Research and practice of building redundant control system using small PLC

introduction

Currently, most PLC manufacturers only launch redundant PLCs in large PLC systems, but not in small PLC systems, and large redundant PLC systems are expensive.

As the requirements for control system reliability in various industries increase, how to use small PLC to achieve low-cost redundant control system solutions has become a topic. This article discusses a solution to achieve redundant configuration using small PLC, and introduces its principle and application.

1 Introduction to the solution of using small PLC to realize redundant configuration

The scheme of realizing redundant control by using small PLC is shown in Figure 1. In the redundant control system built by using small PLC, two small PLCs of exactly the same model are configured, which are called PLCA and PLCB respectively. In addition, a watchdog signal detection device is configured to determine whether PLCA and PLCB are in normal working state and realize the switching of master and slave PLC. PLCA and PLCB receive the same input signal, and the output signal is determined by the watchdog signal detection device to determine which PLC output is used as the output of the redundant control system. The watchdog signal detection device can be a small PLC with a small number of points, or a single-chip microcomputer.

2 Judgment and switching principle of master-slave PLC

In a redundant PLC system, the most critical issue is how to judge the master and slave PLCs and implement the switching between the master and slave PLCs.

2.1 How to determine the status of PLCA and PLCB

PLCA and PLCB each generate a watchdog pulse signal, and send the watchdog pulse signal to the watchdog signal detection device through the Do point. The watchdog signal detection device detects the watchdog pulse signal sent by PLCA and PLCB, and if the signal is abnormal, it determines that the PLC has a fault.

2.2 Master-slave status at power-on and master-slave PLC switching during normal operation

If PLCA and PLCB are both normal, and the startup sequence at power-on is watchdog signal detection device + PLCA + PLCB, the watchdog signal detection device will give priority to detecting the watchdog signal of PLCA, and take PLCA as the host, and then detect the status of PLCB, and take PLCB as the backup machine: If the power-on startup sequence is watchdog signal detection device + PLCB + PLC A, the watchdog signal detection device will give priority to detecting the watchdog signal of PLCB, and take PLCB as the host, and then detect the status of PLCA, and take PLCA as the backup machine: If the power-on startup sequence is PLCA (or PLCB) + PLCB (or PLCA) + watchdog signal detection device, the watchdog signal detection device detects the watchdog signals of PLCA and PLCB at the same time. In this case, PLCA is given priority as the host and PLCB is used as the backup machine.

During normal operation, if the PLCA (or PLCB) serving as the host fails, it stops sending watchdog signals to the watchdog signal detection device. The watchdog signal detection device detects that the watchdog signal of the PLCA (or PLCB) is abnormal, determines that it has failed, and sends a master-slave switching signal.

3 Lo points for redundancy

3.1 PLCA and PLCB are used to realize redundant lo points

Dol:watchdog pulse signal (towatchdog signal detection device):

D11: A is the host, B is the standby (from watchdog signal detection device):

D12: B is the host machine, A is the standby machine (from watchdog signal detection device).

3.2 Lo point of Watchdog signal detection device

D1l: watchdog pulse signal of PLCA (fromPLCA): D12: watchdog pulse signal of PLCB (fromPLCB): Dol: A is the host, B is the backup machine (toPLCA & PLCB): Do2: B is the host, A is the backup machine (toPLCA & PLCB): Do3: Use the output pulse signal of PLCA (to output switching device): Do4: Use the output pulse signal of PLCB (to output switching device).

4 Input and output signal processing

4.1 Processing of switch input signals

The switch quantity input signal can be divided into two signals by using a relay and connected to PLCA and PLCB at the same time.

4.2 Processing of analog input signals

The analog input signal can be divided into two signals by using a one-input and two-output analog signal isolator and connected to PLCA and PLCB at the same time.

4.3 Processing of switch output signals

The switch output signal uses an R/s holding relay as the switching device of the output signal. Do3 of the watchdog signal detection device (using the output of PLCA) is connected to the R end of the R/s holding relay, and Do4 of the watchdog signal detection device (using the output of PLCB) is connected to the s end of the R/s holding relay.

4.4 Analog output signal processing

The analog output signal uses a homemade electronic circuit board as the output signal switching device. The circuit board selects the PLC output of the host according to the Do3 and Do4 signals of the watchdog signal detection device.

5. Program Implementation

This article takes oMRoNPLC as an example to introduce the implementation method of the program.

(1) The program used to generate the watchdog pulse signal in PLCA (PLCB) is shown in Figure 2.

(2) The status judgment procedure of the watchdog signal detection device for PLCA is shown in Figure 3. The status judgment procedure of the watchdog signal detection device for PLCB is consistent with that of PLCA.

(3) The watchdog signal detection device selects the program of the master and slave PLCs, as shown in Figure 4.

6 Conclusion

The redundant control system built with small PLC has been applied in many actual engineering projects, and the feedback effect is good.

As long as you are diligent in thinking, researching and practicing in your daily work, you will be able to find a low-cost, highly reliable redundant solution suitable for small-point (less than 10 points) occasions to achieve unmanned operation. This will not only save users money and investment during the construction period, but also reduce subsequent maintenance costs and save a lot of labor costs.