The hottest PMAC based thermal error compensation

2022-09-28
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Research on thermal error compensation system of CNC lathe spindle based on PMAC

with the wide application of CNC machine tools and precision machining, people put forward higher requirements for the machining accuracy of CNC machine tools. Error compensation technology can effectively improve the machining accuracy of CNC machine tools. The so-called error compensation control is to reflect the error models into the actual machining process of the machine tool after obtaining them, so that the tool and workpiece can produce a certain relative motion along the opposite direction of the error in the machine tool workspace, so as to realize error compensation and improve the machining accuracy of the machine tool

thermal error is the largest error source of CNC machine tools, accounting for about 50% of the total error of machine tools, so reducing thermal error is very important to improve the machining accuracy of machine tools. Reducing thermal error can be achieved by improving design, controlling temperature and error compensation. Error compensation is a simple and effective method, which is receiving more and more attention. Because of the closed structure of the special CNC system, it is difficult for researchers to carry out secondary development on it and integrate the thermal error compensation function. This paper introduces a software compensation system based on programmable multi axis controller (PMAC). Experiments show that the software compensation system can realize real-time thermal error compensation in the process of machine tool machining, and can greatly improve the machining accuracy of machine tool

1 PMAC multi axis motion controller

pmac is a high-performance servo motion controller launched by delta tau data system company in the United States. It is a multi axis, multi-channel open motion controller based on industrial PC and windows operating system. PMAC multi axis motion controller can control 1-8 axes at the same time. It can not only execute the program stored in the controller, but also execute the motion program and PLC program. It is also a real-time, multi task computer, which can automatically distinguish the priority of tasks

2 software compensation system based on PMAC

the software compensation system based on PMAC multi axis motion controller is developed. Its control mode is semi closed-loop feedforward control. Its compensation principle is to use the compensation value calculated by the controller to control the motion of the tool rest, so as to ensure the accuracy of the relative position between the tool rest and the main axis

2.1 control mode of compensation system

the compensation system adopts the control mode of semi closed-loop feedforward compensation, and its principle is shown in Figure 1. In the process of machining, the detected temperature variables and mathematical models are used to calculate the prediction error, and the theoretical value in the NC program is compensated to correct the machining error. When using semi closed loop feedforward compensation, the key to realize real-time and high-precision error compensation is to select correct and reasonable variables and establish a high-precision model

Fig. 1 semi closed loop feedforward control schematic diagram

2.2 compensation system control principle

the compensation principle of the compensation system is shown in Fig. 2. Its working process and function is to first collect the temperature signal at the spindle (related to thermal error) in real time through the intelligent temperature sensor (DS18B20) arranged on the spindle of the NC lathe, and then send the signal to the PC through the input and output interface after being processed by arm. According to the pre established mathematical model of thermal error, Combined with the temperature value of the spindle collected in real time, the compensation value is calculated. Finally, the system sends the compensation value (the opposite number of thermal error difference) to the CNC controller, and the PMAC control card in the controller runs the program to correct the coordinate value of the tool in the original NC program to realize compensation

Figure 2 thermal error compensation control schematic diagram

the displacement sensor (CCD laser displacement sensor) in Figure 2 is used to detect the thermal error displacement of the spindle of the NC lathe during the empty cutting process, that is, the thermal drift error of the spindle relative to the tool caused by the thermal deformation of the NC lathe, According to the thermal error displacement data of the spindle and the corresponding spindle temperature data, the parameters of the spindle thermal error model gb/t 16989 ⑴ 997 "geotextile joint/joint Wide Strip Tensile Test Method" are determined. In the actual processing process, there is no need to use displacement sensors

2.3 software design of compensation system

the flow chart of this compensation system is shown in Figure 3. First, input the temperature variable data and initialize the mathematical model of spindle thermal error; The second step is to collect the temperature signal in real time; The third step is to calculate the thermal error displacement value according to the mathematical model, that is, to obtain the error compensation amount; Finally, the compensation value is sent to the CNC controller. If the machining is not finished, the program will return to the second step to continue to execute. In this cycle, the thermal error of the NC lathe spindle will be continuously calculated and compensated in real time

Figure 3 software flow diagram of thermal error compensation control system

in the compensation process, after the compensation value is input into the CNC controller, the PMAC control card in the controller reads the data, and offsets the coordinate value of the tool in the NC machining program pre input into the controller according to the compensation value, so as to compensate the error. When the processing starts, start the temperature error compensation thread, read the temperature every 1 min (changing the reading cycle can be achieved by changing the cycle of the temperature measurement system), call the function comerrorbytemp (double DT2, double DT4, double dt10) to calculate the thermal error value according to the read temperature value, and then update the original coordinate value of PMAC card, so as to realize the real-time compensation of thermal error

the subroutine used for compensation in PMAC control card is:

p500 - axis real-time thermal error displacement

p5o1 - axis real-time thermal error displacement

q124 - axis real-time coordinate storage variable

q126 - axis real-time coordinate storage variable

open program1000//G code subroutine

read (x, z) //read x, Z-axis coordinate

q124 = q124 - P500

q126 = q126 - P501

ra PID//compensation in fast feed mode (C00)

xq124zq126

return

n1000linear//compensation in linear interpolation (G1)

xq124zq126

return

...P>3 verification of compensation effect

this compensation system is used to compensate the spindle thermal error of a certain type of CNC lathe, After measuring at the same time (when the spindle speed of the CNC lathe is 900r/min), the thermal error displacement curves of the Z-axis and axis direction of the CNC lathe spindle before and after compensation are measured, as shown in Fig. 4 and Fig. 5

Figure 4 thermal error change curve before and after z-axis compensation

Figure 5 thermal error change curve before and after X-axis compensation

it can be seen from Figures 4 and 5 that after compensation, the thermal error displacement value of the spindle of the CNC lathe is greatly reduced. The maximum and average values of the thermal error displacement of the CNC lathe spindle before and after compensation are shown in the table below. After calculation, the accuracy of CNC lathe is improved by more than 90% after compensation. The software compensation system based on PMAC multi axis motion controller is designed to realize the real-time thermal error compensation of NC lathe spindle by running the compensation program. After verification, the compensation system is easy to realize and has high compensation accuracy. It has a wide range of applications and versatility in the research of machine tool industry. It has important research significance for further improving the machining accuracy of CNC machine tools. The final process only needs one drying process. (end)

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