Microprocessor automation a UV-visible monochromator

Malcolm W. Warren, James P. Avery, Daniel W. Lovse, and Howard V. MalmstadtUniversity ofIllinois, School of Chemical Sciences, Department of Chemistry, Urbana, IL 61801, USA.

IntroductionIsolation of selected wavelengths of light with high resolutiongrating monochromators is a common operation in spectro-chemical methods of analysis. Commercial monochromatorsand monochromator controllers are often unable to controlthe monochromator’s operating parameters with the resolu-tion required in automated systems. If a monochromator isto be used in a completely automated system its controllershould provide a number of important capabilities: initialis-ation of wavelength and slitwidth operation; calibration ofthe wavelength and slitwidth; the ability to select any wave-length available; the ability to set any slitwidth available;provide operator interaction directly (stand-alone mode);and permit interaction with other microprocessors in themeasurement system (slave mode).

The controller should be extremely reliable for unattendeduse. In addition, the controller should also be able to operatein high noise environments without loss of accuracy andshould not be the source of noise that will affect the operationof other parts of the instrument. The EU-700 (GCA/McPher-son, 530 Main Street, Acton, MA, 01720, USA.)monochro-mator [1] was used in this study because it can be readilymodified to provide these capabilities.

InstrumentationThe monochromator controller uses both closed-loop andopen-loop control systems. In both the. closed-loop and theopen-loop systems control signals are sent out by the micro-processor, and are modified by the control board to providesignals that can operate the electromechanical components ofthe monochromator. In closed-loop systems the signals

originating in the monochromator are modified by thecontrol board to provide feedback. The movements in theopen-loop control systems used in this controller are assumedto be so accurate that feedback is not required.

Wavelength driveThe wavelength drive is a sine-bar and precision leadscrewassembly driven by a slew motor for large wavelength changes(9 nm/sec) and a stepping motor for scanning and fine wave-length adjustment (zero to 2 nm/sec). The stepping motor isnormally engaged. The slew motor is engaged using an acsolenoid when the slew motor is on.

Stepping motorThe stepping motor used in the wavelength drive (No. 36D300 R7.2, Haydon Switch and Instrument, Inc. 1502 MeridenRoad, Waterbury, CN, 06705, USA.) provides reproduciblewavelength increments of +/-0.01 nm/step at speeds up to200 steps/sec. A four-phase signal is required by the steppingmotor coils for forward and reverse movement. When design-ing the driver circuits like those in Figure 1, the designermust consider the inductive nature of the stepping motorcoils. To eliminate the turn-off transients, diodes are usedto shunt the current and protect the drive transistors.

Slew motorThe slew motor (Hi-torque motor, SPEC 1218, Multi ProductsCompany, 2052 Grove Avenue, Racine, WI, 53405, USA.)provides rapid movement of the wavelength drive betweenwavelengths. An alternative to the slew motor used in thismonochromator design would be a high-speed stepping

+5

IOKSTEP

LIMIT+51

IOK’STEP

LOWER LIMIT

Figure 1. Stepping motor drive circuit

+52v

15v COIL DRIVER-A

+5 IN914ioo

COIL DRIVER- B

CO’L DR’VER-C ICOIL DRIVER-D !_

STEPPINGMOTOR

76 Journal of Automatic Chemistry

Warren et al Automation of a UV visible monochromator.

motor. This would be less attractive because high-speedstepping motors tend to be large, run hot, and require largepower supplies. In general high-speed steppers are muchslower (3 nm/sec max speed) than the slew motor (9 nm/sec)used in this monochromator. Major modifications to themonochromator would have been required for mounting themotor. Venting the heat produced is possible but couldcreate problems with the optical system.

The slew motor is an ac induction motor with shadingcoils and a squirrel-cage rotor [2]. The application of ac linevoltage to the motor and the selection of the proper pair ofshading coils presents a number of design problems. Ifmechanical relays are used to control the slew motor power,the contacts arc, and noise spikes are created on the logicpower supply lines. These noise spikes cause errors in thecontrol logic leading to the loss of wavelength information.A solid state relay, with photoisolation and zero-crossingswitching (S-312 Crydom, 1521 Grand Avenue, E1 Segundo,CA, 90245, USA.) is used to provide power to the slewmotor, eliminating this problem.

