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Training manual

Connection technology

Training manual

You will find further information, data sheets, prices etc. at: www.ifm-electronic.com

Training manual connection technology for binary sensors (March 2003)H:\STV\INTERN\Sc- und Se-Unterlagen alt\DEUTSCH\Sc\SC100\sc100.doc 17.01.07 12:24

Guarantee note

This manual was written with the utmost care. However, we cannot assume any guarantee for the contents.

Since errors cannot be avoided despite all efforts we appreciate your comments.

We reserve the right to make technical alterations to the products so that the contents of the training manual maydiffer in this respect.

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Contents

1 Introduction 4

1.1 Proximity switches in industrial processes 4

1.2 Layout 6

1.3 On the contents 6

2 The basics 7

2.1 Binary sensors 7

2.1.12-wire units 8

2.1.23-wire 10

2.1.3Supply 11

2.1.4Further electrical characteristics 12

2.2 Analogue sensors 17

2.3 Sensors with a built-in interface 18

3 Notes on the practical use 19

3.1 Supply 19

3.2 Circuits 20

3.2.1General 20

3.2.2Series connection 21

3.2.3Parallel connection 22

3.2.4Mechanical and electronic switches 24

3.3 Electrical data 24

Annex 25

Glossary of technical terms 26

Index 28

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1 Introduction

1.1 Proximity switches in industrial processes

Sensors Automated production processes require sensors to supply information.They provide signals about positions, limits, levels or serve as pulse pick-ups. Without reliable sensors even the best controller is not able tocontrol processes.

In general, all these sensors consist of two components: The first oneregisters the change in the physical conditions (basic sensor), the secondone converts the signals of the basic sensor into electrical output signals(signal processing).

Figure 1: Structure of a sensor

In general, a distinction is made between binary sensors which provide adefinite high-low signal and analogue sensors which are preferably usedfor temperature, distance, pressure, force measurement, etc. The sensorsupplies an analogue signal which is further analysed for measurementand control.

Binary and digital In order to avoid misunderstandings this is to give a short explanation ofthe difference. Binary means "two values" also in the original sense ofthe word. An analogue signal which can have any value within certainlimits is often digitalised today so that it can be further processed inelectronic controllers. This is done via an A/D converter (analogue intodigital). It divides the analogue signal into steps. The number of stepsresults from the number of bits used. One bit can only have two values,but with 8 bits there are already 256, with 10 bits there are 1024 steps.This is also called resolution. Fewer than 8 bits are seldom used becausethe resolution is too coarse in this case. More than 12 bits are also rarelyused because it does not make sense if the resolution is much higherthan the measuring accuracy.

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Shaft encoders are an exception. They provide digital signals from thestart, see Training manual Shaft encoders.

This text deals especially with binary electronic sensors as replacement formechanical switches. It is to give an overview of the characteristics andcriteria for the use of such sensor systems. There are many names forinductive and capacitive sensors: Proximity switch, initiator, inductivesensor, non-contact position sensor, but there are also manufacturer-specific names like e.g. efector (registered trademark of ifm electronicgmbh). The expression proximity switch, however, is the standard termwhich will be used in the following.

Division With the initially confusingly large number of sensors with differentoperating principles it helps to keep the overview if they are dividedaccording to their applications.  PositionThis can mean simple presence, reaching of an end stop or end position,exceeding or not reaching a level etc. Thus, in this application moremechanical movements are monitored.In most cases binary sensors are used. They are also called limit switches.Typical examples are inductive and capacitive proximity switches,photoelectric sensors etc. Shaft encoders can also be included.  FluidsIn case of liquid and sometimes also gaseous media levels, but also otherparameters are monitored, like exceeding or not reaching a limit speed, alimit pressure, a limit temperature etc.Thus fluid sensors are also often used as binary sensors. Analogue outputsignals, however, are more important.Typical examples are level, flow, pressure, and temperature sensors.

This division should not be seen as a rigid scheme. There are alsoborderline cases, e.g. a binary level sensor can be seen as position sensoror as fluid sensor.

Binary sensors are mainly connected to electronic controllers. There arealso cases, however, in which other loads are switched or in whichattempts are made to connect the sensors to each other (connection inseries or in parallel). This manual is to give useful information on thistopic.

Further electrical data and characteristics of binary and analogue sensorswill be discussed. There is a series of electrical characteristics which arethe same for all electronic, binary position sensors, e.g. inductive andcapacitive proximity switches, photoelectric sensors, etc. This is also validfor analogue sensors. Therefore they are discussed in detail separately.

