The basic principle of PLC selection is: the selected PLC should be able to meet the functional needs of the control system.

1. Selection of PLC structure

In the case of the same function and the same I / O points, the price of integrated PLC is lower than that of modular PLC.

2. Selection of PLC output mode

Different loads have corresponding requirements for the output mode of PLC. The relay output PLC can carry DC load and AC load; Transistor type and bidirectional thyristor type output modules are used for DC load and AC load respectively.

3. Selection of I / O response time

The response time of PLC includes input filter time, output circuit delay and time delay caused by scan cycle.

4. The choice of network communication

If the PLC control system needs to be connected to the factory automation network, the selected PLC needs to have the communication networking function, that is, the PLC should have the ability to connect other PLC, upper computer and CRT interfaces.

5. Selection of PLC power supply

Power supply is one of the main ways to introduce PLC interference, so high quality power supply should be selected to help improve the reliability of PLC control system. Generally, voltage regulator with small distortion or power supply with isolation transformer can be selected, and bridge full wave rectifier power supply should be selected when DC power supply is used.

6. Selection of I / O points and I / O interface equipment

(1) The input circuit of input module should match the type of output circuit of external sensor or electronic equipment (such as frequency converter), and it is better to connect them directly.

(2) The selection of analog module should consider the use of transmitter, and whether the range of actuator can match the range of analog input / output module of PLC.

(3) When using rotary encoder, we should consider whether the function and working frequency of PLC high-speed counter can meet the requirements.

7. Selection of storage capacity

The capacity of PLC program memory is usually in words or steps. The capacity of user program memory can be roughly estimated. In general, the memory capacity required by the user program can be calculated according to the following empirical formula:

Program capacity = k × Total input points / total output points

For a simple control system, k = 6; If it is a normal system, k = 8; For complex system, k = 10; If it is a complex system, then k = 12. When selecting the memory capacity, there should also be a margin, generally 25% of the running program. We should not simply pursue large capacity. In most cases, the memory capacity of PLC that can meet the I / O points can also be satisfied.

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