Fundamentals of real-time processing in automation and control

Almost all automation and control systems are designed, developed and built using data processors, microprocessors, digital signal processors (DSPs) or any other processing device. Part 1 of a series of articles on real-time control systems.


This is the first part of a series of articles on real-time systems, real-time processing and fundamentals of real-time software design, applied to automation and control systems.

Figure 1: A simple system, Courtesy: Mario TorreToday, almost all automation and control systems are designed, developed and built using data processors, microprocessors, DSPs or any other processing device, which execute instructions derived or compiled from a software program. Just very few and specialized control devices still use plain hardware (or "programmable" hardware, as FPGA) to accomplish the job for which they were designed.

Nevertheless, to design and develop software applications for a controller or automation device, special skills and a good understanding of the automation and control problem are required. Many designers and programmers are around, building web servers, information systems, business processes management systems, and many other important and valuable infrastructures based on computing platforms. But when it comes to industrial process controllers, the designer recognizes that a new design and programming paradigm is required. This is well accepted throughout the automation and control industry.

Real-time processing

This special paradigm is closely related with many concepts involving signals acquisition, transducers, control set points and other aspects mostly related to process state variables, measurements and control. But mostly, such paradigm is closely connected with real-time processing. The concept of real-time processing is the main paramount that any engineer must take into account when a new automation and control system is designed, developed and deployed. This is, in other words, what differentiates automation software designers from designers of any other application software. It's not better or worse, easier or harder; it's just different.

Automation controllers are required to be real-time systems because they must control physical processes or plants that demand real-time control. At this point, the automation controller designer must have a very good idea on what a real-time system is. Hermann Kopetz, in his book, "Real Time Systems: Design Principles for Distributed Embedded Applications" (2011), stated that "A real-time computer system is a computer system where the correctness of the system behavior depends not only on the logical results of the computations, but also on the physical time when these results are produced. By system behavior we mean the sequence of outputs in time of a system." For any physical plant to be governed, it requires a controller capable of acquiring some input signals and producing some specific output signals within a very specific time frame. If the controller outputs occur outside such time frame, those outputs will no longer be valid, and will produce malfunctions or even a catastrophic failure in the physical plant.

Real-time explanation

How can we determine that a computer system is actually a real-time system? Figure 1 (above) can help answer this question. Suppose there is a very simple system, with just one input and one output. Such system generates an output signal every time it receives an input signal. Please note in Figure 1 that the output signal is produced t seconds after the input signal is introduced.

Now suppose that we repeatedly (but not periodically) apply the same input signal to this system. We may expect that this system will generate the same output signal every time, at exactly t seconds after the input signal is applied. Unfortunately, this is not what happens in actual controllers. The same output signal will be generated by the controller every time, but the time t that it takes the controller to produce each output may increase slightly (see Figure 2). If we repeat this experiment, we will find that the controller response times fall into a variation interval, say Δt. Some literature refers to Δt as "latency jitter."

The question is: Who determines this magnitude Δt? It strongly depends on how the controller hardware was designed, which components were used, and how the application software running inside it was designed and developed.

Figure 2: Inserting input signal repeatedly to system. Courtesy: Mario Torre

At this stage, something needs to be mentioned: the time magnitude t is not related to the time variation value, Δt. The former depends on the function that the controller is supposed to perform, while the latter depends on how the designer implemented such function inside the controller.

It is important to point out that no controller system can reach Δt = 0. Therefore, the controller designer must know the maximum that the physical plant would allow without causing any failures or damages, and consequently he/she must build the right real-time controller which takes into account such constraint.

Hard real time, soft real time 

Depending of the physical plant and the type of control to be performed, the controller may be classified as "hard real time" or "soft real time." If the specific characteristics of the plant or process to be controlled are such that a non-compliance of its Δt constraint will produce a malfunction or a failure, then the controller must be a "hard real-time" controller. If, on the other hand, the specific characteristics to be controlled are such that a non-compliance of its Δt constraint will generate a degradation of the plant functionality, but will not produce any malfunctions or failures, then a "soft real-time" controller may be used.

