An ultraprecision positioning system using twist-roller friction drive

扭辊摩擦传动机构是一种具有小导程的丝杠。据报道，一种采用扭辊摩擦驱动机构的定位系统，其定位分辨率低至0.2 nm，其中使用交流伺服电机作为执行器。在本研究中，使用带有运动减速机构的压电式驱动器来代替伺服电机，并研究了扭轮摩擦驱动机构定位精度的局限性。运动减速机构可以看作是执行器变形与工作台位移之间的精确线性转换。该机构的减速比小于千分之二，转台的位移由光纤传感器检测。该光纤传感器的标称分辨率为0.25 nm，频率范围为130 kHz。采用外部低通滤波，可以获得更好的分辨率。检测到的工作台移动被反馈到微计算机，该微计算机控制用于压电致动器的电源电压。从而实现了工作台定位的闭环控制系统。数字控制系统的最低有效位(LSB)为76 PM，大于待研究的定位分辨率。为了实现小于LSB的精确控制，在控制器中采用了脉冲数调制。控制系统的周期约为30μ。利用该控制系统进行的步进定位实验表明，1 nm、0.5 nm和0.1 nm的步进得到了清晰的分辨。然而，噪声水平几乎等于步长，定位分辨率的极限被认为是50 pm。扭轮摩擦驱动机构具有运动转换的线性度和纳米范围内的高重复性，可以精确地控制工作台的位置，可以作为提高超精密加工和测量的比例尺定位装置。

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.