Global Positioning System

The Global Positioning System (GPS) is a global navigation satellite system (GNSS) developed by the United States Department of Defense and managed by the United States Air Force 50th Space Wing. It is the only fully functional GNSS in the world. It can be used freely by anyone, unless the system is technically restricted. These restrictions can be applied to specific regions by the U.S. Department of Defense. GPS can be used almost anywhere near the earth, and is often used by civilians for navigation purposes. An unobstructed line of sight to four satellites is required for non-degraded performance. GPS horizontal position fixes are typically accurate to about 15 meters (50 ft). GPS uses a constellation of between 24 and 32 medium Earth orbit satellites that transmit precise radiowave signals, which allow GPS receivers to determine their current location, the time, and their velocity. Its official name is NAVSTAR GPS.

Since it became fully operational on April 27, 1995, GPS has become a widely used aid to navigation worldwide, and a useful tool for map-making, land surveying, commerce, scientific uses, tracking and surveillance, and hobbies such as geocaching. Also, the precise time reference is used in many applications including the scientific study of earthquakes and as a required time synchronization method for cellular network protocols such as the IS-95 standard for CDMA.

Basic concept of GPS

A GPS receiver calculates its position by precisely timing the signals sent by the GPS satellites high above the Earth. Each satellite continually transmits messages which include

• the time the message was sent
• precise orbital information (the ephemeris)
• the general system health and rough orbits of all GPS satellites (the almanac).

The receiver measures the transit time of each message and computes the distance to each satellite. Geometric trilateration is used to combine these distances with the satellites' locations to obtain the position of the receiver. This position is then displayed, perhaps with a moving map display or latitude and longitude; elevation information may be included. Many GPS units also show derived information such as direction and speed, calculated from position changes.

Three satellites might seem enough to solve for position, since space has three dimensions. However, even a very small clock error multiplied by the very large speed of light—the speed at which satellite signals propagate—results in a large positional error. Therefore receivers use four or more satellites to solve for x, y, z, and t, which is used to correct the receiver's clock. The very accurately computed time is effectively hidden by most GPS applications, which use only the location. A few specialized GPS applications do however use the time; these include time transfer, traffic signal timing, and synchronization of cell phone base stations.

Although four satellites are required for normal operation, fewer apply in special cases. If one variable is already known, a receiver can determine its position using only three satellites. (For example, a ship or plane may have known elevation.) Some GPS receivers may use additional clues or assumptions (such as reusing the last known altitude, dead reckoning, inertial navigation, or including information from the vehicle computer) to give a degraded position when fewer than four satellites are visible (see , Chapters 7 and 8 of , and ).

Position calculation introduction

To introduce the operation of a GPS receiver, this section ignores measurement errors.

A GPS receiver can use the messages from a minimum of four visible satellites to determine

• the times the messages were sent
• the satellite positions corresponding to these times.

The x, y, and z components of position, and the time sent, are designated as where the subscript i is the satellite number and has the value 1, 2, 3, or 4. Knowing the indicated time the message was received , the GPS receiver can compute the indicated transit time, . of the message. Assuming the message traveled at the speed of light, c, the distance traveled, can be computed as .

A satellite's position and distance from the receiver define a spherical surface, centred on the satellite. The position of the receiver is somewhere on this surface. Thus with four satellites, the indicated position of the GPS receiver is at or near the intersection of the surfaces of four spheres. (In the ideal case of no errors, the GPS receiver would be at a precise intersection of the four surfaces.)

If the surfaces of two spheres intersect at more than one point, they intersect in a circle. The article trilateration shows this mathematically. A figure, Two Sphere Surfaces Intersecting in a Circle, is shown below.

Two sphere surfaces intersecting in a circle

The intersection of a third spherical surface with the first two will be its intersection with that circle; in most cases of practical interest, this means they intersect at two points. Another figure, Surface of Sphere Intersecting a Circle (not disk) at Two Points, illustrates the intersection. The two intersections are marked with dots. Again trilateration clearly shows this mathematically.

Surface of Sphere Intersecting a Circle (not disk) at Two Points

For automobiles and other near-earth-vehicles, the correct position of the GPS receiver is the intersection closest to the earth's surface. For space vehicles, the intersection farthest from Earth may be the correct one.

The correct position for the GPS receiver is also the intersection closest to the surface of the sphere corresponding to the fourth satellite.

Correcting a GPS receiver's clock

The method of calculating position for the case of no errors has been explained. One of the most significant error sources is the GPS receiver's clock. Because of the very large value of the speed of light, c, the estimated distances from the GPS receiver to the satellites, the pseudoranges, are very sensitive to errors in the GPS receiver clock. This suggests that an extremely accurate and expensive clock is required for the GPS receiver to work. On the other hand, manufacturers prefer to build inexpensive GPS receivers for mass markets. The solution for this dilemma is based on the way sphere surfaces intersect in the GPS problem.

It is likely that the surfaces of the three spheres intersect, since the circle of intersection of the first two spheres is normally quite large, and thus the third sphere surface is likely to intersect this large circle. It is very unlikely that the surface of the sphere corresponding to the fourth satellite will intersect either of the two points of intersection of the first three, since any clock error could cause it to miss intersecting a point. However, the distance from the valid estimate of GPS receiver position to the surface of the sphere corresponding to the fourth satellite can be used to compute a clock correction. Let denote the distance from the valid estimate of GPS receiver position to the fourth satellite and let denote the pseudorange of the fourth satellite. Let . Note that is the distance from the computed GPS receiver position to the surface of the sphere corresponding to the fourth satellite. Thus the quotient, , provides an estimate of

(correct time) - (time indicated by the receiver's on-board clock),

and the GPS receiver clock can be advanced if is positive or delayed if is negative.

Diagram depicting satellite 4, sphere, p4, r4, and da

Oxygen sensor

An oxygen sensor, or lambda sensor, is an electronic device that measures the proportion of oxygen (O₂) in the gas or liquid being analyzed. It was developed by Robert Bosch during the late 1960s under supervision by Dr. Günter Bauman. The original sensing element is made with a thimble-shaped zirconia ceramic coated on both the exhaust and reference sides with a thin layer of platinum and comes in both heated and unheated forms. The planar-style sensor entered the market in 1998 (also pioneered by Robert Bosch GmbH) and significantly reduced the mass of the ceramic sensing element as well as incorporating the heater within the ceramic structure. This resulted in a sensor that both started operating sooner and responded faster. The most common application is to measure the exhaust gas concentration of oxygen for internal combustion engines in automobiles and other vehicles. Divers also use a similar device to measure the partial pressure of oxygen in their breathing gas.

