How Potentiometric Recorder Performance Affects Analysis DAVID A. AAKER Industrial Products Group, Texas lnsfrumenfs Inc., Houston, Texas
b An understanding of recorder performance is needed when selecting or operating equipment and when analyzing results. The four principal elements in potentiometer recorder performance are dynamic accuracy, deodband, linearity, and interference rejection. Each i s examined to determine how it affects analysis. For example, dynamic accuracy determines the instrument's capability of tracing peak height without overshoot and of following small, rapid peaks. Dynamic performance is dictated by the torque of the servo motor and by the inertia of the pen drive system. It can be measured by observing span step signals of different magnitudes and from the frequency response specifications.
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sometimes purchased as an afterthought, is an important part of analytical instrumentation. I n terms of complexity and cost it represents a significant, if not the major, part of the system. Accurate analysis is dependent upon the recorder's ability to monitor the detector output accurately, rapidly, and reliably. The four principal elements of recorder performance are dynamic accuracy, deadband, linearity, and interference rejection. What factors contribute to these performance elements? How can they be measured? How do they affect analysis? Answers to these questions will provide the user with a more competent and realistic appreciation of the recorder as a contributing part of analysis. POTENTIOMETER RECORDER,
DYNAMIC ACCURACY
VELOCITY
I I
0
Figure 1 .
t maximum pen velocity
1 / PEN TUVEL
100%
0
10~0
Pen velocity vs. pen travel
when the signal changes rapidly as i t does, for example, in capillary column chromatography. Naturally we cannot expect a recorder to respond any faster than its designed pen velocity. The first consideration, then, is the basic span step response time, the time required for the instrument to respond to full scale signal change. Span step response time provides a gross determination of whether the instrument is capable of following a given signal. However, the span step response, normally 0.3, 0.4, 0.5, 1, 2, or 5 seconds, does not tell the whole story. T o obtain an accurate indication of the instrument's ability to perform dynamically, we must look behind this specification. An ideal pen movement system would be a velocity limited system. The servomotor would have infinite torque and the servo system would have zero inertia. The pen would then instantaneously reach its rated velocity and move at that velocity until the end of the span step is reached. Reality, of course, must be something less than a velocity limited system. The shaded area in Figure 1 represents the difference between reality and the ideal. Since the pen requires finite time to
reach rated velocity, the response to rapid signal change is limited bv acceieration as w e l l a s velocity. ff the signal change is small, the acceleration becomes particularly significant. To obtain a measure of the dynamic properties of the recorder, two characteristics can be examined: its ster, response and its frequency response. Before any valid tests of either can be made, the damping must be adjusted to eliminate pen overshoot. a distortion of the signal. The comparison of the step response to abrupt signal changes of different magnitudes provides a good measure of the recorder's ability to accelerate the pen. Since acceleration time is more noticeable on recorders with higher pen speeds, consider an instrument with a full scale response rating of 0.4 second, typical of readily available, faster recorders. Span step response of a full span signal change is shown in Figure 2 as 400 mseconds. If the magnitude of deflection is decreased b y a factor of 10, to 10% of span, the time might be expected to be 40 mseconds. The actual time when compared with 40 mseconds becomes a finite measure of the instrument's ability to approach the ideal velocity limited system. The actual time will be from 70 to well over 100 mseconds, depending upon the exact span step response and the dynamic capability of the recorder. The frequency response is a related consideration. The ability of the pen to follow a sine wave input of increased frequency becomes another measure of acceleration. An actual frequency response trace is shown in Figure 3 for a recorder with an 0.4-second response
The recorder is a dynamic device; its pen must move as a function of a signal which is changing with time. Its dynamic properties can become critical
m = c s f - c _ ,
Figure 2.
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10
Span step response
ANALYTICAL CHEMISTRY
A M P L I T U D E 10% FULL S C A L E 0.4 S E C O N D P E N S P E E D
Figure 3.
Frequency response
Figure 4.
