Hardware Components for Automation
Sections:
1. Sensors
2. Actuators
3. Analog‐to‐Digital Conversion
4. Digital‐to‐Analog Conversion
5. Input/output Devices for Discrete Data
1
Hardware Components for Automation
Computer Process Control System
Input/output interface for different types of process parameters and variables
2
Hardware Components for Automation
Computer‐Process Interface
• To implement process control, the computer must collect data
from and transmit signals to the production process
• Components required to implement the interface:
– Sensors to measure continuous and discrete process
variables
– Actuators to drive continuous and discrete process
parameters
– Devices for ADC and DAC
– I/O devices for discrete data
3
Hardware Components for Automation
Sensor‐ Transducer
A wide variety of measuring devices is available for collecting data from
the manufacturing process for use in feedback control.
In general, a measuring device is composed of two components:
A sensor and a transducers.
Commonly, a sensor‐transducer combination is called transducer.
the sensor detect the physical variable of interest (such as temp.,
pressure or force) and a property related to its characteristic
changes: (e.g strain gauge‐ resistance change with deformation.)
The transducer convert the physical variable into an alternative form
(commonly electric voltage ), quantifying the variable in the
conversion and enabling reading of the conversion.
Thus has an electronic instrumentation (circuit)
The quantified signal can be interpreted as the value of the measured
variable.
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Hardware Components for Automation
Sensor‐ Transducer
Temp &Strain gauge sensor Examples
5
Hardware Components for Automation
Sensor‐ Transducer
Strain gauge
As the wire grid is distorted by
elastic deformation, its length
increases, and its cross‐sectional
area decreases.
These changes causes an increase
in the resistance of the wire of the
strain gauge, vice versa.
An electronic circuit detect the
resistance change and convert it
to strain; or to weight if the
dimensions of the loaded member
is known
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Hardware Components for Automation
Sensor‐ Transducer
In some cases the sensor and transducer are the same device; for
example, a limit switch that convert the mechanical movement of
a level to close an electric contact.
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Hardware Components for Automation
Sensors
A sensor is a transducer that converts a
physical stimulus from one form into a
more useful form to measure the stimulus
• Two basic categories:
1. Analog
2. Discrete
•
•
Binary
Digital (e.g., pulse counter)
8
Hardware Components for Automation
Sensors
Sensors‐analogue
An analogue measuring device produces a continuous analogue
signal such as electrical voltage, whose value varies in an
analogue manner with the variable being measured.
Examples as thermocouples, strain gauges, and potentiometers.
The output signal from an analogue measuring device must be
converted to digital data by an (ADC) in ordered to be used by
digital computer.
Sensors‐discrete
A discrete measuring device produces an output that can have
only certain values
Discrete sensor devices are often divided into two categories:
Binary and digital.
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Hardware Components for Automation
Sensors
Sensor – discrete –binary
A binary measuring device produces an output on/off (1‐0)
signal.
The most common devices operate by closing an electrical
contact from a normally open position
Limit switches operate in this manner.
Other binary sensors include photo electric sensors and
proximity switches
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Hardware Components for Automation
Sensors
Sensor – discrete –digital
A digital measuring device produces a digital output signal,
either in the form of a set of parallel status bits (e.g., photo
electric sensor array or a serious of pulses can be counted
(e.g., an optical encoder)
In either cases, the digital signal represents the quantity to be
measured.
Digital transducers are becoming increasingly common
because they are easy to read when used as standalone
measuring devices, and because they are compatible with
digital computer systems
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Hardware Components for Automation
optical encoder
Sensors
photo electric sensor array
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Hardware Components for Automation
Sensors
Sensors – MEMS
A significant trend in sensor technology has been the
development of very small sensors.
The term micro sensors refer to measuring devices whose
physical feature have dimensions in the micron (μm)
range.
Micro sensors usually fabricated out of silicon using
