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Press Release

(Ref: ZET32)

25th October 2012

How Inductive Sensors Work

Inductive sensors are widely used to measure position or speed, especially in

harsh environments. However, to many engineers, inductive sensor terminology

and techniques can be confusing. Mark Howard of Zettlex explains the various

types and operating principles, as well as their consequent strengths and

weaknesses.

Inductive position and speed sensors come in a wide variety

of shapes, sizes and designs. All inductive sensors can be

said to work on transformer principles and they all use a

physical phenomenon based on alternating electrical

currents. This was first observed by Michael Faraday in the

1830s when he found that a first current-carrying conductor

could ‘induce’ a current to flow in a second conductor.

Faraday’s discoveries went on to deliver electric motors,

dynamos and, of course, inductive position and speed

sensors.

Such sensors include simple proximity switches, variable inductance sensors, variable

reluctance sensors, synchros, resolvers, rotary and linearly variable differential

transformers (RVDTs & LVDTs).



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The different types

In a simple proximity sensor (sometimes referred to as a proximity or prox switch) the

device is supplied with electrical power, which causes an alternating current to flow in a

coil (sometimes referred to as a loop, spool or winding). When a conductive or

magnetically permeable target, such as a steel disk, approaches the coil, this changes

the coil’s impedance. When a threshold is passed, this acts as a signal that the target is

present. Proximity sensors are typically used to detect the presence or absence of a

metal target and the output often emulates a switch. These sensors are widely used inmany industrial applications where electrical contacts in a traditional switch would

otherwise prove problematic – notably where lots of dirt or water is present. You will see

lots of inductive proximity sensors next time you put your car through a car wash.

Variable inductance and variable reluctance sensors typically produce an electrical

signal proportional to the displacement of a conductive or magnetically permeable

object (normally a steel rod) relative to a coil. As with the proximity sensor, the

impedance of a coil varies in proportion to the displacement of the target relative to a

coil energised with an alternating current. Such devices are commonly used to measure

the displacement of pistons in cylinders – for example in pneumatic or hydraulic

systems. The piston can be arranged to pass over the outer diameter of the coil.

Synchros measure the inductive coupling between coils as they move relative to each

other. They are usually rotary and require electrical connections to both moving and

stationary parts (typically referred to as the rotor and stator). They can offer extremely

high accuracy and are used in industrial metrology, radar antennae and telescopes.

Synchros are notoriously expensive and are increasingly rare nowadays, having mostly

been replaced by (brushless) resolvers. These are another form of inductive detector

but electrical connections are only made to windings on the stator.



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LVDTs, RVDTs and resolvers measure the change in inductive coupling between coils,

usually referred to as primary and secondary windings. The primary winding couples

energy into the secondary windings but the ratio of energy coupled into each of the

secondary windings varies in proportion to the relative displacement of a magnetically

permeable target. In an LVDT, this is usually a metal rod passing through the bore of

the windings. In an RVDT or resolver, it is normally a shaped rotor or pole piece that

rotates relative to the windings arranged around the periphery of the rotor. Typical

applications for LVDTs & RVDTs include hydraulic servos in aerospace aileron, engine

and fuel system controls. Typical applications for resolvers include brushless electric

motor commutations.

A significant advantage of inductive sensors is that the associated signal processing

circuitry need not be located in close proximity to the sensing coils. This allows the

sensing coils to be located in harsh environments, which might otherwise preclude other

sensing techniques – such as magnetic or optical – as they require relatively delicate,

silicon-based electronics to be located at the sensing point.

InductiveProximitySensor

VariableInductanceSensor

LinearVariable

DifferentialTransformer

Primary

Secondary

Secondary



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Strengths & weaknesses

Due to the nature of the basic operating elements – wound coils and metal parts – most

inductive sensors are extremely robust. Given their solid reputation, an obvious

question is ‘Why are inductive sensors not used more frequently?’ The reason is that

their physical robustness works as both a strength and a weakness. Inductive sensors

tend to be accurate, reliable and robust, but also big, bulky and heavy. The bulk of

material and the requirement for carefully wound coils makes them expensive to

produce, especially high accuracy devices that require precision winding. Besidessimple proximity sensors, the more sophisticated inductive sensors are prohibitively

expensive for many mainstream, commercial or industrial applications.

