1, According to The Motion of The Machine Tool Control Trajectory Classification
The position control of NC machine tool
The position control requires only control the moving parts of the machine from moving from one point to another point accurate positioning and trajectory for between points is not strictly, in the process of moving without processing, the movement of each axis between is not related. In order to achieve fast and accurate positioning, displacement between two points moving average to move quickly, and then slowly closer to the point positioning, in order to ensure the positioning precision, position control of motion trajectory.
With the point of the control function of the machine tools are CNC drilling, CNC milling machine, CNC punching machine, etc. with the development of CNC technology and the reduction of the price of CNC system, simple for point control of the CNC system has not been seen.
The linear control of CNC machine tools
Straight-line control of CNC machine tools is also known as the parallel control of CNC machine tools, its characteristics are in addition to control between point and point accurate positioning, but also to control the two point correlation between movement speed and line (track), but the movement route just and machine tool coordinate moving parallel to the axis. That is to say axes simultaneous control of only one (i.e. CNC system do not have interpolation function), in the process of shifting tool to specify the feed rate of the cutting, the general can only be processed rectangular, a step shaped parts.
The linear control machine functions are relatively simple CNC lathe, CNC milling machine, CNC grinding machine etc.. The machine tool CNC system, also known for linear control CNC system. Similarly, solely for the purpose of linear control of CNC machine tools also does not see more.
The contour control of CNC machine tools
Contour control CNC machine tools, also known as continuous control of CNC machine tools, its control is characterized by two or two, more than the displacement and speed of the motion of the control of the same time.
In order to meet the relative motion of the tool along the workpiece contour of the workpiece contour meets the requirements, must be the displacement control and speed control in accordance with the provisions of the proportion of the precise coordination of the coordinate movement. Therefore in this kind of control mode, requires the NC device with interpolation function. The interpolation is based on the basic data input program (such as end coordinates, straight line arc coordinates and the coordinates of the center of circle or radius), through CNC system interpolation for mathematical treatment, the linear or arc shape description, it is one side of calculation, according to the calculation results to each axis controller to control the pulse distribution, displacement and the linkage the axis contour is consistent with the movement in the process of cutting tools for continuous cutting on the workpiece surface, can be a variety of straight line, arc, curve processing. corresponding NC device can be divided into the following form:
Second axis: mainly used for CNC lathe machining rotation surfaces or CNC milling machining cylindrical curve. 2 axle linkage: mainly used for more than three axis machine tool control, of which two shafts can linkage, and also an axis can be periodically wins feed.
3 axis: general points into two classes, a class is x /Y/Z three straight line coordinate axis, the more for CNC milling machine, machining center, etc.. Another is addition to controlling x /Y/Z coordinates of two linear, but also control around which a linear coordinate axes rotation of the rotation axis. Such as turning machining center, it in addition to vertical (Z axis), lateral (x axis) two linear coordinate axis, still need to simultaneously control around the Z axis of rotation of the spindle axis (c) of linkage.
The four axis: control of the X /Y/Z three linear axes and one rotary coordinate axis.
In addition to also control x /Y/Z three fertile line axis coordinates outside. At the same time control around the coordinates of these linear axis rotation of a, B, c axis coordinates in the two coordinate axes, forming and control five axis linkage then tool can is in space of any party to five axis linkage.
For example control tool and around the X axis and Y axis two swing in the direction of the cutter in the cutting point remains to be processed the contour surface normal direction, in order to ensure is the smoothness of the surface machining and improve the precision and efficiency of machining, is reduced to the machined surface roughness.
2, according to the way of servo control classification
The open-loop control of CNC machine tools
This kind of machine tool feed servo drive is open loop, i.e., no feedback detection devices, generally the drive motor is a stepper motor, main characteristics of stepper motor is control circuit transforms per a command pulse signal, the motor will rotate a step angle and the motor itself have self-locking capability.
The output of the numerical control system of feed command signal through the pulse distributor is used to control drive circuit, it to transform pulse to control the number of coordinate displacement amount, to transform the pulse frequency to control speed and displacement, to transform the pulse of the allocation sequence to control the displacement.