processing techniques associated with integrated circuit
manufacturing.
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Hardware Components for Automation
Sensors – MEMS
Sensors
Tire – pressure mentoring
Mmes chips for
automatic
switching from
portrait to
landscape mode
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Hardware Components for Automation
Sensors
Sensors – active
Sensors can be classified as active or
passive
An active sensor responds to the
stimulus without the need of any
external power.
An example as a thermocouple.
Which respond to an increase
in temperature by generating a
small voltage (microvolt range)
related to the measured temp.
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Hardware Components for Automation
Sensors
Sensors – passive
Passive sensor require an external
power source in order to operate.
A thermistor illustrates this case.
It also measure the temp., but its
operation requires an electric
current to be passed through it
As the temp increased, the thermistor
electrical resistance change. The
resistance can be determined and
related back to the temp.
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Hardware Components for Automation
Sensor Transfer Function
Sensors
The relationship between the value of the physical stimulus and the
value of the signal produced by the sensor in response to the stimulus
S = f(s)
where S = output signal,
s = stimulus, and
f(s) is the functional relationship between them.
• Ideal functional form is simple proportional relationship:
S = C + ms
C‐ output value at stimulus value at zero
m‐constant of proportionality between S and s. also can be thought as
sensor sensitivity (it is the measure of how much the output of the
sensor is affected by the stimulus.
Eg. A standard chrome/alumel thermocouple generates 40.6 (μv)
microvolt per degree celsius 0C
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Hardware Components for Automation
Sensor Clibration
Sensors
To use any measuring device, a calibration procedure is
requred to establish the relationship between the
physical variable to be measured and the converted
output signal (such as voltage)
The ease with which the calibration procedure can be
accomplished is one criterion by which a measuring
device can be evaluated
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Hardware Components for Automation
See table :
Common measuring devices used in
automation
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Hardware Components for Automation
Actuators
Hardware devices that convert a controller command signal into a
change in a physical parameter
• The change is usually mechanical (e.g., position or velocity)
• An actuator is a transducer because it changes one type of
physical quantity into some alternative form
• An actuator is usually activated by a low-level command signal,
so an amplifier may be required to provide sufficient power to
drive the actuator
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Hardware Components for Automation
Types of Actuators
1. Electrical actuators
– Electric motors
•
•
•
DC servomotors
AC motors
Stepper motors
– Solenoids
2. Hydraulic actuators
– Use hydraulic fluid to amplify the controller
command signal
3. Pneumatic actuators
– Use compressed air as the driving force
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Hardware Components for Automation
Types
of
Actuators
Hydraulic actuator
Hydraulic actuators use a hydraulic fluid to amplify the controller
command signal.
The available devices provide either linear or rotational motion
Hydraulic actuators are often specified when large force are
required
pneumatic actuator
Pneumatic actuators use compressed air as the driving power.
The available devices provide either linear or rotational motion
pneumatic actuators are often specified for low force
applications.
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Hardware Components for Automation
Comparison of hydraulic and pneumatic systems.
System characteristic
Hydrolic system
Penumatic system
Presureized fluid
Oil [or water oil emulsion]
Compressed air
compressibility
incompressible
compressible
Typical fluid l pressure level
20MPa
0.7 MPa
Forces applied by device
high
Low
Actuation speed of device
low
high
Speed control
Accurate speed
Difficult to control accurately
Problem with Fluid leaks
Yes , potential hazard
No,
Relative coast of device
high
low
Device construction and
maintenance
Close tolerance and good
surface finish
O rings used to prevent leaks
instead of highly accurate
components.
Automation applications
Preferred when high forces
and accurate controls are
required
Preferred when low coast and
high speed actuation are
required
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Hardware Components for Automation
Pneumatic cylinders and motors
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Hardware Components for Automation
