Six tips for designing and building a small controls project
When designing and building a small controls project, aspects such as ordering and specifying parts, building the system, and programming should be considered to make the process as smooth as possible.
There are many requirements and skills required for a small project and sometimes it may be quite expensive and time-consuming to do a properly executed controls project. The skills often required to do even a small project such as computer-aided design (CAD), wiring, design, and programming are entirely different skill sets. Following the six steps highlighted below can help make the design and build process much easier.
1. Specifying parts
Describe the project on paper with as much detail as possible. Identify requirements—and potential future/expansion possibilities—in the document. Determine availability of utilities such as pneumatics, 480/120 V power, cubic feet per minute (CFM), and amperage. This should provide enough information to determine whether a programmable logic controller (PLC), individual discrete control components such as temperature controllers, timers, counters, or a human-machine interface (HMI) is needed. A 24 Vdc power supply may also be needed.
This is also a good time to create an input/output (I/O) list if specifying a PLC. This can be a temporary document, but starting a spreadsheet for the project at this point is a good idea. Handwritten notes should be kept in a folder or binder.
Catalogs and online resources are useful for selecting parts. A lot of detail goes into building a control system and there are a lot of small components such as terminal blocks, labels, jumpers, different gauges and colors of wire, etc. If this is to create a proposal rather than for the actual project, these small components can be estimated for cost and selection can wait until later. Use local vendors to help if you don’t understand the specifications.
2. Designing the system
This part requires some knowledge of electrical and mechanical design. Again, for a small project items can be sketched by hand to determine sizes, one-line diagrams can be created for determining electrical requirements. Graph paper can be useful for determining sizing of the enclosure. The Excel spreadsheet used in specifying the system can be modified as part of the documentation process. If possible, AutoCAD or an equivalent design software package should be used for electrical and mechanical drawings. Dimensions will be very important here and any work done in this preliminary step may end up being used in the final documentation.
3. Ordering parts
Procurement of parts may involve some shopping around. Of course price is important, but don’t spend too much time trying to save a few pennies. Lead time is also an important consideration. It is important to keep all documents and paperwork for received components. Some of this may end up in the project binder, and packing lists can be used to reference purchase orders and possible returns later. Open boxes carefully and keep all packing materials for the same reason; some vendors won’t take components back unless it is in the original packaging.
4. Building the system
There are a lot of skills and tools required for wiring, panel fabrication, and bracketry. Among these are panel layout (drilling, tapping, and "pinging" drill points), panel prep (cutting of din-rail, wireway, and component cutouts), wiring (ferrule crimping, wire stripping, and labeling), possible millwork and painting. There are also legal requirements that must be met for wire sizing, grounding, and cabling/conduit outside the enclosure (field wiring).
It is often not economical to purchase some of the tools and components needed for proper panel fabrication, especially if this is a "one-off" project. It is often best to use an outside panel shop or electrician for this step. Also, without experience in this field, the system may not look very good. If the system is going to a customer, they may cast judgment on how it looks.
5. Programming and software design
For systems involving a PLC or HMI, knowledge of the platform’s software is required here. This software can often be quite expensive also, this should be taken into account during the specification design phase. Will the customer have the software? Is there an additional yearly licensing fee involved? Is there local support for troubleshooting or modification, or can the customer do it themselves?
Also, just because someone has done some PLC maintenance work does not mean they are a programmer. Even with years of experience, there are a lot of considerations that should be taken into account for maintenance and operator usage. HMI design is a bit of an art, as is PLC programming. As always, documentation is also critical. There should be LOTS of diagnostics, messages and faults programmed into a properly designed and programmed system, downtime costs money.
6. Startup and debug
Most experienced programmers know not to worry too much about small mistakes during the programming and software phase, they will be discovered and fixed easily after the system is powered up. This is where the system gets fully tested before going into production. For larger systems, there is often a testing phase at the factory acceptance test, (FAT) and another at the customer’s location after it is installed for the site acceptance test (SAT). These events take up to two weeks each in some cases.
Frank Lamb is the founder of Automation Consulting LLC, and is a member of the Control Engineering Editorial Advisory Board. This article originally appeared on Automation Primer’s blog. Automation Primer is a CFE Media content partner. Edited by Chris Vavra, production editor, Control Engineering, CFE Media, firstname.lastname@example.org.