Structure Concept of Exechon PKM

Updated: 2008/9/30

Analyzing existing Parallel Kinematics Machines

      Parallel Kinematics, as an occurrence, is a number of parallel “arms” connected to each other in one end and to a base in the other end and by default this design requires joints with multiple Inactive Degrees Of Freedom (IDOF). The number of parallel arms in a Parallel Kinematics Machine (PKM) depends on the design, read patent, and can vary from three arms up to eight arms but in all cases the target of the design is to combine flexibility and stiffness.

      Significant for all PKM machines is the problem of the joints, which are complex and difficult to manufacture with high stiffness and backlash free to the right cost. This technology issue limits the number of PKM machines on the market and is also the reason why the most successful designs are the PKMs with less number of joints and IDOFs. The Tricept before today is the PKM with fewest number of joints and IDOFs and that"s why the Tricept has 70% of the market of PKMs before.


Analyzing Tricept

      A PKM machine is designed to create a movable box frame and from a spectator point of view this looks great but the consequential number of Inactive Degrees Of Freedoms required, creates problems. Every joint with more than one IDOF acts like a hinge in all directions and is only capable of carrying pure axial loads and a consequence is that applied forces have to be absorbed otherwise e.g. by the centre tube in a Tricept.

      The biggest problem with the centre tube is that the applied forces create both bending and torsion forces on the centre tube and this combination is difficult to handle. Another problem with multiple IDOF joints is that joints with more than two Degrees Of Freedom are very difficult to manufacture with enough stiffness and accuracy for a reasonable cost and such design also affects the axial joint stiffness negatively that seriously affects the overall stiffness.

The new Exechon Concept


      The new Exechon Concept is based on the use of lower joints with no more than one DOF and the use of actuators with two Degrees Of Stiffness (DOS), linear and bending in one direction. This design forms a solid structure that completely takes care of the bending and torsion forces applied to the machine in all directions. 

      The described Exechon design actually forms a pyramid-like moving tripod that transfer the forces from the wrist to the upper platform through the three 2-DOS actuators linked to the wrist with 1-DOF joints without any losses in stiffness. However, due to the influence angles of the upper joints, this design, with one DOF in all lower joints and two DOS in all actuators, is only possible with two DOF in the upper joints of two actuators and three DOF in the upper joint of the third actuator.