PLC sensor operation

1. Sensor/InstrumentSpecificationDefinitions:Range –is the maximumandminimumvalue range over
whicha sensorworkswell.Oftensensorswill workwelloutsidethisrange,butrequire special or
additional calibration.e.g.salinitysensorsdeployedinsalinityof afew PPTin an estuarywhichisbelow
where the PSU scale isdefined(2PSU).However,generallyif youtryto operate asensoroutside its
range,it will notwork(give aconstantoutputat the max,significantlychange sensitivityorgive erratic
results) orbe damagede.g.a 130 m pressure sensordeployedat200 m depth.Accuracy – how well the
sensormeasuresthe environmentinanabsolute sense.Thatishow good the data iswhencompared
witha recognizedstandard.e.g.atemperature sensor accurate to0.001º C isexpectedtoagree within
0.001º C witha temperature standardsuchas a triple-point-of-watercell orthe temperature measured
by a PRT standardizedinrecognizedcalibrationstandardsorbyanothersensorwiththe same accuracy
calibratedproperly.Thisiswhatyouwantto compare resultswithotherobservations.Resolution –the
abilityof a sensortosee small differencesinreadings.e.g.atemperaturesensormayhave aresolution
of 0.000,01º C, butonlybe accurate to 0.001º C. That is youcan believe the sizeof relativesmall
changesintemperature,whichare smallerthanthe accuracyof the sensor.Resolutioninoften
controlledbythe quantizationindigitizingthe signal –e.g.one bitisequal to0.0005º C. Thisisnot a
functionof the sensor,butthe samplingprocess.Repeatability –Thisisthe abilityof a sensortorepeata
measurementwhenputbackinthe same environment.Itisoftendirectlyrelatedtoaccuracy,but a
sensorcan be inaccurate,yetbe repeatable inmakingobservations.Drift –Thisis the low frequency
change in a sensorwithtime.Itisoftenassociatedwithelectronicagingof componentsorreference
standardsinthe sensor.Driftgenerallydecreaseswiththe age of a sensorasthe componentparts
mature.A smoothlydriftingsensorcanbe correctedfordrift.e.g.SeaBird temperature sensorsthatare
driftingabout1 mºC/yr (andhave beensmoothlychangingforseveral years) allow one tocorrectfor the
driftand getmore accurate readings.Driftis alsocausedby biofoulingthatcan’tbe properlycorrected
for,but we oftentry.Hysteresis –A linearupand downinputtoa sensor,resultsinanoutputthat lags
the inpute.g.youget one curve on increasingpressure andanotherondecreasing.Manypressure
sensorshave thisproblem,forbetteronesitcanbe ignored.Itis oftenseeninaCTD whenthe pressure
readingondeckafter recoveryisdifferentfromthe readingbefore itisdeployed.Itisnota problem
withthe response time of the sensor, butisan inherentpropertyof some sensorsthatisundesirable.In
a CTD italsomay be a temperature sensitivityproblem.Stability –isanotherway of statingdrift.That is,
witha giveninputyoualwaysgetthe same output.Drift,shortand longterm stabilityare reallywaysof
expressingasensor’snoise asa functionof frequency.Sometimesthisisexpressedasguaranteed
accuracy over a certaintime period.Driftisoftenaproblemwithpressure sensorsunderhighpressure.
All sensorsdriftwithtime –hence the standardizationof PRTsintriple-point-of-waterandgalliummelt
cells.Response time–a simple estimateof the frequencyresponseof asensorassuminganexponential
behavior.We will discussinmore detailbelow.RefertoSeaBirdtemperature specificationsheets:Self
heating– to measure the resistance inthe thermistortomeasure temperature,we needtoputcurrent
throughit.Currentflowingthrougharesistorcausesdissipationof heatinthe thermistor,whichcauses
it to warmup, or self-heating.Thisisespeciallyimportantintemperature measurement.If the water
velocitychanges,the amountof advectivecoolingwill change,andthe temperature sensedwill change
as a functionof velocity –anemometereffect.Settlingtime –the time forthe sensortoreach a stable
outputonce it isturnedon. Therefore,if youare conservingpowerbyturningoff the sensorsbetween
measurements,youneedtoturnon the powerandwaita certaintime forthe sensorto reacha stable
output.Voltage required –the voltage supplyrange overwhichthe sensorhasa stable output.Toolow

