Design Method of PLC Sequence Control System

 

Introduction

In the field of automatic control of production machinery, PLC sequence control system has a wide range of applications. However, different manufacturing machines require different design of ladder diagrams of control systems. At present, many electrical designers still use the experience design method to design the PLC sequence control system, which is not only inefficient in design, but also easy to make mistakes during the design stage, debugging and modification are required to meet the design requirements. The four simple design methods proposed in this paper can be used to quickly design a PLC sequence control system.

Features and design ideas of the Sequence Control System

1. The feature Sequence Control System is an automatic control system based on the predefined action sequence of the controlled execution organization and the corresponding forwarding conditions. The controlled device is usually a production machine with the same operation sequence or relatively fixed. Most of the main signal of this type of control system is the travel switch (including the touch or non-contact travel switch, photoelectric switch, reed switch, Hall element switch and other position detection switch ), sometimes, a signal conversion component such as a pressure relay and a time relay is used as the main signal for certain steps.

In order to make the sequence control system work reliably, a step-by-step sequence control circuit structure is usually used. Step-by-step sequence control means that the electric power of any program step (hereinafter referred to as StEP) of the control system must be obtained from the previous step, and the main signal of the current step has been sent as a condition. For production machinery, whether or not the mechanical action of the controlled device is performed depends on whether the output signal and the controlled mechanical action have been completed in the previous step of the control system. If the action in the previous step is incomplete, the action in the next step cannot be executed. The mutual lock of the control system is strict. Even if the signal component fails or misoperations occur due to the redirection, the operation sequence will not be disordered.

2. the four simple design methods proposed in this paper are to first design the step ladder, and then implement the signal control via the switch main command in the step ladder to help the relays gain and lose electricity; then design the output step according to the step ladder, in the output step, the auxiliary relay controls the power loss of the output relay. The ladder diagram circuit structure and corresponding commands designed by these four design methods should be applicable to most PLC models and be universal.

Due to the different programming element codes and serial numbers of various PLC models, this article stipulates: Input relays, output relays, and auxiliary relays (also called internal relays) in all trapezoid diagrams for convenience of elaboration) the codes are X, Y, and M. Some functional commands used in the design, for example, the set instruction convention is "s *", the reset instruction is "R *", and the shift instruction is "Sr. "X" indicates the number of the programming component, expressed in decimal number. When designing the actual control system using these methods, the code and number of the programming element should be transformed into the code and number corresponding to the selected PLC model.

Figure 1 Sequential Control Process

The following describes the various design methods. The first three methods are designed based on the sequential control flow shown in Figure 1. As shown in the figure, x0 is the input relay connecting the start button (for the sake of conciseness, the subsequent forwarding signal saves the words "input relay, (input signal only). X1 is the in-situ switch signal, and X2, X3, and X4 are the switch main switch signals in steps 2, 3, and 4 respectively. M1 ~ M5 is the controlled auxiliary relay of each step. Y1 ~ Y4 is the output relay controlled by each step.

I. Design Method of step-by-step Sequence Control System for Electric synchronization power loss

As shown in 2, this design method is based on the basic logical relationship between "and", "or" and "Non" to design a circuit structure of series, parallel or series, and parallel compound.

Figure 2 step-by-step electrical synchronization out of power ladder diagram

1. Step ladder design step ladder Structure

2a. The electrical condition of M1 in step 1 is that the controlled mechanical in-situ switch X1 is in the pressed state (if the controlled mechanical has multiple implementing mechanisms, the in-situ switch of each implementing mechanism is required to be in the pressed State ), after the in-situ conditions are met, press the start button x0 to obtain power. After the M1 is powered, it is self-locked and provides stepping condition signals for Step 2 (the Common open contact of M1 ). The travel switch signal X2 triggered when the execution of step 1 is completed is used as the turning condition signal of step 2. After the input of M2 in step 2 meets the stepping and switching conditions, the electric self-locking is obtained, and the stepping condition signal is provided for Step 3. According to this rule, the power acquisition and self-locking of the auxiliary relay in each subsequent step can be realized. The stepping condition signal and the switching condition signal of the Stop step M5 are: the stepping condition signal sent by the M4 in the last working step (the Common open contact of the M4) and the x1. Because M5's telecommunications number control system is out of power, the M5 loop is not self-locked, and its normally closed contacts must be connected to the leftmost end of step 1 loop. The loop in the subsequent steps from step 2 constitutes a branch loop. Once M5 is powered, the entire system loses power. You can also use the loop shown in Figure 3 without the branch loop structure. That is, the M5 normally closed contacts are connected in series on the back of each step of the auxiliary relay. It should be noted that, no matter the work step or stop step, if there are multiple forwarding main signal of a step, multiple forwarding main signal should be connected to each other.

Figure 3 step-by-step electric synchronization power loss trapezoid diagram

2. Design and output step of the output step

2b. The design method is as follows: (1) in the control flowchart, find out where the output relay M starts to generate electricity and where it starts to lose power, in this way, determine the electrical signal (the auxiliary relay that enables m to start to be electric) and the electrical loss signal (the auxiliary relay that causes m to start to lose power in the step ); (2) connect electrical signals, power loss signals, and controlled output relay coils. If an output relay loses power multiple times in a working cycle, the series signals of each gain and loss are connected in parallel. For example, in figure 1, the output relay Y1 is required to generate power in steps 1 and 3, and power loss in other steps. In Figure 2B, the first electrical signal M1 and the first power loss signal m2 are connected in series, and the second power signal M4 and the second power loss signal are connected in series, then the two are connected in parallel, and then connected with the Y1 coil to form the Y1 control loop. The rest.

