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Chemical instrumentation feature Edited by GALEN W. W I N G , Seton Hall University, So. Orange, N. J. 07079
These articles are intended to serve ~e reoflm o f m r s JOURNAL
by calling atlention to new developments i n the theory, deaign, or availubilitg of chemical laboratory instrumentation, or by presenting useful insighls and ezplanations of topics that are of practical importance to those who use, or reach the use of, modern instrumentation and instrumenlal techniques. The editor invites correspondence from prospective contributors.
XC. Laboratory Recorders"
which increases with the angle, becoming 1% at about 14' and 2% at 20'. The reading error in a small reeorder is likely to be at least as may be great as valid. this, SO that the linear approximation (Continued on page A364)
Galen W. Ewing INTRODUCTION
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One of the most universal functions of any scientific or engineering laboratory is the gathering of data to provide answers to immediate questions or information to he filed for future reference. Such data gathering may he achieved in various wavs. . . denendine on the nature and qunntrty of rhe iniornlatron. The n m r prevalent d surh data gathering methods is undoubtedly analog recording. Electrical analog recorders are available in a variety of sizes, speeds, sensitivities, and prices. They are suitable for recording any signal which is in, or can be converted to, electrical form. These recorders are found in
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adequately diagnose a heart ailment without a cardiographic recorder, or obtain infrared or magnetic resonance spectra on any praeticable basis without a strip-chart recorder? True, various curves that arenow traced automatieallv with a recorder can he dotted manually from point-hy-point measurements. This procedure, however, is not only time consuming, but may cause valid bits of infannation to be overlooked entirely, simply because the points were taken too far apart. Another factor favoring the use of reeorders is the ability to pinpoint faulty operation of the data-gathering system. Artifacts that might not be observable at all in point-bypoint observations will often be readily identifiable on a recording. Asymmetry of a peaked curve, for example, is only clearly evident in a recording. Recorders can he categorized in many ways. They may be capable of plotting one variable against another ( X - Y recorders) or only against time (Y-Tor simply "stripchartBrecorders).They may be equipped to plot a single variable or two or more simultaneously (one-, two-, or multiple-pen recorders), or they may he designed to plot Abstracted from "The Laboratory Recorder," by G. W. Ewing and H. A. Ashworth, Plenum Press, New York, 1914, by permission of the copyright holder.
several variables consecutively as a series of coded points. Perham most fundamentally, recorders are classed hy the kind of signals they will accept. Thus we have millivolt recarden. recordins ~~~~~e ammeters or wattmeters, and many others. The DC millivolt or microampere recorder is the most versatile, since input circuitry can adapt it to almost any application.
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DEFLECTION RECORDERS The most straightforward type of recorder depends upon deflection of the pen by a conventional meter movement. The signd to be measured appears as a current flowing through a coil of wire pivoted or suspended between the poles of a magnet. The interaction of the magnetic field produced by the current with the static field tends to produce circular motion. This motion must be counteracted by some restoring force which is usually supplied by a pair of hairsprings, one above and one below the coii, that also serve to make electrical connections. If the coil is suspended rather than pivoted, the restoring force must be supplied through torsion of the suspension. Rigidly attached to the coil is a pen or stylus which leaves a trace on a moving strip of paper. Figure 1suggests how this may be done. This mechanism is inexpensive and capable of fairly high speed of response. It suffers, however, from the requirement that the power necessary to move the pen must be supplied by the signal itself. Particularly in the mare sensitive ranges, this is far from negligible. The deflection mechanism must be modified if serious distortion is to be avoided. There are several devices that convert angular to linear displacements.One of these, used in same low cost instruments, merely approximates the deflection arc by a straight line (Fig. 2). The length of arc subtended hy the angle of deflection, a,is proportional to the angle, while the corresponding semichord is proportional to the sine of the angle. Hence the relative error inherent in this approximatim is
Figure 1. A coii carrying me signal current is suspended from hairsprings between the poles of a permanent magnet.