Scientists use oxygen sensors to measure respiration or production of oxygen and use a different approach. Oxygen sensors are used in oxygen analyzers which find a lot of use in medical applications such as anesthesia monitors, respirators and oxygen concentrators.

There are many different ways of measuring oxygen and these include technologies such as zirconia, electrochemical (also known as Galvanic), infrared, ultrasonic and very recently laser. Each method has its own advantages and disadvantages.

Automotive applications

A 3-wire oxygen sensor (spare part) suitable for use in a Volvo 240 or similar.

Automotive oxygen sensors, colloquially known as O₂ sensors, make modern electronic fuel injection and emission control possible. They help determine, in real time, if the air fuel ratio of a combustion engine is rich or lean. Since oxygen sensors are located in the exhaust stream, they do not directly measure the air or the fuel entering the engine. But when information from oxygen sensors is coupled with information from other sources, it can be used to indirectly determine the air-to-fuel ratio. Closed-loop feedback-controlled fuel injection varies the fuel injector output according to real-time sensor data rather than operating with a predetermined (open-loop) fuel map. In addition to enabling electronic fuel injection to work efficiently, this emissions control technique can reduce the amounts of both unburnt fuel and oxides of nitrogen from entering the atmosphere. Unburnt fuel is pollution in the form of air-borne hydrocarbons, while oxides of nitrogen (NO_x gases) are a result of combustion chamber tempuratures exceeding 1300 Kelvin due to excess air in the fuel mixture and contribute to smog and acid rain. Volvo was the first automobile manufacturer to employ this technology in the late 1970s, along with the 3-way catalyst used in the catalytic converter.

The sensor does not actually measure oxygen concentration, but rather the amount of oxygen needed to completely oxidize any remaining combustibles in the exhaust gas. Rich mixture causes an oxygen demand. This demand causes a voltage to build up, due to transportation of oxygen ions through the sensor layer. Lean mixture causes low voltage, since there is an oxygen excess.

Modern spark-ignited combustion engines use oxygen sensors and catalytic converters as part of an attempt by governments working with automakers to reduce exhaust emissions. Information on oxygen concentration is sent to the engine management computer or ECU, which adjusts the amount of fuel injected into the engine to compensate for excess air or excess fuel. The ECU attempts to maintain, on average, a certain air-fuel ratio by interpreting the information it gains from the oxygen sensor. The primary goal is a compromise between power, fuel economy, and emissions, and in most cases is achieved by an air-fuel-ratio close to stoichiometric. For spark-ignition engines (such as those that burn gasoline, as opposed to diesel), the three types of emissions modern systems are concerned with are: hydrocarbons (which are released when the fuel is not burnt completely, such as when misfiring or running rich), carbon monoxide (which is the result of running slightly rich) and NO_x (which dominate when the mixture is lean). Failure of these sensors, either through normal aging, the use of leaded fuels, or fuel contaminated with silicones or silicates, for example, can lead to damage of an automobile's catalytic converter and expensive repairs.

Tampering with or modifying the signal that the oxygen sensor sends to the engine computer can be detrimental to emissions control and can even damage the vehicle. When the engine is under low-load conditions (such as when accelerating very gently, or maintaining a constant speed), it is operating in "closed-loop mode." This refers to a feedback loop between the ECU and the oxygen sensor(s) in which the ECU adjusts the quantity of fuel and expects to see a resulting change in the response of the oxygen sensor. This loop forces the engine to operate both slightly lean and slightly rich on successive loops, as it attempts to maintain a mostly stoichiometric ratio on average. If modifications cause the engine to run moderately lean, there will be a slight increase in fuel economy, sometimes at the expense of increased NO_x emissions, much higher exhaust gas temperatures, and sometimes a slight increase in power that can quickly turn into misfires and a drastic loss of power, as well as potential engine damage, at ultra-lean air-to-fuel ratios. If modifications cause the engine to run rich, then there will be a slight increase in power to a point (after which the engine starts flooding from too much unburned fuel), but at the cost of decreased fuel economy, and an increase in unburned hydrocarbons in the exhaust which causes overheating of the catalytic converter. Prolonged operation at rich mixtures can cause catastrophic failure of the catalytic converter (see backfire). The ECU also controls the spark engine timing along with the fuel injector pulse width, so modifications which alter the engine to operate either too lean or too rich may result in inefficient fuel consumption whenever fuel is ignited too soon or too late in the combustion cycle.

When an internal combustion engine is under high load (e.g. wide open throttle), the output of the oxygen sensor is ignored, and the ECU automatically enriches the mixture to protect the engine, as misfires under load are much more likely to cause damage. This is referred to an engine running in 'open-loop mode'. Any changes in the sensor output will be ignored in this state. In many cars (excepting some turbocharged ones), inputs from the air flow meter are also ignored, as they might otherwise lower engine performance due to the mixture being too rich or too lean, and increase the risk of engine damage due to detonation if the mixture is too lean.

Carbon monoxide detector

A carbon monoxide detector or CO detector is a device that detects the presence of the carbon monoxide (CO) in order to prevent carbon monoxide poisoning. CO is a colorless and odorless compound produced by incomplete combustion that is lethal at high concentrations. If a high concentration of CO is detected, the device sounds an alarm, giving people in the area a chance to ventilate the area or safely leave the building.

CO detectors do not serve as smoke detectors and vice versa. However, dual smoke/CO detectors are also sold. Smoke detectors detect the smoke generated by flaming or smoldering fires, whereas CO detectors can alarm people about faulty fuel burning devices. In the home CO can be formed, for example, by open flames, space heaters, water heaters, blocked chimneys or running a car inside a garage.

Installation

The devices, which retail for $20-$60USD and are widely available, can either be battery-operated or AC powered (with or without a battery backup). Battery lifetimes have been increasing as the technology has developed and certain battery powered devices now advertise a battery lifetime of over 6 years. All CO detectors have "test" buttons like smoke detectors.

CO detectors can be placed near the ceiling or near the floor because CO is very close to the same density as air^.

