FORCE SENSING, LOW-COST MANIPULATOR IN MOBILE ROBOTICS
The ability to sense tactile feedback is important for many robotic grasping and manipulating tasks. However, common force sensors are expensive. Therefore, we investigate various force measuring principles and review existing sensor concepts for mobile robotic applications. The results lead to a concept of a low-cost, two-finger manipulator with force sensing ability using embedded hall sensors in flexible material. The gripper is produced by rapid prototyping. A calibration method is presented. Furthermore, we evaluate and present the idle state, local sensitivity, and material influence of the twofinger manipulator. As a result, it is possible to successfully handle sensitive objects with the sensitive areas of the fingers, although a bias was measures while handling ferromagnetic materials. With a priori knowledge of the objects, this effect can be compensated.
Resistive force sensors measure the change in electric resistance R resulting from an induced force. According to Ohm’s law, the resistance is the quotient of voltage and current. Therefore, a resistive transducer registers resistance variations by operation on either constant voltage or current and measuring the other quantity. The electrical resistance varies by changes in geometry, piezo-resistive effects, or deformations of resistive gels. In general, bending beams equipped with strain gauges are used to detect geometric changes because the strain gauge resistance varies when length variations occur. This technique is applied to the DLR-Hand II , . It uses three symmetrical bending beams for force sensing. The quality of the measuring strongly depends on the placement of the gauges and on the structure of the beams. Instead of using strain gauges, thin layers of conductive polymers are places between two electrodes to reduce bending influences. The resistance of resistive gels changes under pressure. Two sensors are usually arranged crossbreeding. Therefore, only forces perpendicular to the surface are detected. This technique is used in the WTS sensors  of weiss robotics.
This paper presents the development of a low-cost and practical force sensor system integrated into a two-finger gripper. Force measuring is a commonly used method and the basis for more complex sensor systems. Therefore, force measuring principles are reviewed regarding cost and usability, followed by the design and evaluation of a twofmger gripper.
After a review of existing force sensing principles and implementations in robotic grippers, we developed a lowcost tactile sensor and evaluated its applicability using a twofmger robotic gripper that was built utilizing rapidprototyping techniques. With the developed system, we were able to successfully grasp delicate objects, like an egg (see Fig. 9), using force feedback from the tactile sensors. However, the evaluation of several experiments has shown that the concept is subject to some restrictions. One major issue is the influence of ferromagnetic materials on the measured forces. In applications where tactile grasping of objects with unknown materials is necessary, a different tactile sensor might be more feasible. The second aspect to consider is the influence of the contact point with the grasped object on the measured force. In applications where grasped objects have a large contact area with the gripper, this influence can be neglected. In other applications, results might be improved by an optimized arrangement of magnets and hall sensors in the gripper fingers. An intelligent fusion of individual sensor signals could be implemented to locate the contact point and to differentiate influences that affect the measurement.
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