proximity sensors 2922

Proximity Sensors

2–9

Principles of Operation for Inductive Proximity Sensors

Coil Oscillator Trigger Circuit

OutputCircuit

Inductive proximity sensors aredesigned to operate by generating anelectromagnetic field and detecting theeddy current losses generated whenferrous and nonferrous metal targetobjects enter the field. The sensorconsists of a coil on a ferrite core, anoscillator, a trigger-signal level detectorand an output circuit. As a metal objectadvances into the field, eddy currentsare induced in the target. The result is aloss of energy and a smaller amplitudeof oscillation. The detector circuit thenrecognizes a specific change inamplitude and generates a signal whichwill turn the solid-state output “ON” or“OFF.”

Target Position

Oscillator Response

Output Voltage

Operating LevelReleasing Level

On

OffOutput

Off

A metal target approaching aninductive proximity sensor (above)absorbs energy generated by itsoscillator. When the target is in closerange, the energy drain stops theoscillator and changes the outputstate.

Standard Target for Inductive Proximity Sensors

MotionDirection

Proximity Switch

Target

Active Face

1 mm

The active face of an inductiveproximity switch is the surface where ahigh-frequency electro-magnetic fieldemerges.

A standard target is a mild steel, 1mmthick, square form with side lengthsequal to the diameter of the active faceor 3X the nominal switching distance,whichever is greater.

Target Correction Factors for Inductive Proximity SensorsTo determine the sensing distance formaterials other than the standard mildsteel, a correction factor is used. Thecomposition of the target has a largeeffect on sensing distance of inductiveproximity sensors. If a targetconstructed from one of the materialslisted is used, multiply the nominalsensing distance by the correctionfactor listed in order to determine thenominal sensing distance for that target.Note that ferrous-selective sensors willnot detect brass, aluminum or copper,while nonferrous selective sensors willnot detect steel or ferrous-type stainlesssteels.

The correction factors listed below canbe used as a general guideline.Common materials and their specificcorrection factors are listed on eachproduct specification page

(Nominal Sensing Range) x (CorrectionFactor) = Sensing Range.

Correction Factors

Target MaterialApproximate

Correction Factor

Mild Steel 1.0

Stainless Steel 0.85

Brass 0.50

Aluminum 0.45

Copper 0.40

The size and shape of the target mayalso affect the sensing distance. Thefollowing should be used as a generalguideline when correcting for the sizeand shape of a target:

� Flat targets are preferable

� Rounded targets may reduce thesensing distance

� Nonferrous materials usually reducethe sensing distance for all-metalsensing models

� Targets smaller than the sensingface typically reduce the sensingdistance

� Targets larger than the sensing facemay increase the sensing distance

� Foils may increase the sensingdistance

Principles of Operation forCapacitive Proximity Sensors

Probe Oscillator RectifierFilter

OutputCircuit

Capacitive proximity sensors aredesigned to operate by generating anelectrostatic field and detectingchanges in this field caused when atarget approaches the sensing face.The sensor’s internal workings consistof a capacitive probe, an oscillator, asignal rectifier, a filter circuit and anoutput circuit.

In the absence of a target, the oscillatoris inactive. As a target approaches, itraises the capacitance of the probesystem. When the capacitance reachesa specified threshold, the oscillator isactivated which triggers the outputcircuit to change between “on” and “off.”

The capacitance of the probe system isdetermined by the target’s size,dielectric constant and distance fromthe probe. The larger the size anddielectric constant of a target, the moreit increases capacitance. The shorterthe distance between target and probe,the more the target increasescapacitance.

Standard Target and Groundingfor Capacitive Proximity SensorsThe standard target for capacitivesensors is the same as for inductiveproximity sensors. The target isgrounded per IEC test standards.However, a target in a typicalapplication does not need to begrounded to achieve reliable sensing.

Introduction

Proximity Sensors

2–10

Shielded vs. UnshieldedCapacitive SensorsShielded capacitive proximity sensorsare best suited for sensing low dielectricconstant (difficult to sense) materialsdue to their highly concentratedelectrostatic fields. This allows them todetect targets which unshieldedsensors cannot. However, this alsomakes them more susceptible to falsetriggers due to the accumulation of dirtor moisture on the sensor face.

