Mechatronics
Interfaces
Electrical/Electronic
Relays
Relays have a wide range of applications in industry. Even with the
advent of electronic controls such as programmable logic controllers
and other solid state devices such as thyristors (solid state relays),
the electromechanical relay still retains a high level of acceptance.
One reason for this is that for simple types of control, relays offer a
cost effective solution. They are available in a large range of
configurations from small relays of only 10 mm in length with multiple
contact pairs for data communications, to large relays called
contactors for carrying heavy current loads in applications such as
switching 3 phase motors.
A relay has certain characteristics:




Low maintenance
Able to switch a number of independent circuit paths
Easily adaptable to various operating voltages
High operating speed, i.e. short switching times
A small amount of energy applied on the relay coil can control a
larger energy flow through the relay contacts.
In the
electropneumatic circuit, relays are generally used as signal
processors. Rather than switch solenoids directly via limit switches
and potentially overloading contacts, the relay contacts act as a
buffer, carrying the larger amount of current. Another important
function of the relay in circuitry is as a logic or interlocking device.
Relay construction
In practice there are many types of construction for the relay but the
functional principle is the same.
Fig. 1
Principle of relay operation
Festo Didactic  Mechatronics
v1.0
4.2 Page 1
Mechatronics
Interfaces
Electrical/Electronic
Operation
Fig. 2
Relay
When voltage is applied to the coil (5), an electric current flows
through the winding; a magnetic field builds up and pulls the
armature (3) against the core (7) of the coil. The armature is
mechanically joined to a contact 1 and is pulled against the contact 4.
This switching position is maintained as long as the voltage is
applied. When the voltage is removed the armature is restored to its
original position by a spring (6). In the initial position the contact 2 is
active.
In practice, symbols are used to represent relays in circuit drawing
(refer Fig.3). K1 relay in this case operates four NO contacts.
Fig. 3
Festo Didactic  Mechatronics
v1.0
Relay contact configuration
4.2 Page 2
Mechatronics
Interfaces
Electrical/Electronic
contactor coil
Symbol for a contactor
large voltage
small
voltage
K
Motor,
etc
Fig. 4
Relays and contactors can allow a small control
signal to switch
a much
larger power
Relays/contactor
switching
a different
circuit circuit.
Electrical Interface Signals
Because all interface devices either convert a physical signal or
parameter into an electrical signal or convert an electrical signal into
a physical signal or quantity, it is important to understand the different
types of electrical & electronic signals that are used in control
systems. These signal types are binary, digital & analogue.
Generation of binary signals
Computers and PLC's operate using binary or digital signals. By a
binary signal, we understand a signal which recognises only two
defined values or distinguishes between only two types of data.
Festo Didactic  Mechatronics
v1.0
4.2 Page 3
Mechatronics
Interfaces
Electrical/Electronic
Fig. 5
Binary signal
These values are termed “0” or “1”, the terms “low” and “high” or “off”
and “on” are also used. Binary signals can be generated simply and
reliably. An example is signals that are output from contacting
components. An actuated normally open contact corresponds to a
logic 1-signal and an unactuated one to a logic 0-signal. When
working with contactless components, this can give rise to certain
tolerance bands. For this reason, certain voltage ranges have been
defined as logic 0 or logic 1 ranges.
Fig. 6
Festo Didactic  Mechatronics
v1.0
Voltage ranges
4.2 Page 4
Mechatronics
Interfaces
Electrical/Electronic
IEC 1131-2 defines a value range of -3 V to 5 V as logic 0-signal, and
11 V to 30 V as logic 1-signal (for contactless sensors). This is
binding for PLC's, whose device technology is to conform to IEC
1131-2. In current practice, however, other voltage ranges can often
be found for logic 0- and 1-signal. Widely used are: -30 V to +5 V as
logic 0, 13 V to 30 V as logic 1.
Analogue signals
An analogue signal is a signal generated by physical variables which
change over a continuum. It is a continuous signal which varies in
level. It is a type of signal where a specific signal is assigned to each
value within a range of values. The signal always has a definite
value.
Fig. 7
Analogue signal
Digital signals
In the case of the analogue signal, a specific signal is allocated to
each value in a range of values. This is not the case with the digital
signal which can assume only a finite number of values. Every
possible value is a whole number multiple of a specific basic unit.
