In order to achieve the goal of maximum flexibility and performance our philosophy is the miniaturization and complete integration of all components of the hand and also the massive reduction of cabling. As on DLR's Hand I the main aspects in developing the new hand were maximum performance to improve autonomous grasping and fine manipulation possibilities and the use of fully integrated actuators and electronics without a forearm. This is the only possibility to use an articulated hand on different types of robots which are not specially prepared to be used with hands. Hands with forearms or hands with just grasping abilities allow for a much smaller and thus more anthropomorphic design due to the possibility of using the additional space in the forearm for actuators and electronic components, but restrict the usability with e.g. industrial robots. Farther displacement of those components as known from the MIT-Utah Hand nearly disables the use on mobile robots. Furthermore the hand must be easy to maintain and use and even economically rebuild-able in case of any damage by daily research usage.
Due to maintenance problems with Hand I and in order to reduce weight and production costs the fingers and base joints of Hand II were realized as an open skeleton structure. The open structure is covered by 4 semi shells and one 2-component fingertip housing realized in stereolitography and vacuum mold. This enables us to test the influence of different shapes of the outer surfaces on grasping tasks without redesigning finger parts.
Kinematic Design of DLR’s Hand II
The design process started on an anthropomophic base by evaluation of different workspace/manipulability measures to get optimal ratios of link lengths of one finger. The desired objects to be manipulated and technological restrictions resulted in absolute link lengths. The second step was to get suitable hand kinematics. The main target developing Hand II beside the ability for fine manipulation has been the improvement of the grasping performance in case of precision and power grasp. Therefore the design of Hand II was based on performance tests with scalable virtual models. Soon it turned out very important to be able to change the position of the 4th finger and the thumb as well. To perform power grasps it is absolutely necessary to have a nearly parallel position of the second, third and forth finger. On the other hand performing precision grasps and fine manipulation requires huge regions of intersection of the ranges of motion and the opposition of thumb and ring finger. Therefore Hand II was designed with an additional minor degree of freedom which enables to use the hand in 2 different configurations. This degree of freedom is a slow motion type to reduce weight and complexity of the system. The motion of the first and the fourth finger are both realized with just one brushed dc motor using a spindle gear. The realized finger positions for both types of grasping were designed virtually and mapped to each other using the positions of 2nd and 3rd finger. Realizable kinematics were calculated and imported to the two virtually found configurations and optimized unless the actual configuration with an overall number of 13 DOF was found.
The three independent joints (there is one additional coupled joint) of each finger are equipped with appropriate actuators. The actuation systems essentially consist of brushless dc-motors, tooth belts, harmonic drive gears and bevel gears in the base joint. The configuration differs between the different joints. The base joint with its two degrees of freedom is of differential bevel gear type, the harmonic drive gears for geometric reasons being directly coupled to the motors. The differential type of joint allows to use the full power of the two actuators for flexion or extension. Since this is the motion where most of the available torque has to be applied, it allows to use the torque of both actuators jointly for most of the time. This means that we can utilize smaller motors. The actuation system in the medial joint is designed to meet the conditions in the base joint when the finger is in stretched position and can apply a force of up to 30 N on the finger tip. Here the motor is linked to the gear by the transmission belt. The motor in the medial joint has less power than the motors in the base joint, however there is an additional reduction of 2:1 by the transmission belt. Thus we achieve the torque which corresponds to the torque created by the two motors in the base joint for an external force of 30 N on the finger tip. The harmonic drives used are of the same type for all joints, since the smallest appropriate type can stand the torque for both types of actuators.
A dexterous robot hand for teleoperation and autonomous operation needs as a minimum a set of force and position sensors. Various other sensors add to this basic scheme. Each joint is equipped with strain gauge based joint torque sensors and specially designed potentiometers based on conductive plastic. Besides the torque sensors in each joint we designed a tiny six dimensional force torque sensor for each finger tip. The potentiometers, each with an analogous filter of third order, would not be absolutely necessary, since one may calculate the joint position from the motor position, however they provide us with a more accurate information of joint position, and they can by the way eliminate the necessity of referencing the fingers after power up. In case of not using the potentiometers one would have to consider the elasticity of the transmission belt and the harmonic drive. With the potentiometer we achieve a resolution for the joint angles of 1/10 degree, this means approximately 10 bits for the joint.
Since the base joint is of differential type, one has to calculate the joint position of the base joint from the potentiometer values. There was no way to measure the joint position directly due to space restrictions. For increasing the controllability of the actuators we appreciate speed sensors. Like in DLR's first generation hand we utilize so called Tracking Converters. In contrast to the old version the complete calculation is done by software since there is enough computing power available now. The sensor itself is basically a position sensor with very high resolution, where the speed can be calculated by differentiation of the position signal. Each motor is equipped with two linear Hall effect sensors which are used for commutation of the motors as well. These sensors supply two sinusoidal signals with a phase shift of 120 degrees. The position within the magnetic cycle of the motor is calculated from these signals. By additionally counting the cycles the position can be calculated. This type of sensor gives us just a relative position of the motor and has thus to be referenced after power up.
One major goal of the design of the new DLR Hand was to fully integrate the electronics needed in the fingers and the palm in order to minimize the amount of cables needed for a multisensory hand, to increase the reliability by minimizing the amount of cables moved crossing the joints. By this design we get a hand which can be freely combined with different robot arms. In case of joints with a single degree of freedom we solved the problem of reliability by using flexible printed circuit boards (PCB) with appropriate bending space within the links. Tests showed after 100,000 cycles no visible or measurable effect on the flexible PCB. In each link at least one serial ADC with 8 channels and 12 bit resolution converts the sensor signals as near as possible to the sensor circuitry into digital data. Thus only digital data is crossing any joint of the finger. The power converters for driving the motors are located directly beside the motors and they are galvanically decoupled from the sensor electronics in order to minimize any noise induced by the running motors. Moreover the different fingers are galvanically decoupled from each other, too.
The control of the fingers and the hand is done by an external computer. In order to use the hand freely on different manipulators and to reduce cables and the possibility of noise in the sensor signals, we decided to design a fully integrated serial communication system. Each finger holds one communication controller in its base unit. This controller is responsible for the collection and distribution of all information of interest within the finger. Furthermore it does some reasonable signal processing. It collects the data of all five ADCs per finger with together 40 channels of 12 bit resolution each and transmits these data to the communication controller in the hand base. On the other hand it distributes the data from the control scheme to the actuators for finger control. The communication controller in the hand base links the serial data stream of each finger to the data stream of the external control computer. By this hardware architecture we are able to limit the number of external cables of DLR's Hand II to a four line power supply and an eight line communication interface since the data is transmitted via differential lines. This interface even provides the possibility of using a quick-lock tool retainer for autonomous tool exchange. Reducing external cabling from 400 (in Hand I) to 12 here, is one of the major steps forward in our new hand.