Electrical Actuation System Lecture 8 (Chapter 9)

SME 3252: Mechatronics Lecture 8 Introduction The electrical systems used as actuators for control are: Switching devices – mechanical switches and solid-state switch Solenoid type device Drive system – motors (AC, DC, Stepper) SME 3252: Mechatronics Lecture 8

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

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. SME 3252: Mechatronics Lecture 8 Sources: Wikipedia, the free encyclopedia

SME 3252: Mechatronics Lecture 8 Figure 9.1 (a) A relay, (b) a driver circuit SME 3252: Mechatronics Lecture 8

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 SME 3252: Mechatronics Lecture 8

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

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: Diode Thyristor and triac Bipolar transistor Power MOSFET SME 3252: Mechatronics Lecture 8

SME 3252: Mechatronics Lecture 8 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

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 SME 3252: Mechatronics Lecture 8

SME 3252: Mechatronics Lecture 8 Figure 9.3 (a) Diode characteristic, (b) half-wave rectification SME 3252: Mechatronics Lecture 8

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 SME 3252: Mechatronics Lecture 8

SME 3252: Mechatronics Lecture 8 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. SME 3252: Mechatronics Lecture 8

SME 3252: Mechatronics Lecture 8 SME 3252: Mechatronics Lecture 8 Figure 9.4 (a) Thyristor characteristic, (b) thyristor circuit SME 3252: Mechatronics Lecture 8 SME 3252: Mechatronics Lecture 8

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 SME 3252: Mechatronics Lecture 8

Figure 9.5 Triac characteristic

SME 3252: Mechatronics Lecture 8 Several thyristors and triacs Symbols and pin placements for: a - thyristor, b - triac, c - diac SME 3252: Mechatronics Lecture 8

SME 3252: Mechatronics Lecture 8 Figure 9.6 Voltage control: (a) thyristor, (b) triac SME 3252: Mechatronics Lecture 8

SME 3252: Mechatronics Lecture 8 Figure 9.7 Thyristor d.c. control SME 3252: Mechatronics Lecture 8

SME 3252: Mechatronics Lecture 8 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 SME 3252: Mechatronics Lecture 8

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). PNP NPN SME 3252: Mechatronics Lecture 8

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 SME 3252: Mechatronics Lecture 8

SME 3252: Mechatronics Lecture 8 To identify a specific transistor configuration is to follow three simple steps: Identify the element (emitter, base, or collector) to which the input signal is applied. Identify the element (emitter, base, or collector) from which the output signal is taken. The remaining element is the common element, and gives the configuration its name. SME 3252: Mechatronics Lecture 8

SME 3252: Mechatronics Lecture 8 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. SME 3252: Mechatronics Lecture 8

SME 3252: Mechatronics Lecture 8 Figure 9.9 (a) Transistor symbols, (b), (c), (d), (e) transistor switch SME 3252: Mechatronics Lecture 8

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. SME 3252: Mechatronics Lecture 8

SME 3252: Mechatronics Lecture 8 MOSFET SME 3252: Mechatronics Lecture 8

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 SME 3252: Mechatronics Lecture 8

SME 3252: Mechatronics Lecture 8 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. SME 3252: Mechatronics Lecture 8

SME 3252: Mechatronics Lecture 8 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 SME 3252: Mechatronics Lecture 8

SME 3252: Mechatronics Lecture 8 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. SME 3252: Mechatronics Lecture 8

SME 3252: Mechatronics Lecture 8 3/2 valve (cont.) 3/2-valve usually for controlling single-acting cylinder Usually use poppet valve SME 3252: Mechatronics Lecture 8

SME 3252: Mechatronics Lecture 8 5/2 valve (cont.) 5/2-valve usually for controlling double-acting cylinder Usually use slide valve SME 3252: Mechatronics Lecture 8

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 SME 3252: Mechatronics Lecture 8

End of Lecture 8