. Wiring according to the figure is not an exaggeration to say that this is the highest standard. First of all, you must carefully read the drawings before wiring and fully understand the designer's intentions, rather than connecting based on your personal rich experience. If you find unknown or contradictions, you should contact the designer to confirm immediately until the wiring is correct.
. The wiring sequence should be clear and clear, the process should be simple and checkable. In reality, there are very few things that can be done, basically the thread end is connected and the box is covered.
. Learn more about wiring skills and be good at using professional tools flexibly. For example:
Q: When we are working on PLC cabinets, there are many terminals for . If we are not dealing with it well, there will be looseness, burrs, etc., should we directly peel off the wire and press it in, use pins, or stick tin.
A: Single-core wire is peeled and pressed directly. Multi-core wire uses cold press terminals. It is not recommended to tin engrave;
Q: PLC When there are many expansion modules, how to deal with the wiring between the common end and the power supply terminal? Is it directly connected to the next module through the terminals on each PLC module, or is it connected to the terminal, and short-connected on the terminal strip ?
A: We hope that the power supply power supply will be assigned and shorted on the terminal and introduced the user points (using a wire number tube or marking the destination on the terminal). This is intuitive and clear, with little influence between each other. We do not want to go from one point to another, and we do not want to connect more than two wires under one terminal. For power terminal strips, you prefer to use terminals with safety or the type that can be disconnected from the upper and lower ends, which is very convenient when finding short circuit faults.
1
PLC internal and external circuit
. External circuit wiring
Figure 1 is the contactor electrical control circuit controlled by motor full voltage start control. The control logic is achieved by AC contactor KM coil, indicator lights HL1, HL2, thermal relay normally closed contact FR, stop button SB2, start button SB1 and contactor normally open auxiliary contact KM through wire connection.
presses the start button SB1, and then the coil KM is powered on and locked itself, and the auxiliary contact KM of the branch where the indicator light HL1 is located and the main contact in the main circuit, HL1 is on and the motor M is started; press the stop button SB2, then the coil KM is powered off, the indicator light HL1 is off, and M stops.
▲Figure 1 Electrical control line for motor full voltage start
Figure 2 is an external wiring diagram for the full voltage start control of motor using SIEMENS. The main circuit remains unchanged, and the thermal relay normally closed contact FR, stop button SB2, start button SB1, etc. are connected to the input device of the PLC on the input interface of the PLC, and the AC contactor KM coil, indicator lights HL1, HL2, etc. are connected to the output device of the PLC on the output interface of the PLC. The logic of the process is realized by executing user programs written and stored in program memory according to the motor full pressure control requirements.
▲Figure 2 Motor full voltage start PLC control wiring diagram
. Establish an internal I/O image area
has opened an I/O image storage area in the PLC memory, which is used to store the I/O signals, which are called the input image register and the output image register respectively. In addition, other PLC programming components also have corresponding image memory, called component image registers. The size of the I/O image area is determined by the PLC system program. For each input point of the system, there is always a bit of the input image area corresponding to it, and for each output point of the system, there is also a bit of the output image area corresponding to it, and the address number of the input and output points of the system also corresponds to the image register address number of the I/O image area.When the PLC is working, the collected input signal status is stored in the bits corresponding to the input image area, and the calculation results are stored in the bits corresponding to the output image area. The data that requires the PLC to describe the equivalent contacts of the input relay or the equivalent contacts and equivalent coil status of the output relay when executing the user program is taken for the I/O image area without directly having a relationship with the external device. The establishment of the I/O image area of
I/O is so that when the PLC is working, it only relates to the status data stored in the address unit related to the memory, and the system output only sets a status data for a certain address unit in the memory. This not only speeds up the execution of the program, but also separates the control system from the outside world, improving the system's anti-interference ability.
. Internal equivalent circuit
Figure 3 is the internal equivalent circuit of the PLC. Taking the start button SB1 as an example, its access interface I0.0 is connected to a trigger I0.0 in the input image area. When SB1 is turned on, the trigger is I0.0 is triggered to the "1" state, and this "1" state can be directly referenced by the user program as the state of I0.0 contact. At this time, the on-off state of I0.0 contact and SB1 are the same, then SB1 is turned on, and the I0.0 contact state is "1", and SB1 is disconnected, and the I0.0 contact state is "0";
Since the I0.0 trigger function is the same as relay coil and does not require hard connection wires, the I0.0 trigger is equivalent to an I0.0 soft relay coil inside the PLC. The I0.0 contact directly referenced to the I0.0 coil state is equivalent to a normally open contact (or called an actuating contact) controlled by the I0.0 coil.
▲Figure 3 The internal equivalent circuit of PLC
Similarly, the stop button SB2 is connected to a soft relay coil I0.1 inside the PLC, SB2 is closed, the state of the I0.1 coil is "1", otherwise it is "0". The state of the relay coil I0.1 is reversed by the user program and referenced as the state of the I0.1 contact, so I0.1 is equivalent to a normally closed contact (or dynamically broken contact) controlled by the I0.1 coil. The output contacts Q0.0 and Q0.1 are physical normally open contacts of the internal relay of the PLC. Once closed, the corresponding external KM coil and indicator light HL1 will be turned on. The PLC output terminal has a public interface COM for output power supply.
