U.S. patent application number 14/910687 was filed with the patent office on 2016-10-20 for method for determining hydraulic pressure parameters in a displacement pump.
The applicant listed for this patent is PROMINENT GMBH. Invention is credited to Fabian Kennel, Steven Liu.
Application Number | 20160305419 14/910687 |
Document ID | / |
Family ID | 51454657 |
Filed Date | 2016-10-20 |
United States Patent
Application |
20160305419 |
Kind Code |
A1 |
Liu; Steven ; et
al. |
October 20, 2016 |
Method for Determining Hydraulic Pressure Parameters in a
Displacement Pump
Abstract
The invention relates to a method for determining hydraulic
parameters in a displacement pump, wherein the displacement pump
has a movable displacement element, which bounds the metering
chamber, which is connected to a suction and pressure line by means
of valves, wherein a drive is provided for the oscillating motion
of the displacement element. In order that pumped fluid can be
alternately sucked into the metering chamber via the suction line
and pressed from the metering chamber via the pressure line by
means of an oscillating motion of the displacement element, a
physical model having hydraulic parameters according to the
invention is established for the hydraulic system, the force
exerted by the displacement element on the fluid located in the
metering chamber or the pressure in the metering chamber is
determined and the position of the displacement element is
determined, and at least one hydraulic parameter is calculated by
means of an optimization calculation
Inventors: |
Liu; Steven;
(Kaiserslautern, DE) ; Kennel; Fabian;
(Kaiserslautern, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
PROMINENT GMBH |
Heidelberg |
|
DE |
|
|
Family ID: |
51454657 |
Appl. No.: |
14/910687 |
Filed: |
August 21, 2014 |
PCT Filed: |
August 21, 2014 |
PCT NO: |
PCT/EP14/67817 |
371 Date: |
May 19, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F04B 17/042 20130101;
F04B 49/10 20130101; F04B 2203/0402 20130101; F04B 2203/0401
20130101; F04B 17/04 20130101; F04B 43/04 20130101; F04B 49/065
20130101; F04B 2203/0201 20130101; F04B 17/03 20130101; F04B 51/00
20130101; F04B 49/02 20130101 |
International
Class: |
F04B 49/06 20060101
F04B049/06; F04B 49/10 20060101 F04B049/10; F04B 51/00 20060101
F04B051/00; F04B 49/02 20060101 F04B049/02; F04B 17/03 20060101
F04B017/03; F04B 43/04 20060101 F04B043/04 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 29, 2013 |
DE |
10 2013 109 411.2 |
Claims
1.-16. (canceled)
17. A method of determining at least one physical variable in a
positive displacement pump, wherein the positive displacement pump
has a moveable displacer element delimiting the metering chamber
which is connected by way of valves to a suction and a pressure
line so that delivery fluid can alternately be sucked into the
metering chamber by way of the suction line and urged out of the
metering chamber by way of the pressure line by an oscillating
movement of the displacer element, wherein there is provided a
drive for the oscillating movement of the displacer element,
wherein for the displacer element a differential equation is
established based on a physical model, at least the position of the
displacer element is measured and the physical variable is
determined by means of the differential equation, wherein the fluid
pressure p of a delivery fluid in a metering chamber of a positive
displacement pump is determined as the physical variable,
characterized in that if the actual fluid pressure reaches or
exceeds a predetermined maximum value a warning signal is output
and the warning signal is sent to an automatic shut-down
arrangement which shuts down the metering pump in response to
reception of the warning signal.
18. A method of determining at least one physical variable in a
positive displacement pump, wherein the positive displacement pump
has a moveable displacer element delimiting the metering chamber
which is connected by way of valves to a suction and a pressure
line so that delivery fluid can alternately be sucked into the
metering chamber by way of the suction line and urged out of the
metering chamber by way of the pressure line by an oscillating
movement of the displacer element, wherein there is provided a
drive for the oscillating movement of the displacer element,
wherein for the displacer element a differential equation is
established based on a physical model, at least the position of the
displacer element is measured and the physical variable is
determined by means of the differential equation, wherein the fluid
pressure p of a delivery fluid in a metering chamber of a positive
displacement pump is determined as the physical variable,
characterized in that for a movement cycle of the displacer element
a target fluid pressure curve, a target position curve of the
displacer element and/or the target current pattern through the
electromagnetic drive is provided and the actual fluid pressure is
compared to the target fluid pressure, the actual position of the
displacer element is compared to the target position of the
displacer element and/or the actual current through the
electromagnetic drive is compared to a target current through the
electromagnetic drive and a warning signal is output if the
differences between the actual and target values satisfy a
predetermined criterion.
19. A method of determining at least one physical variable in a
positive displacement pump, wherein the positive displacement pump
has a moveable displacer element delimiting the metering chamber
which is connected by way of valves to a suction and a pressure
line so that delivery fluid can alternately be sucked into the
metering chamber by way of the suction line and urged out of the
metering chamber by way of the pressure line by an oscillating
movement of the displacer element, wherein there is provided a
drive for the oscillating movement of the displacer element,
wherein for the displacer element a differential equation is
established based on a physical model, at least the position of the
displacer element is measured and the physical variable is
determined by means of the differential equation, characterized in
that the mass m of the displacer element, the spring constant k of
the spring prestressing the displacer element, the damping d and/or
the electrical resistance R.sub.Cu of the electromagnetic drive is
determined as the physical variable.
