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lthough the PLC has a Level 1 HMI with all its screens, the Level 2 for this system typically has a separate PC dedicated to running just the Level 2 for calculating those temperatures and the most efficient use of fuel for achieving the desired output temperatures with minimal fuel and minimal scaling.

It's not unusual for customers to request the two HMIs to run on the same computer terminal, and we often create a button on the Level 1 HMI capable of launching the Level 2 HMI. In rare occasions some parts of the Level 2 HMI (such as the simulated isotherm image) are requested to be embedded into the Level 1 screens; but recently, we've seen an increase in the number of customers that are insisting that the Level 2 HMI be thoroughly embedded within the Level 1 HMI. We've been creating custom one-of implementations within the customer's Level 1 SCADA system, borrowing or creating custom one-of controls for this purpose, but after finishing two of these fully mixed systems, it seemed like a good time to investigate a new design philosophy for our HMI in general.

This is an on-going effort for us. The first step is to examine what's possible. Since the two most common SCADA systems for which our customers request this type of implementation has been WinCC and WonderWare's Indusoft, our research (and this article) will focus primarily on these two systems, but there are likely issues discussed here that will apply to designs of this type in any SCADA system. Likewise, any suggestions or feedback found here are likely to contribute to our final design.

Definitions

HMI

A Human Machine Interface or HMI is the graphical interface for a plant to control some industrial equipment. The rest of the world would call this a Graphical User Interface or Gooey (GUI), but Human Machine Interface just sounds more -- down-to-business.

PLC

A Programmable Logic Controller or PLC is a specialized, hardened computer that reads industrial sensors and controls industrial equipment.

Level 1, 2, and such

Automation Levels are based on the ANSI/ISA Batch Standards (S88). Level 0 generally refers to MITL (Man In The Loop) controls. This is where some guy hand-cranks valves and pushes a start button on a pump, watches the glass level fill up, then pushes the stop button. Level 1 is where that guy's supervisor gets tired of paying someone to stand around all day watching the level on a tank, and buys a PLC to monitor an electronic level gauge, and programs the PLC to automatically send start and stop signals to the pump. Level 2 is where that supervisor's manager realizes that the water in the tank costs more between 6 AM and 5 PM, and decides it makes sense to fill the tank from half to full after 5 PM, but only fill it to half from a quarter between 6 AM and 5 PM.

SCADA

A Supervisory Control and Data Acquisition system or SCADA system is a type of application development environment intended to make it easier for non-programmers to create an HMI.

WonderWare, Indusoft

WonderWare's Indusoft is a SCADA system. For more information on this system, check out Invensys' WonderWare Indusoft Product Page.

WinCC

WinCC is a SCADA system. For more information on this system, check out Seimen's Simatic WinCC Product Page. Use caution here when looking for information on WinCC, because Seimens also has another, unrelated SCADA system with similar capabilities called WinCC Flexible. (If you Google information on WinCC, I suggest appending -Flex -Flexible filters at the end of your query.)

Setpoint

A setpoint is easiest to understand in terms of a home's thermostat. You don't typically turn your heater on and off; rather, you set the temperature to a specific setpoint, such as 72 degrees F, and the thermostat turns the heater on when it's too cold and turns it off when it's warm enough.

PID

A Proportional-Integral-Derivative controller or PID works similar to the cruise control in some modern automobiles. It is a sophisticated output calculation (how much gas to feed the engine) that is based on your setpoint (the current mph or kph you desire), the rate of acceleration or deceleration, and how long it's taking to speed up or slow down.

Tags

Tags are special variables that are common to PLCs and SCADAs. External tags are control points that correspond to some physical piece of equipment such as a pressure indicator or a valve's percentage open. Some, such as the pressure indicator, are read-only. Some are writable only under certain circumstances; the valve's percentage open, for example, is only writable when the valve is in manual. If the PLC is controlling the valve via a PID controller, then a setpoint is the writable value, but the percentage open is controlled by the PID calculated output. Internal, or memory tags mimic external tags, but are not tied to a specific physical piece of equipment. Rather, memory tags are much more like a variable for the SCADA system, with their values set by the SCADA systems programming logic.

Microsoft's Interop Forms Toolkit

"Microsoft's Interop Forms Toolkit is a free Visual Studio add-in that simplifies the process of displaying .NET forms and controls in a Visual Basic 6 application." [Microsoft, 2010] This tool is primarily designed to allow customers to extend the lifetime of existing Visual Basic 6 applications and allow them to incorporate new VB.Net controls and forms into existing applications of that type. However, it has been successfully applied to other applications which make use of the same technologies.

SOA

Service-Oriented Architecture or SOA is a design philisophy for providing data on one computer to be consumed by one or several computers for purposes that may not be known at the time the data service was created. The easiest way to understand SOA services is in terms of Web pages. The HTML language returned from a HTTP request was initially only used for displaying web pages, but increasingly is used to provide raw data that can be displayed in various ways by a wide array of devices. Pod Casts, RSS feeds and Geo-spatial data from services like Google Maps are examples of SOA Data Services.

VMS

Virtual Memory System or VMS was the Cadillac of computer operating systems. However, it consumed too much gas and was quickly displaced by Windows NT when computers were switched to all electric. For more information on VMS, check out the Hewlett-Packard, OpenVMS Product Page.

OPC

Object-linking and embedding (OLE) for Process Control or OPC was originally created as a Windows device driver for PLCs. The current name is a misnomer in that more modern OPC Standards Specifications do not rely on Object-Linking and Embedding for implementation. In fact, there is evidence of some movement within the current standards that offer to open the drivers to non-Windows operating systems. For more information on OPC, check out the OPC Foundation's web site.