Integrated HMI and PLC
The heart of a “lean automation” solution
By Rich Harwell
The integration of a human machine interface (HMI) and programmable logic controller (PLC) provides a lean automation solution. Lean manufacturing is a proven, powerful method to boost efficiencies in production processes. Similar concepts and practices that eliminate “waste”—unnecessary equipment and process steps—can be applied to the design, construction, and support of automation systems to enable increased productivity and reliability, yielding increased efficiency. Combining visualization and control means:
- Faster machine design by providing an integrated development environment
- Reduced machine construction costs by eliminating components and wiring
- Reduced machine support cost and improved operation by centralizing remote access and administration
More than any other time, there are a range of trends in both control system architecture and manufacturing that are coming together to support an integrated HMI-PLC. For OEMs and control engineers alike, this means it is easier to build smaller, smarter machines faster—freeing both OEMs and engineers from having to use controllers and equipment simply because of a familiarity and a prohibitive cost to change.
Control system basics
To understand better the trends driving HMI and PLC technology, it is useful to first examine the basic architecture of a control system and how the control system itself is evolving. Fundamental changes in control system architecture are making HMI-PLC technology a compelling alternative—streamlining functionality, reducing equipment and costs, and propelling the next generation of machine control.
The basic control system structure includes sensing, actuation, operator interface, and logic control devices. Sensors measure a physical quality, like temperature, and convert that information into an electrical signal; the actuation device acts on that information; the operator (or user) interface is where the interplay between equipment and people occurs; and the logic device controls machine operation. The logic control device examines input from the operator and sensor, sending signals to the actuation device. This simple model of sensors, actuators, human interface, and logic applies to control systems in both the discrete and process control space.
Today, the basic functionality of a control system remains largely intact, but the architecture that achieves that functionality is shifting. All the devices in each category are evolving—both in terms of their intelligence and how they interface to the logic device. Industrial networking technologies are driving changes in the latter—how devices interface. Simple device networking is replacing I/O points and wiring on the machine, while Ethernet-based system networking is connecting machines to the enterprise and enabling remote access—both of these trends are propelling rapid change in sensors, actuators, logic controllers, and operator interface devices. And the most dramatic changes are happening at the operator interface and controller levels—where functionality overlap makes a combined device a compelling alternative.
Device-level networks drive HMI-PLC technology
While somewhat overshadowed by the advances in enterprise networking, device-level networks have also progressed, enabling more cost-effective connection of even simple devices through a network to the logic control within the system. These advances have taken place both within the control cabinet and outside of it—to sensors and actuators mounted on the machine. The impact of moving from traditional I/O cards and wiring to simple device-level networking in a control cabinet can be seen in Figures 4 and 5.
Traditionally, as seen in Figure 4, basic devices in the cabinet are connected through discrete I/O cards and point-to-point wiring to the logic controller. Today, as seen in Figure 5, this wiring is being replaced by a simple networking system that connects all the devices to a single communications port on the controller and also incorporates the device power connections. Device-level networks yield multiple advantages: reducing time to wire and commission the control panel, removing the I/O cards from the system, and improving operational diagnostic information for more reliable systems.
The move away from I/O to networking for connecting sensors, actuators, and human interface in machine control is a key enabler of moving to a combined HMI-PLC. Once the role of the I/O cards in a system is removed or significantly reduced, the value of having a separate PLC from the electronic operator interface (EOI) is greatly reduced. The processing power available in today’s microprocessors makes handling both the operator interface task and the programmable logic control from a single device a manageable task. Further, as remote access is increasingly important, the case for a combined HMI-PLC is strengthened, changing the basic control system and enabling a leaner automation approach that combines logic and visualization.
Enterprise networking and remote access propels HMI-PLC technology
The manufacturing community stands to make significant productivity gains with the emergence of industrial Ethernet applied to the factory floor. The expanding web of Internet networking and smart devices that can interface over it are driving improved diagnostics, faster troubleshooting, and overall improvement in machine reliability. With the information that Ethernet connectivity is enabling, there are increasing opportunities to reduce the time and costs involved in diagnosing and fixing issues quickly before they cause downtime or delay commissioning. These changes also impact the automation systems on machines, demanding a fresh look at the automation architecture and the role of the PLC and HMI in that system.
To understand better the convergence of HMI-PLC functionality and equipment options, a historical perspective of the PLC and operator interface (OI) technology and development is useful. Traditionally, the PLC was developed to provide control, sequencing, and safety functionality for manufacturing processes, replacing systems that involved numerous devices—multiple control relays, timers and drum sequencers, and closed loop controllers. Largely the I/O cards used to interface to the sensors, actuators, and simple operator interface devices, such as push buttons and pilot lights, made up the controller.
The electronic operator interface (EOI) was introduced as a complement to this simple PLC. By connecting to the PLC through a simple serial network connection, the EOI added the ability to present much richer information to the machine operator, replacing some of the simple push button and pilot lights in the process. This historical relationship between the EOI and PLC is shown in Figure 7.