2. a voltage andthe sensordoesn’tworkproperly,toohighandsomethingburnsout.Currentdrawn –
whatthe sensordrawsat each voltage.Generallythisvariesinverselywithvoltage (the sensordrawsa
nearlyconstantpower- current timesvoltage).Therefore,youneedtosupplyaknownvoltage and
current.If youdo thisfroma battery,beware thatthe voltage decreasesasthe batterydischarges,and
the current rises. Also,the amountof poweryoucanget out of a batterychangeswiththe amount of
currentdrawn.Therefore,whenyoucalculate powerrequirements,youneedtoknow the currentover
the full voltage range togetpowerand batteryrequirements.We willdiscussthisinmore detail laterin
a lab.Output– a voltage range e.g.0 to 5 voltsfor an inputrange of 0 to 30º C,or a frequency
modulatedsine wave,ora square wave of frequencyrange 6 to 12 kHz,etc. Fitof calibrationsto
equation –thermistorsare logdevices,sofitasan inverse polynomialsof logof sensoroutput
(frequency),whichchange fromKtoCelsiusbythe 273.15 SensorNoise Estimation: - Besidesdigitizing
noise,there isanotherlimitationinameasurementdue tothe inherentnoise in asensoritself.Today
we have the technologytoreduce the digitizingintervalordigitizingnoisetobelow the sensornoise,so
the limitingfactorina measurementisgenerallythe physicsof the sensoritself.Ideally,one wouldput
the sensorina noise free environmentandmeasure the spectraof the sensor’soutputtogetthe sensor
noise.However,thiswouldonlyworkif the sensor’snoise wasnotrelatedtosignal level,andif you
couldfinda noise free environment.
DynamicCalibrationsorFrequencyResponse:A sensorisa filterthataffectsthe frequencycontentof
the data that passesthroughit.It isup to the experimentalisttomake sure thatthese filteringeffects
are understoodanddonot affectthe resultsthathe istryingto obtain.To measure the frequency
response function,L(ω),of a sensor,one couldinputanimpulse,measure the response,transformitto
obtainthe frequencyresponsefunction.Animpulse isagoodinputsince itcontainsall frequencies.
Hysteresisof anInductive proximityswitch
The switchinghysteresisof an Inductive proximityswitch describesthe distance betweenthe turn-on
pointwhile approachinganobjectandthe turn-off pointduringthe separationof itfromthe sensor.The
hysteresisbringsaboutastable switchingsignal evenwhenthere are vibrations,temperature drift,or
electrical variations.The hysteresisisdefinedinEN60947-5-2 as a maximum20 % fromthe effective
operatingdistance,andcarriesavalue of typically10 % fromthe effective operatingdistance srforour
sensors.
Hysteresis
The switching hysteresis describes the distance between switching on and switch off. So if there was
a level sensor and you had a high level alarm configured, then the alarm may switch on at 90%, but
in order for the alarm to clear the level would need to fall below (for example) 85%.
Inductive Proximity Sensors
Principles of Operation for Inductive Proximity Sensors

3. Inductive proximity sensors are designed to operate by generating an electromagnetic field and
detecting the eddy current losses generated when ferrous and nonferrous metal target objects enter
the field. The sensor consists of a coil on a ferrite core, an oscillator, a trigger‐signal level detector
and an output circuit. As a metal object advances into the field, eddy currents are induced in the
target. The result is a loss of energy and a smaller amplitude of oscillation. The detector circuit then
recognizes a specific change in amplitude and generates a signal which will turn the solid‐state output
“ON" or “OFF."
A metal target approaching an inductive proximity sensor (above) absorbs energy generated by the
oscillator. When the target is in close range, the energy drain stops the oscillator and changes the
output state.
Standard Target for Inductive Proximity Sensors