Outputs To switch the output signal semiconductor switches like transistors andthyristors are now widely accepted on the market. They have clearadvantages over mechanical switches as regards life, the number ofreliable switching operations, the switching frequency, and the bounce-free switching characteristics.

The few disadvantages, i.e. leakage current in the switched-off state,voltage drop in the switched state and higher sensitivity as regardsovervoltage and overcurrents can usually be tolerated or to a large extent

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avoided by means of suitable protective measures. This manual is toprovide helpful information.

1.2 Layout

For a better understanding a few explanations regarding terms used inthe text will be given to make reading the text and location ofinformation easier.

Keywords Keywords are given in the left margin. They refer to the topic to be dealtwith in the following section.

What does FAQ mean? This means Frequently Asked Questions. This term is also used in modernelectronic media. Almost every beginner has the same questions.Occasionally they will be put before a section instead of a keyword. Todifferentiate them from simple keywords, they are written in italics.

(4) A figure in round brackets in the left margin refers to a formula used inthe following text, e.g. see (4). Of course these formulas are not meantto be learnt by heart. They are to make understanding the subject easierbecause a formula, similar to an illustration, describes a relation morebriefly and clearly than many words.

1.3 On the contents

This manual is to give basic information on (binary) proximity switches.Important terms and connections will be explained, the state of the artwill be described and technical data of ifm units will be presented. Thisresults in the following structure.

1. Introduction The introduction is followed by the chapter:

2. The basics A few basic terms and their context will be described. Consequences forpractical use will follow in the next chapter:

3. Notes on the practical use The requirements as regards current and voltage supply are stated.Connection in series or in parallel of electronic sensors should be avoidedif possible. If it is unavoidable you can refer to the notes in chapter 3. Theknowledge of these features, the advantages and disadvantages, is aprerequisite for successful use.

Annex This manual is to also help you with your self-study. Therefore importantterms will be explained again briefly in the short technical glossary. Thepoints which are important for ifm sensors will be discussed in detail inthe preceding chapters. The index helps to look them up. The type keyand the code for the production date will also be briefly presented.

Much success! These basics should enable everybody to benefit from the chance thatelectronic sensors offer and to use them successfully.

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2 The basics

The physical basics are not treated in this chapter; this is done when theindividual sensor types are discussed. Here the basics of connectiontechnology are explained and classifications are made.

2.1 Binary sensors

Binary sensors can be compared with mechanical switches. Thecharacteristics of these systems are explained in detail e.g. in the Trainingmanual Inductive proximity switches.

If you compare all features you can see clearly that electronic proximityswitches have advantages over mechanical limit switches so that the useof non-contact sensors is an advantage for the user in any case. Itincreases the reliability of the plant while at the same time reducing theoperating costs. Thus it results in increased competitiveness.

Figure 2: Mechanical switch

These reasons have led to the electronic proximity switch replacing themechanical limit switch to a large extent in industrial applications. It hasspecific characteristics, however, which have to be taken into account.

Proximity switches are offered in the so-called 2-wire, 3-wire and (forspecial cases) 4-wire technologies.

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2.1.1 2-wire units

This is the system which is most similar to the mechanical switches(shown as symbol in Figure 3). The load is connected in series to thesensor.

Figure 3: 2-wire

Voltage For 2-wire switches the operating voltage is the common voltageavailable for the connection in series of the proximity switch and theload. In this case a slightly lower voltage is applied to the connections ofthe proximity switch because some of the voltage already drops at theconnection, depending on the internal resistance of the load.An important criterion in this case is the voltage drop over the sensor inthe switched state. It can be found in the data sheet, see e.g. www.ifm-electronic.com and depends on the type. Typical values are:  2.5 V for current DC units  6.5 V / 6 V for UC units in AC / DC operation  8.5 V for AC unitsThese three types are described in 2.1.3.

Current In the unswitched state the leakage current which the electrical circuitrequires for its own operation flows over the load. Depending on thesensitivity of the load this can for example lead to problems (cf.connection in series and parallel, 3.2.2. and 3.2.3). For older units it wasa few mA. If required, a resistor or an RC combination must be switchedin parallel to the load.

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Figure 4: Leakage current DC

Figure 5: Leakage current AC

In the past usually 3-wire units were used to ensure optimum protectionagainst incorrect switching, e.g. in connection with a plc. For newerunits, especially the quadronorm units, it was possible to reduce theleakage current to typically 0.4 to 0.6 mA. The voltage drop in theswitched state could also be reduced.

There is a trend to replacing 3-wire units with 2-wire units. This enablesconsiderable savings as regards cabling. It has to be considered, however,that especially 2-wire units cannot be compared with mechanicalswitches. Each step of development, however, tries to make them moresimilar.

