Sensors Basics

8/3/2019 Sensors Basics

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Industrial Sensors

Programmable Logic Controllers

Industrial Controls

Sensors are the eyes and ears of the PLC system. Without sensor the PLC has no

idea what the current state is of the process it is trying to control.


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Overview

Classification of Sensors Sensor Types

Analog

Digital

Wiring

Example Applications

There are several ways to classify sensors. Some of these classification schemes

will be discussed. The output of a sensor can fall in two different categories. If the

sensor senses when its input is above or below a threshold the sensor type is digital

or discrete. If the sensor senses a continuum of values, the sensor is analog. Wiring

of sensors is shown so that the sensor may be powered and a signal returned. Some

typical applications of sensors is discussed at the end of this presentation.


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Sensor Types

ContactContact type mustbe activated byprocess beingmonitored

NonNon--contactcontact typeindirectly activatedby process beingmonitored

Some sensors come in direct contact with the object, whose property it is being

detecting. If contact is permissible this is the most common type of sensor used.

The primary reason being that cost is the lowest. Examples of contact type sensors

are limit switches. When the object is in position the limit switch is energized

allowing a voltage to be present or not present on the input modules node. Whether

the signal is present or absent when the switch is energized depends on whether the

switch is normally-open or normally-closed.

Other times it is impossible or to costly to make direct contact with the object.

Then the object is monitored in a remote and non-contact way. Often this involves

using a transducer. Because of the added transducer the cost of non-contact sensor is

usually higher.


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Sensor Types

DigitalDigital sensors interface with discreteI/O modules on PLC lowest cost sensor and I/O module

only detect discrete state changes

AnalogAnalog sensors interface to ADC orDAC modules on PLC higher cost sensor and I/O module

detects continuum of values

An other way to classify sensors is whether the output is a digital level or a

continuum of values. In the case of digital sensor the signal is in one of two

different states. When the measured parameter is below a specified threshold, the

output is false. When it is above that threshold it is true. Many digital sensors have

a knob to adjust the threshold level. This can be a blessing and a curse. It is a

blessing in that it allows the sensors application to be tailored to the applications

needs. The curses is in that threshold may drift or be maliciously changed. This

requires the threshold to be set again by a trained and qualified person.

Analog sensor produce a current or voltage proportional to the parameter being

measures. As an example a thermal couple may be used to measure temperature.

This signal may be amplified and applied to an analog in module on the PLC or a

special thermocouple/millivolt input module may be used.


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Digital Sensors

OpticalOptical monitoring Detectors

Light sensing (light-on) with or without delay

Dark sensing (dark-on) with or without delay

Detector and source Diffusion reflective type

Polarizing Photo-sensors

Retroreflective

Thru-beam

Optical sensor are used to monitor the presents or absences of an object between the

light source and the detector. A light sensing optical sensor produces a logic-1 our

when light is falling on the detector. Their may or may not be a time delay

associated with the sensor. With a delay, the sensor will ignore interruptions less

than the delay period. Without delay, the sensor output will have 1-0-1 glitches

caused by the short interruption in the light path.

Dark sensing is the opposite of light sensing in the the output is a logic-1 when the

beam is broken and logical-0 when the beam unbroken. Dark sensing device may or

may not have a delay. The delay works similar to that of the light sensing devices.

When light hits the sensor for short period the output changes quickly from 0 to 1 to

0 when no delay is provided by the device. With the delay the 0-1-0 glitch is

masked from the sensor output.

Polarized photo sensors have a polarizing filter on the source and the detector. The

polarizing filter on the source blocks all light except that that has a given E-H

polarization. Lets say horizontal polarization. The sensor has a matching

polarizing filter. This only allows light with a horizontal polarization to enter the

detector. Stray light source are unlikely to also produce horizontally polarized light.

This reduces the chances of noise in the system with sporadic pulses in the sensor

output.

