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How Ultrasonic Sensors Work

Ultrasonic sensors are common parts of many automated factories and other robotic areas of endeavor, in part because they’re easy to understand and in part because they’re easy to use. All ultrasonic sensors have this much in common: they release a pulse of sound too high in pitch for human ears to hear, and they have a sensor in place that ‘hears’ the echo of the pulse after it has bounced off of any objects in the immediate environment. The distance between the sensor and the object is calculated based on the time it takes for the sonic pulse to return to the point of origin.

The sonic pulse is created by a part called a ‘transducer’ that converts electrical energy into mechanical energy, and then mechanical energy into sonic energy. There are two kinds of transducers: piezo, and electrostatic.

Piezo transducers involve a ceramic or crystalline material bonded to a metallic cone or case. The materials in piezo transducers are special insofar as they exhibit the unusual property of converting electricity into mechanical force or vice versa: a piezoelectric crystal will generate electricity if it is forced to bend, or it will bend when electricity is applied. It’s the latter principle used in the piezo transducer: electricity is applied in extremely rapid on-off cycles, causing the crystal (or, rarely, ceramic structure with the same property) to vibrate rapidly against the metallic cone or case, causing a sound. The faster the on/off cycle of the electricity, the higher in pitch the sound created. Obviously, in ultrasonic sensors, the cycle is extremely fast and the sound produced is beyond human hearing.

In an electrostatic transducer, two plates are fixed very near to each other. One plate is aluminum, and the other consists of two layers: an inner layer of a polyimide film called Kapton, and an outer layer of gold. Kapton is a strong, light, insulative plastic that is primarily there to take up space. When an electric charge is introduced to both the gold and aluminum, they become attracted to one another and slap together. Much like the piezo transducer, this means that the more rapidly you pulse the electricity going into the electrostatic transducer, the higher the soundform that comes out.

Using Inductive Proximity Sensors Even At Range

When you hear the term ‘proximity sensors’, you know one thing right away: whatever you’re sensing, it had better be proximate — as in, close. You Google ‘proximity sensors,’ and you’ll see vast scope of sensing ranges from two millimeters all the way up to…forty millimeters. It’s easy to assume, then, that the thing you’re sensing had better be no more than a few inches away from your proximity sensor. And you’re right…ish.

What might not be incredibly obvious at first brush is just how easy it is to bring the distant object to the sensor, in a manner of speaking. Or, more accurately, you can bring a proxy of the distant object to the sensor — turning it into a proximity proxy sensor.

Side note: before we continue, let’s note precisely what we’re talking about below: an inductive proximity sensor is a metal coil with electricity oscillating within the coil, causing an electromagnetic field. If a ferrometallic object moves through the field, it causes an electromagnetic feedback which forces the oscillation of the electricity to slow down, which triggers the sensor.

For example, let’s say you have a conveyor belt, and you want to know whenever a box passes through a certain location on the belt — but there’s no convenient place to put a proximity sensor near the belt itself. If, instead, you put a simple wheel on the edge of the belt where it will be turned by a box as it passes by — and then attach that wheel to a belt that turns another wheel several feet away — you can sense the rotation of that second wheel with a proximity sensor. Say, you put a metal rivet on the second (non-ferrous) wheel, and you have a inductive proximity sensor that pings every time the ferrous rivet on the second wheel passes by. Then each box will rotate the wheel which will cause one ping from the proximity sensor.

With the aid of a few simple machines, the inductive proximity sensor with a range measured in millimeters can be used several feet away from the actual item being detected. This kind of system is in use in industrial applications all around the world; the ‘proximity’ of the proximity sensor is much less limiting than one might first expect.

Know Your Magnetic Field Sensors: Reed, Hall, and Beyond

When you’re dealing with automated pneumatic machinery, having reliable sensors that detect the position of your pistons is an absolute necessity. The most common sensors for accomplishing that goal are magnetic field sensors that sense simple magnets implanted on the inside of the pneumatic pistons.

Magnetically actuated switches mounted on the sides of a cylinder will detect the magnets as they come within range. Generally the magnets are placed so that the field sensor activates at one or both extreme ends of the actuator’s stroke, though there are applications where an actuator may need to be stopped at any of several points along its route and this is easily accomplished by arranging several magnetic field sensors at different points along the stroke.

