4.20
Recorders, Oscillographs, Loggers, Tape Recorders F. D. MARTON
(1970)
P. M. B. SILVA GIRÃO
818 © 2006 by Béla Lipták
G. F. ERK
(1985)
B. G. LIPTÁK
(1995)
(2005)
Types of Designs and Movements:
1. Portable or laboratory, bench-top or flatbed 2. Strip-chart 3. Multipoint 4. Circular chart 5. X–Y recorder 6. Oscillograph/paperless recorder (light beam recorder) 7. Tape recorder 8. Loggers 9. Charts and accessories
Sensor mechanisms:
A. Potentiometric B. Galvanometric C. Linear array recorder
Costs:
A portable drum chart recorder for temperature or humidity costs $500. A portable 2.375-in. (60-mm) strip-chart recorder for event, mA, mV, V DC, V AC, or thermocouple inputs costs from $400 to $650. A 4-in. (102-mm) portable circular chart recorder is available with 2% full-scale error for temperature ($250), pressure ($350), or temperature and humidity ($500). An 8-in. (203-mm) portable, circular chart recorder with 0.75% full-scale error for temperature or 4 to 20 mA costs $750. A 12-in. (305-mm) digital circular chart recorder for direct temperature, 4 to 20 mA, or voltage costs from $1000 for one channel up to $2000 for four channels. A programmable 4-in. (102-mm) folding strip-chart recorder for millivolts, volts, or direct temperature with 0.5% error outfitted with one pen costs $1250; with two pens $1700; and with three pens $2200. A portable bench-top flatbed recorder with a 10-in. (254-mm) chart and 0.35% full scale error can be obtained with mV and V input for one ($1000), two ($1500), three ($2200), or four ($3100) channels; built-in RS-232C interface adds $500. X-Y recorder/plotters with 0.25% error and RS-232C and IEEE-488 interface cost about $4000; 24-point, 10-in. (254-mm) strip-chart potentiometers range from $4000 to $7000, depending on features. A data logger for eight channels costs around $100 and for 96 channels about $12,000. Paperless recorders cost from $650 to $4000 or more.
Partial List of Suppliers:
ABB Group (1, 2, 3, 4, 5, 6, 8, 9, A) (www.abb.com/global/USABB/usabb045.nsf! OpenDatabase&mt=html&l=us) Agilent Technologies (8) (www.agilent.com) Ampex Data Systems (7) (www.ampexdata.com) Anderson Instrument Co. (4, A) (www.andinst.com) Astro-Med Inc. (1, 2, 3, 8, 9, C) (www.astro-med.com) Avalon Electronics Ltd. (7) (www.avalon-electronics.com) Barton Instrument Systems LLC (1, 4, B) (www.barton-instruments.com) Bristol Babcock (1, 2, 3, 4, A, B) (www.bristolbabcock.com) Campbell Scientific Inc. (7, 8) (www.campbellsci.com) Chart Specialties Inc. (9) (www.chartspecialties.com/products.htm) Chino Corp. (1, 2, 3, 6, A) (www.chinoamerica.com) Cole Parmer Instrument Co. (1, 2, 3, 4, 5, 6, 8, 9, A) (www.coleparmer.com) Cooper Instruments & Systems (1, 2, 5, A) (www.cooperinstruments.com) Cybernetics (7) (www.cybernetics.com) Dickson (4, 8, 9) (www.dicksonweb.com/info/home.php)
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Dresser Instruments (4,8) (www.dresserinstruments.com) Dwyer Instruments Co. (1, 2, 4, 8, 9, A) (www.dwyer-inst.com/htdocs/DATA/qs-tocdata.cfm) Endress+Hauser (2, 3, 6, 9, A) (www.us.endress.com) Enercorp Instruments Ltd. (2, 3, 6, A, B) (www.enercorp.com) Graphtec Instruments (1, 2, 3, 5, 6, 8, 9, A, B, C) (www.westerngraphtec.com) Gulton (9) (www.gulton.com) Honeywell Industrial Measurement and Control (1, 2, 3, 4, 5, 6, 9, A) (www.content. honeywell.com/imc) Invensys Eurotherm (1, 2, 3, 4, 5, 6, 7, 9, A, C) (www.eurotherm.com) Invensys Foxboro (1, 2, 3, 4, 6, A, C) (www.foxboro.com/instrumentation) Jumo Process Control Inc. (1, 2, 3, 6, 9, A, B) (www.jumoprocesscontrol.com) Kipp & Zonen (1, 2, 5, 9, A) (www.kippzonen.com) Linseis International (1, 2, 3, 4, 5, 8, 9, A, C) (www.linseis.com) Martel Electronics Sales Inc. (7) (www.martelelectronics.com) Metrum Information