Proximity Probes
Proximity sensors are all around us. Most of us carry a device with several of them in our pocket, but few of us know how they work or have given them a second (or even first) thought.
Measuring distance electronically is far less practical than we would initially think. For something so easy for humans to do, judging distance by eye, machines, until recently, were very bad at it. Measuring distance without having physical contact with the object in question was quite a problem to solve and remains so if machine vision is not an option.
One solution to this conundrum is the proximity probe. LVDTs, string encoders, etc will all have their time in the spotlight, but for this week’s Transducer Thursday we’re looking at the humble prox-probe, and not only because they happen to be one of the most photogenic sensors we’ve managed to get our hands on.
Proximity probes work in several ways, all designed to measure the distance between the probe and an object without having to make contact with it. Typically four types are used as engineering sensors, magnetic reed switches, hall effect, inductive, and capacitive, whilst infrared optical sensors are often used in consumer products.
Each could have its own transducer Thursday (and perhaps they will) as each have quite ingenious and different ways of working. So how do we choose which one we use?
Let’s say we have a tidal turbine that we want to instrument. We want to know how the bearings and the main shaft are holding up to the huge loads on the main turbine. As the bearings degrade the shaft will start to vibrate, a measurable movement. Certain vibrations will be tolerable, large vibrations will begin to indicate the machine is close to causing itself permanent damage or catastrophic failure.
We don’t want to touch the shaft, which might influence the vibration and would mean adding a new bearing; the perfect tool is the proximity probe. If the shaft is made from steel, a ferromagnetic material, we might be tempted to use a magnetic proximity probe, they are extremely simple, sensitive, and cheap. These offer good resolution, in the tens of micrometers, appropriate to capture large, low-frequency vibrations which we would expect on the main shaft. However, this shaft is connected to a generator, and the changing magnetic field of the generator will cause the small magnet in the reed switch to vibrate, adding noise. On top of this, the reed switch is a moving part under cyclical load, which can lead to failure, not ideal if we want to monitor the bearings over a long time period. Hall effect sensors, whilst robust over extended periods still suffer from the issue with the nearby magnetic field.
Instead, we can utilise an inductive sensor. These sensors work based on electric fields, rather than the interaction of a magnetic and electric field, when the distance to the shaft changes, the inductive field changes which alters the sensor’s output. They are very hardy against magnetic fields, meaning they can be used near our generator. Even better, if our client wants to change the shaft to aluminium to save weight, the inductive sensor still performs. The only reason is that they are slightly more costly than Hall effect sensors and significantly more expensive than magnetic sensors.
However, what if our client wants to try an experimental composite drive shaft? Plastic is famously non-conductive and so doesn’t have much effect on electric fields.
Luckily we still have another horse in our stable. The capacitive proximity probe works by sensing changes to the dielectric field between two electrodes. When an object enters or moves within this field, it can detect the change as a change in capacitance. Whilst many materials do not affect an inductive sensor, such as plastics and ceramics, they do have dialectic properties, allowing detection by a capacitive proximity probe. A capacitive probe has its disadvantages, it’s sensitive to temperature and humidity, and we expect these to vary in a marine context. We can take steps to mitigate this, another option could be to add a metallic sleeve to the drive shaft, thin and just in the instrumented region, this brings inductive proximity probes back into the picture, which are more resistant to these changes.
Another option would be to attempt to compensate, by having a second capacitive proximity probe that is not detecting anything, variations in capacitance due to temperature and humidity should be the same for both probes meaning we can filter out the effects for the one we are using to study the drive shaft.
You can see, just in this example, the complexities of different probe types and how and why we might use each one effectively. But there is a type I’ve neglected and one that is utterly ubiquitous.
Optical sensors are built into many mobile phones, when you place the phone to your ear the presence of an object alters the response that a small infrared LED receives and turns off the touch screen so you don’t tap it accidentally with your face. Granted, it doesn’t always work.
But in every touchscreen phone, every touchscreen at all actually, is a proximity sensor, many sensors actually, woven together. Human fingers have specific dielectric properties that capacitive probes can detect with extreme accuracy. Using that effect, touchscreens can detect the slightest tap and yet largely ignore contact with other materials. The reason that water droplets sometimes wreak havoc on a touchscreen is that they can nearly match the dielectric pattern of a finger and generate loads of false touches.
Here at Flintmore, we have a strong affection for proximity probes, because they just work. Other sensors often require a lot of faff, but a prox-probe, robust, simple, with no moving parts, does its job every time.
Do you have a job requiring proximity probes? Flintmore can help with sensor selection, placement, and data collection and processing, we have the experience and expertise you need. Get in contact today.
Further Reading
RS’s Article on Proximity Probes is one of the best I’ve found for a quick overview, and get’s into a some more practical considerations on selecting an individual model of proximity probe, although it fails to cover the hall effect type despite RS selling several examples of such type. It does touch on the ultra-sonic type, which we don’t cover here.
https://uk.rs-online.com/web/content/discovery/ideas-and-advice/proximity-sensors-guide
I always recommend the wikipedia articles on transducers as they tend to provide good content. The articles for proximity sensing is no exception although it approaches the subject in a much broader sense before branching off into separate pages dealing with each sensor type rather than examining the main types in use in the instrumentation field as we and RS do:
