Pumping system

An improved chromatographic pump control circuit for compensating for pulsation in pump flow over a wide range of flow-rates and pressures and for providing improved protection against pump pressure and motor torque fault conditions. The control circuit is coupled to a pump motor to provide motor speed-up during the repressurization portion of the pump cycle as a function of the set point pump flow rate. The circuit also includes a summing circuit for developing a repressurization signal based on the flow-rate and detected pressure. The signal is coupled to compensate for pulsation by controlling the duration of repressurization with respect to both pressure and the set point of the flow rate. The circuit is additionally coupled to sense pump pressure and motor torque to initiate alarm and pump shutdown during high and low pressure and motor over-torque fault conditions.

BACKGROUND OF THE INVENTION 
The present invention relates to pump motor control circuitry and more 
particularly to improved circuitry for compensating for pulsations in pump 
output flow in chromatographic systems. 
Chromatography is a technique for separating a mixture of components, known 
as a sample, by distributing the sample in dynamic equilibrium between two 
phases in ratios characteristic of each component. The sample is normally 
dissolved in a flowing mobile phase and forced through a stationary phase, 
as by pumping, to cause each component of the sample to migrate through 
the stationary phase at a characteristic rate. After a period of time, the 
migration results in the separation of the components into individual 
zones which can be detected by a detector to identify individual 
components. 
In order to provide for resolution of the components within the sample, 
various characteristics of the system (e.g., flow-rate, chemical 
composition, temperature) can be changed to improve system performance. Of 
particular importance is the ability to maintain a relatively pulse free 
pump output flow for any particular system flow-rate. In operation, a 
system may be controlled to provide a constant or variable flow-rate with 
a single pump, constant flow-rate with a programmed chemical composition 
in a gradient system, a constant chemical composition with a programmed 
flow-rate in a gradient system, or a programmed flow-rate and chemical 
composition in a gradient system. In each case, however, the sensitivity 
of detection and quantitation of the zones depends upon the noise level of 
the detector. Since detector noise is aggravated by pulsations in the 
flow-rate, the sensitivity of analysis, and thus the resolution and 
reproducibility of system performance depends on the capability of 
maintaining the pump output flow relatively free of pulsations caused by 
pump refill and repressurization over the operating range of flow-rates 
and pressures for the chromatographic system. 
In the prior art various techniques have been proposed to minimize or 
eliminate pulsations in pump output flow. In one such system, pump speed 
is increased during refill and repressurization and pump pressure, 
detected as a function of motor torque, is used to produce a signal for 
controlling the length of the piston stroke through which the motor is 
speeded up for rapid repressurization. While in this arrangement some 
compensation is provided, the control fails to provide sufficient 
compensation over the desired range of flow-rates and pressures in a 
chromatographic system. Such system fails to provide tracking for 
repressurization as a function of varying flow-rates. Thus, if the 
compensation circuit is set to minimize pump flow pulsations at low 
flow-rates, the pulsations will be under-compensated at high flow-rates 
and high pressures. More specifically, as the flow-rate setting is 
increased, the average output flow-rate actually drops off at high 
flow-rates since the time interval for actual physical repressurization of 
the pump, as determined by the prior art circuitry, tends to become a 
larger fraction of the pumping cycle as the flow-rate increases. In 
addition, since the repressurization signal is derived from a measure of 
motor torque rather than actual pressure, pulse compensation does not 
accurately track system pressure. 
In the noted prior art system, the detected motor torque is also used to 
indicate fault conditions due to over and under pressures at which the 
chromatographic system will not properly operate. Such system, however, 
provides circuitry which does not enable the setting of accurate reference 
limits to control alarm and pump shutdown during fault conditions. In 
addition, since the motor torque is used as a measure of pressure, the 
system may respond to torque conditions causing alarm and pump shutdown 
which are not actual pressure faults. In still other instances, the 
circuitry is not capable of detecting pressure fault conditions that 
should signify alarm and pump shutdown. 
Accordingly, the present invention has been developed to overcome the 
shortcomings of the above known and similar techniques and to provide pump 
control circuitry for allowing improved pulse compensation and fault 
detection in chromatographic systems. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a pump 
control circuit for compensating for flow pulsations over a wide range of 
flow-rates. 
Another object of the invention is to provide pump control circuitry for 
providing improved pulse compensation in output flow over a range of 
operating pressures and particularly at low output pressures. 
A further object of the invention is to provide a pump control circuit 
which provides improved tracking and linearity of pulse compensation with 
respect to flow-rate. 
Still another object of the invention is to provide a pump control circuit 
which controls pump speed during repressurization as a function of 
flow-rate and pressure. 
A still further object of the invention is to provide a pump control 
circuit that may be used to reduce flow pulsations in both single pump and 
gradient chromatographic systems. 
Yet another object of the invention is to provide a pump control circuit in 
a chromatographic system which provides an accurate detection of pressures 
above and below present limits to produce alarm and pump shutdown. 
Still another object of the invention is to provide a pump control circuit 
in a chromatographic system which provides alarm and pump shutdown in 
response to a pump motor over-torque condition. 
