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Joint Torque Sensor


Low cost Force/Torque Sensor
   - Research Background        
  • As robots are required to perform tasks involving contact with the environment, accurate sensing of the external load at the end-effector of manipulators is increasingly important.

   - Research Objectives
  • Low-cost F/T sensing system based on strain gauge
  • Automatic calibration algorithm for compensation of cross-torque error
  • Force control algorithm using the low-cost F/T sensing system
                
     * Calibration algorithm - Actual sensing frame method
  • Crosstalk calibration algorithm using coordinate frame transformation method
  • Coordinate transformation from nominal sensing frame to actual sensing frame
  • Nominal sensing frame: strain gauge is supposed to generate the force or torque components
  • Actual sensing frame: strain gauge actually generates the force or torque components
Joint Torque Sensor
   - Research Background
  • In the field of manipulation, force control of robots has been studied by measuring joint torques
   - Research Objectives
  • Low-cost joint torque sensor based on strain gauge
  • Embedded amplifier with digital output
   - Principles
  • The structure of the sensor is designed so that it deforms linearly as external force is applied up to the elastic limit(Hooke’s law).
  • The structure is only sensitive to the torque applied with respect to the rotational axis.
  • FEM analysis(stress, strain and deformation analysis) is used to find the optimal attaching position of strain gauges(about 1000 μm/m) and to calculate the rotational and bending stiffness.
  • The strain of a sensor can be measured using strain gauges.
  • Four strain gauges are placed in a Wheatstone bridge configuration.
  • Full bridge circuit is used to amplify the signal while minimizing the possible errors due to heat, misplacement and so on.
                                         
                 <Deformation of elastic structure by external torque: (a) side view, and (b) top view.>

                                        
         <Measurement of external force: (a) installation of strain gauge in spoke, and (b) full-bridge circuit.>

    - Research Output
  • Joint torque sensor
  • Sensing range: 0 ~ 10 Nm
  • Limit torque: 30 Nm (safety factor: 3)
  • Rotational stiffness: 500 Nm/deg
  • Amplifier
    • 2 channels
    • 400 times signal amplification
    • Embedded ARM processor
    • 485 communication
    • 1.5 kHz bandwidth
                                  
                                           <Joint torque sensor: (a) structure, and (b) amplifier.>