Subject:
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RE: 9v Gear Motors
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Newsgroups:
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lugnet.robotics
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Date:
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Fri, 9 Jun 2000 20:06:28 GMT
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Original-From:
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Jim Thomas <jim.thomas@trw.&IHateSpam&com>
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Viewed:
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660 times
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Well I guess I'm not too surprised. As has been mentioned, PWM control is much more effective if it is driving a heavy load not a light one. This is exacerbated by the presence of the flywheel. When you turn off the electrical power and float the motor, the flywheel dumps it's energy into the drive train and keeps the speed from slowing as much. The rate of slowing will be directly dependant on the drivetrain load. If you actually brake the motor you can control the speed better when you don't have the feedback because you are adding a known electrical load to help slow it down at a known rate. Actually the effect of the flywheel plus braking is to make the drivetrain load less significant in determining overall speed and therefore makes open loop speed control possible for lighter loads.
My suggestion for people trying to control speed without wasting battery power is to hang a rotation sensor on there and make it closed loop. That way you can provide only the power needed to keep it going at the speed you want.
The gearing down the motor solution only works if you want only one speed. In that case you just run the motor at full power to get the motor in its RPM limited mode and then gear appropriately. Assuming you have enough gearing for the load you will always get about the same speed even under varying load conditions.
JT
-----Original Message-----
From: John Barnes [mailto:barnes@sensors.com]
Sent: Friday, June 09, 2000 2:53 PM
To: lego-robotics@crynwr.com
Subject: Re: 9v Gear Motors
I am very surprised to learn that the coded speed control function
actually uses "on" and "off" rather than "on" and "float". When the motor
is turned "on", it speeds up, consuming substantially more power than
it will once it reaches its final full speed because it is transfering
energy to its flywheel amonst other things.
When the motor is "float"ed, it is the flywheel energy that is then used
and which slowly runs down until the motor is again supplied with power.
The pulse power function incorporated in the RCX is just that. It is
not a speed control, it is a power control, so a lightly loaded motor
will not slow down much as the power is reduced. But a mechanically
loaded motor will slow noticably as the power level is reduced.
If you "off" or "brake" the motor between "on" periods, you are actually
dumping the stored energy and disposing of it as heat in both the motor
and the drive electronics. So if you use this method, you will be wasting
a lot of valuable battery power. If you want something that goes slowly,
gear the motor down to the desired speed.
JB
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