Sequential control is in accordance with the pre-defined sequence of the production process, under the role of various input signals, according to the internal state and time sequence, so that the various processes in the production process automatically and orderly work. When using the PLC to control the sequence of the production process, first draw a sequential function diagram according to the system process, and then convert the sequential function diagram into a ladder diagram or an instruction statement table. Sequential function charts consist of steps, directed connections, transitions, transition conditions, and actions (or commands). Figure 1 Motor full-voltage start process function diagram 2) Actions and Commands Figure 2 The representation of several actions “Action†refers to the command issued by the PLC to the controlled object or the action to be performed by the controlled object when a step is active. Actions are represented by text or symbols in a rectangular box, which should be connected to the corresponding step rectangle. If there are several actions for a step, its representation is shown in Figure 2. According to the transformation form between step and step, sequence function diagram has three basic forms: single sequence structure, selection sequence structure and parallel sequence structure. Figure 3 Sequential function diagram basic structure If there is more than one sequence after an active step, sequence control can only select one of the sequences whose conversion conditions are true to be executed. The rest of the sequence conversion conditions are false and cannot be performed. The structure of this sequential control process is called the selection sequence, see Figure 3b). The control of the CNC machine tool is divided into two parts: the position control of the coordinate axis movement and the sequence control of the numerical control machining process. The logic control and the switch quantity control involved in the control process are all completed by the PLC in the numerical control machine tool. PLC used in CNC machine tools is divided into two types: built-in type and independent type. Figure 4 built-in PLC input and output connection diagram Built-in type PLC with CRT (Cathode Radiation Tube - cathode ray tube display), MDI (Manual Date Input - manual data input) panel, numerical control machine tool operation panel, strong electric control circuit, pneumatic control circuit, hydraulic pressure through I/O and CNC machine tools Transmission control circuit, magazine servo control unit, spindle servo unit, feed servo unit, etc. perform signal transmission. Figure 5 Independent PLC input and output connections The stand-alone PLC must exchange signals with the machine-side I/O, and must also perform signal connection with the I/O of the CNC system. Therefore, both the CNC system and the PLC have I/O interface circuits. The independent PLC adopts a modular structure and is installed on the card rack. The number of I/O points and other function modules can be flexibly configured according to the requirements of the specific numerical control machine tools. The PLC's various functional modules installed on the plug-in rack are passed through the bus. Connected to each other. The CNC side of the CNC machine tool mainly includes the hardware and software of the CNC system and the peripheral equipment connected to the CNC system. The MT side of the CNC machine tool includes the mechanical parts, auxiliary devices, machine tool operation panel, and machine tool power lines. The PLC of the CNC machine tool is located between the CNC and the MT and processes the input and output signals of the CNC side and the MT side. Will correctly draw the sequence of function charts; describes the NC machine tool PLC signal content. Date boxes are popular products of our factory,we produce nearly 60000pcs boxes every year to hold dates,The date boxes made of different materials, such as Wooden Box, Leather Box and paper box.LOGO,size,color and material can be customized,MOQ is 500pcs for wooden date box and leather date box,2000pcs for paper date box.
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1) Step <br> and divided according to a change state of the control system outputs a duty cycle of the system is divided into a plurality of sequentially connected stages that called step (the Step), available programming elements (e.g. auxiliary relay M) representative of Each step. The step is divided according to the state change of the PLC output. In the first step, the status of each output (ON or OFF) remains unchanged, and the status of the adjacent two-step output is different. As long as the output status of the system changes, the system enters a new step from the original step.
Initial Step The step corresponding to the initial state of the system is called the initial step, and the initial state is generally a relatively static state in which the system waits for a start command. The initial step is represented by a two-line box, and each sequential function chart has at least one initial step.
Active step <br> When the system is in the stage of step, this step is active, and this step is called active step. When the step is active, the corresponding action is executed; when the step is inactive, the corresponding non-stored command is stopped.
Example of Step Division <br>In the case of the motor full voltage start PLC control as an example, the motor M is stopped at the beginning, the start button SB1 is pressed, the motor M is rotated, and once the stop button SB2 is pressed, the motor returns to the initial stop state.
According to the change of the state of the motor M, it is apparent that one cycle consists of stopping the initial step and the rotating step, as shown in FIG. 
a) Action horizontal connection representation b) Action vertical connection representation
3) directed to the line connecting between a connection with the conversion step by step <br> there, and with a separate conversion step, the active state is the progression step to the connection according to a predetermined route. When there is no arrow on the directed connection, the progress direction is from top to bottom or from left to right, otherwise proceed in the direction indicated by the arrow on the connection line.
The progress of the activity status of the step is completed by the conversion. The conversion is represented by a dash that is perpendicular to the directed connection. It is not allowed to connect directly between steps and steps. It must be separated by conversion, and conversion and conversion can not be connected directly. It must be separated by steps.