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2.1.2 3-wire

With 3-wire switches the operating voltage is applied between +UB and0 V and the switching signal is led to the load via an additional wire.

Figure 6: 3-wire

Voltage The voltage drop in the switched state is considerably lower, it is typically1 V.

Current In the unswitched state the output then is virtually de-energised, nocurrent can flow except for the leakage current of a few µA of thetransistor and the protective circuitry.The own requirement of the sensor is not important here. It has to betaken into account, however, when dimensioning the power supply.

4-wire An exception are some 4-wire units. They have two complementaryoutputs which can both be used for normally open and normally closedfunction.

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Figure 7: 4-wire

2.1.3 Supply

This is an overview and an explanation of the designations. For furtherinformation see 3.1.

Basically there are 3 types:

DC units This is the most common type.The operating voltage for sensors which is common in industrialinstallations is 24 V, usually DC. There are also installations, however,which are operated with alternating voltage. Other nominal operatingvoltages like 12, 48 or 60 V are found as well.In practice it is not only the nominal voltage which has to be taken intoaccount, but also the operating voltage range in which the sensor worksreliably (see 3.1.). It can also be found in the data sheet. Typical valuesare: 10...36 V, 10...50 V, 5...36 V, 18...36 V.A wide range is interesting for several reasons:The units can also be operated with unusual voltages, e.g. with a 12 Vbattery.In industrial applications voltage fluctuations are to be expected. Thisshould not lead to faulty signals.Usually current and voltage fluctuations of any kind are permissible witha DC supply, if the values are not below or above the minimum andmaximum voltage of the indicated operating voltage range.

AC units This can be 230 V or 115 V. The importance of these units is decreasing,however. If no AC unit is available a UC unit (see below) can be usedinstead.

UC units UC stands for universal current. These units can be used for direct currentor alternating current. Typical ranges are 20... 250 V AC/DC. If units forboth voltages are used it simplifies storage of spare parts for the user.

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The total harmonic distortion (interference in the range of more than 50Hz) should not exceed approx. 10% for AC units.

The standard IEC 60947-5-2 defines utilisation categories. They defineswitchgear in order to better describe the characteristics which are typicalof these categories.

Utilization categories They are listed and explained in the table below.

Switch elementsCategory Typical applications:AC �12 control of resistive loads and solid state loadsAlternating

voltage AC �140 control of small electromagnetic loads withholding current < 0.2A, e.g. contactor relays

DC-12 control of resistive loads and solid state loadsDirect voltageDC �13 control of electromagnets

Category AC-140 applies to efectors in AC/DC version, category DC-13applies to 2 and 3-wire DC efectors and category DC-12 to NAMURtypes.

Different types of application are permissible if it is agreed between ifmas manufacturer and the user or if it is mentioned in the catalogue.

For further information about noise immunity see the Training manual CEmarking.

2.1.4 Further electrical characteristics

Energy requirement Energy is of course needed for the own supply of the electronics. As anexample the 24 V DC units are described assuming that the 24 V areconstant. For 2-wire unit the leakage current flows via the sensor (andthe load) in the open condition, see 2.1.1. It is typically at approx. 0.5mA. For 3-wire units the current consumption has to be taken intoaccount, see 2.1.2. It is typically at approx. 15 mA. This results in typically10 mW for 2-wire units and 150 mW for 3-wire units. This considerabledifference is a further reason why one should consider using 2-wire unitsinstead of 3-wire units. This concerns mainly standard inductive sensors.For photoelectric sensors this is of course not easily possible. In this caseeven higher values have to be expected.

Load capacity The load capacity of the sensors cannot be compared with the loadcapacity of electronic relays, among other things due to the housing type.The maximum current of DC units is between 100 and 400 mA,depending on the type. For UC units the maximum current is differentdepending on the operating voltage. Typical values are e.g. 100 mA DCand 350 mA AC.

Protective circuitry Most DC units have short-circuit protection. In this case they are alsoreverse polarity and overload protected.

Short-circuit protection In technical terms the short-circuit protection of proximity switches isnormally designed as follows: A precision resistor is inserted into the loadcircuit and the voltage drop at this resistor is monitored. If the currentexceeds a fixed limit value the switch is blocked.

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Two disadvantages of the short-circuit protection have to be mentioned:On the one hand, the resistor causes a little higher voltage drop in theswitched state than a comparable proximity switch without short-circuitprotection. On the other hand, the short-circuit protection may respondunintentionally, for example when there are temporary higher switch-oncurrents(contactors, incandescent lamps), capacitive loads (e.g. whenoperated with long cables)or transients.The latter case leads to sporadic interferences which are difficult tolocate. For this reason some users refuse to use units with short-circuitprotection. Should the sensor be destroyed due to overload the fault islocalised and appropriate measures can be taken.

