Haptic Feedback Rotary Control Dial/Knob

Please help! I need some opinions and peoples thoughts on the following project:

I need to design a rotary dial which has 'haptic feedback' as part of a project for a medical application. The idea is that there is a rigid plate which moves along a slider and compresses a part of the human body. A load cell measures the amount of force exerted by the compressor plate on the human body part.

The operator controlling this system moves this plate along the slider by rotating the dial (lets say at this point there will be a frequency encoder on the dial to measure the speed and direction of dial movement). As the operator adjusts the position of this plate, and the plate comes into contact with the body, and the force starts to increase, the feeling of turning resistance in the dial should increase in order to give the operator a 'feeling' of how much force is being applied.

There s pretty much like one or 2 products on the market that could potentially do this - although it would be like buying a computer when all you need is a calculator.

I was thinking of a sort of electromagnetic braking disk which could apply increasing force to a spinning metal disk attached to the dial shaft, thus making it more difficult to turn. Any thoughts? If so I need to buy a suitable electromagnet. (buying components rather than constructing them is almost 100% preferred for this project). I would like to use an arduino to do the processing and control (receive signal from load cell and drive braking actuator).

All thoughts and comments are very much appreciated!

I was thinking of a sort of electromagnetic braking disk

They're called electromagnetic clutches; typically, they are mounted on a shaft, and you apply a voltage (12 volts is common) to "lock" it up. They tend to use quite a bit of current, so you would need a hefty transistor or MOSFET interface, like controlling a relay or solenoid (remember the blocking diode).

Potentially, you could control this using PWM - that would be the first thing to try; if that wasn't satisfactory, then maybe some form of PWM to voltage or current controller would work better.

Another option might be to attach the shaft to a DC motor, or maybe a stepper motor, then actuate that as needed; this might be an issue, though, as if the operator let go of the knob, it would start spinning in the opposite direction he was turning it in - not sure if there would be a way around this or not.

Hope this helps...