Let’s move on to a more complex system. Many of the basic principles are still the same. You still have information in the controller for the operator that needs to be displayed and you still have actions by the operator that need to be conveyed to the controller. Instead of a simple keypad and text display, let’s look at a graphical style O/I. This type of Operator Interface has some type of screen similar to a computer monitor that allows some form of graphics to be displayed. The most basic of these systems may use a small monochrome display of just a few inches with simple line graphics, while the more advanced systems may have a large color display with very advanced graphics capabilities. Both are useful in the right settings and of course the fancier the display the more costly the system. Depending upon the memory available in the system, the controls specialist can design multiple screens or pictures modeling the process or machine being controlled that can be paged through by the operator. There are also a variety of options available to handle the keypad’s responsibility described in the previous example. The simple, low cost graphical systems employ a very similar keypad built into the display. However, even on the cheapest systems you can usually control a large number of devices as you can assign different devices to the same buttons. Thus depending on what page or screen the operator is viewing, a different group of field devices are being controlled by the keypad buttons. There are also touch screen Operator Interfaces that allow the operator to touch an item on the screen to control its actions or move to other screens. One similarity of all industrial grade Operator interfaces is that they are tough. They usually get put in dirty areas and have to hold up to corrosive chemical vapors and of course the abuse some operators can deliver. They generally have no moving parts (i.e., hard drives, cd roms, etc.) and are expected to go into an area and work without interruption.
In the most basic sense the graphical Operator Interface is very similar to the simple text display type. You still have a keypad or on screen pushbuttons to take the place of physical pushbuttons. Instead of a text messages such as “Valve Open” from our text display example, you might have a picture of a valve. On color displays this valve might show one color for closed and one color for open. Chances are there is room on the operator interface for many more keys. Your control options might include pressing a key that selects the valve on the screen. This would lead to additional information on the screen tying other buttons to functions such as “open valve” or “close valve.” With the touch screen option, the operator would touch the valve on the screen. This could pop up a window with open and close pushbuttons on it that would allow the operator to open or close the valve by touching these buttons.
Making all this work with the PLC would be similar to the text display. The push button elements are linked to PLC memory locations and the Operator Interface is able to manipulate the values in those locations. A simple pushbutton would just toggle a bit on or off. However, Operator Interfaces usually will have the capability to manipulate the bit in several different manners. The bit could have a momentary action that could operate directly with the operator’s actions. If the operator pushes the button the bit changes to a 1 – when he releases it the bit returns to a 0. The operator may press too quickly in this case not allowing the bit to perform its required operation due to communication delays or a slow PLC program. In this case the pushbutton can be setup to have a minimum on time or a minimum off time. It may be set up to normally set the bit to a 1 and reset it to a 0 when pressed. Finally it might toggle the bit so that the value changes from 1 to 0 or 0 to 1 depending on the value of the bit when the button is pressed. So all this bit discussion may lead you to believe that Operator Interfaces are only good for discrete applications. Not so. They are capable of grabbing and inserting information of all types from the controller. A PLC may have a temperature probe hooked to an analog input card that allows it to turn on a fan if the temperature goes too high. The Operator Interface can be configured to get the temperature information and display it on the screen. It might have a graphic of a vessel with a fan and the temperature shown next to it all. The operator could select the vessel bringing up an input screen that would allow him to enter a temperature setpoint. This information would be pushed down to the PLC where it would act as the point at which the fan would turn on. The operation of the fan could be shown on the screen to indicate that it is running. In fact using animation techniques the fan might even look like it’s running on the screen. The variations are endless.