The shading coils are connected in pairs, the directionof movement being determined by which pair of shadingcoils is shorted. The shading coils present a special problembecause they operate at 9 volts ac peak to peak at 1.75 amps.A photoisolated solid state relay, operable at these low volt-ages and high current, is not presently available. However,the RCA SK3506 triac can conduct at low ac voltages andhigh currents, and requires low gate currents. It also providesgood isolation from high voltages when off. Thus, while themonochromator controller is not completely isolated fromthe shading coils, the low voltages involved and the zero-crossing switching of the triac combine to prevent the creationof noise spikes. The combination of a photoisolated solidstate relay on the main coil and triacs on the shading coilsas shown in Figure 2 has eliminated the noise spikes and re-sulting errors in the wavelength counter due to slew motoroperation.

Slitwidth driveThe slits are straight knife edges adjustable from 5 to 2000/am.The entrance and exit slits are connected to a single controlshaft. A stepping motor of the same type used on the wave-length drive is directly coupled to the control shaft using apair of miter gears.

Filter selectionTo decrease the amount of stray-light passing through themonochromator an optional filter module with a ten-positionfilter wheel is available. The filter wheel is positioned behindthe entrance slit. The proper filter is moved into position

when the filter’s select line is shorted to the internal commonusing one of several SK3506 triacs as shown in Figure 3. Thestray-light filters can be selected in three different modeswith the GCA/McPherson EU-700 filter module: a manualmode, an internal automatic mode, and an external mode.The monochromator controller can select the proper filterfor preselected wavelength regions stored in a table in themonochromator program when the filter module is operatedin the external mode. The filter selection subroutine deter-mines the correct filter and outputs the encoded filternumber to the filter selection circuit.

Encoder devicesIn this system several encoding devices are used. These includea relative wavelength encoder, single-value absolute encoderson the wavelength and slitwidth drives, limit switches on thewavelength and slitwidth drives, and a "filter-change-in-progress" signal on the filter mechanism.

Absolute VS relative encodersTo provide the required resolution for the wavelength drive,an absolute encoder with at least one part in ten thousandresolution (14 bits) across the wavelength range is needed.This would be prohibitively expensive.

The high cost of absolute encoders can be avoided bymaintaining a wavelength counter. This counter must beset each time the instrument is turned on. A relative encoderwith the proper resolution is used to update the wavelengthcounter. This method would preclude the complete auto-mation of the monochromator because it normally requiresthe operator to initialise the counter. For this controllerthe relative encoder provided with the monochromator isused across the wavelength range, and an automatic absoluteencoder is used for the calibration sequence at a single wave-length.

Single-value absolute encodersThe wavelength and slitwidth absolute encoders are singlevalue encoders. These encoders allow the automatic encod-ing of the wavelength to within 0.01 nm and encoding of theslitwidth to within 0.25 /am. This is as accurately as themechanical counters on the monochromator can be manuallyread and set. The absolute encoders were constructed fromthe counters on the top of the monochromator module. Thewavelength and slitwidth counters are removed from themonochromator module. The wavelength and slitwidthcounters are removed from the monochromator and modifiedfollowing the design of Cordos and Malmstadt [3]. Themechanical counters are then replaced in their normal positionin the monochromator.

AC HI

+5

IOK +5"RCA SK 5506UPPERLIMIT 452 7407 GMTI17408)

i00+5

Figure 2. Slew motor drive circuit

L_

--WAVELENGTH DRIveL/’SOLENOID-

Volume 3 No. 2 April 1981 77

Warren et al Automation of a UV visible monochromator

The encoder as diagramed in Figures 4a, b and c, functionsas follows: the incandescent lamp is turned on during cali-bration. At the calibration setting light can pass from theincandescent lamp to the photocell through the hole drilledthrough the counter rings. An interrupt transition is thengenerated to the microprocessor calling the proper interruptsubroutines for the wavelength and slitwidth counter. Theinterrupt subroutines set the wavelength and slitwidthcounters for their respective encoders.

