S B SINGH ROLL NO 17

SAKEC, MUMBAI

INTRODUCTION

Actuators are the muscles of robots.

Electric motors

Servomotors

Stepper motors

Direct-drive electric motors

Hydraulic actuators

Pneumatic actuators

Shape memory metal actuators

Magnetostrictive actuators

INTRODUCTION

COMPARISON AMONG VARIOUS ACTUATORS

COMPARISON AMONG VARIOUS ACTUATORS

CHARACTERISTICS OF ACUATING SYSTEM

Characteristics of actuating system is criteria of selecting the kind of actuator.

Weight, Power-to-Weight Ratio, Operating Pressure

• The weight of the actuating system and Power-to-Weight Ratio is important.

• Pneumatic and hydraulic system are composed of two system,

actuating system and power system.

Stiffness vs. Compliance

Use of Reduction Gears

• To increase the torque and make it to slow it down to manageable speeds.

• It increase the cost, number of parts, backlash, inertia of the rotating body and resolution of the system.

CHARACTERISTICS OF ACUATING SYSTEM

Inertia and torque relationship between a motor and a load.

HYDRAULIC ACTUATOR

It offers a high power-to-weight ratio , large forces at low speeds compatibility with microprocessor and tolerance of extreme hazardous environments.

Linear Cylinder F = p X A

Rotary Cylinder dA = t . dr

2 2

1 1

2

1

2 2

2 1

. . . . .

1.

2

r r

r r

r

r

T p r dA p r t dr

pt r dr pt r r

where, A is the effective area of the piston,

p is the working pressure. A rotary hydraulic actuator.

HYDRAULIC ACTUATOR

where, q is the flow rate, C is a constant, x is the spool’s displacement, y is piston’s displacement and A is the area of the piston (d/dt denoted as D).

( ) ( ) ( )

q Cx

q dt d vol A dy

Cx dt A dy

Cy x

AD

Schematic of a hydraulic system and its components. Schematic of a spool

valve in neutral position.

PNEUMATIC DEVICES It is very similar to hydraulic

system, but power-to-weight ratio is much lower than hydraulic system.

Because the air is compressible, it compressed and deforms under load.

Differential valve makes the pneumatic cylinder position-controlled

ELECTRIC MOTORS

Sorts of electric motors used in robotics

DC motors

Reversible AC motors

Brushless DC motors

Stepper motors

Important design factor is like below.

Dissipation of heat -> Size, Power of motors

Heat dissipation is generated from below.

Resistance of the wiring to electric current

Heat due to iron losses

Eddy current losses, hysteresis losses

friction losses, brush losses, short-out circuit losses

Basic Principle of all electric motors;

Basic Principle of all electric motors; F I B

Basic Concept of

Electric Motors

D C MOTOR V/S GENERATOR DC motors is reliable, sturdy and relatively powerful.

Schematic of a disk motor.

M t rotor

M t f rotor stator

T k I

T k k I I

Mathematical expression of DC motors

AC Motors The changing flux is provided by the AC current(60Hz) and

commutators and brushes are eliminated.

AC motors can dissipate heat more favorable than DC motors, yielding

more power.

Brushless DC Motors This types of motors are a hybrid between AC motors and DC motors.

It is not necessary that line frequency as 60Hz.

For smooth operation and almost constant torque, the rotor usually

has three phases in it.

Direct-Drive Electric Motors These motors are designed to deliver a very large torque at very low

speeds and with very high resolution, but to be used directly with a joint

without any gear reduction.

ELECTRIC MOTORS

Servomotors servo is an automatic device which uses error-sensing feedback to

correct the performance of a mechanism

The back electromotive force or back-emf is most important factor for all electric motors.

Back-emf voltage

The motor’s velocity is

governed by

The motor’s torque

emfV nkE

in emf EV IR V IR nk

T L f DT IK T T nk

Stepper motors Stepper motors are long-lasting, versatile and used without feedback.

It has permanent magnet rotors and multiple winding stator housing.

Most industrial steppers run between 1.8 to 7.5 degrees at full stepping.

The number of poles means more precise resolution, but it has physical limitation.

MICROPROCESSOR CONTROL OF ELECTRIC MOTORS A robot is supposed to be a manipulator that is controlled by

computers or microprocessor and actuated by the electric motors with controller, which

Pulse Width Modulation

PWM is used for DC motor speed control with microprocessors.

It requires only high voltage(5V) and one output bit.

Average output voltage of PWM is like below. is computer or microprocessor.

PWM timing Sine wave generation with PWM

1out CC

tV V

t

Direction Control of DC Motors with an H-Bridge

It is desirable to change the direction of current flow in a motor for changing its direction of rotation with only two bits of information.

It means one should change the direction of the flow by changing bit information from the microprocessor.

Application of H-bridge for

motor direction control

ACTUATORS MAGNETROSTRICTIVE ACTUATORS

A terfenol-D is placed near a magnet, this special rare-earth-iron material will change its shape slightly, which is called magnetostriction effect and is used to make linear motors with micro inches displacement.

SHAPE-MEMORY-TYPE METALS

One particular metal alloy, called Biometal shortens about 4% when it reaches a certain temperature.

The major disadvantage of the wire is that the total strain happens within a very small temperature range and it is very difficult to accurately control the strain.

THANK YOU!!!