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Originally Posted by phrontist
Okay, new question.
If I have a 1000 rpm motor gear down 1:2, and a 500 rpm motor running in tandem, and the system is put under load, won't situations occur in which one motor is driving the other? Wouldn't this mean that a two motor system set up this way can acutally decrease speed and/or torque?
Even worse, if one motor is, in effect, "spinning" the other, won't this cause backcurrent to damage the victor?  |
Unless you have a ridiculous mismatch between the motors' torque-speed curves (at the output of the matching gear-box), the back drive situation won't occur. You would have to have the motor going beyond its free speed in order to get no useful work out of it.
All motors have some "back emf." When back emf = driving voltage, you get your maximum speed. If you drive the motor beyond this speed, the back emf will exceed the driving voltage and you will be "back driving" the speed controller. Up to a point, this shouldn't damage the Victor. However, I'm sure if you created a net 15v at the Victor, you will find yourself into a condition that the designers didn't consider.
On the motor balancing issue, consider it this way.
The Victor supplies a current to your motor. The load provides a torque/speed curve into which the motor is driving.
When your motor acts on the load, it provides a torque which accelerates the load. As the load accelerates, your motor's speed increases, which increases the back emf. The back emf reduces the current supplied, which reduces the torque. At some point, your motor is supplying enough torque to run at the balance speed for a given current supply.
The load that your motor sees is a combination of the torque provided by the other motor and the other load torques provided by the robot.
As the load changes (for instance, your robot accelerates and then reaches steady speed for a particular PWM current), your motor will move up and down the torque-current curve until you reach a particular steady state.
You can actually have a pretty broad mismatch between motor speeds and have the system work. The down side is that your motors don't necessarily run at their optimum efficiency points.
But, where do you want the max efficiency point to be? When you're cruising at top speed? When you're pushing? Since load conditions in a FIRST robot are constantly changing, you will not be running at the most efficient point in any motor for any significant length of time. So, you might as well match the free speeds of your motors.
If you really tweak out your system, you might be able to balance the motors and gear boxes so that you get a little extra top speed or so that you get a little extra pushing power. But, that would take a lot of engineering effort (including verifying measurements to validate your theory).
Based on our experiences with multiple motor drive systems, I suspect that a little software load balancing would be worth the effort.