Servo Motors: Precisely Control Force & Pressure

[KingNothing]

A normal servo motor gives a highly accurate response to an input, with the output being an angular position of the shaft. For example, give it a 2ms pulse and it might go to +63 degrees.

I am wondering what sort of motor (or system) is used to control an amount of force or pressure. For example, say I have two metal plates which I want to press together with a very specific force. How is this accomplished?

 

[sparkey]

it sounds to me as if you are talking about a press. whether to stamp out a part from steel or press liquid out of a slurry i would not try and do it with a direct mechanical linkage to a motor. this is a high stress situation in which, it is my opinion that hydraulics are the best option. a simple AC induction motor drives a hydraulic pump which sends hydraulic pressure to a piston with some flow control and pressure relief valves on your pressure lines to control the system.

if it is NOT a high stress application you could go the rout of transferring the angular positioning into linear positioning using a screw shaft (lead screw) and a ballnut on a carriage or trolley. the server would provide and highly accurate positional response that is converted to linear consistently and measurably.

here is a website showing what i am talking about with the screw shaft (lead screw)

http://www.thomsonbsa.com/

did i understand what you were trying to accomplish correctly?

 

[schip666!]

Just a bit of clarification...

Your description of a "normal servo motor" sounds more like what many call, with justification, a "hobby servo motor". These are used in a radio controlled airplane to adjust flaps and such-like. They move to a particular angular position based on the width of an input pulse. They don't have a way to adjust the output power -- pretty much whatever the motor is rated for is what you get.

The more general type of servo motor is, usually, a DC motor with an encoder that senses the output shaft's position and speed. The controller for such a motor can then supply a varying amount of power to move the motor's shaft at a particular speed or to a particular position using feedback from the encoder. The input to this system could be step-pulses (like a stepper motor) or a voltage proportional to the desired speed. You can probably imagine how one would make a hobby-servo using such a thing, but that's only one possible application.

Using the "raw" servo capability one can indirectly adjust the power output of the motor, but it's very dependent on the load and motor capabilities. You could combine a strain-gauge or accelerometer with the encoder signal to provide force feedback to accomplish your variable press application.

 

[TheAnalogKid]

angular position motors measure their current position with a sensor (rotary encoder, hall sensors, potentiometer, etc.) and then use a feedback system to adjust the voltage in the motor until the position they measure is stable and at the value that they want. The same can be done for a motor if you change the sensor to a force sensor and put the measurement into a feedback system.

Torque is a better property to talk about than force with a motor, and most motor datasheets have a value called the torque constant Kt in units of N-m/Amp. You can use a current sense resistor, current mirror, or some other current sensor that tells you how much current is going into the motor, and then you can calculate the amount of Torque the motor is applying. Combine this with your position sensor, and you can determine over how many Newtons are applied per distance, and combining these two measurements you can put them into a feedback system that will vary the voltage into the motor to keep the force at the value you command it.

So a motor can control both angular position and force applied depending on your load.