2
PLC control system
PLC is used to realize the motor full-voltage starting electrical control system, and its main circuit remains basically unchanged, while PLC is used to replace the electrical control circuit.
.PLC control system composition
Figure 4 is a basic structure diagram of the PLC control system for full-voltage starting of the motor, which can be divided into three parts: input circuit, internal control circuit and output circuit.
▲Figure 4 Basic composition block diagram of the PLC control system
Input circuit
Input circuit The function of the input circuit is to send the input control signal to the PLC, and the input devices are buttons SB1, SB2 and FR normally closed contacts. The externally input control signal is inputted to a corresponding input relay via the PLC. The input relay can provide any number of normally open contacts and normally closed contacts for programming the PLC content control circuit.
output circuit
output circuit function is to convert the output control signal of the PLC into a signal that can drive the KM coil and the HL1 indicator light. There are many output relays in the PLC internal control circuit. In addition to the PLC internal control circuit, each output relay provides a normally open contact and a normally closed contact for programming, and also provides an normally open contact for the output circuit to be connected to the output port. The contact is called an internal hard contact and is an internal physical normally open contact. The contact drives the external KM coil and the HL1 indicator light and other loads, and the KM coil then controls the start and stop of the motor M through the KM main contact in the main circuit. The power supply for the driving load is provided by the external power supply, and the output port of the PLC also has a COM common terminal for the output power supply.
internal control circuit
internal control circuit is formed by a user program written in accordance with the actual control requirements of the controlled motor. Its function is to calculate, process and judge the status of the input and output signals according to the logical relationship specified by the user program, and then obtain the corresponding output control signal, and drive the output device: motor M, indicator light HL1, etc. through the control signal.
User program writes all program statements into the PLC user program memory through personal computer communication or programmer input. To modify the user program, you only need to change certain statements in the memory through programmer and other devices, and will not change the internal wiring of controller , achieving control flexibility.
.PLC control ladder diagram
ladder diagram is an equivalent control line that equals the inside of the PLC into an equivalent control line composed of coils, normally open contacts, normally closed contacts or functional program blocks of many internal relays. Figure 5 is the commonly used equivalent control component symbols for PLC ladder diagrams.
▲Figure 5 Commonly used equivalent control element symbols for ladder diagrams
a) coil b) normally open contact c) normally closed contact
Figure 6 is a PLC control ladder diagram of full-pressure starting of motor, composed of equivalent control element symbols corresponding to FR normally closed contacts, SB2 normally closed buttons, KM normally open auxiliary contacts and parallel units of SB1 normally open buttons, KM coils and other parts. The motor full-pressure start control trapezoid is similar in form to the contactor electrical control circuit diagram, but there are many differences from the electrical control circuit diagram.
▲Figure 6 Motor full pressure start control ladder diagram
ladder diagram The physical structure of the relay component is different from the electrical component
PLC ladder diagram. The coils and contacts in the PLC ladder diagram are only functionally equivalent to the coils and contacts of the electrical component. The coils and contacts in the ladder diagram are just one memory bits in the input and output memory in the physical sense, which are different from the physical structure of electrical components.
On-off state of the relay element in the ladder diagram is different from that of the electrical component
On-off state of the relay element in the ladder diagram is related to the saved data on the corresponding storage bit. If the data of the storage bit is "1", the component is in the "on" state, and if the data of the bit is "0", it means that it is in the "off" state. It is different from the actual on-off state of electrical components.
The state switching process of relay components in ladder diagram is different from that of electrical components
The state switching of relay components in ladder diagram is just the operation of PLC on the state data of the storage bits. If the PLC assigns the equivalent storage bit data of the normally open contact to "1", the dynamic and closing operation process will be completed. Similarly, if the equivalent storage bit data of the normally closed contact is assigned to "0", the dynamic and interrupt operation process can be completed, and there is no time delay in the switching operation process. When the coils and contacts of electrical components are activated or switched, there must be a time delay, and generally it must go through the operation process of first opening and then closing.
The number of contacts to which the relay belongs is different from the electrical components
If the PLC extracts the bit data "0" from the corresponding storage bit of the input relay I0.0 and stores it in a storage bit in another memory, the stored storage bit becomes a normally open contact controlled by the I0.0 relay, and the stored data is "0"; if the inversion operation is performed after the bit data "0" is retrieved first, and then stored in a storage bit in a memory, the data stored in this bit is "1", and the storage bit becomes a normally closed contact controlled by the relay I0.0.
As long as there is enough internal memory of the PLC, this bit data transfer operation can be performed infinitely. Each operation is performed, a relay contact in the ladder diagram can be generated. It can be seen that in principle, the relay contact in the ladder diagram can be used repeatedly indefinitely.
However, the coil inside the PLC can usually only be referenced once. If you need to reuse the coil with the same address number, you should be very cautious.Unlike PLC, the number of contacts in electrical components is limited.
ladder diagram draw rules for each line are to start from the left bus, pass through the contacts and coils (or functional boxes), and terminate at the right bus. Generally, the parallel unit is drawn on the left side of each line, the output coil is drawn on the right side, and the remaining series elements are drawn in the middle.
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