Since CO is colorless, tasteless and odorless (unlike smoke from a fire), detection in a home environment is impossible without such a warning device. It is a highly toxic inhalent and attracts to the hemoglobin(in the blood stream) 200x faster than oxygen, producing inadequate amounts of oxygen traveling through the body. In North America, some state, provincial and municipal governments have statutes requiring installation of CO detectors in construction - among them, the U.S. states of Alaska, Connecticut, Florida, Georgia, Illinois, Maryland, Massachusetts, Minnesota, New Jersey, New York, Rhode Island, Texas, Vermont, Virginia, Wisconsin and West Virginia, the Canadian province of Ontario, and New York City.^[4]

When carbon monoxide detectors were introduced into the market, they had a limited lifespan of 2 years. However technology developments have increased this and many now advertise up to 7 years. Newer models are designed to signal a need to be replaced after that timespan although there are many instances of detectors operating far beyond this point.^{[citation needed]}

According to the 2005 edition of the carbon monoxide guidelines, NFPA 720 , published by the National Fire Protection Association, sections 5.1.1.1 and 5.1.1.2, all CO detectors “shall be centrally located outside of each separate sleeping area in the immediate vicinity of the bedrooms,” and each detector “shall be located on the wall, ceiling or other location as specified in the installation instructions that accompany the unit.”

Installation locations vary by manufacturer. Manufacturers’ recommendations differ to a certain degree based on research conducted with each one’s specific detector. Therefore, make sure to read the provided installation manual for each detector before installing.

CO detectors are available as stand-alone models or system-connected, monitored devices. System-connected detectors, which can be wired to either a security or fire panel, are monitored by a central station. In case the residence is empty, the residents are sleeping or occupants are already suffering from the effects of CO, the central station can be alerted to the high concentrations of CO gas and can send the proper authorities to investigate.

Sensors

Early designs were basically a white pad which would fade to a brownish or blackish colour if carbon monoxide were present. Such chemical detectors are cheap and widely available, but only give a visual warning of a problem. As carbon monoxide related deaths increased during the 1990s, audible alarms became standard.

The alarm points on carbon monoxide detectors are not a simple alarm level (as in smoke detectors) but are a concentration-time function. At lower concentrations (eg 100 parts per million) the detector will not sound an alarm for many tens of minutes. At 400 parts per million (PPM), the alarm will sound within a few minutes. This concentration-time function is intended to mimic the uptake of carbon monoxide in the body while also preventing false alarms due to relatively common sources of carbon monoxide such as cigarette smoke.

There are four types of sensors available and they vary in cost, accuracy and speed of response. All three types of sensor elements typically last up to 10 years. At least one CO detector is available which includes a battery and sensor in a replaceable module. Most CO detectors do not have replaceable sensors.

Opto-Chemical

The detector consists of a pad of a coloured chemical which changes colour upon reaction with carbon monoxide. They only provide a qualitative warning of the gas however.

Biomimetic

A biomimetic (chem-optical or gel cell) sensor works with a form of synthetic hemoglobin which darkens in the presence of CO, and lightens without it. This can either be seen directly or connected to a light sensor and alarm. Battery lifespan usually lasts 2-3 years. Device lasts on the average of about 10 years.

Electrochemical

A type of fuel cell that instead of being designed to produce power, is designed to produce a current that is precisely related to the amount of the target gas (in this case carbon monoxide) in the atmosphere. Measurement of the current gives a measure of the concentration of carbon monoxide in the atmosphere. Essentially the electrochemical cell consists of a container, 2 electrodes, connection wires and an electrolyte - typically sulphuric acid. Carbon monoxide is oxidised at one electrode to carbon dioxide whilst oxygen is consumed at the other electrode. For carbon monoxide detection, the electrochemical cell has advantages over other technologies in that it has a highly accurate and linear output to carbon monoxide concentration, requires minimal power as it is operated at room temperature, and has a long lifetime (typically commercial available cells now have lifetimes of 5 years or greater). Until recently, the cost of these cells and concerns about their long term reliability had limited uptake of this technology in the marketplace, although these concerns are now largely overcome.

Semiconductor

Thin wires of the semiconductor tin dioxide on an insulating ceramic base provide a sensor monitored by an integrated circuit. This sensing element needs to be heated to approximately 40 deg C in order to operate. Oxygen increases resistance of the tin dioxide, but carbon monoxide reduces resistance therefore by measurement of the resistance of the sensing element means a monitor can be made to trigger an alarm. The power demands of this sensor means that these devices can only be mains powered although a pulsed sensor is now available that has a limited lifetime (months) as a battery powered detector.

Device usually lasts on the average of 5-10 years.

Digital

Although all home detectors use an audible alarm signal as the primary indicator, some versions also offer a digital readout of the CO concentration, in parts per million. Typically, they can display both the current reading and a peak reading from memory of the highest level measured over a period of time. These advanced models cost somewhat more but are otherwise similar to the basic models.

The digital models offer the advantage of being able to observe levels that are below the alarm threshold, learn about levels that may have occurred during an absence, and assess the degree of hazard if the alarm sounds. They may also aid emergency responders in evaluating the level of past or ongoing exposure or danger.

Wireless

Wireless home safety solutions are available that link carbon monoxide detectors to vibrating pillow pads, strobes or a remote warning handset. This allows those with impediments such as hard of hearing, partially sighted, heavy sleepers or the infirm the precious minutes to wake up and get out in the event of carbon monoxide in their property.

Legislation

House builders in Colorado will be required to install carbon monoxide detectors in new homes in a bill signed into law in March 2009 by the state legislature.

House Bill 1091 requires installation of the detectors in new and resold homes near bedrooms as well as rented apartments and homes. It takes effect from July 1, 2009. The legislation was introduced after the death of Denver investment banker Parker Lofgren and his family. Lofgren, 39; his wife Caroline, 42; and their children, Owen, 10, and Sophie, 8, were found dead in a multimillion-dollar home near Aspen, colorado on Nov. 27, 2008, victims of carbon-monoxide poisoning.

Manufacturers

• System Sensor
• DuPont
• First Alert
• Kidde
• KWJ Engineering, Inc.
• Sprue Aegis
• Duomo (UK) Ltd.

Carbon dioxide sensor

A carbon dioxide sensor or CO₂ sensor is an instrument for the measurement of carbon dioxide gas. The most common principles for CO₂ sensors are infrared gas sensors (NDIR) and chemical gas sensors. Measuring carbon dioxide is important in monitoring Indoor air quality and many industrial processes.

