# Design and Testing of Two Haptic Devices Based on Reconfigurable 2R Joints
## 4 Experimental Setup Design  
### Mechanical Design  
The haptic device was used to control the 2-RFR ultraflexible planar manipulator, developed by our CompMech research group (see Figure 2b). The manipulator consisted of two flexible nylon bars with active revolute joints on one side and a passive revolute joint in the end-effector. Two Nema 32-sized stepper motors were used as the actuators. The manipulator was placed between two acrylic sheets (transparent in the front and white in the back) that were attached to a square frame with a 2 m side, made of an aluminum profile. The rear acrylic sheet had a coordinate mesh of 10 cm in size for better visualization of the end-effector position. This 2-RFR manipulator was kinematically equivalent to a 5-bar mechanism; however, it was more \"human friendly\" as it was safer due to the flexible nature of its bars. The joint prototypes were manufactured using fused deposition modeling 3D printing technology. The material of the majority of the printed parts was acrylonitrile butadiene styrene (ABS). The rest of the printed parts were manufactured with Onyx from the Mark-forged company. Both reconfigurable joints are used in the proposed haptic device in a joystick mode. A schematic representation of the 2R spherical mechanism-based joint and its practical implementation is presented in Figure 4. The setup was built around joint 1, which was connected to force feedback actuators 2 and 3 through the belt gears 4 and special torque-generating bushings 5. The absolute rotary encoders 6 determined the current handle position. The clamp 7 was able to lock one DOF of the joint in order to provide precise horizontal motion. In the proposed setup, stepper motors were used as actuators of the force feedback system. In order to transform steps into torque, special bushings (5 in Figure 4) were designed (see Figure 5a). The bushing consisted of inner 1 and outer 2 drums. The inner drum was connected to the input shaft of the joint. The outer drum was connected to the pulley 3 and was able to turn around the inner drum within a certain range. There were two springs 4 between the drums, of which one side was placed in the slots of the inner drum and the other side was pressing on the metal partition 5 that was fixed on the outer drum. When the drums rotated with respect to each other, the compression of the springs created an output torque. This happened when the handle was fixed in a certain position and the actuators of the force feedback system were turning. The reconfiguration ability of the proposed haptic device was implemented by means of a manual clamp (see Figure 5b). The clamp contains three aluminum strips 1 (in Figure 5b one is removed for better visibility) of 4 mm in thickness, that can move around the axle 2, when the eccentric 3 is turned with the handle 4. In this moment, the upper strips are pressing upon the bushing 5, which connects the rotary encoder to the joint shaft to the lower strip, thus preventing it from rotation. The handle of the haptic device and its scheme are presented in Figure 6.  
The handle contained two tension-compression load cell sensors that were placed in two equal modules made of aluminum profile. The sensor 1 was fixed between the static part 2 of the module, and the moveable part 3. The moveable part can bend due to the slot 4. This design is intended to prevent the sensor from taking on non-axial loads that can damage it. Knowing the distances (see Figure 5b) from the center of the joint sphere (in dashed line in Figure 5b) to the sensors and the center of the handle ball, it is possible to calculate the forces acting on the handle. The experimental setup built for the differential gear-based joint design (see Figure 7) was similar to the previous one and had several common elements. The joint 1 was placed in the center of the haptic device. Its force feedback system was driven with the actuators 2 and 3 through the belt drives 4 and torque-generating bushings 5.