Mechatronics: You can’t just say a system is reliable

Here's why understanding the physics of failure and having a systems approach are essential to modern engineering. As we become more dependent on complex mechatronic systems, it is insufficient to understand just how they work; we must also understand how they fail. Fault-tolerant system design has become ...

01/01/2011


“A man’s got to know his limitations.” This is one of the more memorable lines delivered by Clint Eastwood in the movie “Magnum Force” and it possesses great wisdom. But how does this relate to engineering system reliability?

What do we mean when we say that a person is reliable? Is it possible to say that a person is reliable all the time or just sometimes, in all circumstances or in just some circumstances? The same questions need to be applied to an engineering system design because reliability cannot be an after-thought.

As we become more dependent on complex mechatronic systems, it is not sufficient to understand just how they work; we must also understand how they fail. Fault-tolerant system design, not just fault-tolerant component or subsystem design, has become paramount. Reliability is the probability that an item performs a required function under stated conditions for a stated period of time. So an engineer needs to define the functions a system must perform, the boundary conditions under which the system will operate and the time duration during which reliability is required.

To better understand reliability, I spoke with Tim Kerrigan, fluid power consulting engineer at Milwaukee School of Engineering’s Fluid Power Institute, where he works to ensure industrial and government systems are designed for reliability.

A physics-of-failure approach to reliability is consistent with the model-based approach of modern mechatronic system design. It uses modeling and analysis to design reliability into a system, perform reliability assessments and focus reliability tests where they will be most effective. The approach involves understanding and modeling the potential failure mechanisms (e.g., fatigue, wear, and temperature), the failure sites and the failure modes (the activation of the failure mechanisms).   The failure modes of a mechatronic system include those of mechanical, electrical, computer and control subsystems, i.e., hardware and software failures. A physics-of-failure approach can improve reliability, reduce the time to field systems, reduce testing and costs, and increase customer satisfaction.

As mechatronic systems become more complex, the interactions among the subsystems — mechanical, electrical, computer and control — become more difficult to manage and the overall system reliability is impacted by this integration. Therefore, an assessment of overall system reliability must have an adequate margin for safety. A useful analogy here is the feedback control system. It provides great benefits, but feedback control systems can become unstable if there is an imbalance between strength of corrective action (gain) and system dynamic lags (phase lags). Model uncertainty is quantified by assuming that either gain changes or phase changes occur and the tolerances of gain or phase uncertainty are the stability margins, gain margin and phase margin. Real systems must have adequate stability margins. Real systems must also have adequate reliability margins.

Mechatronics can enhance the reliability and fault-tolerance of a system with prognostics, diagnostics and built-in test capabilities. The additional sensors and control elements must be very reliable and do add additional cost. But the long-term cost of unreliability is huge compared to the initial design cost of reliability. In addition, designing for reliability enhances energy efficiency and sustainability. Reliability and fault-tolerance is a competitive advantage in the commercial market and an absolute requirement in the health care, military and transportation sectors.

Kevin C. Craig, Ph.D., Robert C. Greenheck Chair in Engineering Design and Professor of Mechanical Engineering, College of Engineering, Marquette University.

Visit the Mechatronics Zone for the latest mechatronics news, trends, technologies and applications.



No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2013 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Leaders Under 40 program features outstanding young people who are making a difference in manufacturing. View the 2013 Leaders here.
The new control room: It's got all the bells and whistles - and alarms, too; Remote maintenance; Specifying VFDs
2014 forecast issue: To serve and to manufacture - Veterans will bring skill and discipline to the plant floor if we can find a way to get them there.
2013 Top Plant: Lincoln Electric Company, Cleveland, Ohio
Case Study Database

Case Study Database

Get more exposure for your case study by uploading it to the Plant Engineering case study database, where end-users can identify relevant solutions and explore what the experts are doing to effectively implement a variety of technology and productivity related projects.

These case studies provide examples of how knowledgeable solution providers have used technology, processes and people to create effective and successful implementations in real-world situations. Case studies can be completed by filling out a simple online form where you can outline the project title, abstract, and full story in 1500 words or less; upload photos, videos and a logo.

Click here to visit the Case Study Database and upload your case study.

Bring focus to PLC programming: 5 things to avoid in putting your system together; Managing the DCS upgrade; PLM upgrade: a step-by-step approach
Balancing the bagging triangle; PID tuning improves process efficiency; Standardizing control room HMIs
Commissioning electrical systems in mission critical facilities; Anticipating the Smart Grid; Mitigating arc flash hazards in medium-voltage switchgear; Comparing generator sizing software

Annual Salary Survey

Participate in the 2013 Salary Survey

In a year when manufacturing continued to lead the economic rebound, it makes sense that plant manager bonuses rebounded. Plant Engineering’s annual Salary Survey shows both wages and bonuses rose in 2012 after a retreat the year before.

Average salary across all job titles for plant floor management rose 3.5% to $95,446, and bonus compensation jumped to $15,162, a 4.2% increase from the 2010 level and double the 2011 total, which showed a sharp drop in bonus.

2012 Salary Survey Analysis

2012 Salary Survey Results

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.