The electrostatic field of an unshieldedsensor is less concentrated than that ofa shielded model. This makes them wellsuited for detecting high dielectricconstant (easy to sense) materials orfor differentiating between materialswith high and low constants. For theright target materials, unshieldedcapacitive proximity sensors havelonger sensing distances than shieldedversions.

Unshielded models are equipped with acompensation probe which allows thesensor to ignore mist, dust, smallamounts of dirt and fine droplets of oil orwater accumulating on the sensor. Thecompensation probe also makes thesensor resistant to variations in ambienthumidity. Unshielded models aretherefore a better choice for dustyand/or humid environments.

Unshielded capacitive sensors are alsomore suitable than shielded types foruse with plastic sensor wells, anaccessory designed for liquid levelapplications. The well is mountedthrough a hole in a tank and the sensoris slipped into the well’s receptacle. Thesensor detects the liquid in the tankthrough the wall of the sensor well. Thisallows the well to serve both as a plugfor the hole and a mount for the sensor.

Target Correction Factors forCapacitive Proximity SensorsFor a given target size, correctionfactors for capacitive sensors aredetermined by a property of the targetmaterial called the dielectric constant.Materials with higher dielectric constantvalues are easier to sense than thosewith lower values. A partial listing ofdielectric constants for some typicalindustrial materials follows. For moreinformation, refer to the CRC Handbookof Chemistry and Physics (CRC Press),the CRC Handbook of Tables forApplied Engineering Science (CRCPress), or other applicable sources.

Dielectric Constants of Common Industrial MatierialsAcetone 19.5Acrylic Resin 2.7–4.5Air 1.000264Alcohol 25.8Ammonia 15–25Aniline 6.9Aqueous Solutions 50–80Bakelite 3.6Benzene 2.3Carbon Dioxide 1.000985Carbon Tetrachloride 2.2Celluloid 3.0Cement Powder 4.0Cereal 3–5Chlorine Liquid 2.0Ebonite 2.7–2.9Epoxy Resin 2.5–6Ethanol 24Ethylene Glycol 38.7Fired Ash 1.5–1.7Flour 1.5–1.7Freon R22 & 502 (liquid) 6.11Gasoline 2.2Glass 3.7–10Glycerine 47Marble 8.0–8.5Melamine Resin 4.7–10.2Mica 5.7–6.7Nitrobenzine 36Nylon 4–5Oil Saturated Paper 4.0Paraffin 1.9–2.5Paper 1.6–2.6Perspex 3.2–3.5Petroleum 2.0–2.2Phenol Resin 4–12Polyacetal 3.6–3.7Polyamide 5.0Polyester Resin 2.8–8.1Polyethylene 2.3Polypropylene 2.0–2.3Polystyrene 3.0Polyvinyl Chloride Resin 2.8–3.1Porcelain 4.4–7Powdered Milk 3.5–4Press Board 2–5Quartz Glass 3.7Rubber 2.5–35Salt 6.0Sand 3–5Shellac 2.5–4.7Shell Lime 1.2Silicon Varnish 2.8–3.3Soybean Oil 2.9–3.5Styrene Resin 2.3–3.4Sugar 3.0Sulphur 3.4Teflon 2.0Toluene 2.3Transformer Oil 2.2Turpentine Oil 2.2Urea Resin 5–8Vaseline 2.2–2.9Water 80Wood, Dry 2–7Wood, Wet 10–30

Principles of Operation for Ultrasonic Proximity SensorsUltrasonic sensors detect objects byemitting bursts of high-frequency soundwaves which reflect or “echo” from atarget. These devices sense thedistance to the target by measuring thetime required for the echo to return anddividing that time value by the speed ofsound. This allows these devices todetect objects of any shape andmaterial that can sufficiently reflect anultrasonic pulse.

Analog models provide an outputvoltage proportional to the distancefrom the sensor face to the target, whiledigital/discrete output models changeoutput state when this distance crossesa pre-set threshold.

Because ultrasonic sensors depend ona reflected sound wave for properoperation, the correction factors andtarget requirements used for inductiveproximity sensors do not apply. Referto the Bulletin 873C product pages inthis catalog for target considerations.

