SHADE (A SPHERICAL 3-DOF HAPTIC DEVICE)

A 3-DOF haptic device, called SHaDe, an acronym standing for Spherical Haptic Device, was developed in our laboratory to allow a human operator to control motions while being subjected to force feedback. The mechanism presents the particularity of having only three degrees of freedom, leading to a simpler design and a more ergonomic utilization. Moreover, the use of a spherical geometry in this haptic device offers several advantages, namely, a pure rotation around a point located inside the user's hand (no translations at this point), a large workspace, a comfortable use, and precise manipulation while the arm is resting.

The prototype makes use of a particular design in which only revolute joints are used, based on a spherical geometry. Indeed, it is a spherical parallel mechanism with two spherical linkage chains of type RRR and one chain of type RRRR. Kinematically, however, the parallel mechanism is equivalent to a spherical 3-RRR one. The RRR(RR) chain was used in order to minimize the link interferences. In SHaDe, all joint axes, passive and active, intersect at a common point which is the center of rotation of the end effector. Such a spherical geometry has also been used in the design of the high-performance camera orienting device, referred to as the Agile Eye.

Numerical analysis was used to optimize the prototype's characteristics with respect to given performance criteria. To this end, a weighted combination of indices was used, including the size of the workspace, the minimal dexterity, the average dexterity, etc. The prototype was built using a Fused Deposition Modeling (FDM) rapid prototyping machine using a commercially available CAD package.

The force control involves an intelligent multi-axis force sensor communicating at a high speed through a serial link with the sensor control sub-program. This sub-program is in turn communicating with the motor torque control and running under QNX, a real-time micro-kernel operating system. Different control laws were created to simulate a robot arm's behaviour or distant hazardous environments. The force control itself is based upon a classical PID scheme enhanced with static compensation plus a feedforward term in order to improve the performance. 

A poster was prepared in 2002 on the SHaDe mechanism. The poster can be downloaded with the PDF file below.

The following video clip illustrate the prototype of the haptic device.