A sensor is a device used to supplement, enhance, or replace the human ability to perceive things. Sometimes sensors are used to detect things that the human eye cannot, such as radio waves or very high frequency sound. Sometimes they are used to detect things in an area where there are no humans around, such as the surface of Mars or a warehouse after closing hours. Sometimes they are used to report measurements with a precision humans are incapable of, as in the case of a barometer or bubble level. Other times they report information to a computer or PLC to automate systems when they would be too complicated for a human to control, or require faster reaction time, or result in repetitive motions, or require constant monitoring.

Whatever the need, a sensor generally has a much more limited scope of operation than a human but compensates with enhanced reliability and precision. A photoeye, for example, will detect an object that passes in front of it at quite nearly the exact same position every time, within the limits of transistor switching time and computer processing power. However it would be unable to tell the difference between a metal plate which needs a hole punched through it and a human hand, which does not.

Therefore it is important to remember that sensors are dumb, and can be fooled easily. Sometimes they are used in combinations that reduce the potential for error; for example a photoeye and an inductive sensor can be used together to detect an object but make sure it is not made of metal. Other times the system will need to be set up to minimize problems that could fool a sensor; for example air bubbles crossing the path of a consistency meter will affect the reading because it provides an unexpected medium for the propagation of the microwaves.

Sensors can be generally divided into two groups, active and passive. An active sensor sends out a signal and generally receives a reflection from its target to detect it. A passive sensor, on the other hand, simply waits for a signal generated by the target itself. A radar gun used by traffic officers, for example, is an active sensor. It transmits a radar pulse and detects the reflected pulse off of its target. A radar detector, which some drivers use for early warning against radar guns, is a passive sensor. It simply waits for the radar gun's signal and warns the diver. Active sensors can detect a wider range of targets and perform a larger variety of functions, but their transmitted signals are detectable and in some cases, they may even affect the behavior of the target. Passive sensors, because they do not transmit anything, are unobtrusive.

Below are some of the sensors available today. An exhaustive list, or one that takes into account all the variations on the basic themes, would be impossible to provide, especially as new designs are being produced constantly as the technology to create them becomes more and more advanced. On the cutting edge is the vision system, which is an attempt to duplicate and surpass the capabilities of the human eye using a digital camera and a computer. One day it may make many of the others obsolete.

An accelerometer is any of several different styles of sensor, ranging from ring laser gyro to a simple pendulum, which detects acceleration or gravity, forces between which it is incapable of differentiating. One application is in cars, to trigger the airbag system in the event of a crash (essentially a very sudden deceleration).

Ammeters use a variety of methods to detect and measure the electric current passing through it. To measure the current in a circuit, the circuit must be broken so that the ammeter can be placed in series with it. Ideally, it will have a very small input resistance, to minimize its effect on the circuit it is measuring. One simple method is to detect the magnetic field created by the current passing through a small resistor in the ammeter. Another type of ammeter is the amp clamp, which operates similar to a current transformer and does not require the circuit to be broken to measure it.

An antenna is a thin, conductive rod which experiences a standing electric wave along its length due to the presence of electromagnetic waves at resonant frequency with it. They are most popularly used in radios to receive AM and FM news and music broadcasts.

A barometer consists of two sections, one open to the atmosphere and one not, sealed off from each other by the liquid, usually water or mercury, in the barometer. The empty space in the sealed section above the liquid level will be a vacuum, and the atmospheric pressure on the unsealed section will push the liquid level up into this vacuum. By monitoring how high into the vacuum the liquid is pushed, the atmospheric pressure can be measured. A special application of the barometer is the altimeter.

Bubble level
A bubble level is a tube almost filled with colored water, except for a small bubble of air which floats to the top of the tube if it is not held level. They are used to ensure an installation is straight, flat, or vertical.

Capacitive proximity sensor
A capacitive prox senses the dielectric strength of an object in front of it, and triggers an output if it is high enough. They can have trouble detecting objects with a dielectric constant similar to air.

A compass is a magnetized needle which is free to spin and orient itself with the magnetic field of the Earth. They are widely used for navigational purposes while hiking or otherwise traveling through unfamiliar territory. It is important to keep in mind that a compass will point to magnetic North, which is close to, but not exactly the same as, true North.

Consistency meter
Similar to a flowmeter, a consistency meter sends an electromagnetic or sonic pulse through a material in a pipe. The consistency meter uses the time of flight delay to determine the percentage of materials in the flow which affect the time of flight of the pulse, because the speed of propagation is dependent on the medium through which it is propagating. Pure water will slow down a microwave pulse more than wood fiber, for example, so a faster time of flight would indicate a higher percentage of wood fiber in the slurry.

