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BERO Sensors

BERO is the trade name used by Siemens to identify its line of

“no-touch” sensors. Siemens BERO sensors operate with no

mechanical contact or wear. In the following application, for

example, a BERO sensor is used to determine if cans are in the

right position on a conveyor.

T ypes of BERO Sensors There are four types of BERO sensors: inductive, capacitive,

ultrasonic, and photoelectric. Inductive proximity sensors use an

electromagnetic field to detect the presence of metal objects.

Capacitive proximity sensors use an electrostatic field to detect

the presence of any object. Ultrasonic proximity sensors use

sound waves to detect the presence of objects. Photoelectric

sensors react on changes in the received quantity of light. Some

photoelectric sensors can even detect a specific color.

Inductive Proximity Sensors

Theory of Operation

In this section we will look at BERO inductive proximity

sensors, and how they detect the presence of an object without

coming into physical contact with it. Inductive proximity sensors

are available in a variety of sizes and configurations to meet

varying applications. Specific sensors will be covered in more

detailed in the following section.

Electromagnetic Coil and The sensor incorporates an electromagnetic coil which is used Metal T arget to detect the presence of a conductive metal object. The sensor

will ignore the presence of an object if it is not metal.

ECKO Siemens BERO inductive proximity sensors are operated using

an Eddy Current Killed Oscillator (ECKO) principle. This type of

sensor consists of four elements: coil, oscillator, trigger circuit,

and an output. The oscillator is an inductive capacitive tuned

circuit that creates a radio frequency. The electromagnetic field

produced by the oscillator is emitted from the coil away from

the face of the sensor. The circuit has just enough feedback

from the field to keep the oscillator going.

When a metal target enters the field, eddy currents circulate

within the target. This causes a load on the sensor, decreasing

the amplitude of the electromagnetic field. As the target

approaches the sensor the eddy currents increase, increasing

the load on the oscillator and further decreasing the amplitude

of the field. The trigger circuit monitors the oscillator’s amplitude

and at a predetermined level switches the output state of the

sensor from its normal condition (on or off). As the target moves

away from the sensor, the oscillator’s amplitude increases. At a

predetermined level the trigger switches the output state of the

sensor back to its normal condition (on or off).

Operating Voltages Siemens inductive proximity sensors include AC, DC, and AC/

DC (universal voltage) models. The basic operating voltage

ranges are from 10 to 30 VDC, 15 to 34 VDC, 10 to 65 VDC, 20

to 320 VDC, and 20 to 265 VAC.

Direct Current Devices Direct current models are typically three-wire devices (two-wire

also available) requiring a separate power supply. The sensor is

connected between the positive and negative sides of the

power supply. The load is connected between the sensor and

one side of the power supply. The specific polarity of the

connection depends on the sensor model. In the following

example the load is connected between the negative side of

the power supply and the sensor.

Output Configurations Three-wire, DC proximity sensor can either be PNP (sourcing) or

NPN (sinking). This refers to the type of transistor used in the

output switching of the transistor.

The following drawing illustrates the output stage of a PNP

sensor. The load is connected between the output (A) and the

negative side of the power supply (L-). A PNP transistor

switches the load to the positive side of the power supply (L+).

When the transistor switches on, a complete path of current

flow exists from L- through the load to L+. This is also referred

to as current sourcing since in this configuration conventional

current is (+ to -) sourced to the load. This terminology is often

confusing to new users of sensors since electron current flow (-

to +) is from the load into the sensor when the PNP transistor

turns on.

The following drawing illustrates the output of an NPN sensor.

The load is connected between the output (A) and the positive

side of the power supply (L+). An NPN transistor switches the

load to the negative side of the power supply (L-). This is also

referred to as current sinking since the direction of conventional

current is into the sensor when the transistor turns on. Again,

the flow of electron current is in the opposite direction.

Normally Open (NO)Outputs are considered normally open (NO) or normally closed Normally Closed (NC)(NC) based on the condition of the transistor when a target is

absent. If, for example, the PNP output is off when the target is

absent then it is a normally open device. If the PNP output is on

when the target is absent it is a normally closed device. Complementary Transistor devices can also be complementary (four-wire). A

complementary output is defined as having both normally open

and normally closed contacts in the same sensor.

Series and Parallel In some applications it may be desirable to use more than one Connections sensor to control a process. Sensors can be connected in series

or in parallel. When sensors are connected in series all the

sensors must be on to turn on the output. When sensors are

connected in parallel either sensor will turn the output on.

There are some limitations that must be considered when

connecting sensors in series. In particular, the required supply

voltage increases with the number of devices placed in series.

Shielding Proximity sensors contain coils that are wound in ferrite cores.

They can be shielded or unshielded. Unshielded sensors usually

have a greater sensing distance than shielded sensors.

Shielded Proximity Sensors The ferrite core concentrates the radiated field in the direction

of use. A shielded proximity sensor has a metal ring placed

around the core to restrict the lateral radiation of the field.

Shielded proximity sensors can be flush mounted in metal. A

metal-free space is recommended above and around the

sensor’s sensing surface. Refer to the sensor catalog for this

specification. If there is a metal surface opposite the proximity

sensor it must be at least three times the rated sensing

distance of the sensor from the sensing surface.