How does the proximity switch know after being blocked by the short-circuit protection and with (almost) no current flowing through the loadthat the short circuit is removed and it can switch on again?

There are two different operating principles on the market:

For principle 1 the proximity switch remains blocked after the short circuithas been noticed until the operating voltage is interrupted which leads toa resetting of the short-circuit protection circuit. According to the EN50178 switches which work according to this principle can only be called"partially short-circuit protected" because action is required to restoreoperation.

For principle 2 the switch is only blocked for a certain period of time(typically approx. 100 ms) and is switched on again automatically afterthis period. If there is still a short circuit the current at the precisionresistor will again exceed the defined limit value and block the switchagain after which the cycle starts again ("automatic checking"). If theshort circuit has been removed the proximity switch is operational againimmediately (i.e. after 100 ms at the latest) without any furthermeasures. This is the principle of ifm efectors which are protected againstshort circuits.

1 short circuit 2 short circuit removed

Figure 8: Automatic checking

Proximity switches fitted with this type of short-circuit protection complywith the requirements of DIN 57 160 as regards short-circuit-proofelectrical equipment.

Polarity As mentioned above semi-conductor outputs have considerableadvantages over mechanical contacts, but other characteristics have to betaken into account, e.g. the polarity. In many countries pnp-switching

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units are common, in some countries however, npn-switching units arecommon. In both cases reverse polarity is possible of course due to awiring fault.

Reverse polarity protection If a proximity switch is protected against reverse polarity its connectionscan be reversed without any damage to the switch. It cannot beexpected, however, that the switch functions in all these cases.There are only two ways of connecting a 2-wire proximity switch so thatan error can be handled easily (for example by means of a built-in diodein series with the switch which blocks when the polarity is wrong or bymeans of built-in rectification which permits either polarity).

Reversed connections With 3-wire systems there are a number of ways to make a wrongconnection of at least 2 wires. The reaction of the switch to the wrongconnection, i.e. whether it remains open or permanently closed dependson the type of protective circuitry. A detailed example is given in thetable:

Connection of the cablebrown black blue

Reaction of the efector

L+ load L- normal functionL+ L- load short-circuit protection is activatedload L+ L- switch blocked, no functionload L- L+ switch blocked, no functionL- load L+ switch blocked, no functionL- L+ load switch blocked, no function

Switches protected against reverse polarity must also be short-circuitproof for 3-wire units because otherwise reversing the output and the 0V wire would destroy the switch.

Overload protection There is a variable difference between the maximum current which ispermissible for a certain proximity switch according to the data sheet andthe current at which the short-circuit protection becomes effective. Thisoverload range is due to component tolerances. Normally, the proximityswitch should not be operated in this range because the data specified bythe manufacturer in the data sheet are only guaranteed up to thenominal current. Furthermore the overload range usually depends on theambient temperature (this effect is called "derating") and varies from oneunit to the next.

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IL continuous current rating [mA]T ambient temperature [°C]1 AC units 3 IF / IFA AC2 AC/DC for AC 4 short-body types IFB, IGB, IIB

Figure 9: Derating

A proximity switch is protected against overload if it can be operated inthis current range for any period of time and over the whole temperaturerange. Thus the switch can be connected to any resistor between 0 and without being damaged.

If the short-circuit protection operates to the automatic checkingprinciple there is, however, the reservation that normally, in the case ofhigh inductive loads, the overload protection cannot be ensured withoutan external no-load diode.

Standard sensors All efectors of the ifm standard range having the letter "K" at the 11thposition of the type key are short-circuit protected according to DIN 57160. They are also protected against overload over the whole permissibletemperature range, see www.ifm-electronic.com.

They are also capable of switching capacitive loads of at least 20 nF inparallel to an ohmic load. As a rule, this value corresponds at least to acable length of 200 m.

The short-circuit protection has been designed following the automaticchecking principle. For inductive loads with a time constant >> 1 ms theproximity switch may get damaged in the event of an overload withoutexternal protection.

When the short-circuit protection circuitry was rated special care wastaken that transients frequent in industrial power supplies do not triggerthe short-circuit protection. Thus the short-circuit protection does notimpair the high noise immunity of the efectors.

Order of connection AC and AC/DC switches can be connected in either way. They are fullyoperational with both connections. All efectors are protected againstreverse polarity. So quadronorm switches cannot be damaged by false

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connection either. However, the way they are connected determines theiroutput function.

