SHaDe (a spherical 3-DOF haptic device)

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The Prototype

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.

Poster

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

Two spherical mechanisms: The Agile Eye and SHaDe (12 Mb)

Video clips

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

Haptic device and Agile Eye Video clip showing the use of the SHaDe haptic device to control the Agile Eye mechanism. Format: mpg    Length: 0:12    Size: 2.2 Mb