Study on Optimization Method of Cam Grinding Accuracy

Cam and camshaft are one of the most common and key parts in mechanical equipment. In the process of cam operation, the rotation of the cam pushes the air valve stem to move in accordance with the law, so the accuracy of cam contour curve determines the performance of mechanical equipment. For example, the camshaft in automobile engine, its product quality directly affects the dynamic performance of engine. It has great practical significance to improve the machining accuracy of mechanical equipment, especially for the key parts. In order to improve the accuracy of cam grinding, many scholars have done a tremendous amount of research on compensation control [1], [2], but there is little research on the design of speed controller from the contour angle error. In order to improving the precision of cam contour machining, the emphasis of this paper is to design a speed optimization controller which considers the cam contour erroras a efficient control.The cam manufacturing process based on CNC grinding wheel is usually completed by the linkage of the rotating axis (C axis) and the advancing and withdrawing axis (X axis) of the grinding wheel. Typically, the input data of the system is the lift meter which describes the cam contour curve, but the lift meter is not directly input the data of the two-axis servo system, so it is necessary to process the lift meter data [3]. In this paper, a contour curve generation model is developed by the inversion method to generate cam contour curve. Then, modeling the input sequence values of the system for the input sequence value of the cam grinding platform. Finally, a contour error model is developed for error analysis to improve the preparation accuracy.In cam machining, there are many factors affecting cam precision, the biggest one is the cam rotation angle and grinding wheel feed speed [4]. Constant angular velocity and constant linear velocity grinding are often used in the existing machining methods, but when the precision of machining parts is high, constant angular velocity grinding cannot meet the requirements. In the cam grinding, servo system hysteresis leads to the tracking errors in the C axis and X axis,which in turn synthesizes the contour error [5]. Therefore, it is very necessary to adjust the speed in the grinding process. The results of theoretical derivation and constant angular velocity grinding simulation show that the big grinding speed leads to big cam contour error and the small grinding speed leads to small cam contour error. According to the final value theorem, when the position loop gain keeps the system stable, the grinding speed V is proportional to the tracking error E. Following this principle, the fuzzy control method is utilized to adjust the grinding speed, on the basis of constant angular speed grinding, the grinding speed is re-planned. The error of parts can be reduced by increasing the grinding time in the area with large error. But blindly increase grinding time, will affect the efficiency of processing. Therefore, in reducing the error of parts, but also to take into account the overall grinding time. Therefore, according to the error, the main task of the algorithm is to increase the grinding time in the area with large error and decrease the grinding time in the area with small error. This needs to carry the effective control on the grinding speed. The formulation of the control rules consiste of the following three aspects. Firstly, the grinding speed can be reduced in the area with large contour error, that means the grinding time can be increased in this area, while the grinding speed can be increased in the area with small contour error. Secondly, the grinding speed is reduced in the area where the variation rate of contour error is large, that is the grinding time is increased in this area, and the grinding speed is reduced in the area where the variation rate of contour error is small. Thirdly, setting a threshold for the contour error, which is the maximum contour error specified by the control algorithm, when the absolute value of the contour error exceeds the set threshold. When the absolute value of the contour error is less than the set threshold value, the controller does not work, and the original grinding speed remains unchanged. Since there is no feedback value of the contour error at the beginning of the system, the fuzzy controller can not work on the system, the controller controls the second grinding cycle. After the controller adjusts the grinding speed, when the absolute value of the contour error is less than the threshold value, the controller stops working to abtain the final results of the whole grinding process.In the grinding system, the given values of the cam rotating shaft and the grinding wheel feed shaft correspond to the grinding time of the adjacent grinding points respectively. Because of the change of grinding time between grinding points, the grinding process is no longer constant angular velocity grinding. In order to realize the change of grinding speed, a variable time-length Zero-order hold is proposed on the basis of Zero-order hold. The output characteristic curve of variable Zero-order hold is shown in Fig. 1. The sampling time T of the Zero-order hold is a constant value. If the value of the sampling time T is changed during the control process, the variable time Zero-order hold can be obtained. The grinding speed can be changed by the action of time-varying Zero-order hold. The function of time-varying Zero-order hold is realized by cubic spline interpolation.Taking the MK8312C CNC cam grinding machine tool as the research object. The CNC incorporates a three-closed-loop control system, where a position loop is added to the traditional double-closed-loop control system. The position signal output from the system is fed back to the input, which improves the response speed of the two axes and the tracking precision of the system. Simulink is used to build the two-axis simulation system of cam grinding, and MATLAB is used to design the fuzzy controller, and the fuzzy rules are input into the controller. The table of fuzzy rules is shown in Table 1, as shown in Fig. 2, the input of the fuzzy controller is the contour error and the contour error change rate, and the output is the time interval of the adjacent grinding points after fuzzy control The constant angular speed grinding and the speed optimization simulation grinding of fuzzy control are carried out on the cam. The simulation results are shown in Fig. 3. The blue curve is the contour error curve of constant angular velocity grinding, and the red curve is the contour error curve after the first optimization of fuzzy control algorithm. The two curves have roughly the same trend. The optimized grinding period T=3.631 s, the maximum absolute value of contour error is 1.023364938 mm, and the average absolute value of contour error is 0.220922157 mm. Compared with the constant angular speed grinding, the grinding time is increased by 0.86% , the maximum value of contour error is reduced by 22.83% , and the average value of contour error is reduced by 17.93%. The results of simulation show that compared with constant angular velocity grinding this method not only ensures the machining efficiency, but also greatly reduces the maximum and the average value of the absolute value of cam contour error.