Current transformer
Operating on a similar principle of magnetic induction used by a standard AC electrical transformer, a current transformer is a loop of wire around a larger electrical conductor which induces a current in the wire proportional to the current in the larger conductor. They are often used as ammeters in high current systems, such as power transmission and distribution systems.

An encoder is a sensor intended to be installed on a rotating piece of equipment, such as a hinge or a motor, which measures the rotational position of the device. They typically use a slotted wheel with a photosensor or a magnetically encoded wheel to count how far the encoder has turned. An encoder will usually have two sets of sensing marks which are slightly offset from each other, so it can tell which direction it is rotating. One use is in the older mechanical ball mouse pointers, in which the encoder tracks the movement of the ball to relay the pointer's position to the computer.

Float sensor
A float sensor is a floating device which triggers a mercury switch or limit switch as it rises with the level of a liquid. They are often used to turn on sump pumps when a low area is flooding, and also control the flush valve on a toilet.

A flowmeter is a device which measures the flow of a liquid or gas based on its effect on a sonic pulse or electric field created in the liquid. They are often used to control the flow of water, chemicals, or raw material slurry into an industrial process, such as paper manufacturing.

Geiger counter
A Geiger counter is a Geiger-Müller tube that briefly conducts electricity when a particle or photon of radiation briefly makes the gas conductive. They are used to detect ionizing radiation, including alpha particles, beta particles, and gamma radiation. They will not detect neutron radiation, however, as it does not ionize the gas in the tube.

A gyroscope is a spinning wheel which resists changes in its roll and yaw orientations due to the gyroscope effect, easily demonstrated with a bicycle tire. Gyroscopes are generally used as a stable reference point from which the orientation of other objects are measured.

Hall effect sensor
The Hall effect is a deflection of electric current to one side of a conductor due to the presence of a magnetic field. A Hall effect sensor detects this deflection as a voltage difference across the conductor in order to sense the presence of a nearby magnetic field.

Inductive proximity sensor
An inductive prox senses only metallic objects though magnetic induction.

Infrared sensor
An infrared sensor is usually a charged coupled device or a photoelectric sensor designed to detect electromagnetic radiation of a frequency below the visible light spectrum. They are useful for detecting heat and for transmitting information with light invisible to the human eye. Televisions use an infrared sensor to detect the infrared signals from the remote control.

Laser rangefinder
A laser rangefinder, also called LIDAR, operates similar to RADAR in that it measures the time of flight delay between transmitting a laser pulse and receiving the reflection to determine distance to the measured object. There are rangefinders for use in golf which use these. It can also incorporate doppler effect measurement to determine the speed of the target, which is the principle behind laser speed detectors used for traffic law enforcement.

Limit switch
Very similar in principle to an ordinary light switch, a limit switch is a mechanical lever arm, usually with a spring return, which opens or closes a set of electrical contacts.

Linear variable differential transformer
An LVDT is a transformer which changes the ratio of its input to its output due to the location of a moveable ferromagnetic core. They can be used to make very precise measurements of a distance between two objects, and were recently used to monitor the expanding cracks in Mount Rushmore.

Magnetoresistive sensor
Very similar in principle to the inductive sensor, magnetoresistance measures a change in a wire's resistance rather than inductance due to varying magnetic field strength. They are more sensitive and faster acting than inductive sensors and have replaced them in magnetic media digital storage devices such as hard drives. They are also used in metal detectors.

Mass flowmeter
A mass flowmeter is a scale placed under falling granular material which records the mass per unit time flow. They are used to load material such as cereal grains by weight rather than volume.

Mercury switch
A mercury switch is a tube filled with conductive liquid mercury which makes an electrical connection when oriented such that the mercury pools around electrodes protruding into the tube. They are often used as orientation sensors, for example in digital cameras to detect when they are held in portrait or landscape mode. Since mercury is a toxic heavy metal, mercury switches are only found on older devices. Other conductive liquids are used in modern devices.

A microphone is a device which uses a magnet sitting on a diaphragm to convert sound waves into electrical current by action of diaphragm moving the magnet through a loop of conductive wire. The reverse hookup, in which a magnet sitting on a diaphragm is oscillated by the electrical current to produce sound waves, is a speaker.

Moisture sensor
A moisture sensor measures some feature of a substance, such as capacitance or resistance, which is substantially affected by its moisture content. One application is in irrigation systems and sprinklers.

Motion detector
A motion detector will typically be a passive infrared sensor which responds to rapid changes in the amount of infrared radiation it receives, or the direction from which it is received. This allows it to detect movement of warm objects, such as people, animals, and vehicles, while ignoring stable or slow-changing sources of heat, including a person who is moving very slowly or standing still, or just the ambient temperature changing after sunset.

