PLC Programming: Technical Summary Using Siemens Products as Examples

A programmable logic controller (PLC) is a rugged, microprocessor-based electronic device that is essential to all modern automation, including:

• Heavy processing sectors such as oil and gas, nuclear power, steelmaking and wastewater treatment

• Industries that emphasize controlling discrete tasks—including general factory automation, automated warehousing, packaging, food, beverage, and medical device manufacturing

Of course, PLCs are not the only choice for automation. Relay-based systems are still essential in a wide range of applications, but programmable automation controllers (PACs) or industrial PCs (IPCs) and panel PCs (human-machine interfaces with control electronics) are alternatives in many machine designs and systems that require varying degrees of distributed control. PACs and IPCs running industrial-grade Microsoft Windows operating systems in particular offer great design flexibility.

All of these control systems are configured and programmed using a variety of sophisticated software, making all types of control designs more advanced and user-friendly than ever before. This in turn enables OEM machine builders and plant engineers to quickly build, upgrade and migrate systems with maximum efficiency, productivity and IIoT connectivity.

Tools for programming controllers - including PLC

Figure 1: PLCs have all the benefits of dedicated hardware—including reliability. PACs, by contrast, offer the highest reliability. Some vendors allow engineers to program both control types in the same unified software environment. In such an environment, engineers also have the freedom to use a wide range of digital automation, engineering, and operations monitoring tools. (Image source: Siemens)

Today, almost all PLCs are configured and programmed through PC-based software. Large suppliers that have a broad range of programmable motion control, sensing, actuation, and machine interface components in addition to general automation and PLC products typically allow all of these components to be programmed in their proprietary unified programming environment—a PC-based Windows-compatible software and design, configuration, programming, and even operation and management modules. This is especially true when the supplier's product lineup includes pre-integrated products, such as smart motors or HMIs with PLC functions.

While the learning curve can be daunting, once mastered its unified programming environment can greatly speed up machine design.

One benefit of this software environment is the availability of an accurate and universally applicable database of programmable symbol, variable or tag names. These are readable alphanumeric names assigned to component (including PLC) addresses, improving the direct use of complex register addresses that was previously standard practice. Complementing these sortable and searchable device tags are information-rich machine and work cell tags, as well as those for common machine functions such as automatic, manual, motor run, fault or reset.

Let's consider Siemens STEP 7 Totally Integrated Automation (TIA Portal) software. This software includes packages for various specific purposes and is accessed through the Siemens SIMATIC (SiemensAutomatic) software management environment. STEP 7 software is convenient for illustrating the most common method of PLC programming because it is the most widely used industrial automation software in the world - with a large number of functions and reliability verification. According to most estimates, nearly 1/3 of all PLC installations in the world use Siemens PLCs.

With this software, engineers can create process control, discrete automation, energy management, HMI visualization, or simulation and digital twin programming related to PLC and other industrial controller functions. On the PLC side, Siemens' STEP 7 (TIA Portal) engineering software has evolved from the traditional SIMATIC STEP 7 software to support the programming of S7-1200, S7-1500 and S7-1500 controllers - as well as ET 200SP I/O CPUs, traditional S7-300 CPUs (a long-standing industry mainstream product) and S7-400, SIMATIC WinAC controllers. Professional and specially authorized copies of STEP 7 include additional functions, logic editors and integrated traditional engineering software.

While beyond the scope of this article, it is worth noting that industrial control alternatives to multi-function PLCs can be configured and programmed using complementary software. The vast ecosystem of Siemens controllers provides numerous examples.

1. LOGO! logic modules meet small and moderate automation applications, bridging the gap between relays and microprocessor-based industrial controllers. Logic modules are easily configured and designed through Siemens LOGO! software and Soft Comfort engineering software, LOGO! access tools and LOGO! network editors.

2. The process control system uses Siemens SIMATIC PCS 7 controller products and can be programmed using SIMATIC PCS 7 system software.