Nondispersive Infrared (NDIR) CO₂ Sensors

NDIR sensors are spectroscopic sensors to detect CO₂ in a gaseous environment by its characteristic absorption. The key components are an infrared source, a light tube, an interference (wavelength) filter, and an infrared detector. The gas is pumped or diffuses into the light tube, and the electronics measures the absorption of the characteristic wavelength of light. NDIR sensors are most often used for measuring carbon dioxide. The best of these have sensitivities of 20-50 PPM. Typical NDIR sensors are still in the (US) $100 to $1000 range. Most are used for carbon dioxide, because no other sensing method works reliably for this gas. New developments include using MEMS to bring down the costs of this sensor and to create smaller devices (for example for use in air conditioning

Chemical CO₂ Sensors

Chemical CO₂ gas sensors with sensitive layers based on polymer- or heteropolysiloxane have the principal advantage of a very low energy consumption and can be reduced in size to fit into microelectronic-based systems. On the downside, short- and long term drift effects as well as a rather low overall lifetime are major obstacles when compared with the NDIR measurement principle^.

Applications

• Since the danger of fires comes mostly from its quickly spreading CO₂ output, these sensors can be used to detect fire and certain other air related problems safely without false alarm.
• For air conditioning applications these kind of sensors can be used to monitor the quality of air and the tailored need of fresh air, respectively.
• In applications where direct temperature measurement is not applicable NDIR sensors can be used. The sensors absorb ambient infrared radiation (IR) given off by a heated surface.

Throttle position sensor

A throttle position sensor (TPS) is a sensor used to monitor the position of the throttle in an internal combustion engine. The sensor is usually located on the butterfly spindle so that it can directly monitor the position of the throttle valve butterfly.

The sensor is usually a potentiometer, and therefore provides a variable resistance dependent upon the position of the valve (and hence throttle position).

The sensor signal is used by the engine control unit (ECU) as an input to its control system. The ignition timing and fuel injection timing (and potentially other parameters) are altered depending upon the position of the throttle, and also depending on the rate of change of that position. For example, in fuel injected engines, in order to avoid stalling, extra fuel may be injected if the throttle is opened rapidly (mimicking the accelerator pump of carburetor systems).

More advanced forms of the sensor are also used, for example an extra closed throttle position sensor (CTPS) may be employed to indicate that the throttle is completely closed.

Some ECUs also control the throttle position and if that is done the position sensor is utilised in a feedback loop to enable that control.

Related to the TPS are accelerator pedal sensors, which often include a wide open throttle (WOT) sensor. The accelerator pedal sensors are used in "drive by wire" systems, and the most common use of a wide open throttle sensor is for the kickdown function on automatic transmissions.

Modern day sensors are Non Contact type, wherein a Magnet and a Hall Sensor is used. In the potentiometric type sensors, two metal parts are in contact with each other, while the butterfly valve is turned from zero to WOT, there is a change in the resistance and this change in resistance is given as the input to the ECU.

Non Contact type TPS work on the principle of Hall Effect, wherein the magnet is the dynamic part which mounted on the butterfly valve spindle and the hall sensor is mounted with the body and is stationary. When the magnet mounted on the spindle which is rotated from zero to WOT, there is a change in the magnetic field for the hall sensor. The change in the magnetic field is sensed by the hall sensor and the hall voltage generated is given as the input to the ECU. Normally a two pole magnet is used for TPS and the magnet may be of Diametrical type or Ring type or segment type, however the magnet is defined to have a certain magnetic field.

Speedometer

A speedometer is a device that measures the instantaneous speed of a land vehicle.Now universally fitted to motor vehicles, they started to be available as options in the 1900s, and as standard equipment from about 1910 onwards.

Speedometers for other vehicles have specific names and use other means of sensing speed. For a boat, this is a pit log. For an aircraft, this is an airspeed indicator.

The speedometer was invented by the Croatian Josip Belušić in 1888, and was originally called a velocimeter.

Operation

Eddy current

The eddy-current speedometer has been used for over a century and is still in widespread use. Until the 1980s and the appearance of electronic speedometers it was the only type commonly used.

Originally patented by a German, Otto Schulze on 7 October 1902, it uses a rotating flexible cable usually driven by gearing linked to the tail shaft (output) of the vehicle's transmission. The early Volkswagen Beetle and many motorcycles, however, use a cable driven from a front wheel.

A small permanent magnet affixed to the rotating cable interacts with a small aluminum cup (called a speedcup) attached to the shaft of the pointer on the analogue instrument. As the magnet rotates near the cup, the changing magnetic field produces eddy currents in the cup, which themselves produce another magnetic field. The effect is that the magnet "drags" the cup, and thus the speedometer pointer, in the direction of its rotation with no mechanical connection between them.

The pointer shaft is held toward zero by a fine spring. The torque on the cup increases with the speed of rotation of the magnet (which, recall, is driven by the car's transmission.) Thus an increase in the speed of the car will twist the cup and speedometer pointer against the spring. When the torque due to the eddy currents in the cup equals that provided by the spring on the pointer shaft, the pointer will remain motionless and pointing to the appropriate number on the speedometer's dial.

The return spring is calibrated such that a given revolution speed of the cable corresponds to a specific speed indication on the speedometer. This calibration must take into account several factors, including ratios of the tailshaft gears that drive the flexible cable, the final drive ratio in the differential, and the diameter of the driven tires.

Electronic

Many modern speedometers are electronic. A rotation sensor, usually mounted on the rear of the transmission, delivers a series of electronic pulses whose frequency corresponds to the rotational speed of the driveshaft. The sensor is typically a toothed metal disk positioned between a coil and a magnetic field sensor. As the disk turns, the teeth pass between the two, each time producing a pulse in the sensor as they affect the strength of the magnetic field it is measuring.

A computer converts the pulses to a speed and displays this speed on an electronically-controlled, analog-style needle or a digital display. Pulse counts may also be used to increment the odometer.

Another early form of electronic speedometer relies upon the interaction between a precision watch mechanism and a mechanical pulsator driven by the car's wheel or transmission. The watch mechanism endeavors to push the speedometer pointer toward zero, while the vehicle-driven pulsator tries to push it toward infinity. The position of the speedometer pointer reflects the relative magnitudes of the outputs of the two mechanisms.

MAP sensor

A manifold absolute pressure sensor (MAP) is one of the sensors used in an internal combustion engine's electronic control system. Engines that use a MAP sensor are typically fuel injected. The manifold absolute pressure sensor provides instantaneous manifold pressure information to the engine's electronic control unit (ECU). The data are used to calculate air density and determine the engine's air mass flow rate, which in turn determines the required fuel metering for optimum combustion (see stoichiometry). A fuel-injected engine may alternately use a MAF (mass air flow) sensor to detect the intake airflow. A typical configuration employs one or the other, but not both.

MAP sensor data can be converted to air mass data using the speed-density method. Engine speed (RPM) and air temperature are also necessary to complete the speed-density calculation. The MAP sensor can also be used in OBD II (on-board diagnostics) applications to test the EGR (exhaust gas recirculation) valve for functionality, an application typical in OBD II equipped General Motors engines.

