actuator nptel slides

1. ACTUATORS

2. Actuators:
 Actuation is the process of conversion of energy to mechanical form. A device that
accomplishes this conversion is called actuator.
 Actuator plays a very important role while implementing control. The controller provides
command signal to the actuator for actuation.
 The control codes aims at “deriving the actuator when an event has occurred”
Actuators for Robots:
1. Actuators are used in order to produce mechanical movement in robots.
2. Actuators are the muscles of robots. There are many types of actuators available depending on
the load involved. The term load is associated with many factors including force, torque, speed of
operation, accuracy, precision and power consumption.

3. CHARACTERISTICS OF ACTUATING SYSTEMS:
Weight, Power-to-weight Ratio, Operating pressure:
1) Stepper motors are generally heavier than servomotors for the same power.
2) The high the voltage of electric motors, the better power-to- weight ratio.
3) Pneumatic systems delivers the lowest power-to-weight ratio.
4) Hydraulic systems have the highest power-to-weight ratio. In these systems, the weight is
actually composed of two portions. One is the hydraulic actuators, and the other is the hydraulic
power unit (pump, cylinders, rams, reservoirs, filter, and electric motor). If the power unit must
also move with the robot, the total power-to-weight ratio will be much less.

4. Types of Actuators:
1. Electric Actuators.
• Servomotor
• Stepper Motor
• DC Motor
2. Hydraulic Actuators.
3. Pneumatic Actuators.
4. Magnetostrictive Actuators.
5. Shape Memory Metal Actuators.

5. Electrical actuators:
 The electric actuators generally require reduction gears of high ratios.
 The high-gear ratio linearizes the system dynamics and reduces the coupling effects.
 This is an added advantage of the electric actuators but at the cost of increased joint friction,
elasticity and backlash.
 On the author hand use of hydraulic or pneumatic actuators to directly drive the joint
minimizes the drawbacks due to friction, elasticity, and backlash.
1) Easy to control
2) From W to MW
3) Normally high velocities 1000 - 10000 rpm
4) Several types
(5) Accurate servo control
6) Ideal torque for driving
7) Excellent efficiency
8) Autonomous power system

6. ELECTRICAL ACTUATORS:
1. Mainly rotating but also linear ones are available.
2. Linear movement with gear or with real linear motor.
ELECTRICAL ACTUATOR TYPES
1. Servo Motor
2. Stepper Motor
3. DC-motors
4. Brushless DC-motors
5. Asynchronous motors
6. Synchronous motors
7. Reluctance motors.

7. Servo Motor:
 The servo motor is most commonly used for high technology devices in the industrial
application like automation technology.
 It is a self contained electrical device, that rotate parts of a machine with high efficiency and
great precision.
 The output shaft of this motor can be moved to a particular angle.
 Servo motors are mainly used in home electronics, toys, cars, airplanes, etc.

8. Types of Servo Motors
 Servo motors are classified into different types based on their application, such as AC servo
motor, DC servo motor, brushless DC servo motor, positional rotation, continuous rotation and
linear servo motor etc.
 Typical servo motors comprise of three wires namely, power control and ground.
 The shape and size of these motors depend on their applications.
 Servo motor is the most common type which is used in hobby applications, robotics due to
their simplicity, affordability and reliability of control by microprocessors.

9. DC Servo Motor
 The motor which is used as a DC servo motor generally have a separate DC source in the field
of winding & armature winding.
 The control can be archived either by controlling the armature current or field current.
 Field control includes some particular advantages over armature control.
 In the same way armature control includes some advantages
over field control.
 Based on the applications the control should be applied to
the DC servo motor.
 DC servo motor provides very accurate and also fast
respond to start or stop command signals due to the low
armature inductive reactance.
 DC servo motors are used in similar equipment's and
computerized numerically controlled machines.

10. AC Servo Motor
AC servo motor is an AC motor that includes
encoder is used with controllers for giving
closed loop control and feedback.
This motor can be placed to high accuracy and
also controlled precisely as compulsory for the
applications.
Frequently these motors have higher designs of tolerance or better bearings and some simple
designs also use higher voltages in order to accomplish greater torque.
Applications of an AC motor mainly involve in automation, robotics, CNC machinery, and other
applications a high level of precision and needful versatility.