20. A method as set forth in claim 17, characterized in that the
positive displacement pump is an electromagnetically driven
metering pump.
21. A method as set forth in claim 20 characterized in that besides
the position of the displacer element the current through the
electromagnetic drive is measured and the differential equation
uses both the position of the displacer element and also the
current through the electromagnetic drive as measurement variables,
wherein the differential equation does not have any further
measurement variables to be detected.
22. A method as set forth in claim 18 characterized in that a
weighted sum of the relative deviations from the target value is
determined and the criterion is so selected
23. A method as set forth in claim 18 characterized in that a
plurality of criteria are predetermined, a fault event is
associated with each criterion and, if a criterion is fulfilled,
the associated fault event is diagnosed.
24. A method as set forth in claim 17 characterized in that a
model-based closed-loop control is used for the drive.
25. A method as set forth in claim 24 characterized in that a
non-linear state space model is selected as the model, wherein the
non-linear closed-loop control is effected either by way of
control-Lyapunov functions, by way of flatness-based closed-loop
control methods with flatness-based precontrol, by way of
integrator backstepping methods, by way of sliding mode methods or
by way of predictive closed-loop control, wherein non-linear
closed-loop control by way of control-Lyapunov functions is
preferred.
26. A method as set forth in claims 24 characterized in that the
difference between the detected actual position profile of the
displacer element and a predetermined target position profile of
the displacer element is detected during a suction-pressure cycle
and the difference of at least a part of the detected difference
and the predetermined target position profile is used as the target
value profile for the next suction-pressure cycle.
27. A method as set forth in claim 17 characterized in that
hydraulic parameters in the positive displacement pump are
determined, for the hydraulic system a physical model is
established with hydraulic parameters, the force exerted by the
displacer element on the fluid in the metering chamber or the
pressure in the metering chamber as well as the position of the
displacer element is determined and at least one hydraulic
parameter is calculated by means of an optimization
calculation.
28. A method as set forth in claim 27 characterized in that the
density of the fluid in the metering chamber and/or the viscosity
of the fluid in the metering chamber is determined as the hydraulic
parameter.
29. A method as set forth in claim 27 characterized in that the
physical model is set up for the situation where the valve to the
suction line is opened and the valve to the pressure line is closed
and/or for the situation where the valve to the suction line is
closed and the valve to the pressure line is opened, wherein if the
physical model is set up both for the situation where the valve to
the suction line is opened and the valve to the pressure line is
closed and also for the situation where the valve to the suction
line is closed and the valve to the pressure line is opened, the
valve opening times are determined, and the physical model is
selected in dependence on the result of determining the valve
opening times.
30. A method as set forth in claim 28 characterized in that after
determination of the hydraulic parameter same and the physical
model is used for determining the force exerted by the delivery
fluid on the displacer element and the force determined in that way
is used in a closed-loop control of the movement of the displacer
element.
31. A method as set forth in claim 18, characterized in that the
positive displacement pump is an electromagnetically driven
metering pump.
32. A method as set forth in claim 19, characterized in that the
positive displacement pump is an electromagnetically driven
metering pump.
33. A method as set forth in claim 18 characterized in that a
model-based closed-loop control is used for the drive.
34. A method as set forth in claim 19 characterized in that a
model-based closed-loop control is used for the drive.
Description
[0001] The present invention relates to a method for determining
hydraulic parameters in a displacement pump. The displacement pump
has a movable displacement element, which bounds the metering
chamber, which is connected to a suction and pressure line via
valves, with the result that pumped fluid can be alternately sucked
into the metering chamber via the suction line and pressed out of
the metering chamber via the pressure line by means of an
oscillating motion of the displacement element. Displacement pumps
additionally have a drive for the oscillating motion of the
displacement element.
[0002] There are, for example, electromagnetically driven diaphragm
pumps, in which the displacement element is a diaphragm, which can
be moved backwards and forwards between two extreme positions,
wherein in the first extreme position the volume of the metering
chamber is at a minimum, while in the second extreme position the
volume of the metering chamber is at a maximum. Therefore, if the
diaphragm is moved from its first position into the second
position, the pressure in the metering chamber will fall, with the
result that pumped fluid is sucked into the metering chamber via
the suction line. In the case of the movement back, i.e. the motion
from the second position into the first position, the connection to
the suction line is closed, the pressure of the pumped fluid will
increase because of the reducing volume in the metering chamber,
with the result that the valve to the pressure line is opened and
the pumped fluid is transported into the pressure line. Through the
oscillating motion of the diaphragm, on an alternating basis pumped
fluid is sucked from the suction line into the metering chamber,
and pumped fluid is sucked out of the metering chamber, and pumped
fluid is conveyed from the metering chamber into the pressure line.
The flow of pumped fluid into the pressure line is also called the
metering profile. This metering profile is essentially determined
by the motion profile of the displacement element.
[0003] In the case of electromagnetically driven diaphragm pumps,
the diaphragm is connected to a thrust member, which is usually
mounted pretensioned in a spring-loaded manner at least partially
within an electromagnet. As long as the electromagnet does not have
a current flowing through it, with the result that no magnetic flux
is built up inside it, the spring-loaded pretensioning ensures that
the thrust member and thus the diaphragm remains in a predetermined
position, e.g. the second position, i.e. the position at which the
metering chamber has the greatest volume. If a current is now
impressed on the electromagnet, a magnetic flux forms, which brings
the correspondingly formed thrust member inside the electromagnet
from its second position into the first position, which causes the
pumped fluid located in the metering chamber to be conveyed from
the metering chamber into the pressure line.