Hydraulic cylinders and motors
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Hardware Components for Automation
Electric Motors
The motor consists of two basic components,
Stator and rotor
The stator is the ring-shaped stationary
components, and the rotor is the cylindrical
part that rotate inside the stator.
The rotor is assembled around a shaft that is
supported by bearings, and the shaft can be
coupled to the machinery components such as
gears, pulleys, lead screws or spindles.
Electric current supplied to the motor generate
a contentiously switching magnetic field that
causes the rotor to rotate in its attempt to
always align its poles with the opposite poles
of the stator
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Hardware Components for Automation
A Rotating Electric Motor
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Hardware Components for Automation
DC Electric Motors
Dc motor are powered by constant voltage.
The continuously switching magnetic field is achieved by means of a
rotary switching device, called a commutator, which rotate with the
rotor and picks up current from a set of carbon brushes that are
components of a stator assembly.
Its function is to continually change the relative polarity between the
rotor and stator, so that the magnetic field produce torque to
continuously turn the rotor.
The use of a commutator represent the traditional construction of the
DC-Motor .
The use of a commutator is a disadvantage because it results in
arcing, worn brushes, and maintenance problems.
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Hardware Components for Automation
DC Electric Motors
Special type of DC motor avoid the use of a commutator and
brushes, called a brushless DC motor, it used a solid sate circuitry
to replace the brushes and commutator components.
Elimination of these parts has the added benefit of reducing the
inertia of the rotor assembly, allowing higher speed operation .
DC motors are widely used for two reasons:
 the convenience of using of using direct current as power
source for example, the small electric motors in automobiles
are DC because the car’s battery supplies direct current.
 the traditional popularity of DC motors is that their torque –
speed relationship are attractive in many applications
compared to AC-motors.
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Hardware Components for Automation
DC Electric Motors
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Hardware Components for Automation
DC Electric Motors
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Hardware Components for Automation
DC Electric Servomotors
DC servomotors are a common type of DC motor used in
mechanized and automated systems.
The term servomotor simply means that a feedback loop is used to
achieve position speed control.
In a DC servomotor, the stator typically consists of two permanent
magnet on opposite side of the rotor.
The rotor, called the armature in DC motor, consists of three sets of
copper wire windings around a ferrous metal core.
Input current is provided to the windings through the commutator
and interact with the magnetic field of stator to produce the torque
that derive the rotor
32
Hardware Components for Automation
DC Electric Servomotors
The magnitude of the rotor torque is a function of the current passing through the
windings, and the relationship can be modeled with the following equation.
T  KT I a
T=Motor torque (N.m), KT,=torque constant of motor
(N.m/amp), Ia =net armature current (amp)
The reason for defining I as a net current is :
Rotating the armature in the magnetic field of the stator produces voltages across
the armature terminals, called the back emf. In effect the motor acts as a
generator and the back emf increases with rotational speed as follows:
Eb  KV 
Eb=Back emf (V), KV,=the voltage constant of motor
[V/(rad/sec)], ω =angular velocity (rad/sec)
The effect of the back‐emf is to reduce the current flowing through the armature
winding.
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Hardware Components for Automation
DC Electric Servomotors
Note that, the angular velocity, ω in rad/sec can be converted to more
familiar rotational speed N(rpm)
60
N
2
The relationship between Horsepower and Watts (T‐N.m, ω-rad/sec)
T
HP 
745.7
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Hardware Components for Automation
DC Electric Servomotors
Given the resistance of the armature Ra and an input voltage Vin
supplied to the motor terminals, the resulting armature current well be
Vin
Ia 
Ra
This is the starting current and it
produce the starting torque as c
T  KT I a
But as the armature begin to rotate it generate the back -emf Eb
which reduce the available voltage.
Thus the actual armature current depend on the rotational speed
of the rotor.
Vin  Eb Vin  K v 
Ia 
Ra