In order to accomplish these and other objects, a pump control circuit is 
coupled to receive a signal indicative of the output pressure of the 
mobile phase in a chromatographic system. The output pressure is smoothed 
to be proportional to the actual delivery stroke pressure and summed with 
a signal representing the flow-rate setting of the pump. The combined 
signal is coupled to a comparator to control the volumetric duration of 
motor speed-up during the repressurization interval to provide for pulse 
compensation in the pump output flow. The summing circuit includes an 
adjustment capable of varying the signal baseline level to compensate with 
respect to both flow-rate and actual pressure. A signal indicative of 
actual pressure is also provided to individual comparator circuits which 
detect pressure above and below reference values set to be proportional to 
high and low pressure limits. When an output is provided from the 
comparator circuits, alarm and pump shutdown controls are initiated to 
stop system operation. In addition, an over-torque signal is derived by 
sensing armature current above a preselected value. The over-torque signal 
is also coupled to indicate pump fault and cause alarm and motor shutdown.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1, a schematic diagram shows a chromatographic system 
incorporating the features of the present invention wherein like numerals 
are used to identify like elements throughout the drawings. In the present 
instance, the invention will be described with reference to a liquid 
chromatographic system which is capable of providing gradient elution and 
flow programming as is well known in the art. It is to be understood, 
however, that the teachings of the present invention are equally 
applicable to other well known applications of high pressure metering 
pumps. 
Generally, the system includes a typical chromatographic column 10 packed 
with a stationary phase 11 and supplied with column eluent that contains 
solutes dissolved in a mobile phase which is forced through the column 10 
to the detector 13. Multiple sample mixtures containing the solutes are 
located in sample changer 15. The samples removed from the sample changer 
and injected into the stream of mobile phase by conventional means 
includes sample injection valve 10. The detector is responsive to 
individual solutes dissolved in the mobile phase and provides an output 
signal which is displayed on a strip chart recorder 14 or any other 
similar recording device. The detector signal responds to flow-rate 
fluctuations as well as solute concentration. Hence, for maximum solute 
and detection sensitivity, the flow-rate must be free of fluctuations. 
After passing through the detector 13, the eluent is collected by a 
conventional fractional collector (not shown) which may include a 
plurality of vials disposed in a rotating rack that is controlled in 
synchronization with the gradient to receive a portion of the eluent 
containing individual components of the solute as they pass through the 
detector at different times. 
In the present embodiment, the mobile phase in conduit 12 is provided as a 
mixture of mobile phases A and B supplied through conduits 16 and 18 from 
fluid refill reservoirs 20 and 22 respectively. Each reservoir includes a 
mobile phase of a particular chemical composition having the selected 
solute dissolved therein and provided to the conduits 16 and 18 by 
individual pumps 24 driven by motors 26. The motors are controlled by 
separate motor control circuits 28 which are in turn controlled by a motor 
control interface 30. The motor control 28, motor 26, and pump 24, in the 
circuit coupled to deliver mobile phase A, are intended to be of identical 
construction to the corresponding elements in the circuit coupled to 
deliver mobile phase B. Accordingly, subsequent reference to the 
construction and operation of the elements 24, 26, and 28 will necessarily 
apply to those corresponding elements coupled to deliver both mobile 
phases A and B. 
In accordance with the present invention, the motor control interface 30 is 
coupled to receive an input signal from a pressure transducer switch 32 
(FIGS. 1 and 3) positioned to provide an indication of the actual pressure 
in the conduit 12. In addition, the motor control interface 30 is coupled 
through switch 32 to receive signals from a conventional gradient program 
control 34 or from a single pump or gradient chromatographic system. When 
the system is operated in the single pump mode, the flow-rate setting is 
coupled to a flow-digital voltmeter 40 for providing a visual output of 
the flow-rate. The interface control also receives signals from the sample 
changer 15 to provide for automatic sequential injection and 
chromatographic analysis of multiple samples, and provides an output to 
pressure digital voltmeter 38 for monitoring system pressure. These and 
other features of the motor control interface control 30, including 
pulsation compensation, pressure fault detection and display, and pump 
shutdown, will be more particularly described in connection with FIGS. 2 
and 3 below. 
In the prior art, the pumps selected to supply the mobile phase often 
exhibit pulsations in the output flow-rate and pressure during refill and 
repressurization which adversely affect system performance. The pulsations 
in the output flow are created during the time that the pump piston 
retracts to refill its chamber and during the first part of the delivery 
stroke of the piston. At these times, the output flow stops until the 
pressure in the pump chamber rises slightly higher than the outlet 
pressure so that liquid can flow through a check valve at the pump outlet. 
These two contiguous zero-flow periods are known as refill and 
repressurization times and the same result in residual pulsation in the 
pump outlet flow which, if uncompensated, get worse at high pressures. In 
order to reduce such pulsations, the prior art employs a drive cam for the 
pump piston that has a linear spiral contour and rapid return. Thus, if 
the speed of the drive motor is constant, the pump output flow-rate will 
be constant during the delivery stroke but the cam will still act to 
return the piston at a faster rate during refill to reduce the time period 
(and therefore the pulsations) for refill and repressurization. 
