Transcript Introduction - Faculty Of Engineering And Technology

Introduction
A Sensor (also called detector or encoder) is a device that produces a
measurable response to a change in a physical condition, such as temperature,
speed or position.
It is a converter that measures a physical quantity and converts it into a signal
which can be read by an observer or by an electronic instrument.
Sensors are particularly useful for making measurements in industrial and drive
process control.
Introduction
The following figure shows the basic topology of an electric drive.
Introduction
“Closed loop" systems - also called feedback control systems - are necessary.
The purpose of the drive controller is essentially to convert the desired drive
torque/speed profile into triggering pulses for the electronic power converter,
taking into account various drive variables (currents, speed, temperature etc.)
fed back by the sensors.
The input to the power converter is controlled (manipulated) by the
controller.
The inputs to the controller consist of:
1. Torque, flux, speed, and/or position commands
2. Rate of variations to facilitate soft start, to preserve the integrity of the
load
3. The actual values of torque, flux, speed, position for feedback control.
4. Limiting values of currents, torque, acceleration, etc.
5. Temperature feedback, instantaneous currents & voltages in the motor
and/or converter.
Introduction
Introduction
A sensor is used to monitor the actual operating state of the system and to
feed back to the input of the controller an analogue or digital signal
representing the output state.
The actual and desired or reference states are continually compared and if the
actual state is different from the reference state an error signal is generated
Sensors can be classified as, either (Passive / Active) or (Analogue / Digital)
Sensors:
-Active Sensors:
they require an external power supply to operate
- Passive Sensors:
Passive sensors don’t need any additional energy source and directly generates
an electric signal in response to an external stimulus.
Introduction
- Analogue Sensors:
Analogue Sensors produce a continuous output signal or voltage which is
generally proportional to the quantity being measured.
Physical quantities such as Temperature, Speed, Pressure, Displacement, Strain
etc., are all analogue quantities as they tend to be continuous in nature as
example below.
Figure 1: Temperature is measured by thermo couple sensor which continuously
responds to temperature changes
Introduction
- Digital Sensors:
Digital Sensors produce a discrete digital output signals or voltages that are a
digital representation of the quantity being measured. Digital sensors produce
a Binary output signal in the form of logic “1” or logic “0”, (“ON” or “OFF”).
Figure 2: the speed of the rotating shaft is measured by using a digital LED/Opto-detector
sensor.
Introduction
Compared to analogue signals, digital signals or quantities have very high
accuracies and can be both measured and “sampled” at a very high clock
speed.
Circuits which measure analogue signals usually have a slow response and/or
low accuracy.
Also analogue signals can be easily converted into digital type signals.
In most cases, sensors and more specifically analogue sensors generally
require an external power supply and some form of additional amplification or
filtering of the signal
(Linear / Angular) Position and
Speed Sensors
Drives operated with closed-loop control are equipped with a measuring system
for the speed and the angular position, so-called angle sensors or encoders, in
order to achieve high speed and positioning accuracy.
In addition, on drives with synchronous motors they also supply the rotor
position information for the current control.
The speed is calculated by differentiation of the angle.
This differentiation is part of the angle evaluation.
(Linear / Angular) Position and
Speed Sensors
*Positional Sensors*
Position Sensors detect the position of something which means that they are
referenced either to or from some fixed point or position. These types of
sensors provide a “positional” feedback.
One method of determining a position is to use either “distance”
or by “rotation” (angular movement).
Inductive
Position Sensors
Linear
Variable
Differential
Transformer
The
Potentiometer
Inductive
Proximity
Sensors
Encoders
Incremental
Encoders
Absolute
Position
Encoders.
(Linear / Angular) Position and
Speed Sensors
1- The Potentiometer
A potentiometer is a resistive sensor used to measure linear displacements as
well as rotary motion.
It has a wiper contact linked to a mechanical shaft that can be either angular
(rotational) or linear (slider type) in its movement, and which causes the
resistance value between the wiper/slider and the two end connections to
change giving an electrical signal output that has a proportional relationship
between the actual wiper position on the resistive track and its resistance
value.
