Overview of interpolation in CNC machining
January 31, 2023
SUMMARY CNC machining generally rectilinear profile parts, arcs, there are some non-circular curved profile, such as high-order curve, curve list, a list of other surfaces, but can be used to approximate a straight line or arc, when the press slide way When the data of each straight line and circular arc segment is programmed into the NC machining program, and the CNC system is input and started, the CNC system performs input processing, interpolation calculation and output processing, and controls the servo mechanism according to the calculation result. The contours of the machined parts are generally composed of straight lines and arcs. There are also some non-circular curve contours such as high-order curves, list curves, list surfaces, etc., but they can all be approximated by straight lines or arcs. When the data is programmed into the NC machining program and the CNC system is input and started, the CNC system performs input processing, interpolation calculation, and output processing on the block, and controls the servo mechanism according to the calculation result, thereby driving the servo mechanism of the CNC machine tool to make the tool. And the parts are precisely matched to the relative motion of each block, and finally processed to meet the requirements Components.
The interpolation calculation is based on the input basic data, such as the linear end point coordinate value, the arc starting point, the center of the circle, the end point coordinate value, the feed speed, etc., through calculation, the shape of the workpiece contour is described, and the calculation result is calculated according to the calculation result. A feed command is issued to each coordinate. Interpolation is actually the work of data densification based on limited information. Whether it is hardware numerical control or CNC numerical control, the interpolation module is indispensable. The module or device that can complete the interpolation function is called interpolator.
Interpolation methods can be classified as follows:
One-time interpolator, quadratic interpolator, and high-order interpolator are divided according to mathematical models. For example, linear interpolation is one interpolation, and circular or parabolic interpolation is secondary interpolation.
Hardware interpolation and software interpolation Generally, the hardware CNC interpolation module is composed of digital circuits, which is faster, but the upgrade is not easy and the flexibility is poor, which is called hardware interpolation.
The CNC CNC interpolation module is implemented by software. Although the speed is not as fast as the hardware interpolation, it is easy to upgrade and the cost is lower. It is called software interpolation.
Reference pulse interpolation and data sampling interpolation
1. Reference pulse interpolation (again: stroke scalar interpolation, pulse incremental interpolation):
The characteristic is that after each interpolation, the numerical control device outputs a reference pulse sequence to the corresponding motion coordinates, each pulse represents a minimum displacement, the frequency of the pulse sequence represents the coordinate motion speed, and the number of pulses represents the motion speed.
This method is simple to implement and easy to implement in hardware because it only involves addition and shift calculation. More commonly used are:
Digital Pulse Multiplier (also: Binary Ratio Multiplier, BRM for short)
Point-by-point comparison method (also known as regional discriminant method)
Digital integration method (referred to as DDA method)
Vector discriminant method comparison integral method minimum deviation method target point tracking method single-step tracking method direct function method encryption discrimination and double discrimination method...
2. Data sampling interpolation (again: time scalar interpolation, digital incremental interpolation)
The characteristic is that the numerical control device generates not a single pulse but a binary word, which is suitable for a closed loop, semi-closed AC/DC servo motor driven control system. It can be divided into two phases:
Coarse interpolation: A small straight line segment is used to approximate a given contour. This tiny straight line segment is related to the speed given by the command and is usually implemented by software.
Fine interpolation: “densification of data points” on the small straight line segment of the above-mentioned tapping spindle head . This stage is actually pulse-interpolated interpolation of straight lines. The calculation is simple and can be realized by hardware or software.
The following are commonly used data sampling interpolation methods:
Linear function method extended DDA method second-order recursive extended DDA method double digital DDA method angle approaching circular interpolation method improved tossing method
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