In connection with the above-noted cams, the prior art systems have also 
employed compensating circuitry designed to speed up the motor during 
refill and repressurization to further reduce their effect in producing 
pulsations. As a general rule, however, such circuitry has produced a 
compensation signal based on a presumed pressure derived from motor torque 
and which does not provide for adequate tracking for repressurization 
compensation as a function of varying flow rates. Thus, while both the cam 
and compensation circuitry have improved output flow, prior art circuits 
have still been limited in the maximum compensation and linearity that 
could be attained over the operating ranges for pump flow-rates and 
pressures, particularly in gradient chromatographic systems. 
Turning now to FIG. 2, a block diagram shows an arrangement suitable for 
use as either of the motor controls A or B indicated at 28 in FIG. 1, when 
operated in conjunction with the motor control interface 30, produces 
improved flow pulsation compensation and linearity of operation. The 
circuit also provides for the generation of an over-torque signal 
representing a fault condition requiring pump shutdown. 
In the present example, the motor 26 is a low inertia disc-armature DC 
motor controlled to operate both thermally and dynamically by the circuit 
28 as a simple DC velocity servo system. The motor should have low inertia 
to provide rapid velocity response. Basically, the motor is controlled to 
provide cyclic motor speeds that will produce the desired flow-rate within 
the operating pressure range while minimizing the effect of pulsations 
during refill and repressurization. The motor is additionally controlled 
to respond to a fault due to an over-torque condition as may be detected 
by the circuit 28. 
In order to control motor speed during pump delivery, refill, and 
repressurization, a signal is developed by pump speed control 42 which is 
indicative of the desired speed of the motor during each part of its 
operating cycle. The signal is developed by combining a flow-rate signal 
from control 46 and a signal representing actual motor speed. The actual 
speed signal is provided by a tachometer disc 44 located on the shaft of 
motor 26 and mechanically coupled to pump speed control unit 42 to produce 
a signal indicative of actual motor speed. The flow-rate signal is 
developed by control 46 in response to various inputs designed to control 
motor speed during each of the delivery, refill, and repressurization 
periods of the pump cycle. Thus, during the delivery stroke, control 46 is 
construed to produce an output control signal from 42 which will produce a 
pump speed corresponding to the flow-rate setting input provided to 
control 46 from motor control interface 30 (not shown in FIG. 2) as will 
be later described. During the refill portion of the pump cycle, a disc 48 
located on the output of the motor shaft and mechanically coupled to 
refill period control 50, causes the generation of a signal output at 
control 50 which, when coupled to control 46, provides a signal to control 
42 to speed up the motor. The signal output from control 46 causes the 
increase in motor speed to be dependent upon the flow-rate setting at the 
input to control 46. Finally, during repressurization, the control 46 
receives a signal from a compressability compensation control 52 which 
causes control 46 to provide a signal to control 42 to cause an increase 
in motor speed for a period to compensate for the mechanical elasticity of 
the system components as a function of system flow-rate and pressure. The 
repressurization period is defined from the end of the refill period to a 
time during the delivery stroke of the piston as determined by the 
repressurization signal provided by motor control interface 30. The 
flow-rate control 46 additionally includes a linearity adjustment 47 for 
controlling system response over a wide range of flow settings. 
In response to the above described signals provided by flow-rate control 
46, the output signal from control 42 represents a signal indicative of 
desired motor speed. This signal is coupled to a servo amplifier 54 which 
in turn produces a high frequency signal proportional to desired motor 
speed. That signal is coupled through a power amplifier 58 to a motor 
drive circuit 60 to control the drive current to the motor 26 and 
therefore the speed of the pump during each part of the pump cycle. 
In addition to the above control, the control 28 makes provisions for the 
control of motor speed and shutdown during various fault conditions caused 
by operation of the pump head. A first circuit includes an output from 
amplifier 58 which represents average motor current which, in turn, is 
proportional to pump head pressure. The output from amplifier 58 is 
coupled to overcurrent control 56 to inhibit delivery of a motor speed 
signal to the motor drive circuits when the average motor current exceeds 
a predetermined value. This circuit causes the motor to slow down until 
the average current falls below the set value thereby providing electrical 
protection and a limit on the maximum head pressure that may be developed 
by the pump. 
A second circuit, including elements 62, 64, 66 and 70, acts to provide a 
control signal responsive to torque during the delivery portion of the 
pump cycle, to provide alarm and motor shutdown during over-torque 
conditions. In this example, the torque detector 62 is coupled to sense 
the armature current of motor 26 as a signal proportional to pump 
pressure. Detector 62 is designed to compensate for static friction in the 
armature to provide a more accurate indication of torque. In addition, 
torque compensation 70 is coupled to detector 62 to compensate for 
parasitic frictional torque known as windage friction. Since the effects 
of windage friction are greater at increased armature speed, the 
compensation provided by 70 is controlled to be responsive to the 
flow-rate setting at control 46. 
