CHAPTER 5: OUTPUT PRESENTATION UNIT

Transcription

1 CHAPTER 5: OUTPUT PRESENTATION UNIT 5.1 INTRODUCTION All analogue electrical and electronic instruments can be broadly classified into two categories, namely: (1) Instruments with pointer movements, and (2) Instruments with graphical displays. All analogue electrical and electronic meters belong to the first category. Included in this very wide group category are the basic electromechanical instruments used as panel board or industrial instruments, and the portable electronic instruments. There are four types of these meters built around the following principles of operations: 1. The permanent magnet moving coil (PMMC) movement mechanism, which responds to average or direct current only; 2. The moving iron vane (MIV), fixed coil movement mechanism, which responds to d-c or a-c currents. 3. The moving magnet fixed coil (MMFC) movement, which again responds to direct current only; and 4. The electrodynamometer (EDM) movement with one fixed coil and one moving coil, which again responds to direct or alternating currents. We shall next examine briefly the principle of operation of each of the above type of instruments by emphasising the input-output response relationships as discussed in the generalised static and dynamic performance characteristics of instruments. Particular attention will be devoted to the PMMC as this meter movement is still the heart of many simple and complex electronic instrumentation systems. ECEg535/EE507:-Instrumentation Eng g 1

2 5.2 DYNAMICS OF POINTER METER MOVEMENTS Electromechanical instruments have a number of common features in that they basically involve pointer movements rotated against fixed scales. As illustrated in Fig. 5-1, the moving elements of these instruments are mounted on rigid supports (called spindles) which rest pivoted on jewel cups or bronze bearings. To minimise friction losses, the pivots and bearings are polished as thoroughly as possible. The force giving rise to deflecting torques is typically exerted on current carrying conductors, and in principle these can be determined using the laws of electromagnetic dynamics. Thus starting from the instantaneous force acting on a charge of q columb moving with a velocity V in an electric field of strength E and magnetic field of flux density B, the basic force law yields F = q (E + v x B) newtons If E is neglected in comparison to the cross product vxb, we can set F = q ( v x B ) Fig.5.1 ECEg535/EE507:-Instrumentation Eng g 2

3 We consider first the movements of the PMMC. If two parallel conductors (carrying equal but oppositely direct currents) are placed in a magnetic field, then two force couples giving rise to a torque will result as shown in Fig. 5-2, which represents the top view of a PMMC. Fig. 5.2 The deflecting torque will then be given by τ i = where d is the distance between the lines of action of the two couple forces. This deflecting torque will be opposed by three other torque acting on the moving assembly, namely: Fd 1. A restoring or control torque which will be directly proportional to the angular displacement θ, and is also directly proportional to the spring constant K of the spiral spring, and thus can be set equal to K; 2. An accelerating torque which is dependent on the moment of inertial J of the moving 2 d θ assembly, and with the angular acceleration 2 dt Cdθ 3. A damping torque, which is dependent on the angular, speed C being an effective dt damping coefficient. ECEg535/EE507:-Instrumentation Eng g 3

4 Hence, the torque equation τ = 0 Can be written as or equivalentely 2 d θ dθ τ i J kθ = 0 2 dt dt 2 d θ dθ τ i = J + C + kθ 2 dt dt The above equation represents the dynamic performance of a second order instruments. 5.3 THE PERMANENT MAGNET MOVING COIL (PMMC) The permanent magnet moving coil (PMMC) movement mechanism, which responds to average or direct current only; the dynamic performance of second order instruments of PMMC is given by τ i = kθ 5.4 THE MOVING IRON VANE (MIV) While the above procedure for obtaining the dynamic equation of the PMMC is in principle valid for all electromechanical instruments, it could pose difficulties in determining the magnetic forces, which give rise to the deflecting torque. An alternative procedure for determining τ i from energy relations is, however, found helpful for practically all electromechanical and electrostatic pointer movements. The principle behind this method is that torque can be related to rotational systems. Consider the top-view representation of a repulsion type of moving iron vane movement shown in Fig ECEg535/EE507:-Instrumentation Eng g 4

