Digital Chart Driver for a Strip Chart Recorder Dennis E . Wisnoskyl Division of Molecular Biophysics, Laboratory of Molecular Biology, University of Alabama Medical Center, Birmingham, Ala. 35233
THEIMPACT of digital data acquisition techniques in research and test laboratories has been monumental in scope. Some manufacturers have recently gone t o the point of offering new instrumentation devoid of a n analog recording instrument except as a n option. In this case, although available, the recorded signal which enables the scientist t o view his results immediately, without paper tape translation or reading a signal averager or computer memory, bears little physical resemblance to the signal which was recorded digitally. This is because the fixed rate at which the recorder chart moves is vastly different from the acquisition system scan rate. If for some reason the experiment must be repeated, recorder charts must be overlayed for comparison or a n attempt made t o return the pen t o the exact starting point o n the chart manually. Of course, unattended operation is then impossible. Since most older systems already use strip chart recorders, it is indeed justifiable t o expect t o use the same recorder when the instrument is automated or replaced. Described below is a n inexpensive system for replacing the drive motor, in a Bristol potentiometric strip chart recorder commonly found in the laboratory, by a digital stepping motor ( I ) . Associated circuitry, which enables slave operation in conjunction with a data acquisition system, or external time base or internal clock mode which eliminates the need for changing gears in providing a virtually unlimited choice of chart speeds, is fully detailed. Although this particular system is used primarily as a slave recorder t o follow the monochromator scan on a spectrophotometer (2), the principles apply equally as well elsewhere.
Present address, Miami University, c/o AFML/LPA, WrightPatterson Air Force Base, Ohio 45433.
less than 120 Hz, which swings through 2.5 volts to be used as the time base, since the comparator will in any case output a TTL voltage level with the correct rise time t o clock succeeding circuitry. The only precaution is that the signal level should not exceed +15 volts. Typical uses for the external input are for precise time bases which must be accurately repeated, slaving several recorders together from a single time base or driving the recorder from a programmed or event initiated source. In the internal time base mode, the frequency of the internal oscillator is adjusted by a ten-turn potentiometer with a turns counting dial. Range is sufficient to allow the chart t o be driven from zero through the maximum slewing rate of the motor. The internal feature is used for normal recorder operation where this approach allows instantaneous speed selection as well as reverse drive capability with no mechanical manipulations involved. The pulse timing train, from either source, which appears as the pole of Sa is converted t o the two line Gray code (3) necessary for driving the stepping motor by the SN7474 dual D flip-flop ( 4 ) . The two waveforms, output from the flip-flop, have a 50% duty cycle and are 90 degrees out of phase. The direction that the stepping motor turns depends upon the lead-lag relationship between the two waveforms; A leads B in “FORWARD” operation. For the “REVERSE” direction, S 3 causes waveform B to be inverted so that B lags A by 90 degrees. “STOP” is provided by inhibiting A and B from reaching the driver amplifiers. In time base mode, chart direction is solely a function of internal circuitry so that no additional constraints would be placed on an external input. In “slave” operation the stepping motor follows a two-line Gray code signal output by a n incremental shaft encoder (5) which is mechanically coupled t o the monochrometer drive shaft of a Cary Model 60 spectropolarimeter. The encoder was previously added to the Cary as part of a modification involving a data acquisition system manufactured by Datex Division, Conrac Corp., Duarte, Calif. Any encoder may be employed, however, the only restriction being that at least a two-line Gray code signal be available so that direction sense circuitry can be utilized. The Gray code from the shaft encoder is seen not as voltage waveforms, but as two lines which are alternately opened and shorted t o ground. Needless t o say, there is a certain amount of noise attendant to this type of operation, which can cause false triggering of logic circuitry. Therefore, the Gray code signals are fed along two identical paths where they are converted t o TTL logic levels, filtered t o remove noise, and reshaped by zero crossing detectors. In “EXTERNAL FORWARD-REVERSE” operation, SI is closed and the buffered signal is fed to the driver amplifiers by Sf. In this mode, the speed and direction of the recorder
(1) Arthur E. Snowden and Elmer W. Madsen, “Characteristics of an Synchronous Induction Motor,” Applications and Industry, March 1962. (2) J. R. Krivacic, D. E. Wisnosky, and D. W. Urry, “A Simultaneous Absorption and Circular Dichroism Data Acquisition System,” in press.
(3) Montgomery Phister, Jr., “Logical Design of Digital Cornputers,” Wiley, New York, N. y.,1958, pp 232-33, 399-401. (4) “TTL MSI Applications,” National Semiconductor Corp., Santa Clara, Calif., April 1970. ( 5 ) “How to Use Shaft Encoders,” Datex Division, Conrac Corp., Duarte, Calif., 1965.
