Guide to Connecting a Thermal Mass Flow Controller to a PLC

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How to connect and use a Sino-Inst thermal gas mass flow controller and a PLC? The connection between a thermal gas mass flow controller (MFC) and a PLC is mainly achieved through electrical signal transmission to read flow data and issue control commands.

Communication between the MFC and the PLC enables real-time monitoring and control of gas or liquid flow rates, improving the stability and reliability of the production process. Through communication, the PLC can acquire the real-time operating status of the MFC, such as parameters like flow rate, pressure, and temperature, and remotely control the MFC as needed. Furthermore, communication can also enable fault diagnosis and alarm functions, allowing for timely detection and handling of problems in the production process.

Our thermal gas mass flow controllers can be configured with either analog or digital signals, such as 0–5V, 4–20mA, 1–5V, RS232/485, and MODBUS protocol. Therefore, the connection between the thermal gas mass flow controller and the PLC needs to be discussed separately based on the signal.

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Connecting a Thermal Mass Flow Controller to a PLC

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1. Analog Signal Connection

Analog signal connection is simple, reliable, and low-cost.

The PLC’s analog input module receives the MFC’s real-time flow feedback signal, such as 0–5V, 4–20mA, or 1–5V. 4-20mA corresponds to 0%~100% of the measurement range. The PLC’s analog output module sends the setpoint signal to the MFC; similarly, 4-20mA corresponds to the target flow.

Note: 0-5V voltage signals are suitable for short-distance transmission (typically <10 meters), and the wiring method is similar to that of current signals. It should be noted that voltage signals are easily affected by line voltage drops; for long-distance transmission, current signals are preferred.

2. Digital Communication Connection

Digital communication connections offer strong anti-interference capabilities and support multi-device networking. The RS485 Modbus RTU protocol is the most commonly used.

The MFC connects to the PLC’s serial port module via an RS485 interface, supporting multi-point bus topologies. Up to 64 devices can be connected. The PLC acts as the master station, sending Modbus commands, such as function codes to read flow values ​​and write setpoints. RS485 digital communication products are suitable for high-performance scenarios. Both the PLC and MFC must support the corresponding protocol, achieving high-speed data exchange and real-time control through a dedicated interface module.

Whether using analog or digital signals, the wiring requirements are consistent. A four-wire design (power positive and negative + signal positive and negative) is required, ensuring the signal common terminal is correctly grounded to reduce interference.

Multi-MFC Integrated Monitoring and Control

3. Implementation Steps:

  1. Hardware Connection: Connect the signal output terminal to the PLC’s input module, such as an analog input module or a digital input module.
  2. Parameter Setting: Set parameters such as output signal type (analog or digital), signal range, and communication protocol according to actual needs. Simultaneously, set the PLC’s input module parameters, such as sampling rate and data format.
  3. Programming: Write the PLC program to implement control and data processing. For example, a PID control algorithm can be used to achieve precise flow control; a data processing algorithm can be used to achieve real-time monitoring and alarm functions for parameters such as flow rate and pressure.
  4. Debugging and Optimization: During actual operation, debug and optimize the PLC program to ensure stable and reliable communication with the PLC, meeting production requirements.

FAQ

A mass flow controller involves both measurement and regulation. The measurement principle varies depending on the sensor used, such as thermal, differential pressure, or Coriolis principles. After measurement, the mass flow controller displays the instantaneous and cumulative flow rates.

Regulation is achieved through control valves. Users can set flow rates as needed. The control valves adjust the flow rate of the medium based on the sensor feedback signal, maintaining it within the set range. Even fluctuations in system temperature or pressure will not affect the flow rate.

The main functions of a flow controller include:

  • Monitoring and regulating flow: Flow controllers monitor, regulate, and control the flow rate of liquids or gases, ensuring a constant flow rate in pipelines to meet the needs of production processes.
  • Saving resources: By controlling the flow rate, raw material consumption can be reduced, achieving savings and cost reduction.
  • Protecting facilities: Flow controllers can slow down the flow velocity of the medium, protecting facilities from damage caused by flow surges exceeding their capacity.
  • Preventing backflow: In some special cases, flow controllers can effectively prevent backflow of the medium, avoiding damage or contamination of facilities.

Flow controllers are widely used in industries such as chemical, petroleum, pharmaceutical, and food to meet the flow rate requirements of different fluids.

The principle of a thermal mass flow controller is based on the thermal diffusion effect and the principle of thermal equilibrium. It achieves high-precision control of mass flow rate by measuring the disturbance of the thermal field caused by gas flow.

Its core sensor typically consists of a heating element and symmetrically distributed temperature sensors. When the gas is stationary, the thermal field between the heating element and the temperature sensors is symmetrically distributed. When the gas flows, fluid molecules carry away heat, causing the temperature downstream of the heating element to drop, forming a temperature difference related to the flow rate.

According to King’s Law, the gas mass flow rate has a linear relationship with the temperature difference and the heating power. By measuring the temperature difference or adjusting the heating power, the gas mass flow rate can be calculated.

Gas flow controllers require accumulated gas volume values ​​in some applications. How can the accumulated gas volume be precisely controlled?

Our MFC function includes a feature that automatically sets the instantaneous flow rate to 0 once the accumulated flow rate value is reached.

First, set the desired accumulated flow rate (accumulated gas volume) within the meter. Then, set the real-time intake flow rate (instantaneous flow rate). MFC will activate the accumulation function, and automatically stop gas supply once the accumulated flow rate reaches the set target value.

This control method can be operated on the meter’s screen or controlled via a host computer, and is equipped with 485 Modbus communication.

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A thermal mass flow controller is an instrument capable of accurately measuring and controlling gas flow rate, widely used in industries such as petroleum, chemical, metallurgy, and power. A programmable logic controller (PLC), as a widely used control device in industrial automation, possesses powerful functions and flexibility.

Communication between the mass flow controller and the PLC is an important topic in industrial automation. Through proper hardware connection, parameter setting, program writing, and debugging optimization, precise control and real-time monitoring of gas or liquid flow rates can be achieved, improving the stability and reliability of the production process. If you have any technical questions, please feel free to contact us!

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