5 Fig.5.3 Let θ, i, di, L, and dl denote, respectively, the angular displacement of the moving vane, the current applied to the fixed coil, a small increase in current, the self-inductance of the coil, and a small variation in L. During the time that the moving vane carrying the pointer is being deflected, an emf will be induced in the coil, which can be expressed by d e = ( Li) dt di = L + i dt dl dt Which are due to changes in i and L, respectively. The electrical work done against this opposing emf will be given by W done = = i e dt Li di + i 2 dl This work must be equal to the change of stored energy in the whole system. This comprises of the change in potential energy of the spring which is Kθ, and the change in energy stored in the inductance of the coil due to current and inductance changes such that W stored = τθ + ( i + di) ( L + dl) = i L 2 2 ECEg535/EE507:-Instrumentation Eng g 5

6 Equating the above Equations, it can thus be shown that the deflecting torque will be τ i dw d = = ( W dθ dθ 1 2 dl Kθ = i 2 dθ stored W done ), or Where K is the spring constant. which is generally valid for any rotating system. Simply states that the deflection torque is proportional to the product of the square of the current and the change in inductance due to the rotation of the moving vane from its equilibrium position. 5.5 THE ELECTRODYNAMOMETER (EDM) The same procedure can be followed to describe the principle of operation of an EDM. The basic construction of an electrodynamic instrument consists of a moving coil mounted the magnetic field of two series-connected stator coils. Let i l and i 2 denote the current in the stator and moving coil, respectively. The conditions relating the deflecting torque to the magnetising currents i l and i 2 is very similar to the torque relationship applying to the moving-iron vane instrument, but the important variable here is the mutual inductance M between the fixed and moving coils, and the stored magnetic energy i 1 i 2 M. Starting from the expression for the induced emf e = e + e i 2 di1 = M + i1 dt dm dt + M di2 dt + i 2 dm dt where the first and second terms resepresent the emfs induced in the moving coil, and the third and fourth terms give the emfs induced in the fixed coil. By considering the change in stored energy, we obtain W = τ iθ + i2 Mdi1 + i i stored 1 2 It can thus be shown that the deflecting torque for the EDM movement is given by i1i2 dm τ i = 2K dθ dm ECEg535/EE507:-Instrumentation Eng g 6

7 Ignoring the transient parts of the solutions of the basic equations for the three-meter movements, the steady state solutions (θ ss ) for the three basic electromechanical meter movements can then be summarised by the following relationships: (i) For the PMMC, inbld θ ss = = Cons tan t xinput = KI C (ii) For the MIV, θ 2 i dl = dθ 2C 2 SS = Cons tan t x( input) = KI 2 (iii) for the EDM, θ i i dm = dθ C where C is stiffness of the suspension spring. 1 2 SS = Cons tan t x( input) 1 x( input) 2 = KI 1 I ANALOG ELECTRONIC INSTRUMENTS All analogue electronic instruments are built around the PMMC as the basic display mechanism. Such instruments are capable of measuring a-c signals (i.e voltages, currents and non-electrical quantities) which cannot be measured by other standard electrical instruments. As shown in Fig. 5-5, the input signal q i (t) is fed to a signal conditioning circuit before it is applied to a PMMC instrument which contains different scales for different quantities on a single plate. ECEg535/EE507:-Instrumentation Eng g 7

8 Fig.5.4 The signal conditioning circuit involves the use of a suitable range selector or attenuator as well as an interconnection of active (i.e amplifying) and non-linear passive (i.e. rectifying) devices. Depending on the electronic circuitry, an analogue electronic meter belongs to one of the following instruments: 1. Average responding meter 2. RMS responding meter; 3. peak responding rectifier 4. peak-to-peak responding rectifier AVERAGE RESPONDING METER The deflection of the PMMC meter is proportional to the average value for this purpose the instrument are used for dc responding either voltages or currents to safe values. This can be realised in practice by using very large series of resistors (for voltage ), and very small shunt paths (for current ). ECEg535/EE507:-Instrumentation Eng g 8

9 Fig RECTIFIER METERS As shown in Fig. 5-5, rectifier meters can be built with the use of half-wave or full-wave rectifying devices. In both circuits, currents can only flow in one direction. The deflection of the PMMC meter is proportional to the average value of the rectified current (or voltage). Assuming that the input current is sinusoidal, then the average current I av will be I I AVE AVE = 0.45I = 0.9I RMS RMS FOR HALF WAVE FOR FULL WAVE ECEg535/EE507:-Instrumentation Eng g 9

10 Fig.5.6 Typically, the meter scale can also be calibrated in terms of rms values of sinusoidal voltages (or currents). However, the meter indications will be incorrect when non-sinusoidal voltages are measured PEAK AND PEAK-TO-PEAK RESPONDING METERS As shown in Figs 5-6, the peak-responding meter is built with the use of a capacitor and a diode. ECEg535/EE507:-Instrumentation Eng g 10

