Automated Control Systems and Cost of Ownership
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This article summary discusses life cycle cost analysis (the full article, by William M. Goble, was published in Intech, Volume 41, Number 5, pg. 33):

Just How Expensive is Your Automated Control System?

“In these tough economic times and low margins, life cycle costing, the real way to choose the lowest cost system, can mean the difference between business success and failure. “

“To obtain the complete life cycle cost, all yearly costs are converted to present value. All future expenses are converted into their current value accounting for discount rate (interest). Initial costs and the present value of yearly costs are added to obtain total life cycle cost. Given a list of alternative systems, life cycle costs for each alternative can be calculated in a consistent manner. The best control system for a particular application is the one that provides the minimum life cycle cost.”

“High quality products that are easy to use and provide reliable operation will cost less over the product lifetime.”

“A typical automated control system is comprised of input, output, MPU, and rack/power supply modules. Low-cost to high-strength (higher purchase cost) modules are available with varying purchase costs, failure rates, and diagnostic coverage. From these options, several different control system architecture solutions are possible:

    Low-Cost Simplex Automated Control System Architecture:

    The cheapest purchase cost can be obtained by using low-cost modules without any form of redundancy (simplex).

    High-Strength Simplex Automated Control System Architecture:

    The next logical step in improving system MTTF (mean time to failure) is to use high-strength modules in a simplex system.

    High-Strength Dual MPU/PS Automated Control System Architecture:

    A third approach to improving system architecture is composed of redundant MPU modules and redundant power supplies.

    High-Strength Full Dual Automated Control System Architecture:

    In the fourth system architecture, fault tolerance is extended by providing full duplication of all modules.

    High-Strength THIN/Dual Automated Control System Architecture:

    In the fifth system, architecture diagnostic coverage of the input modules is improved by using triplicated input modules.

The life cycle cost optimization curve (Figure 1) illustrates the five system architectures with average failure times and initial purchase costs.  The cost of poor quality (low MTTF) is higher recurring and maintenance costs. As the quality improves, these costs decrease to a point.  As quality levels approach perfection, the costs incurred in achieving that perfection outweigh the savings.”

William M. Goble was manager of electronic systems development at Moore Products Co. He holds a BSEE from Penn State University and an MSEE from Villanova University. He has more than 35 years of experience in hardware design, software development, and engineering management and is author of the ISA book “Evaluating Control System Reliability – Techniques and Applications”.


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