Storage (2, 6, 7, 8, A) (www.metrum.co.uk) Nicolet Gould Instrument Technologies (1, 2, 6, 8, 9, C) (www.niti.com/docs/home. php?id=1) Ohkura Electric Ltd. (2, A) (www.ohkura.co.jp) Omega Engineering Inc. (1, 2, 3, 4, 5, 6, 8, 9, A) (www.omega.com) Palmer Wahl Instruments Inc. (4, 8, A) (www.instrumentationgroup.com) Pyrometer Instrument Co. (1, 2, 3, 6, 8, A) (www.pyrometer.com) The Recorder Co. (1, 2, 5, 8, 9, A) (www.recordercompany.com/page1.htm) Ronan Engineering (8) (www.ronan.com) RMS Instruments Ltd. (1, 2, 7, 8, 9, A) (www.rmsinst.com) Siemens Energy & Automation (6) (www.sea.siemens.com/default.asp) Soltec Corp. (1, 2, 3, 4, 5, 6, 7, 8, A, C) (www.solteccorp.com) Sony Corp. (7) (www.sony.com) Teac America Inc. (7, 8) (www.teac.com) Thermo Electron Corp. (2, 3, 6, 8, A) (www.thermo.com) Toshiba America Inc. (6, 7, 8) (www.toshiba.com) Yokogawa Electric (1, 2, 3, 5, 6, 8, A, B, C) (www.yokogawa.com/cms/com/ep/ home.do)
INTRODUCTION The recording of events, trends, or variables is an important part of process control. With the increase of computer-based process control and information digitized and displayed mainly in cathode-ray tubes (CRTs) and liquid crystal-based displays (LCDs), temporary recordings are often made using the computer’s memory and permanent recordings are made using a mass-storage unit (e.g., hard disk, CD, tape). When information on paper is required, the easiest and cheapest way is to use printers connected to the computer. This type of solution is surely the state of the art and probably seen in control rooms of distributed control systems. Nevertheless, both for local and for less complex applications, other means of variable recording for event and trends detection are used. Information can be recorded either in analog or digital format. In the first case the quantity to be recorded is continuous in time and the recorder (analog type) must have the bandwidth required for reproduction without distortion. Digital recording uses sampled data and the information recorded corresponds to discrete values of time (digital recorders). Some recorders, called hybrid, are of the analog type insofar as printing is concerned but also output data in digital format. Both digital and hybrid recorders operate under microprocessor control, and
© 2006 by Béla Lipták
features presently available include remote operation through instrumentation interfaces (e.g., IEEE488, Fieldbus). This section describes the various recorder designs; printers are covered in Sections 4.17, “Human–Machine Interface Evolution,” and 4.24, “Workstation Designs and Features”; CRTs are discussed in Section 4.5, “CRT Displays.” Recorders can be classified according to their measurement signals (electronic, electrical, pneumatic, thermal), according to their sensor mechanism (potentiometric, open loop, galvanometric, linear array), according to their application (portable, laboratory, flatbed, benchtop, industrial, panel-mounted), according to the type of chart used (strip roll, fanfold, circular, multipoint, X–Y plotter), or according to the recording technique applied (pressure, electrostatic, light beam, ink, electric, thermographic). The discussion below begins with descriptions of the most widely used sensing mechanisms. SENSOR MECHANISMS Practically all the indicator movements that have been discussed in Section 4.18, “Indicators, Analog Displays” can also be used to operate analog recorders. Here only the most widely used designs — the galvanometric (including light-beam),
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Mag
net
S
Pen attached to coil draws curvilinear trace on chart
N
Recorder paper
Rotating galvanic coil
FIG. 4.20a The recorder pen of a galvanometer can be connected directly to the rotating armature.
