Current-Based Impedance Control for Mobile Manipulators Without Force/Torque Sensors

Abstract

Mobile manipulators, which combine a mobile platform with a robotic arm, are versatile robots that can be used for a variety of tasks like logistic pick-and-placing, manufacturing or assembly. Compliant control for mobile manipulators could improve the safety of the users sharing their workspace with these robots. The two general methods of compliant control, admittance control and impedance control, require force/torque sensors, which are often not available on low-cost or lightweight robots. This report presents an adaption of impedance control, including a strategy to compensate for joint friction, that can be used on current-controlled robots without the use of force/torque sensors. A calibration method is designed for the arm, that enables estimation of the actuator's current/torque ratios and frictions, used by the adapted impedance controller. Software is developed to use the controller on a combination of the Kinova GEN3 Lite arm and the Clearpath Dingo Omnidirectional base. Real-world experiments with the arm show that the calibration method is consistent and that the designed controller is compliant while also being able to track targets with five-millimeter precision when no interaction is present. Experiments with the complete mobile manipulator showcase two new modes, both suitable for interaction with a human. The first is a guidance mode where the user can control the robot using interaction with the arm. The second is a tracking mode where the mobile manipulator tracks a moving target while still being compliant.