How the MAP value is used

The manifold absolute pressure measurement is used to meter fuel. The amount of fuel required is directly related to the mass of air entering the engine. The mass of air is proportional to the air density, which is proportional to the absolute pressure and inversely proportional to the absolute temperature. (See ideal gas law.) Engine speed determines the frequency, or rate, at which air mass is leaving the intake manifold and entering the cylinders.

(Engine Mass Airflow Rate) ≈ RPM × (Air Density)

or equivalently

(Engine Mass Airflow Rate) ≈ RPM × MAP / (absolute temperature)

Example

The following example assumes the same engine speed and air temperature.

• Condition 1:

An engine operating at WOT (wide open throttle) on top of a very high mountain has a MAP of about 15" Hg or 50 kPa (essentially equal to the barometer).

• Condition 2:

The same engine at sea level will achieve 15" Hg of MAP at less than WOT due to the higher barometric pressure.

The engine requires the same mass of fuel in both conditions because the mass of air entering the cylinders is the same.

If the throttle is opened all the way in condition 2, the manifold absolute pressure will increase from 15" Hg to nearly 30" Hg (~100 kPa), about equal to the local barometer, which in condition 2 is sea level. The higher absolute pressure in the intake manifold increases the air's density, and in turn more fuel can be burned resulting in higher output.

Anyone who has driven up a high mountain is familiar with the reduction in engine output as altitude increases.

Microwave chemistry sensor

Microwave chemistry sensor or Surface acoustic wave (SAW) sensors consist of an input transducer, a chemically adsorbent polymer film, and an output transducer on a peizoelectric substrate, which is typically quartz. The input transducer launches an acoustic wave that travels through the chemical film and is detected by the output transducer. The Sandia-made device runs at a very high frequency (approximately 525 MHz), and the velocity and attenuation of the signal are sensitive to the viscoelasticity and mass of the thin film . SAWS have been able to distinguish organophosphates, chlorinated hydrocarbons, ketones, alcohols, aromatic hydrocarbons, saturated hydrocarbons, and water . The SAW used in these tests have four channels—each channel consists of a transmitter and a receiver, separated by a small distance. Three of the four channels have a polymer deposited on the substrate between the transmitter and receiver. The purpose of the polymers is to adsorb chemicals of interest, with different polymers having different affinities to various chemicals. When a chemical is adsorbed, the mass of the polymer increases, causing a slight change in phase of the acoustic signal relative to the reference (fourth) channel, which does not contain a polymer. The SAW device also contains three Application Specific Integrated Circuit chips (ASICs), which contain the electronics to analyze the signals and provide a DC voltage signal proportional to the phase shift. The SAW device, containing the transducers and ASICs, is bonded to a piece of quartz glass, which is placed in a leadless chip carrier (LCC). Wire bonds connect the terminals of the leadless chip carrier to the SAW circuits.

Apllication

The Microwave chemistry sensor can detect several chemical materials:

• Lead
• Mercury
• Oxygen

Wheel Speed Sensor

A wheel speed sensor or vehicle speed sensor (VSS) is a type of tachometer. It is a sender device used for reading the speed of a vehicle's wheel rotation. It usually consists of a toothed ring and pickup.

Special purpose speed sensors

Road vehicles

Wheel speed sensors are used in anti-lock braking systems.

Rotary speed sensors for rail vehicles

Many of the subsystems in a rail vehicle, such as a locomotive or multiple unit, depend on a reliable and precise rotary speed signal, in some cases as a measure of the speed or changes in the speed. This applies in particular to traction control, but also to wheel slide protection, registration, train control, door control and so on. These tasks are performed by a number of rotary speed sensors that may be found in various parts of the vehicle.

In the past, sensors for this purpose often failed to function satisfactorily or were not reliable enough and gave rise to vehicle faults. This was particularly the case for the early mainly analogue sensors, but digital models were also affected.

This was mainly due to the extremely harsh operating conditions encountered in rail vehicles. The relevant standards specify detailed test criteria, but in practical operation the conditions encountered are often even more extreme (such as shock/vibration and especially electromagnetic compatibility (EMC)).

Rotary speed sensors for motors

Bearingless motor speed sensors

Although rail vehicles occasionally do use drives without sensors, most need a rotary speed sensor for their regulator system. The most common type is a two-channel sensor that scans a toothed wheel on the motor shaft or gearbox and therefore does not require a bearing of its own.

The target wheel can be provided especially for this purpose or may be already present in the drive system. Modern sensors of this type make use of the principle of magnetic field modulation and are suitable for ferromagnetic target wheels with a module between m =1 and m = 3.5 (D.P.=25 to D.P.=7). The form of the teeth is of secondary importance; target wheels with involute or rectangular toothing can be scanned. Depending on the diameter and teeth of the wheel it is possible to get between 60 and 300 pulses per revolution, which is sufficient for drives of lower and medium traction performance.

This type of sensor normally consists of two hall effect sensors, a rare earth magnet and appropriate evaluation electronics. The field of the magnet is modulated by the passing target teeth. This modulation is registered by the Hall sensors, converted by a comparator stage to a square wave signal and amplified in a driver stage.

Unfortunately, the Hall effect varies greatly with temperature. The sensors’ sensitivity and also the signal offset therefore depend not only on the air gap but also on the temperature. This also very much reduces the maximum permissible air gap between the sensor and the target wheel. At room temperature an air gap of 2 to 3 mm can be tolerated without difficulty for a typical target wheel of module m = 2, but in the required temperature range of from 40°C to 120°C the maximum gap for effective signal registration drops to 1.3 mm. Smaller pitch target wheels with module m = 1 are often used to get a higher time resolution or to make the construction more compact. In this case the maximum possible air gap is only 0.5 to 0.8 mm.

For the design engineer, the visible air gap that the sensor ends up with is primarily the result of the specific machine design, but is subject to whatever constraints are needed to register the rotary speed. If this means that the possible air gap has to lie within a very small range, then this will also restrict the mechanical tolerances of the motor housing and target wheels to prevent signal dropouts during operation. This means that in practice there may be problems, particularly with smaller pitched target wheels of module m = 1 and disadvantageous combinations of tolerances and extreme temperatures. From the point of view of the motor manufacturer, and even more so the operator, it is therefore better to look for speed sensors with a wider range of air gap.