potentiometric, open loop, and linear array type recorders— will be described. Galvanometric Recorders Figure 4.20a illustrates a direct-writing galvanometer on which the pen is connected to the armature of the galvanometer and therefore rotates with the rotating-coil or moving-iron type armature. The writing system can use carbon transfer, thermal, pressure, electro-discharge, ink, or ink-jet methods. The error sources in galvanometers include linearity, tangential error, drift, and hysteresis. The torque capability of galvanometers limits their frequency response to about 100 Hz in a 10 mm (0.4 in.) wide channel. Multichannel recorders can be obtained by placing several galvanometers side by side. Galvanometers are relatively inexpensive, require little maintenance, and are suited for operation in extreme temperature (high or low) and humidity environments. Nevertheless, they are seldom used today. Light-Beam Recorders (Oscillographs) When the armature of the galvanometer is used to rotate a mirror, the unit is called a light-beam-type recorder (Figure 4.20b). As a change occurs in the electric measurement signal, which travels through the coil, this change causes the mirror to rotate. The main components include a light source (usually ultraviolet), the galvanometer with a rotating mirror, an optical system, and a light-sensitive recording chart. A recorder that has several channels, using several galvanometers, can record several measurements simultaneously. When ultraviolet-sensitive recording film or paper is used, a permanent record is obtained instantaneously, without a need for any further processing. A galvanometer (oscillograph) with several channels can print numbers on the edge of the record to identify the corresponding channels. The light-beam-type galvanometer has a much higher frequency
© 2006 by Béla Lipták
response (up to 25,000 Hz) than does the direct recording galvanometer (under 1000 Hz). Recorders based on the above-mentioned principles are also rarely seen in use today. In present usage the word “oscillograph” refers mainly to an instrument that incorporates the functions of an oscilloscope and of a printer and, less commonly, to paperless recorders. Potentiometric Recorders Today, most analog recorders are of the potentiometric type. Figure 4.20c illustrates a servo-operated null-balance potentiometric recorder. Here the amplifier drives the balancing servomotor (usually a DC motor), which is mechanically connected to the writing pen and the feedback slide-wire. Because there is no current flow when the unit is in balance, load resistance variations or lead-wire resistances have no effect. This allows the sensor to be located a long distance from the recorder. Potentiometers are sensitive down to the microvolt level and are accurate to within 0.25% of span. Because most process variables can be detected in terms of micro- or millivolt signals, potentiometers can be used to record most variables. When used with a null-balancing bridge input, it can measure resistance, inductance, and capacitance, in addition to electromotive force (emf). The main limitations of potentiometric recorders are cost and speed of response. The inertia inherent in the recording system limits conventional null-balance recorders to a fullscale pen travel of about 0.5 seconds. Although at low frequencies (few Hz), the potentiometric recorder is the most accurate unit available, its speed of response is a limitation. If the belts, pulleys, gears, and rotary servomotors of a conventional potentiometer are replaced by a high-speed servo (the armature of this servomotor moves back and forth as a shuttle), the speed of response can be increased by a factor of 2 to about 0.2 seconds and hysteresis can be reduced.
4.20 Recorders, Oscillographs, Loggers, Tape Recorders
821
Mirror which rotates with galvanometer coil
Mag
net
S
Light-sensitive recording paper
N
Light source
FIG. 4.20b A recorder design in which a mirror is connected to the rotating armature of the galvanometer and a UV light detects the mirror position is often called a light-beam-type or oscillograph-type recorder.
Open Loop Recorders In digital recorders, signals to be plotted are digitized. The mechanisms that actuate the writing element(s) are then usually driven not by DC servomotors but by step-motors. The drivers are microprocessor controlled and the system operates in open loop. In a typical arrangement, a step-motor produces paper movement in one direction and one or several stepmotors actuate the writing elements in an orthogonal direction. This setup permits the representation not only of time-varying quantities but also an X–Y plot if the paper is allowed to move
in either sense. In fast-responding strip-chart digital recorders the paper can be alternatively actuated by a DC motor. Linear Array Recorders In these recorders the recorder pen has been replaced by a fixed (nonmoving) array of small recording elements. The recorder chart moves under this linear array of “stylii,” each one of which corresponds to a particular measurement value to be recorded. If a 0.25% resolution is desired, a 0 to 100% range will be incremented among 400 fixed stylii, each of 120 V A-C power supply
Low A-C voltage power supply D-C voltage source
Electronic amplifier Syncroverter
Input transformer
120 V A-C power supply
Control winding
Unknown D-C EMF from sensing element
120 V A-C power supply
Balancing motor Potentiometer
FIG. 4.20c One of the traditional potentiometric recorder designs.