4. The active face of an inductive proximity switch is the surface where a high‐frequency electro‐
magnetic field emerges.
A standard target is a mild steel square, one mm thick, with side lengths equal to the diameter of the
active face or three times the nominal switching distance, whichever is greater.
Hysteresis (Differential Travel)
The difference between the operate and the release points is called hysteresis or differential travel.
The amount of target travel required for release after operation must be accounted for when selecting
target and sensor locations. Hysteresis is needed to help prevent chattering (turning on and off rapidly)
when the sensor is subjected to shock and vibration or when the target is stationary at the nominal
sensing distance.
Vibration amplitudes must be smaller than the hysteresis band to avoid chatter.
Hysteresis of an Inductive proximity switch
The switching hysteresis of an Inductive proximity switch describes the distance between the turn-on
point while approaching an object and the turn-off point during the separation of it from the sensor.
The hysteresis brings about a stable switching signal even when there are vibrations, temperature
drift, or electrical variations. The hysteresis is defined in EN60947-5-2 as a maximum 20 % from the

5. effective operating distance, and carries a value of typically 10 % from the effective operating
distance sr for our sensors.
Capacitive Proximity Sensing
Capacitive sensing is a noncontact technology suitable for detecting metals, nonmetals, solids, and
liquids, although it is best suited for nonmetallic targets because of its characteristics and cost relative
to inductive proximity sensors. In most applications with metallic targets, inductive sensing is preferred
because it is both a reliable and a more affordable technology.
Capacitive sensor components
The sensor consists of four basic components:
 A capacitive probe or plate
 An oscillator
 A signal level detector
 A solid-state output switching device
 An adjustment potentiometer
Capacitive proximity sensors are similar in size, shape, and concept to inductive proximity sensors.
However, unlike inductive sensors which use induced magnetic fields to sense objects, capacitive
proximity generate an electrostatic field and reacts to changes in capacitance caused when a target
enters the electrostatic field. When the target is outside the electrostatic field, the oscillator is
inactive. As the target approaches, a capacitive coupling develops between the target and the
capacitive probe. When the capacitance reaches a specified threshold, the oscillator is activated,

6. triggering the output circuit to switch states between ON and OFF.
Capacitive proximity operation
The ability of the sensor to detect the target is determined by the target’s size, dielectric constant and
distance from the sensor. The larger the target’s size, the stronger the capacitive coupling between
the probe and the target. Materials with higher dielectric constants are easier to detect than those
with lower values. The shorter the distance between target and probe, the stronger the capacitive
coupling between the probe and the target.
Standard Target for Capacitive Proximity Sensors
As with inductive proximity sensors, the standard target for capacitive sensors is a square piece of mild
steel 1 mm (0.04 in.) thick with side lengths equal to the diameter of the active face or three times
the nominal switching distance, whichever is greater. The standard target is grounded per IEC test
standards; however, a target in a typical application does not need to be grounded to achieve reliable
sensing.
Dielectric Constants
Materials with higher dielectric constant values are easier to sense than those with lower values. For

7. example, water and air are dielectric extremes. A capacitive proximity sensor would be very sensitive
to water, with a dielectric constant of 80, which makes it ideal for applications such as liquid level
sensing. The same sensor, however, would not be sensitive to air, with a dielectric constant of 1. Other
target items would fall within the sensitivity range, such as wet wood, with a dielectric constant
between 10 and 30, and dry wood, between 2 and 6.
A partial listing of dielectric constants for some typical industrial materials follows. For more
information, refer to the CRC Handbook of Chemistry and Physics (CRC Press), the CRC Handbook of
Tables for Applied Engineering Science (CRC Press), or other applicable sources.
Dielectric Constants of Common Industrial Materials
Material Constant Material Constant
Acetone 19.5 Perspex 3.2…3.5
Acrylic Resin 2.7…4.5 Petroleum 2.0…2.2
Air 1.000264 Phenol Resin 4…12
Alcohol 25.8 Polyacetal 3.6…3.7
Ammonia 15…25 Polyamide 5.0
Aniline 6.9 Polyester Resin 2.8…8.1
Aqueous Solutions 50…80 Polyethylene 2.3
Bakelite 3.6 Polypropylene 2.0…2.3
Benzene 2.3 Polystyrene 3.0
Carbon Dioxide 1.000985 Polyvinyl Chloride Resin 2.8…3.1
Carbon Tetrachloride 2.2 Porcelain 4.4…7
Celluloid 3.0 Powdered Milk 3.5…4
Cement Powder 4.0 Press Board 2…5
Cereal 3…5 Quartz Glass 3.7
Chlorine Liquid 2.0 Rubber 2.5…35
Ebonite 2.7…2.9 Salt 6.0
Epoxy Resin 2.5…6 Sand 3-5
Ethanol 24 Shellac 2.5…4.7
Ethylene Glycol 38.7 Shell Lime 1.2
Fired Ash 1.5…1.7 Silicon Varnish 2.8…3.3
Flour 1.5…1.7 Soybean Oil 2.9…3.5