Therefore the control mode of the biggest feature is easy to control, simple structure, cheap price. CNC system sends out the instruction signal flow is unidirectional, so there is no control the stability of the system, but due to the error of mechanical transmission without feedback correction, so displacement accuracy is not high.
Early NC machine tools are used in this control mode, just failure rate is relatively high, at present due to the improvement of the driving circuit of the still received more applications. Especially in our country, the NC transformation of the old equipment and general economic type CNC system the use of this control mode. On the other, this control method can MCU configuration or a single board computer as CNC equipment, making the price of the whole system reduce.
The closed loop control of machine tool
This kind of CNC machine tool servo drive is according to the closed-loop feedback control of the drive motor can use DC or AC servo motor, and need to be equipped with position feedback and speed feedback, in the processing of ready to detect the moving parts of the actual displacement, and timely feedback to the CNC system in the comparator, it and interpolation operation instruction signal are compared, the difference and as servo drive control signal, in order to stimulate displacement components to eliminate displacement error.
According to the position and the feedback device, it is divided into two kinds of control mode, which is full closed loop and half closed loop.
As shown in the figure, the position feedback device adopts linear displacement detecting element (the grating ruler), installed in the machine tool bed saddle parts, i.e., direct detection machine coordinate the straight-line displacement, through feedback can be eliminated from the motor to the transmission error of machine tool bed saddle of the mechanical transmission chain, resulting in very high machine tool static positioning accuracy.
But because in the control loop, the friction characteristics of many mechanical transmission links, rigid and gap is nonlinear, and the mechanical transmission chain of dynamic response time and electric response time compared with very large. The stability of the whole closed-loop system correction brings very great difficulty, the design and adjustment of the system are also quite complex therefore, this full closed loop control mode is mainly used for high precision CNC coordinate building bed, CNC precision grinding.
Semi closed loop control
As shown in the figure, the position feedback using corner detection element (the encoder), installed directly on the part of the servo motor and ball screw end. Because most of the mechanical transmission parts not included within the system closed loop, hence the name to obtain more stable control characteristics. Screw mechanical transmission error can not be through feedback at any time correction, but the software value compensation method to appropriate to improve its accuracy. At present, most of CNC machine tools by semi closed loop control mode of the hybrid control of CNC machine tools
Will the control characteristics of concentration, can be composed of hybrid control scheme. As mentioned earlier, due to the open loop control method has good stability, low cost, poor accuracy, and the closed-loop stability is poor, so in order to compensate each other for, in order to meet the requirements of some machine tool control, should adopt the mixed control mode. Use more have open loop compensation type and semi closed loop compensation type in two ways
3, according to the functional level of the CNC system classification
According to the functional level of CNC system, usually the CNC system is divided into low,, high school class. This way of classification, in our country is more. Low, medium and high three file boundaries is relative, different period, standard division will be different. Look at the current level of development, according to some function and index, the various types of CNC system is divided into low, and high grade three class. Among them, high-grade commonly known for full function CNC or numerical control.
The metal cutting
Refers to the use of vehicles, milling, collision, hinge, drilling, grinding, planing and other cutting process of CNC machine tools. It can be divided into the following two categories.
Ordinary CNC machine tools such as CNC lathes, CNC milling machines, CNC grinding machines.
II processing center of its main features is automatically change the knife mechanism of tool library, the workpiece by a. Clamping, by automatically replacing all tool, on the same machine on the surface of the workpiece processing continuous milling (car keys), reaming, drilling, tapping and other process of processing, such as (buildings / milling) machining center, turning center, drilling center.
Refers to the use of extrusion, punching, pressing, drawing and other forming process of CNC machine tools, commonly used CNC press, CNC bending machine 1, CNC bending machine, CNC spinning machine.
The special processing class
CNC wire cutting machine, CNC EDM machine, CNC flame cutting machine, CNC laser processing machines and so on.
The measurement, drawing class
There are mainly three coordinate measuring machine, CNC tool, NC plotter.