The output from detector 62 is coupled to sample gate 64 which passes the 
signal to a torque limit sensing circuit 66. The circuit 66 in turn 
provides an output signal when the sensed torque is above set limits. This 
signal is provided to the motor control interface 30 to provide alarm and 
motor shutdown signals, and to the control terminal of gate 64 to allow 
continuous torque sensing during fault conditions. 
In order to restrict the sampling of the pressure signal at detector 62 to 
the delivery portion of the pump cycle, a gate control 68 receives signals 
from the compressability control 52 which inhibits the provision of a 
gating signal from amplifier 54. Accordingly, the sample gate 64 will not 
pass the signal from detector 62 to the sensing circuit 66 until the 
refill and repressurization time periods have passed unless there is an 
over-torque condition indicated by the output at sensing circuit 66. 
Referring now to FIG. 3, a block diagram shows the functional operation of 
the motor control interface 30 as generally depicted in FIG. 1. The motor 
control interface 30 provides the flow-rate settings for pumps A and B to 
control pump speed, and also generates the repressurization signal for 
controlling the period of pump speed up during repressurization. The motor 
control interface 30 also provides for the sensing of pressure and the 
establishment of high and low pressure limits for controlling alarm and 
pump shutdown circuits. 
Generally, the flow-rate settings are provided by gradient program control 
34 for a two-pump gradient system and by single pump flow-rate setting 36 
for a single-pump configuration. In the single pump mode, the flow-rate is 
read directly from the digital voltmeter 40 coupled to receive the 
flow-rate setting from 36. Switch 32 controls the mode of operation by 
selecting either of the signals from 34 or 36. As can be seen, when the 
gradient mode is employed, signals are developed for both the A and B pump 
circuits which include identical circuit elements. For purposes of 
simplicity, therefore, the motor control interface 30 will be described 
with reference to the operation of only one pump, it being understood that 
the same description applies to those elements for control of the second 
pump in the gradient mode. 
With reference to FIG. 3, the flow-rate signal is coupled through a 
flow-rate driver 80 to provide a flow-rate setting signal to gates 82. 
Unless a pump stop signal is present to inhibit gate 82, the same provides 
the flow-rate setting to the flow-rate control 46 (FIG. 2) of the 
appropriate pump for setting the desired pump speed during the delivery 
stroke. The flow-rate setting is also coupled through a flow adjust 
circuit 84 to provide an adjustable output proportional to the flow-rate 
setting. The output from adjust 84 is then coupled to a repressurization 
control 86 which, in conjunction with a pressure signal, produces the 
repressurization signal coupled as input to compressability control 52 in 
FIG. 2. 
The noted pressure signal is derived by sensing the pressure in the conduit 
12 (FIG. 1) with pressure transducer 32, which in the preferred embodiment 
is a switch. The output of transducer 32 is coupled through amplifier 87 
to provide a signal representing pressure. A pressure zero adjust 88 is 
coupled to calibrate amplifier 87 so that there is zero output at zero 
pressure. A pressure range adjust 90 is also coupled to the amplifier 87 
to alter the responsive pressure range. 
The output of the amplifier 87 provides a pressure signal to sample and 
hold 92 which provides a smoothed output representing pressure during the 
delivery portion of the pump cycle only. This is accomplished by providing 
a strobe signal to the control gage of sample and hold 92 to inhibit 
pressure sensing during refill and repressurization. The strobe signal is 
provided at the output of compensation control 52 (FIG. 2) and coupled to 
the gate of sample and hold 92 via gate drive circuit 94. 
The smoothed pressure signal is coupled as the second input to 
repressurization control 86 for developing the repressurization signal. 
The control 86 combines the pressure and flow-rate signals to produce a 
repressurization signal which compensates for compressability based on 
flow-rate and actual delivery pressure. A baseline adjust 98 is provided 
to develop an adjustable pressure-flow baseline level with respect to 
which pulsation compensation can be provided. While the adjust 84 allows 
the circuit to compensate for pulsation over a wide range of flow-rates, 
the same are set to limit the maximum repressurization interval during 
operation in the gradient mode. This is to prevent the gate of sample and 
hold 92 from being strobed constantly off during certain pump operating 
conditions. 
In addition to pulse compensation control, the motor control interface 30 
provides for output pressure display and over and under pressure limit 
detection and display. Such pressure is read by coupling the smoothed 
pressure output from the sample and hold 92 to a pressure limit read 
switch 100. In one position the switch 100 couples the pressure signal to 
a PSI/BAR calibration select switch 102 which in turn provides the 
pressure signal to pressure digital voltmeter 38. The calibration select 
switch 102 allows the reading of delivery pressure in either pounds per 
square inch (PSI) or atmospheres (BAR). 
In order to provide high and low limit pressure detection for alarm and 
motor shutdown control, the actual output pressure from amplifier 87 is 
coupled to high and low pressure limit detectors 104 and 108 respectively. 
The detectors 104 and 108 are constructed to produce an output signal at 
pressures above and below those set by high and low limit sets 110 and 
112. The limit sets 110 and 112 accurately fix a signal representative of 
the pressure at which it is desired to stop the pump. The limit sets are 
also coupled to a select limit read switch 114 which allows either the 
high or low limit setting to be coupled to the digital voltmeter 38 in 
order to display the pressure limits. 