Figure 10: Connection efector quadronorm

One goal of developing the efector quadronorm switches was to providea unit for as many applications as possible. By simply reversing theconnection wires the functions normally closed and normally open arereversed. If the connecting wires are inadvertently reversed, this leads toan undesired change of these functions which can cause spuriousoperations. To prevent this source of error non polarised switches weredeveloped.

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Figure 11: Connection unipolar efector

2-wire DC units except quadronorm switches (programming by reversingthe connections) are permanently conductive at the terminals if thepolarity is reversed.

Output function Some units are available as normally open or normally closed units. Formany units a programming possibility is connected additionally to the

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evaluation stage. In this case the switching function normally open ornormally closed can be selected.

For quadronorm units this is made by reversing the wires (see Figure 10).For 4-wire units (see 2.1.2) both functions are available simultaneously.

2.2 Analogue sensors

The processing of analogue signals was already practised before binaryelectronic proximity switches replaced the mechanical switches. Thus thisapplication has been known for a long time. In this case there are fewerspecial characteristics to be taken into account.

Current and voltage outputs have to be differentiated. The voltageoutput is used more seldom in order to avoid measuring errors due to thevoltage drop in the wires.

Current There are two variants: 0..20 mA and 4...20 mA. The advantage of thesecond solution is the additional possibility of wire break monitoring. Thisis preferred in practise. Thus most of the units with current output areonly available in the version 4...20 mA.

Simple sensor systems, so-called transmitters, virtually consist only of thetransducer which transmits an analogue signal. Processing andimplementation are carried out in the connected units.

Modern sensors with microprocessor integrate this function and help tosave production complexity and thus costs. Some of them also allow anadaptation of the measuring range to the application. Thus for e.g.temperature sensors the lower limit of 4 mA can be assigned to atemperature of 0° C and the upper limit of 20 mA to a temperature of100° C (see Training manual Temperature sensors).

The control and display unit to which the analogue output is connectedhas of course an input resistor. If the resistance is low, even in case of ashort circuit, the current regulator should keep the current constant.More critical is the case of too high a resistance. The current regulator inthe sensor cannot increase the required voltage to any value. Thus thereis an upper limit for the resistance, for efector 600 e.g. max. 500 .

Voltage The range of 0...10 V is common. This is the opposite of the currentoutput. The resistance must not be too low, otherwise the voltageregulator is not able any more to maintain the voltage. In case of a shortcircuit the voltage output is blocked. For efector 600 e.g. the resistanceshould be at least 2000 .

Supply voltage The importance of the voltage interval in which the sensors functionreliably was already explained in the chapter about binary sensors, see2.1.3. There are types, especially among the fluid sensors, which areequipped optionally with binary switching outputs, e.g. for triggering apump or a heating system or with analogue outputs (or mixed), e.g. for acontrol module. If such a unit has an analogue output the requirementsas regards the supply voltage are higher. Thus the permissible range isoften smaller than for a unit with binary outputs.

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2.3 Sensors with a built-in interface

These sensors are only mentioned here for the sake of completeness.There are e.g. inductive proximity switches with AS-i interface or shaftencoders with PROFIBus interface. Thus they have no switching outputsin the conventional sense. The connection must be made according tothe specifications of the bus system. Any standard sensor can also beconnected to a bus system, but this requires for example an I/O module.In this sense each sensor "can be connected to a bus system": This is notthe same as "with a built-in interface".

Intelligent Sensors with a built-in interface often belong to a group called intelligentsensors. This means sensors which provide more information than"object detected" or "object not detected". Photoelectric sensors cansignal e.g. that they are soiled. Capacitive sensors can be adjusted bymeans of a pulse on the programming wire in such a way that theenvironmental influences are compensated for. With conventionalconnection this means an additional output or input, an additional wire,more wiring complexity...thus it is not often used. For sensors with abuilt-in interface, however, the complexity is not higher. Here the sensorcan easily be monitored as regards wire break or readiness for operation.It is to be expected that these sensors with a built-in interface will beused more and more in future.

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3 Notes on the practical use

This chapter deals mainly with binary sensors.

3.1 Supply

For noise immunity and preventive measures in case of interference seethe Training manual CE marking.

Electronic sensors have become less sensitive to interference over theyears. Their use has become a matter of course. The data sheet is noteven consulted. It may also happen that concerning the permissibleoperating voltage range (see 2.1.3) the addition "including residualripple" is not considered.

Residual ripple This can sometimes cause interference. It means that also short spikes ordips of the supply voltage must be within the permissible range.Otherwise reliable signals cannot be ensured. A quality power supplywhich supplies a stable and smooth supply voltage for DC should also beused for the supply of the periphery,i.e. the sensors. For AC or UC unitsthere should not be too much interference of the alternating voltageeither, of course.

