Force control in robotics is conventionally implemented using either impedance control or admittance control. One example of an impedance control feedback loop is illustrated in FIG. 1. With impedance control, positions of the joints of the robot are inputted into the controller and joint torques for controlling movement of the robot are outputted and applied. In other words, the impedance controller determines position and applies (or commands) force/torque. In FIG. 1, the impedance controller applies specific joint torques to the joints. If the robot experiences external force acting on one of the joints, for example, the impedance control system does not calculate or measure such force. Instead, the impedance controller merely re-determines the robot position and re-calculates the requisite force to be applied.
Conventional impedance control may provide stable control when contacting rigid environments and may provide a light feel when engaging soft environments. However, impedance control can give the robot an unstable loose feel and may introduce errors when interacting with stiff virtual constraints, such as haptic boundaries, which limit movement of the robot.
Admittance control, on the other hand, is the inverse of impedance control. One example of an admittance control feedback loop is illustrated in FIG. 2. With admittance control, rather than determining position and commanding force, the controller instead determines applied force/torque and commands position. A force-torque sensor or joint torque measurements are used to detect input force to the system. Based on the detected input force, and knowing a current position of the joints based on measured joint angles, the admittance controller commands a new position of the joints by applying determined joint torques to move the joints accordingly.
Conventional admittance control can give the robot stable rigid feel and may reduce errors when interacting with virtual constraints, such as haptic boundaries. However, a robot subject to admittance control may feel heavy to a user and may overreact when contacting rigid environments. As significantly, using a single admittance controller that utilizes either the force/torque sensor or the joint torques to measure external force(s) acting on one or more of the joints provides significant challenges. Mainly, when the robot experiences such external forces, the location(s) (e.g., the joint(s)) to which the external forces are applied are unknown thereby potentially resulting in undesired dynamic behavior of the robot.