Unformatted text preview: Module
Version 2 EE IIT, Kharagpur 1 Lesson
Displacement and Speed
Version 2 EE IIT, Kharagpur 2 Instructional Objectives
At the end of this lesson, the student should be able to
• Name three methods of displacement measurement using passive electrical sensors. • Sketch the construction and characteristics of LVDT. • Explain the principles of operation of inductive and capacitive types of proximity
sensors. • Distinguish between variable distance and variable area type of capacitance displacement
sensors. • Sketch and explain the principle of operation of a optical type displacement sensor. • Name two methods of noncontact type speed sensing and explain their principles of
Displacement and speed are two important parameters whose measurements are important in
many position and speed control schemes. Error free measurements of these two parameters are
necessary in order to achieve good control performance.
Displacement measurement can be of different types. The displacement may be in the range of
few μm to few cm. Moreover the measurement may be of contact type or noncontact type. Again
displacement to be measured can be linear or angular (rotary). Similar is the case for speed
measurement. Accordingly different measuring schemes are used for measurement of these two
parameters. In this lesson, we shall discuss about few such schemes. Displacement Measurement
Broadly speaking, displacement measurement can be of two types: contact and noncontact types.
Besides the measurement principles can be classified into two categories: electrical sensing and
optical sensing. In electrical sensing, passive electrical sensors are used variation of either
inductance or capacitance with displacement is measured. On the other hand the optical method
mainly works on the principle of intensity variation of light with distance. Interferometric
technique is also used for measurement of very small displacement in order of nanometers. But
this technique is more suitable for laboratory purpose, not very useful for industrial applications. Potentiometer
Potentiometers are simplest type of displacement sensors. They can be used for linear as well as
angular displacement measurement, as shown in Fig. 1. They are the resistive type of transducers
and the output voltage is proportional to the displacement and is given by:
eo = i E ,
where xi is the input displacement, xt is the total displacement and E is the supply voltage. Version 2 EE IIT, Kharagpur 3 The major problem with potentiometers is the contact problem resulting out of wear and tear
between the moving and the fixed parts. As a result, though simple, application of potentiometers
is limited. θi
eo (a) E (b) Fig. 1 Potentiometer
(b) Rotary Linear Variable Differential transformer (LVDT)
LVDT works on the principle of variation of mutual inductance. It is one of the most popular
types of displacement sensor. It has good linearity over a wide range of displacement. Moreover
the mass of the moving body is small, and the moving body does not make any contact with the
static part, thus minimizing the frictional resistance. Commercial LVDTs are available with full
scale displacement range of ± 0.25mm to ± 25mm. Due to the low inertia of the core, the LVDT
has a good dynamic characteristics and can be used for time varying displacement measurement
The construction and principle of operation of LVDT can be explained with Fig. 2(a) and Fig.
2(b). It works on the principle of variation of the mutual inductance between two coils with
displacement. It consists of a primary winding and two identical secondary windings of a
transformer, wound over a tubular former, and a ferromagnetic core of annealed nickel-iron alloy
moves through the former. The two secondary windings are connected in series opposition, so
that the net output voltage is the difference between the two. The primary winding is excited by
1-10V r.m.s. A.C. voltage source, the frequency of excitation may be anywhere in the range of
50 Hz to 50 KHz. The output voltage is zero when the core is at central position (voltage induced
in both the secondary windings are same, so the difference is zero), but increasing as the core
moves away from the central position, in either direction. Thus, from the measurement of the
output voltage only, one cannot predict, the direction of the core movement. A phase sensitive
detector (PSD) is a useful circuit to make the measurement direction sensitive. It is connected at
the output of the LVDT and compares the phase of the secondary output with the primary signal
to judge the direction of movement. The output of the phase sensitive detector after low pass
filtering becomes a d.c voltage for a steady deflection. The output voltage after PSD vs.
displacement characteristics is shown in Fig. 2(c). Version 2 EE IIT, Kharagpur 4 V0
Ferromagnetic Core Secondary
Coil Vref Secondary
Coil Fig. 2(a) Construction of LVDT. vo Secondary
Coil Core Primary
Fig. 2(b) Series opposition connection of secondary windings. Version 2 EE IIT, Kharagpur 5 Voltage
(d.c) Displacement x Linear
Range Fig. 2(c) Output voltage vs. displacement characteristics of LVDT after
Phase sensitive detection. Inductive type Sensors
LVDT works on the principle of variation of mutual inductance. There are inductive sensors for
measurement of displacement those are based on the principle of variation of self inductance.
