Tricept: Hybrid Parallel Robot


Tricept: Hybrid Parallel Robot


Since early sixties, industry has benefited from the continuous advances in industrial robot developments. The first robots were designed mimicking the human arm, where every link follows the previous one in a serial kinematical chain. Still today, the most known manipulators are based in this arm-like configuration (Fig. 1). The benefits from serial kinematics mechanisms (SKM) are based in their flexibility and ability to mechanically transport loads between points within a reasonable big work area. Their bigger disadvantage comes from the fact that the moving load is supported in turn by each of the links of the serial robot. This means that the nearer to the ground a link is, the bigger the load to be supported by this link is (i.e. this link needs to handle not only the carrying load, but also the weight of all the links above it). Therefore, based on this fact, SKM usually are characterized by low accuracy and rigidity.


Articulated Robot

Fig. 1 Articulated Arm. SKM


CNC Machine Tools are also SKM. In contraposition to robot arms, though, they have a very high requirement in both accuracy and rigidity. In order to be able to reach such requirements with a SKM structure, CNC Machine Tools usually are designed with very big masses added to each moving axis. At this point accuracy and rigidity are obtained, but flexibility and dynamics are missed.

In order to combine both high flexibility and accuracy together with an acceptable rigidity and dynamic response, the parallel kinematic mechanisms (PKM) came into play. Here "parallel" means that all joints must act in parallel in order to reach a position in the workspace. It also means that the applied load to the tool center point (TCP) can be supported in parallel by all the legs of the platform, distributing in a more adequate way the load to be supported.

The first six degree of freedom (DOF) machine was developed by Gough, back in year 1949, and it was used to test tires. The next historic reference comes from 1969, when Stewart [1] built a similar platform to be used in a flight simulator. Since then, and during the last 3 decades, PKM in general, and Stewart platform in particular (Fig. 2), have been among the best studied mechanisms. As a consequence of all this research several implementations have come to reality in fields such us telescope precise positioning, drive/flight simulation, medicine or even six axis CNC machine tools (e.g. Hexaglide by Ingersoll, Hexel by Sandia Labs). In this case the CNC machine tools based on Stewart platform (or hexapods) have the drawback of a small workspace, above all considering the angle orientation of the TCP.


Stewart Platform. Parallel robot.

Fig. 2 Stewart Platform. PKM.


As a step forward in development, hybrid parallel kinematic machines where developed. Hybrid parallel machines can be of two types:
1. Parallel legs are composed or two or more serial links (e.g. Delta robot [2])
2. The structure formed by the parallel legs is followed by a serial kinematic chain (e.g. Tricept robot).

Industry has shown that hybrid parallel kinematic machines have been much more successful commercially that all other PKM together. In the first case the Delta robot (Fig. 3) has been commercialized as a high speed positioning device (e.g. ABB IRB 340 Flexpicker). In the second case, the Tricept (Fig. 4), with more than 70% over the parallel kinematic machining machines is a well known mature product already recognized by the market (e.g. Tricepts T605 and T805).


Delta Robot

Fig. 3 Delta Robot. HPKM.


Tricept Robot

Fig. 4 Tricept Robot. HPKM


1. Stewart, D., "A platform with Six Degrees of Freedom", Proc. Of Institution of Mech. Engineers. Vol. 180, Part 1, No. 15(1965), p.371
2. Clavel, R., "Device for the Movement and Positioning of an Element in Space," US Patent No. 4,976,582, December 11.

(c) Last updated, JLO, April '09