The machining accuracy of the outer surface of the piston has a decisive influence on the operating performance, fuel consumption and exhaust emissions of the internal combustion engine. The piston works in the harsh environment of high temperature, high pressure and periodic impact. The deformation and thermal deformation of the piston are very serious and uneven. In order to adapt the external shape of the piston in working condition to the cylindrical body, the outer shape of the piston at normal temperature is usually designed as a profiled surface. Its shape along the axis of its own axis is approximately a waist drum shape, and its cross-section perpendicular to its axis is approximately elliptical. In order to meet the requirements of use, the processing method of the piston outer circle is generally turning. In order to turn a piston with a profiled cross section, a mechanical simulation method is conventionally used. However, the mold is difficult to manufacture, easy to wear, low frequency response, piston change is difficult, and it seriously restricts the improvement of processing accuracy and processing efficiency. In recent years, domestic and foreign researchers have been studying the application of numerical control technology to the machining of piston outer rings, and have made great progress. The PTC series of piston outer circle CNC turning system developed by us has been used by nearly 30 domestic companies and has achieved very good economic benefits. 1 Difficulties in Piston Cylindrical CNC Turning System Piston Profile Input If a general NC program is used to describe the piston profile, the NC program will be very large and complex. The maximum diameter of the cross-section perpendicular to the piston axis and at a distance from the piston stop at Z (generally perpendicular to the axis of the pin hole) D is called the major diameter of the section, and the minimum diameter (usually parallel to the axis of the pin hole) d is called The small diameter of this section, E=Dd, is called the ellipticity of the section, and the shape description P is called the section form. The above three sets of data or functions are given in the form of list-type points or mathematical formulas on the piston drawing as D=f1(Z); E=f2(Z); P=f3(Z,f) where: f It indicates the angle between the radial direction and the major diameter direction of an arbitrary point P on the contour line of the cross-section. These descriptions are given in a piecewise fashion. Each interval is smooth and the interval junction is a jump breakpoint. In our system, if the drawing gives a mathematical formula, the user can select or directly input the function formula in the function list. The control system accepts and checks the correctness of the function expression, the integrity of the segmentation interval, and the function value. The rationality of the domain: if the value point is given, check the rationality of the type value, the integrity of the segmentation interval and the rationality of the range of the fitting function, and according to the situation that the type value point is within the interval The resultant curve extends toward the end of the interval. The fitting function uses a cubic spline with two freeform points, and the extended segment adopts two straight segments with the same slope as the first and last value points. Finally, the processing result is converted into numerical control data, and if necessary, it is merged with the traditional numerical control instructions and the numerical control data is output graphically. The output graphics are consistent with the traditional piston drawings and are easy to check and proofread. Ways to achieve high-frequency response The frequency response of the X-axis of an ordinary CNC system is about 1Hz, and the acceleration is about 1g (gravitational acceleration). It is affected by mechanical crawling and backlash. Because the piston is a profiled cross section, the frequency response of the X axis is 120 Hz or more, and the acceleration is 9 g or more. Therefore, a second X axis with a high frequency response parallel to the X axis must be added. We call this axis a linear axis. We use voice coil linear motors and linear servo components as the linear axis of the gapless transmission actuator, frequency response is higher than 135Hz, acceleration is greater than 13g, to meet the requirements of turning profiled sections. High-resolution implementation The control accuracy of the X-axis of an ordinary CNC system is in the micron range, while the control accuracy in the vicinity of the large-piston cross-sectional area of ​​the piston is required to be in the sub-micron range. The traditional control method cannot meet the requirements. Since the relative displacement of the linear axis of the piston cross-section is less than ±0.5mm, we use a 12- or 16-bit digital-to-analog converter to control the displacement of the linear axis relative to the X-axis to meet the control accuracy requirements. High-frequency four-axis linkage control method If you use a common four-axis linkage CNC system with servo spindle, not only the cost is too high, but also can not meet the real-time control requirements. For example, if the spindle rotates once every 1° to control the linear axis once, then the control frequency should be higher than 15 kHz, and the ordinary numerical control cannot be achieved. We use the control method of the general numerical control system for the X-axis and Z-axis, and the linkage of the spindle rotation angle and the linear axis adopts the rotation-based, dual-axis linkage method. A spindle encoder is installed on the spindle, and the computer detects the rotation angle of the spindle by means of an interrupt. The interrupt handler controls the linear axis based on the current position of the spindle rotation angle, Z axis, and X axis. This not only satisfies the four-axis linkage and real-time requirements, but also can use the general spindle without using a servo spindle. Under this ideological guidance, the control system can be further simplified: since the difference between the maximum diameter and the minimum diameter of the surface of the outer contour of the piston is generally less than 2 mm, the processing of the piston is mass-produced in the assembly line, and a series of pistons can be adjusted using the manual X-axis. The maximum diameter, the relative displacement of the tool tip with respect to the maximum diameter is controlled by a linear axis, and the X-axis is not controlled during the machining process, so that most of the piston processing requirements can be satisfied. In this way, a position detection encoder is installed on the Z-axis of the universal lathe. The computer can control the outer contour of the piston by controlling the linear axis according to the spindle rotation angle and Z-axis position. In this case, both the main shaft and the Z-axis can be driven by an ordinary AC motor. The X-axis is adjusted by the handle, and the system cost can be greatly reduced. Linear axis control parameter adjustment method For the linear axis, the second-order system whose dynamic model is approximated by the sweep frequency and system identification is expressed as the transfer function y(s) = ku(s) T2s2+2Tzs+1 Where: u is the input voltage applied to the control terminal, y is the displacement of the linear axis, k is the static stiffness, T is the time constant, and Z is the damping ratio. Fine design of the mechanical structure of the linear axis, reasonable choice of the static stiffness of the servo components and the quality of the moving part, can be used to meet the requirements of the time constant T and static stiffness k. Since the dynamic performance of the damping coefficient has a decisive influence, if z is too small, the number of step responses to oscillation is large and the amount of overshoot is large, which affects the profile and surface roughness of the workpiece. It is difficult to adjust the damping ratio of the mechanical parts. We installed a speed sensor on the moving parts of the linear axis to provide the system with an appropriate damping ratio. This is not only easy to implement, but also has a wide adjustment range, a stable damping ratio, and a simple adjustment method. Solve the contradiction between the time delay of optical isolation and the high control frequency Because the numerical control system must work in the industrial production line, we have carried on the photoelectrical isolation to all input and output signals. However, due to the high frequency of system control, simply using ordinary optoelectronic isolation devices can not meet the requirements. We have adopted a series of technical measures such as pulse stretching (via 74LS221), high-speed optocoupler (6N137), time slice multiplexing, and reentrant interrupt handlers to ensure the real-time control requirements.
Figure 1 System hardware structure
Figure 2 System hardware structure
Mine Main Ventilator Control System
Mine Main Ventilator Control System,Mine Ventilator,Ventilator Automation,Ventilator Control System
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