These sensors can be used for proximity detection also. Such a typical scheme is shown in Fig. 3.
In this case the inductance of a coil changes as a ferromagnetic object moves close to the
magnetic former, thus change the reluctance of the magnetic path. The measuring circuit is
usually an a.c. bridge.
Target Object X
(Measurand) Fig. 3 Schematic diagram of a self inductance type proximity sensor.
Version 2 EE IIT, Kharagpur 6 Rotary Variable Differential Transformer (RVDT)
Its construction is similar to that of LVDT, except the core is designed in such a way that when it
rotates the mutual inductance between the primary and each of the secondary coils changes
linearly with the angular displacement. Schematic diagram of a typical RVDT is shown in Fig. 4.
θ Fig. 4 Rotary Variable Differential Transformer (RVDT) Resolver
Resolvers also work on the principle of mutual inductance variation and are widely used for
measurement of rotary motion. The basic construction is shown in Fig. 5. A resolver consists of
a rotor containing a primary coil and two stator windings (with equal number of turns) placed
perpendicular to each other. The rotor is directly attached to the object whose rotation is being
measured. If a.c. excitation of r.m.s voltage Vr is applied, then the induced voltages at two stator
coils are given by:
v01 = KVr cosθ
and v02 = KVr sin θ ; where K is a constant. By measuring these two voltages the angular position can be uniquely determined, provided
(0 ≤ θ ≤ 900 ) . Phase sensitive detection is needed if we want to measure for angles in all the
Synchros work widely as error detectors in position control systems. The principle of operation
of synchros is similar to that of resolvers. However it will not be discussed in the present lesson. Version 2 EE IIT, Kharagpur 7 Output
vo2 = Kvref sin θ Rotor AC
vo1 = Kvref cos θ Stator Fig. 5 Schematic diagram of the resolver. Capacitance Sensors
The capacitance type sensor is a versatile one; it is available in different size and shape. It can
also measure very small displacement in micrometer range. Often the whole sensor is fabricated
in a silicon base and is integrated with the processing circuit to form a small chip. The basic
principle of a capacitance sensor is well known. But to understand the various modes of
operation, consider the capacitance formed by two parallel plates separated by a dielectric. The
capacitance between the plates is given by:
ε ε A
C= r 0
where A=Area of the plates
d= separation between the plates
ε r = relative permittivity of the dielectric ε 0 = absolute permittivity in free space = 8.854 × 10 −12 F / m .
A capacitance sensor can be formed by either varying (i) the separation (d), or, (ii) the area (A),
or (iii) the permittivity ( ε r ). A displacement type sensor is normally based on the first two
(variable distance and variable area) principles, while the variable permittivity principle is used
for measurement of humidity, level, etc. Fig.6 Shows the basic constructions of variable gap and
variable area types of capacitance sensors mentioned above. Fig. 6(a) shows a variable distance
type sensor, where the gap between the fixed and moving plates changes. On the other hand, the
area of overlap between the fixed plate and moving plate changes in Fig. 6(b), maintaining the
gap constant. The variable area type sensor gives rise to linear variations of capacitance with the
input variable, while a variable separation type sensor follows inverse relationship.
Version 2 EE IIT, Kharagpur 8 Fixed (a) (b) Fig.6 Capacitive type displacement Sensors:
a) variable separation type,
b) variable area type.