Figure 12: Residual ripple and total harmonic distortion

Furthermore, when planning it should be taken into account that notonly loads, e.g. valves have to be supplied, but also the sensors. Therequirement for inductive sensors is in the mA range. Optical sensorsrequire a little more current simply to produce light. Especially for fluidsensors with more complex conditioning and processing of the measuring

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signal the current consumption can exceed 50 mA. If the power supply isunderdimensioned there is the risk of falling below the permissible limitof the supply voltage in case of a voltage dip. This can lead to wrongpulses.

Periodic higher-frequency AC voltage parts are called residual ripple incase of DC voltage. For AC voltage they are called total harmonicdistortion.

There are also different types of fluctuation with AC.

Figure 13: Voltage fluctuations AC

3.2 Circuits

3.2.1 General

Electronic units are different The characteristics which have an unfavourable effect with mechanicallimit switches, like wear and tear and corrosion, have led to theirreplacement by electronic proximity switches as mentioned above. Thereplacement of contactor controllers by electronic controllers is a similarcase. The current solution has considerable advantages. It has to be takeninto account, however, that electronic sensors may react differently frommechanical switches. This is especially important in the contexts describedbelow.

Clarity It is state of the art in automation technology to combine the binarysignals provided by the sensors in the plant in an electronic controller. Asa rule it is avoided when working with a plc to externally form logicalcombinations by means of series or parallel connection because thismakes tracing of errors more difficult in case of a fault. Sometimes,however, this is done. Thus the wiring complexity in a large-scale plantcan be reduced if the switches are connected logically. It is also possiblethat several switches must be connected together because ofmodifications to existing plants.

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Time characteristics Usually electronic proximity switches have considerably shorter switchingtimes than mechanical switches, but they also have a power-on delaytime which is much longer than the switching time. The time whichelapses between the application of the operating voltage and completereadiness for operation is called power-on delay time. This time must betaken into account if sensors are only supplied with operating voltageafter another sensor is switched. These times can add up if severalsensors are connected in this way.

Better not! For reasons of clarity and because of the characteristics described belowwe advise against the connection of binary electronic sensors with eachother!This should only be done if it cannot be avoided, but the notesconcerning the maximum number must be observed. It is absolutelynecessary to make extensive tests and test runs.

3.2.2 Series connection

2-wire units In principle, the series connection of 2-wire standard inductive units is notrecommended as a safe function cannot be guaranteed. It must beconsidered that the voltage drops of the proximity switches add up andso less voltage is available to the load. When switching inductiveconsumers the phase shift becomes effective. If all these points areconsidered, a maximum of 2 to 3 proximity switches can be connected inseries depending on the type. ifm electronic offers some special units forsuch applications. For information please contact the respectivedepartments.

Figure 14: Series connection 2-wire

In principle, the series connection of 2-wire optical units is notrecommended. As the closed-circuit current consumption of the units canbe different the unit with the higher closed-circuit current is not suppliedsufficiently which causes problems. At the same time the switched unitneeds a higher current; this leads to the output status indication notbeing lit if one of the units has not switched. In principle seriesconnection is not recommended for the units of the new generation withmicroprocessor either.

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3-wire units If 3-wire proximity switches are connected in series, the voltage drops onswitched units between 1 and 2.5 V add up. Care must be taken that theload can still be operated correctly with the remaining voltage. The firstproximity switch must be capable of switching the current consumptionof all subsequent proximity switches in addition to the load current. Asthe operating voltage of the proximity switches connected in series canbe turned on or off, the power-on delay time must be considered (up toseveral 100 ms). If these points are taken into account, a maximum of 5to 10 proximity switches can be connected in series depending on thetype.

Figure 15: Series connection 3-wire

In principle, series connection of 3-wire optical units is notrecommended. Due to their system, photoelectric components have ahigh inrush current. This current causes triggering of the short-circuitprotection of the first sensor connected.

We would like to point out that the circuit logic has the same result ifparallel connection is selected instead of series connection. The switchingfunction "normally open" must be inverted in this case, i.e. it must bereplaced by "normally closed".

3.2.3 Parallel connection

2-wire units If 2-wire proximity switches are connected in parallel, the leakagecurrents of all non-switched units add up. The sum of the leakagecurrents must be clearly below the holding current of the load (which isimportant for the connection to programmable controllers). It also has tobe taken into account that when one proximity switch is switched theoperating voltage is taken off the switches connected in parallel so thatthey can no longer indicate their true damping status. (Exception:quadronorm units). If all these points are taken into account, a maximum

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of 5 to 10 proximity switches can be connected in parallel depending onthe type.As photoelectric units generally have a high leakage current theconnection in parallel of 2-wire photoelectric units is not recommended.When a unit is switched through, operating voltage is taken off the otherswitches so that the function of the units can be impaired.

