Modern sensing for detection and protection

by Andy Oram
April, 2015

This article originally appeared on the International Manufacturing Technology Show site.

Let’s face it—industrial sites are hazardous. Nowadays you’re not likely to fall into a 2000-degree blast furnace or cut your hand off with a lathe; the dangers come more from chemicals or from accidents caused by too much heat or pressure.

Everyone knows the metaphor of a canary in a coal mine. When dangerous gases filter into the mine shaft, the canary would show signs of distress earlier than humans could detect the danger. This allowed the humans to escape in time. Modern sensors allow similar early warnings while eliminating cruelty to animals.

The variety of sensors is huge and exploits a wide range of sophisticated physical, chemical, and electromagnetic phenomena. This article just gives a feel for how they work, along with scattered examples, in order to remove some of the mystery. We’ll investigate each stage of sensing: what detects the presence of a chemical or other stimulus? How is this translated into a signal that a computer can understand and transmit? And how is the information incorporated into the work environment in a useful way?

Some gas sensors actually include an array of different sensors that react to gases in various ways, changing their shapes by different amounts depending on which gas is present. Although the gas could not be identified by any single sensor, the pattern of changes they make collectively is unique to one gas. Thus, the device’s software compares patterns in the incoming sensors to patterns of known gases, somewhat like an anti-virus software on computers compares patterns in mail attachments to patterns in known computer viruses.

One of the key engineering challenges in creating a functional sensor—aside from the science of sensors—is trapping the molecules that are being tested, and feeding them at a steady rate to the materials that sense them. Various meshes are often created from different materials to filter the air or other substance being tested.

Making the sensor convenient to deploy is another challenge. Sensors based on ceramics are rigid and have a noticeable, if small, weight. Various other substances are now being used, such as cellulose, which can create a material like paper that is light and flexible. The radically new kinds of porous materials made available by nanotechnology are leading to new generations of smaller and more sensitive sensors. For instance, one experimental system makes sensors at a highly reduced cost by adopting inkjet printing technology.

When a chemical changes a material in some way, the sensor has to translate the change into a signal that a digital device can receive. Any device that translates input from one form to another is called a transducer, and the ones used by chemical sensors produce electrical or light output.

One familiar, everyday type of transducer is a microphone, which picks up the changes in air pressure caused by your voice or a music performance and translates the signal into an electrical signal. The speakers on a stereo system, or the earbuds on a digital player, make the opposite translation from an electrical signal to air waves.

Some semiconductors react directly to the presence of some gases. Various impurities can be deliberately inserted in the semiconductors to make them sensitive to particular gases. This technology is the basis of metal oxide semiconductor sensors.

Changes in pressure, which may be caused not only by actual pressure but by twisting, temperature changes, or other subtle physical effects, can be translated into an electrical signal by the tendency of some conductive substances to change their electrical currents under pressure, a phenomenon known as a piezoelectric effect.

Sometimes a simple chemical reaction on a very tiny scale produces the desired result. For instance, a sensitive substance is painted on the sensors as a film. A particular gas that hits the film transfers some of its molecules—generally oxygen—to that film. These molecules in turn capture electrons and therefore create a charge that can be turned into a signal captured by an attached microchip.

Other sensors induce a tiny chemical reaction that increases the heat in the sensor, which in turn changes the characteristics of a wire. In this way, the presence of the chemical that caused a reaction can be turned into an electrical current and be recorded.

Sensors have to be considered in conjunction with the environment in which they are deployed. Thus, temperature has a strong effect on the ability of some sensors to detect the presence of a material.

It’s important to choose the right sensors, in terms of accuracy and cost, to effectively patrol a work environment. But none of this will be worth anything unless the sensors are integrated into an alerting or recording system that makes effective use of their information. This is where chemistry ends and user interface begins. How can your factory staff make effective use of sensor input?

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