This article describes a Programmable Logic Control (PLC) trainer constructed using common 19-inch rack components. The assembly shown in Figure 1 provides a functional, compact solution which can be modified to meet your needs. This assembly provides a natural progression from the smaller introductory trainer as described in this article.
The smaller trainer is ideal for the introductory labs exploring pushbuttons, relays, motor starters, safety relays, and small PLCs. The larger trainer may be used for lessons in panel construction, troubleshooting, and programming of larger PLCs.
Figure 1: Image of the PLC trainer ready for DIN rail components, HMI, and 22 mm switches and panel indicators.
Tech Tip: The height of 19-inch rack components is historically measured in 1.75-inch units. The plate pictured in Figure 1 is identified as an 8U with dimensions of 19 by 14 inches. The side-rail spacing is standardized to ensure that spacing and screws line up with all contemporary rack mount equipment.
Advantages of the 19-inch rack system
The 19-inch standard has been with us for approximately 100 years. It’s been used for telephone relays, military radios, computer servers, patch panels, and professional audio. There are several advantages:
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Readily available from multiple manufacturers.
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The cost is relatively low.
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Robust industrial components are easily incorporated. The featured project includes 22 mm switches and conventional DIN rail.
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Additional modules are easily added. As an example, a 3U panel could be added to host an HMI.
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The “built on a plate” construction is representative of most industrial control panels.
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Wide range of options are available ranging from the simple open-frame desktop (Figure 1) all the way to 42U (6.125 ft) enclosed units.
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Expandable with a variety of 19-inch rack mount accessories. For example, the 8U demonstrator could be installed in a larger rack complete with PC, monitor, and keyboard.
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The physical size is a good compromise between size and weight.
The rack system is highly desirable from an education perspective. With the convenient 8U desktop system, your students can learn assembly, wiring, and troubleshooting skills. Once they master these lessons, the panel(s) may be moved to a larger equipment rack to control a larger process.
Tech Tip: Which screw(s) should be removed last in a 19-inch rack system? It’s a simple question that considers the geometry and physics of the equipment. Always remove the lower screws last. This will hold the equipment in position as the lower screws hold against gravity that is pulling down on the equipment. This tech tip will be immediately apparent as you attempt to position heavy equipment.
Components used in the featured PLC trainer
The pre-assembled trainer is shown in Figure 2. It includes the following parts:
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Hammond HDR8UBK 8U open frame desktop rack
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Hammond PBPA19014BK 8U aluminum panel
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Essentra Components GR3150A grommets allow wires to pass through the panel with minimal abrasion risk. The featured trainer has three grommets located to the left just below the upper wire duct as shown in Figure 1.
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Omron PFP-100N DIN rail.
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Phoenix Contact 3240189 wire duct. Note: The blue wire duct is no longer manufactured. This part number is for the low-cost gray counterpart.
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A Hammond Manufacturing PBPA19007BK plate is installed as a shelf in the feet of the desktop frame. It provides a convenient location for a bench power supply (not shown). The added weight adds stability and prevents the unit from tipping over when a front-heavy payload is used.
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A Bud Industries H-9111-B handle is installed on back of the 8U plate. It provides a convenient way to carry the trainer. Note that the 10-32 screw heads are visible through the 22 mm holes in Figure 1.
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A custom laser-cut acrylic switch plate and HMI mount are shown in Figure 1. The closest commercially available product is the 4-position Phase-Dock 10175. An alternative is to cut holes directly into the plate and pass the wires through the grommets. The plate was an important aspect of the featured trainer, as it allows inspection of the 22 mm devices without physically moving the trainer.
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Hex standoffs are selected with sufficient clearance for the chosen 22 mm panel switches and indicators. An alternative is to mount the controls directly on the plate and pass the wires through the grommets.
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User supplied hardware, including 8-32 screws, washers, and nylock nuts are required. Be sure to size the hardware to protect the user again exposed threads.
Figure 2: Image of the PLC trainer prior to assembly.
Assembly of the PLC trainer
The assembly is highly dependent on your individual needs. Figure 3 presents an example showcasing the Schneider Electric TM221CE24T PLC and the associated TMH2GDB HMI. This is but one of many unique panel layouts.
The real assembly challenge involves visualizing how the components will fit together to make the final product. Like their production counterparts, your students must select a harmonious assemblage of components and then construct an orderly and compact system that is easy to assemble and troubleshoot.
As a time saving measure, you may want to preassemble a general-purpose plate suitable for a wide range of experiments. Figure 3 serves as a model for a general-purpose layout. To reduce cost, you may want to use a hole punch to mount the 22 mm controls and indicators directly on the aluminum plate. The wires will then pass through the grommets.
Assembly is easy once the layout has been determined. The wire duct and aluminum DIN rail are easily cut to length. As an electrical engineer, I’m challenged to take the necessary time to properly mark, center punch, and then drill the holes. This is yet another useful skill for future automation work.
Figure 3: Image of the PLC trainer loaded with a Schneider safety relay, Modicon PLC, HMI, and a variety of supporting components.
Tech Tip: I’m reminded of Brooks’ Mythical Man-Month and the famous quote, “plan to throw one away; you will, anyhow.” It’s an honest recognition of our limitations in clearly understanding the complex interaction until the finished unit is built. The rack system is forgiving, as new holes may be drilled to accommodate changing positions. You could leverage this situation by having your students design and construct the panel. The 8U plate then becomes a consumable that must be replaced every few years.
Parting thoughts
The 19-inch rack mount system offers a versatile platform for a PLC trainer. This system provides a beneficial yet controlled way to integrate advanced lessons into your curriculum. Be sure to review DigiKey’s complementary small PLC trainer. Start with the small trainer, and then move to a larger, adaptable system for advanced construction, layout, wiring, and programming lessons.
Please share your thoughts and questions in the space below. Educational tips and insights are especially welcome.
Best wishes,
APDahlen
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About this author
Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (interwoven). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET-accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching