PWM constant current source ... help

Hi folks,

I'm trying to think of a good way to implement a PWM cc source for an
inductive load (a small DC motor). For resistive loads like battery
chargers it's obviously enough to just toss a current-sensing resistor
into the source leg of the low-side driver. But for an inductive load I
need to take the current trough the freewheel diode into account.
Currently my basic current-sensing circuit looks like this:

,---+--+27V
| |
Motor |
V prop | |
/ to I /+|-----+ D1
/ / | | |
--+-R1---<G=1| Rs |
| \ | | |
C1 \-|-----+---'
| |
gnd D
PWM---G Q1
25kHz S
|
gnd

(plus, of course, error amp, sawtooth generator, comparator, gate
driver).

This kind of works (with some , but I need an auxiliary supply for the
diff amp because the drain of Q1 swings between 0V and Vcc + diode drop,
and the other node goes Rs*I higher. I'd like to avoid this. I added
dividers into the diff amp's inputs to circumvent this (CMMR not an issue
due to low accuracy requirements), but things are getting a bit messy
now. I can't put Rs into the top leg of the motor because I can't break
the connection between the +27V line and the motor (it's inside a black
box, quite literally).

Seeing that I need to build about 20 of these I started thinking: Maybe
there's a better way to do all this that I'm not aware of. Especially
since I'm not familiar at all with the electrical properties of DC motors
except that they are inductive (I'm thinking that they behave like a
constant inductance in series with a resistor that increases with rpm).
Perhaps there's some cool exploit to use.

The idea is to have a roughly constant-torque setup. The motor is rated
24V/1A. My first lashup simply used a 24V/25W lightbulb which almost
works well enough but dissipates too much power.

Thanks,
robert
.

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