An ultraprecision positioning system using twist-roller friction drive

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The twist-roller friction drive mechanism is a kind of lead screw having a small lead. It was reported that the positioning resolution of a positioning system using the twist-roller friction drive mechanism is as small as 0.2nm, where an AC servomotor was used as the actuator. In the present research, instead of the servomotor, a piezoelectric actuator with motion reduction mechanism is used, and the limitation of the positioning resolution of the twist-roller friction drive mechanism is investigated. The motion reduction mechanism can be regarded as a precise linear converter of the actuator deformation to the table displacement. The reduction ratio of the mechanism is less than two-thousandth.The displacement of the table is detected by a fiber-optic sensor. Nominal resolution of this fiber-optic sensor is 0.25nm in the frequency range to 130kHz. With external low-pass filtering, finer resolution can be obtained. Detected table movement is fed back to a microcomputer that controls the supply voltage for the piezoelectric actuator. Thus a closed-loop control system for the table positioning is realized. The LSB (Least Significant Bit) of the digital control system is 76pm, which is larger than the positioning resolution to be investigated. For the precise control less than the LSB, PNM (Pulse Number Modulation) is employed in the controller. Cycle time of the control system is about 30μs.Experiments of step positioning using the closed-loop control system shows that steps of 1nm, 0.5nm and 0.1nm are clearly resolved. However, the noise level is almost equal to the step width and the limitation of the positioning resolution is considered to be 50pm. Because of the linearity of motion conversion and high repeatability in nanometer range, the twist-roller friction drive mechanism can control the table position precisely and can be used as a picometer-positioning device for improving the ultraprecision machining and measurement.