Ii. Design Method of step-by-step sequence control system for gradual power loss

1. Step-by-Step Design

According to the control process shown in figure 1, the step-by-step 4A of the design method of the step-by-step electric generation and step-by-step power loss sequence control system is adopted, one of the differences between the circuit structure and Figure 3 is that the power loss of each step is controlled by the normally closed contacts of the next auxiliary relay; the second is the auxiliary relay normally closed contacts that must be in series from step 2 to step 4. In case that the control sequence is disordered due to misoperation and start again during circuit operation. The remaining circuit ends are the same as Figure 3.

2. The output step 4B is shown in the output step design. The control loop of the output relay is intuitively determined according to the control flow. For example, if the output relay Y1 is required to generate power in steps 1 and 3, the common open contacts of M1 and m3 of step 1 and 3 are connected in parallel and connected to the coil of Y1. The control circuit construction method of other output relays is the same.

Figure 4 step-by-step power-out drive control system trapezoid diagram

Iii. Design Method of placement/reset command-type Sequential Control System

1. Step 5 is the step 5 of the sequential control system designed with placement/reset commands. The design basis is also the control process shown in Figure 1. This step structure features that each step of the secondary relay has a placement coil and a reset coil, the numbers are the same. Step 1 uses the position instruction s to enable the M1 position of the auxiliary Relay (that is, the internal self-locking after the M1 coil is powered), establish the Step 1 program, and provide step 2 with stepping condition signals. When Step 2's main route signal is sent (X2 closed), the command s sets m2 to set up step 2, and the reset command R resets M1 to cancel step 1. Similarly, you can draw a step-by-step relay placement/Reset trapezoid diagram. When the last step is completed and returned to the original position (x1 closed), the command R resets M4 and the system's work cycle ends.

2. The output step Design Drawing 5b is the output step structure, which is exactly the same as Figure 4b and will not be repeated.

Figure 5 placement/reset command type Sequential Control Circuit

Iv. Design of shift instruction sequence control system

1. Step-by-Step design is shown in Step 6. Figure 7A shows the step ladder of the sequential control system designed using the shift Instruction Design Method as shown in Figure 6. This step ladder consists of an 8-shift register (the auxiliary relay M20 ~ is defined by the shift instruction ~ As a control component. In the shift register is the input of the shift data, CP is the input of the shift pulse, and r is the reset end. The input signals of these three inputs are valid for the rising pulse. For the sequence control system, the input in signal must be a single pulse signal, that is, the shift data is "1 ". When Step 1 is started, after both in and CP input the pulse rising edge of the button signal x0, the shift data "1" generated on the in side is moved to the M20 bit of the shift register, at this time, this bit has an output (that is, the constant open contact closed signal of the M20 output), establishes Step 1 program, and provides step 2 with stepping condition signal; the normally closed contact of M20 instantly disconnects the in input and the CP Step 1 input, and completes the data "1" input and shift pulse input. Starting from step 2, when the main signal of this step is sent (X2 connected), a shift pulse rising edge is input, so that the data originally migrated into the M20 bit "1" is moved into the m21 bit, create Step 2 programs and provide step 3 with step-by-step conditional signals. After the shift, the status of the M20 bit changes to 0, that is, the corresponding Step 1 is revoked, and the output is 0. With this type of push, the entire step can be gradually powered and gradually lost. When the last step is completed and returned to the original position (X1 is connected), the reset end R of the shift register is switched on to reset the shift register and the entire control system stops power loss.

Figure 6 shift sequence control Flowchart

Figure 7 shift command type Sequence Control Circuit

When designing this step ladder, pay attention to the following issues: (1) In an automatic operation cycle, the shift register's shift data input in only allows one single pulse signal to be input at startup. That is to say, only shift data "1" can be input during start ". The step-by-step operation is based on the input data "1". In the shift register, the step-by-step operation is performed gradually to the high-displacement position for power generation and power loss. Therefore, the input in must be connected to the normally closed contacts of each shift output bit. (2) The shift register is very sensitive to the jitter of the switch at the input of the shift pulse. If the switch jitters once, it is equivalent to inputting a shift pulse, and the shift data "1" moves one more bit. Because the contact switch is triggered, jitter is inevitable. To eliminate this effect, in step 1 input loop of the shift pulse input, the 0-bit (M20 in this example) of the shift register must be in series. Once the shift data is moved to the M20 bit, the input loop of step 1 is disconnected. From step 2, the input loop of each step is also connected to the previous normally open contact. For example, the input circuit in step 2 is connected to a common open contact of the previous M20. In this way, when you shift to the m21 bits corresponding to step 2's main signal, the input loop of step 2 is immediately disconnected. This shift pulse input loop structure ensures that the input signal duration of each step is only one scanning period of the PLC (usually only a few ms ), because the switch jitter time is much longer than a scanning period of PLC. Therefore, the impact of switch jitter can be effectively eliminated.

2. The output step design diagram 7b is the output step. Its structure is the same as Figure 4b, but the auxiliary relay number is different.

Conclusion

The common features of the above four methods are:

(1) The input relay controls the auxiliary Relay (including the auxiliary relay defined by the placement/Reset instruction and shift instruction), and thus forms a step ladder;

(2) the output relay is controlled by an auxiliary relay to form an output step;

(3) Both step-by-step and output step are regular loop structures. No matter how many steps the sequential control system has to be designed or the number of input and output points, you only need to find out the regularity of the step ladder designed by various design methods and the loop structure of the output step ladder, based on the design basis, the circuit structure of any of the design methods can be applied to quickly design a more complex PLC sequence control system.