# Design and Testing of Two Haptic Devices Based on Reconfigurable 2R Joints
## 1 Introduction  
A haptic device is an instrument that is used to perform different kinds of manipulations with objects, providing the operator with feedback, i.e., tactile feedback, vibrational feedback, force feedback, etc. The application of haptic devices varies from handling virtual three-dimensional (3D) bodies [1, 2] to device remote control [3, 4, 5] and surgery [6, 7]. Different types of feedback can be used to indicate the interaction with the controlled object. Some devices provide vibrational feedback [8, 9, 10] and others can implement torque [6] or force feedback [2, 11, 12, 13]. The most widely used architectures of haptic devices are serial [2, 7] parallel [6, 13, 14] or cable drive-based architectures [15].  
Regarding the most common type of joints that are implemented in commercial robots and manipulators, universal and spherical joints are among the most widely used in parallel manipulators. They are utilized in Delta robots and Stewart-Gough platforms and in haptic devices that share a similar architecture [6, 14]. Spherical joints and mechanisms have been applied in commercially available serial haptic devices. The 3D SYSTEMS Company has proposed three models of serial haptic devices [2, 11]. Examples of applications of spherical mechanisms in haptic devices can be found in [16, 17]. A spherical joint was applied in a haptic device with magnetorheological-based feedback in [7].  
However, typically a haptic device design does not provide any reconfiguration ability that can extend the device's capacity. This functionality can be provided by using reconfigurable or active spherical joints that could be adapted for reconfiguration. A reconfigurable joint is a joint that can change its performance according to the necessary task. It can lock a degree of freedom (DOF) [18], changing the direction of an axis of one of the DOFs [19], or combining the functions of several different joint types [20]. The joints can have different working principles. They can be based on a gear-like drive [21, 22, 23, 24], have a direct connection with the actuators [25], form a spherical 2 DOF [26] or 3 DOF mechanism, or be electromagnetically driven [27].  
From the kinematical point of view, reconfiguration is easier to implement in serial mechanisms, where a single joint of the haptic device provides 1-2 DOFs of the controlled object. For instance, locking one DOF of the handle in a Delta robot-based haptic device similar to that in [14] is not possible with a hardware solution; meanwhile, serial haptic devices, proposed by 3D Systems [2,11] would be able to provide this functionality. Thus, serial architecture has greater potential in relation to reconfigurable haptic devices.  
The main target of this paper is the testing of two reconfigurable 2R joints in order to obtain a haptic device that can be used to control an ultraflexible manipulator for human-robot collaboration. The proposed joints provide reconfiguration via the relocation of axis one of the DOF. In the haptic device, this feature ensures the implementation of the precise straight motion of the controlled object. These haptic devices will be equipped with a force feedback system. They will be tested by two groups of participants in order to define the best joint design and the most appropriate working mode.  
The paper is organized as follows. In Section 2, joint design and kinematics will be considered. In Section 3, the experimental methodology will be explained in detail. In Section 4, haptic device designs will be presented. In Section 5, the results of the statistical analysis of the experimental data will be shown. Section 6 provides the interpretation of the statistical analysis and the justification of the best joint design and working mode selection. Finally, the conclusions of the study, along with directions for future work, will be presented.