Hardware design best practices are not static
People in the automation industry understand that the pace of change and improvement on the software side of the business is relentless. The fact that the latest version of software issued yesterday may be obsolete in a week is just the nature of the industry. What many engineers and designers fail to recognize is that hardware design best practices also change, although at a slower pace than software.
Design best practices from 10 years ago may be in direct conflict with the manufacturer’s current recommended installation instructions. Engineers and designers are creatures of habit and like to reuse past design best practices on current projects. Unfortunately, technology changes, and best design practices must be reevaluated to make sure they still represent best practices based on the latest hardware versions and application instructions from the manufacturer.
For example, programmable logic controller (PLC) hardware technologies and form factors have changed due to material changes, the miniaturization of semi-conductors, and the surface mounting of board level electronic components. Think of the form factor difference between legacy input/output (I/O) platforms and the current versions. These form factor changes are driven by component level improvements in the semi-conductors used in these products.
In addition to the form factor changes, there are also significant differences in the logic voltage levels on these boards, which impact the heat dissipation of the I/O cards. In one case, it was a best practice to include air circulation fans within an enclosure to assure individual I/O cards did not overheat when the I/O card was used in difficult environmental conditions. Due to technology changes, best practice recommendations for internal panel fans have changed.
Another example of changing best practices due to technology updates is variable frequency drives (VFDs). Early VFDs utilized silicon controlled rectifier (SCR) or gate controlled thyristor (GCT) output driver technology. These semi-conductor technology families required that any drive power contactor be installed on the line power input to the drive. No contactors were to be installed in the output motor leads from an SCR-based VFD because if the contactors opened while the drive output was energized the final output semi-conductors would be destroyed due to their slow turnoff response time. Best practices at the time dictated the drive stop contactor was always installed on the line side of the drive.
The introduction of insulated gate bipolar transistor (IGBT) semi-conductor technology in VFDs turned this best practice upside down. Because of the speed at which an IGBT could shutdown current flow, opening a contact in the motor leads would no longer cause destruction of the final output semi-conductors in the VFD. Because of this, VFD manufacturers changed their recommendation to have the drive shutdown contactor installed in the motor leads not in the input section of the drive. Again, due to component level technology changes, best practice recommendations for VFD shutdown contactors changed.
The main takeaway here is that in the practice of automation design, the engineer or designer must always check past successful best practices against the manufacturer’s current best practice recommendations. Just because a design practice worked well in the past does not mean it will work again in the future with a slightly different product that may be functionally the same.
This post was written by David Paul, P.E. David is an engineering design manager at Maverick Technologies, a leading automation solutions provider offering industrial automation, strategic manufacturing, and enterprise integration services for the process industries. Maverick delivers expertise and consulting in a wide variety of areas including industrial automation controls, distributed control systems, manufacturing execution systems, operational strategy, business process optimization and more.
Maverick Technologies is a CSIA member as of 5/17/2016