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8/13/2019 1zet32 How Inductive Sensors Work Final
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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 emailinfo@zettlex.com.
--- 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: info@zettlex.com 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: dean@silverbulletpr.co.uk www.silverbulletpr.co.uk