Power management

[koen.paes]

Hello everybody,

I am currently working on a thesis project, investigating an IRB1600 robot (ABB). I am new in the field of robotics and I was wondering about the way power is managed in the robot-controller.

When one of the motors is generating electric power (because the joint is moving with gravity for instance), is this power then redirected to the other motors or does it get lost? And in case all the motors together produce more power than they consume, is power sent back to the net or dissipated somehow?

Any hints or references to relevant literature are  very welcome...

thanks,
Koen

[newInRobotics]

When one of the motors is generating electric power (because the joint is moving with gravity for instance), is this power then redirected to the other motors or does it get lost?

As far as I know, robot motors (servos in this case) do not get powered off until robot is in its 'parked' position, hence none of the motors work as a generator. Reason being is that gravity cannot be trusted as it is an external force and cannot be controlled. So even if robot is lowering something towards the ground, servos are actively working against gravity to keep required velocity of the end-effector and to follow path correctly.

And in case all the motors together produce more power than they consume, is power sent back to the net or dissipated somehow?

Motor can never produce more power than it consumes in the same amount of time or travel distance. So, to raise end-effector to 125cm height will take more power than can be generated by letting robot go 'floppy' and motors being spun by gravity until end-effector touches the ground.

[jwatte]

Temporarily, a motor can generate voltage spikes that need to be dissipated, or the motor controller will be damaged. Typically, you use a flyback diode reverse biased across the appropriate leg of the controlling H bridge.
Some controllers can also short the motor terminals together when not providing power, which dissipates the generated power as heat and braking power.

[Billy]

As far as I know, robot motors (servos in this case) do not get powered off until robot is in its 'parked' position, hence none of the motors work as a generator.

There are two ways to decelerate an active motor, both of which have the motor acting as a generator:

• Short motor windings when when the PWM pulse if off, in which case the current generated by the spinning motor is circulated through the motor and dissipated as heat.
• Allow that generated energy back into the power buss to be used by other loads, and if needed connect a resistor to the buss to prevent the bus voltage from getting too high. You'll hear the term "braking resistor" or "regen resistor" occasionally and the usual implementation is to short them across a buss that is approaching overvoltage from a decelerating motor acting as a generator. Resistors can also be connected directly across the motor leads to stop the motor and I think that was more common in the old days.  These days when you hear about regenerative technology in the hybrid cars, instead of using a resistor, they put the energy back into the battery.
  

You can't slow down a motor without taking energy out of it, and by definition, the motor is acting as a generator.
If you simply disconnect the motor, it will not act as a generator and will take much longer to slow down, so I do not consider that as decelerating. Friction will stop it eventually, in which case the motor is acting as a heater.