Today, I'll give you a basic introduction to the jog control principle of PLCs . This article is mainly for PLC beginners.
Please see the image below:
First, what is inching control? Please see the diagram:
This is a traditional control circuit, which is divided into two parts: the main circuit and the control circuit.
1. The main circuit consists of: QS (circuit breaker), FU (fuse), KM (contactor main contacts) and M (motor).
2. Control circuit (also known as auxiliary circuit): It consists of FU (fuse), SB (normally open contact), and KM (contactor coil).
Everyone, please look at the following image:
When the circuit is working, pressing button SB1 energizes the contactor coil, causing the armature to engage and close the three pairs of main contacts. The motor then connects to the three-phase power supply and starts rotating forward. When we release the button, the contactor coil is de-energized, and the motor stops rotating. This control method is called inching control. It is mainly used for lifting, stationary movement control of equipment, and debugging of production equipment. In practical applications, contactor control circuits are relatively large, and prolonged mechanical movement can reduce the reliability of components such as buttons and contactors. The number of contacts used is also limited. If we want to change the control function, the circuit needs to be rebuilt, which is labor-intensive and prone to errors. To address these shortcomings, we use a PLC. So, what is a PLC? Please look at the following diagram:
Here, we'll use the Mitsubishi FX2NPLC as an example: The area on the left with many screws represents the PLC's signal input points (X), mainly used for input signals from buttons, switches, and sensors. The area on the right represents the PLC's output points (Y), used to send signals to external contactors, solenoid valves, indicator lights, alarm devices, etc. The CPU and memory are located in the middle, primarily controlling the entire system, coordinating the work of various internal parts, and storing programs and data. If the control function needs to be changed, only the internal program needs modification; the external circuitry doesn't require readjustment, facilitating debugging, minimizing hardware errors, and allowing for unrestricted use of internal relays within the PLC program. So, how do we use a PLC to achieve inching control of a motor? Please see the following diagram:
Normally open button SB1 is connected to input point X1 of the PLC. When we press button SB1, the input circuit is connected, and X1 receives an IO signal. The IO signal of X1 is sent to the PLC for processing, and then an output signal Y1 is output. The output signal Y1 connects the output circuit, energizes the KM1 coil, and the corresponding KM main contact in the main circuit is energized. Let's look at the following diagram:
Let's compare this with the diagram above. What does the PLC use for internal control calculations? Let's continue:
Let's look at the diagram. The vertical line on the left is the left busbar, and the one on the right is the right busbar. We can choose to draw the right busbar or not. Let's imagine that the left busbar is connected to the positive terminal of the power supply, and the right busbar is connected to the negative terminal. The input relay X1 is set to a normally open contact and connected in series with the output coil Y1. When X1 is in the NO state, it's as if the circuit between the two busbars is connected. We can consider that an imaginary current flows through this circuit. See the diagram below:
The coil is now energized and conducting. As you can see, the red line in the trapezoidal diagram in the upper right corner corresponds to the Y1 contact (coil), which activates. This causes the contactor KM1 in the main circuit to engage, and the motor starts rotating forward. See? The KM1 main contact on the left is engaged, becoming a straight line, and it turns red, indicating that it's energized. When we release the button, look at the diagram below:
There are quite a few pictures, but the main purpose is to help everyone understand.
As shown in the diagram above, when we release the button, that is, when X1 is OFF, the circuit between the busbars is open, the Y1 coil is de-energized, the main contacts reset and open, and the motor stops working. This completes the design of the jog control. We call this design a "ladder diagram," which is the internal calculation and control of the PLC. Finally, we use GX simulation software to write the designed ladder diagram into the PLC. (See diagram below.)
Connect the external X and Y power supply circuits, press the start button to start the motor, and release the button to stop the motor.
It achieves electric control function.