[0004] Upon activation of the electromagnet, there is an
essentially abrupt stroke of the metering piece and thus of the
metering diaphragm from the second into the first position.
[0005] Typically, such electromagnetically driven diaphragm pumps
are used if the fluid volume to be metered is significantly greater
than the metering chamber volume, with the result that the metering
rate is essentially determined by the frequency, or the timing, of
the current flow through the electromagnet. If, for example, the
metering rate is doubled, at the same time a current temporarily
flows through the electromagnet twice as frequently, which in turn
results in the motion cycle of the diaphragm being shortened and
taking place twice as frequently.
[0006] Such a magnetic metering pump is described in EP 1 757 809,
for example.
[0007] However, the use of this magnetic metering pump reaches its
limits when only low metering rates are necessary, with the result
that the abrupt metering of an entire stroke is not desirable.
[0008] In the named EP 1 757 809, it is therefore already proposed
to provide a position sensor with which the position of the thrust
member, or the diaphragm connected thereto, can be determined. By
comparing the actual position of the thrust member with a
predetermined target position of the thrust member a control of the
motion can then take place, with the result that magnetic metering
pumps can also be used to convey significantly lower fluid amounts,
as the stroke motion no longer takes place abruptly, but rather in
a controlled manner.
[0009] In practice it is difficult to find suitable control
parameters. In fact, different control parameters are determined
empirically in each case for different thrust member position
states and stored in a memory, with the result that the pump can
retrieve and use the corresponding control parameters in dependence
on the position of the thrust member.
[0010] However, it is very laborious to determine the control
parameters. In addition, it depends heavily on the circumstances in
the metering chamber, such as e.g. the density and the viscosity of
the pumped fluid. The control therefore only functions
satisfactorily when the system approximately corresponds to the
desired state. In particular when there are pressure fluctuations
on the suction and/or pressure line, when cavitation occurs, when
air accumulates in the metering chamber or when there are changes
in the viscosity of the pumped fluid, the control parameters stored
in the memory are unsuitable and the control accuracy decreases,
with the result that the actual metering profile differs
significantly from the desired metering profile. However, this is
undesirable in particular in the case of continuous metering of
very small amounts, such as e.g. in the case of the chlorination of
drinking water.
[0011] The control accuracy can, for example, be improved by
measuring the density and/or viscosity of the pumped fluid and
using the measurement result to select the control parameters.
[0012] However, for such a measurement at least one additional
sensor is necessary, which would increase the selling price of the
displacement pump and additionally needs maintenance and repair.
Density and viscosity changes have therefore hitherto not been
taken into account in the control.
[0013] Starting from the described state of the art, the object of
the present invention is therefore to provide a method that allows
hydraulic parameters to be determined, such as e.g. the density or
the viscosity of the pumped fluid, without additional sensors being
required.
[0014] This is achieved according to the invention in that a
physical model having hydraulic parameters is constructed for the
hydraulic system, the force exerted by the displacement element on
the fluid located in the metering chamber or the pressure in the
metering chamber and the position of the displacement element is
determined, and at least one hydraulic parameter is calculated by
means of an optimization calculation.
[0015] By hydraulic parameters is meant any parameter of the
hydraulic system--except the position of the displacement
element--which influences the flow of the pumped fluid through the
metering chamber.
[0016] Hydraulic parameters are thus e.g. the density of the pumped
fluid in the metering chamber as well as the viscosity of the fluid
in the metering chamber. Further hydraulic parameters are, for
example, the hose or pipe length and diameter of hoses and pipes
which are at least at times connected to the metering chamber.
[0017] The required determination of the position of the
displacement element can take place via the position sensor which
is usually present in any case. The speed and acceleration of the
displacement element can be determined from the position of the
displacement element.
[0018] If the method according to the invention is used in an
electromagnetically driven metering pump, and best of all in an
electromagnetically driven diaphragm pump, in a preferred
embodiment the current through the electromagnetic drive can be
measured and the force exerted by the displacement element on the
fluid located in the metering chamber is determined from the
measured current and the measured position of the displacement
element. In this case, a separate pressure sensor is not necessary.
However, the present method can, of course, also be used with a
separate pressure sensor.
[0019] It is an inherent property of the displacement pump that the
hydraulic system always changes significantly when one of the
valves via which the metering chamber is connected to the suction
and pressure line is opened or closed.
[0020] It is easiest to model the system for the case in which the
valve to the suction line is open and the valve to the pressure
line is closed. Namely, a flexible hose which ends in a storage
tank at ambient pressure is usually mounted on the valve to the
suction line.
[0021] This situation exists during the so-called suction stroke
i.e. while the displacement element is moving from the second
position into the first position. This hydraulic system could, for
example, be described by means of the nonlinear Navier-Stokes
equation taking into account laminar and turbulent flows. In
addition to the density and the viscosity of the pumped fluid, the
diameter of the hose which connects the suction valve to the
storage tank, the length of the hose and the height difference
which the fluid in the hose must negotiate are also to be
considered as hydraulic parameters.
[0022] Depending on the system used, further meaningful assumptions
can be made. By means of an optimization calculation, which can,
for example, take place via the known gradient method or
Levenberg-Marquardt algorithms, it is possible to determine the
hydraulic parameters contained in the physical model which best
describe the pressure progression in the metering head and the
motion, or the speed and acceleration determined therefrom, of the
thrust member.
