Chapter 4 Actuator

8/10/2019 Chapter 4 Actuator

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Chapter 4

Department of Mechanical-Mechatronics Engineering

Actuator

Introduction to Actuators. Electro-magnetic,Solenoid valve, electro-static,and fluid-power actuator types,Relay, Servomotors and stepper motor.Piezoelectric actuators, magnetic valves.


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H- bridge DC Motor Control


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PWM

Bright Bulb Dim Bulb

PartialPower

Pulse Width Modulation (PWM) is a technique fordelivering partial power to a load via digital means.

Other devices for delivering partial power potentiometerand rheometer.


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The duty cycle (the width of the signal) is modulated.It is a percentage measurement of how long thesignal stays on.

Duty Cycle - Introduction

Period (T)

DutyCycle (D) V L

V HOn Off

Duty Cycle is determined

by:%100

Period TimeOn

Cycle Duty

Average signal can be

found as: L H avg V DV DV 1

Usually, V L is taken as zero volts for simplicity.


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Q. A customer has a 0.9 stepper and isrunning at 256x microsteps. They would liketo achieve 3 RPS. How many pulses/secshould they send to their drive?


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Conventional permanent magnet step motor


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Conventional permanent magnet step motor


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This type of step motor has an electromagnetic stator with a magnetically soft ironrotor having teeth and slots. Whereas PM motors are basically 2-phase machines, VRmotors require at least 3 phases. Most VR step motors have 3 or 4 phases although5-phase VR motors are available. The motor shown has 12 stator teeth, 8 rotor teeth,and step angle of 15 in case of A3 phase VR stepper motor.

Variable Reluctance Type


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Hybrid Type The hybrid stepper motor provides better performance with respect to stepresolution, torque and speed . Typical step angles for the Hybrid stepper motorrange from 3.60 to 0.90. The hybrid stepper motor combines the best features ofboth the PM and VR type stepper motor. The rotor is multi-toothed like the VR motorand contains an axially magnetized concentric magnet around its shaft. The teeth onthe rotor provide and even magnetic flux to preferred locations in the air gap . This

further increases the dynamic torque characteristics of the motor when comparedwith both the VR and PM types

Torque Generation The torque produced by a stepper

motor depends on several factors. The step rate The drive current in the windings The drive design or type


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Torque Generation

In a stepper motor a torque is developed when the magnetic fluxes of the rotor andstator are displaced from each other. The stator is made up of a high permeabilitymagnetic material. The presence of this high permeability material causes themagnetic flux to be confined for the most part to the paths defined by the statorstructure in the same fashion that currents are confined to the conductors of anelectronic circuit. This serves to concentrate the flux at the stator poles.


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Example 1: Assume we wish to design a 20 kW (output power) roller coaster thatwill reach speeds of 27m/s. The maximum B-field that we can achieve is 0.8 T.

We have a 240 VDC source available. We desire a full-speed efficiency of 92%.

a) what is the required rail resistanceb) what is the source currentc) what is the effective length of the bar

or wire

Hints:


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Relay operation

The basics for all the relays are the same. For example, a 4 pin relay is shownbelow. There are two colours shown. The green represents the control circuit and thered represents the load circuit . A small control coil is connected onto the controlcircuit. A switch is connected to the load. This switch is controlled by the coil in thecontrol circuit. Now let us take the different steps that occour in a relay.

Energized Relay (ON)/Normally Open(NO) The current flowing through the coils represented by pins 1 and 3 produces amagnetic field which causes the closing of the pins 2 and 4. Thus the switch plays animportant role in the relay working. As it is a part of the load circuit, it is used tocontrol an electrical circuit that is connected to it. Thus, when the relay in energizedthe current flow will be through the pins 2 and


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De Energized Relay (OFF) As soon as the current flow stops through pins 1 and 3, the switch

opens and thus the open circuit prevents the current flow through pins2 and 4. Thus the relay becomes de-energized and thus in off position.

Normally closed

In this relays natural state it is in the normallyclosed state. When the coil is energized thecontacts opened


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Normally Open Contact (NO) NO contact is also called a make contact. It closes thecircuit when the relay is activated. It disconnects the circuit when the relay is inactive

Normally Closed Contact (NC) NC contact is also known as break contact. This isopposite to the NO contact. When the relay is activated, the circuit disconnects.When the relay is deactivated, the circuit connects

Change-over (CO) / Double-throw (DT) Contacts This type of contacts are usedto control two types of circuits. They are used to control a NO contact and also a NCcontact with a common terminal. According to their type they are called by the namesbreak before make and make before break contacts.

Relay TypeSingle Pole Single Throw (SPST) This type of relay has a total of f o u r t er m i n a l s .Out of these two terminals can be connected or disconnected. The other twoterminals are needed for the coil.Single Pole Double Throw (SPDT) This type of a relay has a total of fiveterminals . Out f these two are the coil terminals. A common terminal is also includedwhich connects to either of two others.


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Double Pole Single Throw (DPST) This relay has a total of six terminals . Theseterminals are further divided into two pairs. Thus they can act as two SPSTs which are

actuated by a single coil. Out of the six terminals two of them are coil terminals.

Double Pole Double Throw (DPDT) This is the biggest of all. It has mainly eightterminals . Out of these two rows are designed to be change over terminals. They aredesigned to act as two SPDT relays which are actuated by a single coil.

Relay Applications Relays are used to realize logic functions. They play a very important role in

providing safety critical logic. Relays are used to provide time delay functions. They are used to time the delay

open and delay close of contacts. Relays are used to control high voltage circuits with the help of low voltage signals.

Similarly they are used to control high current circuits with the help of low currentsignals. They are also used as protective relays. By this function all the faults during

transmission and reception can be detected and isolated.


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Reed Switch A Reed Switch consists of two ferromagnetic blades (generally composed of iron andnickel) sealed in a glass capsule. The blades overlap internally in the glass capsulewith a gap between them, and make contact with each other when in the presence ofa suitable magnetic field. The contact area on both blades is plated with a very hardmetal, usually Rhodium or Ruthenium. These very hard metals give rise to thepotential of very long life times if the contacts are not switched with heavy loads. The

gas in the capsule usually consists of Nitrogen or some equivalent inert gas.


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Some Reed Switches, to increase their ability to switch and standoff high voltages,have an internal vacuum. The reed blades act as magnetic flux conductors when

exposed to an external magnetic field from either a permanent magnet or anelectromagnetic coil. Poles of opposite polarity are created and the contacts closewhen the magnetic force exceeds the spring force of the reed blades. As the externalmagnetic field is reduced so that the force between the reeds is less than the restoringforce of the reed blades, the contacts open.

Solenoid Valve A solenoid valve is an electromagnetic valve for use withliquid or gas controlled by running or stopping an electricalcurrent through a solenoid, which is a coil of wire, thuschanging the state of the valve. A solenoid valve has two mainparts: the solenoid and the valve. The solenoid convertselectrical energy into mechanical energy which, in turn, opensor closes the valve mechanically. A spring may be used to holdthe valve opened or closed while the valve is not activated.


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When current flows in the coil, magnetic field is generated and it reduces the air gap.Generally spring is used to create gap when coil is not energized.Large back emf is

generated when coil is switched off. Diode is used to suppress the back emf and itprevents the control electronics from damage

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Chapter 4 Actuator