The precision of the display is, therefore, dependent on the value of
the basic unit. Example: digital clock (clock with digital display): If
this clock were designed for the display of hours only, the number of
possible values would be 1 to 12 (24) and the basic unit 1 hour. All
intermediate positions between two full hours would not be displayed.
Festo Didactic  Mechatronics
v1.0
4.2 Page 5
Mechatronics
Interfaces
Electrical/Electronic
Fig. 8
Digital signal
All analogue signals need to be converted into digital quantities so
that they may be understood by a CPU. Provided care is taken over
matching voltage and power levels, digital sensors or transducers are
connected directly to input ports of a controller as either current
sourcing (PNP) or sinking (NPN) devices.
Interfacing Analogue and Digital devices
PLC’s are digital devices. To handle analogue signals special
interfaces based on analogue to digital (A/D) converters, digital to
analog (D/A) converters, multiplexers and de-multiplexers are
required.
Digital to Analogue converter
The digital to analogue converter (DAC) produces an analogue
output from a digital input. In all types of DAC, the analogue voltage
is produced from a reference voltage (Vref). Binary code is input into
the DAC and determines what fraction of Vref is presented at the
output. (NB: Binary coded decimal is explained at end of chapter)
The output from a DAC is not truly continuous but rather a series of
discrete voltage levels.
Festo Didactic  Mechatronics
v1.0
4.2 Page 6
Mechatronics
Interfaces
Electrical/Electronic
Fig. 9
Eight bit digital-analogue converter (DAC)
The output from a DAC is not truly continuous but rather a series of
discrete voltage levels.
Fig. 10
Digital to analogue conversion
For example, the 8-bit DAC shown in Fig. 5 has an output given as
 B 7 B 6 B5 B 4 B 3 B 2 B1 B 0 
Vout = Vref 








 2
4
8
16 32 64 128 256 
Eqn. 1
where the bits B7 to B0 can take values 0 or 1 and are the binary
inputs. B7 is the most significant bit (MSB) and B0 the least
significant bit (LSB).
Festo Didactic  Mechatronics
v1.0
4.2 Page 7
Mechatronics
Interfaces
Electrical/Electronic
Consider an 8-bit DAC with a reference voltage Vref of 10 V. The
binary input of 00000001 generates the smallest discrete output, i.e.
10/256 volts. The next discrete output is 10/128 volts, generated
from the binary code 00000010. Clearly 256 discrete analogue levels
(referred to as quantisation levels) can be produced from the binary
input. The voltage resolution of an N-bit DAC is calculated by
dividing the maximum operating voltage by 2N-1. The factor 2N-1
represents the number of steps between quantisation levels. An 8-bit
DAC with a reference voltage of 10 V has a resolution of 10/255.
The speed of a DAC is determined by how long it takes to settle to a
stable value after a change in input. This is specified as the settling
time. The other main parameters of a DAC are linearity and
accuracy. Linearity is a measure of the deviation from a straight line
of output voltage plotted against binary input. Accuracy is the
variation between voltage plotted against binary input. Accuracy is
the variation between the DAC’s actual output and the intended one.
The operating principle of a DAC is illustrated in Fig. 11. DACs use a
set of binary weighted resistors switched by the binary input to
generate an analogue output from Vref. The highest weighted resistor
generates the smallest discrete value at the analogue output.
Fig. 11
Festo Didactic  Mechatronics
v1.0
Operating principle of a digital to analogue converter
4.2 Page 8
Mechatronics
Interfaces
Electrical/Electronic
Analogue to digital converter
The analogue to digital converter (ADC) produces a digital output
from an analogue input (see Fig. 12). ADCs incorporate start convert
(SC) and end of convert (EOC) connections. When the start convert
signal is pulsed the ADC converts the analogue input at that time into
an equivalent digital value. The ADC then produces an end of
convert signal to indicate that the conversion has finished.
Fig. 12
Eight-bit analogue to digital converter
The simplest type of ADC makes use of a DAC and a comparator as
shown in Fig. 13. Digital data from a counter is fed into the DAC and
generates an analogue voltage which is compared with the incoming
analogue voltage which is to be converted. When both signals
match, the comparator amplifier generates a logic 1 to indicate that
conversion has finished (i.e. the end of convert signal). The digital
value input to the DAC at that time represents the analogue input.