11. Positional Rotation Servo Motor
• Positional rotation servo motor is a most common type of servo motor.
• The shaft’s o/p rotates in about 180o.
• It includes physical stops located in the gear mechanism to stop turning outside these limits to
guard the rotation sensor.
• These common servos involve in radio controlled water, radio controlled cars, aircraft, robots,
toys and many other applications.

12. Continuous Rotation Servo Motor
 Continuous rotation servo motor is quite related to the common positional rotation servo motor,
but it can go in any direction indefinitely.
 The control signal, rather than set the static position of the servo, is understood as the speed
and direction of rotation.
 The range of potential commands sources the servo to rotate clockwise or anticlockwise as
preferred, at changing speed, depending on the command signal.
 This type of motor is used in a radar dish if you are riding one on a robot or you can use one as
a drive motor on a mobile robot.

13. Applications of Servo Motor
 The servo motor is small and efficient, but serious to use in some applications like precise position
control. This motor is controlled by a pulse width modulator signal.
 The applications of servo motors mainly involve in computers, robotics, toys, CD/DVD players, etc.
These motors are extensively used in those applications where a particular task is to be done frequently in
an exact manner.
 The servo motor is used in robotics to activate movements, giving the arm to its precise angle.
 The Servo motor is used to start, move and stop conveyor belts carrying the product along with many
stages. For instance, product labeling, bottling and packaging
 The servo motor is built into the camera to correct a lens of the camera to improve out of focus
images.
 The servo motor is used in robotic vehicle to control the robot wheels, producing plenty torque to
move, start and stop the vehicle and control its speed.
 The servo motor is used in solar tracking system to correct the angle of the panel so that each solar
panel stays to face the sun
 The Servo motor is used in metal forming and cutting machines to provide specific motion control for
milling machines
 The Servo motor is used in automatic door openers to control the door in public places like
supermarkets, hospitals and theatres

14. Stepper Motor:
 A stepper motor is an electromechanical device it converts electrical power into mechanical
power.
 Also it is a brushless, synchronous electric motor that can divide a full rotation into an
expansive number of steps.
 The motor’s position can be controlled accurately without any feedback mechanism, as long as
the motor is carefully sized to the application.
 Stepper motors are similar to switched reluctance motors.

15.  The stepper motor uses the theory of operation for magnets to make the motor shaft turn a
precise distance when a pulse of electricity is provided.
 The stator has eight poles, and the rotor has six poles. The rotor will require 24 pulses of
electricity to move the 24 steps to make one complete revolution.
 Another way to say this is that the rotor will move precisely 15° for each pulse of electricity
that the motor receives.

16. Working Principle:
 Stepper motors operate differently from DC brush motors, which rotate when voltage is applied
to their terminals.
 Stepper motors, on the other hand, effectively have multiple toothed electromagnets arranged
around a central gear-shaped piece of iron.
 The electromagnets are energized by an external control circuit, for example a microcontroller.
 To make the motor shaft turn, first one electromagnet is given power, which makes the gear’s
teeth magnetically attracted to the electromagnet’s teeth.
 The point when the gear’s teeth are thus aligned to the first electromagnet, they are slightly
offset from the next electromagnet.
 So when the next electromagnet is turned ON and the first is turned OFF, the gear rotates
slightly to align with the next one and from there the process is repeated.

17.  Each of those slight rotations is called a step, with an integer number of steps making a full
rotation.
 In that way, the motor can be turned by a precise. Stepper motor doesn’t rotate continuously,
they rotate in steps.
 There are 4 coils with 90o angle between each other fixed on the stator. The stepper motor
connections are determined by the way the coils are interconnected.
 In stepper motor, the coils are not connected together. The motor has 90o rotation step with the
coils being energized in a cyclic order, determining the shaft rotation direction.
 The working of this motor is shown by operating the switch. The coils are activated in series
in 1 sec intervals. The shaft rotates 90o each time the next coil is activated. Its low speed
torque will vary directly with current.