4) transition condition <br> refers to the transition condition logic variable conversions, you can be the literal language, Boolean expressions or graphic symbols, and labeling next dashes represents conversion.
Conversion conditions X and 
Respectively indicate that when the binary logic signal X is in the "1" and "0" states, the condition is satisfied; the transition conditions X and X Ì„ represent respectively when X is from "1" (on) to "0" (off) and from " When 0" to "1", the condition is established.
In the Sequential Function Chart, the progress of the step's activity status is achieved by conversion. The realization of the conversion condition must satisfy two conditions at the same time: First, all the previous steps of the conversion are active steps, and the second is that the corresponding conversion conditions are established. After the conversion is completed, all subsequent steps of the conversion become active steps, and all previous steps become inactive steps.
1) behind each step sequence <br> single sequence by one single converter only after each conversion is only one step, each step along all in series connection to the single row, see Fig. 3a)
2) Selection sequence 
a) Single sequence b) Selection sequence c) Parallel sequences
The beginning of the selection sequence is called the branch, and the dash that represents the branch transformation can only be drawn on the sub-sequence directed line below the horizontal line of the branch. The end of the selection sequence is called merging, and the transition condition dashes before the merging of the branches can only be drawn on each sequence of directed edges above the merging horizontal single line.
3) If after parallel sequences <br> has a plurality of active step sequences, there is after the step transition condition after the establishment of the connection on the activity, while sequentially on all the branch operation sequence and the sequence of the entire branch After the sequential actions are completed, they merge into the same state at the same time. The structure of the sequential control process is a parallel sequence, see Figure 3c).
As soon as the transition condition above the horizontal double line of each branch sequence of the parallel sequence is established, each active step on each branch sequence is activated at the same time, and the progress in each sequence proceeds independently of each other. After all the branch sequences have been completed, they are summarized on the double line of the merge level. Only the transition condition under the horizontal double line is established and the next step can be activated.
1) Built-in PLC
The built-in type PLC is integrated in the CNC device. It can share one CPU with the CNC, or it can be a separate CPU. The built-in type PLC generally does not configure the I/O interface separately, but completes the input and output functions through the I/O circuit of the CNC device itself, and the signal transmission between the built-in type PLC and the CNC system is also performed internally. 
After the CNC device has a built-in PLC, it can use the ladder diagram mode to edit the PLC and transfer complex CNC machine tool control functions. Due to the compact hardware and software structure of the built-in PLC system, the cost-effectiveness of the CNC is improved. At present, most of the world's leading CNC system manufacturers have developed built-in PLC functions.
2) Independent PLC
The independent PLC is completely independent of the CNC device, has complete system hardware and software, can independently complete the control tasks assigned by the CNC system, and cooperates with the CNC system of the CNC machine tool to complete the tool path control and the sequential control of the machine tool. The signal connection between the independent PLC and CNC machine tools and CNC system is shown in Figure 5. 
1) CNC and PLC transmission signals The CNC output data is processed by PLC logic and then transmitted to the MT side. The signals sent from the CNC to the PLC are mainly M, S, T and other function codes, manual and automatic mode signals, and various enable signals. According to different M codes, the PLC controls the positive and negative rotations and stops of the spindles, the shifting of the gears of the spindle gearboxes, the accurate stopping of the spindles, the opening and closing of the cutting fluid, the clamping and release of the chucks, and the picking of the robotic arm. Such as; CNC machine tools must be managed by the PLC library, automatic tool change to complete the T function; through the use of 4 BCD code in the PLC to directly specify the spindle speed to complete the S function processing.
The signals sent by the PLC to the CNC mainly include the response signals of the M, S and T functions and the machine tool reference point signals corresponding to each coordinate axis.
2) the PLC signal <br> MT transmission signal transmitted by the PLC to the machine tool is mainly performed for each element (such as solenoid valves, contactors, relays, status lights, warning lights) control signal.
The signals sent by the CNC machine to the PLC are mainly input signals of the CNC machine tool operation panel, status signals of various switches or buttons, such as the machine start and stop signals, coolant switching signals, override selection signals, coordinate axis jog signals, and knives. Chuck clamping or release signal, the signal of each moving part limit switch, spindle status monitoring signal and servo system operation ready signal.
Information: Sorting and summarizing lecture notes, describing the contents of the CNC machine tool PLC signal content: Determine the control circuit of the motor star delta switching connection decompression starting plan: Determine the sequence function diagram drawing steps Implementation: After the completion of the motor star triangle transformation connection decompression starting process sequence Functional chart review: Team mutual review assessment: panel assessment
PLC sequential function drawing and PLC signal description in CNC machine tools
A, PLC sequential control function drawing