Hysteresis (Differential Travel)The difference between the operate andthe release points is called hysteresis ordifferential travel. The amount of targettravel required for release afteroperation must be accounted for whenselecting target and sensor locations.Hysteresis is needed to help preventchattering (turning on and off rapidly)when the sensor is subjected to shockand vibration or when the target isstationary at the nominal sensingdistance.

Vibration amplitudes must be smallerthan the hysteresis band to avoidchatter.

Hysteresis

Motion Direction

ProximitySwitch

Switch PointWhen Leaving

Switch PointWhen

ApproachingOperatingDistance

Target

Introduction

Proximity Sensors

2–11

Switching FrequencyThe switching frequency is themaximum speed at which a sensor willdeliver discrete individual pulses as thetarget enters and leaves the sensingfield. This value is always dependent ontarget size, distance from sensing face,speed of target and switch type. Thisindicates the maximum possiblenumber of switching operations persecond. The measuring method fordetermining switching frequency withstandard targets is specified by DINEN50010.

ÉÉÉÉÉÉ

ÉÉÉÉÉÉ

ÉÉÉÉÉÉÉÉÉ

MotionDirection

Proximity Switch

Targets of Mild Steel

m = d

m2 x m

m

Sn2

d

Non�magnetic and non�conducting material

RippleRipple is the alternating voltagesuperimposed on the DC voltage (peakto peak) in %.

For the operation of DC voltageswitches, a filtered DC voltage with aripple of 10% maximum is required(according to DIN 41755).

U

t

Ud

Uuss

Mounting Considerations for Weld Field Immune ProximitiesReliable operation is dependent on thestrength of the magnetic field and thedistance between the current line andthe sensor.

Perpendicular Mountingto the Current Line

ÉÉÉÉ

Current Line

Magnetic Field

Sensor

r

Parallel Mountingto the Current Line

ÉÉÉÉ

Sensor

Current Line

r

Use the following chart or formulas todetermine the spacing requirementsbetween the current line and proximitysensor. Select a distance that fallswithin the safe zone.

� H = I2� r

� B = H0.796

� Gauss = 10* B

where:

I = welding current (in kA),

H = field strength (in kA/m),

B = flux (in mT), and

r = distance between sensor andcurrent carrying lines (in meters).

0.50in 1.00in 1.50in 2.00in 2.50in

40kA

Weld Field Immunity

0kA

20kA

30kA

50kA

Cur

rent

( l

)

Safe Zone

Distance from Current Line (r)

10kA

0mm 10mm 20mm 30mm 40mm 50mm

Introduction

Proximity Sensors

2–12

Series Connected SensorsSensors can be connected in serieswith a load. For proper operation, theload voltage must be less than or equalto the minimum supply voltage minusthe voltage drops across the series-connected proximity sensors.

+V DC

–

Load+ –

Wiring Diagram forSeries Connected Current

Sink Sensors (NPN)

+V DC

–

Load+ –

Wiring Diagram forSeries Connected Current

Source Sensors (PNP)

V AC

Load

L1 L2

Wiring Diagram forSeries Connected

AC Sensors

Parallel Connected SensorsSensors can be connected in parallel toenergize a load. To determine themaximum allowable number of sensorsfor an application, the sum of themaximum leakage current of thesensors connected in parallel must beless than the maximum OFF-statecurrent of the load device.Note: Care should be taken when designing

parallel proximity circuits. If too muchleakage current flows into the load it maycause the solid state input to change stateor a small relay not to drop out. Sensorsconnected in parallel do not provide a higherload current capability.

+

V DC–

Load+ –

Wiring Diagram forParallel Connected

Current Sink Sensors (NPN)

+V DC

–

Load–+


Current Source Sensors (PNP)

V AC

Load

L1 L2


AC Sensors

R*

R*

* Add R in series with sensor to maintain minimum voltage when sensor is switching.

TTL Wiring

Sink

Brown

Black

Blue

TTL

( - )

( + ) 10-30 V DC

(+) 5V

Source

(-) 5V GND

( - )

( + ) 10-30 V DC

Note: When using sourcing outputs, groundmust be floating and cannot be common,or short circuit will result.

(-) CommonGND

TTL

PLC WiringFor PLC wiring information for Inductiveand Capacitive sensors, refer topublication 871–4.5, June 1996.

