Parallel Spherical Mechanism with Two Degrees of Freedom

Numerous mechanisms enabling a body to be oriented in three dimensions by being turned about two or three axes are well known. When the body for orienting presents an axis of symmetry, turning the body about its own axis is generally not useful and it is sufficient to use an orienting mechanism having two degrees of freedom.

A wrist mechanism presents a motion of spherical type, i.e. each point of the body moves over a spherical surface with the centre of spherical motion situated inside or outside the body. Under the circumstances, that it is not necessary to perform complete revolutions about the movement axes, a parallel mechanism comprising a plurality of independent kinematic linkages for connecting the body to the fixed base can be used. Parallel mechanisms are small in size and mass, while simultaneously presenting a high degree of stiffness.

To this day, very few parallel wrist mechanisms with two degrees of freedom presenting spherical motion are known. The main drawbacks are the fact that the spherical motion axes are not predetermined, i.e. they are not fixed relative to the base or the body, and the movements in rotation about the axes cannot be decoupled. In addition, they are intrinsically overconstrained leading to high levels of internal mechanical stress and a major risk of jamming. Nevertheless, some known mechanisms exist, that present predetermined axes and enable motion in rotation to be decoupled, but they are penalized by their size. In addition, these mechanisms contain prismatic joints that affect their operation negatively in space applications due to lubrication problems and in micromechanical applications due to high levels of friction.

An object of the offered technology is to provide a parallel spherical orienting wrist mechanism having two degrees of freedom that makes it possible to solve the drawbacks of the prior art.

The axes of the offered mechanism are predetermined, which enables motions in rotation about the axes to be decoupled. Advantageously, the mechanism presents a structure that is simple and compact, operation that is reliable, and a very high degree of stiffness.

The parallel spherical wrist mechanism provides two degrees of freedom for connecting a body to a fixed base in such a manner as to enable it to be oriented in three dimensions by turning about a first axis fixed relative to the base, and about a second axis fixed relative to the body. These two axes intersect at a centre of spherical motion situated in the body. At least four links connect the base to the body. These links are constituted by compression beams or by cables. Because of their flexibility, the use of cables makes it possible to obtain the necessary degrees of freedom without any need of mechanical joints.

The variable length of at least one link enables the mechanism to be retracted and deployed. Each of these variable links includes a linear actuator in order to vary the length in an active manner and has an integrated actuator and sensor for damping vibration or compensating deflections. With two actuators the body can be oriented actively in three dimensions about the two axes. It is advantageous for the connection points of the links to present additional degrees of freedom to avoid accumulating torsion stresses, thus, depending on the movement, they are made using compliant connections, spherical or universal joints.

Innovations and advantages of the offer:

The mechanisms of the offered technology present very great stiffness compared to mechanisms known in the prior art, due to the larger number of parallel kinematic linkages. This reduces the risk of jamming. The mechanism is subjected solely to forces in traction and compression leading to greater stiffness/mass ratio than in the prior art. Mechanisms having exactly four links are not overconstrained. Nevertheless, overconstrained mechanisms of the offered technology having more than four links, have the advantages of better stiffness/mass, stiffness/size, strength/mass, and strength/size ratios, greater resilience, and reduced backlash compared to the offered four-link-mechanisms.

All of the mechanisms do not present surfaces in sliding contact, which makes it possible to avoid using lubricants, to increase the working lifetime, and to guarantee high precision during the whole lifetime. In addition, the absence of sliding contact enables the mechanisms to be miniaturised without any need to modify the structure, making the offered technology suitable for making micromechanical devices and micro-electromechanical systems (MEMS).

Domain of Application:

• Industrial Automation
• Industrial Measurement and Sensing Equipment
• Robotics
• Industrial Equipment and Machinery