Re!ative wavelength encodersThe relative wavelength encoder is a Model B 200X AVSoptical tachometer (H. H. Controls, 16 Frost Street, Arlington,MD, 02174, USA.). The optical tachometer provides quadra-ture output that determines the direction and magnitude ofthe wavelength change. The circuitry that converts the quad-rature outputs to increment or decrement pulses requiresthe detection of an edge from each phase before producingthe pulse. The circuit is shown in Figure 5. The resultingpulses generate interrupts in the microprocessor that incre-ment or decrement the wavelength counter.A relative wavelength encoder would not necessarily be

required if movements were made only through use of thestepper motor. Because of the slew motor’s inertia and thespeed at which movement occurs, only large coarse wavelengthchanges can be made with the slew motor. Even if the slewmotor were not used, however, the relative encoder wouldprovide important assurance that the wavelength was properlyset.

Limit switchesAnother set of absolute encoders is provided by limit switcheson the wavelength and slitwidth drives as shown in Figure 6a.The limit switches for both the slitwidth and the wavelengthdrives are used as a point of reference for the calibrationprocesures. The limit switches also provide important pro-tection for the wavelength and slitwidth drives. Althoughthe limit signals are returned and detected by the micro-processor, the limit signals are also used on the control boardto prevent any movement in the direction of the limit throughthe board’s control logic. This prevents damage to eitherdrive.

When the sine-bar nears its limit of travel on the leadscrew,the wavelength limit switches are actuated by the limitswitch actuator bar attached to the sine-bar. The slitwidthlimit switches are actuated when the slitwidth stops reachtheir mechanical limit.

a)

b)

I Ca r-INCANDESCENT

CdSe "b",l’’[’1’[ [)ORI TRRANSISTORI-TO TRANSISTOR AMP

+Sv

+5

LAMP 2N568 L IK 7414k"’x/

ENcODER ON

c)2N2222 ENCODER OUT

PHOTOCELL

Figure 4. (a) Cross section of a single value absoluteencoder (b) Lamp drive circuit for single value absoluteencoder, (c) Enoder pulse shaping circuit

Filter changeWhen the filter wheel is in a detent position, the filter-changeline from the filter module is low. When a filter change isoccurring, the filter-change line rises between filters. Eachrising edge triggers a retriggerable monostable shown inFigure 6b, which sets the "filter-change-in-progress" signalhigh for one second. When the filter wheel is moving, thefilter-change line pulses more than once per second. Thiscauses the "filter-change-in-progress" signal to remain highuntil the filter wheel stops. The microprocessor inhibitsscanning by the monochromator during the filter changeinterval.

The 8080 microprocessor systemThe microprocessor is the source of the control signals forthe electromechanical system in the monochromator, andreceives feedback signals from the monochromator, as wellas instructions from the keyboard or the master micro-processor. The present status of the monochromator is

FILTERNUMBER,

+5---

D 5c TB

_G2 I0

Figure 3. Filter selection circuit

7406MT2

FILTER SELECT LINEAUTOMATIC MODE

COMMONFILTER SELECT LINE-ONE

FILTER SELECT LINE-TWO

(SAME CIRCUIT REPEATSFOR ALL FILTERS)

78 Journal of Automatic Chemistry

Warren et al Automation of a UV visible monochromator

reported either to the operator through the display or to themaster microprocessor. The microprocessor in the controlleris constantly testing the status of the monochromator andchanging the control signals sent to the electromechanicalsystem according to a set of internal rules determined by themonochromator control program.

The microprocessor system used in this study was designedby Avery [4] and Lovse [51, using the Intel (3065 BowersAvenue, Santa Clara, CA, 95051, USA.) 8080A micropro-cessor and components in the Intel MCS-80 microcomputerfamily. Included are 3K bytes of read-only memory, 256bytes of read/write memory, a keyboard and alphanumericdisplay, an 8251 programmable communication interface, an8253 programmable interval timer, an 8255 programmableperipheral interface, and an 8259 programmable interruptinterface controller. These microprocessor componentswere interfaced using standard Intel methods. A blockdiagram for the microprocessor system is shown on Figure 7.The control signals passed between the microprocessor andthe control board are tabulated in Table 1.

