Introduction to parallel mechanisms

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The most common robot architecture is undoubtedly the serial manipulator (Figure 1), in which the motors and various members are connected in series from a base to an end-effector. This design offers numerous advantages, including a large workspace. The popularity of this architecture in industry is a clear indication of its ability to fulfill a broad variety of needs.

However, serial manipulators also have their drawbacks. Thus, several alternative architectures have been proposed and used in robotics. One well-known example is parallel manipulators (Figure 2). As the name indicates, in this case the motors and members are mounted in parallel from a base to a common platform.

Fig. 1: 6-DOF serial manipulator (courtesy of KUKA Robotics Corporation).

Fig. 2: Example of a parallel manipulator (courtesy of ABB Flexible Automation).

Research on parallel manipulators began in the 1970s and their popularity is rapidly increasing. The main advantages of parallel architectures are the following:

• the structural stiffness (which greatly increases the payload to mass ratio of the robot);
• the dynamic properties;
• the possibility of mounting the actuactors on or close to the base of the robot.

The latter property allows one to consider more powerful actuators since they no longer have to be moved by the robot itself, but are instead fixed to the base or to a nearby member.

Several types of parallel architectures are possible and the most common is that of the Gough-Stewart platform (Figure 3). This 6-DOF manipulator has a platform which is moved by the action of six linear actuators. This platform is most commonly used for flight simulator applications (Figure 4), since it can support the very large mass (up to 15 000 kg) of the cabin which is being subjected to various movements. Flight simulators are used to train pilots on the ground.

Fig. 3: Gough-Stewart Platform (courtesy of Hydra Power Systems).

Fig. 4: Flight Simulator (courtesy of CAE).

Over the years, the laboratory has carried out numerous studies on the analysis of parallel manipulators (workspace, dexterity, etc.) and has developed several innovative architectures, some of which have been patented.