Figure 2. me arc described by me tip of the pointer Pis approximated by the recorded trace me record is made, e.g, by pressure contact at the intersection.
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Volume 53, Number 9, September 1976 1 A361
Chemical instrumentation The distortion appearing in curvilinear recordines can be eliminated bv a simole Iwer mechanism which forces the pointer tip tu fdlow a strataht line (Fig :I,. This device, found in several uell-known lines of record. ers, permits easy observation of waveforms on a rectilinear grid, hut still suffers from the sine error. Botheaual- and uneaual-arm devices, as shown in the figure, possess the same error. Another principle is shown in Figure 4, and is described in the caption. The lengths of the lever arms and relative sizes of the pulley and drum ean be seleded to reduce the nonlinearity error to less than 0.5% of fullscale.
of deflection, as in Figure 2. Ideally, the deflection is independent of a, and for small angles the error is negligible. Where greater precision is needed, these errors can be corrected in a t least two ways: (1) the magnetic field can he designed to be nonuniform in such a way as t o cancel all Or part of the error; (2) the recorder may incorporate an input amplifier whose response is nonlinear, i.e., proportional t o a h a n a. The problem of direct deflection recorders drawing power from the signal being measured is greater than in simple meters because of the greater inertia and friction in the moving system. The effect is to lower the source voltage in the act of measuring it. The difficulty can be overcome by inserting a buffer amplifier between source and recorder. (Continued on page A366)
Figure 3. Linearizing mechanisms: circular motion about point 0 produces straight-line motion of the the pointer P, perpendicular to lhs cent& line far end of the pointer is constrained to move along the center line at B. (a)Equalarm,(b)unequal-arm types. (c)Mechanism from a HewlebPackard recorder. All the recorders discussed thus far are liable to an inherent decrease in sensitivity with increasing deflection because the electromagnetic force tending to move the coil is proportional t o the cosine of the angle between the coiland the magnetic field. Taking this into account, the rectilinear deflection can he shown to be given by tan a CI a where C is a constant of proportionality,I is the current to be measured, and a is the angle A384 1 Journal of ChemicalEducafion
Figure 4. Linearizing mechanism: (a)arm m i s a t tached to the moving coil system and rotates with it around 0. while the coaxial drum Oremains ststlonilry Tne o.ler end of m e arm bears a 01. ey B u m me pmer r godly anacned lo it A beh Cpasses around drum and pulley. The relative diameters, together with lengths of arm and pointer, are chosen so that the pan Pdescribesa WaigM line amass the ~herl.(b)Such a mechanism from aOould recorder.
Chemical Instrumentation This can be a single operational amplifier in the "voltare-follower" eonfieuration which " promdrs more power to #,peralethe recorder wlthout alter~ngthe srgnal m be measured. An inconvenience is the need far a small power supply for the amplifier.
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SERVO RECORDERS
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The most widelv,used laboratorv recorders are those built around a motor-driven servo loop. A motor drivrc the pen across the chart until a position sensor observes that its location agrees with the requirement of the input signal. There are many possible implementations of this hasic principle. The motor, for instance, may be limited to less than one full turn. with direct connection to a pointer carrying the pen or stylus, or it may be n more conrrntionnl motor, rither ac ur dc. A block diagram presenting a universal servo recorder is shown in Figure 5. The signal voltage (preamplified if necessary) and the reference voltage are both passed through attenuators to a differential amplifier. The output of this amplifier is called the "error signal," since it represents the difference between the true signal and the fraction of the reference voltage corresponding to the pen position. These two voltages should be equal if the recorder is in balance. The error voltage drives the motor in such a direction as to diminish the error, and the motor in ~~~~
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A366 I Journal of ChemicalEducation
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Figre 5. Block diagam of a typical selvo recader. The poshional feedback could be connected at any of the mrae points Mrked "X'.
