Precision motion systems with resolutions in a range down to ten nanometers are required in many manufacturing applications, including assembly and alignment of optical transmission and processing equipment. Typically, precision motion is provided by a combination of a DC or AC servo motor, a high precision feedback encoder, and a high precision linear bearing or guide. A 6-axis motion control stage incorporating these sub-systems, however, can be very expensive. These sub-systems are also complicated, having a high part count that can cause reliability problems. Yet another problem with these systems is that they typically require speed reduction devices, such as gear sets or lead screws to create sufficient torque for many precision motion applications. Such speed reduction devices incorporate play or looseness of fit that can cause backlash and dead spots in the motion system, as well as, lateral forces that reduce the precision of the stage motion.
One alternative to these complicated precision systems is a motion system using a friction drive. Typically a friction drive comprises a drive shaft which is rotated by a power source, such as a motor, and a stage which moves in a linear direction, due to the frictional force (thrust) applied to it by the rotation of the shaft. A separate high-precision guide such as an air bearing or hydraulic bearing typically maintains the linear motion of the stage. Thus, the friction drive motor and the motion guide are connected through a bar or rod that is longer than the range of linear motion of the stage, reducing the system stiffness, and consequently, the stage motion resolution. The guide also increases the cost and complexity of the precision motion system.
Yet another problem with using a friction drive for a precision motion system is the need to provide a consistent pre-load force. Typically, the friction motor is fixed and the pre-load force is applied to the linear stage through back-up rollers. Back-up rollers, however, add weight, cost, size, and complexity to the precision motion system. The speed of precision motion systems may also be limited to maintain a consistent pre-load force on the friction drive.
Accordingly, a need exists for a precision motion system that is inexpensive and compact, and can operate at high speed with nanometer-range motion resolution.