Ra
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Hardware Components for Automation
DC Electric Servomotors V  E V  K 
T  KT I a
Ia 
in
b
Ra

in
v
Ra
The torque produced by the DC servomotor at speed ω is:
 Vin  K v  

T  k t 
Ra


The power delivered by the motor is the product of torque and velocity
P  T
P- power in watts (n.m/sec)
T – motor torque n.m
ω - Angular velocity rad/sec
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Hardware Components for Automation
DC Electric Servomotors
The servomotor is connected either directly or through a gear reduction to a
piece of machinery (fan, pump, spindle, table derive, etc.) which is basically a
load that is driven by the servomotor.
The load required a certain torque to operate, and the torque is usually related to
the rotational speed in some way
In general the torque increases with speed.
In simplest case, the relationship is proportional:
TL  K L 
TL -load torque N.m
KL -the constant of proportionality between torque and angular
velocity Nm/(rad/sec)
The functionality between kL and TL may be other than proportional, such that
KL itself depend on angular velocity ω.
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Hardware Components for Automation
DC Electric Servomotors
As example the torque required to derive a fan increases approximately as
the square of the rotational speed, that is
T  2
The torque developed by the motor and the torque required by the load must
be balanced.
That is, T=TL in steady state operation and this amount of torque is called
the operating point.
The motor torque relationship with angular velocity can be plotted as shown
on the torque speed curve.
As shown in fig. the intersection of the two plots is the operating point,
which is defined by the values of torque and angular velocity
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Hardware Components for Automation
DC Electric Servomotors
The intersection of the two plots is
the operating point
Torque‐Speed Curve of a DC Servomotor and Load Torque Plot
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Hardware Components for Automation
DC Electric Servomotors
DC Servomotor operation
A DC servomotor has a torque constant Kt =0095N.m/amp. Its voltage
constant Kv =0.11 V/(rad/sec). The armature resistance Ra =1.6 ohms. A
terminal voltage of 24 volt is used to operate the motor Determine
a) The starting torque generated by the motor just as the voltage is applied
(this is referred to as start torque)
b) The maximum speed at a torque of zero.
c) The operating point of the motor when it is connected to a load whose
torque characteristic is given by TL = KL ω and KL =0.007 N.m/(rad/sec).
culculate the rotational speed in rev/min.
Solution:
a) At ω=0 , then max torque is achieved.
Eb  K V   0
Vin
Vin  Eb 24  0
Ia 

 15 Amp
Ra
1.6
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Hardware Components for Automation
DC Electric Servomotors
Then, the starting torque:
T  K T I a  0.095(15)  1.425 Nm
Vin
b) At T=0; then Max speed is achieved.
 Vin  Eb
T  k t 
 Ra

  0

then Vin  Eb  14  k v   0.11
24

 218.2 rad / sec
0.11
c)
At operating point Tm =TL;
TL  K L   0.007
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Hardware Components for Automation
 Vin  Eb 
24  0.11 

  0.0095
T  k t 

1.6


 Ra 
TL  K L   0.007
 24  0.11 
0.007  0.0095

1.6


  105.3 rad / sec  1006 rev / min
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Hardware Components for Automation
AC induction motors
AC motors can be classified into two categories:
1. Induction motors
2. Synchronous motors
AC induction motor are the most widely used motor in the world, due to their
relatively simple construction and low manufacturing coast (robust, cheap,
brushless).
In operation of this motor type, a magnetic filed is induced by the rotation of
the rotor through the magnetic filed of the stator (the power supplied to the
rotor by means of electromagnetic induction).
Because of this feature, the rotor in most induction motors does not need an
external source of electrical power.
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Hardware Components for Automation
AC synchronous motors
AC synchronous motors operate by energizing the rotor with alternating
current. Which generate a magnetic filed in the gap separating the rotor and
stator. (rotating magnetic field is produced in stator)
This magnetic field create a torque that turns the rotor at the same rotational
speed as the magnetic forces in the stator.
synchronous motors are somewhat more complex than induction motors
because they require a device called an exciter to initiate rotation of the rotor
when power is first supplied to the motor.
The exciter accelerate the rotational speed of the rotor to synchronize with that
of the stator rotating magnetic field, which is required condition for an ac
synchronous motor to function.
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Hardware Components for Automation
AC motors
AC induction motors and synchronous motors operate at constant speed that
depends on the frequency of the incoming current.
Their applications are usually those in which running at fixed speed is a
requirement.
This is a disadvantage in many automation applications because frequent speed
changes are often necessary with much starting and stopping.
The speed issue is sometimes addressed by using adjustable frequency drives
(called inverters) that control the cycle rate of the AC power to the motor
Motor speed is proportional to the frequency, so changing frequency changes
motor speed.
Advances in solid state electronics have also improved speed control for AC
motors and they are new competitive in some application traditionally reserved
for DC motors.
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Hardware Components for Automation
Stepper motors
Stepper motors provide rotation in the form of discrete angular displacements,
called step angles.
Each angular step is actuated by a discrete electrical pulse, and the total
angular rotation is controlled by the number of pulses received by the motor,
and rotational speed is controlled by the frequency of the pulses.
The step angle is related to the number of steps for the motor according to the
relationship:
360