[0023] By an optimization calculation is meant any calculation with
which the optimal parameters of the system are discovered. Optimal
parameters are the parameters which best describe the system, i.e.
for which the difference between the model and the measured
situation is at a minimum.
[0024] The determination method according to the invention could
essentially take place simply through a repeated analysis of the
suction stroke behavior.
[0025] Alternatively, however, the physical model of the hydraulic
system can also be considered for the case that the valve to the
suction line is closed and the valve to the pressure line is open.
As, however, the pump manufacturer as a rule does not know at first
in what environment the metering pump is used, and therefore also
does not know the pipe system attached to the pressure valve which
connects the pressure line to the metering chamber, only a
generalized assumption can be made here. Without knowledge of the
pipeline system attached to the pressure valve, the constructed
physical model can therefore not be constructed as accurately as is
the case in the described simplest form for the hydraulic system
during the suction stroke.
[0026] In a particularly preferred embodiment, physical models for
both described hydraulics systems are used and then the valve
opening time points are measured or determined and the respectively
appropriate physical model is selected in dependence on the result
of the determination of the valve opening time points. Essentially,
the method according to the invention is then carried out
separately for the suction stroke and the pressure stroke. In both
cases, values are obtained for the hydraulic parameters, such as
e.g. the density and the viscosity of the pumped fluid, which do
not correspond exactly in practice. In principle it would therefore
be possible to take the average of the different values, wherein it
must possibly be taken into account that because of the better
description of the actual situation by the physical model during
the suction stroke, the value obtained during the suction stroke is
weighted more heavily in the averaging than the value ascertained
during the pressure stroke.
[0027] Of course, there are also application cases in which the
hydraulics system is also more complex during the suction
stroke.
[0028] After the hydraulic parameters have been determined in the
manner according to the invention, the constructed physical model
can be used with the hydraulic parameters determined in this way in
order to determine, for its part, the pressure in the metering
chamber.
[0029] This knowledge can, in turn, be used to improve the motion
control of the thrust member. In a preferred embodiment it is
provided that a model-based control, in particular a nonlinear
model-based control is used for the drive of the displacement
element.
[0030] In the case of a model-based control, a model of the process
dynamics which is as complete as possible is developed. Using this
model it is then possible, put simply, to make a prediction for
where the system variables will move to in the next moment.
[0031] From this model, a suitable manipulated variable can then
also be calculated. A characterizing feature of such a model-based
control is thus the constant calculation of the necessary
manipulated variables on the basis of measured variables using the
system variables given by the model.
[0032] Essentially, the underlying physical system is described
approximately mathematically by the modeling. This mathematical
description is then used to calculate the manipulated variable on
the basis of the obtained measured variables. Unlike the known
metering profile optimization methods, the drive is thus no longer
seen as a "black box". Instead, the known physical relationships
are used to determine the manipulated variable.
[0033] By this means, a significantly better control quality can be
achieved.
[0034] In a preferred embodiment, the position of the displacement
element and the current through the electromagnetic drive are
measured and a state-space model is used for the model-based
control, which uses the position of the displacement element and
the current through the magnetic coil of the electromagnetic drive
as measured variables.
[0035] In a particularly preferred embodiment, the state-space
model does not have any further measured variables to be detected,
i.e. the model is developed such that it makes a prediction for the
immediately following motion of the thrust member solely on the
basis of the detected thrust member position and the detected
current through the magnetic coil.
[0036] In a preferred embodiment, the determined hydraulic
parameters are used.
[0037] By a state-space model is usually meant the physical
description of a present system state. For example, the state
variables can describe the energy content of the energy storage
elements contained in the system.
[0038] For example, a differential equation of the displacement
element can be used as model for the model-based control. For
example, the differential equation can be an equation of motion. By
an equation of motion is meant a mathematical equation which
describes the spatial and temporal motion of the displacement
element under the effect of external influences. In a preferred
embodiment, displacement pump-specific forces which act on the
thrust member are modeled in the equation of motion. Thus, for
instance, the force exerted on the thrust member by a spring, or
the spring constant k thereof, and/or the magnetic force exerted on
the thrust member by the magnetic drive can be modeled. The force
exerted on the thrust member by the pumped fluid can then be
treated as a disturbance variable. In a particularly preferred
embodiment, this disturbance variable can then likewise be modeled
using the determined hydraulic parameters.
[0039] By means of such a state-space model, when the measurement
variables are detected, a prediction for the immediately following
system behavior can be made.
[0040] If the immediately following behavior forecast in this way
deviates from the desired predetermined behavior, the system is
influenced in a corrective manner.
[0041] In order to calculate what a suitable exertion of influence
looks like, in the same model the influence of the available
manipulated variables on the controlled variable can be simulated.
Using known optimization methods, the control strategy which is
best at that time can then be selected adaptively. Alternatively,
it is also possible to determine a control strategy once on the
basis of the model and to then apply this in dependence on the
detected measured variables.
[0042] In a preferred embodiment, a nonlinear state-space model is
therefore chosen as state-space model and the nonlinear control
takes place either via control-Lyapunov functions, via
flatness-based control methods with flatness-based feedforward
control, via integrator backstepping methods, via sliding mode
methods or via predictive control. Nonlinear control via
control-Lyapunov functions is preferred.
