[Abstract] This paper analyzes the problems that exist in realizing online measurement and control functions in general NC systems. By introducing the "conditional short circuit" function instruction, etc., the problems in various online measurement and control of NC systems can be solved by programming the parts. Taking the online measurement and control of part dimensions in external cylindrical grinding as an example, the implementation principle of this mechanism is discussed in detail. Keywords : Online measurement and control, NC system, part dimensions [b][align=center]Research On Realization On-line Measurement and Control Function in GeneralNC System[/align][/b] [align=center]Cheng Lianghong Gao Ansheng Qian Xinen (Hubei Automotive Industry Institute 442002)[/align] The realization principle of this function is explained in detail in this paper. Keywords: NC system; online measurement and control; Part size 1 Introduction For precision machine tools, such as precision grinding machines, which have high requirements for part machining accuracy and relatively large tool wear rates, the method of using online part dimension measurement and control to obtain high machining accuracy has a long history. The traditional implementation method is to construct the online measuring instrument part dimension automatic control system shown in Figure 1, which is implemented in the machine tool CNC system and non-CNC system. It is necessary to develop special hardware and software control modules for the machined object, and the system is complex and lacks versatility. This paper studies the implementation problem of online measurement and control function in general CNC system, analyzes the problems existing in the implementation, and proposes solutions. A general CNC system design method with online measurement and control function is given for the online direct automatic dimension control system shown in Figure 1. The system constructed has a high degree of openness and flexibility. 2 Problem Description For the convenience of describing the problem, the following explains the two statements used in this paper: Statement 1: The trajectory of the part machining program in the CNC system cannot be changed after programming. The automatic part machining process of CNC machine tools is achieved by sequentially executing part program instructions. The machine tool path and feed rate are set during programming and cannot be changed in real time during program execution. This characteristic of the CNC system is referred to as "immutable path after programming". Proposition 2: Conditional Short Circuit. This paper proposes to introduce a new CNC function instruction. The real-time interpolation module of the CNC system performs servo control once per sampling cycle and performs an interpolation calculation on the current program segment. However, it first tests whether the segment contains a conditional short circuit instruction. If so, it tests whether the condition specified by the instruction is met. If it is met, the current segment interpolation continues; otherwise, the current segment is stopped (short-circuited) and a new program segment begins. The following examples illustrate two arguments, demonstrating the problems in implementing online measurement and control functions in a general CNC system and feasible solutions: Argument 1: The immutable path after programming the CNC part machining program is the problem hindering the implementation of the online measurement and control mechanism. Proof: Taking the grinding of an outer diameter as an example, consider a grinding system with real-time measurement and control as shown in Figure 2. The grinding wheel (tool) has a radius of r, and the workpiece axis is mounted at the origin 0 of the workpiece coordinate system. The required final workpiece size (diameter) is d. The worktable, carrying the grinding wheel, starts from the initial position b and feeds towards the workpiece at three speeds: feed, fine grinding, and finishing grinding along the negative x-axis. When the measuring instrument measures the workpiece size to reach the first threshold set by the system (i.e., the fine grinding size reaches d and the workpiece diameter equals d), the feed speed is switched from fine grinding feed to finishing feed. When the measured workpiece size reaches the second threshold (i.e., the finishing grinding size reaches d and the workpiece diameter equals d), the grinding wheel is quickly retracted to point b. Due to the high wear rate of the grinding wheel, the value of r during the machining process is variable, making it impossible to determine the switching point size of the machine tool feed trajectory segment in advance. Real-time control through online measurement is necessary. That is, to introduce an online measurement and control mechanism, the real-time change of the machine tool tool trajectory size is a necessary condition. Argument 1 is proven. Argument 2: Introducing a conditional short-circuit instruction into the CNC system allows the programmed trajectory to be variable in real time. As shown in Proposition 2, the conditional short-circuit instruction can determine the switching point position between two program segments in real time by detecting whether the specified conditions are met, so that the trajectory can be changed after programming. Argument 2 is proved. Argument 2 provides a solution for introducing an online measurement and control mechanism in a general CNC system. The following example illustrates this point. Suppose that the CNC system has a conditional short-circuit instruction and controls the grinding process shown in Figure 2. Then the automatic machining process based on online measurement and control can be programmed and implemented according to the following operations: (1) Control the center of the tool (grinding wheel) to move rapidly from point b to point b; (2) Instruct the center of the tool to finish grinding the feed point. This segment contains a conditional short-circuit instruction, with the "finish grinding dimension reached" signal as the short-circuit condition; (3) Instruct the center of the tool to finish grinding from the current point to the origin of the coordinates. This segment contains a conditional short-circuit instruction, with the "finish grinding dimension reached" signal as the short-circuit condition; (4) Instruct the center of the tool to move rapidly back from the current point to point b; (5) The program ends. 3 System Design Principles 3.1 System Hardware Structure The hardware structure of the CNC system with online measurement and control mechanism is exactly the same as that of traditional CNC. The workpiece measuring instrument, as an external device of the CNC system, is connected to the CNC system via I/O VI circuit. The measurement threshold level of the measuring instrument is used as an input switch quantity for testing and control conditions of the CNC system. 