ns
α- the step angle in Deg.
ns : the number of steps (per revolution) which must be integer.
Typical values for the step angle in commercially available stepper motors are:
7.5 Deg., 3.6 Deg., and 1.8 Deg. corresponding to 48,100, and 200 steps
(pulses per revolution of the motor.
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Hardware Components for Automation
The total angle through which the motor rotates Am is given by:
Am =np α
Am angle measured in degree.
np the number of pulses received by the motor.
α- the step angle in oC
The angular velocity ω in (rad/sec) and rotational speed N (rpm) are given by:

ns
fp
2f p
ns
N
60 f p
ns
the number steps in the motor (step/rev) or
pulses received by the motor (pulses/rev).
pulse frequency, (pulse /sec) (HZ).
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Hardware Components for Automation
Operation modes of Stepper motors
There are two operating modes, locked step and slewing.
In locked-step mode each pulse received by the motor causes a discrete
angular step to be taken; the motor start and stop(at least approximatly) with
each pulse.
In this mode the motor can be started and stopped, and its direction of rotation
can be reversed.
In slewing mode, usually associated with higher speeds, the motor’s rotation
is more or less contentious and does not allows for stopping or reversing with
each subsequent step.
Nevertheless, the rotor does not respond to each individual pulse; that is, the
relationship between rotating speed and pulse frequency is retained in the
slewing mode.
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Hardware Components for Automation
Stepper motors are used in :
Open loop control systems
For applications in which torque and power requirements are low or modest
Used for position control system, no noise problems
They are widly used in machine tools and other production machines,
industrial robot, x-y plotters, medical and scientific instruments, and computer
peripherals.
Probably the most common application is to derive the hands of analog quartz
watches.
49
Hardware Components for Automation
Typical Torque‐Speed Curve of a Stepper Motor
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Hardware Components for Automation
Solenoids
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Hardware Components for Automation
Cylinder and Piston:
(a) Single‐Acting and (b) Double‐Acting
Q
v
A
F  PA
V: velocity of the piston. (m/sec)
Q: volumetric flow rate (m3 /sec)
A: area of the piston (m2 )
F: applied force (N)
P : fluid pressure
(a)
(b)
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Hardware Components for Automation
A Typical Analog Control Loop for the industrial process
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Hardware Components for Automation
Components of a Direct Digital Control System
54
Hardware Components for Automation
1 Computer‐Process Interface
• To implement process control, the computer
must collect data from and transmit signals to
the production process
• Components required to implement the
interface:
– Sensors to measure continuous and discrete
process variables
– Actuators to drive continuous and discrete process
parameters
– Devices (converters) for ADC and DAC
– I/O devices for discrete data
Hardware Components for Automation
Analog‐to‐Digital interface
2 ‐We use RC filter to
smooth out signal
from random noise
4 –scale signal up or
dowen
Limit comparator to
make the digital signal
within limited rang
Procedure for converting an Analog signal from the process
into digital form
Hardware Components for Automation
Analog‐to‐Digital Conversion
An ADC converts a continuous analog signal from
transducer into digital code for use by computer
• ADC consists of three phases:
1. Sampling phase – converts the continuous signal into a
series of discrete analog signals at periodic intervals as in
fig.
2. Quantization phase – each discrete analog (sample) is
converted into one of a finite number of (previously
defined) discrete amplitude levels
3. Encoding phase – discrete amplitude levels are converted
into digital code, representing the amplitude level obtained
as a sequence of binary digit.
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Hardware Components for Automation
General characteristics of an ADC (how to select it)
• Sampling rate – rate at which continuous
analog signal is polled (recorded)
• Conversion time – how long it takes to convert
the sampled signal to digital code
• Resolution – depends on number of
quantization levels
• Conversion method – means by which analog
signal is encoded into digital equivalent
– Example – Successive approximation method
Hardware Components for Automation
Analog Signal Converted into a Series
of Discrete Data by A‐to‐D Converter
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Hardware Components for Automation
Successive Approximation Method
 A series of trial voltages are successively compared to
the input signal whose value is unknown
 Number of trial voltages = number of bits used to
encode the signal
 First trial voltage is 1/2 the full scale range of the ADC
 If the remainder of the input voltage exceeds the trial