Figure 16: Parallel connection 2-wire

3-wire units: It is possible to connect a maximum of 20 to 30 three-wire switches inparallel without any problem (depending on the type). The only factorwhich must be considered is that the (very small) leakage currents of theswitches in the unswitched state add up. Decoupling diodes are onlyrequired for output stages without open collector circuit.

The current consumption of all non-switched units adds up. Proximityswitches can be used together with mechanical switches (taking intoaccount 3.2.4).

Figure 17: Parallel connection 3-wire units

There is no objection in principle to the parallel connection of 3-wirephotoelectric units. The number of possible components depends on the type.In principle, this is also valid for function check outputs.

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3.2.4 Mechanical and electronic switches

Figure 18: Mechanical and electronic in parallel

An especially critical case is the connection in parallel of a mechanicalproximity switch , S1, and an electronic proximity switch, S2. In this casethe arguments of 3.2.3. have an even stronger effect. Let us have a closerlook at the chronological sequence of events:1. The mechanical switch is closed. Thus the electronic proximity switch

has no operating voltage. It is not operational.2. The proximity switch is damped. Because of 1., however, it does not

react.3. The mechanical switch opens. The power-on delay time of the

proximity switch elapses, see 3.2.1.4. Only when the power-on delay time has elapsed, the proximity

switch switches.

The connection of electronic and mechanical switches can have effectswhich are not easily forseeable. For reasons of safety this should not bedone.

3.3 Electrical data

Some data have already been mentioned in the respective contexts. Herethey are to be summarised and completed.

Power The power consumption which modern proximity switches require formaintaining their sensor function is very low. For 3-wire switches it isapprox. 0.1... 0.5 W; for 2-wire switches there are efectors with powerconsumption as low as 0.003 W (3 mW). Any resistive load can beconnected to a proximity switch within its nominal data. With thetechnical data which are common today the use of programmablecontrollers is possible without any restrictions.

S1 S2

Load

0 V

+24 V

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Inductive loads Inductive loads up to a cos of 0.3 (like e.g. solenoid valves) do notcause any problems either as long as the limit currents are not exceeded.In theory difficulties can arise in case of high switching frequencieswhich, however, are not common in this context. Usually the low leakagecurrent which flows via the load in the unswitched state as well as thevoltage drop over the switch in the switched state do not impair thecorrect function.

Incandescent lamps With incandescent lamps the high inrush current which flows in case ofcold filaments must be taken into account like with AC relays or ACcontactor relays which have a considerably lower impedance before thecontact is closed.

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Figure 19: Inrush current of an incandescent lamp

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Figure 20: Inrush current of a contactor

Thus some types of switches (especially AC switches) are rated accordingto the corresponding standard in such a way that they can have six timesthe nominal current for a short time. For switches with short-circuitprotection it must be ensured that the inrush current does not lead totriggering of the short-circuit protection. Incandescent lamps can bepreheated by means of a resistor in parallel to the switch if necessary, inorder to reduce the inrush current efficiently.

As mentioned in 3.2.3 these characteristics are important for theutilisation category of the unit.

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Annex

Small glossary of technical terms

Current consumption of 3-wire units The current consumption is the internal consumption of the proximityswitch in the blocked state. When the output is blocked a very smallleakage current of approx. 0.1 mA (open collector) flows through theoutput transistor.

Current rating/continuous The continuous current rating indicates the current at which a proximityswitch can be continuously operated.

Current rating/peak The peak current rating is the maximum current which may flow for ashort time when power is applied without destroying the proximityswitch.Especially DC units are rated in such a way that they can be charged withsix times the nominal current for a short time because of the high inrushcurrents of many DC current loads (signal lamps, contactors...).

Leakage current of 2-wire units The leakage current is the current that must flow through two-wire unitsin their open condition in order to supply the electronics with current.This leakage current also flows through the load.

Minimum load current of 2-wire units The minimum load current is the smallest current which must flow in theswitched state to ensure reliable operation of the proximity switch.

Normally closed Principle of normally closed operation; if there is an object in the area ofthe active zone the output is blocked.

Normally open Principle of normally open operation; if there is an object in the area ofthe active zone the output is switched.