8. Freon R22 & 502 (liquid) 6.11 Steel
Gasoline 2.2 Styrene Resin 2.3…3.4
Glass 3.7…10 Sugar 3.0
Glycerine 47 Sulphur 3.4
Marble 8.0…8.5 Teflon 2.0
Melamine Resin 4.7…10.2 Toluene 2.3
Metal Transformer Oil 2.2
Mica 5.7…6.7 Turpentine Oil 2.2
Nitrobenzine 36 Urea Resin 5…8
Nylon 4…5 Vaseline 2.2…2.9
Oil Saturated Paper 4.0 Water 80
Paraffin 1.9…2.5 Wood, Dry 2…7
Paper 1.6…2.6 Wood, Wet 10…30
Materials with high dielectric constants may be sensed through the walls of containers made with
materials with lower dielectric constants. An example is the detection of alcohol or flour through a
glass wall. The alcohol would be detected through the glass while the flour would not.
Capacitive sensing through a tank
Each application should be tested. The list of dielectric constants is provided to help determine the
feasibility of an application. The values shown will vary depending on the size and density of the target
material.
Proximity Sensor Hysteresis

9. In the context of sensors and switches, the term 'hysteresis' refers to the difference between the 'operate'
and 'release' point.
Hysteresis can be demonstrated very easily using a snap-action micro switch. If you carefully and slowly
move the actuator on the switch until you feel it actuate, then reverse direction and slowly release the
actuator until the switch turns off, you'll notice that there's certain amount of travel between the two
points. This difference between the operate and release points is the switch hysteresis. In the case of a
micro switch, it is mechanically controlled, and intentionally built into the design. Hysteresis also exists in
proximity sensors, and it is also an intentional product of the sensor design.
Some hysteresis is necessary in most control systems - the classic example of which is the thermostat
controlling your home furnace. If you set the thermostat to 70°, the furnace might not turn on until the
temperature drops to 68°; and it will stay on until the temperaturereaches, maybe 72°. Without this built-in
hysteresis, the furnace would try to turn on as soon as the temperature dropped below 70°, and turn off
again as soon as 70° was reached. If it operated in such a manner, it would constantly be turning on and off,
wasting energy and requiring much more frequent maintenance.
Proximity sensors' hysteresis prevents them from rapidly turning on and off (chattering) if the target object is
right at the operating threshold point of the sensor. The amount of hysteresis varies with the distance
between the target and the sensor. There is more hysteresis at a greater distance from the sensor
(see Figure 1 below)

10. Proximity Sensor Hysteresis (Light Blue Shaded Area)
Hysteresis can cause application problems when a too-large proximity sensor is used to detect a small
hole. A proximity sensor's sensing field is shaped like a rounded cone - the sensing field becomes smaller in
diameter as the target-to-sensor distance increases. Some users attempt to take advantageof this fact by
trying to use a large sensor to detect a small hole simply by backing the sensor away from the target. The
theory being that the increased target-to-sensor distance will place the small diameter portion or the sensing
field into the hole, allowing it to be detected.
This method will work if the target approaches the sensor head-on, but if the target approaches the sensor
from the side (as it does in most stamping applications), the hole will not be detected by the sensor unless
the hole diameter is larger than both the sensing area as well as the extended area covered by the 'release'
portion of the hysteresis curve.

11. This problem can be avoided by selecting a sensor with a sensing field that closely matches (but is slightly
smaller than) the hole diameter, and installing the sensor as close to the target as possible.