Is there one controller that can do it all?
A single controller can now do the job of multiple dedicated controllers of the past
- In machine control, motion control and automation must work in tandem.
- Motion applications are fast, but speed is nothing if it is not repeatable.
- It is important to have the ability to select program execution speed.
By Mark Harned and Bob Nelson
What kind of controller is best for your application?
Is it a programmable logic controller (PLC)? Perhaps you should use a programmable automation controller (PAC). Maybe a programmable automation device (PAD) would be best.
When it comes down to it, the name is not that important. What is important is developing a clear understanding of your needs and then finding the controller that best targets them.
In the world of industrial automation, technology has evolved leading to innovations in controllers and I/O, enhanced engineering tools, and entirely new system architectures.
The controller we are familiar with can now take virtually any size and shape, from a traditional industrial PLC, to a "soft" PC-based controller, and even a "nano-sized" brick.
Industrial automation as we think of it today began with the development of the PLC 40 years ago.
The relay ladder logic program was only a few hundred bytes in length, looked just like a typical electrical drawing, and could be programmed and maintained by most plant electricians. Since then, program size has ballooned into the mega-bytes; programming tools have become more sophisticated; network communications have evolved to connect "islands" or "silos" of automation into plant-wide networks; and the PLC has grown from being a dedicated box running sequential ladder logic to an open platform for controlling applications across all manufacturing environments.
The automation project has evolved into a full-fledged software development project, needing structured design, extensive diagnostics, and alternate programming languages to achieve project goals.
By 2002, the evolution of the PLC had so progressed that Craig Resnick, director of research at technology consulting firm ARC Advisory Group, proposed changing the name of the PLC. "The label 'PLC' simply understates the capability of current automation systems," wrote Resnick, proposing a new name: "programmable automation controller," or PAC.
"The PLC has evolved by incorporating open standard interfaces, multi-domain functionality, distributed modular architectures, and modern software capabilities integrated as turnkey automation solutions," he said.
"Today, everyone, at least the major companies who make full-functioned PLCs, is making PACs, even if they don't call them PACs. There is no advantage in buying something called a PLC or a PAC; you just buy the controller you need," said another industry commentator.
Regardless of the three letter acronym used, the bottom-line recommendation is "end users should let their application be their guide" in choosing control solutions.
Broad range of requirements
One thing is clear: The modern automation controller can do more than the "Founding Fathers" ever imagined.
A single controller can now do the job of multiple dedicated controllers of the past. Considering the advances in technology, a user has every reason to expect a single controller deliver value through:
- Providing cost-effective and best-of-breed performance across all desired control applications
- Employing open standards to allow full integration in a heterogeneous, multi-vendor control environment
- Offering upgrade and maintenance paths to ensure long-term flexibility, protection against obsolescence and a favorable total cost of ownership
- Delivering sustainable, long-term suppler relationships based on decades of experience and global technology leadership
On paper, an automation controller must cover a broad range of application requirements.
The platform should offer standards-based openness; flexible, single-database integration for the entire system; standard IEC-61131-3 control programming and configuration languages; the ability to create preconfigured libraries of reusable code; and object-oriented design of full system architectures.
The reality is very few controllers can truly provide these capabilities in one package. However, such solutions do exist, and offer both the openness suggested by the PAC promise and the best-of-breed features that address the real-world solutions users need to satisfy requirements, present and future, across multiple types of applications.
Six key control applications
What does it mean when you hear modern automation controllers offer multi-domain control capability that works across application types? One way to look at this is to identify unique control applications, and then look at what the controller should do to help you solve the application. Here are six unique control applications.
Configurable system-wide diagnostics: You can always program diagnostics into your code, but this takes time. Sometimes the project leaders will skip this step to meet project schedules. A controller with easily enabled and built-in diagnostics ensures this important aspect of your solution stays a part of that solution.
Availability of multiple programming languages: Relay Ladder Logic is part of a logic controller, but aspects of your application may work better using graphical function blocks or a high-level programming language. The IEC 61131-3 standard for PLC programming languages defines five languages that range from ultra-efficient "machine code" to the graphical representation of sequences. Compliance with this standard not only gives you implementation options but can also ease training across vendor platforms.