CIRCUIT DESCRIPTION AND OPERATION
Figure 1 shows a block diagram of the chart driver. Switches SI, SZ, and S3 are mechanically analogous t o the modes of operation selected electronically by the function switch on the chart driver schematic, Figure 2. Both figures must be referred t o in the following discussion. The circuitry enables the recorder chart t o be driven in one of three modes; internally from a continuously variable time base, externally from a time base oscillator or externally slaved t o a shaft encoder which outputs a two-bit Gray code referenced to ground. When the recorder is utilized on either time base mode, the function switch sets S? down “TIME BASE” which connects the time base control logic t o the stepping motor drive amplifiers. S4 is then positioned to feed either the output of the UJT oscillator or the external time base to the dual D flip-flop inputs. The external signal is buffered by a n L M 311 comparator. This buffering allows virtually any waveform with a frequency
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ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
LOGIC
BCDIGRAY CONVERTER
INPUT
TIME BASE CONTROL LOGIC
Figure 1. Chart driver block diagram
chart will correspond exactly to the period and relative phase of the Gray code input. Thus, if the monochrometer scan is reversed, the chart will follow accordingly, the signals will then be automatically overlayed at the exact spectrophotometer scan rate. When the function selector switch is positioned in “EXTERNAL FORWARD,” the reverse scan of the recorder is inhibited. Thus, the recorder remains stationary while the monochrometer is reset and begins to follow when a new scan begins. This function is accomplished by utilizing a “D”
+
!5-V
type flip-flop to sense the relative phase of the Gray code signals. A D (data) flip-flop transfers the logic state appearing on the D input to the logic 1 (high) output when a pulse is applied to the clock input. During a reverse scan, signal B lags signal A by 90 degrees, thus the data input is “0” when signal A clocks the flip-flop. In this case, the flip-flop output remains “0” for the duration of the scan and the signals are inhibited from reaching the driver amplifiers by Si. When the scan is in the forward direction, however, the D input remains “1” for each clock. Switch SI is closed and
1
TIME BASE SLAVE
I-
CSCILLAT03
OSCILLATOR IYPllT
Figure 2.
Chart driver schematic
I is DM8004; Al, A2, are MC3001; 01 is MC3003; D1-4 are NH0018CN. Driver transistors are MJE340; Diodes are all 1N4818. All capacitors are in microfarads unless otherwise indicated. All resistors are 1/4 watt carbon unless otherwise indicated. PIN 14 of all IC’s is +5V and PIN 7 is ground unless otherwise indicated ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
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the chart follows the Gray code input. “FORWARDREVERSE” is accomplished as previously described by locking out the inhibit signal from reaching SI. The driver amplifier is comprised of two identical push-pull sections which translate TTL logic levels to the levels necessary for driving the motor. This design allows the components shown to be used for both three-wire ac motors which require a center tap power supply and, with slight modification, for five-wire bifilar wound dc motors. CONSTRUCTION
Of prime importance in the implementation of this system is the availability of a recorder in which the scanning motor is easily accessible and in which the driver mechanism can be reversed without objectionable backlash. The recorder chosen for this project, a potentiometric strip chart recorder manufactured by the Bristol Division, American Chain and Cable Company, Inc., Waterbury, Conn., scored well on both points. It was possible to directly replace the existing motor with a slo-syn SS 25 manufactured by Superior Electric Company, Bristol, Conn. A single gear was utilized to directly couple this motor to the chart gear. The original gears were thereby discarded. Other recorders commonly found in the laboratory such as the Brown and Leeds & Northrup were also examined. In these units, motor replacement can also be accomplished with relative ease; however, backlash might be a problem if the original gears were employed. Although this is not a serious
consideration in many applications, it should not be entirely neglected. The circuit was constructed on three DEC flip-chip blank printed circuit boards. Separate cards were used for the motor power supply, control logic, and the drive amplifiers. Care was taken to separate input and output lines on the drive board and the control board. Use of 0.1-microfarad capacitors on the power supply pins of each device minimized coupling through the power leads. Trim pots are provided on the input buffer amplifiers for adjusting the filtered Gray code for a 5Oz duty cycle at the zero crossing detector input. Other than this, no circuit adjustments are required. CONCLUSION
This system has been in operation for over a year and has proved to be a virtual work horse in the laboratory. The analysis of data automatically acquired proceeds with much greater confidence now that each data point can be related to a real time event. Instrumental errors show up immediately because each scan is automatically overlayed. As the computer plotted spectrum is also related in real time, it can be easily compared with the original data as an additional check on performance and procedure. This would prove to be invaluable in analyzing the first run of a new data reduction program. RECEIVEDfor review February 25, 1971. Accepted July 19, 1971.
New Bubbler Design for Atmospheric Sampling Herman D. Axelrod, Arthur F. Wartburg, Ronald J. Teck, and James P. Lodge, Jr. National Center for Atmospheric Research, Boulder, Colo. 80302
THE USE OF BUBBLERS is a widely accepted technique for gas sampling. The typical apparatus is made entirely of glass, and various parts are sealed with standard tapered ground glass joints. The dispersing device is a straight glass tube with an open or fritted glass end. These bubblers are easily damaged and costly to repair, especially if a glass joint becomes damaged. Some of the many styles of bubblers have been described by Wartburg et al. ( I ) . Wartburg et al. ( I ) also improved upon the previous designs by incorporating a Teflon (Du Pont) top mated to a glass bottom, thereby making the interchangeable parts relatively inexpensive to replace. For airtight seals, this improved design, however, required precise glass tolerances in the diameter and roundness of the bubbler neck. If the glass bottom neck is slightly oversize, the top is difficult to remove and in the process, the frit stem can be damaged; if the neck is undersize, the top will not seal to the bottom. Furthermore, components of the top are sealed with epoxy glue, thus preventing the cleaning of the top in corrosive solutions. The new bubbler design described here has eliminated the above problems through several significant changes in design. Figure 1 shows the details of the top. The entire unit is
(1) A. F. Wartburg, J. B. Pate, and J. P. Lodge, Jr., Eizcirort. Sci. Techtzol., 3, 767 (1969). 1916
KNURLED BRASS C A ?
O - R I N G SEAL PLASllC HOSE CONNEClOR
1EFLON
Figure 1. The bubbler top
machined from l l / ~ - i n .diameter stock Teflon. The top screws onto the bottom of the glass bubbler in a fashion similar to that of a lid onto a jar. The silicone rubber 0ring seals the units along the outer edge of the glass bottom, providing an excellent seal regardless of slight differences in bubbler neck sizes.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