3.2 System Software Design The software design mainly adds support for conditional short-circuit instructions on the basis of traditional CNC system software. Suppose that the conditional short-circuit instruction has the format: Where M95 is the conditional short-circuit instruction word, and Ek represents the k-th condition, which can be the state of any I/O information of the system. Since the condition satisfaction is real-time, the algorithm of the conditional short-circuit instruction is mainly implemented in the real-time interpolation stage. The algorithm is described below. Usually, the CNC system processes the data of the part machining program in an approximate pipeline manner, which is roughly divided into three data processing stages: decoding, tooling, and interpolation. The interpolation module processes the data structure queue shown in Figure 3. This queue is dynamically established by the decoding and tooling modules when processing the part machining program segment by segment. Without loss of generality, we use vectors to represent the coordinate displacements of program segment trajectories, regardless of the trajectory shape. Let the coordinate displacements of the i-th program segment trajectory be X, Y, Z, denoted as S. Similarly, let: S_i be the coordinate displacement completed by the i-th program segment at the current moment; S_i be the coordinate displacement not yet completed by the i-th program segment; is a vector accumulator used to store the accumulated sum of the remaining coordinate displacements of the program segment caused by conditional short-circuit instructions. The accumulator has an initial value of zero when a new part machining program begins. Let the current interpolation be the i-th program segment. The conditional short-circuit algorithm in the current program segment is implemented by executing the following process: (1) If the current program segment has no M95 or Ek is not satisfied, go to ③; (2) If the current program segment has M95 and Ek is satisfied, calculate according to formula (2) and accumulate the remaining coordinate displacement according to formula (3), and then move the interpolation data area pointer to the next data structure (new program segment); (3) If the current program segment is G90 programming and is a new program segment, modify the original set coordinate displacement of the segment according to formula (4), and then clear the S accumulator. Otherwise, go to ④; (4) Execute the normal servo control and interpolation operation; (5) End and return (interrupt return). In the above algorithm, the remaining coordinate displacement of the short-circuited program segment is superimposed on the first G90-programmed trajectory segment that appears afterward. This ensures that the subsequent relative displacement segment (G91 programming segment) is not affected by the preceding short-circuit instruction and can still move the programmed coordinate displacement relative to the current position. However, the absolute displacement segment (G90 programming segment) is affected by the preceding "short circuit" and the entire planned displacement is affected, making the segment unable to reach the specified position. After superposition correction by formula (4), the influence can be eliminated. When writing part machining programs using conditional short-circuit instructions, the G90 or G91 of each program segment must be correctly specified as needed. In addition, the following points should be noted: ① At least one G90-programmed straight trajectory segment should be arranged after the program segment containing the conditional short-circuit instruction and before the end of the machining program. Otherwise, due to the random factor of "short-circuit trajectory loss", the stopping point will be a random point; ② It is emphasized that the subsequent G90 trajectory segment is a straight line because the endpoint coordinates of other line types are not easy to modify randomly. 3.3 Programming Example Problem: Use a general-purpose CNC system with online measurement and control to complete the external cylindrical grinding shown in Figure 2. Try to write a part machining program. Solution: Treat the grinding wheel as a machine tool tool (tool No. 1), and its radius value r. Set it into the machine tool parameter table. Let the displacement in the x direction be represented by the diameter value, and the starting point be b (the origin of the machine tool coordinate system). Then the corresponding part machining program is: The above program code is based on ISO1056-1975E, where fl and f2 represent the feed speeds for smooth grinding and fine grinding, respectively; E1 and E2 represent the conditions of "smooth grinding dimension reached" and "fine grinding dimension reached", respectively. 4 Conclusion (1) The invariable trajectory after programming in a general-purpose CNC system is the obstacle to the realization of online measurement and control mechanism. (2) Introducing conditional short-circuit instructions into the CNC system can make the trajectory variable in real time after programming. (3) Conditional short-circuit instructions provide a solution for realizing online measurement and control mechanism in a general-purpose CNC system. (4) The method presented in this paper enables the online measurement and control functions to be programmable, further enhancing the versatility and openness of the CNC system. Users can decide whether to select these functions as needed. If selected, the details of the machine tool's online measurement and control can be flexibly implemented by programming the parts according to the production process requirements. It has no impact on other functions of the CNC. Using the method described in this paper, we introduced an online measurement and control mechanism into the general CNC system and successfully realized the CNC transformation of the H175 crankshaft grinding machine with a high performance-price ratio. The online measurement device consists of a controllable measuring frame and an Italian Marposs measuring instrument sensor head. The measurement accuracy is 1 m, the machine tool feed resolution is designed to be 1 m, the speed range is 0.05 mm/min to 10 m/min, and the processing index of the shaft diameter of the acceptance part reaches: the surface roughness of the shaft diameter is Ra0.4, the roughness of the shaft diameter end face and arc is Ra0.8; the shaft diameter width is 38±0.1 mm; the size dispersion is ≤0.016 mm. The grinding machine has been put into use for more than a year, the system is running normally, and it has been well received by users. References [1] Chen Dianzheng, Translator, New Progress in Online Detection Technology in Grinding Processing, Grinding Machine and Grinding, 1993, 3: [2] Gao Ansheng, Cheng Lianghong, Qian Xinsi, A New Type of CNC Machine Tool Computer Control System, Mechanical Industry Automation, 1994, 3 [3] Qian Xinsi, Gao Ansheng, Cheng Lianghong, Application of a New CNC System for Crankshaft Grinding Machine, Manufacturing Technology & Machine Tool, 1997.11. About the Authors: Cheng Lianghong, male, professor, born in 1957, from Laohekou City, Hubei Province. He received his Master of Engineering degree from Huazhong University of Science and Technology in 1988. Currently, he is mainly engaged in theoretical and engineering technology research on computer control and application, and automation of electromechanical equipment. Related research project: "Research and Application of a New CNC System for Crankshaft Grinding Machine", which won the fourth prize of China Automotive Industry Science and Technology Progress Award in 1999.