Fig. 13
Operating principle of a comparator type ADC
Festo Didactic  Mechatronics
v1.0
4.2 Page 9
Mechatronics
Interfaces
Electrical/Electronic
The main parameters of ADCs are again resolution, accuracy,
linearity and speed. The comments already made concerning
resolution, accuracy and linearity of DACs apply to ADCs. Note that
for an 8-bit ADC equation (Eqn. 1) works in reverse. Concerning
operating speed, ADCs are generally slower than DACs because the
process involves comparing one signal with another. Successive
approximation of the input value rather than ramping the DAC from a
counter speeds up the conversion process. For high speed, so called
‘flash’ converters are used.
Multiplexer
A multiplexer allows several signal carrying channels to share a
single line. A block diagram of a multiplexer is illustrated in Fig. 14.
This shows that each input channel may be connected to the output
line when one of a bank of switches inside the multiplexer is turned
on. In practice the bank of switches shown in Fig. 14 is a bank of
transistors.
Fig. 14
Multiplexer
Switches controlled by the lines A, B and C. A binary code placed on
the lines labelled A, B and C determines which of the channels is
switched through to the output. De-multiplexers are multiplexers that
work in reverse.
Festo Didactic  Mechatronics
v1.0
4.2 Page 10
Mechatronics
Interfaces
Electrical/Electronic
Interfacing
The general rule when interfacing analogue signals is to match
voltage levels and ensure that the impedance of the sourcing circuit
is less than or equal to that of its load circuit. Impedance matching is
essential for the optimum power transfer to the load circuit. To match
voltage levels you may have to reduce or amplify a voltage level.
When doing so, it is important to ensure that the signal amplifier
amplifies the signal in a reliable manner, i.e. that it does not deform
or distort the signal. Fig. 15 shows a signal amplified by a factor of
1000.
Fig. 15
Amplification of a signal
Analogue Transducers
Physical (analogue) quantities such as :








displacement
velocity
acceleration
force
pressure
temperature
flow
strain
Festo Didactic  Mechatronics
v1.0
4.2 Page 11
Mechatronics
Interfaces
Electrical/Electronic
are converted into analogue voltage or current by transducers. Some
common types of transducers are:











potentiometer
LVDT
strain gauges
displacement capacitors
tachometers
accelerometers
encoders
flow meters
thermocouples
RTD (resistance temperature detector)
Piezo - electric transducers
Potentiometer
The simplest way of producing an analogue input to an ADC is to use
a potentiometer circuit such as that shown in Fig. 16. The position of
the wiper terminal of the potentiometer is converted into a voltage
signal. Linear and rotary potentiometers may be used as low-cost
position transducers.
Fig. 16
Festo Didactic  Mechatronics
v1.0
Principle of Potentiometers
4.2 Page 12
Mechatronics
Interfaces
Electrical/Electronic
Linear relationship between shaft position and signal output.
e.g. (input voltage at AB) and if wiper contact is
at the centre
-
half applied voltage at output AC
at extreme left
-
zero voltage at output AC
at extreme right -
full voltage at output AC
Same principle apply to the angular displacement potentiometer but
in terms of varying angle of displacement.
LVDT (Linear Variable Differential Transformer)
The linear variable differential transformer of LVDT is a displacement
transducer. It consists of nickel-iron rod which is free to move
through primary and secondary coils. The basic arrangement is
illustrated in Fig. 17.
Fig. 17
Basic arrangement of linear variable differential transformer (LVDT)
Festo Didactic  Mechatronics
v1.0
4.2 Page 13
Mechatronics
Interfaces
Electrical/Electronic
The primary coil is fed alternating current so that voltages are
induced in the two halves of the secondary coil. Moving the rod back
and forth changes the phase and voltage in the secondary windings.
The output voltage versus core displacement characteristic in Fig. 18
shows that the phase of the output (secondary winding) relative to the
input (primary winding) changes by 180 degrees as the core is
moved through the central position. Consequently, a phase detector
is used if a unique output is required for each core position.