[0043] All five methods are known from the field of mathematics and
are therefore not explained in more detail here.
[0044] Control-Lyapunov functions are, for example, a generalized
description of Lyapunov functions. Appropriately chosen
control-Lyapunov functions lead to a stable behavior in the
framework of the model.
[0045] In other words, a correction function is calculated which
leads to a stable solution for the model in the underlying
model.
[0046] In general, there are a large number of control
possibilities which lead to the difference between the actual
profile and the target profile in the underlying model becoming
smaller.
[0047] In a preferred embodiment, the model which forms the basis
of the model-based control is used to formulate an optimization
problem, in which, as a secondary condition for optimization, the
electric voltage in the electric motor and thus the energy fed to
the metering pump is as small as possible, but at the same time it
is achieved that the actual profile approaches the target profile
as quickly as possible and with as little overshooting as possible.
Moreover, it can be advantageous if the measured signals are
filtered using a low-pass filter before the processing in the
underlying model in order to reduce the influence of noise.
[0048] In a further particularly preferred embodiment it is
provided that during a suction-pressure cycle the difference
between the detected actual position profile of the displacement
element and a desired target position profile of the displacement
element is detected and a target position profile which corresponds
to the desired target position profile reduced by the difference is
used for the next suction-pressure cycle.
[0049] Essentially, a self-learning system is realized here.
Although the model-based control according to the invention has
already led to a significant improvement in the control behavior,
there can still be deviations between the target profile and the
actual profile. In particular, this cannot be avoided in the case
of energy-minimizing selection of the control intervention. In
order to reduce this deviation further at least for subsequent
cycles, the deviation during a cycle is detected and in the next
cycle the detected deviation is at least partially subtracted from
the desired target position profile.
[0050] In other words, a subsequent pressure-suction cycle is
deliberately provided with a "false" target value profile, wherein
the "false" target value profile is calculated from the knowledge
obtained in the preceding cycle. Namely, if in the subsequent
suction-pressure cycle there is exactly the same deviation between
actual and target profile as in the previous cycle, the use of the
"false" target value profile leads to the actual desired target
value profile being achieved as a result.
[0051] Although it is essentially possible, and also sufficient in
some applications because of the periodic behavior of the system,
to carry out the described self-learning steps only once, i.e. to
measure the difference in the first cycle and from the second and
in all further cycles to correct the target value profile
correspondingly, it is particularly preferred if the difference
between actual and target profile is determined at regular
intervals, best of all in every cycle, and is taken into account
correspondingly in the subsequent cycle.
[0052] Of course, it is also possible to only use a fraction of the
detected difference as profile correction for the subsequent cycle
or cycles. This can be advantageous in particular in the cases in
which the detected difference is very large, in order not to
generate instability in the system by the sudden change in target
value.
[0053] Furthermore, it is possible to determine the size of the
fraction of the detected difference, which is used as profile
correction, using the present difference between target and actual
profile.
[0054] It is also possible that the difference between actual and
target profile is measured over several cycles, e.g. 2, and from
this an average difference is calculated, which is then at least
partially subtracted from the target profile of the subsequent
cycles.
[0055] In a further alternative embodiment any of the functions
dependent on the detected difference can be used for correcting the
next target position profile.
[0056] In a further preferred embodiment, the modeling according to
the invention can be used in order to determine a physical variable
in the displacement pump. Thus, for example, the fluid pressure in
the metering chamber can be determined.
[0057] The equation of motion of the displacement element takes
into account all forces which act on the displacement element. In
addition to the force applied to the displacement element by the
drive, this is also the counterforce applied to the diaphragm and
thus to the displacement element by the fluid pressure in the
metering chamber.
[0058] Thus, if the force applied to the displacement element by
the drive is known, conclusions can be drawn from the position of
the displacement element, or from the speed or acceleration of the
displacement element which can be derived therefrom, about the
fluid pressure in the metering head.
[0059] For example, if the actual fluid pressure reaches or exceeds
a predetermined maximum value, a warning signal can be emitted and
the warning signal can be sent to an automatic shutoff, which shuts
off the metering pump in response to the warning signal being
received. Therefore, if for any reason a valve does not open or the
pressure on the pressure line increases sharply, this can be
ascertained by the method according to the invention without using
a pressure sensor and the pump can be shut off as a precaution.
Essentially, the displacement element, with the associated drive,
additionally takes over the function of the pressure sensor.
[0060] In a further preferred embodiment of the method, a target
fluid pressure curve, a target position curve of the displacement
element and/or the target current progression through the
electromagnetic drive is stored for a motion cycle of the
displacement element. The actual fluid pressure can be compared
with the target fluid pressure, the actual position of the
displacement element with the target position of the displacement
element and/or the actual current through the electromagnetic drive
with a target current through the electromagnetic drive and, if the
differences between actual and target value satisfy a predetermined
criterion, a warning signal can be emitted.
[0061] The idea that forms the basis of this method step is that
certain events, such as for example gas bubbles in the hydraulics
system or cavitation in the pump head bring about a recognizable
change in the fluid pressure to be expected, and conclusions can
therefore be drawn about the named events from the determination of
the fluid pressure.
[0062] The warning signal can, for example, activate an optical
indicator or an audible alarm. As an alternative or in combination
therewith, the warning signal can, however, also be directly made
available to a control unit, which takes the appropriate measures
in response to the warning signal being received.
