braking a DC motor

hi there,

i'm trying to drive some pulley using a DC motor, H-bridge and Arduino. the problem is - once I stop the motors movement (either forward or backward), the the weight on the pulley slips..

I need a way to actively hold the motor (like with a servo, and not just shortening the motor terminals..).

will alternating the motor back/forward fast enough will do the trick ? or is there a more clever way of doing that? :o

thanks in advance ! rubin

A ratchet with a servo-operated pawl?

Richard - what do you mean by "no practical electronic/microcontroller answer" ? -- how do servos manage to hold a torque ?

how do servos manage to hold a torque ?

Good point. A servo does continue to measure it’s actual position compaired to it’s commanded position and applies motor voltage in the proper polarity and amplitude to correct for the positioning error.

For you to do the same would require that you get a real feedback from the motor/pully position and read it into the software then then apply correcting actions to the H-drive. Not a easy project but possible with enough study and research.

A easier solution might be a solenoid operated lock on the motor shaft or pully assembly. However you must make sure your software never trys to move the motor with the lock on or high current will result in damage most likely.

Lefty

Stepper motors are held in place at each step, too. Depending on your design, you might be able to use a stepper motor. They are available in tiny to massive sizes. The larger ones require some pretty spendy controllers, though.

For simplicity - Rich C’s suggestion of a motor with a worm gear is pretty much unbeatable. I use one in an electric gate. Allows you to get crazy torque in a small package.

Good point. A servo does continue to measure it's actual position compaired to it's commanded position and applies motor voltage in the proper polarity and amplitude to correct for the positioning error.

Only if the PPM signal and power are applied...

When they are not applied, though, a servo can still "hold", because a servo's (RC) gear reduction is accomplished using a spur gear drive train. Spur gears, when there are enough involved in a gear reduction system, are difficult (but not impossible!) to backdrive; it can take a lot of torque to do so on the output shaft (so much so that the shaft or teeth on the gear can break).

I do agree, though, that if you need an "impossible to backdrive" transmission that can stay that way with power off, the easiest way would be a worm-gear drive gear motor. Barring that, a mechanical solution using an electromagnetic brake (that is applied only when power is off) is the next best thing.

:)

thanks.

btw: i wasn't looking for a solution to hold the torque without any electrical power or control. on the contrary - i was looking for a way to actively control a DC motor to stay foot and resist some torque.

i don't have all the numbers/specs/details of my build at the moment - and i was just looking for ideas to implement some active-brake-control..

so far, it seems that the stepper motor is the right solution for me.. thanks again for all the help :)