With industrial Ethernet, the advent of remote access in the automation space means increased automation functionality including: historical data capture and exchange, alarm notification and management, and security administration. These features have been introduced into both the PLC and EOI and tend to overlap today, as vendors of both PLC and EOI equipment vie to address new requirements for historical information, alarming, and security (see Figure 8).
More is not always better, and redundancy in EOI and PLC feature sets has several downsides: the needless duplication of hardware (i.e., Ethernet is required on both devices), unnecessary complexity of programming (redundant alarm systems), and increased risks of security holes. Instead of two devices with overlapping functionality, a single device with a feature set that covers requirements without duplication means less equipment to buy, program, and configure (see Figure 9).
Logic programming standardization
A robust development environment underpins efficient automation design and can crucially provide standardization. Standardization promotes interoperability, saves engineers’ time when they work with products from multiple vendors, and drives “lean” efficiencies by enabling reuse (of code) and reducing the retraining effort.
In December 1993, the development of IEC 61131-3 was a ground-breaking effort—bringing standardization to logic programming. It provided a core-programming model with a variety of benefits:
- Structured software based on program organization units (POUs)
- Strong and consistent data typing
- Task-based execution control
Beyond the standardization of program structure, IEC 61131-3 defined a standard set of programming languages: ladder logic, function block, structured text, instruction list, and sequential function chart.
Globally, suppliers and end users of automation products adopted IEC 61131-3. By providing a common programming model and language set, the standard allowed control engineers to work effectively with controllers from different suppliers, enabling engineers to more easily understand programs generated for various logic controllers. Additionally, the task of porting a program from one supplier’s programming software to another’s became faster and easier.
Key attributes of a combined HMI-PLC
There are a variety of HMI-PLC options for OEMs to select from. The two major kinds of HMI-PLC devices are open, industrial PC (IPC)-based and closed platform solutions.
Both types of HMI-PLCs have their place. The open, IPC-based approach provides the flexibility to combine additional programs on the control platform, whereas the closed approach enables a more optimized solution that is easy to administer. Both open and closed devices have similar key features and functionality (see Figure 9).
Logic and graphics processing
The heart of the combined HMI/PLC is the logic and graphic processing capabilities. Specifically, a robust and repeatable performance over the range of intended applications, powerful logic programming, and extensive graphics programming capabilities are critical. In addition to the run-time capabilities, an intuitive and integrated development environment that combines both logic and graphics programming is crucial for automation engineers—making programming and deployment faster. Further, a development environment with well-developed tools for simulation, debugging, and program updates saves time up front and at the back end, speeding up development and maintenance.
Display and touch screen
Combined HMI-PLC devices provide similar display characteristics to standard HMIs:
- A broad range of screen sizes provides flexibility
- Resolution and brightness of the display
- Touch screen technology options, including rugged environment considerations
Enterprise Ethernet and remote access
This goes beyond the presence of a physical Ethernet port and includes the services that are provided through that port, including:
- Remote access for screen viewing
- Remote access from various devices, including smart phones and tablet computers
- Remote program administration
- Remote database access options
- Additional data protocols supports (i.e., OPC)
Cyber security continues to be an area of concern in manufacturing environments, and the enterprise Ethernet connection can be a point of weakness. Important areas to examine:
- Is each of the remote access methods described above adequately secured?
- Are all open enterprise Ethernet ports documented, and can they be shut off if not used?
- Is a method in place to secure the logic/visualization program?
As direct I/O wiring is replaced by device-level networking, the integration of the HMI/PLC with a device-level network—and the information and data points that it is able to provide—is key, saving time during commissioning, expanding diagnostics capabilities, and providing ongoing information that enables smarter, faster machines.
- Does the device provide the network options you need?
- Can you combine multiple network options when needed?
- Is the cost reasonable for the network option you can foresee?
Historical, alarm data management
Today’s machine automation is more than machine control. Machine automation today includes the ability to obtain and evaluate critical performance data and timely notification of any operational issues. Considerations are as follows:
- Do the historical data features offered by the system meet your needs?
- Is the method of historical data access—and format of data—sufficient for your applications?
- Is the alarm system adequate for your needs?
- Do the alarm notification system capabilities meet your needs?
A thorough review of these points will allow you to identify a combination HMI/PLC that meets your application needs.
HMI-PLC enables lean automation
Today, a host of trends are coalescing to support an integrated HMI-PLC. The control system architecture is shifting and networking technology—both at the device and enterprise level—is advancing, while there is an established and portable programming environment for both HMI and PLC devices. Ultimately, the HMI-PLC not only eliminates entire device levels but also enables remote intelligence, reduces training costs, and empowers OEMs to take advantage of a variety of suppliers, driving best-in-class solutions.
Further, the integrated HMI-PLC is enabling lean automation—boosting efficiencies and reducing waste, both in terms of equipment and time. In a highly competitive business environment, providing best-in-class solutions that are also intelligent, intuitive, and elegant means a real business advantage and yielding control systems that are faster to design, commission, and maintain.
ABOUT THE AUTHOR
Rich Harwell (RichardCHarwell@Eaton.com) is an advanced solutions manager at Eaton. He has more than 25 years of experience in the electrical industry, with expertise in a broad range of control and automation solutions.