Development Of A Novel Two Degree Of Freedom Fast Tool Servo

Non-Rotationally Symmetric (NRS) optical surfaces has been widely used in the aerospace, national defense and civilian industry, because it has the advantages of improving the performance of optical system, reducing its bulk and weight. The diamond turning based on Fast tool servo (FTS) is been recognized as the most potential processing method for NRS optical surfaces. As one of the key technology, FTS can meet the processing needs to some extent, but the existing single degree of freedom FTS has some shortcomings which are difficult to overcome. Therefore, developing a high-performance, multi-degree of freedom FTS is positive to improve processing efficiency and surface quality.The dissertation focuses on the development of a two-degree of freedom fast tool servo (2-DOF FTS). After analyzing the X-axis and Z-axis component of the tool path and the systematic error of ultra-precision lathe, a 2-DOF FTS has been designed by flexure hinge. In order to drive the 2-DOF FTS, three PICMA PZT stack are used, which are from PI company. One PZT stack is used to drive X-axis and others is installed in Z-axis. The theoretical displacement of X-axis and Z-axis are 12μm and 60μm respectively. As the FTS's feedback unit, two SPNS1100 capacitive transducer from QUWWNSGATE are used to detect the displacement of two kinematics axes.The key part of 2-DOF FTS is a two-DOF flexure hinge mechanism which is fabricated by WEDM (Wire cut Electrical Discharge Machining). Its two axes (X-axis and Z-axis) is cross distributions. X-axis is a bistable mechanism which is composed by flexure hinge, its theoretical stiffness is 6.246N/μm and natural frequency is 416.7745Hz. Z-axis is fabricated on the X-axial platform by WEDM and composed of a pair of mechanism of lever-amplification and a bistable mechanism. Its theoretical stiffness is 6.658 N/um and natural frequency is 496Hz. The 2-DOF flexure hinge mechanism has a number of advantages:(1) the mechanism is a whole structure which is manufactured by WEDM and has no assembling error and clearance of reverse movement; (2) there is no parasitic motion between two kinematics axes; (3) the two axes are easy to control synchronously, because there is no coupling movement.To determine the reliability of the designed mechanism, ANSYS is used to do some analysis on static and dynamic of structure. After analyzing, the simulation stiffness of X axis and Z axis are 6.1425N/μm and 6.4872 N/μm, and natural frequency of simulation is 413.783Hz and 476.83 Hz, respectively. Build the electromechanical coupling model and do some simulation on time domain and frequency domain in the open-loop state. Finally, a conclusion is got by Nyquist stability criterion, which the FTS systems will be stable in closed-loop.To get the true performance of 2-DOF FTS, SIEMENS IPC, Power PMAC motion control card and a power amplifier are used to establish a experiment system. Experimental results about X-axis and Z-axis of the prototype FTS as follow: 6.7114N/μm and 7.3260N/μm static stiffness,12.72um and 50.14μm stroke, 0.008μm and 0.01μm resolution. A closed-loop control system was established by using PMAC as the servo unit. The results of testing as follow:X-axis:8.85ms and 0% overshoot when track step signal, and less than 0.4% error in tracking a 50.6Hz sinusoid of 10μm p-v; Z-axis:15.24ms and 0% overshoot when track step signal, and less than 1.7% error in tracking a 55Hz sinusoid of 50μm.p-v; Finally, when the two axes (X-axis and Z-axis) follow the two different sinusoidal signal (a 27Hz sinusoid of 8μm p-v and a 27Hz sinusoid of 30μm p-v) simultaneously, the error in tracking are less than 1.2% and 4% respectively.