The primary signal from a Hall sensor loses amplitude sharply as the air gap increases. For sensor manufacturers this means that they need to provide maximum possible compensation for the Hall signal’s physically induced offset drift. The conventional way of doing this is to measure the temperature at the sensor and use this information to compensate the offset, but this fails for two reasons: firstly because the drift does not vary linearly with the temperature, and secondly because not even the sign of the drift is the same for all sensors.

For a new sensor generation it was therefore necessary to find another way: an integrated signal processor now corrects the offset and amplitude of the Hall sensor signals. This correction is so effective that one can almost double the maximum permissible air gap at the speed sensor. On a module m = 1 target wheel these new sensors can tolerate an air gap of 1.4 mm, which is wider than that for conventional speed sensors on module m = 2 target wheels. On a module m = 2 target wheel the new speed sensors can tolerate gap of as much as 2.2 mm. It has also been possible to markedly increase the signal quality. Both the duty cycle and the phase displacement between the two channels is at least three times as stable in the face of fluctuating air gap and temperature drift.

In addition, in spite of the complex electronics it has also been possible to increase the MTBF for the new speed sensors by a factor of three to four. So they not only provide more precise signals, their signal availability is also significantly better.

These new sensors, still with the familiar appearance, thus open up whole new possibilities for the designers of drives for rolling stock. The sensors are attractively priced and operate without wear and tear.

Motor encoders with integrated bearings

There is a limit on the number of pulses achievable by sensors without integrated bearings: with a 300 mm diameter target wheel it is normally not possible to get beyond 300 pulses per revolution. But many locomotives and electric multiple units (EMUs) need higher numbers of pulses for proper operation of the traction converter, for instance when there are tight constraints on the traction regulator at low speeds. Such applications really need encoders with built-in bearings, which can tolerate an air gap many orders of magnitude smaller because of the greatly reduced play on the actual sensor as opposed to that of the motor bearing. This makes it possible to choose a much smaller pitch for the measuring scale, right down to module m = 0.22. There are a number of types of encoder with this property. One of them is used in large numbers in EMUs. It can be used to achieve values from less than 100 to more than 130 000 pulses per revolution. In railway applications however, the maximum possible pulses per revolution are not required.

For even greater robustness and signal accuracy a precision encoder can be used.

The functional principles of the two encoders are similar: a multichannel magneto-resistive sensor scans a target wheel with 256 teeth, generating sine and cosine signals. Arctangent interpolation is used to generate up to 512 rectangular pulses from each of the 256 signal periods per revolution. The precision encoder also possesses amplitude and offset correction functions that are housed in the external interpolation unit. This makes it possible to further improve the signal quality, which has a very positive effect on the traction regulator.

Speed sensors on the wheelset

Bearingless wheelset speed sensors

Bearingless speed sensors may be found in almost every wheelset of a rail vehicle. They are principally used for wheel slide protection and usually supplied by the manufacturer of the wheel slide protection system. These sensors require a sufficiently small air gap and need to be particularly reliable. One special feature of rotary speed sensors that are used for wheel slide protection is their integrated monitoring functions. Two-wire sensors with a current output of 7 mA/14 mA are used to detect broken cables. Other designs provide for an output voltage of around 7 V as soon as the signal frequency drops below 1 Hz. Another method used is to detect a 50 MHz output signal from the sensor when the power supply is periodically modulated at 50 MHz. It is also common for two-channel sensors to have electrically isolated channels.

Occasionally it is necessary to take off the wheel slide protection signal at the traction motor, and the output frequency is then often too high for the wheel slide protection electronics. For this application there is a speed sensor with an integrated frequency divider. There are now products available that are compliant with all the usual standards, with considerably improved technical properties and markedly longer useful lives.

Hall effect magnetic sensors

The Hall effectⁱ refers to the potential difference (voltage) on opposite sides of a thin sheet of conducting or semiconducting material in the form of a 'Hall bar' or a van der Pauw element through which an electric current is flowing, created by a magnetic field applied perpendicular to the Hall element. The ratio of the voltage created to the amount of current is known as the Hall resistanceⁱ, and is a characteristic of the material in the element. Dr. Edwin Hall discovered this effect in 1879.

The Hall effect comes about due to the nature of the current flow in the conductor. Current consists of many small charge-carrying "particles" (typically electrons) which see a force due to the magnetic field. Some of these charge elements end up forced to the sides of the conductors, where they create a pool of net charge. This is only notable in larger conductors where the separation between the two sides is large enough.

Photoresistor

A photoresistor or light dependent resistor or cadmium sulfide (CdS) cell is a resistor whose resistance decreases with increasing incident light intensity. It can also be referenced as a photoconductor.

A photoresistor is made of a high resistance semiconductor. If light falling on the device is of high enough frequency, photons absorbed by the semiconductor give bound electrons enough energy to jump into the conduction band. The resulting free electron (and its hole partner) conduct electricity, thereby lowering resistance.

A photoelectric device can be either intrinsic or extrinsic. An intrinsic semiconductor has its own charge carriers and is not an efficient semiconductor, e.g. silicon. In intrinsic devices the only available electrons are in the valence band, and hence the photon must have enough energy to excite the electron across the entire bandgap. Extrinsic devices have impurities, also called dopants, added whose ground state energy is closer to the conduction band; since the electrons do not have as far to jump, lower energy photons (i.e., longer wavelengths and lower frequencies) are sufficient to trigger the device. If a sample of silicon has some of its atoms replaced by phosphorus atoms (impurities), there will be extra electrons available for conduction. This is an example of an extrinsic semiconductor.

Applications

Photoresistors come in many different types. Inexpensive cadmium sulfide cells can be found in many consumer items such as camera light meters, street lights, clock radios, alarms, and outdoor clocks.

They are also used in some dynamic compressors together with a small incandescent lamp or light emitting diode to control gain reduction.

Lead sulfide and indium antimonide LDRs are used for the mid infrared spectral region. Ge:Cu photoconductors are among the best far-infrared detectors available, and are used for infrared astronomy and infrared spectroscopy.

Transducers are used for changing energy types.

Circuit symbol

Below is a symbol for a photoresistor as used in some circuit diagrams.

Microchip Technology

Microchip Technology (NASDAQ: MCHP) is an American manufacturer of microcontroller, memory and analog semiconductors. The company was founded in 1987 when General Instrument spun off its microelectronics division as a wholly owned subsidiary. Its products include microcontrollers (PICmicro, dsPIC / PIC24, PIC32), Serial EEPROM devices, Serial SRAM devices, KEELOQ devices, radio frequency (RF) devices, thermal, power and battery management analog devices, as well as linear, interface and mixed signal devices. Some of the interface devices include USB, ZigBee/MiWi, Controller Area Network, and Ethernet. In April 2009, Microchip Technology announced the nanoWatt XLP™ Microcontrollers (With World’s Lowest Sleep Current). This industry-leading combination of low power consumption and functionality makes these PIC MCUs ideal for any battery-powered or power-constrained application.

