P r e s s u r e t r a n s d u c e r s c o n t r o l p r e s s u r e ( u s u a l l y e x p r e s s e d i n p

James Lock

Engineering science for Engineers Assignment 4 Information and Energy Control Systems

How Electrical Signals Convey Information Describe the following methods used by electrical signals to convey information: 1.1 frequency modulation In telecommunications and signal processing,frequency modulation conveys information over carrier waves by varying its instantaneous frequency,which contrasts with amplitude modulation where the amplitude of the carrier is varied while its frequency remains at a constant. In analogue applications,the difference between the instantaneous and the base frequency of the carrier is proportional to the instantaneous value of the input signal amplitude. Thus, there are infinitely many possible carrier frequencies. Digital data is sent by shifting the carriers frequency among a set of values which is known a frequency shift keying. The number of possible carrier frequency states is usually a power of 2.In more complex modes, there can be four, eight, or more different frequency states. Each specific carrier frequency represents a specific digital input data state. 1.2 Modulation Amplitude Modulation amplitude or Amplitude modulation (AM) is a type of modulation where the amplitude of the carrier signal is varied in accordance with the information bearing signal. The boundary of the amplitude modulated signal embeds the information bearing signal. The total power of the transmitted signal varies with the modulated signal but the carrier signal remains constant. A non-linear device is used to combine the carrier and the modulating signal to generate an amplitude modulated signal. The output of the non-linear device consists of discrete upper and lower side bands. In AM, the carrier itself does not fluctuate in amplitude. Instead, the modulating data appears in the form of signal components at frequencies slightly higher and lower than that of the carrier. These components are called side-bands The lower side-band (LSB) appears at frequencies below the carrier frequency; the upper side-band (USB) appears at frequencies above the carrier frequency. The LSB and USB are essentially "mirror images" of each other in a graph of signal amplitude versus frequency, as shown in the picture below. The side-band power accounts for the variations in the overall amplitude of the signal. When a carrier is amplitude-modulated with a pure sine wave, up to 1/3 (33 percent) of the overall signal power is contained in the side-bands The other 2/3 of the signal power is contained in the carrier, which does not contribute to the transfer of data. With a complex modulating signal such as voice, video, or music, the side-bands generally contain 20 to 25 percent of the overall signal power; thus the carrier consumes 75 to 80 percent of the power. This makes AM an inefficient mode. If an attempt is made to increase the modulating data input amplitude beyond these limits, the signal will become distorted, and will occupy a much greater bandwidth than it should. This is called over modulation, and can result in interference to signals on nearby frequencies.

James Lock1.3 Morse code Morse code is a method for transmitting telegraphic information, using standardized sequences of short and long elements to represent the letters, numerals, punctuation and special characters of a message. The short and long elements can be formed by sounds, marks or pulses, in on off keying and are commonly known as "dots" and "dashes" or "dits" and "dahs".

Morse code can be transmitted in a number of ways: originally as electrical pulses along a telegraph wire, but also as an audio tone, a radio signal with short and long tones, or as a mechanical or visual signal (e.g. a flashing light) using devices like an Aldus lamp or a heliography Morse code is transmitted using just two states (on and off) so it was an early form of a digital code. However, it is technically not binary, as the pause lengths are required to decode the information. Originally created for Samuel F. B. Morse's electric telegraph in the early 1840s, Morse code was also extensively used for early radio communication beginning in the 1890s. For the first half of the twentieth century, the majority of highspeed international communication was conducted in Morse code, using telegraph lines, undersea cables, and radio circuits. However, the variable length of the Morse characters made it hard to adapt to automated circuits, so for most electronic communication it has been replaced by more machinable formats, such as Baud code and ASCII. The most popular current use of Morse code is by amateur radio operators. Although no longer a requirement for Amateur licensing in most countries, it also continues to be used for specialized purposes, including identification of navigational radio beacon and land mobile transmitters, plus some military communication, including flashing-light semaphore communications between ships in some naval services. Morse code is the only digital modulation mode designed to be easily read by humans without a computer, making it appropriate for sending automated digital data in voice channels, as well as making it ideal for emergency signalling, such as by way of improvised energy sources that can be easily "keyed" such as by supplying and removing electric power (e.g. by switching a breaker on and off,flashing lights etc). 1.4 ASCII Code

The American Standard Code for Information Interchange is a character encoding scheme based on the of the ordering of the English alphabet. ASCII codes represent text in computers, communication equipment, and other devices that use text. Most modern character-encoding schemes are based on ASCII, though they support many more characters than did ASCII. US-ASCII is the internet assigned numbers authority (IANA) preferred char-set name for ASCII.

James LockHistorically, ASCII developed from telegraphic codes. Its first commercial use was as a seven-bit teleprinter code . Work on ASCII formally began on October 6, 1960, with the first meeting of the American Standards Association's (ASA) X3.2 subcommittee. The first edition of the standard was published during 1963 a major revision during 1967, and the most recent update during 1986. Compared to earlier telegraph codes, the proposed Bell code and ASCII were both ordered for more convenient sorting (i.e., alphabetization) of lists, and added features for devices other than teleprinters. ASCII includes definitions for 128 characters: 33 are non-printing control characters(now mostly obsolete) that affect how text and space is processed;94 are printable characters, and the space is considered an invisible graphic. The most commonly used character encoding on the World Wide Web was US-ASCII until December 2007, when it was surpassed by UTF-8.