When the pressure indicated by the signal from amplifier 87 falls below the 
low limit set or rises above the high limit set, the respective outputs of 
detectors 104 and 108 provide a signal which enables high and low limit 
indicators 116 or 118. At the same time, the respective outputs are 
coupled as input to alarm control 120 and pump stop control 122. These 
control circuits in turn provide signals which initiate alarm 124 and pump 
stop switches 126 to produce pump shutdown signals to flow-rate control 46 
(FIG. 2). In connection with the low limit pressure indication, a pump 
reset 128 is coupled to detector 108 to allow the pump to be restarted 
following shutdown due to a low pressure fault. 
As was described with reference to FIG. 2, the torque signal from sensing 
circuit 66 is provided to the motor control interface 30 via limit control 
130. The output of the limit control couples a signal to the alarm control 
120 and pump stop control 122 upon occurrence of an over-torque condition. 
As a result, the system responds to both instantaneous pressure and torque 
(from sensing circuit 66) to shutdown the system upon fault detection. 
As can be seen with reference to FIG. 3, the output from the pump stop 
control 122 is coupled to sample and hold circuit 92 and gates 82 to 
provide additional circuit protection. Thus, when the pump stop signal is 
received at sample and hold 92, the circuit will produce a continuous 
output pressure signal representing actual pressure to be displayed by the 
digital voltmeter 38. At the same time, the signal will insure that the 
pumps stop by inhibiting gates 82 to prevent flow-rate signal delivery to 
control 46 (FIG. 2) during fault conditions. 
As was noted with reference to FIG. 1, the sample changer 15 can 
additionally be used to provide automatic system operation. In the desired 
embodiment, the changer 15 is constructed to provide an output signal to 
the pump stop control 122 in the motor control interface 30 after the last 
vial in the rack is injected. At that time, the pumps will be stopped 
without an alarm indication until the rack is reset for subsequent 
operation. 
In order to produce the functions and control described with reference to 
FIGS. 1-3, more detailed illustrations of exemplary circuits are shown in 
FIGS. 4A, 4B, 5A and 5B. It is to be understood, however, that the 
particular circuits are only examples of those that could be used to 
accomplish the functions and results described herein. In regard to the 
motor control circuit 28 as particularly shown in FIGS. 4A and 4B, the 
same was derived by modification of well-known control circuitry. Thus, 
the elements and values used in such circuitry are the same as included in 
prior art circuits except where modified in accordance with the present 
invention as indicated herein. 
Turning first to FIGS. 4A and 4B, a detailed example of the servo circuitry 
of motor control 28 is illustrated. Referring first to the servo amplifier 
54 of FIG. 2, the same is constructed from a differential comparator 300 
(FIG. 4A) having the signal from the pump speed control 42 supplied to the 
non-inverting input. The inverting input is swept by a triangular waveform 
generated at capacitor C1 by the oscillator 302. In the present instance 
the circuit is constructed to have a frequency of about 20 KHZ with a 
voltage range of 7.5 to 11 volts. In operation, when the voltage at the 
non-inverting input to comparator 300 exceeds the minimum value of the 
triangular wave at the inverting input, an output pulse train is produced 
at the frequency of the triangular wave having a duty factor which 
increases from zero to 100% as the voltage at the non-inverting input 
increases. Thus, a drive signal is supplied to the power amplifier 58 
(FIG. 2) where it is amplified by transistors Q1 and Q2 current limited by 
transistor Q3 and amplified to suitably high power level by transistors Q4 
and Q5. Current through the collector of Q4 and Q5 establishes an average 
current in the motor driven circuit 60 (FIG. 2) through inductor L1 and 
capacitor C2 which controls motor speed. In order to insure that the motor 
will slow down when the turn on time of the transistors Q4 and Q5 rapidly 
decreases, the transistor Q6 is coupled to short the motor windings when 
voltage on C2 falls below the back emf of the motor. Since the +terminal 
of C2 and the motor are tied together, when the back emf exceeds the 
voltage on the capacitor by about 1.3 volts Q6 turns on and shorts the 
motor to decelerate the same in a well-known manner. 
In order to provide for over-current protection, the current pulse train 
flowing through R1 is sensed through resistors R2 and R3 to charge 
capacitor C3 of over-current control 56. If the average motor current (as 
represented by the current pulse train) exceeds a predetermined limit, the 
comparator 304 clamps the bases of transistors Q1 and Q2 to ground thereby 
inhibiting any further signal input to amplifier 58. Since this also stops 
current flow through R1, capacitor C3 will discharge through R1 until its 
value falls below the hysteresis level set by R4 at which time, comparator 
304 will remove the clamps on Q1 and Q2. In operation, the circuit acts to 
oscillate at a high frequency to provide switching mode current limiting 
which provides electrical protection as well as a limit for maximum motor 
torque representing maximum pump head pressure. 
