or its content.
Inductive Proximity Sensors: Design, Selection, Specification, and Implementation Inductive Proximity Sensors: Design, Selection,
Specification and Implementation
Authored by Guerrino Suffi
Proximity Sensor Product Marketing, Omron Electronics LLC
When your application calls for metallic target sensing that falls within an inch of the sensing
surface, the inductive proximity sensor fits nicely into your design criteria. These durable sensors
are suitable for harsh environments. They have dust and dirt materials build up immunity.
Industrial inductive proximity sensors first came out in the early 1960s and today have a proven
track record in the sensing arena. They also generally have standardized behaviors.
This article discusses the rudimentary design of the inductive proximity sensor, and goes on to
show a selection method that accounts for conditional application and device requirements. The
article then teaches key inductive proximity sensor specifications followed by a discussion of
mounting restrictions for the sensors implementation. Together, this information will supply a
designer with the knowledge required for a successful inductive proximity sensor to object
detection design.
Inductive Sensor Design
Inductive proximity sensors operate under the electrical principle of inductance. Inductance is the
phenomenon where a fluctuating current, which by definition has a magnetic component, induces
an electromotive force (emf) in a target object. In circuit design, one measures this inductance in
H (henrys). To amplify a devices inductance effect, a sensor manufacturer twists wire into a tight
coil and runs a current through it.
An inductive proximity sensor has four components; The coil, oscillator, detection circuit and
output circuit. The oscillator generates a fluctuating magnetic field the shape of a doughnut
around the winding of the coil that locates in the devices sensing face.
When a metal object moves into the inductive proximity sensors field of detection, Eddy circuits
build up in the metallic object, magnetically push back, and finally dampen the Inductive sensors
own oscillation field. The sensors detection circuit monitors the oscillators strength and triggers
an output from the output circuitry when the oscillator becomes dampened to a sufficient level.
Designers should consider two types of inductive proximity sensors when selecting an inductive
sensor; shielded and unshielded. When current generates in the sensors coil the doughnut effect
it causes the proximity sensor to trigger when any object comes behind, along side or in front of
the device. Shielding uses a ferrite core to direct the coils magnetic field to radiate only from the
sensors detection face. Unshielded inductive proximity sensors are not completely unshielded. A
peeled back ferrite core shielding in the unshielded case allows for a longer sensing distance,
while still preventing sensing due to objects behind the detection face.
Understanding the operation, the magnetic nature, and the shielding of the inductive proximity
sensor is helpful when considering the influences of target material, environment, and mounting
restrictions on the sensor itself and in your design.
Inductive Sensor Selection
Inductive proximity sensors categorize in five specific types; cylindrical, rectangular, miniature,
harsh environment, and special purpose. 70% of all inductive proximity sensor purchases are of
the standardized cylindrical threaded barrel type. When one considers this statistic, it is easy to
understand why a designer would specify into their application a general-purpose (or
standardized) inductive proximity sensor. 70% of the time, he would be correct. Experience has
shown, however, that applications in need of inductive sensing usually warrant the examination of
a few additional design criteria.
These conditional criteria eliminate (or specify) the more special inductive proximity sensors
available first before falling upon the general purpose inductive proximity sensor. The three
guiding beliefs of inductive proximity sensor selection are target material, environment, and
mounting restrictions. Target Materials
In the world of inductive proximity sensors, not all metals are created equally. A designer can find
himself looking for a quick fix to an inductive sensing problem that would vanish if a special
inductive sensor was selected. The inductive proximity sensor specification that we have all
become familiar with in technical data sheets worldwide references a standard detectable object
made of an iron (ferrous) material. Other metallic materials, such as stainless steel, brass,
aluminum, and copper have different influence over the inductive effect and are usually less
detectable than iron.
Is the target material an iron object? Will the target material change in future runs of the
application?
Examine the sensing distance reductions for typical inductive proximity sensors below.
Stainless Steel = Standard Sensing Distance X .8
Brass = Standard Sensing Distance X .5
Aluminum = Standard Sensing Distance X .4
Copper = Standard Sensing Distance X .3
A designers full line sensor supplier will have a solution for his inductive proximity sensors
detection of troublesome metallic materials. Manufacturers terms for these special inductive
proximity sensors are non-ferrous sensing or all metal sensing. Non-ferrous sensing
Inductive proximity sensors will detect non-ferrous metals such as aluminum better than they
sense iron. All metal sensing inductive proximity sensors will detect all metallic materials at the
same sensing distance.
What separates the non-ferrous sensing and all metal sensing inductive proximity sensor from
the standard (or general-purpose) inductive proximity sensor is the number of separate inductive
coils included in the proximity sensor head.
The non-ferrous sensing or all metal sensing inductive proximity sensor will include two or
three separate coils in the proximity sensor head while the general-purpose inductive proximity
sensor will include only one coil. The main trades between a non-ferrous sensing or an all
metal sensing type proximity sensor and a general-purpose proximity sensor are the cost and
body size. Non-ferrous sensing and all metal sensing proximity sensors tend to be more
expensive due to the increased number of coils required and have larger enclosures than their
traditional inductive proximity sensor counterparts.
Environment
Environmental conditions can have far and sweeping effects upon the inductive proximity sensor.
These effects specifically refer to sensor life, but can only be related to premature failure (false
trigger or otherwise) of the inductive proximity sensor once installed into its component mounting
position. Nevertheless, your full line sensor supplier has many solutions to specific environmental
conditions.
Is the application one in which metallic chips or filings are prone to build up on the side or
face of the inductive proximity sensor?
Intelligent semi-conductor microprocessors found in some modern inductive proximity sensors
have the ability to detect the slow build up of metal filings or chips over time and teach the
inductive proximity sensor to ignore their effects. Sensor suppliers call this specialized inductive
proximity sensor a chip immune type.
Another type of inductive proximity sensor that is resilient against chip build up is the flat-pack
proximity sensor. The slim profile of the flat pack proximity sensor when mounted with its sensing
face exposed vertically is virtually unaffected by chip build up on its slim horizontal component.
Is the inductive proximity sensor exposed to cutting fluids or chemicals for prolonged
periods of time?
In the face of cutting fluids or corrosive chemicals, a traditional inductive proximity sensor may
become brittle and crack, shortening its life. In such cases, a designer must again turn to a specialized inductive proximity sensor. Proximity
sensors dipped, coated or shot from
fluoroplastic suffer no ill effects from the material
in terms of performance or
reliability. Fluoroplastic's stability against cutting oils and corrosive
chemicals outweigh the additional
costs that come with a product manufactured with fluoroplastic.
An additional benefit to
this type of inductive proximity sensor is its ability to prevent
any build up of weld slag.
Is the sensing application in a high temperature environment?
Inductive proximity sensors typically include their silicon amplifiers and detection circuitry inside
the sensor head housing. These proximity sensors are called self-contained devices. Self-
contained proximity sensors are practical for most applications until environmental conditions
begin to exceed the normal operating parameters for a silicon-based circuit. Normal operating
temperatures for silicon based circuitry is within the realm of -25 to 70C (-13 to 158F). Under any
temperature conditions beyond these ranges, the circuitry becomes more prone to operating
failure.
For temperature applications that exceed these requirements, look for inductive proximity sensors
that use separate amplifiers. With separate amplifier inductive proximity sensors, the sensor
head contains the inductive coil and little more. The intelligent amplifier and detection circuitry can
be located safely away in a remote environmentally controlled area. Such sensors can resist
temperatures as high as 200C (392F).
Mo