ASCII specifies a correspondence between digital bit patterns and character symbols This allows digital devices to communicate with each other and to process, store, and communicate character-oriented information such as written language. The ASCII character encoding or a compatible extension is used on nearly all common computers, especially personal computers and workstations. ASCII is, strictly, a seven-bit code, meaning it uses patterns of seven binary digits (a range of 0 to 127 decimal) to represent each character. When ASCII was introduced, many computers used eight-bit bytes (groups of bits), also called octets, as the native data type. In seven-bit ASCII encoding, the eighth bit was commonly used as a parity bit for error checking on communication lines or for other device-specific functions. Machines that did not use parity checking typically set the eighth bit to 0. Except for a few of the ASCII control characters that prescribe some elementary line-oriented formatting, ASCII does not define any mechanism for describing the structure or appearance of text within a document. Other schemes, such as markup languages, address page and document layout and formatting. 2. Describe the inherent functions of the following electrical components used within an aircraft information system: 2.1 transducers

A transducer can be defined as a device that converts one type of energy to another. The conversion can be to/from electrical, electro-mechanical, electromagnetic, photonic, photovoltaic or any other form of energy. While the term transducer commonly implies use as a sensor/detector, any device which converts energy can be considered a transducer. Transducers may be categorized by application:sensor, actuator, or combination. A sensor is used to detect a parameter in one form and report it in another form of energy (usually an electrical and/or digital signal). For example, a pressure sensor might detect pressure and convert it to electricity for display at a remote gauge on an aircraft's instrument panel. An actuator accepts energy and produces movement (action). The energy supplied to an actuator might be electrical or mechanical (pneumatic,hydraulic etc.). An electric motor is a transducer, converting electrical energy into motion for

James Lockdifferent purposes. An aircraft would use a position transducer to present information on the instrument control panel(e.g. positional information of a wing flap).This is achieved by the output signal of the transducer is the difference between the output signals of the systems sensors. The instrument system would also consist of a means of conditioning the signal such as an amplifier and the display screen itself as illustrated below:

transducer

amplifier

Display

Sensor signal

Further example uses of transducers in aircraft sytems could be found in the use of accelerominters, which are one type of transducer which measures acceleration in its principle direction and could be used to measure excessive vibration on helipcoptor drive shaft mounts. 2.2 Amplifiers

An amplifier is a device that is there to control amounts of energy or amplify an energy source. Amplification is done by using gain within the component.Amplifiers are an important component within an aircrafts imformation system as the amplfy weak signals from the sensors. Gain is generally calculated by the ratio or the output power to the input power and is measured in decibels (dBs). One example of a amplifier used in aviation (although now obsolete) would be the Mag Amplifier.Magnetic amplifiers were used in aircraft systems before the advent of high reliability semiconductors. They were important in implementing early autoland systems and Concord made use of the technology for the control of its engine air intakes before subsequent development of a replacement system using digital electronics. 2.3 Digital to Analoge converters. A digital to analogue converter (DAC) is a device that is used for converting a digital (usually binary) code to an analogue signal (current, voltage or electric charge). An analoge to digital converter (ADC) will do the opposite. . They are built up with a network of resistors, simple switches and current sources or capacitors may implement the conversion.One practiucal and widely used method of performing D/A conversions is by using a ladder network. 2.4 ocisators An oscillator is a device that produces a repetitive electronic signal. (Sine wave or square wave). A low frequency oscillator generates an AC waveform between 0.1Hz and 10Hz. A voltage controlled oscillator is specifically designed to be controlled by a voltage input. This means that the frequency of oscillation varies with an applied DC voltage, while modulating signals may be fed into the VCO to generate frequency modulation, phase modulation and pulsewidth modulation.

3 Explain the meaning of the following terms and discuss their significance within an electrical system: 3.1 signal. There are two main types of electrical signal those being digital and analoge.Both types of signal have been used from

