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What’s the right position sensor?

May 9, 2022
With supply chain issues and pandemic woes ongoing, control designs may need a component change

There are signs that our industry is starting to shake off the oppression of this long battle with the coronavirus. We are seeing more people show up for employment orientations, and, generally, the mood is lighter and, shall we say, hopeful. While the economy seems to be on an upsweep, the ability to execute automation projects, however, is definitely becoming more and more difficult.

I don’t think there were many of us that could even imagine that we would be facing the impact of a viral outbreak more than three years later. While the onset of the pandemic brought about delays in production due to people staying at home, the continuous curbing of normal behavior has extended far beyond the completely understandable desire to stay home and protect ourselves from an unpredictable virus.

Also read: How to choose the right presence sensor

The use of the word “normal” seems overused, but, in the normal world, the planning and execution of an automation project could be done with a reasonable expectation of getting the job done. We could stand back proudly and watch our machine contribute to the manufacturing process, help increase the bottom line. While the need to do more and produce more has never been more important, we can’t just pick out our usual components and combine them into our latest creation.

The “usual” components just aren’t available, and we need to spend a great deal of time just figuring out if we can find any components to meet our needs. For me, this has added over 30% more time to my design commitment, and I am constantly working with our purchasing team to figure out how to make this all come together.

Another place where we have been negatively impacted by the long lead times has been in general maintenance. Like any manufacturing entity, we have a constant effort that is put toward maintaining the machines and processes, and we laid down a good plan for how to upgrade older equipment as components start to outlive their life expectancies.

A function that was always a planned event is turning into a panicked response due to conditions that are currently out of our control. We are finding that we are having to borrow components from upcoming projects to serve the immediate needs of breakdowns for which we can’t get replacement parts.

Like most companies, we had a healthy stock of replacements in our inventory room, but we are no longer able to back fill the consumption of those stocked items. Much like the auto sales companies, we are having to look at pulling in used components in the hopes that they can carry us through until the automation providers can get their factories back in production.

Sadly, there doesn’t seem to be a light at the end of the tunnel yet. One only needs to tune into the nightly news to see that it isn’t just our controls industry that is suffering from the lack of components. In a highly technical world, the inability to produce computer chips is shutting down sectors of our manufacturing community.

In my sector of food production, our clients have had to jump through hoops to reformulate traditional recipes for many of our well-known, comfort foods because the component ingredients just aren’t available. Changing a formula isn’t just about using different ingredients. In the need for full-disclosure, packaging materials have to be reprinted to accurately declare the contents of the food product. That means existing supplies of packaging materials need to be pulled back from the manufacturing facilities, and new materials with new graphics need to be rushed to production.

In this environment of uncertainty, who knows how long the substitute ingredients will be necessary or if the original formulas can be used at all. If the original ingredients don’t come back in a timely manner, the original packaging will have to be scrapped, at a great loss to the company producing the product.

In this time-consuming environment, we still have to come up with controls solutions and make them happen. The silent killer in this entire process has been the dwindling inventory for all of our automation component providers.

In the beginning, the greatest impact was from the big-ticket items like variable-frequency drives and servo drives, processors and operation stations. In the past few months, this declining inventory has spread to pretty much every part of the controls platform.

At first, this was a boon to some of the smaller automation companies as they became the mecca of parts availability. When the big guys ran out of parts, these online companies were there to save the day. Sadly, the drawn-out experience that has been this pandemic is taking a toll on every source of automation components, and the supply lines are painfully slim.

I’ve been submerged in the search for components—any components—to complete some very important projects. One sidebar to this effort was exposure to component vendors that I don’t normally have on my list of favorites. That list contains suppliers that we have experience or familiarity with. It’s not really about good or bad products. Most vendors have a good product; they wouldn’t be in business if they didn’t. What they don’t necessarily have is exposure to the industry.

My own experience in this regard has really opened my eyes to options that I wouldn’t have previously considered. We tend to go with what we know. Our designs take on a standard look and feel and, normally—there is that word again—this would make sense.

Your drawings look familiar, and the skilled trades that put your panels and machines together can establish their own standard approaches to the job, based on your use of favorite components. The introduction of challenges with respect to getting the usual complement of components has generated a number of opportunities to re-evaluate how we go about making a controls design.

This applies to all kinds of automation, including position sensing. I have seen spurious errors in position sensing. I have also seen sensing lag that is the result of trying to detect something at speed. In any sensing application, the key goal is to get the current position of the object to the decision-making device—PLC/PAC—so that an accurate response can be made.

When a magnetic field is exposed to a magnetic material, there is a change in the dimension of the material until it reaches the limit, or saturation, of that magnetic material to change. This was first observed by a fellow named James Prescott Joule in 1842 and is called the Joule Effect. This English physicist, two years earlier in 1840, observed that the amount of heat released from a wire when a current is applied to it is equal to the resistance in the wire multiplied by the current squared. We would know that as P (power loss) = I2R or Joule’s Law.

Exposing a sensor that uses the generation of a magnetic field to cause a magnetic object to “grow” in length until it closes a switch is a common way to detect position. The Joule Effect is only one of the ways to use magnetostrictive properties to measure position.

The concept is to move a magnetic rod, imposing angular magnetic fields that can be used to determine the distance from the original position. Collectively, these would be the base principles in linear sensors.

Inductive sensors, on the other hand, measure the disturbance to a magnetic field when a conductive object, or target, is introduced. The known properties of an air core transformer and the application of Faraday’s Law allow us to accurately locate the position of the disturbance in the electrically induced magnetic field.

Faraday’s Law, you might recall, was established by wrapping two wires around opposite sides of an iron core and applying a current to one of the wires. A wave travels through the iron core and induces a voltage in the other wire.

Faraday’s Law has two components. There is a motional electro-motive force (EMF) generated by a magnetic force on a moving wire and the transformer EMF generated by an electric force due to a changing magnetic field.

Inductive position sensing tends to be more accurate because it simply measures a disturbance in the magnetic field, rather than the quantitative properties of a permanent magnet. The nature of a permanent magnet is non-linear, further adding to the inaccuracy of that method of position sensing. Additionally, this is especially important at higher temperatures where a permanent magnet’s properties change.

Any motion will produce heat and change the properties of a permanent magnet, making it less accurate than when the device is cold, or not in motion.

With the relatively unstable conditions of device sourcing that we face, may you always have the choice of the most appropriate sensor for your controls design.

About the author: Rick Rice
About the Author

Rick Rice | Contributing Editor

Rick Rice is a controls engineer at Crest Foods, a dry-foods manufacturing and packaging company in Ashton, Illinois. With more than 30 years’ experience in the field of automation, Rice has designed and programmed everything from automotive assembly, robots, palletizing and depalletizing equipment, conveyors and forming machines for the plastics industry but most of his career has focused on OEM in the packaging machinery industry with a focus on R&D for custom applications. 

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