Introduction

Proximity Sensors

2–13

Shielded vs. Unshielded Inductive Sensors

Shielded construction includes a metalband which surrounds the ferrite core and

coil arrangement.

Shielded Sensor

MetalShield

Ferrite Core

MetalShield

Unshielded sensors do not havethis metal band.

Ferrite Core

Unshielded Sensor

Spacing Between Shielded Sensors (Flush-Mountable) and Nearby Metal SurfacesShielded proximity sensors allow theelectro-magnetic field to beconcentrated to the front of the sensor

face. Shielded construction allows theproximity to be mounted flush in

surrounding metal without causing afalse trigger.

d = diameter or width of active sensing faceSn = nominal sensing distance

ÉÉÉÉÉ

ÉÉÉÉÉ

ÉÉÉÉÉ

ÉÉÉÉÉ

ÉÉÉÉÉÉÉÉ

ÉÉÉÉÉÉÉÉ

ÉÉÉÉÉÉÉÉÉÉÉÉ


3 Snd

ÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ

d d

2d*

ÉÉÉÉÉ

2d

d

d

d

ÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ

d1.3d*

d

ÉÉÉÉÉ

1.8d

* 802PR–LB or 802PR–XB can be mounted side by side.

Cylindrical Style

Limit Switch Style (871L and 872L)

Limit Switch Style (802PR)

*8d for capacitive sensors

Introduction

Proximity Sensors

2–14

Spacing Between Shielded Sensors (Flush-Mountable) and Nearby Metal Surfaces (continued)


Cube Style (871P V ersaCube)

Miniature Flat Pack Style (871FM)

ÉÉÉÉÉÉ

ÉÉÉÉÉÉÉÉÉ

d

ÉÉÉÉÉÉÉÉÉ

dActive Face

ÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉd

d

d

3 Sn

Introduction

Proximity Sensors

2–15

Spacing Between Unshielded Sensors (Nonflush-Mountable) and Nearby Metal SurfacesLonger sensing distances can beobtained by using an unshieldedsensor. Unshielded proximity sensors

require a metal-free zone around thesensing face. Metal immediatelyopposite the sensing face should be no

closer than 3 times the rated nominalsensing distance of the sensor.

ÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ



3 Sn3d �

4d �

ÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ

≥3d

4d �

ÉÉÉÉÉÉÉÉÉd

ÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ

d

d

4d �

d

d

0.4d

1.5d

0.4d

d �

0.4d �

� d for capacitive sensors if mounted in plastic. 3d (12, 18mmmodels) or 1.5d (30, 34mm models) if mounted in metal.

� For capacitive sensors, 3d at medium sensitivity to 8d atmaximum sensitivity.

� 8d for capacitive sensors.� d for capacitive sensors.


Cylindrical Style

Limit Switch Style (871L, 872L, and 875L)

Limit Switch Style (802PR)

Introduction

Proximity Sensors

2–16

Spacing Between Unshielded Sensors (Nonflush-Mountable) and Nearby Metal Surfaces (continued)

2d


Cube Style (871P V ersaCube)

Miniature Flat Pack Style (871FM)

ÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ

ÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ

ÉÉÉÉÉÉÉÉÉ

2d

ÉÉÉÉÉÉÉÉÉ

d

d

d

0.4d

Active Face

0.4d

d

15mm

40mm typ.

6 Sn

L

0.5L

Side View(amount of overhang)

Introduction

Proximity Sensors

2–17

Machine Tools

Lathe

ToolChuck

InductiveProximitySensor


Grinding Machines



Petroleum Industry—Valve Position

A: Open Valve IndicatorB: Closed Valve Indicator

Handwheel Acts as Target

871TM

871TM

A

B

Plating Line


Wood Industry


Saw Blade Returns forAnother Cut

Wood

Capacitive ProximitySensor

Plating Line

ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ

ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ

ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ

InductiveProximity

Sensors forVertical Position

Detection

Inductive ProximitySensor for Horizontal

Position DetectionTrigger

Work to beDipped

Conveyor Belts


Metal Parts

Applications

Proximity Sensors

2–18

ÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ

ËË ËËË

ÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ

ËËËË

Inductive proximity sensor used to detect a foilseasoning bag inside of a cardboard container.