11 Fig.5.7 It can be seen that V o (t) = V in + V c where V in can be taken as a sinusoidal voltage with V in(t) = V m sin ωt and V c = V m is the voltage to which the capacitor C will be charged when the diode D is conducting during the negative half of the input voltage. The primary difference between the peak-responding meter and the average responding meter is the use of a storage capacitor with the rectifying meter. The average voltage read by the meter is given by T 1 V O ( t) av = Vo t dt = V T ( ) 0 Which is equal to the peak amplitude of V in. The peak-responding voltmeter is able to measure signals of frequencies up to hundreds of megahertz. However for unsymmetrical waveforms, the reading will be in error. To overcome this problem, a capacitor and a diode can be added as shown in Fig. 5-7 to obtain a peak-topeak response. m ECEg535/EE507:-Instrumentation Eng g 11

12 Fig.5.8 ECEg535/EE507:-Instrumentation Eng g 12

13 INSTRUMENTS WITH GRAPHICAL DISPLAYS. 5.7 GENERAL PURPOSE CATHODE RAY OSCILLOSCOPES The oscilloscope, or more generally the cathode ray oscilloscope (CRO), is the most versatile electronic measuring instrument. The CRO can mainly be regarded as a true analog instrument in that it serves both as a voltmeter, and as an electrical-to-optical transducer showing a waveform display of the signal under measurement. Still more importantly, the CRO is an indispensable instrument for the measurements of frequency, time duration, and phase differences at audio and higher frequencies The basic structures of a general-purpose cathode ray tube (CRT) are illustrated in Fig. 5-9 and It consists of five major parts, namely: (i) power supply, (ii) vertical amplifier, (iii) triggering circuit, (iv) time base circuit, (v) gate amplifier, (vi) horizontal amplifier, and (vii) the cathode ray tube(crt). The power supply unit indicated in Fig. 5-9 provides d-c voltages to the various electrodes inside the CRT, and also to the amplifier and time-base circuits. Fig.5.9 Laboratory oscilloscopes are classified in many ways. Usually, the distinctions are based either on frequency-response capability or on CRT characteristics. Thus there are lowfrequency oscilloscopes (DC to 10 MHz for vertical amplifier response), high frequency Oscilloscopes (sometimes capable of capturing and displaying single-shot phenomena of less ECEg535/EE507:-Instrumentation Eng g 13

14 than 1-n sec rise time), and sampling oscilloscopes which reconstruct very high frequency signals up to 18 GHz - repetitive waveforms on a dot-sample basis. In terms of the characteristics of the display screen, there are standard refreshed phosphor oscilloscopes, and storage oscilloscopes, Fig.5.10 Depending upon the type of CRT used. With the foregoing general introduction, we shall next try to examine in some detail important features and principles of operation of the CRO under the following major topics: - Beam Generation, Focusing and Deflection inside a CRO, - Saw-tooth Sweep Waveform Generation and Waveform Display - Large Input Signal Attenuation and Attenuators - Bandwidth and Rise-Time features of CRO's - Applications. Without going too much into details, we shall thus attempt to understand firmly the underlying principles and techniques which explain the basic operations and working conditions of general-purpose CRO's in particular, and other similar devices. ECEg535/EE507:-Instrumentation Eng g 14

15 5.8 BEAM GENERATION, FOCUSING AND DEFLECTION Generation and focusing of an electron beam inside a CRT is illustrated in Fig An electron gun assembly mounted at the base of the CRT produces the electron beam. In this section of the tube, continuous beams of electrons emitted by a heated cathode subsequently pass through a series of electrodes. Starting from the control grid, followed by the accelerating grid, the intensity of the beam of the beam is adjusted to produce a very thin line that will strike the CRT screen with sufficient energy. In a laboratory type CRO, the diffused beam is first narrowed into a pencil-sharp ray using an electrostatic method of focusing as illustrated in Fig Fig.5.11 ECEg535/EE507:-Instrumentation Eng g 15