Another reason for the relative scarcity of inductive sensors is that they can be difficult

for a design engineer to specify. This is because each sensor often requires the

associated AC generation and signal processing circuitry to be separately specified and

purchased. This often requires a significant amount of skill and knowledge of analogue

electronics. Since younger engineers tend to focus on digital electronics, they will

regard such disciplines as an unwanted ‘black art’ to be avoided.

Next generation devices

However, a new generation of inductive sensor has entered the market in recent years

and has a growing reputation, not only in the traditional markets, but also in industrial,

automotive, medical, utility, scientific, oil and gas sectors. This new generation of

inductive sensor uses the same basic physics as the traditional devices but uses printed

circuit boards and modern digital electronics rather than the bulky transformer

constructions and analogue electronics. The approach is elegant and also opens up the

range of applications for inductive sensors to include 2D & 3D sensors, short throw

(<1mm) linear devices, curvilinear geometries and high precision angle encoders.

Zettlex technology is the forerunner of this new generation inductive technique and has

grown over recent years thanks to some high profile design wins. The use of printed



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circuits enables sensors to be printed onto thin flexible substrates, which can also

eradicate the need for traditional cables and connectors. The flexibility of this approach

– both physically and from the ability to readily provide customised designs for OEMs –

is a major advantage of this new approach.

As with traditional inductive techniques, the approach offers reliable and precision

measurement in harsh environments. There are also some important advantages:

Reduced cost Increased accuracy

Reduced weight

Simplified mechanical engineering, for example, eradication of bearings,

seals & bushes.

Compact size – notably with stroke length compared to traditional LVDTs.

Simplification of the electrical interface – typically a DC supply and absolute,

digital signal.

Image of traditional LVDT (top) and Zettlex linear sensor (middle). Rule for scale below.

This is nicely illustrated in the above image – showing a traditional 150mm stroke LVDT

and its new generation replacement, which has been produced for a manufacturer of

linear actuators. The parallels to the ‘before’ and ‘after’ dieting photographs are obvious.

This is reinforced when one considers that the new generation device also includes the

associated signal generation and processing circuit (not shown with the traditional

LVDT). By way of comparison, the Zettlex device offers:



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>10 fold increase in accuracy

95% weight saving

75% reduction in occupied volume

50% cost saving

direct generation of digital data – thus eradicating the need for analogue to

digital conversion.

For more information on new generation inductive position sensors, please contact

Zettlex UK Ltd on 01223 874 444 or visit the website at www.zettlex.com or [email protected].

--- ENDS --- [1,318 words]



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Editor’s Notes:

About Zettlex UK Ltd:

Zettlex is a sensors company. The company’s range of sensors measure position or speedaccurately and reliably, even in harsh conditions.

Zettlex designs and manufactures sensors; supplies sensor components and integrated circuits.The company offers bespoke sensor design and development for specific customerapplications.

Unique technology and laminar, printed designs, enables Zettlex to manufacture sensors thathave no contacts, no bearings, no delicate parts and zero maintenance.

Zettlex sells directly to OEMs and system integrators across a broad range of industry sectors.Applications include position measurement, servos, motor controls, and user interfaces. Around50 per cent of the company’s business is safety-related or safety-critical.

Zettlex is ISO 9001 and BS EN 13980 certified for the manufacture of electromagnetic sensors,including sensors for intrinsically safe (ATEX) environments.

Zettlex UK Ltd contact information:

Mark Howard,General Manager,Zettlex UK LtdNewton Court,Town Street,Newton, CambridgeCB22 7PEUKT: +44 (0) 1223 874 444F: +44 (0) 1223 874 111E: [email protected] W: www.zettlex.com

Press Release issued by:

Dean Palmer,Director,SilverBullet PR Ltd,Frith Farm, Ryhall Road, Tolethorpe, Stamford, Lincolnshire PE9 4BJT: +44 (0) 1780 753 000Mobile: +44 (0) 7703 023771E-mail: [email protected] www.silverbulletpr.co.uk

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