Fogure 6 A lyptcal amnustor corcuit provdng lodr mngm A three-aagment swrtch wthfo.f contacts
in each segment is required.
turn drives the pen to the point on the chart corresponding to the input signal. The second feedback path, marked "velocity," will be described later. Not all the blocks depicted in Figure 5 must necessarily appear in all recorders. Many instruments do not include the velocity feedback, and a preamplifier is sometimes not required. The power amplifier may be included with the differential stages in a single unit, the "servo amplifier."
it is compared with a measured fraction of the reference voltage picked off from a slidewire. The amplifier must have a high input resistance (impedance); a FET-input unit is suitable. For higher ranges, thesignal must bescaled down bv a resistive voltage R2). . divider (RI, The input impedance is now R1 R?,which in iq. 1- meeohm. . .. the .... ill~n~trntion ~~- ~ The reference voltage must also be divided down so that the slidewire will be supplied with the appropriate potential.
Attenuators
Reference Voltage Sources
Figure 6 shows a typical attenuation circuit. In the most sensitive ranges, the input signal is led directly to the amplifier, where
In older recorden the reference voltage was supplied hy zinc.carbon dry cells, and fre-
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Chemical Instrumentation quent adjustment was required to validate the stated ranges. A Weston standard cell was usually incorporated for this purpose. In some more recent recorders mercury cells have been substituted; this type of battery maintains its rated voltage throughout its life, and calibration is needed much less frequently. In present-day designs, all batteries are eliminated and constancy of reference voltage is assured through the use of temperature-compensated Zener diodes; calibration is seldom required after initial factory adjustment.
components. Separate output transistors are usually required to provide enough power to control the motor. The amplifier must have a high impedance differential input. In the recorder diagrammed in Figure 5 i t was assumed that the amplifier responds directly to dc potentials. In many recorders, however, the dc signal is converted t o ac by means of a mechanical chopper or its semiconductar equivalent, to minimize zero drift. If the chopping is a t power-line frequency, the output of the servo amplifier can be used directly to control a two-phase ac motor. Line-frequency chopping makes i t difficult t o prevent noise pickup, so some manufacturers have included an oscillator t o drive the chopper st some other frequency; the amplifier output is then rectified and a dc servo motor employed. Phase-sensitive detection will ensure that the motor turns in the appropriate direction t o maintain electrical balance.
Slidewires The slidewire, in servo parlance called a "rebalance pat," takes many forms. In older instruments i t consists of a sin& turn of resistance wire or of a tightly wound helix, wound on a nonconducting drum several inches in diameter. The resistance wire is contacted by a metallic wiper on an arm rotatable around the center of the drum. This contact should be made of a metal softer than the slidewire, since cantactors are cheaper t o replace than precision slidewires. Since precision multiturn potentiometers have become widely available, many recorder manufacturers have adopted them as economical and convenient replacements for the slidewire. Another type of slidewire takes a straight-line geometry. It is conveniently mounted parallel to the track bearing the pen, so that a single carriage can carry both pen and wiper. This straight resistive element can be metallic or can be made of resistive plastic material. The latter has become quite papular because it is highly resistant to wear and is easily cleaned. The resolution possible with different types of slidewires varies considerably. When a compound helix is used for the wire, so that the eontactor touches successive turns, only a stepwise approximation to the true record is possible. The steps are usually well below 1%of full-scale, and so may be negligible. A single-strand wire does not shgw this effect, but may have to be very thin to have enough resistance, and so is especially liable to damage. Plastic strip resistors have infinite resolution. A zero-adjustment potentiometer is conveniently connected in parallel to the slidewire, as in Figure 6. This will determine the pen position when the input is shorted to ground. It can be adjusted to any desired point, for instance, a t either end of the scale or a t its center. The two resistors Rs and R7 in series with the slidewire make i t possible t o set the zero point as much as the equivalent of one full-scale span either above or below the normal scale.