16 Fig 5.12 Electrostatic focusing in a CRT Further, the focused beam spot striking the CRT screen can be deflected vertically and horizontally by means of voltages applied to pairs of vertical and horizontal deflection plates that are placed at right angles to each other, and the focused beam. There are two methods employed in the deflections of electron beams in CRTs: A) In laboratory oscilloscopes, the beam passes through two pairs of orthogonal mounted deflection plates, which deflect the beam vertically up and downwards, and horizontally sideways. These are called, respectively, vertical and horizontal deflection plates. Time varying and direct current potentials are applied across each plate pair, and the deflection method is called electrostatic deflection. B) Another method of beam deflection, which is called magnetic deflection, uses cross-aligned coils. This method of deflection is practically used in the CRTs found in television and wordprocessing terminals. The CROs used in measurement systems are built with electrostatic deflection mechanisms. As illustrated in Fig. 5-13, the important parameters in the deflection mechanisms are D = Separation or spacing of deflection plates at right angles to the beam; Lp = Effective length of deflection plates int he direction of the focused electron beam; S = Length along beam from center of deflecting plates to the CRT screen; ECEg535/EE507:-Instrumentation Eng g 16

17 Va = Beam voltage or the potential to which the electron beam has been accelerated when it enters the deflection region, Vd = Deflecting voltage between plates, D = final deflection distance on the CRT screen, either horizontally or vertically; Z = Coordinate along central axis of the CRT ECEg535/EE507:-Instrumentation Eng g 17

18 Fig Principle of electrostatic deflection in a CRT Detailed analyses on the electrodynamics of the fields and accelerating voltages finally lead L D = S( d From the above Equation, the deflection sensitivity is defined as the ratio of D to V d in millimeters per volt (or centimeters per volt) D e = D/V d (millimeters/volt) Which is independent of both the deflecting voltage V d and the ratio e/m, but varies inversely with the accelerate voltage V a. Inversely the deflection factor V a /D of a CRT Is defined by P V )( V d a ) G = 1/D e Can thus be defined in volts/centimeter for standard measurements, or in volts/millimeter for very sensitive deflections. Typical values of deflection sensitivities range from 1.0mm per volt to 0.1 mm per volt, and with corresponding deflection factors ranging from 10 volts per centimeter to 100 volts per centimeter. 5.9 SWEEP WAVEFORM GENERATION AND DISPLAY To use an oscilloscope properly, it is first very essential to know when or why the various selector switches are operated. The functions of the four important circuits (i.e-vertical input circuits, horizontal amplifier circuits, triggering circuits, and sweep generator circuits) are therefore discussed below. ECEg535/EE507:-Instrumentation Eng g 18

19 5.9.1 VERTICAL INPUT CIRCUITS A typical functional diagram of vertical input circuits is illustrated in Fig The AC/DC switch short-circuits the blocking capacitor when the oscilloscope is to be used to display signals from d-c to an upper frequency limit determined by the bandwidth of the vertical amplifier. A calibrated attenuator placed before the vertical amplifier controls the vertical sensitivity or deflection factor in VOLTS/DIVISION. Fig.5.14Block diagram representation of vertical, input circuits TIME BASE CIRCUIT AND WAVEFORM DISPLAY The most frequent application of an oscilloscope is to display a changing signal y(t), as a function of time. Thus oscilloscopes are provided with time base or sweep-generator circuit which produce saw-tooth voltages as illustrated in Fig The waveform illustrated rises linearly from "A" to "B" during the active sweep period T 1, during which time the electron beam moves across the CRT screen with constant horizontal velocity. the slope of this waveform and hence the horizontal beam velocity is adjusted by setting the time base to the desired TIME DIVISION. During the retrace period, T 2, the electron beam is returned to its starting point and the CRT is "blanked out" during this flyback time. To obtain a stable display on the CRT it is necessary that the start of each sweep by synchronized to the signal being displayed. In other words, the frequency of the signal (yinput) must be equal to or be integer multiples of the sweep frequency (i.e. l/t l ). Otherwise a stable display will not be obtained. ECEg535/EE507:-Instrumentation Eng g 19

20 Fig Typical output of a time-base circuit In conjunction with the sweep circuits, there is also a STABILITY control mechanism, which controls the sensitivity of the sweep circuit to an input pulse. In TRIG (i.e. triggered) operation, this control is set so that the sweep will not operate unless a triggering pulse is received. The synchronized (SYNC) operation for stability control is adjusted such that repetitive sweeps are generated. The function of the sweep circuit is shown in Fig In many respects, the horizontal amplifier circuits are similar to the vertical amplifier circuits. However, the gain and bandwidth of the horizontal amplifier are usually lower than those of the vertical amplifier. Just as the vertical amplifier is provided with different gain settings for different input signal levels, the horizontal amplifier gain can also be adjusted to magnify or expand the horizontal sweep display. This magnification is obtained by increasing the gain of the amplifier by a variable setting or by a certain factor, which in effect increases the display area width by the same factor. Fig.5.16 Functional diagram of a sweep circuit ECEg535/EE507:-Instrumentation Eng g 20

