[Ether]

Originally Posted by kamocat

Here's a way to get around the issue of putting two motors on the same transmission:
Use only Tan Jaguars.

The reason is, Tan Jaguars switch between 12v and open (coast) in their switching.

In the Tan Jags, during the "off" portion of the duty cycle, the circuit is not open in the direction that the current was flowing. The inductance current continues to flow through the Zener diodes as shown in the attached schematic. This inductance current continues to provide assisting motor torque during the off portion of the duty cycle.

In the Tan Jags, only the high side switches, and there is no current path for the back emf to provide reverse current (and resisting motor torque aka "braking") in the event that the inductance current decays before the end of the "off" portion of the duty cycle. They would then be "coasting".

Black Jaguars switch between 12v and 0v (brake).

Black Jags switch between 12V and motor shorted as shown in the attached schematic. During the "off" portion of the duty cycle, the current continues to flow (and provide assisting torque) due to the motor's inductance. This inductance current decays rapidly, but unless there are large step changes in the command, it does not decay to zero before the end of the duty cycle. Thus there is no braking under these conditions. Unlike the Tan Jag however, if the inductance current ever did decay to zero (due to a large rapid command reduction), the back emf would have a path to force a reversal of the current (basically reversing the arrows of the red lines in the schematic), and provide substantial braking due to a reversal of the current.

See the attachments to this thread for analytical and numerical models of these effects:

http://www.chiefdelphi.com/forums/sh...5&postcount=70

Thus, if one motor is driving the other faster, they won't conflict; the slower motor will simply coast along.

That's exactly what you don't want. You want the motors to share the load. If two motors are mechanically linked to the same gearbox, you don't want separate closed-loop speed controls. This won't guarantee load sharing.

This means an encoder can be split off and wired to two Tan Jaguars for closed-loop control without issue.

If the intent is for the motors to share the load, this is not a good approach.