In other words, resistance is proportional to position.
A DC reference voltage is applied across the two outer fixed connections
forming the resistive element.
The output voltage signal is taken from the wiper terminal of the sliding
contact.
(Linear / Angular) Position and
Speed Sensors
This configuration produces a potential or voltage divider type circuit output
which is proportional to the shaft position.
The output signal (Vout) from the potentiometer is taken from the center
wiper connection as it moves along the resistive track, and is proportional to
the angular position of the shaft.
(Linear / Angular) Position and
Speed Sensors
While resistive potentiometer position sensors have many advantages:
- low cost
-low tech
- wear due to moving parts
- low repeatability
-low accuracy,
-limited frequency response.
they also have many disadvantages:
-easy to use
Typical applications for this type of high accuracy position sensor are in :
computer game joysticks, steering wheels, and industrial and robot applications
(Linear / Angular) Position and
Speed Sensors
2) A- Linear Variable Differential Transformer
(LDVT) is an inductive type position sensor which works on the same principle
as the AC transformer that is used to measure movement.
It is a very accurate device for measuring linear displacement and whose
output is proportional to the position of its moveable core
A small AC reference voltage called the “excitation signal” (2 – 20V rms, 2 –
20kHz) is applied to the primary winding which in turn induces an EMF signal
into the two adjacent secondary windings (transformer principles).
(Linear / Angular) Position and
Speed Sensors
The polarity of the output signal depends upon the direction and
displacement of the moving core.
The greater the movement of the soft iron core from its central null position
the greater will be the resulting output signal.
The result is a differential voltage output which varies linearly with the cores
position.
Therefore, the output signal has both amplitude that is a linear function of
the cores displacement and a polarity that indicates direction of movement.
(Linear / Angular) Position and
Speed Sensors
A typical application of a linear variable differential transformer (LDVT) sensor
would be as a pressure transducer
Advantages :
linearity, which is its voltage output to displacement is excellent, very good
accuracy, good resolution, high sensitivity as well as frictionless operation.
They are also sealed for use in hostile environments.
(Linear / Angular) Position and
Speed Sensors
2) B-Inductive Proximity Sensors
Another type of inductive position sensor in common use is the Inductive
Proximity Sensor also called an “Eddy current sensor”.
While they do not actually measure displacement or angular rotation they are
mainly used to detect the presence of an object in front of them or within a
close proximity, hence their name “proximity sensor”.
inductive proximity sensors operate under the electrical principle of Faraday’s
Law of inductance.
The “sensing” range of proximity sensors is very small, typically 0.1mm to
12mm.
(Linear / Angular) Position and
Speed Sensors
An inductive proximity sensor has four main components:
• The oscillator which produces the electromagnetic field
• The coil which generates the magnetic field
• The detection circuit which detects any change in the field when an object
enters it
• The output circuit which produces the output signal, either with normally
closed (NC) or normally open (NO) contacts.
They are also used in machine vibration monitoring to measure the variation
in distance between a shaft and its support bearing.
This is common in large steam turbines, compressors, and motors that use
sleeve-type bearings.
(Linear / Angular) Position and
Speed Sensors
Incremental Encoder
Incremental Encoders, also known as quadrature encoders or relative rotary
encoder, are the simplest of the two position sensors.
Their output is a series of square wave pulses generated by a photocell
arrangement as the coded disk, with evenly spaced transparent and dark lines
called segments on its surface, moves or rotates past the light source.
The encoder produces a stream of square wave pulses which, when counted,
indicates the angular position of the rotating shaft.
Incremental encoders have two separate outputs called “quadrature outputs”.
These two outputs are displaced at 90o out of phase from each other with the
direction of rotation of the shaft being determined from the output sequence.
(Linear / Angular) Position and
Speed Sensors
The number of transparent and dark segments or slots on the disk
determines the resolution of the device and increasing the number of lines in
the pattern increases the resolution per degree of rotation.
Typical encoded discs have a resolution of up to 256 pulses or 8-bits per
rotation.
The “Quadrature” or “Sine wave” encoder is the more common and has two
output square waves commonly called channel A and channel B.