Retro-reflective sensor have the source and detector in the same package. The light

exits the source, travel across the path of object to be detected. The light hits a


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Digital Sensors

Optical monitoring

Detector and source

Convergent photo-sensors

Fiber-optic sensors

Color mark sensor

Laser sensor


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Digital Sensors

Optical monitoring

Encoders Absolute encoders

Relative or incremental encoders

quadrature

tachometer or dingle track

In principle, absolute encoders are similar to incremental encoders, in that a rotating

disk interrupts a

photodetector to produce an output signal. However, absolute encoders are

different in two very important ways:

1. Every position of an absolute encoder is unique. Unlike an

incremental encoder, where position is determined by counting pulses from a

zero mark or home base, the absolute encoder reads a system of coded

tracks to establish position information. No two positions are alike.

2. Absolute encoders do not lose position when power is removed.

Since each position is unique, true position verification is available as

soon as power is up. It is not necessary to initialize the system by returning

to home base. Photodetector Stationary mask LED Light source

Rotating Encoder Disk


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Digital Sensors

Non-optical Monitoring Ultrasonic sensors

Electro-magnetic Monitoring

Inductive sensors or inductive pickup

sensing distance

hysteresis

Ultrasonic sensors are some times used in place of optical sensors. Instead of using

an light beam, a high frequency sound wave is used. This sound wave is above

normal hearing frequencies and are called ultrasonic. Frequencies around 40 KHz

are common.

It the device is ferromagnetic and inductive sensor may be used. As the distance to

the ferromagnetic changes the inductance in the system changes. If a permanent

magnet acts as the source, the flux density will changes which can be detected by a

coil. This changing flux density will induce a voltage on the coils. This voltage is

then


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Analog Sensors

Accuracy -- refers to the fixed amount asensor reading deviates from a knownor calibrated input.

Precision -- refers to the ability of thesensor to replicate is measurement.

Repeatability -- can measurement berepeated to within specified accuracy

and precision

Accuracy relates to the difference between the reading and the know true value. If

the output is within say 3 microvolts of 320 millivolts and the known true value is

322 millivolts, this sensor is accurate. If instead the output is within say 3

microvolts of 320 millivolts and the known true value is 567 millivolts, this sensor

is inaccurate.

Precision relates to the repeatability of the sensor. If the true value is 678 millvolts

and the sensor reading is 677 millivolts 89 millivolts the sensor is imprecise but

accurate. This is because precision relates the deviations about the mean.

A sensor with high repeatability must have high accuracy and high precision.


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Precision Error

Precision error is always present when

successive measurements of anunchanging quantity yield differentnumerical values

True Value

Imprecise but may be accurate

Time

Reading

The figure above indicates that precision is a statistical property of the sensor.

Since the mean is close to the true value the accuracy is high. But since the

variance is high the sensor is imprecise. Precision relates to the variance or distance

squared from the mean of each reading. The standard deviation is the square root of

the variance.


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Accuracy Error

Accuracy error is always present when the numericalaverage of successive reading deviates from theknown correct reading and continues to deviate nomatter how many readings are made.

True Value

Precise but may be inaccurateTime

Reading

The figure above indicates that accuracy is a statistical property of the sensor. Since

the mean is far from the true value the accuracy is low. But since the variance is

low the sensor is precise.


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Analog Sensors

Thermocouples Seebeck thermoelectric effect

Most often Peltier style

Not Thomas style

See text for lettered types

RTD (Resistive Temperature Device) metal wire

positive temperature coefficient

In early 1820, Seebeck searched experimentally for a relation between electricity

and heat. In 1821, he joined two wires of dissimilar metals (copper wire and

bismuth wire) to form a loop or circuit. Two junctions were formed by connecting

the ends of the wires to each other. He then accidentally discovered that if he heated

one junction to a high temperature, and the other junction remained at a cooler

temperature a magnetic field was observed around the circuit of different

temperatures. He did not recognize, believe, or report that an electrical current was

being generated when heat was applied to one junction of the two metals. He used

the term thermomagnetic currents or thermomagnetism to express his discovery.

