Electrical Actuation System

Lecture 8

(Chapter 9)

SME 3252: Mechatronics Lecture 8

Introduction

• The electrical systems used as actuators for control are:

1. Switching devices – mechanical switches and solid-state switch

2. Solenoid type device

3. Drive system – motors (AC, DC, Stepper)

SME 3252: Mechatronics Lecture 8

7.2: Mechanical Switches

• Mechanical switches – devices used as sensors to give input to system

• E.g. – to switch on motor, heating element, current for actuating solenoid valve in hydraulic and pneumatic system

• Example - relay

SME 3252: Mechatronics Lecture 8

7.2.1: Relay• An electrical switch that opens and closes

under the control of another electrical circuit• When a current flows through the coil, the

resulting magnetic field attracts an armature that is mechanically linked to a moving contact

• The movement either makes or breaks a connection with a fixed contact

• When the current to the coil is switched off, the armature is returned by a force approximately half as strong as the magnetic force to its relaxed position

• Usually this is a spring, but gravity is also used commonly in industrial motor starters.

Sources: Wikipedia, the free encyclopedia

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Figure 9.1 (a) A relay, (b) a driver circuit

SME 3252: Mechatronics Lecture 8

• A relay consists of two separate and completely independent circuit

• The bottom - drives the electromagnet

• When the switch is on, the electromagnet attracts the armature (blue)

• The armature – acts as a switch in the second circuit

• When the electromagnet is energized, the armature completes the second circuit and the light is on

• When the electromagnet is not energized, the spring pulls the armature away

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Application – Elevator control system

• Relays are still used in the control panels of current elevator designs

• In the examples, relays are used:

• to control the elevator floor and door signals

• for fire mode operation

• to control the elevator’s car ventilation fan, lighting, and motor cooling fan.

SME 3252: Mechatronics Lecture 8

control the elevator floor and door signals.

for fire mode operation.

to control the elevator’s car ventilation fan, lighting, and motor cooling fan.

SME 3252: Mechatronics Lecture 8

7.3: Solid-state switches

• Examples of solid-state devices to electronically switch circuit:

1. Diode

2. Thyristor and triac

3. Bipolar transistor

4. Power MOSFET

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Diode operation: (a) Current flow is prmitted; the diode is forward biased. (b) Current flow is prohibited; the diode is reversed biased.

SME 3252: Mechatronics Lecture 8

7.3.1: Diode• a semiconductor device, allows current to flow through it in

only one direction • Some applications of diode: as a rectifier that converts AC (Alternating Current) to DC

(Direct Current) for a power supply device to separate the signal from radio frequencies as an on/off switch that controls current

Several different types of single diodes

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Figure 9.3 (a) Diode characteristic, (b) half-wave rectification

SME 3252: Mechatronics Lecture 8

Bridge rectifier• 4 diodes in a single package is called a

BRIDGE or BRIDGE RECTIFIER

• To achieve full-wave rectification

• Convert AC voltage to DC pulsating voltage

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7.3.2: Thyristor and triacs

• Thyristor - A semiconductor device with four layers of alternating N and P-type material

• Act as a switch, conducting when their gate receives a current pulse, and continue to conduct for as long as they are forward biased (that is, as long as the voltage across the device has not reversed)

• Thyristors are mainly used where high currents and voltages are involved, and are often used to control alternating currents, where the change of polarity of the current causes the device to automatically switch off; referred to as Zero Cross operation

• Thyristors can also be found in power supplies for digital circuits, where they can be used as a sort of "circuit breaker" or "crowbar" to prevent a failure in the power supply from damaging downstream components.

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Figure 9.4 (a) Thyristor characteristic, (b) thyristor circuit

SME 3252: Mechatronics Lecture 8

• A TRIAC, or TRIode for Alternating Current is an electronic component approximately equivalent to two silicon-controlled rectifiers (SCRs/thyristors) joined in inverse parallel (paralleled but with the polarity reversed) and with their gates connected together

• bidirectional electronic switch which can conduct current in either direction when it is triggered (turned on). It can be triggered by either a positive or a negative voltage being applied to its gate electrode. Once triggered, the device continues to conduct until the current through it drops below a certain threshold value, such as at the end of a half-cycle of alternating current (AC) mains power

• TRIAC a very convenient switch for AC circuits• Low power TRIACs are used in many applications

such as light dimmers, speed controls for electric fans and other electric motors, and in the modern computerized control circuits of many household small and major appliances

