The human hand is one of the most universal tools in nature. No wonder researchers are eager to apply the advantages of this evolutionary design to a new generation of robotic hands.
To date, use of robotic hands in industrial production has been restricted to rugged two- and three-finger grippers. But now a "close-to-production" multisensory robot hand features four fingers, with four joints and three actuators each. The "thumb" provides an extra degree of freedom for fine manipulation and power grasping.
This new design is intended to create robotic hands for more delicate tasks, rather than executing relatively simple movements, which have proven unsuccessful due to the lack of technical capabilities available.
Joint Development Project
Based on the DLR Hand II design, the German Aerospace Centre (DLR), in cooperation with the Harbin Institute of Technology (HIT), has developed this robotic hand to be similar to a human hand with the aid of miniature actuators and high-performance bus technology.
Using actuators that are commercial, off-the-shelf, brushless dc motors with analog Hall sensors, all the motors are integrated into the fingers and palm of the robot hand. Each joint is equipped with a contactless magnetic joint angle sensor and a strain-gauge based joint torque sensor for precise feedback and control of hand movements.
A total of 12 motors, supplied by MicroMo Electronics (http://rbi.ims.ca/4929-500), help create complex and delicate movements of the robotic hand with a firm grip. Constructing a robotic hand with the capabilities and dexterity of a human hand requires at least four fingers. Three fingers allow the robotic hand to grip conical parts, with the 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.
Design Philosophy and Challenges
Achieving a goal of maximum flexibility required a design philosophy for the new robot hand. Designers aimed to miniaturize and completely integrate all components in the hand, reducing the cabling required.
"The most important accomplishment with this system is the size and power capabilities of the robot hand," says Shawn Thompson, an applications engineer for MicroMo. "In applications such as robotics or prosthetics, weight is always an important issue because there is a desire to mimic the weight and capabilities that a normal limb would produce. In robotics, you want to keep the weight down because that aids in reducing total system response times."
Miniaturizing the system and achieving good methods for efficient feedback and tactile sensing, allowing the hand to grip objects sufficiently but not too much, proved to be important priorities.
In the past, robotic fingers utilized cables and pulleys to create motion. But in contrast, modern-day microengineering has allowed the motor to be fitted directly in the
finger. Supplying the control processor with the requisite position and operating data is an integral part of overall operation and the only way of allowing the actuator to use its strengths to the fullest. But it also created a series of design challenges.
To achieve efficient operation, each finger joint features a company-designed contactless angle sensor, as well as a torque sensor. Since both sensors require extremely high resolution, a high-speed bus is used to transfer the data required. Rapid feedback for comparing setpoint and actual values is crucial to the function of the control system, particularly when performing intricate tasks. Along with high-volume processing, fast system response is essential.
A real time-capable, 25 Mbps high-speed bus, incorporated in the robotic hand itself and developed specifically for this application, is based on FPGAs (Field Programmable Gate Arrays). Only three leads are required for the external serial connection from the hand to the control processor.
The actual control system, a digital signal processor on a plug-in PCI card, is integrated into a standard PC. An operator-friendly software interface allows the hand to be controlled from the computer and, at the same time, all of the sensor data can be displayed on the screen. The engineering team designed the data display, control and the connection of hand to computer, from the outset, with a view to future use in industrial environments.
Miniature Actuators Provide Power
Besides a nervous system and a brain, the functioning hand also requires muscles to give it strength. The design challenge is to achieve complete integration of all components and the tremendous complexity of the new robotic hand.
Each finger requires several separately controllable actuators, and the design uses 12 electronically commutated dc motors with analog 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.
The motors are low-cost, commercially available, standard products with an extremely small footprint. The brushless dc servomotors selected offer a diameter of 16 mm and are connected with a gearbox of the same diameter to form one integrated unit. The motors are available in 12- and 24V versions, and provide power output of 11W and maximum continuous torques up to 2.6 mNm.
Good dynamic response, even when subject to changes in direction of rotation, and pre-stressed ball bearings ensure precise response behavior to control commands. The analog Hall sensors provide signals that communicate exact position to the controller 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, an outer diameter of 16 mm and a weight of 31g.
Standard Faulhaber products reduce the high rotational speeds required for operation of the hand and, at the same time, enhance torque. The biggest concerns were size and performance issues, and the ability to supply good torque capability through the gear train to establish enough output power and the amount of gripping force required by the robot.
The motors idle at 29,900 rpm and the actuators are combined with all-metal planetary gearheads. A broad selection is available with ratios from 3.7:1 to 5647:1 but the ratio used in this application is 159:1 which increased the maximum continuous torque output up to 450 mNm. The gearhead itself weighs 33g, with an overall length of 29.4 mm.
Ongoing Robotics Development
The DLR Institute of Mechatronics and Robotics (http://rbi.ims.ca/4929-501) has experience developing, building and using dextrous robot hands reaching back to 1993. Its work has covered a variety of areas from multisensory mechatronic hand design up to control of the hands including telemanipulation, autonomous grasping and manipulation. The experience gained with real systems has resulted in new requirements to be included in next generation design steps.
Following a mechatronic design approach, the current generation of dextrous robot hands at the DLR lab is called DLR Hand II. The four-finger hand, developed in conjunction with the Harbin Institute of Technology, was designed to be a "close-to-production version" of the DLR Hand II.
The HIT-DLR robotic hand can be controlled very delicately and precisely, using compact actuator technology and high-speed networking technology to provide sensory feedback. By developing an articulated robotic hand with completely integrated actuators and electronics, the design represents a fully integrated mechatronics concept for better performance, robotic grasping and manipulation. DLR says it also provides a promising base for what will be a future series of complex multisensory robots.