18. Types of Stepper Motor:
There are three main types of stepper motors, they are:
1.Permanent magnet stepper
2.Variable reluctance stepper
3.Hybrid synchronous stepper
Permanent Magnet Stepper Motor: Permanent magnet motors use a permanent magnet (PM) in
the rotor and operate on the attraction or repulsion between the rotor PM and the stator
electromagnets.
Variable Reluctance Stepper Motor: Variable reluctance (VR) motors have a plain iron rotor
and operate based on the principle that minimum reluctance occurs with minimum gap, hence the
rotor points are attracted toward the stator magnet poles.
Hybrid Synchronous Stepper Motor: Hybrid stepper motors are named because they use a
combination of permanent magnet (PM) and variable reluctance (VR) techniques to achieve
maximum power in a small package size.

19. Advantages of Stepper Motor:
1.The rotation angle of the motor is proportional to the input pulse.
2.The motor has full torque at standstill.
3.Precise positioning and repeatability of movement since good stepper motors have an accuracy
of 3 – 5% of a step and this error is non cumulative from one step to the next.
4.Excellent response to starting, stopping and reversing.
5.Very reliable since there are no contact brushes in the motor. Therefore the life of the motor is
simply dependent on the life of the bearing.
6.The motors response to digital input pulses provides open-loop control, making the motor
simpler and less costly to control.
7.It is possible to achieve very low speed synchronous rotation with a load that is directly coupled
to the shaft.
8.A wide range of rotational speeds can be realized as the speed is proportional to the frequency of
the input pulses.

20. Applications:
1.Industrial Machines – Stepper motors are used in automotive gauges and machine tooling
automated production equipment's.
2.Security – new surveillance products for the security industry.
3.Medical – Stepper motors are used inside medical scanners, samplers, and also found inside
digital dental photography, fluid pumps, respirators and blood analysis machinery.
4.Consumer Electronics – Stepper motors in cameras for automatic digital camera focus and
zoom functions.
And also have business machines applications, computer peripherals applications.

21. Permanent Magnet DC Motor
In a DC motor, an armature rotates inside a magnetic field. Basic working
principle of DC motor is based on the fact that whenever a current carrying
conductor is placed inside a magnetic field, there will be mechanical force
experienced by that conductor.
Working Principle of Permanent Magnet DC Motor or PMDC Motor
The working principle of PMDC motor is just similar to the general working principle of DC
motor.
That is when a carrying conductor comes inside a magnetic field, a mechanical force will be
experienced by the conductor and the direction of this force is governed by Fleming’s left hand
rule.
As in a permanent magnet DC motor, the armature is placed inside the magnetic field of
permanent magnet; the armature rotates in the direction of the generated force.

22. Here each conductor of the armature experiences the mechanical force F = B.I.L Newton where,
B is the magnetic field strength in Tesla (weber / m2), I is the current in Ampere flowing through
that conductor and L is length of the conductor in metre comes under the magnetic field.
Each conductor of the armature experiences a force and the compilation of those forces produces
a torque, which tends to rotate the armature.
Equivalent Circuit of Permanent Magnet DC Motor or PMDC Motor
As in PMDC motor the field is produced by permanent magnet,
there is no need of drawing field coils in the equivalent circuit of
permanent magnet DC motor.
The supply voltage to the armature will have armature resistance
drop and rest of the supply voltage is countered by back emf of the
motor. Hence voltage equation of the motor is given by,
Where, I is armature current and R is armature resistance of the
motor. Eb is the back emf and V is the supply voltage.

23. Advantages of Permanent Magnet DC Motor or PMDC Motor
PMDC motor have some advantages over other types of DC motors. They are :
1. No need of field excitation arrangement.
2. No input power in consumed for excitation which improve efficiency of DC motor.
3. No field coil hence space for field coil is saved which reduces the overall size of the motor.
4. Cheaper and economical for fractional kW rated applications.

24. Disadvantages of Permanent Magnet DC Motor or PMDC Motor
1.In this case, the armature reaction of DC motor cannot be compensated hence the
magnetic strength of the field may get weak due to demagnetizing effect armature
reaction.
2.There is also a chance of getting the poles permanently demagnetized (partial) due to
excessive armature current during starting, reversal and overloading condition of the
motor.
3.Another major disadvantage of PMDC motor is that, the field in the air gap is fixed
and limited and it cannot be controlled externally. Therefore, very efficient speed control
of DC motor in this type of motor is difficult.
Applications of Permanent Magnet DC Motor or PMDC Motor
PMDC motor is extensively used where small DC motors are required and also very effective
control is not required, such as in automobiles starter, toys, wipers, washers, hot blowers, air
conditioners, computer disc drives and in many more.

