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Posted: Mon Oct 12, 2015 8:17 am
by unclethehornet

I'd like to get the HBridge shield to brake a motor. I think that I should be able to achieve this by setting both the input lines to High, but the A4954 spec says:

The braking function is implemented by driving the device in Slow Decay mode, which is done by applying a logic high to both inputs of both channels, after a bridge-enable Chop command (see PWM Control Truth Table). Because it is possible to drive current in both directions through the DMOS switches, this con- figuration effectively shorts-out the motor-generated BEMF, as long as the Chop command is asserted.

I have no idea what "after a bridge-enable Chop command", "as long as the Chop command is asserted" means.... ?

I'm hoping to get some advice before I start shorting motors. Thanks in advance.



Re: Braking

Posted: Wed Oct 14, 2015 4:16 am
by angusgr
Hi Dave,

I believe that section is referring to braking under PWM control, where the motor stays turning and the current is high but you PWM the motor and reduce the duty cycle gradually, rather than immediately going to a hard stop.

The datasheet shows the motor is in this "chop" mode whenever 10 x Vs > Vref. If you run this through the settings in the HBRIDGE schematic for Vref and Rs, you find it means "the motor is in chop mode whenever there is more current flowing than the current limit setting".

Essentially, I think what they're recommending is that for a large motor you use PWM to slowly reduce the duty cycle of the motor, so it brakes gradually and less energy has to be dissipated directly through the h-bridge. This is probably a bigger problem when there's also a lot of inertia in whatever the motor is driving.

For a small low current motor (or one spinning at low rpm without a lot of inertia) I suspect you can just drive both inputs high, for a "hard stop" and the HBRIDGE will dissipate the heat generated just fine. Alternatively, ramp the speed down with PWM as described.


Re: Braking

Posted: Wed Oct 14, 2015 6:23 am
by unclethehornet
Hi Angus,

Thank you very much. I shall give it a go!