Mini Cim torque in brake mode?

[jfitz0807]

We're looking to use two mini cims to drive our tote lifter mechanism. We've verified that we have enough torque with a 10:1 gear ration to lift. We're including a spring to help hold some of the weight.

We're having trouble finding the torque rating of a mini cim in brake mode. We're wondering if we can just stop the motor and have it hold the stack of totes in place or will we have to include some kind of mechanical brake to help. We'd rather not have to rely on a PID loop to actively hold the lifter at a particular position as this will be more of a draw on the battery.

Any other suggestions are, of course, welcome.

Thanks for the help.

[JamesCH95]

Brake mode is a dynamic brake. That is to say braking torque is generated only while the motor is turning. The faster the motor is turning the more torque is generated. This will slow your elevator, but is incapable of stopping it.

Have you calculated what power is required to hold the elevator in place?

[Chadfrom308]

From what I understand, brake mode shorts the two leads on the motor, or perhaps something similar to that. Try it for yourself. Short the two leads and try to spin the motor. You can, just not very easy or fast. I think that's what the brake system does.

[Al Skierkiewicz]

Torque is really not the correct term here since the motor is not generating any work. It is simply acting as a load for a mechanical to electrical convertor. As James pointed out, the load varies with the shaft speed of the motor. The greater the shaft is spinning, the more current is generated and since the output is shorted, the greater the counter EMF.

[Ether]

Originally Posted by JamesCH95

Brake mode is a dynamic brake. That is to say braking torque is generated only while the motor is turning. The faster the motor is turning the more torque is generated.

Correct.

When the motor leads are shorted, the back EMF created by the spinning rotor causes current to flow through the motor.

And since current in a DC brushed motor is (roughly) proportional to torque, the faster the shaft is being spun, the higher the braking torque created by the motor.

[Richard Wallace]

Ether is correct as usual.

If I had more details of your lift system (e.g., what diameter is the winch drum, or chain sprocket, or belt pulley that is lifting your elevator carriage) then I could provide more useful guidance; however, a Mini-CIM at 10:1 seems very fast. My own team is using prototyping with a Mini-CIM at 50:1 to drive a 24 tooth, #35 sprocket that lifts our elevator carriage. It is fast enough (we think, ~9 inch/sec), has plenty of lifting force (~12 Ampere lifts four totes), and holds its position nicely when the motor is disabled (brake mode).

[theawesome1730]

Depending on your mechanism, and if you have pneumatics or not, you may look into using a pneumatic braking system i.e. bike disc brake and caliper or a "brake shoe" made from rubber that pushes on the rotating shaft you wish to hold still. Probably not worth adding a whole additional subsystem but if you already have air, it could be easy to add.