3. Rack (rail) mounted, panel mounted and box industrial PCs (IPCs) used to implement distributed control and machines requiring IIoT connectivity are inseparable from Siemens SIMATIC IPC software modules, including IPC Image and Partition Creator, IPC DiagMonitor, IPC Remote Manager, IPC FirmwareManager and SIMATIC Industrial Operating System.

4. As a panel-mounted computer for on-machine control, the HMI uses SIMATIC WinCC Unified (TIA Portal) software as well as SIMATIC WinCC (TIA Portal), WinCC flexible, WinCC V7, WinCC OA, ProAgent process diagnostics software, mobile device notification software, etc.

Additional software simplifies the selection between SIMATIC PLCs and other machine controllers in the form of an online cloud-based selection tool (or offline variant), which asks engineers about the physical layout of a particular design (whether a control cabinet or distributed control is required) as well as:

• The expected number of I/Os, including sensors, switches, and actuators.

• Programming languages ​​used: Ladder Diagram (LD), Structured Control Language (SCL) or Function Block Diagram (FBD), higher level Structured Text (ST), graphics-based Sequential Function Chart (SFC) and Continuous Function Chart (CFC) or higher level languages.

• The level of motion control required (if applicable) – from simple speed and position control, to electronic camming and advanced kinematics control.

• Hardware preference and whether the software PLC program running on the IPC is the best fit.

PLC Program Project

Projects typically include PLC programs written in the PLC vendor's software. These are concerned with application-specific operations such as:

• Heating, mixing, filling, dosing and irrigation

• Moving, steering, circulating, positioning and braking

• Grasping, cutting, punching and slicing

• Welding, gluing, marking and dispensing

• Sense, track, sequence and indicate

The most advanced options support digital planning, integrated engineering and transparent operation. This can be easily done via the HMI on a specific user display in the operating state. In other words, this PLC software allows the display of relevant PLC information on different displays to meet the different information needs of machine operators, technicians, plant managers, and even corporate managers.

Simulation tools within the PLC vendor’s software environment can also speed time to market for specific products—and improve finished product yields. Energy management capabilities and diagnostics round out this suite of software-based improvements.

Verify and load PLC with programs written in software

Figure 2: Siemens SIMATIC PLCs and automation systems were first introduced in the 1950s. Today, the SIMATIC S7 products (including the SIMATIC S7-1500 PLC components featured in this article) have evolved to support a wide variety of industrial automation applications. (Image source: Siemens)

At the heart of optimal PLC functionality is the quality of its programming. All code should meet industry standards and best practices for software development. Beyond that, validation processes (manual and automated) can reveal everything from critical errors to code inefficiencies. Rethink programming for SIMATIC S7 products. Within the Siemens ecosystem, the TIA Portal Project Check application automatically compares certain code with the rules defined by the programming style guide for these specific PLCs. Engineers can then export the comparison results as an XML or Excel file. User-defined rule sets (even for complex types) can also be added via the Project Check Software Development Kit (or SDK) for C# or Visual Basic (.NET). This SDK is primarily used to prove program style.

Table 1: Both manual and automated methods can be used to validate PLC programming—the latter being particularly useful for validating styles and techniques. (Chart source: Siemens)

After the program specified for the PLC is written and verified, it must be loaded onto the PLC. In many cases, a personal computer (usually a laptop) is connected to the PLC via an Ethernet cable or a PC-specific USB via a PLC COMM adapter - the program is loaded onto the PLC microelectronics. The PLC then interfaces with the controlled components via I/O modules. After additional verification at startup, the PLC executes its program by commanding the networked actuators (via various signal types) and making real-time adjustments based on feedback from field devices.

Occasionally, a machine or automated work cell requires adjustment, troubleshooting or repair – and the default feedback response of the PLC is overridden by a forcing method (via some type of programming PC connection to the PLC). This “tricks” the PLC into thinking that certain feedback is at a certain value when it is not – for example, this strategy is used when a station downstream of a faulty actuator must be cleared. In other cases, a machine or work cell may require field adjustment of the parameters of an installed PLC by modification. Such adjustments must refer to appropriate triggers, variable values ​​or tables, counters and timers.