The selection of a mechanical positioning stage is extremely complex when extreme precision is required. When there is no specific requirement for accuracies less than 10 microns (0.01 mm), then a satisfactory selection might be made on price alone. However, sub-micron positioning accuracies demand a great attention to detail. The types of metal used for the stage, the type of bearings used, the lead screw material and accuracy, the type of guide rails and the ball screw (if used) are all contributors to system errors.
Entry level stages may use a simple sliding groove without bearings. Higher accuracies require bearings. Ball bearings are adequate for medium accuracy, but crossed roller bearings are required for higher accuracy. Air bearings provide the ultimate levels of accuracy, at much higher prices. For slide mechanisms, accuracy and cost go hand in hand. One thing to bear in mind is that the use of bearings provide increased life as well as increased accuracy.
Aluminum and steel are adequate materials for all but the highest accuracy stages. Special stress-relieved metal formulations may be required when the highest levels of accuracy are important. Types of errors to be aware of and to include in a specification include maximum deviation from a straight line in all six possible directions of motion and temperature effects on motion and useful life. In high precision stages, backlash is minimized through the use of preloaded ball-bearing nut assemblies that couple the stage and leadscrew.
This preload is one of the primary sources of stiction friction and can also cause an unwelcome degree of mechanical noise. Techniques exist for reducing the noise, such as replacement of some of the metal balls with nylon balls. Due to the high forces involved, the wear of the ball nut becomes one of the largest contributors of error in systems that have a high level of use. It is important to know the expected time between preload adjustments and the procedure for making them.
Last updated on ... October 12, 2007