Current control of a permanent magnet DC motor to create constant torque

[MarkFidgeon]

I am trying to use a 24v DC motor in conjunction with a rotational to linear drive (rack and pinon) to drive the clamping force to hold a workpiece in position. So when the clamp is forced up against workpiece, the idea is that i can control the current input into the motor and therefore keep a constant force against the workpiece. (using the fact that current and motor torque have a linear relationship so by fixing the current to say 0.3amps I will know what the motor output torque is and therefore know the clamping force on the workpiece.)
My problem is I do not know what i need to do to build a control circuit to keep the current at a constant set value. Because motor driving against workpiece and therefore not rotating anymore it will look for more current and i need this current to stay at a set value. From discussing with others it was suggested I use a proportional controller where i can input my referance current and have a current monitor feed the actual current from the motor into the controller. And then put some more control cricuitry like a pulse with modulator to drive a two quadrent chopper to modulate voltage to the motor which will then in turn control the current.    All ideas welcome....

[johnwasser]

The LM317 adjustable voltage regulator can be wired as an adjustable current source: http://www.ae5d.com/led-1.html

[LROBBINS]

An alternative to directly controlling current is to read current with a sensor and then use that in a feedback scheme to control PWM to the motor.  If you search up the Roboteq web site and download their Roboteq nxtgen_controllers_userman.pdf you will find a pretty good description of how they do it either as an open-loop approximation or with PID.
Ciao,
Lenny

[PeterH]

using the fact that current and motor torque have a linear relationship.

For a given position of the rotor, they do. However, what that relationship is will vary depending on the position. (Try turning a motor slowly - you will feel the torque changing significantly as coils come in and out of the circuit and the alignment of the coils relative to the magnets change.) If you want to apply a specific force, rather than apply a constant (but unknown) force, then I think you will need to provide a better way of determining the actual force and use a feedback algorithm to control it.