In developing the speed control signal supplied to servo amplifier 54 by 
the pump speed control 42, the tachometer disc 44 (FIG. 2) on the motor 
shaft is coupled to an optical transducer 306 to produce an AC output 
signal proportional to the speed of the armature. This AC signal is 
coupled to an LM2709 frequency-to-voltage converter 308 which produces 
square-topped output current pulses (having a pulse height and width 
independent of input frequency) at the rate of 2 pulses at the output 310 
for each input cycle at input 309. The average current output therefore 
increases with increasing frequency because the duty factor tends toward 
100% as the frequency approaches one-half of the output pulse width. 
The pulse output from 310 is supplied through resistor R5 to a current 
summing node at input 311 of converter 308. An output current signal from 
control 46 representing the desired pump rate is also supplied to the 
summing node through line 312. The signal from control 46 is derived from 
a current source formed by comparator 314 and transistor Q7 (FIG. 4B) 
which provides a current through the collector of Q7 proportional to the 
flow-rate setting supplied to the non-inverting input of comparator 314. 
When the two signals at the summing node are combined, the resulting 
output at line 315 is a signal indicative of the desired motor speed 
during the delivery stroke which is supplied to the non-inverting input of 
300 for motor control as was previously described. Resistor R6 provides an 
adjustment which allows the output of comparator 314 to be set to insure 
that the motor stops at zero flow-rate in spite of any off-set or leakage 
currents in the circuits. 
In order to provide for pulse compensation, the disc 48 (FIG. 2) on the 
motor cam shaft is mechanically coupled to refill period control 50. The 
disc consists of an optical flag phased on the cam shaft so that the 
leading edge coincides with the top dead center of the cam (beginning of 
refill stroke) and has an angular length equal to the refill return 
angular length of the cam. The flag interrupts an optical signal in a 
conventional optical transducer 316 to produce a high output from 
comparator 318 during the refill period. This output is then coupled 
through line 319 to flow-rate control 46 and compressability control 52 to 
control motor speed during refill and repressurization. 
In connection with the control 46, the high output from comparator 318 is 
coupled to the control terminal of analog gates 320 and 322 (FIG. 4B). At 
this time, gate 320 connects the current source transistor Q8 and resistor 
R7 to the summing node to produce an output signal from control 46 which 
speeds up the drive motor during refill. Since the base of transistor Q8 
is coupled to the output of comparator 314 to control collector conduction 
in accordance with flow-rate setting, R7 will cause motor speed up at low 
set points while Q8 will cause motor speed up at high set points due to 
current flow through R8. At the same time, the gate 322 clamps the 
collector of Q7 to 7.5 volts during the duration of the refill cycle. 
In connection with the control 52 of FIG. 2, the high output from 
comparator 318 (FIG. 4A) is coupled to the control pin of analog gate 324 
(FIG. 4B) clamping the output 325 of differential comparator 326 to 
ground. At the same time, the same high signal is coupled to the base of 
transistor Q10 discharging capacitors C4 and C5 to ground. The capacitors 
C4 and C5 are coupled to the non-inverting input of comparator 326 while 
the repressurization signal from interface 30 is coupled to the inverting 
input. During refill, the low at the output of comparator 326 enables 
repressurization current source transistor Q9 and resistor R9 but does not 
cause further motor speed up due to the clamp of 7.5 volts on the 
collector Q7 through gate 322. 
At the end of the refill part of the cycle, the repressurization part of 
the cycle begins as the piston starts to advance. At this time, the 
optical flag causes the output from comparator 318 to go low turning Q10 
off and allowing voltage rise on C4 and C5. The voltage rise on C4 and C5 
is made proportional to the angle of cam shaft rotation past bottom dead 
center (end of refill stroke) by charging through R10 and R11 in the 
emitter of Q11. The proportionality is established by driving the 
transistor combination Q11, Q12 with the average voltage across R5 which 
is in turn proportional to motor speed as represented by the pulse output 
from line 310 of the converter 308. The proportional voltage causes the 
current at the collector of Q11 to be proportional to the instantaneous 
angular velocity of the motor armature. C4 and C5 thereafter integrate 
this current to produce a voltage proportional to angular rotation of the 
armature past bottom dead center of the cam. 
During the initial repressurization interval, the voltage at the 
non-inverting terminal of comparator 326 is less than the repressurization 
voltage provided to the inverting input terminal causing the output of 
comparator 326 to be low. At this time diodes D1 and D2 enable Q9 and R9 
to carry current from the summing node 311 of the converter 308 to speed 
up the motor. The amount of motor speed-up tracks the set point speed 
fixed by the flow-rate setting since the base of Q9 is controlled by the 
output of amplifier 314. The duration of the repressurization speed up, 
expressed in the angular amount of speed-up shaft rotation, is made 
proportional to an adjustable pressure/flow baseline level and provide 
pulse compensation with respect to flow-rate and an accurate pressure as 
will be described with reference to the motor control interface 30. 