21 5.9.3 TRIGGERING CIRCUITS These circuits enable the sweep generator circuits to start the sweep generation in coincidence with selected time spots. Triggering signals can be selected from one of the following three sources: a) Internal source from the vertical amplifier; b) External source from an externally applied signal; c) Line supply, which is a line frequency of 50 or 60 HZ, and usually obtained from a transformer internal to the oscilloscope unit. These are shown abbreviated as INT, EXT, and LINE on the front panel of a CRO. The internal source, whereby the triggering signal is derived from one of the vertical inputs is most frequently used, while line triggering is used to display signals which contain components at line frequency or multiples of line frequency. When the two triggering sources are not satisfactory, external-triggering sources will be required. In standard service or laboratory oscilloscopes, one frequently finds triggering controls or MODE selector switches for +VE or -Ve slope TRIGGERING TRIG LEVEL, AUTO, TV LINE, and TV FRAME. The selection of the triggering point is illustrated in Fig ECEg535/EE507:-Instrumentation Eng g 21

22 Fig Selection of triggering level. Consider the Y-input signal (i.e. vertical amplifier input) to be a triangular waveform. Then, the triggering point can be chosen either from the + ve going or -ve going parts of the signal. Additionally, the TRIG LEVEL required to obtain the pulse, which triggers the sawtooth waveform generator can also, be adjusted. In the AUTO mode, the TRIG LEVEL is not used and the circuit slope selector still operates as indicated above. The AUTO mode is usually preferred as this minimizes the number of adjustments, which must be made to obtain a stable display on the CRT screen. The triggering modes TV LINE and TV FRAME are only found in oscilloscopes used additionally for servicing television equipment, and they are used for synchronizing a composite video waveform display with the horizontal or vertical synchronizing pulses, respectively. It should be mentioned here that more complex triggering switches are also used in advanced oscilloscopes. ECEg535/EE507:-Instrumentation Eng g 22

23 5.9.4 SAW TOOTH VOLTAGE GENERATION The sweep waveform illustrated in Fig is generated with the use of circuits, which can be represented as simple RC circuits as shown in Fig Fig Equivalent circuit for a saw-tooth waveform generator The switch SW represents an electronic switching circuit which can alternatively connect the capacitor to position 1 for charging, and to position 2 for discharging through resistors R 1 and R 2, respectively. If the electronic switch is running free, then the sweep also becomes freerunning. Typical output waveforms are presented in Figs. 5-19(a) and (b) for different duration of sweep or scan times. Actual saw-tooth waveforms can be described by V c = V(1-e -t/r1c ) in which the voltage levels V m, V m1 and V m2 are determined by the values of the capacitor C for fixed values of R 1 and R 2. To obtain T 2 <<T 1, then R 2 C<<R 1 C. For different sweep rates, different capacitors are then connected to the switch S W, which is physically built from a fast active electronic switch such as unijunction transistor or a combination of bistable multivibrators. ECEg535/EE507:-Instrumentation Eng g 23

24 Fig CALIBRATED SWEEPS To obtain reasonably accurate displays of applied waveforms, it is necessary to maintain a precise relation between the electron beam and the sweep generation. This means that the sweep voltage must be highly linear and adjustable between exactly known limits as illustrated in Fig The one important use of such a calibrated sweep is for time calibrations during measurements. Fig.5.20 Linearized sweep waveform needed for calibration Very dependable oscilloscopes actually have two calibrated sweeps. One is produced by the sawtooth voltage generator, and is called the main sweep. The second sweep is produced by a similar but delayed circuit, thus giving a waveform, which is behind that of the main sweep. The speed of the delayed waveform is usually arranged to be 2,5, and 10 times that of the main ECEg535/EE507:-Instrumentation Eng g 24