Keyboard/displayWavelength and slitwidth readout and stand-along controlare provided by a keyboard/display unit. The display usesfive DL-1414 four-digit, 17-segment alphanumeric intelligentdisplays (Litronix, Inc., 19000 Homestead Road, Vallcopark!Cupertino, CA, 95014, USA.). The keyboard is composed of20 encoded SPST keys (0-9 and A-J). The encoded signal isinput through an 8212 8-bit input port by the keyboardsubroutine. The encoded signal is decoded by the keyboardsubroutine and the proper action is taken by the mono-chromator control program.

Monochromator control programThe monochromator control program in the stand-alonemode enters commands through the keyboard. In the slavemode commands enter through a serial I/0 port whichcommunicates with the master microprocessor. The mono-chromator control program can: calibrate and initialise theoperation of the wavelength and slitwidth drivers, set thegrating for any wavelength from 0.00 to 950.00 nm, set anyslitwidth from 5 to 2000 /.tm, and select the proper stray-light filter. The program also detects and reports any errorin the electromechanical operation of the monochromator.

The controller program is organised into an initialisationsequence, a main program loop, keyboard service subroutines,and service subroutines for the main program loop. The mainloop determines if the monochromator needs service and pro-

Table 1. Microprocessor input and output from the 8255and 8259 interfaces.

8255-Programmable peripheral interface

Port A (Output)

A0 Wavelength slew-upA Wavelength slew-downA2 Wavelength step-upA3 Wavelength step-downA4 Slitwidth step-upA5 Slitwidth step-downA6 Wavelength encoder lightA7 Slitwidth encoder light

Port B (Input)

B0 Wavelength forward limitB Wavelength reverse limitB2 Slitwidth forward limitB3 Slitwidth reverse limitB4 Filter-change-in-progress signalB5-B7 No used

Port C (Output)

C0-C3 Filter selectC4-C7 Not used

8259-Programmable interrupt interface controller

IR0 Wavelength encoder upIR Wavelength encoder downIR2 Slitwidth calibrationIR3 Wavelength calibrationIR4 200HZ clockIR5-IR7 Not used

CLOSED)LIMIT SWITCHES_it._,,/; (NORMALLY5K 1 tSK+5 +5

UPPER LIMITLOWER LIMIT

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FILTER CHANGE LINE

Figure 6. (a) Limit switch pull-up circuit (b) Monostablefor filter-change-in-progress signal

74041 7404QUADRATUREB

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OUTPUT FRoMRELATIVEENCODER

7404 -7404I00 33K

+5

ENCODE UP

IK+5

Figure 5. Wavelength relative encoder pulse generator

Volume 3 No. 2 April 1981 79

Warren et al Automation of a UV visible monochromator

vides any needed service by executing one of the servicesubroutines tabulated in Table 2. Input of commands isperformed by the keyboard subroutine in the stand-alonemode and by the slave subroutine in the slave mode. Thesesubroutines then call the instruction subroutine. The in-struction subroutine decodes the instruction and sets up andstarts any move required. The commands and their functionsare tabulated in Table 3. The program was written in assemblylanguage because of speed requirements, since wavelengthinterrupts occur at 900 Hz when slew motor is on. Themicroprocessor must update the wavelength counter 900times a second, and simultaneously must service the display,while continually comparing the desired and actual wave-length.

Power supplyThe control board requires 5 volts at amp and 32 volts at0.5 amps. The required voltages are supplied by an on-board power supply. The microprocessor is supplied with+5, +15, and -15 volts from a Power-One CBB-75W powersupply (P.O. Box 1261, Canoga Park, CA, 19304, USA.).

ResultsThe accuracy of the monochromator controller was testedwith atomic lines from hollow cathode lamps. The measure-ment system used is shown in Figure 8. The controller wastested in both the stand-alone mode and in the slave mode.The master microprocessor used programs written in BASICto control the slave microprocessor.

The wavelength calibration procedure was tested by firstcalibrating the wavelength drive, then moving to a wavelengthbelow the 253.65 nm emission line of mercury hollow cathodelamp and stepping through the emission line. The measuredemission wavelength maximum was then compared to thestandard value [6]. This procedure was repeated one hundredtimes, and the measured peak maximum (253.55 nm)waswithin the accuracy of the lead screw (0.1 nm) and wasreproducible to within +/- 0.01 nm.

