Subject:
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Re: Adder/Subtractor revisited
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Newsgroups:
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lugnet.robotics
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Date:
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Sat, 6 Feb 1999 05:04:16 GMT
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Reply-To:
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bryan.beatty@autodesk.SAYNOTOSPAMcom
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Viewed:
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1249 times
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David Harry wrote:
> Ingenious as it is, this thread and previous similar ones have pointed out the
> primary problems with the A-S drive approach: power transmission through the
> LEGO differential, which is minimal at best, and a rather high frictional loss
> due to complex gear train. For these reasons, I decided that the independently
> driven track approach is preferable, using software, instead of gears to
> synchronize the system.
Actually, I've had pretty good luck with independently driven left and
right drive trains; I haven't had a lot of problems with robots curving
when I want them to go straight. I find the adder/subtracter useful for
a different reason: it makes fine steering control a lot easier at high
speeds.
Right now, we have a little contest going on at work among several of us
who have MindStorms sets: a race to see whose robot can run a certain
track the fastest. The track is roughly rectangular, consisting of
about sixty feet of 3/4" black electrical tape laid down on a white tile
floor.
Anyway, the challenge is to build something that can run the straight
sections very fast, while optically following the tape. The problem I
encounter when trying to build a really high-speed robot is that there's
no good way to control the speed of the motors; they're either on, or
off. Adjusting the power to left and right drive trains changes the
torque, but doesn't significantly change the speed and doesn't work well
for steering the robots. The only way I have of steering left or right
is to momentarily turn off the left or right drive train.
Unfortunately, if the robot is geared for high running speed, even a
very brief stoppage to left or right drive power causes the robot to
jerk much too strongly to left or right, oversteering and screwing up
its tracking. I can mitigate this a bit by giving the robot a wider
wheelbase, but that can only go so far.
Enter the adder/subtracter transmission: Since it separates out the
drive motor from the turning motor, I can gear the turning motor way
down so that steering is very gradual relative to forward speed.
The main disadvantage in working with the adder/subtracter in this
situation is frictional loss through the transmission, but that can be
minimized with careful arrangement:
1. I drive the "propulsion" differential with two motors for extra
torque.
2. Shafts from the "propulsion" differential go straight out to the
drive wheels, so there actually isn't all that much gear loss going on.
The gear train from left axle, through the steering differential, to the
right axle, isn't under much load (only minor steering corrections), so
frictional loss there isn't a big problem.
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Message has 1 Reply:
 | | Re: Adder/Subtractor revisited
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| (...) Here's what I do and it works quite well. I have added a "partial duty cycle" mode to my motor control system. Basically I intermitantly turn off the motor for a very small increment of time. The time-average of a properly synchronized (...) (26 years ago, 7-Feb-99, to lugnet.robotics)
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Message is in Reply To:
| | Re: Adder/Subtractor revisited
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| In article <36B6F795.4B9310A6@n...a.kth.se>, johannes@nada.kth.se says... (...) Ingenious as it is, this thread and previous similar ones have pointed out the primary problems with the A-S drive approach: power transmission through the LEGO (...) (26 years ago, 6-Feb-99, to lugnet.robotics)
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