Study On The Key Technology Of Nano-CMM Measurement And Control System

In recent years, nano 3D measurement has received a great deal of attention. Many fine components recently fabricated by micro system processes, such as MEMS, LIGA or micro machining, are in overall dimensions within meso scale and required accuracy of nanometer scale. Conventional coordinate measuring machines (CMM) are no longer capable of 3D measurements of these fine parts. Some advanced probes, such as SPM, are capable for only 1D sensing to nanometre resolution. Under this background the concept of nano-CMM has been proposed.With a low-cost nano-CMM a series of key technologies are proposed, including high-resolution sensor development, real-time signal correction and subdivision, robust motion control scheme, non-contact and touch-trigger probe calibration and control, two-axis synchronous control and the idea of system software development.A high-resolution displacement sensor LDGI (Linear Diffraction Grating Interferometer) based on the grating Doppler effect, is developed and improved in this study. With a series of polarization and split effects, properly interfering these two light beams leads to modulation similar to Doppler frequency shift, which can be translated to displacement measurement via phase decoding. With real-time signal correction and subdivision, the resolution can reach 1nm. By calibration experiments with laser interferometer, in the travel length of 20mm, the repeatability LDGI is proved within 10nm.For the driving resolution and efficiency, as well as the simplification requirement, a piezoelement-based ultrasonic motor HR4 (Nanomotion Co.) is employed in this study. By integrating three driving modes of HR4, a multi-scale system is built. For the system stability, a BPNN (Back-Propagation Neural Network)-based PID control scheme is developed to generate low-speed motion. With the LDGI feedback, the control system can not only get a stable motion but can also compensate the deformation of the mechanic system and creep effect of the piezoelements. This control scheme can thus lock the target position for long time. Experiments show that the stability of this positioning control is within 1nm.Probe control and calibration are also the key points in this study. With the system of HR4 and LDGI, the calibration of non-contact probes can be carried out. Besides, a surface scanning software is developed in this way. For touch-trigger probes, because of the high sensitivity and narrow responsible range, conventional trigger control method cannot work. A double-trigger method is developed, that is, high speed for approaching, medium speed for first trigger, and low speed for second trigger. With a high-sensitivity switching circuit, the key parameters such as pretravel length and trigger repeatability can be calibrated exactly. This method can also be used for practical trigger control. Experiments show that with this double-trigger method the repeatability is within 10nm.Based on the above key technologies multi-axis motion control is developed. Besides, the systematic software is studied. From the above, this thesis includes the innovative points as follow:1. Grating interferometry with high tolerance and stability.2. Nanopositioning control based on ultrasonic motor and grating interferometry.3. Grating signal process for nanomesurement.4. Calibration and trigger control for nano-CMM probe.5. Multi-axis nanopositioning control based on ultrasonic motor and grating intreferometry.