This device uses two photo detectors, slightly offset from each other by 90o
thereby producing two separate sine and cosine output signals.
(Linear / Angular) Position and
Speed Sensors
The angle of the shaft in radians can be calculated using mathematical
functions.
Generally, the optical disk used in rotary position encoders is circular, then the
resolution of the output will be given as: θ = 360/n, where n equals the
number of segments on coded disk.
Also the direction of rotation is determined by noting which channel produces
an output first, either channel A or channel B giving two directions of rotation,
A leads B or B leads A.
(Linear / Angular) Position and
Speed Sensors
Absolute Position Encoder
- Absolute Position Encoders are more complex than quadrature encoders.
- They provide a unique output code for every single position of rotation
indicating both position and direction.
-Their coded disk consists of multiple concentric “tracks” of light and dark
segments.
-Each track is independent with its own photo detector to simultaneously read
a unique coded position value for each angle of movement.
-The number of tracks on the disk corresponds to the binary “bit”-resolution of
the encoder so a 12-bit absolute encoder would have 12 tracks and the same
coded value only appears once per revolution.
-Typical application of absolute position encoders are in computer hard drives
and CD/DVD drives were the absolute position of the drives read/write heads
are monitored or in printers/plotters to accurately position the printing heads
over the paper.
Optical sensors
-
Optical sensors are based on the modulation of light travelling between a
light source and a light detector.
- The transmitted light can travel along either an air path or a fibre-optic cable.
Light sources suitable for transmission across an air path include tungstenfilament lamps, laser diodes and light-emitting diodes (LEDs).
-The main forms of light detector used with optical systems are photocells
(cadmium sulphide or cadmium selenide).
-These are all photoconductive devices, whose resistance is reduced according
to the intensity of light to which they are exposed.
Optical sensors
Fiber-optic sensors usually incorporate either glass/plastic cables or all plastic
cables and characteristically enjoy long life.
Further advantages are:
-their simplicity
-low cost
-small size
-high reliability
-capability of working in many kinds of hostile environment.
Two major classes of fibre-optic sensor exist:
- Intrinsic sensors: the fibre-optic cable itself is the sensor.
- Extrinsic sensors: the fibre-optic cable is used to guide light to/from a
conventional sensor.
Optical sensors are commonly used to measure proximity, translation
almotion, and rotational motion.
Optical sensors
As an example, the optical sensor for the Brushless Dc Motor consists
of:
-A light source
-Three phototransistors P1, P2 and P3 mounted on the end plate of the motor,
separated by 120o from each other
-A revolving shutter coupled to the shaft of the motor.
Optical sensors
When the shutter revolves, the phototransistors get exposed to the light in the
sequence of their numbers.
In each revolution, the phototransistors generate the pulses PI1, PI2 and PI3
which have duration and phase displacement of 120o.
Optical sensors
Resolvers
Resolvers are magnetically operated angle sensors which comprise a rotary
transformer and the actual resolver part with a single-phase rotor winding and
a two-phase stator winding with the number of pole pairs (p).
The operating principle exploits the angle-dependent coupling between the
windings in the rotor and in the stator.
From the ratio of the induced voltages, the system can then calculate the
angle and rotational speed.
Resolvers are made of the same materials as the servo motors: copper and
iron, As a result, they are significantly more robust than optical angle sensors.
Resolvers
With the aid of the ring transformer, a carrier frequency voltage with the
amplitude Ve is transmitted in the single-phase rotor. The carrier frequency
voltage induces the voltages Vcos and Vsin in both phases of the stator
winding. As the two phases are electricallyoffsetby90°, the amplitudes of the
voltages depend cosinusoidally or sinusoidally on the angle of rotation:
Here, Itr is the transmission ratio of the ring transformer and Irs the ratio
between the rotor and stator windings. The angle of rotation can be
calculated from the ratio of the voltages:
Resolvers can with stand external temperatures and vibrations and are
resistant to voltage faults. The sensitive electronics are housed in the control
cabinet, where they are protected from environmental influences.
