Ron Rekowski
Aerotech Inc.
Pittsburgh, Pa.

Compared to traditional methods, the mechatronic design approach is more of a holistic approach to product design, where the tradeoffs between different functional components (software, hardware, user interface, etc.) are carefully considered for their impact on overall performance. The goal of the process is to arrive at an optimal solution at the conclusion of product design. Mechatronic principles have been successfully deployed in numerous applications such as hard drives, robotic manipulators, temperature control, and automotive systems. Here we consider mechatronics in micropositioning stages.

This goniometer includes an actuator with integral motor. Height and performance requirements called for a low profile direct-drive motor system, which meant embedding a custom linear motor directly into the stage base.

Micropositioning defined

The term “micropositioning” has different meanings that are largely defined by the context of the application. In this discussion, we are talking about actuators capable of motion in the micron to submicron region. This requires that the actuator is capable of achieving submicron repeatability and accuracy. These types of products are widely used to manufacture fiber optic devices, laser ablation processes, and semiconductor, hard drive, and metrology systems.

The application of mechatronic design principles is critical to the successful implementation of such micropositioning devices and systems. When every micron matters, each design element must be carefully considered for its contribution to overall system error, as a single “bad” choice can easily exceed the total system error budget.

Mechanical subsystem design

One would expect that a device designed to move at the micron level with corresponding accuracy and repeatability would necessarily be small in size. However, there are actuators with travel in the range of hundreds of millimeters that are quite capable of being classified as micropositioning devices. The size of the load (mass, dimensions, inertia) and the environment (cleanliness or atmospheric composition) add to the challenge, narrowing design parameters even further as they relate to material and bearing selection. These design selections are also guided by their impact on system response.

The mechatronic aspect of mechanical subsystem design comes into play when considering the interaction between software, control, and electronics. In some applications, it may be necessary to integrate the control and power stages into the mechanical subsystem itself. To minimize overall footprint, for example, it may be necessary to integrate a custom motor assembly into the moving stage.

Think about thermal expansion

When motors or power electronics are integrated directly into a device, it introduces a heat source that can reduce positioning accuracy due to thermal growth. The following table shows the thermal expansion in microns over 1 m of material length for a 1° C rise in temperature. Based on the expansion coefficient, a 100- mm long aluminum actuator will expand 2.4 μm for each 1° C temperature increase. Note that integral power electronics and motors have the potential to raise the temperature much greater than that.

In applications where extreme accuracy is required over a range of operating temperatures, the control subsystem must be capable of measuring or calculating the temperature rise and adjusting accordingly to stay within the allotted error budget. Alternatively, different materials with a lower coefficient of thermal expansion could be considered for the actuator to limit growth. However, it’s important to remember that material selection has implications for actuator stiffness, which can influence the control architecture and processor type.