U.S. patent application number 10/782262 was filed with the patent office on 2005-08-18 for mean pressure estimation for compressible fluid strut.
This patent application is currently assigned to Visteon Global Technologies, Inc.. Invention is credited to Edmondson, Jeremy Richard, Osorio, Carlos Fernando, Song, Xubin.
Application Number | 20050179185 10/782262 |
Document ID | / |
Family ID | 34838798 |
Filed Date | 2005-08-18 |
United States Patent
Application |
20050179185 |
Kind Code |
A1 |
Song, Xubin ; et
al. |
August 18, 2005 |
MEAN PRESSURE ESTIMATION FOR COMPRESSIBLE FLUID STRUT
Abstract
A method and apparatus is provided for estimating the mean
pressure in a compressible fluid strut. A database is employed
containing values for mean pressure variation corresponding to a
specific combination of motor speed and flow demand, and may also
account for strut temperature. The flow demand and the speed of the
motor are determined, and the mean variation corresponding to the
determined combination of motor speed and flow demand is selected.
The estimation of strut mean pressure is updated with the selected
mean pressure variation. In this way, costly pressure sensors are
eliminated as well as the complicated control algorithms which are
used therewith.
Inventors: |
Song, Xubin; (Canton,
MI) ; Osorio, Carlos Fernando; (Whitmore Lake,
MI) ; Edmondson, Jeremy Richard; (Canton,
MI) |
Correspondence
Address: |
VISTEON
C/O BRINKS HOFER GILSON & LIONE
PO BOX 10395
CHICAGO
IL
60610
US
|
Assignee: |
Visteon Global Technologies,
Inc.
|
Family ID: |
34838798 |
Appl. No.: |
10/782262 |
Filed: |
February 18, 2004 |
Current U.S.
Class: |
267/218 |
Current CPC
Class: |
B60G 2500/205 20130101;
B60G 17/0155 20130101; B60G 17/0528 20130101 |
Class at
Publication: |
267/218 |
International
Class: |
F16F 009/14 |
Claims
1. A method of estimating the mean pressure in a compressible fluid
strut, the strut forming a portion of an active suspension system
for a vehicle, the active suspension system including a motor
having a crankshaft driving a cylinder, the cylinder being
responsive to flow demands to deliver or remove fluid in the strut,
the method comprising the steps of: providing a database of values
for mean pressure variation corresponding to a specific combination
of motor speed and flow demand determining the flow demand;
determining a speed of the motor; selecting the mean pressure
variation corresponding to the determined combination of motor
speed and flow demand; updating the estimation of strut mean
pressure with the selected mean pressure variation.
2. The method of claim 1, further comprising the step of
determining the period of the mean pressure variation based on the
motor speed.
3. The method of claim 2, further comprising the step of
determining a mean pressure rate based on the mean pressure
variation and the period.
4. The method of claim 3, wherein the mean pressure rate equals the
mean pressure variation divided by the period.
5. The method of claim 3, wherein the updating step includes
updating the estimation of strut mean pressure with the mean
pressure rate over a length of time equal to the period.
6. The method of claim 3, wherein the estimation of strut mean
pressure is updated according to the equation
SMP.sub.c=SMP.sub.p+MPR, where SMP.sub.c is current strut mean
pressure, SMP.sub.p is prior strut mean pressure and MPR is mean
pressure rate.
7. The method of claim 3, wherein the estimation of strut mean
pressure is updated according to the equation
SMP.sub.c=SMP.sub.p+.lambda.*MPR, where SMP.sub.c is current strut
mean pressure, SMP.sub.p is prior strut mean pressure, .lambda. is
the efficiency of the motor, and MPR is mean pressure rate.
8. The method of claim 3, wherein the estimation of strut mean
pressure is updated according to the equation
SMP.sub.c=SMP.sub.p+.lambda.*MPR*(1+a) for the first half of the
period, and the equation SMP.sub.c=SMP.sub.p+.lambda.*MPR*(1-a) for
the second half of the period, where SMP.sub.c is current strut
mean pressure, SMP.sub.p is prior strut mean pressure, .lambda. is
the efficiency of the motor, MPR is mean pressure rate, and a is a
predetermined constant to allow adjustment of the estimation.
9. The method of claim 1, wherein the database includes values for
mean pressure variation corresponding to a specific combination of
motor speed, temperature and flow demand, and further comprising
the step of determining a temperature, and wherein the selecting
step includes selecting the mean pressure variation corresponding
to the determined combination of motor speed, temperature and flow
demand.
