Servo-control system

A lift apparatus for transferring a substrate between a support and a transfer plane, the lift apparatus comprising:

• a lift pin assembly, comprising:
• a lift pin configured to move the substrate between the support and the transfer plane;
• at least one pneumatic actuator comprising movable member configured to provide a load to the lift pin;
• at least one proportional pneumatic valve configured to control fluid flow between the at least one pneumatic actuator and a pressurized fluid supply or a vent;
• a plurality of pressure sensors each configured to independently measure pressure in a respective supply line to the at least one pneumatic actuator;
• and
• at least one position sensor configured to measure a position of the member;
• and
• a servo-control system in communication with the lift pin assembly, the servo-control system comprising a controller configured to:
• determine an output force based on a commanded force and an estimated force, wherein the commanded force is based at least in part on a position measurement of the at least one position sensor, and wherein the estimated force is based at least in part on pressure measurements of the plurality of pressure sensors and the position measurement;
• generate a control signal based on the output force;
• and
• apply the control signal to the at least one proportional pneumatic valve to move the movable member.