Dynamic accuracy
-Recorder trace . . . .Actual detector output time. With a signal 10% of full scale, the trace begins to fall off substantially a t 8 cycles. If a larger amplitude were considered, the fall off would be more pronounced. Recorder dynamic performance is critical a t points of maximum trace curvature where acceleration demands are greatest. Since maximum acceleration normally occurs a t the top of the peak (Figure 4), i t requires good dynamic properties to trace accurately the peak height without overshoot. Demanding acceleration can also occur a t the initiation of a trace. In this case the dynamic limitation can cause a distortion of the trace shape. The ability of the instrument to follow the signal is particularly important when short, fast peaks are involved. The effect of the acceleration a t the initiation and a t the peak of a trace will combine to cause a fast, 10% peak to be not fully reproduced. DEADBAND
Deadband is defined by the American Standards Association C39.4 as “the range through which the measured quantity can vary without initiating response.” A relatively easy characteristic t o measure, it provides a good test to the many important components of the recorder. The following contribute directly to deadband :
Servomotor Torque. The servomotor is the driving force behind the pen and affects deadband directly. The output torque will vary directly with the response time t h a t is selected. This means t h a t a 5-second instrument can be expected to have better deadband characteristics than an 0.4second instrument. TACTUAL
MOVEMENT
DEADBAND SIGNAL CHANGE WITHOUT PEN RESPONSE
Figure 5.
Deadband measurement
System Inertia and Friction. The motor must overcome the inertia and the friction of the pen drive system. The magnitude of inertia and friction will depend upon the design and the condition of the instrument. Feedback Slidewire. The resolution of the slidewire is a n inherent limitation to the deadband. The deadband cannot be any smaller than the slidewire’s resolution and is usually two or three times larger. Amplifier Gain. I t is the function of the amplifier to increase the error signal t o a level which will drive the servomotor. If the gain is reduced, the amount of error signal needed to move the pen will have to be larger as :will the deadband. The stability of the gain is characterized by the quantity of feedback that exists in the amplifier. If the feedback is not adequate, the gain will tend to drift with temperature, line voltage, and com-
Figure 6.
’E--! TRANSfRSE
I II-
--:+ -t-
. , . .Actual detector output
ponent aging, and produce a change in observed deadband. Interference Rejection. Excessive interference can create deadband. This will be discussed in detail later. Source Resistance. The source resistance or external circuit resistance effectively attenuates the error voltage. By reducing the error voltage it increases the deadband. The ext e n t of deadband increase will depend upon the amplifier design b u t should not exceed a factor of 2 a t the maximum external circuit resistance rating. To measure deadband the recorder’s gain and damping are adjusted as per the manufacturer’s specification and either the inputs are shorted or a steady d.c. signal input is connected to them. The servomotor is offset manually by a few increments as shown in Figure 5 and it is allowed to return slowly to balance. I t should not approach balance a t a substantial velocity. This procedure is then repeated in the opposite direction. The difference between the rebalancing positions as a percentage of full scale is a measure of deadband. Deadband can also be determined electrically as shown on the right side of Figure 5. The signal can vary within the deadband without initiating response. When it exceeds the deadband, the pen and the deadband will move. As the signal moves in the opposite direction, it must exceed the other side of the deadband to again
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B e 3 LONGITUDINAL
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ANALYTICAL ‘INSTRUMENT
Figure
7. A.C.
RECORDER
interference
move the pen. Therefore, a step input just larger than the deadband will generate a perceptible pen movement, both when it is inserted and when it is removed. Deadband inhibits the recorder’s ability to monitor faithfully the signal from the detector. I n some cases it can be observed on the chromatogram as a stepping movement a t the trailing edge of a trace and as a rounded or flat peak. On a slow moving peak, the recorder pen will fall short of t,he actual peak by an amount equal to one half of the deadband as shown in Figure 6. Similarly the pen will fail to return to the actual base line by an amount equal to one half the deadband.
Deadband effects
--Recorder trace
i------
LINEARITY
As the name potentiometer recorder infers, the potentiometer or the slidewire is the heart of the instrument. It is the basic ,feedback element providing the reference voltage. The recorder performance can be no better than that of the slidewire. The slidewire determines the linearity of the instrument. The linearity is limited by the slidewire resolution, its mechanical construction, and resistance loading effects. I n a conventional wire-wound slidewire, the resolution is dictated by the number of convolutions. A 1500-turn slidewire, for example, would have 0.06% resolution. Generally a linearity of 0.1% is available. The effect of linearity is evident when measuring relative peak height. The comparison of peak heights can only be made within the limitation of linearity. Linearity is a simple and obvious consideration but is one of the basic limitations of an instrument and should not be overlooked when considering overall performance.
A-C E L E C T R O M A G N E T I C FIELD
MONITORED EQUIPMENT
Figure 8.
