Simple harmonic waves and synthesized simple harmonic waves are widely used in the test of instruments. However, because of the errors caused by clearance of gear and time-delay error of FPGA, it is difficult to control servo electric cylinder in precise simple harmonic motion under high speed, high frequency and large load conditions. To solve the problem, a method of error compensation is proposed in this paper. In the method, a displacement sensor is fitted on the piston rod of the electric cylinder. By using the displacement sensor, the real-time displacement of the piston rod is obtained and fed back to the input of servo motor, then a closed loop control is realized. There is compensation of pulses in the next period of the synthetic waves. This paper uses FPGA as the processing core. The software mainly comprises a waveform generator, an Ethernet module, a memory module, a pulse generator, a pulse selector, a protection module, an error compensation module. A durability of shock absorbers is used as the testing platform. The durability mainly comprises a single electric cylinder, a servo motor for driving the electric cylinder, and the servo motor driver.
The quality of force-displacement diagram is significant to help evaluate the performance of shock absorbers. Damping force sampling data is often interfered by Gauss white noise, 50Hz power interference and its harmonic wave during the process of testing; data de-noising has become the core problem of drawing true, accurate and real-time indicator diagram. The noise and interference can be filtered out through generic IIR or FIR low-pass filter, but addition phase lag of useful signal will be caused due to the inherent attribute of IIR and FIR filter. The paper uses FRR method to realize zero-phase digital filtering in a software way based on mutual cancellation of phase lag between the forward and reverse sequences after through the filter. High-frequency interference above 40Hz are filtered out completely and noise attenuation is more than -40dB, with no additional phase lag. The method is able to restore the true signal as far as possible. Theoretical simulation and practical test indicate high-frequency noises have been effectively inhibited in multiple typical speed cases, signal-to-noise ratio being greatly improved; the curve in indicator diagram has better smoothness and fidelity. The FRR algorithm has low computational complexity, fast running time, and can be easily transplanted in multiple platforms.
A nano dimensional standard named SIMT100 was designed and fabricated. The standard consists of a tracking area, a step height area, a 1D grating area, and a 2D grating area. All the structures were fabricated with a height of 100 nm in a 3 mm × 3 mm silicon substrate. To calibrate the standard, a white light interference microscope was constructed and integrated to the nano measuring machine (NMM). The height of a 10 μm width step measured by white light interference (WLI) microscope was 100.2 nm with a standard deviation of 0.41 nm. Due to low lateral resolution of the optical microscope, a metrological atomic force microscope (AFM) was used to measure the 1D and 2D gratings. The period length of the 1D grating evaluated by using the fast Fourier transform (FFT) method was 2999.7 nm with a standard deviation of 0.36 nm. The FFT method was also expended for evaluation of the 2D grating. The calibrated value of 2D grating along the x- and y-axis were 3001.2 and 3000.7 nm with standard deviations of 0.73 and 0.64 nm, respectively. All the measurement results are traceable because the data was recorded by three stabilized laser interferometers embedded in the NMM.