© 2006 by Béla Lipták
Recording mechanism
Reference winding
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which represents a particular measurement signal increment. Every print cycle the drivers select the stylii to be activated to print a dot pattern forming the alphanumerics, grids, and signal traces. As this process is repeated every cycle, a continuous recording results. The main advantage of linear array recorders is the absence of moving parts (other than the paper). The array elements can be thermal or electrostatic. When thermal elements are used, the paper is coated with a thin film of thermoreactive material on a substrate. When electrostatic elements are used, the chart paper is dielectric; in addition to the paper, a liquid toner is required. The records resulting from the electrostatic technique last longer and require less energy than do thermally obtained recordings. Paper costs are similar for the two methods: about 5 to 10 cents per foot of paper. The maximum width of the print ranges from 10 to 15 in. (254 to 314 mm), chart speeds range from 1 mm/hr to 0.5 m/sec, and the printing speed can approach 2000 lines per second.
RECORDING METHODS There are many ways of leaving a mark on a chart, including the various ink-writing systems and the inkless methods of thermographic, electric, pressure, light-beam, and electrostatic marking. Ink-Writing Systems The early pen-and-ink recorders used bucket, V, fiber-tipped, or ballpoint pens and pressurized or gravimetric capillary-type feeding systems. Refilling was performed by the use of disposable ink cartridges, and maintenance was performed by the use of replacement pen assemblies. In high-speed oscillographs either pressurized inking systems are used (requiring special coated paper) or high-cost ink-jet hardware is installed. Impact printing initially used a carbon ribbon and later slow-drying inks stored in porous pads; when the pointer mechanism presses down, it leaves a single-color or multicolored mark. More recently, the pointer has been replaced with a traveling print wheel, which is engraved with numbers or symbols that identify the variables being recorded. This type of marking is used on most multipoint strip chart recorders, which are usually provided with an 11-in. (279-mm) chart and can simultaneously record about 20 variables. Inkless Systems Thermographic recorders use a heated stylus that melts or chemically changes the heat-sensitive coating on the paper. In the linear array version, some 420 fixed thermal-printing elements (stylii) can be used on a 4.5-in. (114-mm) paper chart. Each element is capable of generating some 100 dots per inch (4/mm). When used with up to six channels as a multipoint recorder, the channels are identified by different
© 2006 by Béla Lipták
letter characters, while the date and time are periodically printed onto the recording. Pressure-type recorders were one of the earliest designs, in which a sharpened stylus was used to remove smoke or other special coating from the chart, thereby exposing a contrasting color underneath. In electric recorders an electric discharge generated by the stylus etches the record into an aluminum coating on a black paper, thereby exposing the black substrate. The light-beam-type recorder is illustrated in Figure 4.20b. In this recorder a photosensitive paper or film is used to record the location where the high-intensity light impinges on the chart. In order to eliminate the effects of ambient light, ultraviolet sources rather than visible light sources have been used. If a programmable light gate array is used to record directly on ultraviolet (UV)-sensitive paper, the error contributions of linearity, beam deflection, tangential error, inertia, and overshoot can be reduced. In the electrostatic recorder, there is an imaging head with some 1000 wire elements, spaced at 4/mm, giving a total length of 10 in. (254 mm). As the recording chart moves over the image head, a negative voltage is applied to selected wires while positive voltage is applied to “shoes” on the other side, giving a positive point charge to the paper chart at the activated points. Next the chart proceeds to the toner head, where negatively charged ink particles adhere to the charged points on the paper. Following this step a vacuum knife removes the excess toner particles, and when the remaining particles are exposed to air, they permanently bond to the paper. Paperless Systems Paperless recorders (Figure 4.20d) are multichannel data acquisition systems that present the information on a display similar to a computer monitor, usually of the LCD type. Some of them have also paper printing and permanent data storage capabilities. They are light-beam-type recorders, much like digital sampling oscilloscopes (DSOs), but as they are acquisition systems they can also be considered data loggers. Thus, paperless recorders can be found in suppliers identified both by 6 and 8 on the partial list of suppliers at the beginning of this section.
