| Pneumatic technology is a kind of engineering technology which uses compressed air to transfer energy or signal.It is widely used in automobile manufacturing industry,food and beverage industry and various non-standard automation industries because of its environmental protection,pollution-free,simple and light structure,low pressure level and high reliability.Due to the influence of non-linear factors such as friction,linear cylinders often have problems such as unstable low-speed motion and low servo control accuracy.Therefore,in this paper,the piezoelectric stack and the cylinder are combined to improve the friction characteristics of the cylinder through the vibration antifriction principle,so as to improve the position control accuracy of the pneumatic cylinder.Friction force is the key factor that affects the low-speed and smooth operation of the cylinder.Among the common friction models,the Coulomb model and the Dahl model are selected.Based on the vibration equation of the rod,the two models of vibration reduction theory are modeled.Through numerical simulation,the influence of the relationship between the vibration speed and the sliding speed on the friction reduction effect was compared and compared with the experiment,and the following law was obtained: the essence of vibration friction reduction is the reduction of the average friction force per unit period;under vibration conditions,only When the vibration speed is greater than the sliding speed,the anti-friction phenomenon occurs,and the anti-friction effect becomes more obvious as the vibration-slip speed ratio increases.In addition,compared with the Coulomb model,the vibration reduction theory based on the Dahl model is more in line with the actual working conditions due to the consideration of the pre-displacement phenomenon.Among the common cylinder vibration modes,from the perspective of feasibility,longitudinal vibration is selected as the excitation mode of the cylinder,and then a friction reduction cylinder using the piezoelectric stack inverse piezoelectric effect to realize the longitudinal vibration of the cylinder is proposed.The resonance frequency that the piezoelectric stack can excite under longitudinal vibration is determined by modal analysis.Afterwards,the harmonic analysis and voltage excitation analysis were carried out to obtain the particle motion trajectory map,which verified the rationality and feasibility of the vibration reduction cylinder structure.At the same time,it is found that there is a linear relationship between the voltage and the amplitude,that is,the voltage is proportional to the friction reduction effect.According to the theory of vibration reduction and finite element analysis,a prototype of friction reduction cylinder based on longitudinal vibration mode was developed,and the actual resonance frequency of the prototype was obtained by impedance analyzer.A test system capable of testing the static and dynamic friction of the cylinder prototype was built.According to the experiment,the relationship between the vibration frequency and the friction was obtained.It was found that the friction of the cylinder was significantly reduced under the resonance state.Then,the two-chamber air pressure,excitation voltage and Cylinder movement speed on anti-friction effect.The results show that the static and dynamic friction forces of the cylinder at the resonance frequency are reduced by about 25% and 18% respectively;the friction force of the cylinder decreases with the increase of the excitation voltage and increases with the increase of the pressure of the two chambers;when the excitation voltage and If the vibration frequency is constant,gradually increase the movement speed of the cylinder.It is found that the anti-friction effect is gradually weakened until it completely disappears.That is,the condition of the vibration reduction of the cylinder is that the movement speed is less than a certain critical value(that is,the vibration speed).In order to explore the effect of vibration anti-friction effect on pneumatic servo control,a mathematical model of pneumatic system was established,and an experimental platform of cylinder trajectory tracking control was built.In the controller design,PID algorithm and sliding mode control algorithm are selected,and trajectory tracking experiments are conducted based on these two control algorithms respectively.It is found that the performance of the model-based sliding mode control algorithm is significantly better than the PID control algorithm,and the control accuracy is about An increase of about 30%;in addition,the introduction of high-frequency vibration improves the tracking performance of the cylinder at low speed,especially in the commutation phase,the track tracking accuracy is increased by about 20%. |
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