T..............................................................
..................
.....S--T...................................................................
....................
Note that the time delay between the start of one pulse and the start of the
next is only the amount of time the processor needs to setup for it which
may be only a couple of instructions. By ordering which motors are adjacent
to each other in the sequence, you can minimize the effect of being "out of
sync" (all swing motors on one side are adjacent followed by swing motors on
the other side followed by lift motors on the first side, etc).
Computers multitask the same way but because the switching happens at such a
fast rate, it appears that everything is happening simultaneously. If you
had a hundred legs you would probably need a faster CPU but you could still
handle them in basicly the same way. Using a well designed interrupt-based
timer you can even reduce the time spent is handling the leg motor motions
and perform some sensing. If you reduce the flexibility of your walking
gaits and only use the alternating tripod gait then two legs on each side
can receive exactly the same signal since they always move exactly together.
That would reduce the number of individual motor sequences you would have to
manage.
I hope I didn't ramble on too long and that this helps answer your
questions.
Mike
-----Original Message-----
From: Mike Davis <ipscone@halcyon.com>
To: handyboard@media.mit.edu <handyboard@media.mit.edu>
Date: Thursday, July 16, 1998 10:46 PM
Subject: Simultaneous Control of Multiple Servos
I have asked this before and not received much regarding this question. So,
here goes again.
In building a walking robot, how important is it to control the motion of
all legs
simultaneously?
Seems like that pulsing one at a time creates the situation where one leg
drags
the others. I know there is a delay between the time an mcu command is
issued and the time the pulse goes out, but still it seems like one leg
would
drag the others without some kind of parallel controller. I also wonder
about the
ability of the serial controllers to perform simultaneous servo control.
Seems like each leg should be programmed to walk identically, and that each
would have a phase relationship with each other. But all have to be moving
in
sync and not one at a time. Is the HC11 a poor choice for this kind of
motion?
Or is it just fine? Or does it require some kind of servo controller?
Mostly I'm
interested in the walking algorithm and synchronization control.
Is this at all an important issue? How is it addressed? Any web info on
this? I
have looked at many walking robot pages but have not seen this addressed.
Thanks
--
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Message has 1 Reply:
 | | Re: Simultaneous Control of Multiple Servos
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| That's the way I thought it had to be done. What I was considering was routines that moved each leg the same short distance (for straight line walking). Walking would consist of very short increments of movement. Now, this seems like it would move (...) (26 years ago, 17-Jul-98, to lugnet.robotics.handyboard)
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