Method for applying constant force with nonlinear feedback control and constant force device using same

An apparatus positioned by a robot using robot position parameters proximate to a workpiece surface, the apparatus to apply estimated tool force substantially equal to a reference tool force on the workpiece surface, comprising:

• an actuator for exerting force on the tool, said actuator having a range of motion;
• a tool holder coupled to said actuator for holding the tool;
• accelerometer means coupled to said actuator for measuring tool force acceleration effects;
• potentiometer means coupled to said actuator and the tool holder, said potentiometer means for maintaining said actuator substantially centered within said range of motion;
• load cell means coupled to said actuator and having a load cell means output for measuring force applied by said actuator;
• a low-pass filter coupled to said load cell means for filtering said load cell output;
• a processor coupled to said low-pass filter, said potentiometer means and said accelerometer means, said processor calculating the estimated tool force, including force applied by said actuator and said actuator acceleration effects;
• a memory for storing processor programs for controlling operation of said processor, and the reference tool force, said memory being coupled to said processor;
• a processor interface for entering the reference tool force and robot position parameters into said processor;
• and
• actuator force control means coupled to said actuator for rendering the estimated tool force substantially equal to the reference tool force on a workpiece surface under control of said processor.

An apparatus positioned by a robot using robot position parameters proximate to a workpiece surface, the apparatus to apply estimated tool force substantially equal to a reference tool force on the workpiece surface, comprising:

• a double-acting pneumatic actuator for exerting force on the tool, said actuator having a first gas chamber and a second gas chamber, a movement axis, and a range of motion;
• a tool holder coupled to said actuator for holding the tool, said tool holder being capable of movement substantially parallel to said movement axis;
• accelerometer means coupled to said actuator for measuring a tool force acceleration component;
• potentiometer means coupled to said actuator and the tool, said potentiometer means for locating said actuator within said range of motion;
• load cell means coupled to said actuator and having a load cell means output for measuring force applied by said actuator;
• a low-pass filter coupled to said load cell means for filtering said load cell output;
• a processor coupled to said low-pass filter, said potentiometer means and said accelerometer means, said processor calculating the estimated tool force, including force applied by said actuator and said actuator acceleration effects;
• a memory for storing processor programs for controlling operation of said processor, and the reference tool force, said memory being coupled to said processor;
• a processor interface for entering the reference tool force and robot position parameters into said processor;
• and
• actuator force control means coupled to said first and second gas chambers for rendering the estimated tool force substantially equal to the reference tool force on a workpiece surface under control of said processor.

A method of applying an estimated tool force substantially equal to a reference tool force at a workpiece surface, comprising:

• providing an actuator for applying an actuator component of tool force through a mechanical linkage, said actuator having a first side input and an opposite-acting second side input;
• estimating a tool force acceleration component due to acceleration of said actuator;
• estimating a tool force friction component due to friction in said mechanical linkage;
• measuring actuator force applied by said actuator;
• low-pass filtering said actuator force;
• estimating actual tool force based on said tool force acceleration component, said tool force friction component, and said filtered actuator force;
• comparing said estimated actual tool force with the reference tool force to obtain an error signal;
• producing said first side actuator input by processing said error signal through a proportional-integral-derivative controller;
• producing said second side actuator input by combining the the reference tool force and the tool force acceleration component;
• and
• applying said first and second side actuator inputs to said actuator to obtain said estimated actual tool force substantially equal to the reference tool force.

A method of using an actuator controlled by first side and second side signals, the actuator acting through a linkage to a tool to apply an estimated actual tool force substantially equal to a reference tool force at a workpiece surface, comprising:

• estimating a tool force acceleration component due to acceleration of said actuator;
• estimating a tool force friction component due to friction in the linkage from actuator to tool;
• measuring actuator force applied by said actuator;
• low-pass filtering said actuator force with a filter having a half-power point between about 1 and 100 Hz;
• estimating actual tool force based on said tool force acceleration component, said tool force friction component, and said filtered actuator force;
• comparing said estimated actual tool force with the reference tool force to obtain an error signal;
• processing said error signal through a proportional-integral-derivative controller to form the first side signal;
• calculating the second side signal using a second side equation which includes the reference tool force and said tool force acceleration component as terms;
• and
• combining the first side and second side signals to control the actuator to apply through the linkage an estimated actual tool force substantially equal to the reference tool force.