James Lockthe earliest days of electrical communication. Analogue systems came around first with the telephone, whereas digital systems first appeared when telegraph systems introduced the Morse code. In analogue systems the information or data is given as an electrical signal that varies in direct proportion to the information or data. It follows that the variation must be continuous and, between the limits of operation of the system, the variation can have any value from an infinite number of values. Such variation is associated with the production of sound (as in the telephone) in radio receivers or vision in television sets. An analogue system in its most basic form has an input electrical signal that is either a voltage or a current varying directly in proportion to the input information. The input information is converted into the electrical signal by a transducer. Digital signals have one of a limited number of discrete values. In most applications, the applications have two values which in crude terms could be explained as ON and OFF. These values are usually described as 1 and 0, being the presence and absence of the supply voltage or current. As previously mentioned in question 1.3, the first form of digital signal in common use was that associated with the Morse code. 3.2 Noise. Electrical noise is is a random fluctuation in an electrical signal, a characteristic of all electronic circuits. Noise generated by electronic devices varies greatly, as it can be produced by several different effects. Thermal( generated by thermal agitation of the charge carrier)and shot noise are unavoidable and due to the laws of nature, rather than to the device exhibiting them, while other types depend mostly on manufacturing quality and semiconductor defects. In comms system, the noise is an error or undesired random disturbance of a useful information signal, introduced before or after the detector and decoder. The noise is a summation of unwanted or disturbing energy from natural and sometimes man-made sources. Noise is, however, typically distinguished from interface (e.g. deliberate jamming or other unwanted electromagnetic interference from specific transmitters), for example in the signal to noise ratio (SNR),signal to interference ratio(SIR) and signal noise plus interference ratio (SNIR) measures. Noise is also typically distinguished from distortion, which is an unwanted alteration of the signal waveform, for example in the signal to noise and distortion ratio (SINAD). In a carrier-modulated passband analogue communication system, a certain carrier to noise ratio (CNR) at the radio receiver input would result in a certain signal to noise in the detected message signal. In a digital communications system, a certain normalized signal-to-noise ratio would result in a certain bit error rate (BER). While noise is generally unwanted, it can serve a useful purpose in some applications, such as random number generation. How Electrical Signals Control Energy Flow Describe the following methods used by electrical signals to control energy flow: 4.1 Temperature sensing and control

A good method and example of temperature control with in an electrical system can be found with the thermostat. A thermostat can maintain a system at a pre set temperature as set by the user.The thermostat achieves this by turning heating or cooling devises on or off, or by regulating the flow of heat transfer fluid as required to keep the temperature at a constant pre determined level,thus creating a regulated flow of energy. Pictured below is a double valve engine thermostat.

4.2

humidity sensing and control.

Humidity sensing and control devices use devices called humidistat s. A humidistat is a device used to measure and control relative humidity. It can be set for a desired humidity level, with the humidistat signalling to the humidifier to turn off the water supply once that level is attained. A typical household humidistat includes a sensing element, made of a material that is sensitive to air moisture, and a relay amplifier. Increases or decreases in indoor humidity strengthen or weaken the electrical resistance occurring between the metal conductors of the sensing element. These variations are in turn gauged by the relay amplifier.

James Lock4.3 speed control of AC and DC machines

The examples I shall use for an AC and DC machine will be an AC and DC motor. One method of controlling an AC induction motor is to use a pulse width modulated drive. In this type of drive, a diode bridge rectifier provides the intermediate DC circuit voltage. In the intermediate DC circuit, the DC voltage is filtered in a LC low-pass filter. Output frequency and voltage is controlled electronically by controlling the width of the pulses of voltage to the motor. Essentially, these techniques require switching the inverter power devices (transistors or IGBTs) on and off many times in order to generate the proper RMS voltage levels.

With the use of a microprocessor, these complex regulator functions are effectively handled. Combining a triangle wave and a sine wave produces the output voltage waveform.

The triangular signal is the carrier or switching frequency of the inverter. The modulation generator produces a sine wave signal that determines the width of the pulses, and therefore the RMS voltage output of the inverter. AC drives that use a PWM type schemes have varying levels of performance based on control algorithms. There are 4 basic types of control for AC drives today. These are Volts per Hertz,Flux Vector Control, and Field Oriented Control. V/Hz control is a basic control method, providing a variable frequency drive for applications like fan and pump. It provides fair speed and torque control, at a reasonable cost. Sensor-less Vector control provides better speed regulation, and the ability to produce high starting torque. Flux Vector control provides more precise speed and torque control, with dynamic response. Field Oriented Control drives provide the best speed and torque control available for AC motors. It provides DC performance for AC motors, and is well suited for typical DC applications. Volts/Hertz Volt/Hertz control in its simplest form takes a speed reference command from an external source and varies the voltage and frequency applied to the motor. By maintaining a constant V/Hz ratio, the drive can control the speed of the connected motor. Speed control of a DC motor: As the speed of a DC motor is directly proportional to the supply voltage, so if we reduce the supply voltage from 12 Volts to 6 Volts, the motor will run at half the speed. How can this be achieved when the battery is fixed at 12 Volts? The speed controller works by varying the average voltage sent to the motor. It could do this by simply adjusting the voltage sent to the motor, but this is quite inefficient to do. A better way is to switch the motors supply on and off very quickly. If the switching is fast enough, the motor doesn't notice it, it only notices the average effect. Other common methods for controlling DC are as follows: 1. Varying the flux, i.e. the excitation current(concerning the saturation in the excitation circuit, only a weakening of the flux is possible) the regulation of the rotational speed at a constant armature voltage is possible only to speed values above the rated rotational speed, i.e. beyond the rotational speed at maximum flux. Maximum permitted excitation current. Limit: mechanical stress (centrifugal force) and commutation (brush fire, sparking). 2. Reducing the armature voltage Right arrow the regulation of the rotational Speed is possible only to speeds below the rated rotation speed, to avoid Possible fire on the brushes at higher voltages; (voltage switching, e.g. from

James Lock220 V to 110 V or supply at DC motor controller, Leonard set). 3. Increasing Rtot with an additional series resistance R(starter) in the armature Circuit. This possibility is rarely used due to the additional losses and strong load Dependency of the speed.

