Analysis of full closed loop control of the hottes

With the rapid development of modern manufacturing industry, CNC machine tools are more and more widely used. At the same time, the positioning accuracy and repeated positioning accuracy of CNC machine tools are also increasing. The original semi closed-loop control system of precision ball screw plus encoder can no longer meet the needs of users. The semi closed-loop control system can not control the transmission error caused by the transmission mechanism of the machine tool, the thermal deformation error caused by the transmission mechanism during high-speed operation, and the error caused by the wear of the transmission system in the machining process, which has seriously affected the machining accuracy and stability of the NC machine tool. The linear grating ruler carries out the full closed-loop control of each linear coordinate axis of the NC machine tool to eliminate the above errors and improve the positioning accuracy, repeated positioning accuracy and accuracy reliability of the machine tool. As a key component to improve the position accuracy of the NC machine tool, it is increasingly favored by users

the following discusses the selection of linear grating ruler, the design and installation of special tools for installation, as well as the measurement and adjustment of CNC system parameters. There may be some shortcomings, so please don't hesitate to comment

I. selection of linear grating ruler

(1) selection of accuracy grade. The configuration of linear grating ruler on CNC machine tool improves the setting accuracy and re positioning accuracy of linear coordinate axis, so the accuracy grade of grating ruler should be considered first. The Accuracy grades of grating ruler include 0.01mm, 0.005mm, 0.003mm and 0.02mm. When designing CNC machine tools, we choose the accuracy level according to the design accuracy requirements. It is worth noting that when selecting high-precision grating ruler, we should consider the thermal performance of the grating ruler, which is the key link of the working accuracy of the machine tool, that is, the thermal expansion coefficient of the reticle carrier of the grating ruler is required to be consistent with the thermal expansion coefficient of the mounting substrate of the machine tool grating ruler, so as to overcome the thermal deformation caused by temperature

in addition, the maximum moving speed of the grating ruler can reach 120m/min, which can fully meet the design requirements of CNC machine tools at present; The maximum length of a single grating ruler is 3040mm. If the linear coordinate axis is controlled to be greater than 3040mm, the grating ruler can be butted to achieve the required length

(2) selection of measurement method the measurement method of grating ruler is divided into incremental grating ruler and absolute grating ruler. The so-called incremental grating ruler is that the grating scanning head obtains position information by reading the relative moving distance to the initial point. In order to obtain the absolute position, this initial point must be engraved on the scale of grating ruler as a reference mark, so the position control can be carried out only when the machine tool is turned on. The absolute grating ruler makes the absolute position data directly to the grating in the form of coding with the flashing grating lines of different widths and distances. When the grating ruler is powered on, the subsequent electronic equipment can obtain the position information, without moving the coordinate axis to find the position of the reference point, and the absolute position value is obtained directly from the grating scribed line

the cost of absolute grating ruler is about 20% higher than that of incremental grating ruler. Considering the cost performance of CNC machine tools, machine tool designers generally choose incremental grating ruler, which can not only ensure the motion accuracy of machine tools, but also reduce the cost of machine tools. However, the advantage of absolute grating ruler without returning to the reference point after starting up is unmatched by incremental grating ruler. When the machine tool is started up after shutdown or power failure, it can directly execute the processing program from the interruption, which not only shortens the non processing time, improves the production efficiency, but also reduces the scrap rate of parts. Therefore, it is the most ideal to choose a pair of grating rulers in the automatic production line composed of multiple numerical control machine tools or in the production pace requirements grid to see whether there are equipment with relatively large amplitude around

(3) selection of output signal the output signal of the grating ruler is divided into four types: current sine wave signal, voltage sine wave signal, TTL rectangular wave signal and TTL differential rectangular wave signal. Although the different waveforms of the output signal of the grating ruler have no impact on the positioning accuracy and re positioning accuracy of the linear coordinate axis of the NC machine tool, they must match the NC machine tool system, If the waveform and number of the output signal do not match the control machine system produced by no manufacturer of this kind of experimental machine at present, the machine system cannot process the output signal of the grating ruler, and the feedback information and compensation error are impossible for the full closed-loop control of the linear coordinate axis of the machine tool. In practice, the waveform of the output signal does not match the CNC machine tool system, but there are also ways to deal with this situation. As long as a digital electronic device is installed between the output signal and the machine tool system (such as the subdivision and digital electronic device of heidenhainde's ibv600 Series), it is easy to solve it

second, the structural design of the linear light acquisition data grating ruler

the structural design and installation of the grating ruler are more important than the selection of the grating ruler. No matter which link is handled improperly, it will seriously affect the control accuracy of the grating ruler, and there may be the phenomenon that the accuracy of the full closed loop is not as good as that of the semi closed loop

1. The influence of walking posture on the detection accuracy of grating ruler

(1) the coincidence between the driving axis and the midpoint of the load is well known. When pushing an object to move, if it is not pushed to the midpoint position, it is easy to cause the object to rotate, swing and other unstable phenomena. The moving axis is required to coincide with the midpoint of the load as much as possible. In practice, due to the limitation of structure and processing errors, there is a certain distance between the driving axis and the midpoint of the load, which leads to the swing phenomenon when the lead screw drags the load

(2) the consistency of the damping characteristics of the two guide rails is also a very important factor. The inconsistent damping characteristics of the two guide rails can also easily cause the object to swing, as shown in Figure 1