CHARTS AND COORDINATES The process variable actuates a recording mechanism, such as a pen, that moves across a chart. The chart moves constantly with time. These two motions produce an analog record of variable versus time. Any point on the continuous plot obtained in this manner can be identified by two values, called coordinates. Many coordinate systems are in use, with the Cartesian coordinates most widely used in industry. If the reference lines are straight and cross each other at right angles, they are called rectilinear coordinates. If at least one of the reference lines is an arc of a circle, the coordinates are curvilinear.
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FIG. 4.20d Front and rear views of a touch-screen paperless recorder with input/output handling and communications capabilities, as well as flexible data storage options. (Courtesy of Thermo Electron Corporation.)
The shape of the chart provides a primary means of classification into (1) circular charts and (2) rectangular charts, in sheet or strip form. Strip charts can be torn off and can be stored in rolls or folded in Z folds. Strip chart lengths vary from 100 to 250 feet (30.5 to 76.2 m). Circular Chart Recorders Circular chart recorders are still used because they are simple and, consequently, low priced. Chart speeds are a uniform number of revolutions per unit time. Chart drives making one revolution in 24 hours are standard, but instruments are also
equipped with drives of 15 minutes; 1, 4, 8, 12, or 48 hours; and 7, 8, or 28 days per revolution. The best applications for portable circular chart recorders are temperature or humidity recordings used in heating and ventilating systems. The circular chart recorders shown in Figure 4.20e produce concentric circles crossed by circular arcs whose center, when the pen crosses the arc, is the same as that of the recording arm. The tangents at the intersection points of the two curves should be as near to a right angle as possible for best readability. The concentric circles form the scale on which the variable is read. The “time arcs” divide the
FIG. 4.20e Circular chart recorder. (Courtesy of Honeywell Industrial Measurement and Control and Omega Engineering, Inc.)
© 2006 by Béla Lipták
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full circles into appropriate uniform intervals of the total period. Chart diameters vary from 3 in. (75 mm), 4 in. (100 mm), and 8 in. (200 mm) to up to 12 in. (300 mm). Circular charts offer the advantage of a flat surface. Special features, such as automatic chart changers, allow collection of daily records in a continual manner. Contacts can also be mounted in the pen mechanism for alarms on preset high or low signal conditions. One of the limitations of circular chart recorders is that the recording of more than two variables on one chart poses design problems. Strip-Chart Recorders Strip-chart recorders are characterized by the uniform linear motion of the paper either horizontally (Figure 4.20f) or vertically (Figure 4.20g). The measurement lines can be straight or curved, furnishing either rectilinear (Figure 4.20g) or curvilinear recordings. The curvilinear method uses very simple linkage geometry and offers better readability than is obtained with circular charts. Figure 4.20h shows a two-pen recorder (second from left) and a recorder controller. Whereas the two-pen recorder records two independent variables, the recorder controller accepts one sensor signal, compares that signal with a manually operated set point, and provides a signal output for closing a control loop by actuating a final control element. Strip-chart recorders are available in several standardized sizes, including 4-in. (Figure 4.20i), 6-in., and 11-in. (102-mm, 152-mm, and 279-mm) charts. The standard miniature stripchart recorder is used either in the wide (6 by 6 in. or 152 by 152 mm) or in the narrow format (3 by 6 in. or 76 by 152 mm). Chart speeds are available from 1 in./hr to 30 in./min in adjustable increments (1.0 in. or 25.4 mm). These strip-chart recorders are available with up to three pens and with up to two alarm contacts per pen. The accuracy of recording is usually within 0.5 to 1% of span. Multiple Recorders When several variables are to be recorded on the same chart, such as several temperatures from thermocouples in various locations, multiple recorders are used. Circular chart record-
FIG. 4.20g Strip-chart recorders with vertical movement.