Unshielded Proximity An unshielded proximity sensor does not have a metal ring Sensors around the core to restrict lateral radiation of the field.

Unshielded sensors cannot be flush mounted in metal. There

must be an area around the sensing surface that is metal free.

An area of at least three times the diameter of the sensing

surface must be cleared around the sensing surface of the

sensor. In addition, the sensor must be mounted so that the

metal surface of the mounting area is at least two times the

sensing distance from the sensing face. If there is a metal

surface opposite of the proximity sensor it must be at least

three times the rated sensing distance of the sensor from the

sensing surface.

34Mounting Multiple Sensors

Care must be taken when using multiple sensors. When two or more sensors are mounted adjacent to or opposite one another,interference or cross-talk can occur producing false outputs. The following guidelines can generally be used to minimize interference.?Opposite shielded sensors should be separated by at least four times the rated sensing range ?Opposite unshielded sensors should be separated by at least six times the rated sensing range ?Adjacent shielded sensors should be separated by at least two times the diameter of the sensor face ?Adjacent unshielded sensors should be separated by at least three times the diameter of the sensor face These are general guidelines. BERO proximity sensors have individual specifications which should be followed. For instance,

some devices are rated as suitable for side-by-side mounting.

Standard T arget A standard target is defined as having a flat, smooth surface,

made of mild steel that is 1 mm (0.04”) thick. Steel is available

in various grades. Mild steel is composed of a higher content of

iron and carbon. The standard target used with shielded sensors

has sides equal to the diameter of the sensing face. The

standard target used with unshielded sensors has sides equal

to the diameter of the sensing face or three times the rated

operating range,whichever is greater.

If the target is larger than the standard target, the sensing range

does not change. However, if the target is smaller or irregular

shaped the sensing distance (S n) decreases. The smaller the

area of the target the closer it must be to the sensing face to be

detected.

T arget Size A correction factor can be applied when targets are smaller than Correction Factor the standard target. To determine the sensing distance for a

target that is smaller than the standard target (S new), multiply

the rated sensing distance (S rated) times the correction factor

(T). If, for example, a shielded sensor has a rated sensing

distance of 1 mm and the target is half the size of the standard

target, the new sensing distance is 0.83 mm (1 mm x 0.83).

S new = S rated x T

S new = 1 mm x 0.83

S new = 0.83 mm

T arget Thickness Thickness of the target is another factor that should be

considered. The sensing distance is constant for the standard

target. However, for nonferrous targets such as brass,

aluminum, and copper a phenomenon known as “skin effect”

occurs. Sensing distance decreases as the target thickness

increases. If the target is other than the standard target a

correction factor must be applied for the thickness of the target.

T arget Material The target material also has an effect on the sensing distance.

When the material is other than mild steel correction factors

need to be applied.

Rated Operating Distances The rated sensing distance (S n) is a theoretical value which

does not take into account such things as manufacturing

tolerances, operating temperature, and supply voltage. In some

applications the sensor may recognize a target that is outside of

the rated sensing distance. In other applications the target may

not be recognized until it is closer than the rated sensing

distance. Several other terms must be considered when

evaluating an application.

The effective operating distance (S r) is measured at nominal

supply voltage at an ambient temperature of 23°C ± 0.5°. It

takes into account manufacturing tolerances. The effective

operating distance is ±10% of the rated operating distance. This

means the target will be sensed between 0 and 90% of the

rated sensing distance. Depending on the device, however, the

effective sensing distance can be as far out as 110% of the

rated sensing distance.

The useful switching distance (S u) is the switching distance

measured under specified temperature and voltage conditions.

The useful switching distance is ±10% of the effective

operating distance.

The guaranteed operating distance (S a) is any switching

distance for which an operation of the proximity switch within

specific permissible operating conditions is guaranteed. The

guaranteed operating distance is between 0 and 81% of the

rated operating distance.

Response Characteristic Proximity switches respond to an object only when it is in a

defined area in front of the switch’s sensing face. The point at

which the proximity switch recognizes an incoming target is the

operating point. The point at which an outgoing target causes

the device to switch back to its normal state is called the

release point. The area between these two points is called the

hysteresis zone.

Response Curve The size and shape of the response curve depends on the

specific proximity switch. The following curve represents one

type of proximity switch.

Review 2

1)An ____________ sensor uses an electromagnetic field

and can only detect metal objects.

2)Which of the following is not an element of an inductive

proximity sensor.

a.Target

b.Electrical Coil

c.Oscillator

d.Trigger Circuit

e.Output

3)An area surrounding an unshielded inductive proximity

sensor of at least ____________ times the area of the

sensing face must be metal free.

4)Shielded inductive proximity sensors mounted opposite

each other should be mounted at least ____________

times the rated sensing area from each other.

5) A standard target for an inductive proximity sensor is

made of mild ____________ and is 1 mm thick.

6) A correction factor of ____________ should be applied to

a shielded inductive proximity sensor when the target is

made of brass.

7)The guaranteed operating distance of an inductive

proximity switch is between 0 and ____________ % of

the rated operating distance.

Inductive Proximity Sensor Family

In this section we will look at the 3RG4 and 3RG04 families of

inductive proximity sensors. 3RG4 refers to the first part of the

part number that is used to identify this line of sensors.