Capacitance sensors are also used for proximity detection. Such a typical scheme is shown in
Fig. 4. Capacitive proximity detectors are small in size, noncontact type and can detect presence
of metallic or insulating objects in the range of approximately 0-5cm. For detection of insulating
objects, the dielectric constant of the insulating object should be much larger than unity. Fig. 7
shows the construction of a proximity detector. Its measuring head consists of two electrodes,
one circular (B) and the other an annular shaped one (A); separated by a small dielectrical
spacing. When the target comes in the closed vicinity of the sensor head, the capacitance
between the plates A and B would change, which can be measured by comparing with a fixed
The measuring circuits for capacitance sensors are normally capacitive bridge type. But it should
be noted that, the variation of capacitance in a capacitance type sensor is generally very small
(few pF only, it can be even less than a pF in certain cases). These small changes in capacitor, in
presence of large stray capacitance existing in different parts of Target x A B Fig. 7 Capacitance Proximity Detector. Version 2 EE IIT, Kharagpur 9 the circuit are difficult. So the output voltage would generally be noisy, unless the sensor is
designed and shielded carefully, the measuring circuit should also be capable of reducing the
effects of stray fields. Optical Sensors
Optical displacement sensors work on the basic principle that the intensity of light decreases with
distance. So if the source and detector are fixed, the amount of light reflected from a moving
surface will depend on the distance of the moving surface from the fixed ones. Measurement
using this principle requires proper calibration since the amount of light received depends upon
the reflectivity of the surface, intensity of the source etc. Yet it can provide a simple method for
displacement measurement. Optical fibers are often used to transmit light to and from the
measuring zone. Such a scheme with bundle fibers is shown in Fig. 8. It uses two bundle fibers,
one for transmitting light from the source and the other to the detector. Light reflected on the
receiving fiber bundle by the surface of the target object is carried to a photo detector. The light
source could be Laser or LED; photodiodes or phototransistors are used for detection.
Laser, LED, etc.
Processor Photodetector Receiving
Fibers (Measurand) Fig. 8 A Fiber optic position sensor. Speed Measurement
The simplest way for speed measurement of a rotating body is to mount a tachogenerator on the
shaft and measure the voltage generated by it that is proportional to the speed. However this is a
contact type measurement. There are other methods also for noncontact type measurements.
The first method is an optical method shown in Fig. 9. An opaque disc with perforations or
transparent windows at regular interval is mounted on the shaft whose speed is to be measured.
A LED source is aligned on one side of the disc in such a way that its light can pass through the
transparent windows of the disc. As the disc rotates the light will alternately passed through the
transparent windows and blocked by the opaque sections. A photodetector fixed on the other
side of the disc detects the variation of light and the output of the detector after signal
conditioning would be a square wave (as shown) whose frequency is decided by the speed and
the number of holes (transparent windows) on the disc. Version 2 EE IIT, Kharagpur 10 vo
Light Source Transparent
Opaque Disk Fig. 9 Schematic arrangement of optical speed sensing arrangement.
Fig.10 shows another scheme for speed measurement. It is a variable reluctance type speed
sensor. A wheel with projected teethes made of a ferromagnetic material is mounted on the shaft
whose speed is to be measured. The static sensor consists of a permanent magnet and a search
coil mounted on the same assembly and fixed at a closed distance from the wheel. The flux
through the permanent magnet completes the path through the teeth of the wheel and cut the
search coil. As the wheel rotates there would be change in flux cut and a voltage will be induced
in the search coil. The variation of the flux can be expressed as:
φ (t ) = φo + φm sin ω t
where ω is the angular speed of the wheel. Then the voltage induced in the coil is: e = −N dφ
= − Nω φm cos ω t
dt (3) where N is the number of turns in the search coil. So both the amplitude and frequency of the
induced voltage is dependent on the speed of rotation. This voltage is fed to a comparator circuit
that gives a square wave type voltage whose amplitude is constant, but frequency is proportional
to the speed. A frequency counter is used to count the number of square pulses during a fixed
interval and displays the speed.
d Yoke Permanent
Coil Fig. 10 Variable reluctance type speed sensor.
Version 2 EE IIT, Kharagpur 11 Conclusion
Few techniques commonly used for displacement and speed measurements have been discussed
in this lesson. The selection of the sensing scheme depends on the requirement, environment and
accessibility. Displacement/position sensing can be done in two ways. One method is to convert
the displacement signal into variation of inductance or capacitance and then use suitable
measuring circuit to measure their variation. On the other hand, in optical method the intensity of
the light reflected from a moving surface is measured and calibrated in terms of the distance. An
important application of displacement measurement is proximity sensing. Few such schemes
have been discussed in this lesson. However eddy current type proximity sensing has not been
discussed in this lesson.
