A.H. Slocum, M. Basaran, R. Cortesi, A.J. Hart. J. Int’l. Soc. Precision Engineering and Nanotechnology 27:382-394, 2003. [http://dx.doi.org/10.1016/S0141-6359(03)00037-0]
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A fundamentally precise and simple linear motion axis design is discussed where the attractive force from the linear motor is used to preload a carriage supported by six rigidly attached porous carbon air bearings. The air bearings are assembled by vacuuming the pads to two orthogonal planes, positioning the carriage above the pads, and then injecting epoxy between the bearings and the carriage. Control of flatness tolerances of the bearings and planes permits over-constraint of the carriage by the bearings, which leads to a high degree of accuracy and moment load capacity via elastic averaging. Good dynamic stiffness is also obtained because of large bearing area and squeezed-film damping. The two orthogonal planes represent an accurate and cost effective geometry that can be created to guide linear motion; and by placing the open-face iron-core motor at the desired position and angle with respect to the bearings, preload forces with equal or otherwise desired relative magnitudes are obtained. Because the attractive force is typically 3–5 times the axial force generated by the motor, the system is inherently stable even in the presence of large externally applied moments that might otherwise induce excessive pitch, yaw, or roll errors.
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