Operating voltage The nominal operating voltage is a value for which electrical equipment israted. For proximity switches it is common to indicate an operatingvoltage range which determines an upper and a lower limit value. Withinthese limit values the function of the proximity switch is guaranteed.For DC units the residual ripple of the operating voltage must be includedin these limits.If the residual ripple falls below the limit value of the operating voltage ofthe proximity switch, a smoothing capacitor must be used. A rule ofthumb for this is: 1000 mF per 20 A current.

Power-on delay time The power-on delay time is the time which elapses between theapplication of the operating voltage and the readiness of the device togenerate the correct switching signal. Within this time the internalvoltage supply must be stabilised and e.g. in case of the inductiveproximity switch the oscillator must start to oscillate.During this time (5 ms to more than 200ms depending on the type) theoutput is blocked by means of technical measures in the circuitry.

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Programming For some efector types the output function normally open or normallyclosed can be programmed. Depending on the type of efector the outputfunction is programmed via a wire link, a connector or by selecting thepin connection.

Overload protection The output of a proximity switch is protected against overload if it cancarry any current between nominal load current and short-circuit currentcontinuously without damage.

Reverse polarity protection A switch is protected against reverse polarity if the wires of the proximityswitch can be connected in any combination without the switch beingdamaged. As a rule 3-wire switches which are protected against reversepolarity must be short-circuit protected because otherwise reversing theoutput and the frame earth terminal (0 V) would lead to the destructionof the unit.

Short-circuit protection The output of a proximity switch is short-circuit protected according toVDE 0160 if it withstands a short circuit of the load or a short circuit toground at the output permanently without damage and if it isoperational again without any switching operation being required afterthe short circuit has been removed.In the event of a short circuit the output transistor is blockedimmediately. When the short circuit has been removed, the unit isimmediately ready for operation again. Reversing the connecting wiresdoes not lead to destruction of the units. Short-circuit proof units are atthe same time protected against overload and reverse polarity.

Voltage drop (ON-state voltage) As the switching output of the proximity switch is equipped with asemiconductor switch (transistor, thyristor or triac), in the switched statea (small) drop in the voltage in series to the load occurs. In two-wiretechnology the voltage drop also serves to provide energy to theelectronics of the proximity switch. The amount of the voltage dropdepends on the type (for the IG-2005-ABOA e.g. it is 6.5 V at maximumload).

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Index

2

2-wire units ............................................. 8, 21, 22, 26

3

3-wire units ........................................... 10, 22, 23, 26

4

4-wire ............................................................... 10, 17

A

AC ...........................................................................11analogue............................................................. 4, 17AS-i ..........................................................................18automatic checking ..................................................13

B

basic sensor................................................................4basics .........................................................................7binary.....................................................................4, 7built-in interface.......................................................18

C

capacitive loads ........................................................15common voltage ........................................................8connected to a bus system .......................................18current .......................................................... 8, 10, 17current consumption ................................................26current output..........................................................17current rating ...........................................................26

D

DC .................................................................... 11, 12derating ...................................................................14

E

electrical data...........................................................24

F

FAQ............................................................................6fluid ...........................................................................5

G

guarantee ..................................................................2

I

incandescent lamps ..................................................25inductive loads .................................................. 15, 25information ................................................................2inrush current...........................................................25Intelligent.................................................................18

L

leakage current ............................................. 8, 22, 26level sensor................................................................ 5load........................................................................... 8load capacity ........................................................... 12

M

mechanical switch ................................................... 24minimum load current............................................. 26

N

non polarised .......................................................... 16normally closed ................................................. 16, 26normally open ................................................... 16, 26

O

operating voltage .................................................... 26operating voltage range .................................... 11, 19output....................................................................... 5output function....................................................... 16overload protection ................................................. 27

P

parallel connection ............................................ 22, 23partially short-circuit protected ................................ 13polarity.................................................................... 13position ..................................................................... 5power ..................................................................... 24power supply .......................................................... 20power-on delay time ................................... 21, 24, 26programming .......................................................... 27protected against overload ...................................... 15protective circuitry................................................... 12

Q

quadronorm.................................................. 9, 15, 16

R

residual ripple.................................................... 19, 26resistance ................................................................ 17reverse polarity protection ................................. 14, 27

S

sensor ....................................................................... 4series connection............................................... 21, 22short circuit ............................................................. 17short-circuit protection ................................ 12, 25, 27smoothing capacitor................................................ 26standard sensors ..................................................... 15Supply ..................................................................... 11supply voltage ................................................... 17, 19

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T

time characteristics...................................................21total harmonic distortion..........................................20tracing of errors .......................................................20transmitters..............................................................17

U

UC .....................................................................11, 12

V

voltage...........................................................8, 10, 17voltage drop ................................................10, 21, 27voltage output .........................................................17