The most popular type of speed sensor is the tachogenerator. The tachogenerator is mounted on
the shaft and the voltage induced that is proportional to the speed is measured. But there are
other noncontact methods also in which the speed signal is converted into frequency signal and
the frequency is measured. Two such techniques have been discussed in this lesson. References
1. J.P. Bentley: Principles of Measurement Systems (3/e), Longman, U.K., 1995.
2. E.O. Doeblin: Measurement System Application and Design (4/e), Mcgraw-Hill,
3. L.K.Baxter: Capacitance Sensors Design and Applications, IEEE Press, New Jersey,
4. D. Patranabis: Sensors and Transducers (2/e), PHI, New Delhi, 2003.
5. C.W. de Silva: Control Sensors and Actuators, Prentice Hall, New Jersey, 1989. Review Questions
1. What is the function of a Phase Sensitive Detector (PSD) in a LVDT circuit?
2. Discuss the construction and operating principle of a LVDT.
3. Distinguish between variable gap and variable area type capacitance displacement
4. What are the advantages and limitations of optical displacement (position) sensors?
5. Name two method of noncontact type speed measurement. Explain with a schematic
diagram the principle of operation of any one of them.
6. An optical type speed sensor has a disc with 36 rectangular holes placed at regular
intervals on the periphery of the disc. The frequency of the photodetector output is 360
Hz. Find the speed of the shaft in rpm on which the disc is mounted. Answer
6. 600 rpm. Version 2 EE IIT, Kharagpur 12 Module
Version 2 EE IIT, Kharagpur 1 Lesson
Introduction to Industrial
Automation and Control
Version 2 EE IIT, Kharagpur 2 Lesson Objectives
• To define Automation and Control and explain the differences in the sense of the terms • To explain the relation between Automation and Information Technology • To underline the basic objectives of a manufacturing industry and explain how
automation and control technologies relate to these • To introduce the concept of a Product Life Cycle and explain how Automation and
Control technologies relate to the various phases of the cycle • To classify Manufacturing plants and categorise the different classes of Automation
Systems that are appropriate for these Understanding the Title of the Course
Let us first define the three key words in the title, namely, Industry
In a general sense the term “Industry” is defined as follows.
Definition: Systematic Economic Activity that could be related to Manufacture/Service/ Trade.
In this course, we shall be concerned with Manufacturing Industries only. Automation
The word ‘Automation’ is derived from greek words “Auto”(self) and “Matos”
(moving). Automation therefore is the mechanism for systems that “move by itself”.
However, apart from this original sense of the word, automated systems also achieve
significantly superior performance than what is possible with manual systems, in terms
of power, precision and speed of operation.
Definition: Automation is a set of technologies that results in operation of machines
and systems without significant human intervention and achieves performance
superior to manual operation A Definition from Encyclopaedia Britannica
The application of machines to tasks once performed by human beings or,
increasingly, to tasks that would otherwise be impossible. Although the term
mechanization is often used to refer to the simple replacement of human labour by
machines, automation generally implies the integration of machines into a selfgoverning system. Version 2 EE IIT, Kharagpur 3 Point to Ponder: 1
A. Why does an automated system achieve superior performance compared to a manual
B. Can you give an example where this happens? Control
It is perhaps correct to expect that the learner for this course has already been exposed to
a course on Control Systems, which is typically introduced in the final or pre-final year
of an undergraduate course in Engineering in India. The word control is therefore
expected to be familiar and defined as under.
Definition: Control is a set of technologies that achieves desired patterns of variations
of operational parameters and sequences for machines and systems by providing the
input signals necessary. Point to Ponder: 2
A. Can you explain the above definition in the context of a common control system, such as
temperature control in an oven?
B. Is the definition applicable to open-loop as well as closed loop control?
It is important at this stage to understand some of the differences in the senses that these two
terms are generally interpreted in technical contexts and specifically i...
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- Spring '18
- Prof. S. S. Bhatlawande