How to use PLC to control the heating of electric kettle

1. Description of case task control requirements

Electric kettles are everywhere in our daily life, but they can only be boiled once. If they are not used for a long time, they will become cold. In order to solve this problem, PLC is used for heating control in this case.

Control requirements: Press the start button for the first time to heat the water. When the water temperature reaches 100℃, stop heating. In the following process, if the water temperature in the cup is lower than 30℃, start the heating rod to heat the water. Press the stop button to stop heating regardless of the state.

Required hardware: thermal resistor (Pt100), temperature transmitter, PLC (FX5U-32MT/ES), solid-state relay, heating rod, as shown in Figure 1-1.

Figure 1-1 Kettle heating system

2. Related basic knowledge

The case in the task is based on the application case of Mitsubishi FX5U series PLC. Before studying this case, please evaluate whether you have the following basics. If you are not familiar with the relevant basic knowledge, please follow the QR code provided below to pre-study the course.

01 Use of Mitsubishi FX5U analog module

Through this link, you can learn about the setting of FX5U analog input parameters. I hope that students can learn this live class after watching the linked course. Here are some important or difficult to understand parameters for everyone to explain.

Basic settings (see Figure 2-1)

Figure 2-1 Basic settings for analog input

(1) A/D conversion enable/disable setting:

A/D conversion can only be performed on analog input after A/D conversion enable is set.

(2) A/D conversion method:

Sampling processing: Each time a scan is performed, data is collected once;

Time average: digital output value = total value of time collection ÷ number of time collections;

Average of times: digital output value = total value of times collected ÷ times collected;

Moving average: Digital output value = total value of the number of times set in each scanning cycle ÷ number of times. When the number of times is set to 4, as shown in Figure 2-2.

Figure 2-2 Moving Average

02 Application Settings (see Figure 2-3)

Figure 2-3 Application Settings

(1) Alarm output function: When the digital operation value is not within the process alarm value range, the alarm output flag is ON;

(2) Scale out detection: Detection occurs when the input analog voltage value exceeds 10.2V.

(3) Scaling setting: Convert the digital value to be output as shown in Figure 2-4.

Figure 2-4 Scaling settings

(4) Shift function: shift the digital value to be output as a whole.

(5) Digital clipping settings: digital output range when disabled (0~4095), digital output range when enabled (0~4000).

III. Implementation of the Task

This case will explain the five aspects of electrical component selection - IO address allocation - hardware wiring - software parameter setting - program design.

01 Electrical component selection

According to the description of task control, in this example, a PLC, two buttons, a thermal resistor (Pt100), a temperature transmitter, a solid-state relay, and a heating rod are required, as shown in Table 3-1 below.

Table 3-1 BOM

02 I/O address allocation

According to the description of task control, in this example, 2 input points and 1 output point are required. The I/O allocation table is shown in Table 3-2 below.

Table 3-2 I/O address allocation table

03 Hardware Wiring Diagram

FX5U PLC is compatible with source and sink inputs. When S/S is short-circuited with 24V, the input is sink input as shown in Figure 3-1; when S/S is short-circuited with 0V, the input is source input. In this case, two buttons are needed, one is the start button connected to X0, and the other is the stop button connected to X1. Since the selected model is FX5U-32MT/ES, the output type is sink output, so its common terminal is connected to 0V. Y0 output is to the coil of the solid-state relay, the other end of the solid-state relay coil is connected to 24V, the normally open contact of the solid-state relay is to the neutral line of the heater rod, the other is to the neutral line of the power supply, and the other end of the heater rod is connected to the live wire.

Figure 3-1 PLC input and output wiring

First, provide a 24V power supply to the temperature transmitter. The thermal resistor has three wires, two silver wires are short-circuited and connected to the temperature transmitter, and the red wire is connected alone. The voltage signal output by the temperature transmitter is connected to V1+ in the PLC analog input, and V- is connected to the common terminal 0V. For specific wiring methods, please refer to Figure 3-2.

Figure 3-2 Analog input wiring

04 Software parameter settings

(1) Basic settings for analog input:

Change the A/D conversion enable/disable setting to enable, and the averaging processing instruction can select sampling, as shown in Figure 3-3.

Figure 3-3 Basic settings for analog input

(2) Analog input application settings:

When the temperature is 0℃, the analog input voltage is 0V; when the temperature is 150℃, the analog input voltage is 10V. Expand the obtained value by 100 times and fill it into the table shown in Figure 3-4. After filling it in, you can use SD6021 to monitor the current temperature.

Figure 3-4 Analog input application settings

05 Programming

Write a traditional start-stop program, use X0 for start, X1 for stop, and M0 for run, so that the program requirements can be met first. When the start button is pressed for the first time, it counts once, and the normally open contact of counter C0 is turned on. At this time, when the current temperature is less than 100℃, the heating rod is started. When the current temperature is greater than or equal to 100℃, the counter C0 is reset. In the following process, the heating rod is started as long as the current temperature is less than 30℃.

The program is shown in Figure 3-6.

Figure 3-6 Program design