Reed Switches, the simplest kind of magnetic field sensor, contain two ferrous nickel-and-rion ‘reed’ elements (thin wafers) inside of a vacuum-filled glass tube. The reeds generally don’t touch, but when a strong enough magnetic field comes close, the reeds come together, closing the circuit and thus activating the switch. Reed switches are inexpensive, simple, and require close proximity to the activating magnet. They require no standby power, but they respond relatively slowly and they have a finite life expectancy.

Hall-Effect Sensors, on the other hand, are solid-state devices with no moving parts, and thus a much longer life expectancy. They require continuous power, as they’re electronic, but they activate (and deactivate) much more quickly than reed switches. A Hall-effect sensor is a thin wafer that, under normal conditions, allows electricity to flow directly across it. But when a magnet approaches, the field and the wafer interact in such a way that the electricity flowing into the wafer instead veers off to both sides, into different circuitry.

Giant Magneto-resistance Sensors, or GMR sensors, are actually quite tiny — they’re molecular in nature. In a GMR sensor, two layers of ferrous conductive material are laid out on either side of a thin layer of nonconductive material. As long no magnetic field exists, the ‘top’ conductive layer directs electricity in one direction, while the ‘bottom’ layer directs it in the opposite direction — both 90 degrees to the input. When even a weak magnetic field appears on-scene, however, the field causes the molecules in the ferrous layers to align with the field, causing both layers to ‘point’ 180 degrees away from the input, forming a straight line and thus activating the circuit. Because GMR sensors are literally a few dozen molecules thick, they can be placed almost anywhere. Because there’s no contact between moving parts, they have a lifetime measurable in centuries or more. And because they respond well to even weak magnetic fields, they are much more sensitive and rapidly-actuating then Hall-effect sensors.

The Quick-And-Dirty Guide To Troubleshooting Photoelectric Sensors

You have a photoelectric sensor of some type. It’s not working. What’s the quickest way to fix it? Start right here.

Identify the Sensor Type
There are three basic kinds of photoelectric sensor:

  • Through-beam sensors have an emitter and a receiver, and trigger whenever the beam between the two is interrupted. They offer the longest operational range.
  • Retro-reflective sensors have an emitter and receiver in a single unit, and require a reflector to be placed in such a way that the beam is bounced back into the unit. They’re the most common type of photoelectric sensor.
  • Diffuse sensors rely on the small percentage of light that reflects back into the sensor from a nearby object to trigger; they have the shortest detection range of all, but are also the least expensive and the easiest to install.

Identify the Problem
There are a few basic kinds of problem you can be troubleshooting. In short, is the sensor going off when there’s nothing to sense, or is it not going off when there is something to detect?

Clean the Apparatus
If it’s the first case, and the sensor is registering false positives, start by cleaning the entire sensor. Clean the beam output, the receiver, and if present, the reflector. A soft clean dry cloth and, if the sensor is visibly dirty, a non-abrasive, non-corrosive cleanser are the best tools. After cleaning the sensor parts thoroughly, test the sensor to see if it’s working.

Re-align the Parts
If they’re still not working, carefully re-align the entire system. This requires a string and two people (exception: a diffuse scanner works at such a small range that it should be visually obvious of it’s misaligned.) Have one person stand at one end of the arrangement and another stand at the reflector/receiver, and pull a string taut between the two. If the photo eyes are misaligned, line them up with the string, first in the left-right dimension and then in the up-down dimension. Once they are roughly aligned, proceed to make minor adjustments to the emitter only until the sensor is functioning properly.

Check the Inputs
The inputs for a photoelectric detector are electrical. Check the sensors’ data sheets and ensure that they are receiving the correct voltage, amperage, and AC or DC current. You will need a multimeter or other measuring tool to ensure that the correct amounts are making it all the way through the circuit to the emitter and receiver.

Contact the Dealer
If all else fails, contact the dealer who sold you the sensors — they can talk you through other more complex procedures that you can attempt including altering the gain adjustment and narrowing the beam.

Our feature product. Read more about Ace Gas…

Our feature product. Read more about Ace Gas Springs at www.acecontrols.com