Torque Sensors for Robot Joint Control

Abstract

In the field of service robotics, there is a growing need for robots capable of physical interaction with humans to assist with daily life tasks. The desired coexistence of robotic systems and humans in the same physical domain (sharing the same workspace and actually cooperating in a physical manner) poses very fundamental problem of ensuring safety to the user and robot. Even without wrong programming, a robot, moving freely in a human environment, is potentially dangerous because of its large moving masses, powerful actuators, and unpredictably complex behavior. Design and programming of the robots exhibiting intrinsically safe behavior in a human domain are great challenges in robotics because such robots have to deal with unstructured time-varying environment. Several humanoid robots aimed at integration into people environment were developed (Sakagami et al., 2002), (Kaneko et al., 2004). However, despite the splendid means for sensing the environment (visual, audio, and haptics), the 6-axis force/torque sensors attached at the tip of the robot arm and a stereo vision system which is slow to track the changing environment in real-time, are only the abilities to anticipate and handle the collision. The rest parts of the robot body (forearm, elbow, upper arm, shoulder, and torso) are presenting the significant danger not only for human being, but also for the robot structure itself. Effective methods on enhancement of contact detection ability of manipulator were reported. To avoid collisions in time-varying environment, Lumelsky & Cheung (2001) proposed to cover manipulator with a sensitive skin capable of detecting nearby objects. Mitsunaga et al. (2006) progressively improved the tactile ability of the robot through covering its entire body with piezoelectric-film-based tactile sensors. Since this device integrates a huge amount of small sensors incorporated into soft layer and requires the complicated wiring and signal processing hardware, it has high cost and reliability issues. The high-speed vision system attached to the robot arm aimed at real-time collision avoidance (Morikawa et al., 2007) presumes usage of expensive detectors, complex signal processing techniques, and issues of self-body extraction from the camera view area. Is should be noted, that such tasks under human supervision as transporting the object, leading the robot tip via force-following, performing the assembling tasks, require the processing algorithm of contact state. Finding the technical solution for trade-off between performance and safety is the target of a new manipulation technology. To cope with this issue, an active compliance control implying fast joint torque controlling based on measuring the applied external torque in each joint was developed. The first embodiment of torque measurement is the integration of a torque sensor into each joint of the manipulator. O pe n A cc es s D at ab as e w w w .in te ch w eb .o rg