Robotic hand uses multiple sensors to mimic human functions

Citation

(2007), "Robotic hand uses multiple sensors to mimic human functions", Sensor Review, Vol. 27 No. 4. https://doi.org/10.1108/sr.2007.08727daf.003

Robotic hand uses multiple sensors to mimic human functions

Robotic hand uses multiple sensors to mimic human functions

The Institute of Mechatronics and Robotics at the Deutsches Zentrum für Luft-und Raumfahrt, the German Aerospace Centre (DLR), has been designing and building dextrous robot hands since 1993. Working with the Chinese Harbin Institute of Technology (HIT), it has developed an advanced four-fingered robotic hand, the “DLR-HIT Hand” based on the earlier “DLR Hand II” design. By exploiting miniature actuators, high performance bus technology and a multitude of sensors, some having been developed specifically for this application, the hand scores over earlier two- and three-finger gripper designs as with four fingers it can conduct far more complex and delicate tasks (Figure 3).

Figure 3 The robotic hand (DLR)

A critical aim of the design was to create a hand that would mimic closely the capabilities and dexterity of a human hand and to achieve this it was necessary to miniaturise and minimise the weight of all the component parts. Further, a robotic hand with this capability requires at least four fingers; three allow the hand to grip conical parts, with the thumb acting as a support. Consequently, the new hand consists of three fingers, each with four joints with three degrees of freedom, and a fourth, designed as a thumb, with four degrees of freedom. In total the hand has 13 degrees of freedom but unlike many earlier designs which used cables and pulleys to create motion, this is provided here by miniaturised, brushless DC motors, coupled to harmonically-driven gears. Control is achieved with a high speed, real-time serial communication bus, running at 25Mbps, which has been implemented using field programmable gate array technology and only three cables are required for the communication between the hand and an external CPU.

Sensors play a central role in the design and 16 are used on each individual finger. Three potentiometric sensors based on conductive plastic technology measure joint position with an angular resolution of 0.1 degree and are coupled to a pre-amp and a third- order Butterworth filter; three strain gauge-based sensors measure joint torque; three thermistors monitor the motors and a further three temperature sensors are used for thermal compensation; three Hall effect position sensors monitor the motors' speed by electronic differentiation of the position signal; and a unique six- dimensional force/torque sensor based on strain gauge technology is located on each finger-tip (Figure 4). Patents are pending on this sensor which was developed specifically for the hand. It has a diameter of 20mm and a height of 16mm and the force and torque measuring ranges are 10N for Fx and Fy, 40N for Fz and 150Nmm for Mx, My and Mz. With a mass of only 7g, it features fully integrated electronics and a digital serial interface with 12 bit resolution and has 200 per cent mechanical overload protection.

Figure 4 Miniaturised strain gauge force/torque sensor developed for the robot hand (DLR)

A version of the DLR-HIT Hand is now commercially available from German tooling and automation specialist Schunk. Known as the Schunk anthropomorphic hand, this differs somewhat from the original design in that the potentiometric joint angle sensors have been replaced by non-contacting magnetic sensors.