Ohmmeters use a variety of methods to detect and measure the resistance value of an item. It will generally do this by passing a small, known current through the item and measuring the resulting voltage drop. For this reason, it cannot be used to detect resistance in a circuit while the circuit is energized.

Oxygen sensor
A chemical reaction between oxygen and platinum can create a voltage difference detectable by a voltmeter which reports the level of oxygen in a system. One application is in automobile engines to make sure the fuel:air ratio is correct, which helps reduce pollution in the exhaust.

Photoelectric sensor
Also called a photoeye, a photoelectric sensor is an LED or laser which detects objects blocking or reflecting the light beam. They can have trouble detecting highly reflective or clear objects.

A photoresistor is a semiconductor device whose resistance decreases with exposure to increasingly intense light. These are the principle components in a light meter used for professional camera work.

Plumb bob
A plumb bob is a pointed weight at the end of a string. It is used to provide a vertical reference for an installation, such as a wall, to ensure it is straight. The point at the bottom of the weight can be oriented over a known spot on the ground to provide a reference for placing something directly above that spot.

Pressure sensor
One kind of pressure sensor has a flexible diaphragm, which deflects as pressure is applied to it. Sensing circuitry or a mechanical action measures the deflection of the diaphragm to report the applied pressure. Another kind of pressure sensor is simply a spring-loaded button which requires some minimum pressure to activate.

Proximity fuse
Like a simplified RADAR, a proximity fuse sends out an electromagnetic field which reflects off of a target. However, rather than measuring time of flight, it measures the strength of the reflected signal to determine when it is close enough to the target. Proximity fuses were developed during World War II for anti-aircraft guns.

A RADAR is a radio frequency transmitter which sends out a signal, which is reflected off some target, and is then received by a parabolic reflector. By measuring the time of flight delay the distance to the target can be determined, and by measuring doppler effect the speed of the target can also be determined. The most familiar application is in airport air traffic control towers, where they are used to keep track of the airplanes in the area. The doppler effect measurement is used in radar speed detectors for traffic law enforcement.

RADAR detector
A RADAR detector is an antenna tuned to receive transmitted signals commonly used in RADAR transmitters, such as those used by traffic law enforcement, airport control towers, and anti-aircraft missile launchers. A RADAR detector can typically detect a transmitted signal from farther away than the transmitter can detect the detector, since the transmitter must detect the reflection, which has to travel twice as far.

Smoke detector
A smoke detector uses a very small amount of the alpha emitter americium-241 to ionize air molecules. When smoke is present, its detection circuitry cannot detect these ionized air molecules and it sounds the alarm.

Similar to RADAR, SONAR measures the time of flight delay from omnidirectional pulses of sound traveling through water to calculate the distance to an object solid enough to reflect the sound. They are often used to detect submarines because radio waves do not travel well underwater.

Strain gauge
A strain gauge is a length of wire looped back and forth several times and bonded to something which will be deformed by an applied force. The deformation will affect the length of the wire bonded to it, magnified several times by the back and forth looping, changing the resistance of the wire. This change in resistance is measured to calculate the deformation and thus, applied force. They can be found in scales used to measure very heavy objects, such as trucks.

A thermistor is a resistor whose resistance increases significantly along with ambient temperature. They can be found in electronic thermometers such as remote units that report outside temperature to a station inside the house.

A thermocouple utilizes the Seebeck effect to create an electrical voltage from the temperature difference at the junctions of two dissimilar metals.

A thermometer consists of a liquid which expands with temperature in a graduated tube calibrated to relate the expansion of the liquid to its temperature. Since mercury is a toxic heavy metal, it is only found in older thermometers. Other liquids such as alcohol are used in modern thermometers.

Tilt sensor
A tilt sensor is a metal pendulum surrounded by a conductive ring which makes an electrical connection when the device is tilted far enough to swing the pendulum into contact with the ring. These are often found on pinball machines to prevent people from cheating.

Ultrasonic probe
An ultrasonic probe measures the time of flight delay from pulses — of a frequency higher than the human ear can detect — echoing off a target to calculate the distance to the target. Devices based on this principle, which can estimate lengths difficult to measure with a tape measure, are available at some hardware stores.

Vision system
A vision system consists of a digital camera which takes pictures of an object and a computer which processes the image. The computer then looks for patterns in the image to detect complex features of an object in a manner similar to human visual inspection.

Voltmeters use a variety of methods to detect and measure voltage across their probes, which are connected in parallel with the item they are to measure. Ideally, it will do this with a very large input resistance, to minimize its effect on the circuit it is measuring. One simple method is to detect the magnetic field created by the detected voltage creating a small current through a large resistor in the voltmeter.

Sen"sor (?), a.

Sensory; as, the sensor nerves.


© Webster 1913.

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