The value of Δt to be used as the main constraint for a process controller design depends heavily on the nature of the process that is to be controlled. Each physical process has its own "latency," that is, the mean time the process reacts from a change in one or several of the inputs. This latency is tied to the physical, chemical and electrical laws governing such process. For instance, the latency of an oil production process is very different from the latency of an electric power transmission system.

The table below shows latency times for some process types.

Table 1: Average Latencies. Courtesy: Mario Torre

Based on experience, a basic "rule of thumb" for automation controller design is that the controller's Δt must be at least 5 times smaller than the latency of the process such controller is intended to govern. Of course, this also depends of many other considerations, like power consumption, heat dissipation, available space and many other constraints, not discussed here.

Designing a hard real-time controller is a lot harder than building a soft real-time controller. Depending on the physical process and the specific application, the designer must chose to build one or the other.

Most controllers for industrial applications available in the market are soft real-time controllers. The good news about this is that almost all industrial automation and control applications may be governed by soft real-time controllers. Just in a few, very specific situations, a hard real-time controller is justified and installed. But, as long as a good controller is designed or chosen, in which its latency jitter Δt is small enough compared to the process latency, a reliable automation solution may be provided.

Next up: Reliability criteria

What are the criteria to adequately design a reliable controller? This interesting subject will be the content of the next articles of this series.

Mario Torre is a real-time systems design and architect specialist, focused on digital oil field automation, control and real time information systems. He is an Electronic Engineer with a master degree in Systems Engineering. Mario is currently a Consulting Advisor on Intelligent Operations Solutions (IOS) Global Group, in Halliburton, Landmark Software and Services in Houston, TX. Edited by Brittany Merchut, Project Manager, CFE Media,

See the Control Engineering PLC and PAC page.

Please post any questions or comments using the interface below.

- See part 2 of Fundamentals of real-time processing in automation and control below.

PAVEL , CA, United States, 07/02/14 11:22 AM:

Very interesting series of articles. Looking forward for the rest.
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2015 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
2015 Top Plant: Phoenix Contact, Middletown, Pa.; 2015 Best Practices: Automation, Electrical Safety, Electrical Systems, Pneumatics, Material Handling, Mechanical Systems
A cool solution: Collaboration, chemistry leads to foundry coat product development; See the 2015 Product of the Year Finalists
Raising the standard: What's new with NFPA 70E; A global view of manufacturing; Maintenance data; Fit bearings properly
Special report: U.S. natural gas; LNG transport technologies evolve to meet market demand; Understanding new methane regulations; Predictive maintenance for gas pipeline compressors
Cyber security cost-efficient for industrial control systems; Extracting full value from operational data; Managing cyber security risks
Drilling for Big Data: Managing the flow of information; Big data drilldown series: Challenge and opportunity; OT to IT: Creating a circle of improvement; Industry loses best workers, again
Migrating industrial networks; Tracking HMI advances; Making the right automation changes
Understanding transfer switch operation; Coordinating protective devices; Analyzing NEC 2014 changes; Cooling data centers
Upgrading secondary control systems; Keeping enclosures conditioned; Diagnostics increase equipment uptime; Mechatronics simplifies machine design

Annual Salary Survey

After almost a decade of uncertainty, the confidence of plant floor managers is soaring. Even with a number of challenges and while implementing new technologies, there is a renewed sense of optimism among plant managers about their business and their future.

The respondents to the 2014 Plant Engineering Salary Survey come from throughout the U.S. and serve a variety of industries, but they are uniform in their optimism about manufacturing. This year’s survey found 79% consider manufacturing a secure career. That’s up from 75% in 2013 and significantly higher than the 63% figure when Plant Engineering first started asking that question a decade ago.

Read more: 2014 Salary Survey: Confidence rises amid the challenges

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
This article collection contains several articles on the vital role that compressed air plays in manufacturing plants.