Corporate Headquarters is located at Chandler, Arizona with wafer fabs in Tempe, Arizona and Gresham, Oregon.

Among its chief competitors are Atmel, Infineon, Freescale, STMicroelectronics, Texas Instruments, Analog Devices and Maxim Integrated Products.

Linear Actuator

A linear actuator is a device that applies force in a linear manner, as opposed to rotationally like an electric motor. There are various methods of achieving this linear motion. Some actually convert rotational motion into linear motion. Several different examples are listed below.

Mechanical Actuators

Mechanical actuators typically convert rotary motion of a control knob or handle into linear displacement via screws and/or gears to which the knob or handle is attached. A jackscrew or car jack is a familiar mechanical actuator. Another family of actuators are based on the segmented spindle. Rotation of the jack handle is converted mechanically into the linear motion of the jack head. Mechanical actuators are also frequently used in the field of lasers and optics to manipulate the position of linear stages, rotary stages, mirror mounts, goniometers and other positioning instruments. For accurate and repeatable positioning, index marks may be used on control knobs. Some actuators even include an encoder and digital position readout.[1] These are similar to the adjustment knobs used on micrometers except that their purpose is position adjustment rather than position measurement.

Hydraulic actuators

Hydraulic actuators or hydraulic cylinders typically involve a hollow cylinder having a piston inserted in it. The two sides of the piston are alternately pressurized/de-pressurized to achieve controlled precise linear displacement of the piston and in turn the entity connected to the piston. The physical linear displacement is only along the axis of the piston/cylinder. This design is based on the principles of hydraulics. A familiar example of a manually operated hydraulic actuator is a hydraulic car jack. Typically though, the term "hydraulic actuator" refers to a device controlled by a hydraulic pump.

Piezoelectric actuators

The piezoelectric effect is a property of certain materials in which application of a voltage to the material causes it to expand. Very high voltages correspond to only tiny expansions. As a result, piezoelectric actuators can achieve extremely fine positioning resolution, but also have a very short range of motion. In addition, piezoelectric materials exhibit hysteresis which makes it difficult to control their expansion in a repeatable manner.

Biological sensors

All living organisms contain biological sensors with functions similar to those of the mechanical devices described. Most of these are specialized cells that are sensitive to:

• Light, motion, temperature, magnetic fields, gravity, humidity, vibration, pressure, electrical fields, sound, and other physical aspects of the external environment
• Physical aspects of the internal environment, such as stretch, motion of the organism, and position of appendages (proprioception)
• Environmental molecules, including toxins, nutrients, and pheromones
• Estimation of biomolecules interaction and some kinetics parameters
• Internal metabolic milieu, such as glucose level, oxygen level, or osmolality
• Internal signal molecules, such as hormones, neurotransmitters, and cytokines
• Differences between proteins of the organism itself and of the environment or alien creatures

Artificial sensors that mimic biological sensors by using a biological sensitive component, are called biosensors.

Hydrogen sensor

A hydrogen sensor is a gas detector that detects the presence of hydrogen. They contain micro-fabricated point-contact hydrogen sensors and are used to locate leaks. They are considered low-cost, compact, durable, and easy to maintain as compared to conventional gas detecting instruments.

Level sensor

Level sensors detect the level of substances that flow, including liquids, slurries, granular materials, and powders. All such substances flow to become essentially level in their containers (or other physical boundaries) because of gravity. The substance to be measured can be inside a container or can be in its natural form (e.g. a river or a lake). The level measurement can be either continuous or point values. Continuous level sensors measure level within a specified range and determine the exact amount of substance in a certain place, while point-level sensors only indicate whether the substance is above or below the sensing point. Generally the latter detect levels that are excessively high or low.

There are many physical and application variables that affect the selection of the optimal level monitoring method for industrial and commercial processes. The selection criteria include the physical: phase (liquid, solid or slurry), temperature, pressure or vacuum, chemistry, dielectric constant of medium, density (specific gravity) of medium, agitation, acoustical or electrical noise, vibration, mechanical shock, tank or bin size and shape. Also important are the application constraints: price, accuracy, appearance, response rate, ease of calibration or programming, physical size and mounting of the instrument, monitoring or control of continuous or discrete (point) levels.

This article discusses level sensing from the perspective of the phase of the material - solid, liquid, and slurry-type - and how their physical and electrical properties may affect the performance of the sensor.

Molecular sensor

A molecular sensor or chemosensor is a molecule that interacts with an analyte to produce a detectable change. Molecular sensors combine molecular recognition with some form of reporter so the presence of the guest can be observed.The term supramolecular analytical chemistry has recently been coined to describe the application of molecular sensors to analytical chemistry.

Early examples of molecular sensors are crown ethers with large affinity for sodium ions but not for potassium and forms of metal detection by so-called complexones which are traditional pH indicators retrofitted with molecular groups sensitive to metals. This receptor-spacer-reporter concept is a recurring theme often with the reporter displaying photoinduced electron transfer. One example is a sensor sensitive to heparin.

Geophysical MASINT

Geophysical MASINT is a branch of Measurement and Signature Intelligence (MASINT) that involves phenomena transmitted through the earth (ground, water, atmosphere) and manmade structures including emitted or reflected sounds, pressure waves, vibrations, and magnetic field or ionosphere disturbances.

According to the United States Department of Defense, MASINT is technically derived intelligence (excluding traditional imagery IMINT and signals intelligence SIGINT) that – when collected, processed, and analyzed by dedicated MASINT systems – results in intelligence that detects, tracks, identifies, or describes the signatures (distinctive characteristics) of fixed or dynamic target sources. MASINT was recognized as a formal intelligence discipline in 1986. Another way to describe MASINT is a "non-literal" discipline. It feeds on a target's unintended emissive by-products, the "trails" - the spectral, chemical or RF that an object leaves behind. These trails form distinct signatures, which can be exploited as reliable disriminators to characterize specific events or disclose hidden targets.

As with many branches of MASINT, specific techniques may overlap with the six major conceptual disciplines of MASINT defined by the Center for MASINT Studies and Research, which divides MASINT into Electro-optical, Nuclear, Geophysical, Radar, Materials, and Radiofrequency disciplines.

