So when you command 600 whatevers to the servo the motor is going
maximum speed, and when you command 2900 whatevers to the servo the
motor is going at minimum speed? What ever is your unit of measure?
The conceptually easy way to handle this is to translate your motor
speed commands into servo commands, e.g. if your maximum speed is
12000RPM then you calculate the offset and slope using good old y = ax + b:
y1 = 2900, x1 = 0
y2 = 600, x2 = 12000
Then b = 2900, and a = (600 - 2900) / (12000 - 0) ~ -0.192. Then make a
regular old controller.
The least-processing way to handle this is to operate your controller in
reverse -- bring your speed (or position) command to the subtracting
port of your summing junction, bring your speed (or position) feedback
to the adding port, and have at it. All the signals in your controller
will be negative of what intuition says they should be, but the system
will work fine.
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