Ferrous selective inductive proximity sensor used tosort ferrous and nonferrous can tops.


Food Industry Stainless Steel Sheet Welder

ÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ

ÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ



BottleCap

MetalCan

Conveyor

Conveyor

Hole toIdentify Weld

+ (Anode)

Insulation

- (Cathode)

871Z

871Z

Food Processing Printing

Capacitive ProximitySensors for �Container

Full" Verification

Capacitive ProximitySensors for �Low Paper

Level" Indication

Applications

Proximity Sensors

2–19

On Line Parts Sorting

�DETAIL�

1″

7/8″ Dia.

7/16″1/2″

3/4″ Dia.

871C

Bad

Good

Up and Downslope Control of Continous Tube Welder

Transformersand

Rectifiers

PressureRoll

871P

DownslopeSensor

+ (Anode)

Insulation

- (Cathode)

UpslopeSensor

871P

Detect Presence of Bushing in Piston

A = Sensing Path

871C

A

Railroad Yard Position Sensing

InductiveProximitySensors

Nut Placement on Transformer

Transformer

871Z

871Z

Coolant Resistant Sensing

871TM

Coolant

Closed Barrier Indicator

871TM IndicatesWhen Barrier Is

Closed

ÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊ

Applications

Proximity Sensors

2–20

Control Presence of Mild Steel Bars in Grate Welding

Check Parallelism

+ (Anode)

- (Cathode)

Look for Positionof Steel Bar

with WFI Control

of Transversal Barwith WFI Control

871Z

871P

871Z 871Z871Z

Elevator Positioning

ProximityRail Guide

Sensor

2nd Floor

1st Floor

Target

Target

Allen-Bradley produces rail guide inductive proximity sensors for the positioning of elevator cars. These sensors offerincreased accuracy and longer life when compared to typical mechanical switches. They are a cost effective solution forlowering your repair costs and down-time. Contact your local Allen-Bradley salesperson for a proximity tailored to yourrequirements!

Applications

Proximity Sensors

2–21

Material Handling

PlasticCanister

ÉÉÉÉÉÉÉÉÉÉÉÉÉÉ

UltrasonicProximitySensor

Conveyor

Liquid Level Detection

Capacitive ProximitySensors for High andLow Level Detection

ÏÏÏÏÏÏÏÏÏÏÏÏÏÏÏÏÏÏÏÏÏÏÏÏÏÏÏÏÏÏ



ÏÏÏÏÏÏÏÏÏÏ

Liquid

Printing

Ultrasonic proximity sensor used toindicate when paper supply is almostexhausted.

Level Detection


ÉÉÉÉÉÉÉÉ

ÉÉÉÉÉÉ

ÉÉ

ÉÉÉ

ÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎ

ÎÎÎÎÎÎÎÎÎÎÎÎÎÎÎ

GranularFill

Capacitive ProximitySensors for High andLow Level Detection

Applications

Proximity Sensors

2–22

Top 23 Reasons to Use the 871TM

Stainless Steel Face� Superior Strength of Impact and Abrasion

Resistant Stainless Steel

� Superior Chemical and Cutting Fluid Resistance

� Leakproof

LED Lens and Grommet� Provide Mechanical Seals for Primary

Fluid Barrier

� Chemical Resistant

Stainless Steel Barrel� Superior Strength of Impact and Abrasion

Resistant Stainless Steel

� Superior Chemical and Cutting Fluid Resistance

� One Piece Barrel for Mini Q.D. Eliminates

�Joint" Leaking

� Increased Mounting Torque

SOOW-A ToughLink� Cable� Superior Abrasion and Chemical Resistance

� Cutting Fluid Resistant

� Superior Strength of #18AWG

Conductors and Jacket Materials

� Fire Retardent

� Outdoor Approved� Plastic Filler Provides Increased

�Fluid Wicking" Resistance

Robust Electrical Design� Your choice of AC/DC, 2�wire or 3�wire DC

� Short circuit protection �

� Overload protection �

� Reverse polarity protection (DC models)

� Radio frequency protection

� Transient noise protection� False pulse protection

� Epoxy potted for protection against mechanical

shock and vibration

� DC and long barrel AC/DCmodels only.

Applications

proximity sensors 2922

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