© 2006 by Béla Lipták
FIG. 4.20f Horizontal strip-chart recorder.
ersmay handle only up to four variables, whereas strip-chart recorders handle as many as 24 to 36 measurements. To identify each variable, symbol or numerical coding or color printing is used, as well as full digital alphanumeric printouts on the chart margin. With the advent of microprocessor technology, multivariable recorders became available. These allow recording of a multitude of variables, such as flow and the associated temperature and pressure. Likewise, for comparison of several variables on the same time scale without line crossing or overlapping, multichannel recorders are available.
4.20 Recorders, Oscillographs, Loggers, Tape Recorders
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FIG. 4.20h Two-pen recorder and recorder controller.
Figure 4.20j shows the cut-out requirements and the front dimensions of typical 6-in. and 11-in. (152-mm and 279-mm) multipoint strip-chart recorders. With the addition of microprocessors, high-speed hybrid recorders became available with up to 30 recording channels. Inputs can be DC volts or millivolts, thermocouples, and resistance temperature detectors. On these units the digital measurement data are printed on the left margin of the chart in up to six colors. The basic accuracy of these units is 0.25% of span. Most hybrid recorders are provided with a keyboard on the front panel, which allows for the programming of chart speed, alarm set points, full scale range, and other features.
FIG. 4.20i Four-inch (100-mm) strip-chart, multipoint recorder. (Courtesy of Honeywell Industrial Measurement and Control.)
© 2006 by Béla Lipták
X–Y Recorders X–Y recorders plot two variables simultaneously, such as stress versus strain or temperature versus pressure. Either the chart is stationary and the scriber is moved along both the abscissa and the ordinate by the two signals, or the chart is moved in one direction while the stylus slides on an arm in the other direction. The signals entering the function plotter can be analog or digital. Digital signals require transducers to obtain an analog plot. Likewise, digital records can be provided with analogto-digital conversion and conventional digital printout. There are also combination function plotters, called X–Y–Z1 recorders. Some allow the pen to be driven along either axis at a constant speed, thus making recordings of X versus Y, Y versus T (time), and X versus T possible. Recorders with three independent servo systems allow the recording of two variables against a third. X–Y recorders usually have flat beds with measurements 1 ranging from 8 /2 by 11 in. (216.5 by 279 mm) to 45 by 60 in. (1143 by 1524 mm). Some record on drums. Recorders that print from the back or on a glass screen do not obstruct visual observation. X–Y plotters can be connected to computers using either RS-232C (serial) or IEEE-488 (parallel) ports and can send or receive data with 0.02 mm resolution. Data are most often 1 recorded with thermal or disposable ink pens on either 8 /2 by 11 in. (216 by 279 mm) or 11 by 17 in. (279 by 432 mm) leaf or roll paper. When the X–Y recorder is used to measure variables other than voltage (for example, current, frequency, or temperature),
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241 mm 9.5"
229 mm 9"
Panel cut-out
267 mm 10.5"
257 mm 10.12"
6" (150 mm) strip chart
362 mm 14.25"
Panel cut-out
394 mm 15.5"
257 mm 10.12"
267 mm 10.5"
11" (275 mm) strip chart
FIG. 4.20j Panel cutout and front dimensions of multipoint strip-chart recorders. (Courtesy of Honeywell Industrial Measurement and Control.)
plug-in amplifiers are installed to make the required conversions. The addition of a second servo-actuated measuring element and recording pen to an X–Y plotter yields an X–X1–Y or an X–Y–Y1 recorder that can plot the relationship among three variables.
to their baseline. Event recorders are available with up to 128 inputs and are well suited for the analysis of the sequence of events leading up to industrial accidents.
Event Recorders
Today, permanent storage of information, either in analog or digital formats, uses different technologies such as batteryoperated random access memories, magnetic discs, tapes, and compact optical discs (CDs). In process control, magnetic recording on tape is frequently used either for analog or digital form signals. Videotape recording is similar to magnetic tape recording with the additional feature of reproducing both picture and sound. Industrial application is found in closed-circuit television monitoring. Magnetic tape recording is done by magnetizing fine iron oxide particles, which are coated onto a polyester plastic tape. The data can be recorded by fixed heads on multitrack longitudinal recordings in analog or pulse-code-modulated (PCM) form, or the data can be recorded by helical scan techniques, which record analog or digital data in VHS, Beta, or R-DAT formats. For process-control-related recording the multi-track fixed-head recording has been the most popular. On a 0.5 in. videocassette tape, usually no more than 24 recording tracks are available. Helical scanning reduces this track width of 25 mils to about 2 mils and thereby allows a track density of about 250 tracks per 0.5 in. in the VHS format.