Temperature Sensors
There are many different types of Temperature Sensor available and all have
different characteristics depending upon their actual application.
A Temperature Sensor consists of two basic physical types:
• Contact Temperature Sensor Types
• Non-contact Temperature Sensor Types
Typical applications for temperature sensors include:
-HVAC - room, duct, and refrigerant equipment
-Motors - overload protection
-Electronic circuits - semiconductor protection
-Electronic assemblies - thermal management, temperature compensation
-Process control - temperature regulation
-Automotive - air and oil temperature
-Appliances - heating and cooling temperature
Temperature Sensors
Resistive Temperature Detectors (RTD)
The Resistance Temperature Detectors or RTD’s are precision temperature
sensors made from high-purity conducting metals such as platinum, copper or
nickel wound into a coil and whose electrical resistance changes as a function
of temperature.
However, they have very poor thermal sensitivity that is a change in
temperature only produces a very small output change.
A typical RTD has a base resistance of about 100Ω at 0oC, increasing to about
140Ω at 100C with an operating temperature range of between -200 to +600C.
RTD’s are passive resistive devices and by passing a constant current through
the temperature sensor it is possible to obtain an output voltage that increases
linearly with temperature
Temperature Sensors
The Thermocouple
The Thermocouple is by far the most commonly used type of all the
temperature sensor types.
Thermocouples are popular due to its simplicity, ease of use and their speed of
response to changes in temperature, due mainly to their small size.
Thermocouples also have the widest temperature range of all the temperature
sensors from below -200C to well over 2000C.
Thermocouples are thermoelectric sensors that basically consist of two
junctions of dissimilar metals.
When the two junctions are at different temperatures, a voltage is developed
across the junction which is used to measure the temperature
Temperature Sensors
The Thermocouple
The operating principal of a thermocouple is very simple and basic.
When fused together the junction of the two dissimilar metals such as copper
and constantan produces a “thermo-electric” effect which gives a constant
potential difference of only a few millivolts (mV) between them.
The voltage difference between the two junctions is called the “Seebeck
effect” then the output voltage from a thermocouple is a function of the
temperature changes.
Temperature Sensors
The Thermostat
The Thermostat is a contact type electro-mechanical temperature sensor or
switch, that basically consists of two different metals such as nickel, copper,
tungsten or aluminum that are bonded together to form a Bi-metallic strip.
The different linear expansion rates of the two dissimilar metals produce a
mechanical bending movement when the strip is subjected to heat.
The bi-metallic strip can be used itself as an electrical switch or as a
mechanical way of operating an electrical switch in thermostatic controls and
are used extensively to control hot water heating elements in boilers, furnaces,
hot water storage tanks as well as in vehicle radiator cooling systems.
Temperature Sensors
The Thermostat
-The thermostat consists of two thermally different metals stuck together
back to back.
-When it is cold the contacts are closed and current passes through the
thermostat.
-When it gets hot, one metal expands more than the other and the bonded
bi-metallic strip bends up (or down) opening the contacts preventing the
current from flowing..
Hall effect sensors
 When a current-carrying conductor is placed into a
magnetic field, a voltage will be generated
perpendicular to both the current and the field. This
principle is known as the Hall effect.
In other word it’s a magnetic field sensor.
Hall Effect sensor is a sensor that varies its output voltage in response to a
magnetic field.
Hall Effect sensors are used for proximity switching, positioning, speed
detection, voltage and current sensing applications.
Hall Effect Sensors are activated by an external magnetic field
the sensor detects it and generates an output voltage called the Hall Voltage,
Vh.
Hall Effect Sensors are available with either linear or digital outputs.
The output signal for linear (analogue) sensors is taken directly from the
output of the operational amplifier with the output voltage being directly
proportional to the magnetic field passing through the Hall sensor.
This output Hall voltage is given as:
VH = RH (I * B) / t
Where:
VH is the Hall Voltage in volts
RH is the Hall Effect co-efficient
I is the current through the sensor in amps
t is the thickness of the sensor in mm
B is the Magnetic Flux density in Teslas
Most Hall Effect devices can not directly switch large electrical loads as their
output drive capabilities are very small around 10 to 20 mA .
