Motion control systems, which normally control the position of actuator or stage, are at times given the task of providing both position and force control. These systems operate in a dual mode, in which either the position or the force applied is controlled by the motion controller.
In the force-control mode, it is essential for the motion stage to apply a controlled force while there may be a change in the position. In position control mode, closed-loop servo is used for controlling the stage position, with a position encoder as feedback. Upon being coupled with a direct-drive linear motor, there is no friction in an air bearing, implying that the driving system (motor, controller, drive) can directly transduce electrical current into applied force. The lack of friction in the bearing means that there are no parasitic forces, and it is possible to directly control the force through the closed-loop servo, with no need for an external force sensor.
Example. A-131 Frictionless precision linear actuator with air bearings
The value of force that can be applied is a function of the power available in the motor and drive system. The resolution and precision with which force can be controlled is a function of the bandwidth and control system fidelity. It is possible to apply force in horizontal or vertical directions. A counterbalance system is needed when it comes to vertical motion. The counterbalance system must be designed in such a way that it is friction-free to prevent parasitic friction from being introduced into the system.
In a typical force control application, it is essential for the stage to be positioned very close to the point at which the stage comes into contact with the target substrate. The motion controller dynamically shifts to force control mode (without losing servo lock) and moves the stage until a target force is attained. Subsequently, the control system can maintain a constant force or change the force to adhere to a programed profile. In this mode, the position of the stage is permitted to move freely while the applied force is controlled.
Common applications of force control:
- Polishing of non-flat or flat surfaces
- Assembly operations in which it is necessary for a force to be applied during a bond or weld operation
- Brake friction testing
- Life cycle and durability testing of touch interfaces and push-button components
- Touch-screen sensitivity testing
Depending on the system, there will be a difference in the repeatability and precision levels to which the applied force can be controlled. PI has developed a number of systems for customers that enable forces as small as 0.5 N to be controlled with an accuracy and repeatability greater than ±0.02 N.
Highly repeatable force control. PI has experience with air bearings and force control down to the 0.5 N range with accuracy and repeatability of better than ±0.02 N.
Stability. Constant force can be maintained with little or no drift over long periods of time
This information has been sourced, reviewed and adapted from materials provided by PI (Physik Instrumente) LP, Piezo Nano Positioning.
For more information on this source, please visit PI (Physik Instrumente) LP, Piezo Nano Positioning.