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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: Rick J. Bickle rbickle at swbell.net
Date: Mon Dec 29 13:31:00 CST 2003

Without having done any testing on this, my first impression is that the
battery voltage would behave like a coefficient change for each PID
term. Normally you would have the coefficients:

P_gain
I_Gain
D_Gain

With a lower battery voltage, each coefficient would be reduced as in:

P_Gain * Vbat
I_Gain * Vbat
D_Gain * Vbat

Where Vbat is normalized to 1 at maximum voltage.

So as the battery voltage is reduced, each PID coefficient is
effectively reduced too.
You might be able to compensate for the voltage drop by using an A/D to
monitor voltage and correct for the drop like:

P_Gain * 1/Vbat
I_Gain * 1/Vbat
D_Gain * 1/Vbat

As Kip mentioned, this all assumes that the system never hits the limits
of the PID. (In other words the PWM never gets to 100%)

Rick


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