Describe the functions of the following electrical components used within an energy flow control system: 5.1 relays: A relay is an electrically operated switch. Current flowing through the coil of the relay creates a magnetic field which attracts a lever and changes the switch contacts. The coil current can be on or off so relays have two switch positions and most have double throw (changeover) switch contacts as shown in the diagram.

Relays allow one circuit to switch a second circuit which can be completely separate from the first. For example a low voltage battery circuit can use a relay to switch a 230V AC mains circuit. There is no electrical connection inside the relay between the two circuits, the link is magnetic and mechanical. The coil of a relay passes a relatively large current, typically 30mA for a 12V relay, but it can be as much as 100mA for relays designed to operate from lower voltages. Most ICs (chips) cannot provide this current and a transistor is usually used to amplify the small IC current to the larger value required for the relay coil. The maximum output current for the popular 555 timer IC is 200mA so these devices can supply relay coils directly without amplification. Relays are usually SPDT or DPDT but they can have many more sets of switch contacts, for example relays with 4 sets of changeover contacts are readily available. Most relays are designed for PCB mounting but you can solder wires

James Lockdirectly to the pins providing you take care to avoid melting the plastic case of the relay. Relay coils produce brief high voltage 'spikes' when they are switched off and this can destroy transistors and ICs in the circuit. To prevent damage you must connect a protection diode across the relay coil. 5.2 Thyristors: Thyristors or silicon controlled rectifiers (SCR) as they are sometimes known may appear to be unusual electronics components in many ways, but they are particularly useful for controlling power circuits. As such these electronics components are often used for applications such as light dimmers, and there may be thyristor circuits used in many power supply applications. Thyristors are simple to use and cheap to buy and often thyristor circuits are easy to build and use. All these reasons make thyristors ideal components to consider for many applications. The thyristor may be considered a rather an unusual form of electronics component because it consists of four layers of differently doped silicon rather than the three layers of the conventional bipolar transistors. Whereas conventional transistors may have a p-n-p or n-p-n structure with the electrodes named collector, base and emitter, the thyristor has a p-n-p-n structure with the outer layers with their electrodes referred to as the anode (n-type) and the cathode (p-type). The control terminal of the SCR is named the gate and it is connected to the p-type layer that adjoins the cathode layer.

Thyristors are usually manufactured from silicon, although, in theory other types of semiconductor could be used. The first reason for using silicon for thyristors is that silicon is the ideal choice because of its overall properties. It is able to handle the voltage and currents required for high power applications. Additionally it has good thermal properties. The second major reason is that silicon technology is well established and it is widely used for a variety of semiconductor electronics components. As a result it is very cheap and easy for semiconductor manufacturers to use. The way in which a thyristor operates is different to other devices. Normally no current flows across the device. However if a supply is connected across the device, and a small amount of current is injected into the gate, then the device will "fire" and conduct. It will remain in the conducting state until the supply is removed. For the sake of an explanation, the thyristor circuit can be considered as two back to back transistors. The first transistor with its emitter connected to the cathode of the thyristor is an n-p-n device, whereas a second transistor with its emitter connected to the anode of the thyristor, SCR is a p-n-p variety. The gate is connected to the base of the n-p-n transistor as shown below.

When a voltage is applied across a thyristor no current flows because neither transistor is conducting. As a result there is no complete path across the device. If a small current is passed through the gate electrode, this will turn "on" the transistor TR2. When this occurs it will cause the collector of TR2 to fall towards the voltage on the emitter, i.e. the cathode of the whole device. When this occurs it will cause current to flow through the base of TR1 and turn this transistor "on". Again this will now try to pull the voltage on the collector of TR1 towards its emitter voltage. This will cause current to flow in the emitter of TR2, causing its "on" state to be maintained. In this way it only requires a small trigger pulse on the gate to turn the thyristor on. Once switched on, the thyristor can only be turned off by removing the supply voltage. It can be seen that current will only flow in one direction through the thyristor. If a reverse voltage is applied, then no

James Lockcurrent will flow, even if some gate current is applied. In this way for thyristor circuits used for AC, operation only occurs over one half of the AC waveform. For the other half of the cycle the device remains inoperative and no current can flow. 5.3 Triacs: The triac is in the same family as the thyristor. The formal name for a TRIAC is bidirectional triode thyristor. It can be thought of as two reverse-parell SCR`s with a common gate terminal. The triac operates bi-directionally and behaves as a thyristor would when forward biased and triggered(on).The triac can be triggered from either positive or negative polarity pulses at the gate.

