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[DPRG] Driving an H-Bridge with a switching power supply

Subject: [DPRG] Driving an H-Bridge with a switching power supply
From: Chuck McManis cmcmanis at mcmanis.com
Date: Fri Dec 26 17:43:01 CST 2003

I've been thinking about this off and on ever since Dave posted about it.

What you've done is the equivalent of putting a crystal in an oven on the 
mother board to stabilize its temperature drift.

I tend to agree with Kip that if you're feedback system can't compensate 
for the change in input voltage then it isn't operating on the fundamental 
control loop, and that would be a place to start looking.

What Dave is saying is that the nodes on his feedback function change with 
respect to voltage. And thus the parameters that control how well damped 
his system is, also need to be tuned.

Motor torque is proportional to current, and by Ohms law, the higher the 
voltage across the motor the more current can flow and more torque can be 
produced.

Dave's PID algorithm infers current, and thus torque, by PWM'ing voltage.

This is probably the fundamental reason that people switch to chopper 
drives vs voltage drives. By leaving the voltage quite high, you can always 
develop maximum current in your motors.

The fundamental issue seems to be, again as Kip mentioned, "head room." If 
you tune your algorithm based on the maximum amount of torque you can get 
>from your motors, then you need to insure that you can always get that 
torque. One way to do that is to use a chopper circuit that takes a very 
high voltage to insure voltage is available, and a current limiter to limit 
the torque to an allowed value.

Another way is clearly to create a voltage that doesn't change and let the 
resistance of the motors set the torque point.

I'm guessing that choppers are used in motion control applications rather 
than SMPS solutions because the resistance of the motor can change from 
motor to motor, but its current/torque characteristics are tightly 
controlled. That would make it possible to manufacture without 
changing/tuning for each set of motors.

To wrap it around to the original thread, I'm guessing that if Dave were to 
use his PWM algorithm to feed the current cut-off circuit of a chopper 
drive, then his motor control would be controlling current (and torque) 
directly, and as long as the batteries had enough voltage to generate the 
maximum torque requirement his system would remain remarkably stable.

--Chuck



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