Potential for reusable code: Programmers in all industries embrace the concept of creating, testing, and reusing common functions. The goal is to be more efficient and to improve program quality. For industrial automation, leveraging a library (from the supplier or defined by you) of common program elements and using these elements repeatedly reduces implementation time and improves program consistency, maintainability and quality.
One program across a variety of controller platforms (scalability): Ideally, you would have one engineering environment to configure all of your applications, which, in turn can download to the target platform meeting your requirements. This separation of program and platform allows you to focus on solving the application problem, followed by selecting the best, most cost-effective platform for your specific application.
Performance/processing speed: In machine, factory, or process control, controller performance is a key element in productivity gains. As you strive to improve the mechanical performance of your application to yield higher throughputs, the performance of your automation system must keep pace. The execution time, high-speed interrupt handling, and segmentable scan times of your controller all contribute to the performance of your application.
The controller should make it easy to engineer motion applications: The ability to propagate libraries and templates throughout the application is very important to minimize rework and promote the use of standards. The use of standards-based, pre-defined, tested functions saves significant time. Access to standard algorithms (like circular interpolation and 3D movement) frees you to focus on configuring your application rather than programming base motion functionality.
Tight integration of motion and automation: The reality of machine control is motion control and automation must work in tandem; nobody likes the task of integrating between dedicated control devices. Not only is manual integration time consuming, it is also prone to errors and maintenance challenges. Ideally, motion and automation engineering would take place in the same software with the same languages.
High-speed, deterministic performance: Motion applications tend to be very fast and therefore require real-time, high performance controllers. However, speed is nothing if it is not repeatable. For proper operation, the controller needs to execute your defined tasks the same way every time. This is determinism with minimum jitter. Real time indicates a process or action happens in a defined time. Determinism is where this response time is fixed and does not deviate significantly. Jitter is a measure of the deviation from the defined time. Optimally, you want fast, deterministic execution with very small jitter of all elements of the motion solution from controller to network to drive to the feedback mechanism.
Tools to optimize performance: Motion applications tend to be very fast. To optimize performance, tuning and trace tools are absolute requirements. These tools must look at the entire motion solution to perform accurate tuning. Beyond visualization, the tuning tools may provide wizards to guide you to the best dynamic parameters.
Open or embedded control
Improved integration to achieve higher performance and lower engineering costs: Frequently, the ideal application solution requires special specific hardware and custom software technology to tightly integrate with your controller. Using a traditional PLC architecture, this could involve costly engineering integration along with performance limiting communication interfaces. An open architecture makes it possible to embed the custom program directly with the automation control program. Additionally, you can achieve high-performance due to the ability to directly plug interface modules into the platform bus.
When an application is outside the capabilities of classical controller architecture, open architectures are essential. Connecting non-standard communication busses, PCI-bus based servo controllers, specialized vision applications, and applications written in non-PLC programming languages are not easy to do in a classical architecture. Automation platforms utilizing an open architecture allow the user to develop their own integration solutions that are a part of the controller execution.
Another example is when a machine produces a customized product and it must change often (possibly, for each product produced). For this, it is not realistic to rely on human intervention to change the parameters or to load a new recipe. In this instance, a direct connection to a database is preferred. In a traditional controller solution, this usually involves complex network communications, middleware applications, database interfaces, etc. A controller using an open architecture allows for direct connections to the database, possibly even on the same platform.
Leverage continuous platform innovation: We are all probably familiar with Moore's Law. Intel's Gordon Moore has proven to be a visionary in predicting the pace of innovation for computing. Computer processor speed directly affects controller program performance in an open architecture controller. Coupled with continuous performance improvements to the controller engine and its associated peripherals and the result is dramatic improvements in overall performance. A traditional controller cannot offer the ability to leverage this rapid innovation.
Availability of pre-engineered solutions, templates, and extensive libraries: Process users look to leverage pre-defined objects from libraries (i.e., cascade PID) rather than "rolling their own." This speeds up configuration and makes the resulting project more consistent and maintainable. It would be advantageous if you could also create your own library objects based on your unique control strategy.
Ability to select program execution speed: The normal reaction to controller speed is faster is better. However, for process applications regulatory control loops normally scan in the 100-to-500 millisecond range. In some cases, it could be detrimental to have control logic execute any faster-possibly causing excessive wear on final control elements such as valves, resulting in premature maintenance and process issues. It is important to have the ability to select program execution speed.