Rotor position can be determined by a Hall Effect device (or devices),
embedded in the stator, which provide an electrical signal representing the
magnetic field strength. The amplitude of this signal changes as the magnetic
rotor poles pass over the sensor.
Hall Effect devices can also be used for current sensing
Hall sensors are commonly used to time the speed of wheels and shafts, such
as for internal combustion engine ignition timing, tachometers and anti-lock
braking systems.
They are used in brushless DC electric motors to detect the position of the
permanent magnet.
In the pictured wheel with two equally spaced magnets, the voltage from the
sensor will peak twice for each revolution. This arrangement is commonly used
to regulate the speed of disk drives.
NO magnetic field no voltage
When magnetic field is present
voltage will be indused
Some hall effect sensor application
in motor drives :
 Brushless DC motor
 RPM sensors
 Current sensors
Brushless DC motor sensors
 Hall effect sensors detect the magnetic field coming
from the magnets on the rotor shaft and can know it’s
position this way better control of stator field
switching is achieved.
RPM sensors
 The RPM sensor is one of the most common
applications for a Hall sensor effect
 With RPM sensors the following can be achieved :
Speed control, Control of motor timing, Zero speed
detection, Tape rotation, Under or overspeed
detection, Disk speed detection, Automobile or tractor
transmission controller, Fan movement
Shaft rotation counter, Bottle counting, Radical position
indication, Drilling machines, Linear or rotary
positioning, Rotary position sensing, Flow-rate meter
Tachometer pick-ups
Current sensors
 Linear output Hall effect sensors can be used to sense
currents ranging from 250 milliamperes to thousands
of amperes
 The sensitivity of the simple current sensing system
shown in the figure can be increased by adding a flux
concentrator to the sensor. With the addition of a flux
concentrator, these sensors can be used to check over
or under speed, overload (current surges),
undercurrent and phase failure for large motors or
generators.
 This approach consists of a torroid core with a linear
sensor positioned in the gap. The core encloses the
sensor and acts as an additional flux concentrator
sensitive current sensor
The most common torque measuring principle uses
bonded strain gauge technology where the strain
gauges are bonded to a suitably designed shaft.
 Torque sensors with a circular shaft and with strain
gauges applied at 45deg is a design that has been
around for many years
 Typically four strain gauges are bonded and connected
into a Wheatstone bridge configuration with
temperature compensation components included
within the bridge circuitry. With an excitation voltage
applied to the bridge and torque induced into the
shaft, an electrical output linearly proportionate to
that torque will result.
 The completed Wheatstone Bridge requires a stable
DC supply to excite the circuit. This is usually 5Vdc or
10Vdc, but can be any value from 1Vdc up to 18Vdc
Static vs. Rotary Torque
 Torque measurements can be either Static (also known
as Reaction) or Rotary.
 A static or reaction torque measurement involves little
or no rotation of the item being measured, such as
during torque spanner testing
 A rotary torque measurement would, as the name
suggests, be subject to continuous rotation. This
rotation could be constant or periodical
Dynamic Torque
 A dynamic torque is one that is subject to variation or
change for instance the torque on the motor drive
shaft of an electric hover mower
 When selecting a transducer to measure dynamic
torque, it is important to ensure that Is has a
sufficiently wide frequency response bandwidth so
that the electrical output will react fast enough to
capture the changes (peaks or troughs) as they occur,
rather than smoothing or filtering them out.
Power & Signal Transmission in
Rotary Torque Sensors
 Slip Ring
 Rotary Transformer (Inductive Loop)
 Wireless Radio
Slip Ring
 The conductive rings rotate with the sensor and use a
series of sprung brushes to contact the rings and
transmit the electrical signal
 At low speed the electrical connection between the
rings and brushes are relatively noise free, however at
higher speeds electrical noise will eventually degrade
their performance.
