Steps to run and debug plc

Many electrical novices are not sure how to start debugging after designing electrical control cabinets and PLC programs. Or some people have caused problems such as PLC burnout due to inappropriate debugging methods. So how should the designed electrical system be debugged? You can follow the following seven steps.

1. Check the circuit according to the drawing (without power supply)

Generally, the drawings of PLC systems include drawings inside the cabinet and drawings outside the cabinet. Drawings inside the cabinet refer to the wiring diagram inside the cabinet; drawings outside the cabinet refer to the wiring diagram of all the connections to the electrical cabinet. What needs to be checked in this part are:

1. Whether the drawing design is reasonable, including the capacity of various components, etc.

2. Check whether the components are connected strictly according to the drawings.

In this process, the most important thing to pay attention to is to check the power supply

1. Make sure there is no short circuit in the circuit.

2. Make sure strong and weak electricity are not mixed together

Because the PLC power supply is 24V, once 220V is connected to the PLC due to wiring errors, it is easy to burn the PLC or expansion module.

2. Check the PLC external circuit, which is commonly known as "dotting"

After the power supply is confirmed, power is supplied to test the input and output points, which is commonly known as "dotting". The IO points need to be tested one by one, including the operation button, emergency stop button, operation indicator light, cylinder and its limit switch, etc. The specific method is that one person operates the button on the site, and the other person monitors the input and output signals in the PLC; for large systems, a test table should be established, that is, mark it after the test. If there is a wiring error during the construction process, it needs to be dealt with immediately.

In this step, it should be noted that after backing up the program, the program in the PLC needs to be cleared or disabled to avoid equipment movement due to testing.

3. Check the mechanical structure and test the motor load

This step needs to check whether the mechanical structure is tight, etc., whether the motor load is properly protected to avoid accidents caused by unexpected events. After the inspection, it is necessary to manually test the operation of the equipment. For example, for forward and reverse motors, it is necessary to test whether the circuit is intact and conduct a live test run. For inverters, set corresponding parameters and perform motor optimization, static identification or dynamic identification, etc.

It should be noted here that for some special loads, such as vertical loads that move up and down, they need to be carried out by professionals to avoid testing accidents due to improper control.

4. Debugging manual mode/semi-automatic mode and related logical relationships

After testing the IO points and the load side, the next step is debugging in manual mode. The manual mode here can also be called semi-automatic mode. It does not mean directly pressing the solenoid valve or contactor by hand, but driving the device through buttons or HMI buttons, which corresponds to the automatic state.

Manual mode testing can decompose the automatic mode according to human will, which is convenient for testing procedures.

The most important thing in this link is to test the safety functions, that is, to test whether the safety functions such as emergency stop and safety light curtain play the corresponding role when the equipment is running.

5. Debug the automatic mode according to the production process

After completing the semi-automatic debugging, you can further debug the automatic work. This link is the most important. It is necessary to test various interlocks according to the production process, including logical interlocks, safety interlocks, etc., and test several working cycles to ensure that the system can work continuously and correctly.

6. Testing of special processes

In addition to logic control, the PLC system also has many expanded functions, such as PID control. When these logic debugging is basically completed, you can start debugging analog and pulse control.

The most important thing is to select the appropriate control parameters. Generally speaking, this process is relatively long. You need to be patient and make multiple choices of parameters, and then choose the best one. For some PLCs, its PID parameters can be obtained through self-tuning. But this self-tuning process also takes a considerable amount of time to complete.

7. Complete all the above steps

The entire debugging is basically completed, and the next step is pre-production. Pre-production is a work test before production. During this stage, some special tests can be carried out in conjunction with production, such as whether the production rhythm is met, whether the safety function can still work under load, etc. Generally, the project can be handed over after a certain period of continuous production.

Newbies especially need to pay attention to the power supply. I remember when I was debugging my first project many years ago, the construction unit connected the 220V contacts and 24V contacts of the large belt pull-cord switch incorrectly (the belt pull-cord switch is a safety device with two sets of contacts, one set for 220V to disconnect the control loop, and the other set for 24V to enter the PLC). This caused a digital input template to burn out. Later, I learned my lesson and made sure to distinguish between 220V and 24V when debugging, and there has been no problem since then.