Fig. 18
Linear variable differential transformer (LVDT) output phase and
voltage core position
LVDT's have higher
potentiometers.
Festo Didactic  Mechatronics
v1.0
accuracy
and
reliability
compared
4.2 Page 14
to
Mechatronics
Interfaces
Electrical/Electronic
Thermocouples
A thermocouple consists of two dissimilar wires which are arranged
as shown in Fig. 15. Voltage is produced by thermoelectric effects as
the hot junction is heated. Thermocouple types which conform to
British standards are designated letters. These letters determine the
metals used in the thermocouple junction (see Fig. 19).
Fig 19
Type
E
J
K
T
Fig. 20
Diagrammatic representation of a thermocouple
Metal A/Metal B
Chromel/constantan
Iron/constantan
Chromel/alumel
Copper/constantan
Thermocouple and designation
Thermocouples are non-linear devices which means that their output
voltage is not proportional to temperature. Indeed, a thermocouple
will by supplied with a calibration table which must always be referred
to when converting an output voltage into temperature.
The voltage produced by a thermocouple needs to be amplified
before it can be fed into an ADC unit.
Festo Didactic  Mechatronics
v1.0
4.2 Page 15
Mechatronics
Interfaces
Electrical/Electronic
Strain gauge
A strain gauge is a device which changes resistance when stretched
or compressed. The relationship between the change in resistance
(R/R) and corresponding change in strain (i.e. length change (L/L)
is given as
G
R / R
L / L
Eqn. 2
where G is called the gauge factor.
The gauge factor G is about 2 for metal alloy strain gauges and
about 100 for semiconductor strain gauges. Although it is possible to
measure the strain directly using equation (Eqn. 2), it is normal
practice to use a balancing bridge circuit of the type shown in Fig. 21.
The analogue output of the bridge is nulled using the variable
resistance RV when no strain is applied to the gauge. When the
gauge is strained a voltage in the bridge circuit is observed because
the bridge is no longer balanced. This voltage is usually amplified
and fed back into an ADC so that it may be compared with a
calibrated value. Because of their large gauge factor, semiconductor
strain gauges produce much larger signals compared with metal
types. However, they are more sensitive to temperature variation.
Fig. 21
Festo Didactic  Mechatronics
v1.0
Strain gauge bridge
4.2 Page 16
Mechatronics
Interfaces
Electrical/Electronic
The decimal number system
Characteristic of the decimal number system used in general is the
linear array of digits and their significant place. The number 4344, for
instance, can be represented as follows:
4344 = 4 x 1000 + 3 x 100 + 4 x 10 + 4 x 1
Number 4 on the far left is of differing significance to that of number 4
on the far right.
The basis of the decimal number system is the availability of 10
different digits (decimal: originating from the Latin ‘decem’ = 10).
These 10 different digits permit counting from 0 to 9. If counting is to
exceed the number 9, this constitutes a carry over to the next place
digit. The significance of this place is 10, and the next carry over
takes place when 99 is reached.
The number 71,718,711 is to be used as an example:
107
106
105
104
103
102
101
100
7
1
7
1
8
7
1
1
As can be seen from the above, the significance of the “7” on the far
left is 70,000,000 = 70 million, whereas the significance of the “7” in
the third place from the right is 700.
The digit on the far right is referred to as the least significant digit,
and the digit on the far left as the most significant digit.
Any number system can be configured on the basis of this example,
the fundamental structure can be applied to number systems of any
number of digits. Consequently, any computing operations and
computing methods which use the decimal number system can be
applied with other number systems.
The binary number system
We are indebted to Leibnitz, who applied the structures of the
decimal number system to two-digit calculation. As long ago as
1679, this created the premises essential for the development of the
computer, since electrical voltage or electrical current only permits a
calculation using just two values: e.g. “current on”, “current off”.
These two values are represented in the form of digits: “1” and “0”.
If one were to be limited to exactly 2 digits per place of a number,
then a number system would be configured as follows:
Festo Didactic  Mechatronics
v1.0
4.2 Page 17
Mechatronics
Interfaces
Electrical/Electronic
27=128
26=64
1
0
25=32 24=16
1
1
23=8
22=4
21=2
20=0
0
0
0
1
The principle is exactly the same as that of the method used to
create a decimal number. However, only two digits are available,
which is why the significant place is not calculated to the base 10x,
but to the base 2x. Hence the lowest significant number on the far
right is 20 = 1, and of the next place 21 = 2 etc. Because of the
exclusive use of two digits, this number system is known as the
binary or also the dual number system.