[0063] In the most simple case, the difference of the actual and
target value is determined for one or more of the measured, or
determined, variables, and if one of the differences exceeds a
predetermined value a warning signal is emitted.
[0064] However, in order to not only detect the possible error
events, such as e.g. gas bubbles in the metering chamber or the
occurrence of cavitation, but also to distinguish between them, it
is possible to define an individual criterion for each error
event.
[0065] In a preferred embodiment, a weighted sum of the relative
deviations from the target value can be determined and the
criterion can be chosen such that a warning signal is emitted if
the weighted sum exceeds a predetermined value.
[0066] Different weighting coefficients can be assigned to the
different error events. In the ideal case, precisely one criterion
is satisfied when an error event occurs, with the result that the
error event can be diagnosed.
[0067] Using the described method it is therefore possible to
determine the pressure in the metering head without resorting to a
pressure sensor, and from the pressure determined in this way,
conclusions can be drawn about certain states in the metering head,
which can then in turn trigger the introduction of certain
measures.
[0068] With the method according to the invention, changes in
pressure can be determined very precisely.
[0069] In a further embodiment the temporal gradient of a measured
or determined variable is therefore ascertained and, if this
exceeds a predetermined threshold value, the valve opening or valve
closing is diagnosed.
[0070] In an alternative embodiment, the mass m of the displacement
element, the spring constant k of the spring which pretensions the
displacement element, the damping d and/or the electrical
resistance R.sub.Cu of the electromagnetic drive is determined as
physical variable.
[0071] In a particularly preferred embodiment, all of the named
variables are actually determined. This can take place, for
example, by a minimization calculation. All of the named variables,
with the exception of the pressure in the metering chamber,
represent constants which can be determined by experiment and which
as a rule do not change during the operation of the pump.
Nevertheless, symptoms of fatigue of the different elements can
occur which change the value of the constants. For example, the
measured pressure-path progression can be compared with an expected
pressure-path progression. The difference of both gradients
integrated over a cycle can be minimized by varying the constant
variables. If e.g. it is established that the spring constant has
changed, a defective spring can be diagnosed.
[0072] Such a minimization could also be carried out in the
unpressurized state, i.e. when there is no fluid in the metering
chamber.
[0073] Further advantages, features and application possibilities
of the present invention are made clear using the following
description of a preferred embodiment and the associated figures.
There are shown in:
[0074] FIG. 1 a schematic representation of the suction line
attached to the displacement pump,
[0075] FIGS. 2a-2e example of hydraulic parameters and the
time-dependent development thereof,
[0076] FIG. 3 a schematic representation of an ideal motion
profile,
[0077] FIG. 4 a schematic representation of the self-learning
function,
[0078] FIG. 5 a schematic representation of a pressure-path diagram
and a path-time diagram for the normal state and
[0079] FIG. 6 a schematic representation of a pressure-path diagram
and a path-time diagram for a state with gas bubbles in the
metering chamber.
[0080] Through the design of a physical model, in particular a
nonlinear system description of the hydraulic process in the
metering chamber, or in the line connected to the metering chamber,
of an electromagnetic metering pump system, it is possible to use
model-based identification methods in real time. For this, the
hydraulic parameters, i.e. the state variables of the hydraulic
models, are evaluated and the system dynamics and the parameters of
the hydraulic process are determined.
[0081] The position of the displacement element, or the speed and
acceleration of the displacement element determined therefrom, and
the pressure in the metering chamber which can be determined via
the force exerted on the pumped fluid by the diaphragm serve as
measured variables, or external variables to be determined.
[0082] Because, as a rule, in the named displacement pumps the
suction line consists of a hose which connects the suction valve to
a storage tank, for the suction stroke, i.e. while the pressure
valve is closed and the suction valve is open, the hydraulic system
can be described in a simplified manner, as is represented in FIG.
1. The suction line consists of a hose with the diameter D.sub.S
and the hose length L. The hose bridges a height difference Z.
[0083] The nonlinear Navier-Stokes equations can be simplified if
it is assumed that the suction line has a constant diameter and is
not expandable and that an incompressible fluid is used.
[0084] Using known optimization methods, such as e.g. the gradient
method or the Levenberg-Marquardt algorithms, the hydraulic
parameters are now determined which can best describe the measured
position of the thrust member and the measured or determined
pressure in the metering chamber using the constructed model as a
basis.
[0085] In the FIGS. 2a to 2e here, using the example of glycerol as
pumped fluid, in each case a hydraulic parameter (dotted line) as
well as the values resulting from the method according to the
invention (continuous line) are represented over time.
[0086] Thus, for example, FIG. 2a shows the density of the pumped
fluid. This is approximately 1260 kg/m.sup.3 (dotted line). It can
be seen that the method according to the invention is able to
determine the density within approximately 100 seconds. Although at
the time point where t=0 seconds the determined value is still
clearly below the actual value, the continuous optimization results
in the value for the density determined by the method according to
the invention approaching the true value very rapidly (continuous
line).
[0087] The same is true for the hose length L (see FIG. 2b), the
height difference Z (see FIG. 2c), the hose diameter (see FIG. 2d)
and the viscosity (see FIG. 2e).
[0088] The parameters determined by the method according to the
invention can then, in turn, be used together with the constructed
physical model to determine the force exerted on the thrust member
by the hydraulic system.
