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Micro-Actuators Move Sensitive 4-Finger Hand
MICROMO Posted 10/07/2013
To date, use of robotic hands in industrial production has been restricted to rugged two and three-finger grippers. They are being used for the purpose of executing relatively simple movements. Robotic hands for more delicate tasks have proven unsuccessful due to the lack of available technical capabilities. Positive interaction of microelectronics and micromechanics has now produced the much sought-after breakthrough. Indeed, technological advances within this field are continuously growing. Thus, robotic hands with separately controllable fingers and joints based on human hands are no longer fiction and will probably be available soon on a day-to-day basis within the industrial sector.
The human hand is one of the most universal tools in nature. No wonder that researchers are eager to apply the advantages of this evolutionary design to a new generation of robotic hands. The German Aerospace Centre (DLR), in cooperation with the Harbin Institute of Technology (HIT), has already developed a robotic hand similar to a human hand - with the aid of miniature actuators and high-performance bus technology.
Constructing a robotic hand with the capabilities and dexterity of a human hand requires at least four fingers: three fingers to allow the robotic hand to grip conical parts, and a thumb used as a support. Consequently, the new robotic hand consists of three fingers, each with four joints in three degrees of freedom. The fourth finger, designed as a thumb, has four degrees of freedom. It goes without saying, that the diverse movements made possible by this solution have to be controlled and monitored in a practical manner. Within this context, high-performance information channels are an essential function of the control system, particularly when performing intricate tasks. Therefore, along side high-volume processing, time is of the essence. The real time-capable 25 Mbps high speed bus is incorporated in the robotic hand itself and developed specifically for this application.
In the past, robotic fingers were moved using cable pulls. In contrast, modern-day microengineering allows the motor to be fitted directly in the finger. In this case, supplying the control processor with the requisite position and operating data. This is an integral part of the overall operation - and the only way of allowing the actuator to use its strengths to the fullest. Each finger joint features a company-designed contactless angle sensor as well as a torque sensor. Since both sensors require an extremely high resolution, a bus is used to transfer the wealth of data required. Rapid feedback for comparison of setpoint and actual value is crucial to the FPGAs (Field Programmable Gate Arrays). Only three leads are required for the external serial connection from hand to control processor.
The actual control system, a signal processor on a plug-in PCI card, is integrated in a standard PC. An operator-friendly interface allows the "hand" to be controlled from the computer. All sensor data can be displayed on the monitor. Data display, control and the connection of hand to computer were designed from the outset, with a view to future use in industrial environments. Besides the "nerves" and the "brain", a functioning hand also requires "muscles" to give it strength.
The tremendous complexity of the new robotic hand has its price. Each finger requires several separately controllable actuators. In this particular case, there are twelve electronically commutated DC motors (EC motors), including analogue Hall sensors, per hand. The team of engineers opted for actuators developed by miniature motor specialist FAULHABER since they covered the full range of specifications required. They are low-cost, commercially available, performance-packed products with an extremely small footprint. Brushless DC servo-motors with a diameter of 16 mm were selected for the four-finger robotic hand. They can be connected with gear systems of the same diameter to form one integrated unit. The motors are available in 12 V versions and 24 V versions and feature an output of 11 W with a maximum continuous torque of up to 2.6 mNm. A good dynamic response, even when subject to changes in direction of rotation, and pre-stressed ball bearings ensure precise response behaviour to control commands. The analog Hall sensors fitted as standard, signal the exact position to the control and deliver the requisite feedback information with a resolution of at least 8 bits. The Hall sensors and motor form a compact unit with a length of only 28 mm and an outer diameter of 16 mm with a weight of a mere 31 g. The motors idle at 29,900 rpm. The actuators are combined with all-metal planetary gearheads. These standard FAULHABER products reduce the high rotational speeds for use in the hand and, at the same time, enhance torque. A broad selection is available with ratios from 3.7:1 to 5647:1. The ratio used in this application is 159:1. The permitted torque thus increases to a maximum value of 450 mNm. The gearhead itself weighs a mere 33 g, with an overall length of 29.4 mm. The new HIT-DLR robotic hand can be controlled very delicately and precisely thanks to the compact actuator technology with feedback and fast data forwarding by bus. In this way, microengineering and microelectronics complement each other perfectly. Armed with standard components and a good concept, engineers can now manufacture products that would have been inconceivable several years ago, even with the most expensive customized components.