When measuring the linear movement of CNC machining, linear detection elements are generally used, known as direct measurement. The position closed-loop control formed by this is called full closed-loop control, and its measurement accuracy mainly depends on the accuracy of the measuring elements, which is not affected by the transmission accuracy of the machine tool. Due to the precise proportional relationship between the linear displacement of the machine tool worktable and the rotation angle of the driving motor, the method of indirectly measuring the worktable's movement distance by driving the detection motor or screw rotation angle can be used. This method is called indirect measurement, and the position closed-loop control formed by it is called semi closed loop control.
The measurement accuracy depends on the precision of the detection components and the feed transmission chain of the machine tool. The CNC machining accuracy of closed-loop CNC machine tools is largely determined by the accuracy of position detection devices. CNC machine tools have very strict requirements for position detection components, and their resolution is usually between 0.001 and 0.01mm or less.
1. Requirements for position measurement device in feed servo system
The feed servo system has high requirements for position measurement devices:
1) Less affected by temperature and humidity, reliable operation, good accuracy retention, and strong anti-interference ability.
2) Can meet the requirements of accuracy, speed, and measurement range.
3) Easy to use and maintain, suitable for the working environment of machine tools.
4) Low cost.
5) Easy to achieve high-speed dynamic measurement and processing, and easy to automate.
Position detection devices can be classified into different types according to different classification methods. CNC machining can be classified into digital and analog types based on the form of output signals; According to the type of measurement base point, it can be classified into incremental and absolute types; According to the motion form of the position measuring element, it can be classified into rotary type and linear type.
2. Diagnosis and elimination of faults in detection devices
The probability of detecting component failures is relatively high compared to CNC devices, often resulting in cable damage, component fouling, and collision deformation. If there is suspicion of a malfunction in the detection component, the first step is to check for broken, contaminated, deformed wireless cables, etc. The quality of the detection component can also be determined by measuring its output, which requires proficiency in the working principle and output signals of CNC machining detection components. Taking SIEMENS system as an example for explanation.
(1) Output signal. The connection relationship between the position control module of SIEMENS CNC system and the position detection device.
There are two forms of output signals for incremental rotary measuring devices or linear devices: the first is a voltage or current sine signal, where EXE is a pulse shaping interpolator; The second type is TTL level signal. Taking the sine current output grating ruler of HEIDENHA1N company as an example, the grating is composed of grating ruler, pulse shaping interpolator (EXE), cable, and connectors.
During the CNC machining process, the machine tool outputs three sets of signals from the scanning unit: two sets of incremental signals are generated by four photovoltaic cells. When two photovoltaic cells with a phase difference of 180 ° are connected together, their push-pull motion forms two sets of approximately sine waves, Ie1 and Ie2, with a phase difference of 90 ° and an amplitude of about 11 μ A. A set of reference signals is also connected in a push-pull form by two photovoltaic cells with a phase difference of 180 °, and the output is a peak signal Ie0 with an effective component of about 5.5 μ A. This signal is only generated when passing through the reference mark. The so-called reference mark refers to a magnet installed on the outer shell of the grating ruler, and a reed switch installed on the scanning unit. When approaching the magnet, the reed switch is turned on, and the reference signal can be output.
