**5.1.3 Gripper configuration**

The developed gripper device was configured with a two parallel finger design for its wide applications in spite of its precise control need. One finger is fixed and the other is movable to ease the control and minimize the cost as shown in figure 4. The fingers are flat and rectangular in shape. The housing of the gripper and fingers were made of aluminum sheet for light weight consideration with proper thickness to ease the machining and holes puncture through edges. This gives simple assembly and ease in maintenance. The movable finger is driven on a lead screw and guided by a linear bearing system with the advantage of self-locking capability, low cost and ease of manufacture.

To control the gripping of the object, we need to measure both the force applied to the object and the object slip. A standard commercial force sensor resistor FSR (Flexiforce A201 working in the range of 0-1 lb (4.4N)) is used to measure the applied force. Also Phidget vibrator sensor is adapted as slip sensor to give information about object slip rate in m/sec. These two sensors are tactile sensors. The actuator used to drive the movable finger is a permanent magnet brushless dc motor (BLDC). It has the advantage of high power density, ease of control, high efficiency, low maintenance and low rotor inertia. BLDC servo motor used is an internal rotor motor "BLD3564B" from Minimotor inc. with its drive circuit "BLD5604-SH2P" .

The design of the gripper fingers must take some restrictions into consideration. Long fingers require high developed torque and short fingers impose restrictions on object dimensions. Hence fingers are selected to be 15 cm long. Also, a contact rubber material area between the fingers and the object of 25 mm by 25 mm is used to decrease the pressure on the object, increase the friction, and avoid deformation from centric concentrated force. With this gripper configuration, we succeeded to verify all previous design guidelines except guideline no.4 as our proposed gripper doesn't fully encompass the object in order to be able to grasp a greater variety of objects, although this imposes more difficulty in the control during gripping.