[0089] This information can be used for the control. In particular
when model-based nonlinear control strategies are used for the
control of the motion of the thrust member, the model developed
here can physically model the effect of the hydraulic system and
take this into account in the form of a disturbance variable
feedforward.
[0090] The method according to the invention has been developed in
connection with a magnetic metering pump. In a preferred
embodiment, such a magnetic metering pump has a movable thrust
member with a connecting rod firmly connected thereto. The thrust
member is mounted to be axially movable in the longitudinal axis in
a magnetic casing which is firmly anchored into the pump housing,
with the result that when the magnetic coil in the magnetic casing
is electrically triggered, the thrust member with connecting rod is
drawn into a hole of the magnetic casing against the action of a
pressure spring, and after the magnet is deactivated the thrust
member travels back into the starting position by means of the
pressure spring. The result of this is that when the magnetic coil
is continuously activated and deactivated, the thrust member and a
diaphragm actuated by this carries out an oscillating motion,
which, in the metering head arranged in the longitudinal axis, in
cooperation with an outlet and inlet valve leads to a pumping
stroke (pressure stroke) and an intake stroke (suction stroke). The
activation of the magnetic coil takes place by a voltage being
applied to the magnetic coil. The motion of the thrust member can
thus be established by the temporal progression of the voltage on
the magnetic coil.
[0091] It is understood that the pressure stroke and the suction
stroke do not necessarily have to last for the same amount of time.
On the contrary, as no metering occurs during the suction stroke,
but rather the metering chamber is merely filled again with pumped
fluid, it is advantageous to carry out the suction stroke as
quickly as possible in each case, wherein care is nevertheless to
be taken there is no cavitation in the pressure chamber.
[0092] On the other hand, the pressure stroke can last a very long
time, in particular in application cases in which only very small
fluid amounts are to be metered. This results in the thrust member
moving only gradually in the direction of the metering chamber. In
order to achieve a motion of the thrust member as is represented in
an idealized manner in FIG. 3, the motion of the thrust member must
be controlled. Only the position of the thrust member and the size
of the current through the magnetic coil are customarily available
as measured variables.
[0093] According to the invention, a (nonlinear) model is therefore
developed, which describes the state of the magnetic system.
[0094] The following model results for a preferred embodiment:
x . = [ x . x .PHI. . ] = [ x . 1 m ( - d x . - kx - F vor + F p +
K L ( .delta. ) .PHI. 2 ) 1 N 1 ( - R cu R m ges ( .delta. , .PHI.
) N 1 .PHI. + u ) ] ##EQU00001##
[0095] wherein
[0096] m: mass of the thrust member
[0097] .PHI.: magnetic flux
[0098] K.sub.L(.delta.).PHI..sup.2: magnetic force
[0099] N.sub.1: number of turns
[0100] u: voltage
[0101] d: damping
[0102] k: spring constant
[0103] F.sub.var: force on thrust member through spring
pretensioning
[0104] F.sub.P: force on thrust member through fluid pressure in
conveying chamber
[0105] R.sub.m.sub.gec(.delta., .PHI.): magnetic resistance
[0106] R.sub.Cu: ohmic resistance of the coil
[0107] x: position of the thrust member
[0108] .delta.: size of gap between anchor and magnet
[0109] This is a nonlinear differential equation system which
allows a prediction to be made about the immediately following
behavior of the system, starting from a starting point.
[0110] Using this model, it is therefore possible to identify
future or actually already existing deviations between the target
curve and the actual curve. In addition, the model can be used in
order to calculate the likely effect of a control intervention.
[0111] Because of the measurement of the strength of the current
and the position of the thrust member, it is determined in real
time how the system is likely to develop. It can additionally be
calculated through which control intervention, i.e. through which
changes in voltage on the magnetic coil, the system can be moved
back into the desired direction.
[0112] Of course, there are a large number of possibilities to
intervene in the system in terms of control. At every time point,
stable solutions for the dynamical system can thus be sought. This
calculation step is constantly, i.e. as frequently as the available
calculation power allows, repeated in order to obtain an optimal
control.
[0113] In the case of the model proposed here, it is generally not
necessary to determine new stable solutions for the dynamical
system at every time point. As a rule it is sufficient to determine
the suitable correction function in dependence on the measured
variables, i.e. in dependence on the position of the thrust member
and the voltage on the magnetic drive, once and to use this the
correction function from then on for the control.
[0114] Despite this control, there will inevitably be deviations
between the target and actual value, as the chosen model always
represents an idealization. In addition, the detected measured
variables always contain errors (noise).
[0115] In order to further reduce the difference between actual and
target profile, this difference is measured during a
pressure-suction cycle and the sum of the measured difference and
the desired target profile is used as target profile for the
subsequent cycle. In other words, the fact that the pressure-stroke
cycle repeats is utilized. In the subsequent cycle a target value
profile which differs from the actual desired target value profile
is thus specified.
[0116] This self-controlling principle is represented schematically
in FIG. 4 for the purpose of clarification. The position of the
thrust member is represented on the y-axis and the time is
represented on the x-axis.
[0117] In the first cycle, a target profile used for the control is
represented with a dashed line. This target profile corresponds to
the desired target profile which is modeled as a reference profile
for comparison in the third cycle. Despite the model-based control
according to the invention, the actual profile will deviate from
the target profile. In the first cycle of FIG. 4, an actual profile
is therefore represented by way of example with a continuous line.
For clarification, the deviations between actual and target profile
are represented in a more pronounced manner than they occur in
practice.