In order to provide torque limit detection for the delivery part of the 
cycle, the output torque is derived by monitoring the armature current 
with detector 62 through lines 327 and 238 as was described with reference 
to FIG. 2. In the detector 62, transistor Q13 (FIG. 4B) develops a 
collector current proportional to the voltage drop across R12 in the motor 
drive circuit 60 which in turn is proportional to the armature current and 
dependent on motor torque. Due to various amounts of static friction in 
the armature, the armature current is greater than zero while the motor is 
rotating with zero output torque. A resistor R13 is therefore incorporated 
along with transistor Q14 and R14 to cause a voltage drop across resistor 
R13 to compensate for this error. The output current in the collector of 
Q13 is then coupled through R15 to produce a voltage signal proportional 
to motor torque. To overcome the effect of a parasitic frictional torque 
that increases with armature speed (windage friction), a transistor Q15 is 
used to drain off a portion of the current through Q13. The collector 
current of Q15 is made proportiional to the flow-rate setting (pump speed) 
by coupling its base to the output of amplifier 314 to allow increased 
compensation at increased speeds. 
The output signal from detector 62 is coupled to an analog gate 330 which 
controls application of the signal to a sample and hold capacitor C6 
through the resistor R16 and diode D3 combination. This voltage is coupled 
through operational amplifier 332 having a resistor R17 coupled to allow 
zeroing of the output of amplifier 332 at zero output torque. The 
adjustment R17 is coupled to a current source at Q16 to enable zeroing 
adjustment rather than a change of the gain of the operational amplifier. 
The signal output from amplifier 332 is coupled to an over-torque limit 
circuit incorporating comparator 334. In operation the output of 
comparator 334 is normally low until the output of amplifier 332 exceeds 
an upper limit set by comparator 334 at which time it goes high. The 
output high is provided to the motor control interface 30 to initiate 
alarm and motor shutdown and as input to the control terminal of gate 330 
to cause continuous torque sensing during an over-torque condition. 
The analog gate 330 is also controlled by signals received from the servo 
amplifier 54 through control gate 68 to remain on during the delivery 
stroke. Control gate 68 includes diode D4 which causes a voltage from the 
capacitor C7 to be coupled to the control terminal of gate 330 through 
line 335 during the delivery stroke. During refill, however, when the 
signal from the comparator 318 clamps the output of comparator 326 to 
ground, the capacitor C7 discharges to ground through diode D4 thereby 
inhibiting gate 330 during refill unless an over-torque output signal from 
comparator 334 is present. 
Referring now to FIGS. 5A and 5B, the pressure transducer 32 consists of a 
four arm strain gage mounted on the back of a pressure sensing diaphragm. 
Its differential output voltage representing operating pressure is 
amplified by amplifier 87 (FIG. 3) consisting of differential amplifiers 
200 and 202 (FIG. 5B) having zero adjust and pressure range adjust 
potentiometers 88 and 90 to provide a zero output at zero pressure and an 
adjustment for pressure range. The pressure signal at the output of the 
amplifier 87 is coupled as input to sample and hold circuit 92 consisting 
of analog gate 204 and the amplifier 210. The gate 204 has its control 
terminal coupled through line 203 to receive a strobe signal via the gate 
drive circuit 94 from the compensation control 52 to inhibit the gate 
during refill and repressurization. As a result of this sampling, the 
output of the sample and hold circuit 92 produces a value indicative of 
the delivery pressure only which is a smoothed pressure signal (due to the 
removal of pulsations caused by refill and repressurization). A balance 
adjustment is provided by R18 to enable the output of the sample and hold 
to be set at zero volts when the output of the amplifier 87 is zero. 
The output of the sample and hold circuit 92 is coupled through line 205 to 
switch 100 (FIG. 5A) which allows system pressure to be read on the 
digital voltmeter 38. As was previously noted, the PSI/BAR switch 102 
allow calibration of the DVM to enable reading of the pressure in pounds 
per square inch or atmosphere. 
The actual pressure from the output of amplifier 87 is coupled through line 
207 to high and low pressure limit detectors 104 and 108. The high limit 
detector generally includes comparator 214 and transistor Q17 while the 
low limit detector includes comparator 216 and transistor Q16. The high 
and low limit sets are generally constructed from potentiometers R19 and 
R20, respectively, providing voltage reference levels to the designated 
inputs of 214 and 216, respectively. The voltage reference levels are also 
coupled through switches S1 and S2 forming the select limit read switch 
114 to enable the levels to be read individually on the pressure DVM. 
In operation, when an over or under pressure signal is received at the 
inputs to 214 and 216, the appropriate comparator will cause the base of 
one of the transistors Q17 or Q16 to turn on causing current flow in the 
appropriate collector. At that time, current flow will cause the 
appropriate indicator lights I1 or I2 to light and also cause a low to 
appear at the inputs of NAND gate 218 of the alarm control 120. The NAND 
gate will produce a high output to the circuit 220 which in turn will 
initiate an audio alarm 124. 
The low signals from the output of the limit detectors 104 and 108 are also 
coupled to the pump stop control 122 which includes NAND gates 222 and 
224. The high output from NAND gate 222 is coupled to NAND gate 224 to 
provide a low output signal to pump switches 126 formed by transistors Q19 
and Q20 to provide motor shutdown signals to gate 322. This signal causes 
the collector of transistor Q7 to be clamped to 7.5 volts thereby stopping 
the motor. 
