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.>