10. The method of claim 1, wherein the updating step includes
determining a time delay and delaying the update for the time
delay.
11. The method of claim 1, the active suspension system including
two cylinders being responsive to flow demands to deliver or remove
fluid in the strut, and wherein the flow demand is determined for
each of the two cylinders.
12. The method of claim 11, wherein the selecting step includes
selecting the mean pressure variation corresponding to the
determined combination of motor speed and the two flow demands.
13. An active suspension system for a vehicle comprising: a motor
having a crankshaft a cylinder driven by the crankshaft, the
cylinder having high pressure and low pressure valves, a
compressible fluid strut fluidically connected to the cylinder for
increasing or decreasing the pressure in the strut; a vehicle
dynamics controller generating a requested pressure for the strut;
a device control for regulating the pressure in the strut, the
device control including a valve controller, a mean pressure
estimator and a flow demand creator, the valve controller
regulating the high and low pressure valves of the cylinder, the
mean pressure estimator providing an estimation of the mean
pressure in the strut, the flow demand creator sending flow demand
signals to the valve controller based on the difference between the
requested pressure and the estimation of current mean pressure; and
wherein the mean pressure estimator receives data on the speed of
the motor and the flow demand signals, determines a mean pressure
variation corresponding to the motor speed and flow demand, and
updates the estimation of strut mean pressure with the mean
pressure variation.
14. The active suspension system of claim 13, further comprising a
database having mean pressure variation values corresponding to
specific combinations of motor speed and flow demand.
15. The active suspension system of claim 13, further comprising a
database having mean pressure variation values corresponding to
specific combinations of motor speed, temperature and flow
demand.
16. The active suspension system of claim 15, wherein the mean
pressure estimator receives data on the temperature, and wherein
the mean pressure estimator determines a mean pressure variation
corresponding to the motor speed, temperature and flow demand.
17. The active suspension system of claim 13, wherein the mean
pressure estimator determines the period of the mean pressure
variation based on the motor speed.
18. The active suspension system of claim 17, wherein the mean
pressure estimator determines a mean pressure rate based on the
mean pressure variation divided by the period.
19. The active suspension system of claim 18, wherein the mean
pressure estimator updates the estimation of strut mean pressure
with the mean pressure rate over a length of time equal to the
period.
20. The active suspension system of claim 18, wherein the
estimation of strut mean pressure is updated according to the
equation SMP.sub.c=SMP.sub.p+.lambda.*MPR, where SMP.sub.c is
current strut mean pressure, SMP.sub.p is prior strut mean
pressure, .lambda. is the efficiency of the motor, and MPR is mean
pressure rate.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a method and apparatus for
estimating the mean pressure in a compressible fluid strut, forming
a portion of an active suspension system in a motor vehicle.
BACKGROUND OF THE INVENTION
[0002] An active suspension system for a motor vehicle utilizes
actuable struts at each wheel of the vehicle whereby the pressure
within the struts may be controlled to actively regulate the
damping and spring effect of the suspension system. One key
component of such an active suspension system is a pressure
detector or sensor that provides a reading of the strut mean
pressure for each strut. As used herein, the strut mean pressure is
the static pressure variation a strut can have after executing flow
demands.
[0003] Typically, a high level vehicle dynamics controller creates
a desired pressure for a particular strut, and based on a
comparison between the detected strut mean pressure and the desired
pressure, an actuator increases or decreases the pressure within
the strut to meet the desired pressure level. It can therefore be
seen that the pressure sensor is a very important component of the
active suspension system.
[0004] When a strut is exposed to payload, vibration and the
execution of flow demands, the strut pressure is composed of
payload-dependent pressure (i.e. precharged pressure),
vibration-dependent pressure, and pulsation-dependent pressure.
Additionally, the strut load also includes friction due to
vibration. Unfortunately, all of these pressure components are not
desirable from the standpoint of controlling the pressure within
the strut. Specifically, if a pressure sensor is used, the control
algorithm needs to include a complicated estimation algorithm to
figure out the achieved controllable pressure when a flow demand is
executed. The complicated estimation algorithm must factor out
certain pressure components such as those previously mentioned.
[0005] Accordingly, there exist a need to provide a method and
apparatus for estimating the strut mean pressure in a strut forming
a portion of an active suspension system, the method and apparatus
eliminating the need for a costly pressure sensor and the
complicated estimation algorithm which are required to determine
the achieved controllable pressure.
BRIEF SUMMARY OF THE INVENTION
[0006] The present invention provides a method and apparatus for
estimating the mean pressure in a compressible fluid strut without
the use of a pressure sensor or complicated estimation algorithms.