During the following two years, 1822-1823, he reports on his continuing

observations to the Prussian Academy of Sciences, where he describes this

observation as "the magnetic polarization of metals and ores produced by a

temperature difference.

The basic concept behind thermoelectric modules (TEMs) is the Peltier effect which

was discovered in 1834. The Peltier effect occurs whenever current passes through

the circuit of two dissimilar conductors; depending on the current direction, the

junction of the two conductors will either absorb or release heat. The amount of heat

pumped is in direct proportion to the current supplied. The Peltier effect is utilized

to its maximum when thermocouples are made of material of different conductivity.

Today Kryotherm primarily uses Bismuth Telluride doped with Selenium and

Antimony as semiconductor material. Thoroughly refined ingredients are alloyed

together to result in polycrystalline semiconductor material with anisotropicproperties. Kryotherm vast experience in this field allows us to obtain high-quality

thermoelectric material which greatly contributes to our products reliability. Ingots


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Analog Sensors

Thermistor semiconductor devices

negative temperature coefficient

IC Temperature Sensor uses PN junction

voltage or current proportional totemperature

Hall effect sensors

A thermistor is a metal oxide semiconductor whose resistance varies with

temperature. For a conductor, as its

temperature is increased, its resistance will increase. However, the resistance of a

semiconductor will decreasewith an increase in temperature. Over a wide range of temperature, this change in

resistance is very non-linear.

However, in a restricted range of 10EC or less, it may appear fairly linear. Because

of this, thermistors are

employed in a wide range of applications as temperature sensors.

The basic physical principle underlying the Hall effect is the Lorentz force. When

an electron moves along a direction perpendicular to an applied magnetic field, itexperiences a force acting normal to both directions and moves in response to this

force and the force effected by the internal electric field. For an n-type, bar-shaped

semiconductor, the carriers are predominately electrons of bulk density n. We

assume that a constant current Iflows along the x-axis from left to right in the

presence of a z-directed magnetic field. Electrons subject to the Lorentz force

initially drift away from the current line toward the negative y-axis, resulting in an

excess surface electrical charge on the side of the sample. This charge results in the

Hall voltage, a potential drop across the two sides of the sample. (Note that the

force on holes is toward the same side because of their opposite velocity and

positive charge.) This transverse voltage is the Hall voltage VH

and its magnitude is

equal to IB/qnd, where Iis the current, B is the magnetic field, dis the sample

thickness, and q (1.602 x 10-19 C) is the elementary charge. In some cases, it is


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Analog Sensors

Strain Gage used in bridge configuration

compensate for temperature

The gage is made of a special alloy which changes resistance with strain. This

change in resistance is greater than that caused by changes in diameter of the gage

components. A standard ohmmeter is not sufficient to measure the small changes in

resistance developed in the gage. In order to detect such small resistance changes,

the gage is made an arm of a Wheatstone bridge circuit. Output voltage from the

Wheatstone bridge are interpreted as strains in the specimen.


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Analog Sensors

Linear Variable Displacement Transformer(LVDT)

additive or series adding connection

differential or series opposing connection

The main advantage of the LVDT transducer over other types of displacement

transducer is their high degree of robustness. This is derived from their very

principle in which there is no physical contact across the sensing element and so

there is zero wear in the sensing element. This also means that RDP Electronics

LVDTs can be made waterproof and in a format suitable for the most arduous

applications.

The LVDT principle of measurement is based on magnetic transfer which also

means that the resolution of LVDT transducers is infinite. The smallest fraction of

movement can be detected by suitable signal conditioning electronics.

The combination of these two factors plus other factors such as accuracy and

repeatability has ensured that this technology is still at the forefront of displacement

measurement after over 90 years.

An LVDT comprises a coil former or bobbin onto which three coils are wound. The

first coil, the primary is excited with an a.c. current, normally in the region of 1 to

10kHz at 0.5 to 10V rms. The other two coils, the secondaries are wound such that

when a ferrite core is in the central linear position, an equal voltage is induced into

each coil. However, the secondaries are connected in opposition so that in the

central position the outputs of the two secondaries cancel each other out.