Thermostat

A thermostat is a device for regulating the temperature of a system so that the system's temperature is maintained near a desired setpoint temperature. The thermostat does this by controlling the flow of heat energy into or out of the system. That is, the thermostat switches heating or cooling devices on or off as needed to maintain the correct temperature.

A thermostat may be a control unit for a heating or cooling system or a component part of a heater or air conditioner. Thermostats can be constructed in many ways and may use a variety of sensors to measure the temperature. The output of the sensor then controls the heating or cooling apparatus.

Common sensors include:

• Bi-metallic mechanical or electrical sensors
• Expanding wax pellets
• Electronic thermistors and semiconductor devices
• Electrical thermocouples

A Honeywell electronic thermostat in a retail store

These may then control the heating or cooling apparatus using:

• Direct mechanical control
• Electrical signals
• Pneumatic signals

Variable reluctance sensor

A variable reluctance sensor (VRS) is used to measure position and speed of moving metal components. This sensor consists of a permanent magnet, a ferromagnetic pole piece, a pickup coil, and a rotating toothed wheel.

As the wheel rotates, the reluctance of the flux path through the coil changes, and the flux linkage through the coil changes, which results in a change in voltage that is measured by an external circuit.

Inductive sensor

An inductive sensor is an electronic proximity sensor, which detects metallic objects without touching them.

The sensor consists of an induction loop. Electric current generates a magnetic field, which collapses generating a current that falls asymptotically toward zero from its initial level when the input electricity ceases. The inductance of the loop changes according to the material inside it and since metals are much more effective inductors than other materials the presence of metal increases the current flowing through the loop. This change can be detected by sensing circuitry, which can signal to some other device whenever metal is detected.

Common applications of inductive sensors include metal detectors, traffic lights, car washes, and a host of automated industrial processes. Because the sensor does not require physical contact it is particularly useful for applications where access presents challenges or where dirt is prevalent. The sensing range is rarely greater than 6 cm, however, and it has no directionality.

Image Sensors

An image sensor is a device that converts an optical image to an electric signal. It is used mostly in digital cameras and other imaging devices. An image sensor is typically a charge-coupled device (CCD) or a complementary metal–oxide–semiconductor (CMOS) active-pixel sensor.

CCD vs CMOS

Today, most digital still cameras use either a CCD image sensor or a CMOS sensor. Both types of sensor accomplish the same task of capturing light and converting it into electrical signals.

A CCD is an analog device. When light strikes the chip it is held as a small electrical charge in each photo sensor. The charges are converted to voltage one pixel at a time as they are read from the chip. Additional circuitry in the camera converts the voltage into digital information.

A CMOS chip is a type of active pixel sensor made using the CMOS semiconductor process. Extra circuitry next to each photo sensor converts the light energy to a voltage. Additional circuitry on the chip may be included to convert the voltage to digital data.

Neither technology has a clear advantage in image quality. CMOS can potentially be implemented with fewer components, use less power and/or provide faster readout than CCDs. CCD is a more mature technology and is in most respects the equal of CMOS.

Industrial Sensors

Use

Sensors are used in everyday objects such as touch-sensitive elevator buttons and lamps which dim or brighten by touching the base. There are also innumerable applications for sensors of which most people are never aware. Applications include cars, machines, aerospace, medicine, manufacturing and robotics.

A sensor's sensitivity indicates how much the sensor's output changes when the measured quantity changes. For instance, if the mercury in a thermometer moves 1 cm when the temperature changes by 1 °C, the sensitivity is 1 cm/°C. Sensors that measure very small changes must have very high sensitivities. Sensors also have an impact on what they measure; for instance, a room temperature thermometer inserted into a hot cup of liquid cools the liquid while the liquid heats the thermometer. Sensors need to be designed to have a small effect on what is measured, making the sensor smaller often improves this and may introduce other advantages. Technological progress allows more and more sensors to be manufactured on a microscopic scale as microsensors using MEMS technology. In most cases, a microsensor reaches a significantly higher speed and sensitivity compared with macroscopic approaches.

Classification of measurement errors

A good sensor obeys the following rules:

• Is sensitive to the measured property
• Is insensitive to any other property
• Does not influence the measured property

Ideal sensors are designed to be linear. The output signal of such a sensor is linearly proportional to the value of the measured property. The sensitivity is then defined as the ratio between output signal and measured property. For example, if a sensor measures temperature and has a voltage output, the sensitivity is a constant with the unit [V/K]; this sensor is linear because the ratio is constant at all points of measurement.

Sensor deviations

If the sensor is not ideal, several types of deviations can be observed:

• The sensitivity may in practice differ from the value specified. This is called a sensitivity error, but the sensor is still linear.
• Since the range of the output signal is always limited, the output signal will eventually reach a minimum or maximum when the measured property exceeds the limits. The full scale range defines the maximum and minimum values of the measured property.
• If the output signal is not zero when the measured property is zero, the sensor has an offset or bias. This is defined as the output of the sensor at zero input.
• If the sensitivity is not constant over the range of the sensor, this is called nonlinearity. Usually this is defined by the amount the output differs from ideal behavior over the full range of the sensor, often noted as a percentage of the full range.
• If the deviation is caused by a rapid change of the measured property over time, there is a dynamic error. Often, this behaviour is described with a bode plot showing sensitivity error and phase shift as function of the frequency of a periodic input signal.
• If the output signal slowly changes independent of the measured property, this is defined as drift.
• Long term drift usually indicates a slow degradation of sensor properties over a long period of time.
• Noise is a random deviation of the signal that varies in time.
• Hysteresis is an error caused by when the measured property reverses direction, but there is some finite lag in time for the sensor to respond, creating a different offset error in one direction than in the other.
• If the sensor has a digital output, the output is essentially an approximation of the measured property. The approximation error is also called digitization error.
• If the signal is monitored digitally, limitation of the sampling frequency also can cause a dynamic error.
• The sensor may to some extent be sensitive to properties other than the property being measured. For example, most sensors are influenced by the temperature of their environment.

All these deviations can be classified as systematic errors or random errors. Systematic errors can sometimes be compensated for by means of some kind of calibration strategy. Noise is a random error that can be reduced by signal processing, such as filtering, usually at the expense of the dynamic behavior of the sensor.

Resolution

The resolution of a sensor is the smallest change it can detect in the quantity that it is measuring. Often in a digital display, the least significant digit will fluctuate, indicating that changes of that magnitude are only just resolved. The resolution is related to the precision with which the measurement is made. For example, a scanning tunneling probe (a fine tip near a surface collects an electron tunnelling current) can resolve atoms and molecules.