Slitwidth calibration was tested by moving to 100, 50,and 20 /.tm, after calibrating the slitwidth. The intensity oflight passed through the monochromator to the detector wasmeasured 100 times at each slitwidth. The measured intensitynever varied more than one per cent for any slitwidth setting.This corresponds to an error of 0.2 ktm in the slitwidthsetting.

Accuracy during unattended operation was tested for aslong as 8 hours while the monochromator controller was inthe slave mode. The monochromator wavelength drive was incontinuous movement during the test period. At the end ofthe test period the monochromator controller wavelengtherror was less than 0.01 nm. This is within the resolution ofthe monochromator. The error for the slitwidth movementwas less than the step resolution or less than 0.25/am.

Because of the high precision of this controller the wave-length accuracy error caused by the leadscrew run-out errorfor a portion of the wavelength ,range could be measured.The measured peak maximums were compared to the knownwavelengths for specific lines, and the error was calculated asshown in Table 4. Although some of the measured errors are

SYSTEM BUS

|| 2,048MHZ SEE TEXT FOR DESCRIPTIONTO MASTER OF INPUT/OUTPUT FORMICROPCESSOR 8255 AND 8259

Figure 7. Microprocessor system

Table 2. Main program loop service subroutines

KEYBD

SLAVE

SMOVIN

MOVING

DISPLA

MCSLIT

MCWAV

ERROR

FILTER

Determines if a valid input has occured from the keyboard and takes any required action when enabled.

Determines if a valid input has occured from the slaveinput port when in the slave mode.

Supervises any change in the slitwidth.

Supervises any change in the wavelength.

Displays the current values for the slitwidth and thewavelength.

Supervises the slitwidth calibration procedure.

Supervises the wavelength calibration procedure.

Determines if an error in electromechanical operationhas occured.

If enabled selects the proper stray-light filter.

Table 3. Microprocessor commands

COPY (B)

CSLIT (B)CWAVE (B)DISLW (B)ENSLW (B)FILDIS (B)FILENA (B)FILSEL (M)

GOTO (B)

Sets wavelength counter equal to value in the inputbufferStarts slitwidth calibration procedureStarts wavelength calibration procedureDisables wavelength slewEnables wavelength slewDiables filter selectionEnables filter selectionUses filter number stored in the input buffer to selectthe correct filterStarts wavelength change to wavelength in the inputbuffer

OUTFIL (M) Outputs the current filter number to the mastermicroprocessor

OUTSLI (M) Outputs the current slitwidth to the master micro-processor

OUTWAV (M) Outputs the current wavelength to the master micro-processor

SCOPY (B) Sets slitwidth equal to value in the input bufferSGOTO (B) Starts slitwidth change to value stored in the input

bufferSHOFF (B) Turns keyboard shift offSHON (B) Turns keyboard shift onSLAVOF (M) Returns microprocessor to the stand-alone modeSLAVON (M) Places microprocessor in slave modeSTATUS (M) Outputs current status to the master microprocessorSSTEPD (B)SSTEPU (B)STEPDN (B)STEDUP (B)STOP (B)WAITW (B)

WAITS (B)

Steps the slitwidth down 0.25/JanSteps the slitwidth up 0.25/.tmSteps the wavelength drive down 0.01 nmSteps the wavelength drive up 0.01 nmStops any monochromator movementSets wavelength scanning speed using value in theinput bufferSets slitwidth scanning speed using value in the inputbuffer

B Commands available to both the keyboard and the master mircro-processor

M Commands available only to the master microprocessor

SLAVE 8080 MASTER 8080

CONTROL BOARD

MONOCHRoMATOR

Figure 8. Experimental measurement system

80 Journal of Automatic Chemistry

Warren et al Automation of a UV visible monochromator

greater than the monochromator’s specification, it should benoted that the monochromator used in this study is over tenyears old. Because this error is a rather smooth and stablefunction of wavelength, it is possible to calculate the wave-length of an unknown emission line in a complex sample toan accuracy better than the monochromator specification.The measured wavelength is slightly dependent on temper-ature, as shown in Table 4. Thus, for the best accuracy, themonochromator should be calibrated at the t.emperature atwhich it is to be used. Also, data can be collected for a fewpoints in the region of interest, and the wavelength shift canbe calculated for the operating temperature by comparingthese data to the calibration values at a standard temperature.The microprocessor controller enables the overall performanceof the monochromator to be improved beyond its basicspecifications, as well as providing the automation features.