The over-torquue limit signal from the output of comparator 334 in FIG. 4B 
is also coupled to provide a control signal to the alarm and pump shutdown 
circuits via limit controls 130. Limit controls 130 includes transistors 
Q21 and Q22 which receiver the high signal output from comparator 334 at 
their respective bases. The collectors, upon receipt of the high signal, 
short the voltage through diodes D5 and D6 to ground to produce a low at 
the input of the alarm and pump stop control circuits. At the same time, a 
current path is completed to light indicator lamp I3 and I4 to identify 
the particular pump in the over-torque condition. Since it is possible for 
motor torque in the pump to build up to a dangerous amount without a 
corresponding pressure increase sensed by the pressure transducer 32 (as 
when the pump head filter is plugged) the above circuitry provides a 
safety cutoff and an indication of the faulty pump. 
If it is desired to employ automatic operation under the control of the 
sample changer 15, a low signal from the sample changer 15 should be 
provided as a second input to the pump stop control NAND gate 222. This 
produces a stop signal upon termination of system operation but does not 
cause an alarm indication. 
The high output from the NAND gate 222 of the pump stop circuit 122, which 
occurs during a fault condition, is coupled through lines 207 and 203 to 
the control terminals of the analog gate 204 (FIG. 5B) in sample and hold 
92. This causes the circuit 92 to continuously read actual pressure during 
a fault condition. At the same time, the stop signal is provided via line 
211 to gates 82 to inhibit flow-rate signal passage to the control 46 as 
was previously described with reference to FIG. 3. 
As has been previously mentioned, improved pulsation compensation is 
provided in the present circuit by developing a repressurization signal 
which compensates with respect to flow-rate and pressure. With respect to 
present circuit, the repressurization signal is developed by providing the 
smoothed pressure signal and a flow-rate signal to summing amplifiers 226 
and 228. Resistors R21 and R22, forming baseline adjustors 98, provide an 
adjustable baseline offset potential to compensate properly for residual 
pulsations at low pressures and low flows. 
The flow-rate signals used to form the other inputs to controls 86 are 
provided at the output of drive amplifiers 230 and 232 forming the 
flow-rate driver 80. Such signals are selected from the single pump 
flow-rate setting 36 or gradient program 34 as have been previously 
described. The flow-rate setting is then coupled through the appropriate 
flow adjustments 84, which include diodes D7-D10 and potentiometers R23 
and R24, to the summing input at the non-inverting terminal of amplifiers 
226 and 228 in the control circuits 86. The signal at the output of each 
summing amplifier represents a repressurization signal for delivery to the 
corresponding compensation control 52, which compensates with respect to 
the flow-rate and pressure from an adjustable pressure/flow baseline. The 
potentiometers R23 and R24 provide an adjustable compensation for flow and 
in cooperation with the diodes D7-D10 allows improved tracking of 
pulsation compensation with respect to flow-rate variations. 
In operation, the compensation control 52 provides a repressurization limit 
that is increased by the incorporation of capacitor C5 which allows 
increasing the maximum angular rotation of the pump during the compensated 
repressurization interval over that of the prior art. This allows each 
pump to be essentially flat-compensated over the flow-rate range. In a 
gradient chromatograph operating in the "flow" gradient mode (both pumps 
increasing in speed during the operation of a gradient program), however, 
a high flow-rate and high pressure can develop at the end of the program. 
Accordingly, while maximum repressurization compensation (maximum 
repressurization interval) is desired, the two pumps could cause system 
malfunction by strobing off the sample and hold gate 204 continuously if 
the pumps were turning at the same speed and were 180.degree. out of 
phase. Thus, the maximum repressurization interval is limited by the 
flow-rate adjustments 84 to prevent such a condition while allowing 
improved compensation to the maximum practical extent. 
In accordance with the above circuits, improved pulsation compensation is 
provided which prevents the output flow-rate from dropping off at high 
flow-rates. This is accomplished since the actual cyclical flow delivery 
interval is not as greatly preempted by the actual repressurization 
interval as occurred in prior art compensation circuitry. For this reason 
much less linearity compensation is also needed and the linearity 
adjustments 47 are therefore able to be much less sensitive. Such 
linearity adjustments can thus be made with high resistance potentiometers 
47 coupled in the emitter circuit of Q7 in FIG. 4A. 
It should be noted that while the detailed circuits of FIGS. 5A and 5B have 
not included specific values and an identification of the elements chosen 
to form the circuits, the same are selected in accordance with normal 
design practices to provide proper biasing and signal levels to perform 
the functions indicated and to interface with the voltage levels of the 
particular apparatus and circuitry with which the control interface may be 
employed. In addition, the elements may be selected from a variety of 
prior art devices capable of accomplishing the functions described as 
would be apparent to one of ordinary skill in the art. Thus, any values 
and specifically illustrated elements are only considered exemplary of the 
operable circuitry that may be employed in the practice of the present 
invention. Obviously, many other modifications and variations of the 
invention are possible in light of the above teachings. It is therefore to 
be understood that within the scope of the appended claims, the invention 
may be practiced otherwise than as specifically described.