[0118] In the second cycle, the difference between the actual
profile of the first cycle and the reference profile is then
subtracted from the target profile used for the first cycle and the
difference is used as target profile for the control during the
second cycle. The thus-obtained target profile is represented
dashed in the second cycle.
[0119] Ideally, in the second cycle the actual profile deviates
from the target profile used to the same extent as was observed in
the first cycle. This results in an actual profile (drawn in with a
continuous line in the second cycle) which corresponds to the
reference profile.
[0120] By measuring the position of the thrust member and the
current through the magnetic drive, F.sub.P, i.e. the force on the
thrust member through the fluid pressure in the conveying chamber,
is the only unknown variable. Using this model, the force on the
thrust member through the fluid pressure in the conveying chamber
can therefore be determined. As the surface area of the thrust
member to which the fluid pressure is applied is known, the fluid
pressure can be calculated from the force.
[0121] Through the described design of a nonlinear system
description of the electromagnetic metering pump system, it is
possible to use model-based diagnosis methods. For this, the state
variables of the system models are evaluated and the pressure in
the pump head of the electromagnetic metering pump is determined.
The necessary current and position sensors here are already built
in the pump system for the purposes of control technology, with the
result that the information is already available without the
construction of the metering pump needing to be supplemented. Using
the temporal change in the state variables and the pressure in the
metering head of the pump, the diagnosis algorithms can then be
performed.
[0122] Thus, for example, the model-based diagnosis of excess
pressure in the process and the automated pump switch-off can be
realized.
[0123] The valve opening and valve closing time points can, for
example, be identified via the determination and evaluation of
temporal gradients of coupled state variables of the system model.
It can be detected when the state gradients overshoot or fall short
by means of predetermined limits, which leads to the identification
of the valve opening and valve closing time points.
[0124] As an alternative, the pressure can also be determined in
dependence on the position of the thrust member and the valve
opening and valve closing time points can be derived from an
evaluation. A corresponding pressure-path diagram is represented on
the left in FIG. 5. The associated path-time diagram is represented
on the right in FIG. 5. The path-time diagram shows the
time-dependent motion of the thrust member. It can be seen that the
thrust member first moves forwards from a starting position 1 (x=0
mm) and the volume of the metering chamber decreases (pressure
phase). At time point 3, the thrust member passes through a maximum
and then moves back into the starting position (suction phase).
[0125] The corresponding pressure-path diagram is shown on the left
in FIG. 5. It will proceed in the clockwise direction, starting at
the coordinate origin at which the thrust member is located in the
position 1. During the pressure phase, the pressure in the metering
chamber will first increase sharply until the pressure is able to
open the valve to the pressure line. Once the pressure valve is
open, the pressure in the metering chamber remains essentially
constant. The opening point is indicated with the reference number
2. From this time point, which is also recorded on the right in
FIG. 5, a metering takes place. With each further motion of the
thrust member, metering fluid is pumped into the pressure line.
Once the thrust member has reached the maximum position (time point
3), the motion of the thrust member reverses, the pressure valve
closes immediately and the pressure in the metering chamber drops
again. Once a minimum pressure is reached (time point 4), the
suction valve which connects the metering chamber to the suction
line opens, and metering fluid is sucked into the metering chamber
until the starting position is reached again.
[0126] The valve closing time points can be determined from the
path-time diagram, as they lie at the displacement maxima of the
thrust member. The time points 2 and 4, i.e. the valve opening time
points are not as easy to determine, especially as in practice the
pressure-path diagram has rounded "corners". Starting from position
1 in the pressure-path diagram, for example, when 90% of the
pressure maximum is reached (known from position 3), the path can
therefore be read off and the increase of the pressure-path diagram
between points 1 and 2 can be determined. The 90% curve is drawn in
dotted. The straight line resulting from this intersects the curve
p=p.sub.max at the valve opening time point. The time point 4 can
also be determined in the same manner. This determination can take
place in each cycle and the result used for a later cycle. Changes
in the opening time points can thereby also be detected.
[0127] By a comparison of the target and actual trajectories of the
individual state variables of the system models, gas bubbles in the
hydraulics system, cavitation in the pump head of the metering unit
and/or valve opening and valve closing time points of the metering
units can be diagnosed. In particular when a predetermined error
limit is exceeded between the target and actual trajectories, this
can trigger a warning signal and corresponding measures.
[0128] An example is shown in FIG. 6. Here too, the pressure-path
diagram is represented on the left and the path-time diagram is
represented on the right. The right-hand figure is identical to the
corresponding diagram of FIG. 5. If there are gas bubbles in the
hydraulics system which are compressible, that will lead to the
pressure valve only opening at time point 2' and the suction valve
only opening at time point 4'. A clear shift in the valve opening
time points can thus be used to diagnose the state "air in the
metering chamber". In the case of cavitation, only the valve
opening time point 4' shifts and not the valve opening time point
2, with the result that such a behavior can be used to diagnose the
state "cavitation".
[0129] Through the analysis of the individual coupled system state
variables, the model-based methodology presented enables an
essentially more comprehensive and more valuable diagnosis than has
been realized to date.
[0130] Moreover, this can be realized with low costs in terms of
sensors and high reliability and dependability. Through the higher
diagnosis quality, the field of use of electromagnetic metering
pump systems can possibly be expanded, as the metering accuracy can
now be vastly improved.
* * * * *