Software for the control programs, artwork for the PC boardsand complete documentation are available from the authors.

REFERENCES[1] GCA Corporation, "Scanning Monochromator EU-700 and

EUE-700 Series", Acton, MA, USA, 1968.[2] Fractional and Subfractional Horsepower Electric Motors.

C.G. Veinott, McGraw-Hill Book Company, New York, 1970.[3] Cordos, E., and Malmstadt, H.V., (1975) Anal. Chem. 45,

425(2), "Programmable Monochromator for Accurate HighSpeed Wavelength Isolation".

Table 4. Wavelength accuracy reproducibility and temperaturedata for the. EU-700 monochromator

Temperature(C)24.0

25.1

27.1

Averagevalue

253.54365.06404.73435.90546.10253.55365.09404.77435.91546.12253.57365.10404.77435.92546.13

Literaturevalue (6)

253.65365.01404.66435.84546.08253.65365.01404.66435.84546.08253.65365.01404.66435.84546.08

Errornm

-0.11+0.05+0.07+0.06+0.02-0.10+0.08+0.11+0.07+0.04-0.08+0.09+0.11+0.08+0.05

[4] Avery, J., Ph D. Thesis, University of Illinois, School of ChemicalSciences, Urbana, IL, 1978.

[5 Lovse, D., Ph D. Thesis, University of Illinois, School of ChemicalSciences, Urbana, IL, 1977.

[6] The Chemical Rubber Company, Handbook of Chemistry andPhysics, Cleveland, OH, 1970, p E-220.

Novel apparatus for the automation ofsolvent extraction

John G. Williams, Peter B.. Stockwell*, Michael Holmes, Derrick G. Porter.Laboratory of the Government Chemist, Stamford St., London SE1 9NQ, UK.

IntroductionChemical analyses usually require pretreatment of the sampleprior to measurement. When solutions are employed as trans-port media for the analytes, convenient minimisation ofphysical and chemical interferences may be brought aboutby solvent extraction. Previously, automatic solvent extrac-tion systems have been based on manual techniques, wherebythe eye of the human operator is replaced by some form ofphase boundary detector and the tap of the separating funnelis replaced by an electromechanical valve. Most phase bound-ary sensors have broadly similar characteristics. Apart fromoperating problems they all rely on the differential trans-mission of electromagnetic radiation on either side of theboundary. Trowell has described phase boundary detectorsbased on differential conductivity [1] and differential capa-citance [2]. Associated with the latter, relying on a dielectricchange, are methods using a change of refractive index [3] tocontrol a bistable valve as an interface flows past a fixedpoint in the system. Recent, unpublished work performedat this Laboratory has shown that ultrasonic transducers arealso capable of phase boundary detection.

Other well tried forms of solvent extraction (other thanchromatographic) involve the migration of the species ofinterest across a semi-permeable membrane under the influ-

*Present address: Plasma Therm Ltd., 6 Station Road, Penge, UK.

Volume 3 No. 2 April 1981

ence of either a concentration gradient or a potential gradi-ent, or a combination of the two. Methods relying on thegravity separation of two completely immiscible phases aresometimes employed in continuous-flow air-segmentedanalytical systems; when well designed they are relativelytrouble free. Vallis [4] designed a somewhat differentapproach to automated solvent extraction based on a rotat-able cup-shaped-vessel with a porous lid attached to the lip.The cup is placed inside a collecting vessel; if the porouslid is made from hydrophilic material such as sintered glass,water will pass into the collecting vessel at low rotationspeeds leaving the organic phase in the cup. An increasedrotation speed then ejects the organic phase. The use of ahydrophobic material such as sintered PTFE enables thepreferential rejection of the organic phase.A prototype separator which .has been designed and

built at this Laboratory using a completely new approach, iscurrently the subject of a patent application [5]. In principle,separation is effected by absorption of both phases into aporous nickel-chrome alloy disc mounted on a motor-drivenshaft. Controlled angular acceleration and centripetal forceon the droplets within the pores enables one phase to beseparated from the other. The speed of rotation of theporous disc is coupled microelectronically to the verticalcomponent of its motion so that separated droplets leavingthe disc tangentially are trapped by hitting the walls of

81