Low powered Triacs can be used in applications such as light dimmers, speed control for fans and electric motors. 5.4 Transistors A transistor is a semiconductor that uses a small amount of voltage or current to control a larger change in voltage or current. It is used in a large number of applications including amplification, switching, voltage stabilization, signal modulation and it also can be used as an oscillator. It does so by sandwiching one semiconductor between two other semiconductors. Because the current is transferred across a material that normally has high resistance (i.e. a resistor), it was a "transfer-resistor" or transistor. Transistors are split into 2 categories. These categories are bipolar junction transistors (BJTs) and field effect transistors (FETs). Transistors are used in switches, amplifiers and computers. Demonstration of System Operation 6 Choose a recognised communication system, e.g. television, radio, RS232, satellite communications, an EFTPOS system, mobile phone to satellite uplink, broadband,etc. The communication system I shall write about is Broadband. 6.1 Evaluate and justify the method used to convey information in your chosen system: In general, broadband refers to telecommunication in which a wide band of frequencies is available to transmit information. Because a wide band of frequencies is available, information can be multiplexed and sent on many different frequencies or channels within the band concurrently, allowing more information to be transmitted in a given amount of time (much as more lanes on a highway allow more cars to travel on it at the same time). Related terms are wideband (a synonym), baseband (a one-channel band), and narrowband (sometimes meaning just wide enough to carry voice, or simply "not broadband," and sometimes meaning specifically between 50 cps and 64 Kbps). Currently, when it comes to home broadband, advertised download speeds range from 8Mb to 50Mb, but this is rising at a pretty quick rate - you can expect a broadband download speed of between 100Mb and 200Mb to become commonplace over the next few years.

James LockVarious definers of broadband have assigned a minimum data rate to the term. Here are a few: Newton's Telecom Dictionary: "...greater than a voice grade line of 3 KHz...some say [it should be at least] 20 KHz." Jupiter Communications: at least 256 Kbps. IBM Dictionary of Computing: A broadband channel is "6 MHz wide."

Broad band information can be conveyed in a variety of ways such as via satellite, modem, cable, wireless and fibre optic cables. Fibre optic is currently the fastest medium. Cabled broadband speed will diminish the further you are from the local telephone exchange where as fibre optic does not suffer this problem. It is generally agreed that Digital Subscriber Line (DSL) and digital t.v(e.g. sky) are broadband services in the downstream direction. 6.2 Within your chosen system consider/discuss the following: 6.3 Security: Broadband has an always on nature due to this computers can be open to the threat of viruses, spyware,ad-ware and diallers. According to Spy-bot, as of Feb. 06, 2009 there are 287,524 viruses and growing. Due to this when signing up for broadband you should look into what security you ISP (internet service provider) will provide you with to protect your PC and any information you have on it. It is also advised that you install your own firewall and virus scanning software packages to further protect your PC. 6.4 Encyrption: Encryption refers to algorithmic schemes that encode plain text into non-readable form or cypher text, providing privacy. The receiver of the encrypted text uses a "key" to decrypt the message, returning it to its original plain text form. The key is the trigger mechanism to the algorithm. Until the advent of the Internet, encryption was rarely used by the public, but was largely a military tool. Today, with online marketing, banking, healthcare and other services, even the average householder is aware of encryption. Web browsers ext automatically when connected to a secure server, evidenced by an address beginning with https. The server decrypts the text upon its arrival, but as the information travels between computers, interception of the transmissions will not be fruitful to anyone "listening in." They would only see unreadable gibberish. 6.5 Bandwidth: In electronic communication, bandwidth is the width of the range (or band) of frequencies that an electronic signal uses on a given transmission medium. In this usage, bandwidth is expressed in terms of the difference between the highestfrequency signal component and the lowest-frequency signal component. Since the frequency of a signal is measured in hertz(the number of cycles of change per second), a given bandwidth is the difference in hertz between the highest frequency the signal uses and the lowest frequency it uses. A typical voice signal has a bandwidth of approximately three kilohertz (3 kHz); an analog television (TV) broadcast video signal has a bandwidth of six megahertz (6 MHz) -some 2,000 times as wide as the voice signal. In computer networks, bandwidth is often used as a synonym for data transfer rate- the amount of data that can be carried from one point to another in a given time period (usually a second). This kind of bandwidth is usually expressed in bits (of data) per second (bps). Occasionally, it's expressed as bytes per second (Bps). A modem that works at 57,600 bps hastwice the bandwidth of a modem that works at 28,800. 6.6 Cost: The cost of Broadband hugely varies depending on the requirement of the user for example for up to 10Mb speed including the line rental you can expect to be paying around 20 pound per month. Where as for slower speeds you can pay considerably more. Resently home broadband internet has more commonly become an entiser for custemers and is given away free with phone and entertainment packages.

James LockBusiness users on the other hand will either opt for Very high bit Digital Subscriber Line (VDSL) or fibre optics if cost is relative. These services can be expensive but give a very high amount of channels and high frequencies which can be very beneficial with 100 plus users on the internet at one time. 6.7 Reliability: The reliability of broadband depends on the broadband entry level cost. A low cost usually does not have a service level agreement, therefore if a fault occurs your connection may be unavailable for part or most of the working day. Also if the ISP is busy you may not get a connection as fast or as quick as you like. Because of this it would be worth thinking about having a backup connection if it fails such as a modem.

