Top 5 Control Engineering stories, Feb. 2-9, 2014

Were you out last week? Miss something? Here are Control Engineering's five most clicked articles from last week, Feb. 2-9, 2014, including articles about PLCs and PACs; variable frequency drive selection; machine design; thermal PID loops, and PCs vs. PLCs.


1. PLC vs. PAC 

These technologies continue to evolve, making differences harder to distinguish. Here are some thoughts on what does what, and how to choose between a PLC and a PAC for your next application.

2. How to select a VFD 

These guidelines dispel the confusion about matching variable frequency drives (VFD) and motors to fans and pumps that are typically encountered in commercial building applications. While the motivation to increase energy efficiency could be financial (reduced energy costs) or ethical (reduce greenhouse gas emissions associated with power production), it is taken for granted that VFDs are an easy way to improve energy efficiency in a motor application. And with these noble intentions in mind, the engineer will specify a VFD for his client. Oftentimes, that isn't the end of the story for the engineer.

3. Do not guess and check your machine design 

Controls engineers are often brought in for the first time during the final steps of machine building to help tune everything. To achieve the desired performance, the machine must be designed so that mechanical resonances do not exist, or exist far above the desired control bandwidth. Earlier collaboration is better.

4. Tuning thermal PID loops 

When dealing with critical temperature applications, PID controllers are a common regulatory approach, but tuning these often requires a different strategy than other types of loops. Auto-tuning features can help if you understand them.

5. Inside Machines: PC versus PLC: Comparing control options 

To choose between a PLC or PC, analyze and compare characteristics that could differentiate the two technologies, such as operation, robustness, serviceability, hardware integration, security, safety, programming, and cost. Graphics illustrate some key considerations.

The list was developed using CFE Media's web analytics for stories viewed on Feb. 2-9, 2014.

- Jessica DuBois-Maahs, Jordan Schultz, associate content managers, CFE Media, and

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