Figure 9.5 Triac characteristic

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Several thyristors and triacs

Symbols and pin placements for:

a - thyristor,

b - triac,

c - diac

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Figure 9.6 Voltage control: (a) thyristor, (b) triac

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Figure 9.7 Thyristor d.c. control

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7.3.3: Bipolar Transistor

• A Bipolar Transistor essentially consists of a pair of PN Junction Diodes that are joined back-to-back

• Two kinds of Bipolar sandwich, the NPN and PNP varieties

• The three layers of the sandwich are conventionally called the Collector, Base, and Emitter

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NPN and PNP Transistors• The vertical line represents the base (B), the angular line

with the arrow on it represents the emitter (E), and the other angular line represents the collector (C)

• The direction of the arrow on the emitter distinguishes (graphically) the NPN from the PNP transistor

• If the arrow points in, (Points iN) the transistor is a PNP• On the other hand if the arrow points out, the transistor is

an NPN (Not Pointing iN).

PNPNPN

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Transistor Configuration• 3 basic configurations:

common emitter (CE), common base (CB), and common collector (CC)

• common - denote the element that is common to both input and output circuits

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To identify a specific transistor configuration is to follow three simple steps:

1. Identify the element (emitter, base, or collector) to which the input signal is applied.

2. Identify the element (emitter, base, or collector) from which the output signal is taken.

3. The remaining element is the common element, and gives the configuration its name.

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Transistor as a switch• If the circuit uses the transistor as a switch, then biasing is arranged

to operate on regions of the output curves known as saturation (red) and cut-off (yellow).

• "cut-off" region - are zero input base current, zero output collector current and maximum (supply rail) collector voltage

• "saturation“ - BJT will be biased, maximum base current is applied, maximum collector current flow and minimum collector emitter voltage.

•

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Figure 9.9 (a) Transistor symbols, (b), (c), (d), (e) transistor switch

SME 3252: Mechatronics Lecture 8

7.3.4: MOSFET• Acronym for metal-oxide semiconductor field-effect transistor, a common type of transistor in which charge carriers, such as electrons, flow along channels

• The width of the channel, which determines how well the device conducts, is controlled by an electrode called the gate, separated from channel by a thin layer of oxide insulation

• The insulation keeps current from flowing between the gate and channel

• MOSFETs are useful for high-speed switching applications and also on integrated circuits in computers.

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MOSFET

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MOSFET and Motor Control• This power MOSFET is

working at a higher switch frequency of 20 kHz

• The MOSFET transistor in this motor driver requires a special driver circuit between the PWM circuit and the MOSFET itself because switching the gate voltage of the MOSFET transistor requires high transient current (2 A) due to relatively high capacitive load

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7.4: Solenoid• Solenoids are actuators capable of linear motion• They can be electromechanical (AC/DC), hydraulic, or

pneumatic driven - all operating on the same basic principles

• Give it energy and it will produce a linear force• E.g. - for pushing buttons, hitting keys on a piano, valve

operators, and even for jumping robots• DC solenoids operate on the same basic principles as a

DC motor• The difference between a solenoid and a motor is that a

solenoid is spring loaded and cannot rotate.

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How Solenoids Work

• Inside a solenoid is motor wire coiled in a special way (see image above)

• When an electric current is sent through this wire (energized), a magnetic field is created

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• The inner shaft of a solenoid is a piston like cylinder made of iron or steel, called the plunger or slug (equivalent to an armature)

• The magnetic field then applies a force to this plunger, either attracting or repelling it

• When the magnetic field is turned off, a spring then returns the plunger to its original state

SME 3252: Mechatronics Lecture 8

Push and Pull Type Solenoids

• In pull type solenoids, the plunger is normally outside the solenoid because the spring naturally forces the plunger out. Yet when energized, the force 'pulls' the plunger into the solenoid

• Push type solenoids are the opposite, in that the spring forces the plunger into the solenoid, but when energized the plunger is 'pushed' out.

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3/2 valve (cont.)

• 3/2-valve usually for controlling single-acting cylinder

• Usually use poppet valve

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5/2 valve (cont.)

• 5/2-valve usually for controlling double-acting cylinder

• Usually use slide valve

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Mechanical Considerations • One very important thing to

consider with a solenoid is the stroke

• The stroke distance (maximum distance a plunger can travel) is sufficient for your application

• Also able to handle the sudden non-linear high speeds and high forces expected from such an actuator

End of Lecture 8