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Description  |
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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.
BRIEF DESCRIPTION OF THE DRAWINGS
Still other objects, advantages, and novel features of the invention will
become apparent from the following detailed description when considered
with the accompanying drawings wherein:
FIG. 1 is a block diagram of a chromatographic system employing the pump
control circuitry in accordance with the present invention.
FIG. 2 is a block diagram schematically showing the functional operation of
the motor control shown in FIG. 1.
FIG. 3 is a block diagram schematically showing the functional operation of
the motor control interface shown in FIG. 1.
FIG. 4A and 4B is a detailed diagram showing exemplary circuits forming the
motor control of FIG. 2.
FIG. 5A and 5B is a detailed diagram showing exemplary circuits forming the
motor control interface of FIG. 3.
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 | | |