7 Investigate and analyse an energy flow control system as follows:

7.1 Draw and appropriately label the general block diagram of an open-loop transfer system, giving two suitable aircraft examples :

Input Transducer

Amplifier

M

Load

Figure 1 open loop system

As we can see from figure one the open loop system has no position feedback so this type of system is totally unsuitable as a precision control system This type of arrangement could however be suitable for cabin pressure control systems. The motor may be part of an open-loop system for operating a butterfly valve that may regulate outflow from the aircraft cabin. Another example of an open loop system can be found within an aircraft's cooling system. 7.2 Draw and appropriately label the general block diagram of a closed-loop transfer system, giving two suitable aircraft examples:Velocity feedback TG

Input Transducer

+

_

Amplifier

M

Load

Output Position Feedback Transduce r

Figure 2 Closed Loop System

James Lock

This system has both velocity and positional feedback capabilities the load will continue to be driven by the error signal until the demanded position is reached,Thus the closed loop system is suitable for the aircraft's auto pilot system. The flow volume of used air to be discharged from an aircraft cabin can also be controlled by a closed-loop system Describe the relative advantages and disadvantages of each of these control systems : The most obvious disadvantage of the open loop system is that it lacks the feedback signals of its counter part closed loop system. Therefore any corrections that would have to be made manually imputed by an operator. This alone can be a draw back as even the most experienced of operators can make mistakes and without feedback, there is no guarantee that the control inputs applied to the process will actually have the desired effect. The principal drawback/ disadvantage of open-loop control is accuracy loss. Although the lack of a feedback controller and feedback signal can have its advantages if used on a suitable system. There are many applications where experienced operators can make manual corrections faster than a feedback controller can. Using knowledge of the process' past behavior, operators can manipulate process inputs now to achieve the desired output values later. A feedback controller, on the other hand, must wait until the effects of its latest efforts are measurable before it decides on the next appropriate control action. Predictable processes with long time constants or excessive dead time are particularly suited for open-loop manual control. The biggest/principle advantage of the closed loop system is the feedback controller/signal. The feedback can for example keep an aircraft on a steady heading when used within an autopilot system. Every feedback controller has a different strategy for accomplishing its particular target, but all use some variation on the closed-loop control algorithm, this is measure a process variable, decide if its value is acceptable, apply a corrective effort as necessary, and repeat the whole operation infinitely. Error = reference value measured value signal. However feedback controllers must operate in the open-loop mode on occasion should a sensor fail to generate the feedback signal or an operator may take over the feedback operation to manipulate the controller's output manually. Problems as mentioned above may then occur with these manual inputs. 7.4 Choose one of your examples of a closed-loop control system and give an in-depth analysis as to the function of: 7.4.1 The individual elements: The example I shall concentrate on is the closed loop system of the Autopilot system, with the load as the tail rudder.(ref figure 2 closed loop system).Also see diagram below for a more Auto pilot specific view.

Input transducer:Input Transducers convert a quantity to an electrical signal (voltage) or to resistance (which can be converted to voltage). Input transducers are also called sensors. Error detector: (auto pilot computer)Denoted in the system diagram as a x within a circle, the error detector works by comparing the demand and feed back signals. The two signals are added together algebraically and the resultant output is used to drive the motor. In practice the error detection is achieved through a summing junction and op-amp. Amplifier:(not illustrated in above diagram) The amplifier is used to amplify the weak electrical signal it receives from the error detector. Amplification is done by using gain within the component.Gain is generally calculated by the ratio or the output power to the input power and is measured in decibels (dBs).

James LockTachogenerator: The TG is an electromechanical device which produces a signal proportional to the speed off rotation. The TG connects to the motor via the shaft and its role within the sysytem is to prevent a problem called hunting. This is best explained using an example, the load moves to its demanded position however it cannot stop exactly at the correct position due to inertia. The load therefore overshoots causing an error signal so the motor sends the load back the other way, it again overshoots and the load tick-tocks around the demand position. Output transducer: This device is connected to the systems motor via a shaft and provides the error detector with position feedback signals. Motor: The Motor in this arrangement is connected to a servomechanism which is in turn connected to the load. The Motor provides the required force to move the servomechanism which it turn moves the load. Servomechanism: Used typically to move control surfaces,radar antennae and are used extensively in autopilot and auto-stabiliser systems. In short Servos are used to move a mechanical load to a desired position with a high degree of accuracy and using a small control signal. Servos can be implemented in various forms including: Electrical Hydraulic Pneumatic Electro-hydraulic

7.4.2 The system as a whole: Let's consider the example of a pilot who has activated a single-axis autopilot: The pilot sets a control mode to maintain the wings in a level position. 1. 2. 3. 4. 5. 6. 7. 8. However, even in the smoothest air, a wing will eventually dip. Position sensors on the wing detect this deflection and send a signal to the autopilot computer. The autopilot computer processes the input data and determines that the wings are no longer level. The autopilot computer sends a signal to the servos that control the aircraft's ailerons. The signal is a very specific command telling the servo to make a precise adjustment. Each servo has a small electric motor fitted with a lip clutch that, through a bridle cable, grips the aileron cable. When the cable moves, the control surfaces move accordingly. As the ailerons are adjusted based on the input data, the wings move back toward level. The autopilot computer removes the command when the position sensor on the wing detects that the wings are once again level. The servos cease to apply pressure on the aileron cables.