(3) the installation position of the grating ruler should be as close as possible to the drive axis. The linear coordinate axis drive system of most machine tools generally uses precision ball screw pairs. Theoretically, the grating ruler should be installed as close as possible to the axis of the screw pair. In this way, the installation of the grating ruler conforms to the principle of Abbe error minimization, that is, the installation position of the grating ruler should be close to the working datum of the control axis. The closer it is, the smaller the Abbe error will be, The higher the position accuracy controlled by the grating ruler, the better the positioning accuracy of the machine tool. However, in practice, due to the limitation of structure and space, there are only two ways to install the grating ruler, one is to install it near the side of the lead screw pair, and the other is to install it outside the guide rail. In order to obtain the minimum Abbe error, it is recommended to choose the first installation method as far as possible. On the contrary, the grating ruler with high accuracy is selected, but it does not meet the accuracy required by the CNC machine tool

although the installation position of the grating ruler is relatively close to the driving axis, there is a certain distance between the installation position and the driving axis after all. The combination of this distance and the swing of the object during driving brings great trouble to the detection and control of the grating ruler. When the driving object swings towards the installation side of the grating ruler, the grating ruler mistakenly believes that the moving speed is insufficient during the detection, and the system will give an acceleration signal, while the driving object immediately swings towards the other side, and the grating ruler mistakenly believes that the moving speed is too fast during the detection, and the system will give a deceleration signal. This repeated operation does not improve the control of each linear coordinate axis of the NC machine tool, but intensifies the vibration of the driving object, This leads to the strange phenomenon that the full closed loop is not as good as the semi closed loop. From this point of view, the position design of the driving axis of the driving object, the installation position of the grating ruler and the damping characteristics of the two guide rails are very important, which must be paid great attention to by the machine tool designer. When designing the machine tool, we must carefully consider the equipment factors, which is bound to achieve good results that meet the design requirements

COMAC has invited relevant Chinese and foreign potential suppliers to participate in the scheme design 2 The installation surface of the grating ruler and slide ruler and the slide ruler support have enough rigidity and strength

the installation position of the grating ruler should have enough rigidity and strength, which is also the key link to ensure the normal operation of the grating ruler. The grating ruler works through the photoelectric scanning principle, so the grating ruler cannot be in a strong vibration state. The vibration causes the instability of the light source and affects the control accuracy of the grating ruler. Therefore, the installation position should preferably be integrated with the solid casting of the machine tool. Even if the connector is required due to structural reasons, the whole connection between the connector and the machine body is required to be in good contact, and the connection is rigid enough, so as to prevent the weak links at the connection and connection from causing strong vibration, affecting the normal work of the grating ruler, and ultimately leading to the reduction of the positioning accuracy of the machining center

3. The installation position of the grating ruler should be as far away from the heat source of the machine tool as possible

the installation position of the grating ruler should be as far away from the heat source of the machine tool as possible to avoid the influence of temperature. The grating ruler is not afraid of heating, and the overall environmental temperature has little influence on the grating ruler, but the heat source of the machine tool (such as the ball screw pair) produces an error due to uncertain temperature rise locally, and this error is difficult to control and it is difficult to correct and compensate in real time, If the grating ruler is close to these places, it will inevitably affect the control accuracy of the grating ruler

4. The protection of the installation position of the grating ruler is very important

(1) in modern machine tools, users generally require large flow cooling. When washing with large flow, cutting fluid will splash on the grating ruler. The working environment of the grating ruler is also full of moist air with cooling spray. In this environment, the grating is easy to condense, and a layer of film is easy to form on the scanning head. In this way, the light projection of the grating ruler will be poor, and the grating is easy to leave water traces, which will seriously affect the measurement of the grating. If the processed chips accumulate near the grating, resulting in poor chip removal and drainage, the grating ruler will be soaked in cutting fluid and impurities, which will affect the use of the grating, and more seriously, the grating ruler will be damaged and the whole machine will be paralyzed

(2) if the grating ruler is in strong cooling spray or dust, it can be treated by compressed air, but the compressed air must be purified by filter and meet the following impurity quality grade requirements according to ISO 8573 1, as shown in the attached table

III. installation of linear grating ruler

the structure of grating ruler is shown in Figure 2, which is composed of fixed ruler body 1 and moving ruler reading head 2. The ruler of the grating ruler is an aluminum shell, which is used to protect the ruler, scanning unit and its guide rail from chips, dust and splashing water. The moving ruler reading head consists of a scanning unit, a precision connector and a mounting block. The precision connector connects the scanning unit with the mounting block to compensate for a small amount of guide rail mechanical error

1. The design of the installation tool

it is not difficult to see from Figure 2 that the dimensions of 1.5mm, 0.2mm and 2mm are very important for the measurement of the grating ruler, and must be guaranteed during installation. On the other hand, the fixed ruler body and the moving ruler reading head are not rigidly connected, and the scanning unit and the mounting block are elastically connected with precision connectors. Therefore, the correct installation of the grating ruler is not an easy thing. For this reason, an economical and practical special installation tool has been designed after many studies, as shown in Figure 3

2. Adjustment and installation

the adjustment and installation of grating ruler is a link that cannot be ignored in the use of grating ruler. The good or bad adjustment and installation directly affects the quality of grating ruler detection and control, so it must be paid enough attention. Taking the z-direction linear coordinate axis of XH768 horizontal machining center as an example, this paper introduces the adjustment and installation process of grating ruler

(1) processing of grating ruler installation datum plane processing of grating ruler installation datum plane of fixed ruler and moving ruler, ensuring that the parallelism with the guide rail is within 0.02mm, and machining the fixed ruler combined screw hole according to the coordinate size

(2)