Operations or event recorders mark the occurrence, duration, or type of event. They record multiple incidents, such as ontime, downtime, speed, load, and overload, on the chart. The records that are produced are usually in the form of a bar, with interruptions in a continuous line indicating a change. Microprocessor technology allows scores of points to be scanned every millisecond, with high-speed printouts made for the events that occur. The classic event recorder is a simple instrument (Figure 4.20k) in which the pen is deflected when the associated electromagnet is energized. The electromagnets can be operated by AC or DC signals from 2 to 250 volts. The pen remains deflected until the associated electromagnet is deenergized. The resulting recording shows not only that an event has occurred, but also its time, duration, and possible consequences in initiating other events. Figure 4.20k also shows a typical event recording chart where open rectangles correspond to times when electromagnets were energized (events occurred). When the event is ended, the electromagnet is deenergized and the pens return
© 2006 by Béla Lipták
TAPE RECORDING
4.20 Recorders, Oscillographs, Loggers, Tape Recorders
827
Comb
Electromagnet assembly Operating lever Inkwell Counterweight
Writing pen
FIG. 4.20k The design of an event recorder and the appearance of a typical event record chart. (Courtesy of Esterline Angus Instrument Corp.)
DATA LOGGERS Data loggers are electronic instruments that record measurements of different quantities such as temperature and relative humidity or events over time. They can be software packages loaded into personal computers, DCS systems, or other computing devices that serve to operate printers, but more often they are stand-alone battery-powered units that are equipped with a microprocessor, data storage, a printer and sensors. Most data loggers utilize turn-key software on a personal computer to initiate the logger and view the collected data. High-speed recording is achieved by combining electronic readouts with electrostatic printing. Digital recorders can print as many as 1 million characters per minute on paper in digital form. For applications such as electric power generation, the high-speed digital recorder is the best choice. Some stand-alone microprocessor-based data loggers, commonly called paperless recorders, record data in a processor’s
© 2006 by Béla Lipták
memory; they do not incorporate printing capabilities but are provided with a screen where charts are presented. A low-cost solution to data logging is to use a personal computer to control the printer, which shows the trend of the process or the results of a test. Most data-logging systems are provided with capability both for front panel programming and for switch-selectable remote programming, implemented over RS-232C, IEEE-488 fieldbus connections from remote terminals. The logged data can be linearized or scaled; the data can be averaged by time or by groups; and up to four alarm set points can be programmed onto each channel being monitored. The typical scanning capacity of stand-alone data loggers is up to 128 channels. These units are also provided with an automatic restart feature that functions after a power failure and with RS-232C and passive current loop ports for computer interconnection. Central data loggers are usually integrated into the total control system of the plant and consist not only of a complete data acquisition system but also of some control capability.
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Stand-alone battery-operated data loggers are an interesting alternative to analog chart recorders because they are smaller, less expensive (base price and no consumables required), more accurate, and more reliable. On the other hand, because information is digitized, data analysis, presentation, and storage are simplified. The reduced size of some models allows their operation in areas inaccessible to other recording systems.
Bibliography Anderson, L., “X-Y Recorders,” Measurements and Control, October 1992. Bailey, S., “Recorders and Indicators Exert a Cohesive Force in Online Quality,” Control Engineering, January 1991. Chiranky, L., “A Case for Linear Array Recording,” Instruments and Control Systems, July 1982. Dobrowolski, M., “Guide to Selecting Strip and Circular Chart Recorders,” InTech, Vol. 28, No. 10, October 1981. Dyer, S.A. (Ed.), Survey of Instrumentation and Measurement, New York: John Wiley & Sons, 2001. Gadberry, B. E., “Designing Integrated Control System Displays,” Paper # 91-0354, 1991 ISA Conference, Anaheim, October 1991.
© 2006 by Béla Lipták
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