The strut forms a portion of an active suspension system for a
vehicle, the system further including a motor having a crankshaft
driving a cylinder, the cylinder being responsive to flow demands
to deliver or remove fluid from the strut.
[0007] One embodiment of the method includes the steps of providing
a database of values for mean pressure variation corresponding to a
specific combination of motor speed and flow demand. The flow
demand and the speed of the motor are determined, and the mean
variation corresponding to the determined combination of motor
speed and flow demand is selected. The estimation of strut mean
pressure is updated with the selected mean pressure variation. In
this way, costly pressure sensors are eliminated as well as the
complicated control algorithms which are used therewith.
[0008] According to more detailed aspects, the method further
includes the step of determining the period of the mean pressure
variation based on the motor speed. A mean pressure rate may then
be determined based on the mean pressure variation and the period.
The mean pressure rate equals the mean pressure variation divided
by the period. The updating step preferably includes updating the
estimation of strut mean pressure with a mean pressure rate over a
length of time equal to the period. The estimation of strut mean
pressure is preferably updated according to the equation
SMP.sub.c=SMP.sub.p+.lambda.*MPR where SMP.sub.c is current
estimated strut mean pressure SMP.sub.p is prior estimated strut
mean pressure, .lambda. is the efficiency of the motor (including
electric and hydraulic sub-systems), and MPR is mean pressure rate.
The quantity expressed by .lambda.*MPR may also be adjusted by a
factor (1+a) for the first half of the period and the factor (1-a)
for the second half of the period.
[0009] The method also preferably adjusts for temperature variation
of the strut. That is, the database may include values for mean
pressure variation corresponding to a specific combination of motor
speed, temperature and flow demand. Further, the updating step may
be delayed by a period of time corresponding to the travel time of
fluid flow from the cylinder to the strut.
[0010] An active suspension system constructed in accordance with
an embodiment of the present invention includes a motor, a cylinder
and a compressible fluid strut. The motor has a crankshaft and the
cylinder is driven by the crankshaft. The cylinder has high
pressure and low pressure valves for supplying and removing fluid
from the strut. The strut is fluidically connected to the cylinder
for increasing or decreasing the pressure in the strut. A vehicle
dynamics controller generates a requested pressure for the strut. A
device control is provided for regulating the pressure in the
strut. The device control includes a valve controller, a mean
pressure estimator, and flow demand creator. The valve controller
regulates the high and low pressure valves of the cylinder. The
mean pressure estimator provides an estimation of the mean pressure
in the strut. The flow demand creator sends flow demand signals to
the valve controller based on the difference between the requested
pressure and the estimation of current mean pressure. The mean
pressure estimator receives data on the speed of the motor and the
flow demand signals, and based thereon determines a mean pressure
variation. The estimation of strut mean pressure is updated with
the mean pressure variation.
[0011] According to more detailed aspects, a database is provided
having mean pressure variation values corresponding to specific
combinations of motor speed and flow demand. When temperature of
the strut is accounted for, the database has mean pressure
variation values corresponding to specific combinations of motor
speed, temperature and flow demand. The mean pressure estimator
determines the period of the mean pressure variation based on the
motor speed. Then, the mean pressure estimator determines a mean
pressure rate based on the mean pressure variation divided by the
period.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The accompanying drawings incorporated in and forming a part
of the specification illustrate several aspects of the present
invention, and together with the description serve to explain the
principles of the invention. In the drawings:
[0013] FIG. 1 is a schematic illustration of an embodiment of an
active suspension system constructed in accordance with the
teachings of the present invention;
[0014] FIG. 2 is a schematic diagram showing a device controller
forming a portion of the active suspension system which is in
communication with the vehicle dynamics controller;
[0015] FIG. 3 is a schematic flow diagram showing an algorithm for
updating the estimation of strut mean pressure in accordance with
the teachings of the present invention;
[0016] FIG. 4 is graph showing a comparison of the pressure as
detected through a pressure sensor versus the estimation generated
in accordance with the present invention; and
[0017] FIG. 5 is also a graph showing a comparison of the pressure
as detected through a pressure sensor versus the estimation
generated in accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0018] Turning now to the figures, FIG. 1 depicts a schematic
illustration of an active suspension system 20 constructed in
accordance with the teachings of the present invention. The active
suspension system 20 includes, among other components not listed or
shown here, a motor 22 driving a cylinder 28, which in turn
supplies and returns pressurized fluid to a compressible fluid
strut 40. The motor 22 is preferably a digital displacement pump
motor which allows execution of discrete flow demands. The motor 22
includes a shaft 24 which in turn drives a crankshaft that 26
translates the rotational motion of the motor 22 and shaft 24 into
an axial motion for driving the cylinder 28.
