29. HUMAN MACHINE INTERFACES (HMI)
29.1 INTRODUCTION
For simpler control systems, buttons and switches are quite suitable for operator interfaces. However as the number of operator options increases, or the interface becomes more complicated it may be preferable to replace many of the buttons, dials, and indicators with a Human Machine Interface (HMI). These units can be as simple as a single line of text and a couple of push buttons. More complicated units use large color monitors with touch screen capabilities. Ultimately these units are very powerful because the display contents can be changed to match the mode of operation.
An HMI is a simple to program graphical interface, very much like modern computer software. The simplest control pair are a button and indicator. Consider the example in Figure 437 A Simple HMI Application. The button can be used as a simple input to the PLC, while the output status can be shown with an indicator. The programmer will set up the Ethernet connection to pass tag/variable data between the HMI and PLC so that a change in one appears in the other. So, if the button is touched on the screen of the HMI, the value is changed in the memory of the HMI. On the next data update cycle it is sent to the PLC. The program in the PLC reads the value change and then sets a new indicator value. The updated indicator is then sent to the HMI on a subsequent communication update. The newly changed value in the HMI is then used to update the indicator on the screen.
Obviously the first example is quite simple and only replaces and output or input, but much more capabilities are possible. Typical elements found on HMIs include,
· Multiple screens - each screen is given a function such as idle, maintenance, auto, and manual
· Logical inputs and indicators - simple on off inputs and outputs. These will be the most common component, often used by operators to start and stop operations.
· Numeric inputs and indicators - analog inputs and outputs. These can be used for applications such as monitoring temperatures, or setting part counts.
· Graphs - often to see trends over time for counts or analog values
· Text and Images - pictures and text are often used for operator instructions and help.
· Colors - some units have colors that can be used to highlight or differentiate items. For example a bright red object on the screen can indicate a fault.
· Sounds - some more advanced HMIs offer sound outputs
The general implementation steps for implementing and HMI are listed below. To control the HMI from a PLC the user input will set bits in the PLC memory, and other bits in the PLC memory can be set to turn on/off items on the HMI screen.
2. Enter or load the PLC variable/tag names that the HMI will use.
3. Layout screens, buttons, etc. on the programing software.
4. Download the program to the HMI unit.
6. Run the system - read and write to the HMI using PLC memory locations to get input and update screens.
29.2 HMI/MMI DESIGN
There are a few basic approaches that will help when designing any Graphical User Interface (GUI), such as those running on HMIs. As normal the design process begins with gathering information, providing a structure, and then implementing the structure. A good set of introductory questions are given below.
1. Who needs what information?
2. How do they expect to see it presented?
3. When does information need to be presented?
4. Do the operators have any special needs?
6. What choices should the operator have?
After this information gathering stage the HMI Functionality can be designed using a state diagram and a list of input/output requirements for each screen. A simple example is given in Figure 438 State Diagram for a Simple HMI. Each of the four states in the diagram will become one of four HMI screens. The transitions between the screens are normally touchscreen buttons, but could also be values set by the PLC. For each of the screens a requirements list must be developed that shows the important information and actions available to the user.
HMI design also requires a limited amount of artistic skill to increase usability. A few simple rules-of-thumb a good HMI design are given.
Common Look and Feel - each screen should look very similar.
Components with similar function should appear in the same place on each screen
Colors and logos should be consistent between screens
Use colors conservatively - especially green, red, and yellow
Provide helpful notes (or popup help)
Indicate the current state of operation clearly on each screen
buttons - change the shading or similar when pressed
Adjust the interface to the user
maintenance screen have more technical content
operator screens simpler language, task oriented
other languages for non-english speakers
icons instead of words for reduced literacy
A clear interface design can be presented to customers and non-technical people to get feedback before the implementation. These should include i) the State Diagram, ii) a list of Screen Requirements, and iii) a Look-and-Feel in general, and for each screen. Once these have been set the process of programming the HMI becomes a trivial matter.
29.3 SUMMARY
· HMIs allow customized controls that can change based upon the operation mode of a machine.
· State Diagrams and Screen Requirements can be used with a Look-and-Feel before implementation.