25 sweep. Consequently, two time axes are produced in advanced oscilloscopes, each using a time scale of its own MULTIPLE TRACE DISPLAYS In many CRO measurements, it is necessary to compare one waveform (signal) with another waveform simultaneously. To facilitate such applications, dual (and multiple) trace displays are obtained using one of the following three methods: a) dual beam oscilloscope, (b) alternate mode, and (c) chopped mode. These are illustrated in Fig Fig 5.21 Examples of multiple traces : (a) dual beam oscilloscope; (b) dual traces with alternate mode; (c) dual traces with chopped mode The use of two electron guns complete with two independent sweep generators can produce an instrument known as a dual beam oscilloscope. The same effect may also be produced by a single electron gun, but with the output being split into two independent controllable electron beams. Employing a single electron gun, a double trace can also be produced by switching the Y deflection plates from one input signal to another in an alternate mode of operation (Fig. 5-21(b). By building dot traces of two (or more) channel inputs as shown in Fig. 5-21(c), the instrument can also be operated in a chopped mode. In such a mode, the Y deflection plates are switched from one input signal amplifier to another at a rate, which is faster than the standard sweep rate. ECEg535/EE507:-Instrumentation Eng g 25

26 APPENDIX FOR CHAPTER APPENDIX A 5.1 CHARACTERISTICS OF CRT SCREEN PHOSPHORS Trace Type PER- Relative Type or Fluorescence SISTENCE Writing Speed Application P1 Yellow-green Medium 35% CROs, Radar P2 Blue-green Medium 70% CROs P4 White Medium to 75% Black & white television Medium short Medium Radar, Medical P7 Blue 95% Medium Photographic recording P11 Blue-Violet 100% Medium short General purpose replacement for P1 P31 Yellow-green 75% Very long Radar P33 Orange Medium to 7% Medical, graphics medium long P39 Green 4 0% ECEg535/EE507:-Instrumentation Eng g 26

27 Note:- 1. By Fluorescence of a phosphor is meant the light emission observed when an electron beam hits the phosphor. 2. By Phosphorescence is meant the colour of the light left after the phosphor has been stimulated into light emission and the excitation has been removed. While the phosphorescences of most phosphor screens are the same as their fluorescences types P2 and p7 have yellow-green phosphorescences. 3. By Persistence is meant an approximate measure of the time it takes for the phosphorescence to decay to l.e of the excitation level during fluorescence. For short-persistence, decay time is less than 1 msec. If the decay-time is less than 2 sec., the phosphor is said to have medium-persistence. Phosphors with decay times of the order of a minute or more are said to have long-persistence. 4. Writing-speed is a measure of the fastest deflection rate of a single beam trace (or single-short wave-form display) that is visible on a film when a photograph is taken of a waveform display with P11 taken as a reference phosphor. ECEg535/EE507:-Instrumentation Eng g 27

28 DATA ACQUISITION SYSTEMS In transducer instrumentation, data acquisition systems are used to measure and record information for later study and analysis. The recorded data takes the form of signals with either continuous or pulse waveforms. Accordingly, data acquisition systems are divided into analog and digital recorders. The important analog data storing devices are: (1)X-Y recorders, (2) graphical recorders with moving coil movements, and (3) analog magnetic tape recorders. Digital recording, reproduction and processing of massive data, which is the standard feature of computing systems, is commonly made with instrumentation magnetic tapes and disks, X-Y RECORDERS Fig.5.22 X-Y recordings are similar to CRT waveform displays, with the x-coordinate provided by a ramp generator. The Y- input receives low-frequency analog signals. Accuracy and resolution, functions of an X-Y recorder s electronic and mechanical characteristics, determine the static performance of such an instrument. The slew speed and acceleration in response to the capture of rapid and transient signal input determine the dynamic response. Other important features, which are also common to other graphical recorders, include chart size, number of pens, time base capability, preamplifiers and filters GRAPHICAL RECORDERS Graphical recorders measure variations of electrical or non-electrical quantities with respect to time taking place over many seconds, minutes, hours, or days. In many instances, the recording is needed to check the performance of industrial processes or electrical power generation and distribution systems. Mostly, these recorders are built around a moving coil instrument where ECEg535/EE507:-Instrumentation Eng g 28