This loop, shown above in the block diagram, works continuously, many times a second, much more quickly and smoothly than a human pilot could. Two- and three-axis autopilots obey the same principles, employing multiple processors that control multiple surfaces. Some air-planes have auto thrust computers to control engine thrust. Autopilot and auto thrust systems can work together to perform very complex manoeuvres.

James Lock8 Block Diagram Reduction

Reduce the system described by the following diagram to a single block and determine the transfer function of that block.

G1 (s)

G2 (s)

G1(s)G2(s)

G3 (s)

+ +

G3(s)G4(s)

G4 (s)

+

+ +

G1(s)G2(s)

G3(s)G4(s)

H1(s)

H2(s)

James Lock

+ +

G1(s)G2(s)

H1(s)

=

G1( s )G 2( s ) 1 G1( s )G 2( s) H 1( s)

+=

G 1(s )G 2 (s ) 1 G 1(s)G 2 (s ) H 1(s)

G3(s) +G4(s)

_H2(s)

=

G 1(s )G 2 (s ) 1 G 1(s )G 2 (s )H 1(s )

G3(s)+G4(s)

=

K

The overall gain of the series element:

K =

(G1( s )G 2(2) )(G3( s ) +G 4( s ) )1 G1( s )G 2( s ) H 1( s )

James LockThen we can get the final block diagram:

+(G1(s)G 2(2))(G3( s) + G 4(s))1 G1(s )G 2( s ) H 1(s )

_H2(s)

The above diagram displays negative feedback. Transfer function(s)=

G1( s ) 1 + G1( s ) H ( s )

1+G1(s)H(s)=

1+

(G1( s )G 2(2) )(G 3( s ) +G 4( s ) ) H 2(1 G1( s )G 2( s ) H 1( s )

=

(1 G1( s )G 2( s ) H 1( s ) ) +(G1( s )G 2( s )G 3( s ) +G1( s )G 2( s )G 4( s ) )H 2 [1 G1( s)G 2( s ) H 1( s)]=

(1 G1( s )G 2( s ) H 1( s ) )[G1( s )G 2( s )G 3( s ) H 2( s ) +G1( s )G 2( s )G 4( s ) H 2( s )] [1 G1( s )G 2( s ) H 1( s )]=G1( s ) =T n ra sferfu 1 +G ( s ) H ( s ) n n ctio s (s)

=

G1( s )G 2( s )G 3( s ) +G1( s )G 2( s )G 4( s ) G1( S )G 2( S )G 3( S ) H 2( S ) + [1 G1( s )G 2( s ) H 1( s )] 1( s )G 2( s )G 4( s ) H 2( s ) G

James Lock

9 Derive an equation describing the relationship between the inputs)(),1sd and)(2sdand the output)(so) for the system described by the following diagram.

The " symbol @ input = [ s ) H 2( s ) +d 2( s )] H 1( s ) " ( (i ( s ) [ s ) H 2( s ) +d 2( s )]( H 1( s ) )G1( s ) +od 1( s ) G 2( s ) = s ) ( ( [i ( s ) s ) H 2( s ) H 1( s ) d 2( s ) H 1( s ) ]G1( s ) +d1( s ) G 2( s ) = s ) ( ( [i ( s)G1( s ) ( s ) H 2( s ) H 1( s)G1( s) d 2( s ) H 1( s)G1( s) +d1( s )]G 2( s) = ( s ) i ( s )G1( s )G 2( s ) s )G1( s )G 2( s ) H 1( s ) H 2( s ) d 2( s )G1( s )G 2( s ) H 1( s ) +d1( s )G 2( s ) = s ) ( ( i ( s )G1( s )G 2( s ) +di ( s )G 2( s ) d 2( s )G1( s )G 2( s ) H 1( s ) = s )[1 + G1( s )G 2( s ) H 1( s ) H 2( s )] ( G 2( s )[i ( s )G1( s ) +d1( s ) d 2( s )G1( s ) H 1( s ) ] = s )[1 + G1( s )G 2( s ) H 1( s ) H 2( s ) ] (

The above equation separately breaks down separate sections between:

( s ), 1( s ), 2( s )onLH i d d SAnd

( s)onRH S10 Control System Damping A series RLC circuit has values for R = 100, L = 2.0H and C = 20F. The voltage, Vo, measured across the capacitor is given by:

d 2Vo V R dV o 1 + + Vo = i 2 L dt LC LC dtwhen there is a step input of:

Vi

James Lock10.1 Find the natural frequency of the circuit : For a RLC Circuit natural frequency Here L is inductance in henries and C is capacitance in Farads. Given that: R=100. L=2H. C= . .

= =25.16Hz.

.

10.2 Determine whether the system is over damped, critically damped or under damped

The type of damping depends on If If If the circuit is under damped. the circuit is over r damped.

.

the circuit is critically damped.