voltage, then a bit value of 1 is entered, if less than
trial voltage then a bit value of zero is entered
 The successive bit values, multiplied by their
respective trial voltages and added, becomes the
encoded value of the input signal
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Hardware Components for Automation
Successive Approximation Method
Example for
input
voltage of
6.8 V
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Hardware Components for Automation
Digital‐to‐Analog Conversion
Converts the digital output of the computer
into a continuous analog signal to drive an
analog actuator (or other analog device)
• DAC consists of two steps:
1. Decoding – digital output of computer is
converted into a series of analog values at
discrete moments in time
2. Data holding – each successive value is
changed into a continuous signal that lasts
until the next sampling interval
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Hardware Components for Automation
Data Holding Step in DAC:
(a) Zero‐Order Hold and (b) First‐Order Hold
(a)
(b)
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Hardware Components for Automation
Characteristic of manufacturing process data
 continuous analog signal: as
force, pressure, temp, flow
rate, velocity etc.
Discrete binary data: as on or
of; open or closed; etc
 pulse data (discrete data that
are not restricted to binary;
i.e. more than two values are
possible: as digital
tachometers and turbine flow
meters; to run stepping
motors etc.
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Hardware Components for Automation
Process data input/output
Computer‐process Interface
65
Sequence control and programmable controllers
Binary input and output devices
Device
INPUT
(Sensors)
OUTPUT
(Actuators)
Limit switch
Photo-detector
Pushbutton switch
Timer
Control relay
Circuit breaker
Motor
Valve
Clutch
Alarm buzzer
Control relay
Solenoid
Lights
one/zero interpretation
Contact/no contact
Contact/no contact
On/off
On/off
Contact/no contact
Contact/no contact
On/off
Open/closed
Engaged/not engaged
on/off
Contact/no contact
energized /not energized
On/off
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Hardware Components for Automation
Input/Output Devices for Discrete Data
Binary data:
• Contact input interface – input data to computer
• Contact output interface – output data from computer
Discrete data other than binary:
• Contact input interface – input data to computer
• Contact output interface – output data from computer
Pulse data:
• Pulse counters ‐ input data to computer
• Pulse generators ‐ output data from computer
Hardware Components for Automation
Contact Input/Output Interfaces
Contact input interface:
series of contacts that are open or closed to
indicate the status of individual binary devices such
as limit switches and valves
– The computer periodically scans the contacts to update
values in memory
– Can also be used for discrete data other than binary
(e.g., a photoelectric sensor array)
Contact output interface:
communicates on/off signals from the computer to
the process
– Values are maintained until changed by the computer
Hardware Components for Automation
Pulse Counters and Generators
Pulse counter:
Converts a series of pulses (pulse train) into a digital
value
– Digital value is then entered into the computer through
its input channel
– Most common – counting electrical pulses
– Used for both counting and measurement applications
Pulse generator:
A device that produces a series of electrical signals
– The number of pulses or frequency of the pulse train is
specified by the computer
Hardware Components for Automation
Multiplexers
In electronics, a multiplexer (or mux) is a device that
selects one of several analog or digital input signals and
forwards the selected input into a single line. A multiplexer
of 2n inputs has n select lines, which are used to select
which input line to send to the output. Multiplexers are
mainly used to increase the amount of data that can be
sent over the network within a certain amount of time
and Bandwidth. A multiplexer is also called a data
selector.
Hardware Components for Automation
Multiplexers
Schematic of a 1‐to‐2 Demultiplexer. Like a
multiplexer, it can be equated to a controlled switch.
Schematic of a 2-to-1 Multiplexer. It can be
equated to a controlled switch.
71
Hardware Components for Automation
Multiplexers
The basic function of a multiplexer: combining
multiple inputs into a single data stream. On the
receiving side, a demultiplexer splits the single data
stream into the original multiple signals.
72
Sequence control and programmable controllers
HARDWARE FOR IMPLEMENTING COMBINATIONAL SYSTEMS
Binary sensors : for example limit switches and photodetector switch.
Solenoid
Electromechanical relay
73
Sequence control and programmable controllers
Electromechanical relay
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Sequence control and programmable controllers
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Sequence control and programmable controllers
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Sequence control and programmable controllers
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