regenerative motor driver ?

I am considering the Sabertooth 2x10RC Motor Controller as advertised in the latest Servo magazine. I am curious about the following text I quoted from their website (www.dimensionengineering.com) .
How exactly do the batteries get recharged by a slowing or stopping motor ? Does it just mean that momentum keeps the bot moving after current is cutoff, and the motors act as a generator until it stops ?
"Sabertooth is the first synchronous regenerative motor driver in its class. The regenerative topology means that your batteries get recharged whenever you command your robot to slow down or reverse."
Thanks ! JCD
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I don't know anything about the Sabertooth in particular, but yes, that's generally what the term "regenerative" means when applied to a motor. This is how regenerative brakes work in a (good) electric or hybrid car, for example.
I'm skeptical that you'd get any noticeable benefit from this in a robot, however. A car will coast a considerable distance without braking; that's all energy the brakes can recover. But a robot usually doesn't coast; it doesn't have all that much momentum to begin with, and friction quickly brings it to a stop, so what's to recover?
Best, - Joe
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wrote:

Yeah that's kinda what I thought, too. I could really care less about the regenerative part ... just trying to understand things, though.
Thanks ! JCD
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Almost any mosfet motor driver is capable of regenerative braking under the correct circumstances, that is, when pwm duty cycle is reduced below the motor's current speed. (but not to zero and not reversed)
As Joe mentioned it may be unlikely that the amount of energy recovered is significant. However, if you want to use your motor and drive electronics for braking, you're going to have to get rid of that energy somehow. It is better to dump it into your battery than into the drive transistors and *much* better than to let the motor coils ring it off. For that matter, it's also important that if you're going to do this with a big motor your power system can handle it; rapidly stopping a large motor can result in a nontrivial power surge.
-chris
pogo wrote:

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e c kern wrote:

Sorry, but that's not true. If the maximum motor voltage is say 12V, and the motor driver is driving full ahead, then goes open circuit, the motor will generate *less* than 12V, which won't be enough to regenerate into the battery even if there had been a current path (like a diode).
If the driver uses locked anti-phase, and the driver switches negative, some regeneration may be possible. But in general, motor drivers do not provide 4-quadrant control unless they're specifically designed for that purpose.
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During the off phase a mosfet pwm driver output is essentially a dead short. During that period current begins to flow backwards through the armature coil, storing the motor's rotational kinetic energy in the coil's magnetic field. When the driver switches to the on-phase, the current continues to flow, because the armature coil is an inductor, and that is what inductors do. The battery voltage is indeed greater than the motor emf, but the energy stored in the armature coil's field is drained during the on phase to make up for that. In effect, it functions like a dc-dc upconverter.
-chris.
Clifford Heath wrote:

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