Rotary Transformer
 For higher speed (rpm) torque measurement
applications the rotary transformer system is used
 The rotary transformer system consists of two coils,
one static coil that is attached to the transducer's
housing and one rotating coil that is attached to the
transducer shaft. This offers the distinct advantage of
there being no contact between rotor and stator and
incorporates both transmission of power to and signal
back from the rotating strain gauge bridge circuit.
 inaccuracies of ±0.2% and are capable of speeds up to
50,000rpm.
 The rotary transformer system is versatile enough for
use in special torque transducers and those where
space is limited.
Wireless Radio Telemetry
 The avoidance of rotor and stator coils means that
installing wireless torque sensors is a simplified
process over other types as there is no cabling
provision or coil alignment to
 the measurement signal can be transferred over
considerable distances (up to 120m) easily. consider
 The data stream provided by the wireless telemetry
system is broadcasted and so can be read by multiple
receiver units with different functions such as digital
displays, analogue outputs and PC-based USB
acquisition making multi-featured systems quick and
easy to set up.
Torque Measurement Summary
 To measure torque effectively it is important to
understand the factors involved in the creation of the
torque as well as those that may alter the torque
measurement: nominal torque rating, speed, available
space to mount the sensor, duration of measurement
and the environment within which the measurement
is taking place
 When measuring dynamic torque, the location of the
measurement is vital to ensure that the true torque is
measured and false measurements are avoided, caused
by either adjoining components in the drive train or
those that dampen the torque measurement system,
including the measurement system itself.
 The selection of the correct transducer rating,
transmission system and mechanical connection will
ensure the measurement solution is no more expensive
than it need be and maximum long-term accuracy and
reliability will be achieved
Summary
- Sensors are “Input” devices which convert one type of energy or quantity into
an electrical analogue or digital signals.
- Sensors are used to monitor the actual operating state of the electric drive
system and to feed back to the input of the controller an analogue or digital
signal representing the output state.
- The most common forms of sensors are those that detect Position,
Temperature, Light, Pressure and Velocity.
- Some sensors called “Self-generating” sensors generate output voltages or
currents relative to the quantity being measured
- Some sensors called “Modulating” sensors change their physical properties,
such as inductance or resistance relative to the quantity being measured and
need to be biased to provide an output voltage or current.
- Sensors act as feedback as they sense position of the rotor. It converts the
information of rotor position into a suitable electrical signal. This signal is used
to switch ON and OFF of various semiconductor devices of electronic switching
circuitry as in Brushless Dc motor.
Summary
-There are many types of (Linear / Angular) Position, Current,
Temperature and Speed Sensors that are used widely in electric
drives to adjust and control systems such as:
-Positional sensors (Potentiometer, Inductive Position Sensors,
Rotary Encoders)
-Resolvers
-Optical sensors
-Hall Effect sensors
-Thermocouples
-Thermostat
-Resistive Temperature Detectors (RTD)
Summary
-Factors to consider when choosing a sensor:
Accuracy: The statistical variance about the exact reading.
Calibration: Required since measuring systems readings will drift over time.
Cost: Economic consideration should be taken.
Environmental: Sensors typically have temperature and/or humidity limits.
Range: Limits of measurement or the sensor.
Resolution: The smallest increment the sensor can detect.
Repeatability: The variance in a sensor's reading when a single condition is
repeatedly measured.
References
1- “Measurement and Instrumentation Principles”, Alan S Morris, Third
Edition.
2- “Electric Drives and Electromechanical Systems”, Richard Crowder.
3- “Control of Electrical Drives”, Werner Leonhard, Third Edition.
4- “Electric Drives”, Ion Bolden and S.A. Nasar, 2nd edition.
5- Basic Electronics Tutorials Site by Wayne Storr., www.electronicstutorials.ws
6- Ed Ramsden (2006). Hall-effect sensors: theory and applications
7- Petruk, O.; Szewczyk, R.; Salach, J.; Nowicki, M. (2014). "Digitally
Controlled Current Transformer with Hall Sensor"
8- “Measurement, Instrumentation, and Sensors Handbook”, John G.
Webster, HalitEren, Second Edition
9- “MATLAB Library site”, www.mathworks.com /about-the-electric-driveslibrary.html