Quote:
Originally Posted by i_am_samus
One thing that I don't think anyone mentioned is that if you have two CIM motors attached to one gear box, you cannot have one running one way while the other runs the opposite. The gears work together unfortunately.
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Be careful. That is absolutely not a universal truth. The need for the all motors to turn in the same or different directions is entirely dependent upon the design of the gear train that combines the motor outputs. We have had many cases in the past where the multi-motor setups we have designed have required counter-rotating motors. This may be caused by physical limitations on the available space which force the use of smaller gears, resulting in an extra idler in the system, or many other reasons.
In some cases, this is actually a very desirable design option. Most here are familiar with the concept of motor bias, in which the motor will perform better when turning in one direction that the other. Consider a typical set up with paired motors on each side of the robot providing mechanical input to the robot mobility system. When the robot is driving, one side is always running "backwards" and against the bias. When attempting to drive full speed straight forward, it is under-running the other side, which is favored by the bias. We usually deal with that by adding software to curtail the input voltage of the "stronger" side, effectively limiting the performance until it matches the "weaker" side. This is a simple and effective solution, but not necessarily an optimum one.
A nice alternative is to throw an extra idler in the gear train coupling the pair of motors on each side. Then when the robot is running "flat out" each side has one motor that is running full speed with the bias, and one motor running full speed against the bias. The net result is that each side has the same speed/torque output, and one side is not out running the other. So the robot runs straight, and doesn't drive in big arcs when both joysticks are full forward (assuming a dual-joystick OI setup) without any special software.
-dave