The human hand is one of the body’s most complex structures. Researchers have made progress in the design of delicate grippers with fingers and thumbs. However, they are not yet available. Currently, only simple two or three-fingered grippers are used, and they accomplish only a limited range of tasks. Work is underway to develop more robust, five-fingered versions to take on more intricate work.
Progress in microelectronics and micromechanics has paved the way for producing a multi-fingered hand with separately controllable fingers and joints that replicate the human hand. In fact, the German Aerospace Centre (DRL) has developed a robotic hand with the Harbin Institute of Technology (HIT).
Thanks to micro and precise drive technology and high-performance bus technology, this development sets new standards for sensitive gripper hands that replicate human hands. The new DLR-HIT Hand II has five fingers, each with four joints and three degrees of freedom. The hand is also smaller and lighter than former versions. Four fingers are required for clasping conical parts and a thumb is used as an outer support. The mechanical range of movement must be properly controlled and monitored to enable full use of the hand.
The motors in the DRL-HIT Hand II fit directly into the fingers. Particular attention was paid to the control processor’s information with respect to positioning and operating data. This configuration facilitates the discrete drive to operate effectively. Every finger joint is fitted with a self-developed, non-contacting angle sensor and a torque sensor.
A high-speed bus transmits the data flow. Rapid feedback that compares target and actual value is crucial for the function of the controller, particularly in precise and delicate applications. Besides data volume, transfer speed is also vital, which is why an internal real-time 25 Mbps high-speed bus, based on field programmable gate arrays (FGPA)s, was developed for the application. Three leads are required for the external serial connection of hand and control processor. The actual controls consisting of a signal processor on a PCI insert card is integrated into a standard PC. The interface provides a way for the hand to be controlled at the PC with all sensor data displayed on the screen.
Each finger needs several drives which are controlled separately. In this instance, 15 brushless dc motors with Hall sensors are used for each hand. Maxon motor’s EC 20 flat drives were included because they are inexpensive, commercially available, and offer high power in a compact space. The motors and Hall sensors create a unit that is 10.4-mm long with an outer diameter of 21.2-mm. Each motor weighs 15 g. They are mounted with harmonic drive gears from the HDUC 05 range, which have the same diameter as the motor. The 3 W motors are available in a 12 or 24 V version and provide maximum torque of 8.04 mNm. Good dynamic behavior and preloaded ball bearings ensure precise response behaviour of control commands including changing the direction of rotation. The digital Hall sensors report the actual position to the controller accurately. The motors idle at 9,300 rpm. Thanks to compact drive technology with feedback and rapid data transfer per bus technology, the new DLR-HIT Hand II can be controlled very sensitively and precisely.