Amplifiers Modern servo recorders use solid-state amplifiers, often utilizing integrated circuit A370 / Journal of Chemical Education
Motors Most servo recorders use conventional rotary motors connected t o the slidemre and pen through mechanical linkages. This necessitates the use of a cord or cable with the pen carriage firmly fixed to it, wound around n drivine drum. The lavout of the cable drive .--..... .-~.--~ ~~~~
free and easily replaced (Fig. 7). To simplify the design, linear motors have been developed, in which the prime motion is in a straight line rather than circular. This is rather exiensive, and has not been widely adopted. ~ i g u r 8 e shows an example (Continued on page A372)
Figure 7. A typicel stringing diagram. The knob provides a means of adjusting the string tension.
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Figure 8. Linear servo motor of the permanentmagnet DG type. (Esterline-Angus).
Chernical Instrumentation In any servo recorder, the moving system must be protected from overdrive. This is sometimes accomplished in the electronics through amplifier limiting, but can be achieved mecbanicallv. .. either hv Limit switches or a slip clutch. In the first alternative, the carriage closes a microswitch a t each end of its safe travel, thus cutting off motor power. The disadvantage is that power must be restored somehow when the signal drops to safe levels, and this is not simple to do. Hence the friction clutch is more favored, even though it is prone to wear
Dynamic Properties The simple servo system with positionsensing feedback may not always show the response desired, especially if called on to adjust rapidly to changes in input. The three types of response likely t o he observed are shown in Figure 9. The underdamped response (a) appears as an oscillation of diminishing amplitude, known as hunting or ringing. At the other extreme is curve ( b ) , corresponding to averdamping, a sluggish response. Curve ( c ) displays critical damping, the least which will prevent overshooting. Usually the optimum is a slight und(Conrinued on page A374)
Multichannel recorders For many applications it is desirable t o record two or more functions simultaneauslv on the same timr smle. Thecwcral channrli ~
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ihc,uld share th? snmp p.~pwdrivemrrhn"ism and power wppliet, lrut must br provided with separate range and sensitivity controls. A choice must be made between overlapping operation and side-hy-side location. In the former, the time base is necessarily offset slightly between channels to permit the pens passing each other. In the latter alternative, each channel covers only a portion of the width of the paper, so that the reading error is increased.
A372 / Journal of Chemical Education
Figure 9. Dynamic responseof a servo systemtoa step impulse: (a) undamped. ( b ) overdamped. ( c ) critically damped. (d) optimum, slightly underdamped.
Chemical Instrumentation erdamping, as in c u n . ~( d l , where slight o w prshwx docs not take the response r u r iideuf the accuracy tolerance band; this gives the maximum speed of response. Damping can be controlled mechanically with a dashpot, electrically with an RC-network of hieh enoueh time-constant.. mae.. nrrirnlly hy the frictionlrijdrag prtdurrd by eddy currenrs, or dwmnically b y means of the \,elonty feedtmk l w p otF:yure 5. In rhr latter, a small generator (a tachometer) must he included in the system t o give a feedback signal proportional to the speed of motion. The result is that the power delivered to the servo motor is diminished a t bieh meeds but . not changed at lhw speeds, near balnncr, thus inrreaqing rhs dynamir *tal,ility
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X-Y RECORDERS Since an X - Y recorder can accept any arbitrary functions on both its axes, multiplevalued as well as single-valued curves are possible. All such recorders are potentiometric servo types with separate input networksand amplifiers for the twoaxes. Many models are provided with modular plug-in attenuators and preamplifiers t o give maximum flexibility.
Drive Mechanisms The majority of X-Y recorders currently in use require individual sheets of graph paper held on a flat bed. The paper is traversed by an a m moving parallel to the Xaxis while the pen travels dong the arm in the Y-direction (Fig. 10).The arm isdriven by a
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Figure 10. A representative X-Y recorder (HewlenPackard).
stationary motor through a system of pulleys and cords, while in most designs the Y-drive motor is mounted directly on the moving arm, which greatly simplifies the pen drive.
A374 / Journal of Chemical Education