25. Hydraulic Actuators:
Hydraulic actuators can produce large force/torque to drive the manipulator joints without the use
of reduction gearing and are easily applied for robotic position control. But hydraulic systems are
cumbersome and messy and require a great deal of equipment such as pumps, actuators, hoses,
and servo valves. In application where position and/or torque must be accurately controlled,
hydraulic actuators prove disadvantageous due to friction of seals. Leakage. Viscosity of oil, and
its complex temperature dependence.

26. Hydraulic system generally consists of the following parts:
1. Hydraulic linear or rotary cylinders and rams to provide the force or torque needed to move the
joints and are controlled by servo valve or manual valve.
2. A hydraulic pump to provide high pressure fluid to the system
3. Electric motor to operate the hydraulic pump.
4. Cooling system to get rid of heat (cooling fans, radiators, and cooled air).
5. Reservoir to keep fluid supply available to the system.
6. Servo valve which is a very sensitive valve that controls the amount and the rate of the fluid to
the cylinders. The servo valve is generally driven by a hydraulic servomotor.
7. Sensors to control the motion of the cylinders (position, velocity, magnetic, touch,..)
8. Connecting hoses to transport the pressurized fluid.
9. Safety check valves, holding valves.

27. Pneumatic Actuators:
Pneumatic actuators possess all the disadvantages of hydraulic actuators except that these are
relatively cleaner. Pneumatic actuators are difficult to control accurately due to high friction of
seals and compressibility of air.
Like hydraulic except power from compressed air.
Advantages:
 Fast on/off type tasks.
 Big forces with elasticity.
 No hydraulic oil leak problems.
Disadvantage:
 Speed control is not possible because the air pressure depends on many variables that are out of
control.

28. Basic Robot Motions: A robot must have a control system to operate its drive system, which is
used to move the arm, wrist, and body of a robot at various paths. When different industrial robots
are compared with their control system, they can be divided into four major types. They are:
 Limited Sequence Robots
 Playback Robots with Point – Point Control
 Playback Robots with Continuous Path Control
 Intelligent Robots
Limited Sequence Robots:
The limited sequence robots are incorporated with the mechanical stops and limit switches for
determining the finishing points of its joints. These robots do not require any sort of
programming, and just uses the manipulator to perform the operation. As a result, every joint can
only travel to the intense limits. It is considered as the smallest level of controlling, and it will be
best for simple operations like pick & place process. This type of robots is generally equipped
with the pneumatic drive system.

29. Playback Robots:
The playback robots are capable of performing a task by teaching the position. These positions are
stored in the memory, and done frequently by the robot. Generally, these playback robots are
employed with a complicated control system. It can be divided into two important types, namely:
 Point to Point control robots
 Continuous Path control robots
Playback Robots with Point to Point Control:
The point to point robots are shortly called as PTP. It has got the capability to travel from one
position to another. The desired paths are taught and stored in the control unit memory. These
robots do not move from the desired location for controlling its path. It can be moved in a small
distance only with the help of programming. This type of robots can be used for spot welding,
loading & unloading, and drilling operations.

30. Playback Robots with Continuous Path Control:
The continuous path control is also known as CP control. This type of robots can control the path,
and can end on any specified position. These robots commonly move in the straight line. The
initial and final point is first described by the programmer, and the control unit defines the
individual joints. This helps the robot to travel in a straight line. Likewise, it can also move in
a curved path by moving its arm at the desired points. In these robots, the microprocessor is used
as a controller. Some of the applications are arc welding, spray painting, and gluing operations.
Intelligent Robots:
The intelligent robots can play back the defined motion, and can also work according to their
environment. It uses digital computer as a controller. The sensor is incorporated in these robots
for receiving the information during the process. The programming language will be based
on high level language. This kind of robots is capable of communicating with the programmers in
the work volume. It will be best for arc welding, and assembly purposes.

31. Thanking You