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Actuators • Sensors provide only half of the interaction required between an electronicsystem and its surroundings. In addition to being able to sense physicalquantities in their environment, systems must also be able to affect the outsideworld in some way so that their various functions can be performed. • This might require the system to move something, change its temperature orsimply provide information via some form of display. All these functions areperformedbyactuators. • Actuatorsaretransducerssince they convert one physical quantity into another. We areinterested in actuators that take electrical signals from our system and usethem to vary some external physical quantity.
Actuators • HeatActuators • Most heating elements may be considered as simple resistive heaters,which output the power that they absorb as heat.For application requiringonly a few watts of heat output, ordinary resistors of the appropriate powerratingmay be used. • LightActuators • Most lighting for general illumination is generated using conventional incandescentor fluorescent lamps. The power requirements of such devices canrange from a fraction of a watt to hundreds or perhaps thousands of watts.For signalling and communication applications, the relatively low speedof response of conventional lamps makes them unsuitable, and other techniquesarerequired.
Actuators • Light-emittingdiodes • One of the most common light sources used in electronic circuits is thelight-emitting diode or LED. This is a semiconductor diode constructedin such a way that it produces light when a current passes through it. A rangeof semiconductor materials can be used to produce infrared or visible lightof various colours. Typical devices use materials such as gallium arsenide,gallium phosphide or gallium arsenide phosphide.The light output from an LED is approximately proportionalto the current passing through it; a typical small device might have anoperating voltage of 2.0 V and a maximum current of 30 mA. • Infrared LEDs are widely used with photodiodes or phototransistorsto enable short-range wireless communication. Variations in the currentapplied to the LED are converted into light with a fluctuating intensity,which is then converted back into a corresponding electrical signal by thereceiving device. This technique is widely used in remote control applicationsfor televisions and video recorders. In these cases, the informationtransmitted is generally in a digital form.
Actuators • Liquid crystaldisplays • Liquid crystal displays (LCDs) consist of two sheets of polarised glasswith a thin layer of oily liquid sandwiched between them. An electric fieldis used to rotate the plane of polarisation of the liquid in certain regions,making some parts of the display opaque while others are transparent. Thedisplay segments can be arranged to create specific patterns (such as thoseof seven-segment displays) • A great advantage of LCDs (compared with LEDs) is that they areable to use ambient light, greatly reducing power consumption and allowingthem to be used in a wide range of low-power applications. When insufficientambient light is available they can also be backlit, although this increasestheir power consumption considerably. • Fibre-opticcommunication • For long-distance communication, the simple techniques used in televisionremote control units are not suitable as they are greatly affected by ambientlight, that is light present in the environment. This problem can be overcomeby the use of a fibre-optic cable, which captures the light from the transmitterand passes it along the cable to the receiver without interference fromexternal light sources. Fibres are usually made of either an optical polymeror glass. The former are inexpensive and robust, but their high attenuationmakes them suitable for only short-range communications of up to about 20 metres.
Actuators • Force, displacementandmotionactuators • Meters • Panel meters are important output devices in many electronic systems providinga visual indication of physical quantities. Although there are variousforms of panel meter, one of the simplest is the moving-iron meter, whichis an example of the rotary solenoid described above. Here a solenoidproduces a rotary motion, which is opposed by a spring. This producesan output displacement that is proportional to the current flowing throughthe coil. A needle attached to the moving rotor moves over a fixed scale toindicate the magnitude of the displacement. Moving-iron meters can be usedfor measuring AC or DC quantities. They produce a displacement that isrelated to the magnitude of the current and is independent of its polarity.Although moving-iron meters are used in some applications, a more commonarrangement is the moving-coil meter.
Actuators • Motors • Electric motors fall into three broad types: AC motors, DC motors and steppermotors. AC motors are primarily used in high-power applications andsituations where great precision is not required. Control of these motors isoften by simple ON/OFF techniques, although variable power drives arealsoused. • DC motors are extensively used in precision position-control systemsand other electronic systems, particularly in low-power applications. Thesemotors have very straightforward characteristics, with their speed beingdetermined by the applied voltage and their torque being related to theircurrent. The speed range of DC motors can be very wide, with some devicesbeing capable of speeds from tens of thousands of revolutions per minute downto a few revolutions per day. Some motors, in particular DC permanentmagnetmotors, have an almost linear relationship between speed and voltageand between torque and current. This makes them particularly easy to use. • Stepper motors, as their name implies, move in discrete steps. Themotor consists of a central rotor surrounded by a number of coils (orwindings). The form of a simple stepper motor is shown in Figure 12.5, anda typical motor is shown in Figure 12.6
Actuators • Sound actuators • Speakers • Most speakers (or loudspeakers) have a fixed permanent magnet and amovable coil connected to a diaphragm. Input to the speaker generates a currentin the coil, which causes it to move with respect to the magnet, therebymoving the diaphragm and generating sound. The nominal impedance of thecoil in the speaker is typically in the range 4 to 15 Ω, and the power-handlingcapacity may vary from a few watts for a small domestic speaker to severalhundreds of watts for speakers used in public address systems. • Ultrasonictransducers • At very high frequencies, the permanent-magnet speakers described earlierare often replaced by piezoelectric actuators. Such transducers are usuallydesigned to operate over a narrow range of frequencies.
