Introduction

A Stewart Platform (hexapod) has been successfully completed in the Department of Mechanical Engineering in 2003. This parallel-link positioning device has a high pay-load and rigidity compared to a serial-link robotic arm. The device is operated by six stepper-motors controlled by a PC and has high repeatability and accuracy. The platform can be used to position parts for an assembly operation, or when it is inversely mounted, as a tool post for metal removal operations on an NC machine. Figure 1 shows this stewart platform.

Figure 1. Stewart Platform completed in 2003.

In a newly initiated project, the objective is to build a micro-Stewart Platform of three or six legs for positioning devices in micro-machining or micro-assembly operations. It can also be used in medical applications to control the movement of a laser cutter for a surgical operation. The principle of operation and modeling is largely similar to a normal size Stewart Platform, while the components selected will be scaled down considerably. The overall size of the platform is expected to be contained within a space of 150 x150 x 150mm, and its operations within micron accuracy. However, due to the availability of the actuator in the market, the platform is finally designed in the space of 250 x 250 x 250 mm. A modular design methodology is introduced for the construction of the micro Stewart Platform.

A possible configuration of a micro-Stewart Platform is shown in Figure 2. The minimum height it can be achieve with this system is 250mm. By installing the actuator parallel to the base joint instead of the usual way of installing the actuator on top of the base joint, the height of the platform can be reduced to 250 mm. However, the kinematic calculation of the stroke of the actuator will be more complicated since the actuator is not in line with the base joint and the spherical joint. Nevertheless, there are certain advantages of installing a 3-DOF of Stewart Platform. The controlling algorithm will be less complex and the rigidity of the systems can be increased. Furthermore, the modular design concept can be introduced to the 3-DOF Platform. By fabricating interchangeable parts for the system, the 3-DOF Stewart Platform can be modified from a translational platform to a rotational platform. If additional budget is available in the future, extra legs or active joints can be bought to increase the DOF of the micro-Stewart Platform.

Figure 2. 3-leg micro-Stewart Platform.