Galil Motion Control note 2319 Page 1
DMC-1000
DMC-1500
DMC-1300
Application Note 2319: Force Feedback
Many applications require some sort of force feedback.
The typical application for force feedback includes a sensor that can output
a voltage ( +/- 10 volts ) as a function of pressure or compression and a
DC motor to move the mechanism.
There are two basic approaches to closing the loop in a force feedback
system:
Close the loop around the force feedback sensor directly:
This is done by configuring the controller to accept analog feedback and
simply plugging the sensor into the card. Once connected, the loop is
tuned like any servo system. One drawback of this configuration is that the
sensor must always be in contact with the object being applied the force. If
the contact is lost, the feedback is removed and the system is out of control
( much like pulling the encoder signal out of a servo motor loop ). Another
potential problem may be the gain of the system. Tuning may prove
difficult in some sensor configurations because of the high gain of the
sensor. The main advantages of this configuration are loop update speed
and ease of setup. Typical update times are 1ms and the setup is as simple
as plugging in the sensor and tuning the loop.
Use an encoder on the motor and close an outside sensor loop
In this configuration the motor is always in control. Sensor input is read by
a program on the controller from the analog inputs. This program then
tells the motor to jog forward or back to maintain the desired force value.
There is no need for the sensor to touch the object and tuning is a simple
task. Drawbacks to this setup include the need to write a program in the
controller and the slow loop update rate (from 2-20ms).
Control Diagram
Here is a simple control diagram of a system with an outside force sensor
loop:
