SenSation : A New Translational 2 DOF Haptic Device with Parallel Mechanism

Abstract

We propose a new two -degree of freedom para llel mechanism for a haptic device and will refer to the mechanism as the SenSation. The SenSation is designed in order to improve the kinematic performance and to achieve static balance. We use the panto- graph mechanisms in order to change the location of active joints, which leads to transform a direct kinematic singularity into a non- singularit y. The direct kinematic singular configurations of the SenSation occur near the workspace boundary. Using the property that position vector of rigid body rotating about a fixed point is normal to the velocity vector, Jacobian matrix is derived. Using the vector method, two different types of singularities of the SenSation can be identified and we discuss the physical significance of each of the three types of singularities. We will compare the kinematic performances(force manipulability ellipsoid, kinematic isotropy) of the SenSation with those of five-bar parallel mechanism. By specifying that the potential energy be fixed, the conditions for the static balancing of the SenSation is derived. The static balancing is accomplished by changing the center of mass of the links. Recently, one interesting application in robotics research is the haptic device. A haptic device is the machinery that is designed to provide user with a sense of touch. The device is essentially a robotic arm that is able to both sense its pose at that location and apply a force at that location. A user can take hold of the end-effector of the device and, by means of the use of haptic control algorithm, feel as if he/she is actually touch- ing and interacting with the virtual model or remote site. The force supplied by the device provides instantaneous and intuitive feedback on the task being performed. The haptic device must not distort the reflected forces by its mechanical characteristics. To provide user with a precise sense of touch, the haptic device should be able to have performances such as low apparent mass /inertia, high structural stiffness, wide sin- gular-free workspace, static balancing(or counterbalancing), high force bandwidth, backdriveability, very low friction and backlash, high force dynamic range, good transportability (1)(2)(3). These demands are often conflicting and difficult to achieve. It can be seen that parallel mechanisms are suitable for structure of haptic device because they have a number of ad- vantages such as high structural stiffness and force bandwidth, low inertia. Other demands can be achieved through the use of haptic control. The haptic device with parallel mechanism is in (4)-(11) The kinematic isotropy is very important since it leads to a high stiffness and force bandwidth, wide singular-free iso- tropic workspace, a good position and orientation accuracy. Unfortunately, typical serial and parallel mechanisms have not constant high stiffness and force bandwidth over the entire workspace. The reason is that the isotropy performance tends to be degraded as the mechanism goes to near singular con- figurations. There are three types of singular configu-rations in parallel mechanisms: direct kinematic singularity (DKS), in- verse kinematic singularity(IKS)(12)(13), architecture singu- larity (AS)(14). The DKS, one of three singularities, exists inside of workspace and separates its workspace into a number of regions. Therefore, the workspace of parallel mechanisms becomes small compared to link size. To deal with this singularity(DKS) and improve the kin e- matic isotropy, (15) and (16) had proposed the method of ac- tuator redundancy. (17) used the method to eliminate this type singularit y. (6) also added one actuator to obtain a more sym- metrical workspace and more force of end-effector. However, this method makes a controller design complex.