We know that data quality control is an essential part of online research, follow this guide to learn the on-line data collection and quality control, improving your processes and collect better responses.
Overview of Quality Control Methods
Product quality is the degree to which its quality characteristics meet customer needs. Quality control is to take certain measures to make the quality characteristics of products within the specified standard range For the machining system, the commonly used quality control methods include process improvement, online quality control and post process quality control. The following is an introduction and comparison of these 3 methods.
– Process improvement is a quality control method to eliminate the possibility of quality defects It is to ensure that the product quality characteristics are within the specified range and achieve the purpose of quality control by eliminating the causes of process changes and making the process always stable. Process improvement is a positive method to control product quality and the most direct way to control product quality, because it eliminates the root causes of process changes. Generally, online quality control or post process quality control is not necessary for the improved process.
– The above process improvement is the best way to conduct quality control, but not all process changes can be eliminated through process improvement For process changes that cannot be eliminated, in order to achieve excellent quality characteristics, it is often necessary to use online quality control technology to control process changes in parallel The so-called online quality control refers to the control of product quality in the production process, that is, online monitoring of process parameters and detection of quality characteristics, online adjustment of process parameters, so that the quality characteristics of products are within a certain range.
– For the process that has undergone online quality control, subsequent quality control is generally not required The so-called post process quality control is the post process inspection and testing. It is a control method to inspect the parts after the process is completed and eliminate the unqualified products to ensure the product quality.
Online Quality Data Acquisition and Control
In the on-line quality control method, the most important is the on-line quality data collection and the on-line adjustment of process parameters On line quality data acquisition completes the collection of process information and product information, and sends them to the control computer through signal preprocessing and data interface According to the relationship between quality characteristics and process parameters, the computer makes a decision to adjust the process parameters, and then the controller adjusts the process parameters through the actuator to achieve online quality control.
Online quality data collection methods include process parameter monitoring and online product quality inspection Process parameter monitoring is realized by measuring the parameters related to the product quality characteristics. Online product quality inspection is realized by directly measuring the product quality characteristics during processing or at the interval of processing steps. Common process measurement parameters include cutting force, temperature, main shaft motor current change, vibration noise signal, etc For online quality control, the relationship between process parameters and final product quality characteristics shall be established to ensure the final product quality through parameter adjustment.
Example of Online Quality Data Collection and Quality Control
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On line cycle measurement of CNC machine tools Most CNC machine tools are equipped with a closed loop position feedback control system, that is, the actual moving position is measured by a position sensor installed on the guide rail or moving parts, and then fed back to the controller of the machine tool. If there is a deviation, the controller will automatically adjust the pulse signal of the drive motor to eliminate the position error. At first sight, this seems to be an automatic process control. During each program run, it can ensure the correct size of the parts In fact, to ensure the correct size of parts, there are many factors to consider besides the position accuracy of the machine tool itself. For example, the position, size and shape of the cutting tool are also the main factors that cause the processing quality. Therefore, the information about the tool condition should also be entered into the machine controller in a timely manner, and the tool compensation should be carried out in a timely manner. If this cannot be done, the influence of such factors as tool wear, breakage, and deformation caused by cutting force will lead to dimensional error of parts. With the addition of qualified parts, people have adopted different online quality inspection and control methods. For example, the vibration and noise signals in the cutting process can be used to determine the wear condition and damage generation of tools. The change of spindle current can also be used as a process parameter to determine the tool condition. Perhaps the most useful is the online cyclic measurement technology for directly measuring the size of the workpiece or tool, which is a typical example of online quality control.
– Online cyclic measurement is to use the three-dimensional probe installed on the CNC machine for online quality data measurement and quality control. The 3D probe is a surface contact measuring device. The controller moves the coordinates according to the positions of the machine in three directions to obtain the measurement results of the measured workpiece. The 3D probe is just like the CNC machine. It is usually placed in the tool magazine of the machine tool. When testing is needed, it is taken out by the manipulator and installed in the spindle hole for testing.
– The principle of online cycle quality control is to measure the position of the workpiece before each key cutting, and then send the measurement results to the controller. The controller modifies the compensation table in the machine tool memory through macro commands according to the comparison to achieve the purpose of online compensation. Figure 6-8 is an example of online cycle measurement of workpiece and tool using a three-dimensional probe.
– Online cyclic measurement utilizes the coordinate control function of CNC machines to integrate the processing and detection of parts, which can realize online measurement at the machining interval, and processing compensation according to the measurement results to achieve online quality control In addition to online quality compensation control, the 3D probe can also determine the precise positioning of the machine tool and the position and orientation of the workpiece in advance, measure the allowance of the blank, and determine the feed during the processing.
– The disadvantage of online cycle measurement is that the same coordinate system is used for processing and detection, and it is difficult to detect the processing error caused by the positioning error of the machine tool. In addition, the detection step is added in the processing process, thus increasing the processing cycle time.
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For online compensation control of thermal deformation of CNC machines, thermal deformation is one of the most important factors that cause machining errors. According to statistics, machining errors caused by thermal deformation account for 40%~70% of the total errors of the machine tools. The methods to eliminate and reduce the thermal deformation error include improving the machine tool structure, adding complex heat source control devices, selecting materials with high thermal stability, and using advanced motors and bearings. These methods generally require major improvement of the machine and large investment. The other method is the software generating accuracy method, that is, when the hardware structure of the machine tool remains unchanged, the machine tool processing error caused by thermal deformation is compensated by detecting the thermal condition of the machine tool and using software.
Compensating the thermal deformation error of machine has not been completely accepted by the industry until now, because the current CNC machine controllers generally do not have complex real-time error compensation capabilities. On the other hand, it is very complex and difficult to carry out online measurement of the thermal error of machine tools and establish the thermal error model of machine. In recent years, people measure the thermal state of machine tools online by adding external microcomputers, and realize the compensation control of thermal deformation errors of machine tools through the method of software generating accuracy.
The principle of online thermal deformation compensation control is to first measure the thermal condition of the machine, send the measurement results to the computer, calculate the thermal deformation compensation amount according to the thermal error model, then send the compensation signal to the controller of the machine, and complete the thermal deformation error compensation control through the execution file There are two main methods to implement the thermal deformation compensation control. One is to directly modify the analog voltage signal of the machine servo system circuit through analog quantity, and the other is to modify the CNC program of the machine tool through the digital serial port of CNC.
The typical CNC machine thermal deformation compensation system measures the thermal condition of the machine in real time through the thermocouples distributed on the machine, and establishes a thermal error model based on neural network to carry out thermal error compensation control. The experiment shows that the thermal deformation online compensation system has achieved good compensation effect.