Fir the given circuit . Circuit is under damped.

10.3 Find the damped oscillation frequency : Damped oscillation frequency:

.

James Lock= Damped oscillation frequency=24.85 Hz. 10.4 Determine the solution to the differential equation, if when t=0 both

V0 and

dV 0 are 0 dt

We already know that the circuit is critically damped. The general solution for critically damped case = Where = The Values of constantsA1,A2 are determined from the initial conditions . = Substitute above values in Equation for = = Applying initial condition 0= . . . .

.

. Setting initial condition at t=0, 0= + . .=-0.01 . Substituting in the Equation for = = . .

V0 =Vi (1 (cos 2500 t + 0.01 sin 2500 t )) e 25 t

James Lock11 Aircraft System Response : Explain the derivative/integrative and proportional/integrative control methods as applied to the response of an aircraft system of your choice :

A good example method for a derivative/integrative and proportional/integrative control method is the PID controller.

P -Proportional, I - Integral, D - Derivative. These terms describe three basic mathematical functions applied to the error signal , V error = Vset - Vsensor. This error represents the difference between where you want to go (Vset), and where you're actually at (Vsensor). The controller performs the PID mathematical functions on the error and applies the their sum to a process (motor, heater, etc.).I'll explain the three components (proportional, integral, and derivative) of a PID controller next Proportional All three components of the PID algorithm are driven by the difference between the process value (i.e. the current speed) and the reference point (i.e. the target speed.) We will call this difference error) for one particular time step:

enFor that same time step, we call the process value:

ynand the reference point:

rnTherefore:

e n = rn y nthe output value is:

un

James LockThe proportional component simply calculates the output value, based on the error term by multiplying it by a constant term, so we get:

u n = K p enFor simple situations, this all by itself can be a very effect control algorithm. Typically this works best when you know that when both the error and output value =0.For example, imagine a simple wing leveler in an aircraft. The process value is going to be bank angle, the reference point is going to be zero (zero bank angle means the wings are level.) Assume a well trimmed aircraft with neutral stability so that when the ailerons are zero there is no change in bank. A proportional only control would set the aileron deflection inversely proportional to the bank angle. As the bank angle gets closer to zero, the aileron deflection gets closer to zero. Something as simple as this (a formula with one multiply operation) can be an amazingly effective and stable controller. Integral Even in the case of a simple wing leveler, you encounter situations where the aircraft isn't perfectly trim and zero aileron deflection does not always equal zero roll motion. In an aircraft such as a Cessna 172, the amount of aileron deflection needed to keep the wing level can vary with speed. In these cases, a proportional only controller will stabilize out quickly, but will stabilize to the wrong value. We need a way to drive the error in the proportional only controller to zero. Enter the Integral component of the PID algorithm. Integral refers to the area under a curve. If you have a function, the integral of that function produces a second function which tells you the area under curve of the first function. At each time step we know:

enwhich is the difference between the process value and the reference point. If we multiply this distance times:

dt(the time step) we get an area which approximates the error under the curve just for this time step. If we add these areas up over time, we get a very reasonable approximation of the area under the curve. Essentially what this does is that the longer time passes with us not at our target value, the larger the sum of the (error dt)'s becomes over time. If we use this sum to push our output value (i.e. our accelerator position) then the longer we don't quite hit our target speed, the further the system pushes the accelerator pedal. Over time, the integral component compensates for the error in the proportional component and the system stabilizes out at the desired speed.

Derivative The derivative of a function implies the rate of change of the function output. If you know the function, you can take the derivative of that function to produce a second function. For any point in time, the derivative function will tell you the rate of change (or slope) of the first function. Conceptually, this makes sense in the context of a controller. How quickly we are closing on our target value (i.e. the rate of change from each time step to the next) is an important piece of information that can help us build a more stable system that more quickly achieves the target value. For an Aircraft's cruise control, we are measuring velocity at each time step. The rate of change of velocity is defined as acceleration.

James LockCombining P + I + D The proportional component is very stable. The Integral and Derivative components are very unstable. If we build a proportional only controller, it will be very stable but will stabilize to the wrong value. (i.e. if we want to go 90km/hr, it might stabilize out to 82km/hr.) If we build an integral only controller it will quickly hit the target value, but will overshoot, then overcompensate, and will oscillate wildly around the target value(hunt). It is very unstable. The trick then is to combine these components together by summing them. The actual output is equal to what the P component says the output should be plus what the I component says the output should be plus what the D component says the output should be. Mass value should be added to each component to increase or decrease it's relative power to influence the final output value. The final equation is then:

u n = k p uPn + K i uI n + K D uD n

The actual maths involved in a PID controller (while rooted in some deep theory) is actually quite simple to implement. The real trick for creating a well behaved PID controller and a well behaved autopilot is tuning the relative weights of each of the P, I, and D components.

James LockBibliography Yeovil college moodle course notes-Roger Macey www.ehow.com www.wikipedia.org www.howstuffworks.com Barry college course notes module 3,electrical fundamentals.-A.Davies Barry college course notes module 4, electronic fundamentals.-A.Davies

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