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Analog Sensors

Resolvers based on transformer coupling

uses special I/O modules

Synchros and resolvers, World War II-era technology, still are widely used in

modern-day electronic motion-control applications. Essentially, they are

transformers. Just like a traditional transformer, they have a primary winding and

multiple secondary windings. And just like a transformer, their primary is driven by

an AC signal.

Synchros and resolvers are very similar; however, there are some differences. As

shown in Left Figure (above) a synchro has one primary winding and three

secondary windings, with each secondary winding mechanically oriented 120 apart.

In contrast, as shown in Right Figure (above) a resolver has two primary windings

and two secondary windings oriented at 90 to each other.

While a synchro and a resolver are electrically very similar to a transformer, they

are mechanically more like a motor. The primary winding in a synchro or a resolver

can be physically rotated with respect to the secondary windings. For this reason,

the primary winding is called the rotor. The secondary windings, which are fixed,

are called stators.

Synchros are often used to track the rotary output angle of a closed-loop system,

which uses feedback to achieve accuracy and repeatability. A synchro can be turned

continuously and, since its secondary winding outputs are analog signals, provide

infinite resolution output.

As the shaft of a synchro turns, the angular position of its rotor winding changes

with respect to its secondary (stator) windings. The relative amplitude of the

resulting AC output signals from the secondary windings indicates the rotary


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Analog Sensors

Pressure Sensors based on many technologies

measure pressure difference

Pressure Sensors are based on many technologies and are used to measure pressure

difference. If one side is vented to the ambient, the gage measure relative pressure

called the gage pressure. The other side of the gage is connected to a chamber

whose pressure you are trying to measure. If the one side is not vented to the

ambient, the gage is measuring absolute pressure.


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Wiring

Load powered sensors2-wire

Line powered sensor3-wire

Load

or

PLC

input

Sensor

Some sensor are passive, such as RTD, and you are only measuring and electrical

parameter, resistance, that varies with the physical parameter, temperature, that you

are measuring. This can be done with two wires. Other sensors are active and

require their own power sources. This is a three or four wire sensors. A three wire

sensor shares the power supply ground and the sensor signal ground. With a four

wire sensor the grounds are not common and often lead to a sensor signal that is not

corrupted by supply ground noise.


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Wiring

PLC

Sensor

PLCSensor

PNP or Sourcing

NPN or Sinking

V

V

PLC can have sinking or sourcing inputs. Allen Bradley and most other P{LC

manufactures us the sinking approach for inputs and sourcing for outputs. With

sinking inputs the two wire sensor is placed between the power rail and the PLC

input node. The sensor must then change from a high resistance to a low resistance.

For a three or four wire sensor the sensor signal output is attached to the PLC input

node. The signal ground is then attached to the PLCs common node for the input

module.


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Wiring

Sensor

PNP or Sourcing

NPN or Sinking

V

Sensor

V

PLC

PLC

Sourcing PLC required sourcing or PNP type sensors. The term PNP comes from

the fact that the PLC input node would most likely be connected to the base of a

PNP bipolar junction transistor. When the sensor is in its low resistance state the

base of the transistor is grounded. This causes the pnp transistor to turn on which

the PLC sees as a logic one.

NPN sensor work in a complementary manner. The PLCs input node is assumed to

be an npn BJT and the low resistance sensor pulls the base high and the BJT turns

on. This is detected as a 1 by the PLC.


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Wiring

Consideration dont confuse wiring circuits

mount to protect measurement surface

Don't confuse the sensor signal and the power supply lead. Interchanging leads

more often then not cause the electronics in the sensor to be partially destroyed.

Mount so the the surface that is making the measurement is protected. Also Protectthe rest of the sensor.


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Examples

Packaging system Monitoring station

Documents

Sensors Basics