Up to a maximum of
28 - 1 = 256 - 1 = 255
can be calculated with eight places, which would be the number
1111 11112.
The individual places of the binary number system can adopt one of
the two digits 0 or 1. This smallest possible unit of the binary system
is termed 1 bit.
In the above example, a number consisting of 8 bits, i.e. one byte,
has been configured (in a computer using 8 electrical signals
representing either “voltage available “ or “voltage not available” or
“current on” or “current off”.) The number considered, 1011 00012,
assumes the decimal value 17710.
1 x 27
= 128
0 x 26
1 x 25
+ 32
1 x 24
+ 16
0 x 23
0 x 22 0 x 21
1 x 20
+1
= 177
Example
The BCD code
For people used to dealing with the decimal system, binary numbers
are difficult to read. For this reason, a more easily readable numeral
representation was introduced, i.e. the binary coded decimal notation,
the so-called BCD code (binary coded decimal). With this BCD code,
each individual digit of the decimal number system is represented by
a corresponding binary number:
Festo Didactic  Mechatronics
v1.0
4.2 Page 18
Mechatronics
Interfaces
Electrical/Electronic
Table 1
010
0000BCD
110
0001BCD
210
0010BCD
310
0011BCD
410
0100BCD
510
0101BCD
610
0110BCD
710
0111BCD
810
1000BCD
910
1001BCD
Representation of decimal numbers in BCD code
4 digits in binary notation are therefore required for the 10 digits in
the decimal system. The discarded place (in binary notation, the
numbers 0 to 15 may be represented with 4 digits) is accepted for the
sake of clarity.
The decimal number 7133 is thus represented as follows in the BCD
code:
0111 0001 0011 0011BCD
16 bits are therefore required to represent a four digit decimal
number in the BCD code. BCD coded numbers are often used for
seven segment displays and coding switches.
The hexadecimal number system
The use of binary numbers is often difficult for the uninitiated and the
use of the BCD code takes up a lot of space. This is why the octal
and the hexadecimal system were developed. Three digits are
always combined in the case of the octal number system. This
permits counting from 0 to 7, i.e. counting in “eights”.
Alternatively, 4 bits are combined with the hexadecimal number
system. 4 bits permit the representation of the number 0 to 15, i.e.
counting in “sixteens”. The digits 0 to 9 are used to represent these
numbers in digits, followed by the letters A, B, C, D, E and F where A
= 10, B = 11, C = 12, D = 13, E = 14 and F = 15. The significant
place of the individual digits is to the base 16.
Festo Didactic  Mechatronics
v1.0
4.2 Page 19
Mechatronics
Interfaces
Electrical/Electronic
163=4096
162=256
161=16
160=1
8
7
B
C
Example
The number 87BC16 given as an example therefore reads as follows:
8 x 163 + 7 x 162 + 11 x 161 + 12 x 160 + 34 74810
Signed binary number
Up to now, we have dealt solely with whole positive numbers, not
taking into account negative numbers. To enable working with these
negative numbers, it was decided that the most significant bit on the
far left of a binary number is to be used to represent the preceding
sign: “0” thus corresponds to “+” and “1” corresponds to “-“.
Hence 1111 11112 = -12710 and 0111 11112 = +12810
Since the most significant bit has been used, one bit less is available
for the representation of a signed number. The following range of
values is obtained for the representation of a 16 digit binary number:
Table 2
Integer
Range of values
unsigned
0 to 65535
signed
-32768 to +32767
Range of values for binary numbers
Real number
Although it is now possible for whole positive and whole signed
numbers to be represented with 0 or 1, there is still the need for
points or real numbers.
In order to represent a real number in computer binary notation, the
number is split into two groups, a power of ten and a multiplication
factor. This is also known as the scientific representation of digits.
The number 27.3341 is thus converted into 273 342 x 10-4. Two
whole signed numbers are therefore required for a real number to be
represented in a computer.
Festo Didactic  Mechatronics
v1.0
4.2 Page 20