Emphasis on design using a top-down approach to engineering: Process users spend a lot of time on up-front design and overall program structure. This focus on upfront design minimizes costs, compresses project schedules, and creates applications that plant personnel can maintain over the long term. Since many process applications are large and plant-wide in scope, the ability to propagate libraries and templates throughout the application is very important to minimize rework and promote the use of standards.
Ability to function in a hostile environment: Many process applications are in hazardous (i.e., explosive) environments. Additionally, the environment may be moist or corrosive, potentially leading to damage of critical electronics. The automation platform and associated peripheral must withstand such environments without undue installation costs or complexity.
Safety systems in parallel
The use of a single controller for both standard and safety functionality: When your application requires both standard automation and machine safety, you can install parallel automation and safety systems. However, eliminating the need for two controllers saves significant cost and reduces complexity. The optimal solution is a single controller that handles both tasks. This opens new possibilities when designing architectures such as a shared controller, shared bus system, and shared I/O serving standard automation and the safety.
One engineering platform to program standard as well as safety logic: Learning engineering environments takes time you may not have. A controller that uses the same engineering tools for standard automation and machine safety configuration not only simplifies learning the tool, but also dramatically simplifies the integration of diagnostic information. The same diagnostic visualization you already use for your automation system can work for maintenance and operations to troubleshoot safety incidents.
Safety as part of a distributed architecture, even with existing systems: Implementing programmable safety on top of an established automation system is a challenge. Rather than putting in place a parallel, safety-only system, you should consider whether the safety controller could serve as a slave to the established automation system, handling the safety functions. Doing so has a significant impact on reducing integration time and effort.
Redundancy deep or deeper
Uninterrupted control of a process or machine: It may sound obvious, but redundancy implies a backup system, a safety net to protect your process from unexpected controller failures. Do you want controller redundancy only, or do you want to take redundancy to other levels? What about the power to the controller, or the networks you rely on, or the inputs and outputs connecting your process? Not only must you determine the depth of redundancy you require, but also you need to determine how your application behaves during switchover. Is a switchover time in seconds OK, or do you need milliseconds? The automation platform should be able to meet your requirements, regardless of your answers to the preceding questions.
Minimize engineering complexity: Implementing redundancy should be a simple task. If you take on the challenge of designing your own redundancy solution, you must make sure you have considered all modes of failure and how the system responds. You also need to consider keeping the primary and backup controllers synchronized.
A better approach may be to take advantage of redundancy from the supplier, whether embedded in the operating system of the controller or implemented in software (for less critical applications). The important thing is to let the automation platform take care of redundancy for you while you focus on solving your application problem.
Quickly resolve a failure: When you have a failure, your redundancy strategy will keep your process or machine running. However, how do you know a failure has occurred, and how do you fix the problem? This is where diagnostic notification comes into play pinpointing the problem. Once you locate the problem, it is very beneficial to be able to hot-swap the failed components. If the failed component happens to contain a program, is the program automatically loaded, or does this require manual action from you? All of this combined serves to minimize the time to repair a failure, getting you back to a redundant state.
Beyond hardware is software
With so much expected from a single controller platform, the engineering tools to support all the disciplines involved in a typical automation system become more important then the controller itself. A true multi-domain controller platform includes the multidisciplinary tools required to support traditional logic applications, as well as, motion control, process control, and all the other applications previously discussed.
A complete suite of engineering software gives you the tools to manage the entire engineering life cycle-from design right through ongoing maintenance. Capabilities you should look for include:
- An integrated engineering environment for logic, motion, process, and others
- A single point for system-wide engineering with a project view
- A modular structured approach where you can design your automation around your process
- Flexibility provided by support for the IEC 61131-3 languages
Today's best-of-class engineering software includes programming and configuration, advanced diagnostics, simulation, remote maintenance and plant documentation for the controller, and other system components like HMI and drives. It also allows for the development of interfaces to non-standard or industry-specific devices.
It is important to base a controller decision on the requirements of the entire automation system, no matter what kind of TLA (three-letter acronym) is on the label.
ABOUT THE AUTHORS
Mark Harned (firstname.lastname@example.org) is vice president of Control, Research and Development at Astec Industries in Chattanooga, Tenn. Bob Nelson (email@example.com) is a manager for Siemens Energy & Automation has over 20 years experience in the automation and control industry.