Functions and working principles of human-machine interface

The most basic function of the human-machine interface is to display the switch status of the field device (usually PLC) and the value of the digital variable in the memory, use the monitoring screen to send switch commands to the PLC, and modify the parameters in the PLC memory.

1. Configure the screen

The word "configuration" means configuration and parameter setting. The human-machine interface uses the configuration software running on the personal computer to generate monitoring screens that meet user requirements, uses the graphic objects in the screens to realize their functions, and uses projects to manage these screens.

The configuration software can easily generate the human-machine interface screen, and dynamically display the state of the switch quantity and the value of the digital quantity in the PLC with text or graphics. Through various input methods, the operator's switch quantity command and digital quantity setting value are transmitted to the PLC. The screen generation is visual, and generally does not require user programming. The configuration software is simple and convenient to use and easy to master.

After generating the graphic object in the screen, you only need to associate the graphic object with the memory address in the PLC to realize automatic data exchange between the PLC and the human-machine interface when the control system is running.

The screen consists of static objects and dynamic objects that form the background. Static objects include static text, numbers, symbols and static graphics. Graphics can be generated in the configuration software or other drawing software.

Dynamic objects are displayed as numbers, graphic symbols, bar graphs or trend graphs connected to variables in the PLC. During operation, you can use function keys to switch screens. You can also configure the human-machine interface to monitor the PLC's alarm conditions and alarm screens, as well as the information that needs to be printed when an alarm occurs.

2. Communication function of human-machine interface

The human-machine interface has a strong communication function, and some are equipped with multiple communication interfaces. Using various communication interfaces and communication protocols, the human-machine interface can communicate with PLCs of major manufacturers, as well as with computers running configuration software. The number and type of communication interfaces are related to the model of the human-machine interface. Siemens' current human-machine interface devices basically have Ethernet interfaces, and some also have RS-485 serial communication interfaces (serial ports for short) and USB interfaces. The serial port can use MPI/PROFIBUS-DP communication protocols. It can realize communication between one touch screen and multiple PLCs, or multiple touch screens and one PLC.

3. Compile and download project files

Compiling a project file means converting the created screen and setting information into a file that can be executed by the human-machine interface. After successful compilation, the executable file in the configuration computer needs to be downloaded to the Flash EPROM (flash memory) of the human-machine interface. This data transfer is called downloading (see figure). To do this, you should first select the communication protocol in the configuration software, set the communication parameters on the computer side, and also set the communication parameters of the human-machine interface through the control panel of the human-machine interface.

4. Operational phase

When the control system is running, the HMI and PLC exchange information through communication, thereby realizing various functions of the HMI. There is no need to program the communication between the PLC or HMI. You only need to set the communication parameters in the configuration software and the HMI to realize the communication between the HMI and PLC.

5. Operation and maintenance of human-machine interface

Only use your fingers or a stylus to touch the touch screen of the HMI device, and only use your fingers to operate the membrane keys of the HMI device. Using hard, sharp or pointed objects, or operating the touch screen and keys in a rough manner, may greatly reduce their service life or even cause complete destruction.

The HMI device is designed for maintenance-free operation. Nevertheless, the touch screen or keyboard protective film and the display must be cleaned regularly. Before cleaning, the HMI device should be switched off to avoid unintentional triggering of functions.

You can use a cloth moistened with detergent to clean the device. Use only a small amount of liquid soap or screen cleaner. Do not spray the detergent directly on the HMI device. Use a cloth dipped in it. Do not use corrosive solvents or scouring powder. Do not use compressed air or a jet blower when cleaning.

In order to clean the HMI device when it is powered on and the project is running, the configuration engineer can configure an operator control object (such as a button) to call the "clean screen" function. After the "clean screen" function is activated, the touch screen operation and function key operation will be locked for the configured period of time. The configuration engineer can lock the operation for 5 to 30 seconds. The progress bar indicates the remaining time until the operation lock ends.

HMI devices generally do not provide screen protection films. You can order screen protection films for HMI devices. The screen protection film is a self-adhesive film that prevents scratches and dirt on the screen. When you remove the screen protection film, no sticky residue will remain on the screen. Do not use sharp or pointed tools (such as knives, etc.) to remove the screen protection film, as this may damage the touch screen.