H Bridge help

[dallypost]

Hi All,

This is my first post on this forum. I have a product that we have marketed for a few years now. I use an AVR 644P, an off the shelf motor controller, and a resistive divider to control a 1/4 horse DC motor speed and direction.

I want to release a portable version that runs on a 12 volt car battery. I want to create an h bridge circuit to provide motor control via pulse width modulation.

Here is my problem. I am not an engineer. I have found a tutorial ( http://hades.mech.northwestern.edu/index.php/Driving_a_high_current_DC_Motor_using_an_H-bridge )that may work for me but I have been able to find an answer to one question. I need the motor to aggressively break to a stop. The second paragraph of this tutorial states that "To keep the motor stationary, forward voltage is applied half of the time and reverse voltage is applied half of the time (PWM=50% duty). If the voltage reversals are at a high enough frequency, the cycling is unnoticeable." I know that this is not regen braking, but will it accomplish the same result?

Thanks.

 

[Resqueline]

I believe that will quikcly brake and keep any motor type quite stationary.
To brake a PM motor fast initially you can also just keep the lower bridge transistors on. This won't keep it from crawling from a constant external force though.

 

[Harald Kapp]

Many years have passed since I had to study electrical machines (motors). But from what I remember and what seems logical:

1) Pulsing the motor alternatingly with positive and negative voltage is equivalent to operating the DC motor with AC. If done fast enough, the motor will not turn, so far o.k. But: the motor and the H-bridge both will consume power even if there is no motion, thus draining your battery faster than necessary.
Since U=L*di/dt, if you force a reverse voltage on the motor (high Delta-U), you will induce a high di/dt, which is probably unwanted and may cause havoc.

2) Breaking the motor by opening both lower bridge transistors (or both upper bridge transistors, but by no means two transistors within one path, left or right, simultaneously) will bring the motor to a halt without using energy from your battery (apart from the power needed to control the transistors).
I'd sugggest this technique over /1/.

None of these techniques will completely counter any external force/torque applied to the motor. Even if you short-circuit the motor terminals (which is what no. 2 does) there will be movement of the motor's axle due to external torque. Anyway this technique (2) should work better than applying AC.
If you want the stopped motor to resist external torque, you would need to apply a voltage depending on the torque whcih in turn requires sensors and so on. Alternatively the stopped motor could be arrested by a mechanical brake.

Regards,
Harald

 

[dallypost]

Thanks everyone.

External force, causing the motor to creep is not a problem. Our product is a programmable / remote controllable moving target that is used by law enforcement for firearms qualifications. The motor has a drive pulley attached and an idler pulley that is about 80 to 100 feet away from the drive pulley. A cable runs around the drive pulley, out the the idler pulley and back. A target is suspended from the cable so when the motor turns, the target moves. Therefore, external loads on the motor are insignificant. Last night I found another Mosfet driver that will allow me independent control of all four Mosfets in the h-bridge. I think I will use it and get my breaking by opening the two upper Mosfets and closing the bottom Mostets.

Problem solved, I hope. I will post my schematic when ready and see if you can find mistakes.

Thanks