First-Place Winner - Platinum Award of Excellence!

Siemens design team: Helmut Windl with Helmut Fritz, Harald Gebauer, Wolfgang Sperl, and Eberhardt Ohlert.

Constantine & Lockwood consulting team: Larry Constantine with James Noble and Meilir Page-Jones.

Purpose and Introduction

Step 7 Lite is an integrated development environment (IDE) for use by automation specialists to engineer and program solutions to automation problems. In order to understand the design of the Step 7 Lite system and the significance of its performance-centered features, some basic aspects of the processes by which automation systems are programmed need to be grasped. To set the stage, we offer the following brief background.

Automation and PLC programming

Most modern automation equipment is controlled by software running on specialized computers called Programmable Logic Controllers (PLCs). The systems controlled by PLCs vary tremendously, with applications in manufacturing, chemical process control, machining, transit, power distribution, and many other fields. Automation applications can range in complexity from a simple panel to operate the lights and motorized window shades in a conference room to a completely automated brewery in which the machinery for everything—from dispensing and mixing ingredients to controlling the brewing process and even filling and sealing of bottles—is under programmed control.

To support such highly varied needs, companies such as Siemens AG produce lines of modular computer components that can be combined and configured in many different ways to monitor and operate various equipment. In a representative system (Figure 1), a Central Processor Unit (CPU) is combined with a number of input/output (I/O) modules that connect with sensors and actuators in the automation equipment itself. The programs that run on the PLC are planned, written, and debugged by automation programmers using development software running on conventional desktop or laptop computers or workstations. The programs produced through this development software must be downloaded to the PLC itself to operate the equipment.

Figure 1 - Schematic automation system.

Automation programming is an exacting engineering profession requiring great attention to detail, mastery of specialized concepts and terminology, and thorough and systematic discipline to avoid or eliminate errors. The risks associated with undiscovered errors in PLC code can be enormous. A lurking bug in a PLC program could destroy an expensive piece of material being machined, require the repair of costly equipment, or even lead to injury of personnel.

Performance issues

Step 7 Lite is a new product in the established Simatic Step 7 line of IDEs for automation programming. The system supports programming of several Siemens product lines—S7 300, ET200S ET200X series—comprising comparatively simple PLC components. It is intended for more or less basic problems at the lower end of the range of PLC applications. Siemens hopes that an innovative and highly usable product will help it gain access to new markets internationally.

Nearly all existing IDEs for PLC programming have a very long learning curve. The slow accretion of features implemented by diverse teams at various times and the resulting lack of consistency among the various subsystems have added to difficulties in learning and mastery and to complications in use. Features and functionality have been the driving forces in PLC programming tools with relatively little attention paid to usability during design. Moreover, the roles of actual users and the detailed nature of their tasks have not been fully articulated to be reflected in user interface design.

The key performance issues driving the design of the Step 7 Lite system were:

• immediate usability by qualified but inexperienced users,
• flexible accommodation to a wide range of work styles and programming practices,
• reduction of user errors,
• increases in productivity through reduced user steps as well as reduced errors,
• closer fit between the user interface and the way actual PLC programmers think and work, and
• simplification of tasks where consistent with the need for great flexibility and with recognition of the inherent complexity and unpredictability of the PLC programming process.

This last point bears emphasis, as it makes the problem quite unusual in the application of a performance-centered design philosophy. No step-by-step process template or other rigidly structured performance model would be acceptable within this kind of application or by the targeted users.

In order to achieve the necessary high-levels of usability, flexible support, and performance efficiency, Step 7 Lite was developed through usage-centered design, a systematic model-driven process for designing applications based on the structure of user tasks. At the core of usage-centered design is a comprehensive and fine-grained task model representing user intentions and system responsibilities in the form of so-called use cases. This task model guides every aspect of the visual and interaction design for the user interface, including the design of custom components and novel behaviors.