RECORDER
A.C. stray inductive coupling
VOL. 37, NO. 10, SEPTEMBER 1965
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MONITORED EQUIPMENT
Figure
RECORDER
9. Longitudinal interference INTERFERENCE REJECTION
Perhaps the most critical factor to good recorder performance is that the instrument be designed, manufactured, and utilized to minimize electrical interference. Interference is defined by the American Standards Association C39.4 as “any spurious voltage or current appearing in the circuits of the instrument.” It will occur either as transverse interference, “a form which appears as a voltage between measuring circuits,’’ or longitudinal interference, “a form which appears between the measuring circuits and ground” (Figure
the input transformer secondary. The resulting current in L,, the input transformer secondary, produces an undesirable signal to the amplifier. Direct current longitudinal interference, sometimes called d.c. common mode, contributes undesirable circuit currents in the same manner by using leakage resistance from the recorder circuits to ground. Several methods are used by recorder designers to minimize the effect of transverse and longitudinal interference. Possibly most obvious is the inclusion of a filter in the input circuit of the recorder to reduce directly the transverse interference. The filter, typically an RLC or R C type tuned to 60 cycles, will attenuate the 60-cycle signal that is introduced at the recorder input. The transverse interference rejection depends upon the attenuation achieved by the filter. Another technique is a guard shield to enclose portions of the circuit with a n electrostatic shield. The guard shield, shown in Figure 10, has the
7).
Transverse interference is often caused by inductive coupling. Figure 8 shows interference being introduced into the circuit by a x . electromagnetic fields. Inductive coupling is somewhat like a transformer with the primary being the interference source and secondary being the input leads. Another source of transverse interference is resistive coupling from the power supply in bridge-type detectors. Alternating current longitudinal interference is primarily introduced by capacitk e and resistive coupling in the circuits of the analytical instrument. To be harmful it must be converted t o transverse interference by returning to ground through the measuring circuit of the amplifier. One way to obtain a complete circuit is by using the leakage capacitance from the recorder circuits to ground. The net leakage capacitance to ground is represented in Figure 9 as C,. The voltage drops caused by the current (arrows) flowing through the circuit components is transverse interference. Another circuit route to ground is through C1, the leakage capacitance to
MONITORED EQUIPMENT
RECORDER
Figure 10.
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Guard shield
ANALYTICAL CHEMISTRY
(5 OF
Figure curve
1 1.
Amplifier
SPAN)
characteristic
function of diverting current caused by longitudinal interference. The ax. current will still use leakage capacitance to ground, CBin Figure 10, to obtain a complete electrical circuit. The d.c. current will similarly use a leakage resistive path to ground. Instead of passing through the measuring circuit, however, the currents will now flow harmlessly through the guard. Current flowing in the guard circuit will not have the opportunity to generate the harmful transverse interference. The guard shield does not prevent current from flowing through leakage capacitance, now C4,to the transformer secondary. To minimize C4,an isolation shield is added to the input transformer. The transformer isolation shield and the guard shield combine to provide the recorder longitudinal rejection. The recorder transverse and longitudinal rejection specifications are ultimately given as ratios. The ratio, sometimes in decibel form, is the interference voltage a t the recorder input in R M S volts to the error on the
-7-Figure 12.
Interference effects
--Recorder trace
. .. .Actual detector output
pen in d.c. volts. For example, if a 1-mv. interference voltage appears at the input of a 1-mv. span and the effect is 0.1% or 1 pv. the rejection ratio is 1000: 1 or 60 decibels. Rejection ratios can be obtained experimentally but are generally available from the recorder manufacturer. What effect does interference have on recorder operation? First, if it appears at the amplifier input and is of the same phase as the signal, it will be amplified and emerge as if it had been a legitimate signal. On the trace i t will appear as zero shift. To understand other effects of interference on recorder operation, consider the amplifier. Ideally the amplifier is a device that will multiply a voltage, in this case the error signal, by a constant. I n reality, the amplifier is linear only in part of its characteristic curve, Figure 11. At some input level, normally 2 or 3% of span, the amplifier is extended into the area in which the output per unit input or gain is reduced. Interference which appears at the amplifier input out of phase with the signal does not cause pen movement but still is considered input by the amplifier. It forces the amplifier to operate at an inefficient point on its characteristic curve. The result of operating effectively at reduced gain is increased deadband and sluggish operation. Figure 12 shows how increased deadband, sluggish operation, and zero offset appear on a chromatogram. There are precautions that can be taken during operation to minimize the interference. One procedure is to extend the recorder guard by a good, shielded cable to the output of the instrument being monitored. Another is to watch for obvious sources of interference and physically move them away from the recorder. A third consideration is ground loops. The input circuit should be tied to the ground a t one point and that one point only. Grcunls are rarely a t exactly the same voltage and voltage differential will pass through the recorder if given an opportunity.
RECEIVED for review ,March 31, 1965. Accepted July 6, 1965. Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., March 1965.