29 the indicating pointer is replaced by a writing pen movement on a graduated paper moving at a constant speed. As shown in Fig. 5-22, the tip of the pointer leaves marks on the paper thereby forming a permanent record of the amplitude of the input signal with respect to time. Fig.5.23 Maximum sensitivity is of the order of 4mV/cm within a narrow bandwidth of d-c to about 10Hz. Important features of the basic graphical recorders are: (1) input impedance, (2) time scale, (3) event markers, and (4) writing mechanism. The use of amplifier ensures the input impedance to the recorder is maintained relatively high. The writing mechanism can be an ink pen with a capillary feed system, or heated stylus recording the variations of the input (V in ) on a heat sensitive paper. Other recorders, commonly known as photographic or ultraviolet (UV) recorders, use a light beam as a pointer leaving traces on photographic papers. Recorders known as multi-channel recorders contain a number of writing pens in all making marks simultaneously on a wide roll of paper thereby permitting easy comparison of several simultaneous functions MAGNETIC TAPE RECORDERS The principle of recording with instrumentation magnetic tapes is illustrated in Fig There are three main components: (1) a core with a small nonmagnetic gap, (2) a coil wound on the core, and (3) a thin magnetic coating sitting on a base. The latter can be a wire, but in most application it consists of a flexible plastic tape. ECEg535/EE507:-Instrumentation Eng g 29

30 Fig.5.24 The magnetic coating is a thin layer of iron oxide(fe2 3) particles, and the core is made of laminated steel alloys. The core assembly carrying the winding is called a tape recording head. With current (1) flowing in the coil, a magnetic flux will bridge cross the non-magnetic gap, thus magnetizing the iron oxide particles as they pass the gap. Because the magnetic coating is purposely selected for its high remanence, the iron particles will remain magnetized in the direction of tape travel with the magnitude of flux impressed upon the tape as it moves in front of the non-magnetic gap. Hence, a recording of the applied signal will have been realized. When the same tape is passed through the front gap of a similar playback head, it will cause variations in the reluctance of the winding, and thereby inducing a voltage which ideally is required to be a faithful reproduction of the recorded signal. A functional diagram of a complete magnetic tape-recorder is shown.in Fig While two different heads are needed for accurate instrumentation recording and reproduction, only one combined head is used for coarse instrumentation works as well as for audio recordings. With one head, one needs only to switch from a record to reproduce (playback) mode. At the same time the direction of tape travel has accordingly to is set by the sense of rotation of the motor moving the tape transport. ECEg535/EE507:-Instrumentation Eng g 30

31 Fig.5.25 Four factors contribute to the accuracy of magnetic tape recording, particularly for instrumentation and data processing purposes. These are: (1) the magnetization characteristics of the magnetic recording medium, (2) the tape speed, (3) the bandwidth of the recorded signal, and (4) the gap width. All these factors are considered together to minimize errors and distortions. ECEg535/EE507:-Instrumentation Eng g 31

32 DIGITAL RECORDING Magnetic tapes are also commonly used for digital recording of binary data encoded. in terms of "1s" or "0s", both in instrumentation and data processing systems. Three broad methods of digital recording can be briefly described as follows: a) Return to zero (RZ) systems which employ one direction of saturation for" 1", while magnetization to saturation in the opposite direction then represents "0". b) Non-return to zero (NRZ) systems which regard changes in the direction of magnetization as "1", while no changes in magnetization are taken as "0". c) Phase encoding (FE) systems in which one sense of magnetization is regarded as" 1", while the opposite sense is regarded as "0". For each method, the density.of recording is measured in terms of so many bits per tape length. The unit often used in bits per inch (BPI), typical values being 8000 BPI, and 1600 BPI for NRZ and phase encoded systems, respectively. The recorded binaries represent decimals or alphanumeric characters using one of the standard binary codes. Also, the record and reproduce amplifiers are very simple, and the only requirements are maintenance of accurate tape and head alignments. For computer application, recording is mostly made in a block form, meaning that a block of information is recorded at one time, in place of conventional or incremental recording where data is continuously stored on the tape. As the recorded binaries are typically obtained by digitizing input analog signals, the latter can be recovered by use of digital to analog converters. Alternative recording systems which are gaining wide applications use the same principles as already described for magnetic recording, but with the flexible tapes replaced by rigid disk packs, diskettes or floppy disks, all operated with use of movable heads to scan magnetically coated surfaces. Storage capacities measured in kilo (i.e. thousand), millions, and possibly billions of bytes (i.e 8-bit words), and access times ranging from 35 to 100 ms are the two important features of these recording devices. Data recorded on these devices is therefore much more voluminous than those recorded in tapes, and can also be accessed much ECEg535/EE507:-Instrumentation Eng g 32

33 faster. Further, magnetic tapes are also commonly used for very large digital data recording and processing. ECEg535/EE507:-Instrumentation Eng g 33