[0019] The cylinder 28 generally includes a piston rod 30 connected
to a piston 32. The piston rod 30 is driven by the crankshaft 26,
and the piston 32 reciprocates within the cylinder 28 to pressurize
fluid contained therein. The cylinder 28 includes a high pressure
valve 34 and a low pressure valve 36. The high pressure valve 34 is
fluidically connected via a conduit 38 to the compressible fluid
strut 40. The low pressure valve 36 is fluidically connected to an
accumulator 50 via a conduit 48. The accumulator 50 is utilized to
store a charge of fluid which may be provided to the strut 40, or
alternatively which may have been removed from the strut 40. The
strut 40 generally includes a cylinder 42 having a piston 44 fitted
therein to divide the cylinder 42 into upper and lower portions
which are filled with a fluid 46 such as a composition of liquid
silicone as is known in the art. It will be recognized that
numerous fluid mediums 46 may be utilized in conjunction with the
present invention.
[0020] Turning to FIG. 2, the cylinder 28 and its valves 34, 36 are
regulated by a low level device controller 60 in order to supply or
remove fluid to or from the strut 40. The device controller 60
generally includes a flow demand creator 62, a valve controller 66,
and a mean pressure estimator 70. The valve controller 66 is the
actuator responsible for controlling the valves 34, 36 of the
cylinder 28, and hence the flow of fluid to or from the strut 40.
The valve controller 66 receives a command 64 from the flow demand
creator 62 which opens or closes the valves 34, 36 in order to
achieve the desired pressure within the strut 40.
[0021] The vehicle dynamics controller. 56 sends a signal 58 to the
device controller 60 that is indicative of a desired or requested
pressure in the strut 40. The mean pressure estimator 70 outputs a
signal 72 indicative of the current estimated mean pressure in the
strut 40 which is compared to the requested pressure 58 at
subtractor 74. Based on the difference between the requested
pressure 58 and the current estimated mean pressure 72, the flow
demand creator 62 generates a signal 64 which is used by the valve
controller 66 to operate the valves 34, 36 of the cylinder 28 to
adjust the pressure within the strut 40. In this way, the device
controller 60 makes the actuation system a smart actuator for
active suspension control.
[0022] It can be seen in FIG. 2 that the mean pressure estimator 70
also receives the signal 64 from the flow demand creator 62. Using
this data 64, as well as other data such as the speed of the motor
22 and the temperature of the strut 40, the mean pressure estimator
70 utilizes a database 76 having stored values of mean pressure
variation 78 corresponding to the particular combination of flow
demand, motor speed and temperature. Using the mean pressure
variation 78 from the database 76, the mean pressure estimator 70
updates the current estimation of mean pressure 72 for continued
use by the device controller 60.
[0023] The process or algorithm 80 employed by the mean pressure
estimator 70 will now be described in detail with reference to
FIGS. 3-5. The algorithm 80 used by the mean pressure estimator 70
receives several pieces of information including the flow demand 64
as previously discussed. The algorithm 80 also receives information
on motor speed 82, strut temperature 84 and the shaft trigonometry
86 which is representative of the positioning of the crankshaft 26
in thus the cylinder 28.
[0024] Generally, there are five flow demands to control each
cylinder 28. The five flow demands are full pumping (FP), partial
pumping (PP), partial motoring (PM), full motoring (FM), and idle.
Each one of these flow demands represents a particular combination
of high pressure valve 34 position and low pressure valve 36
position. Pumping generally refers to providing pressurized fluid
to the strut 40, while motoring generally refers to removing
pressurized fluid from the strut 40, thus driving the motor 22 as a
generator. Each strut 40 generally includes two cylinders 28 linked
thereto. Accordingly, there are 14 combined flow demands available
for each strut.
[0025] The database 76 may be constructed by testing a particular
vehicle by setting up the compressible fluid strut 40 and the
active suspension system 20 to represent an on vehicle
installation. The motor 22 is then run at a certain nominal speed
that is specified for production system requirements. During the
testing, a series of FP, PP, PM, FM, IDLE or their combination are
sent to the device controller 60, and in particular the valve
controller 66. At the same time, the motor speed, strut pressure,
shaft trigonometry and strut temperature are monitored to provide
collected testing data which characterizes the variation of the
strut mean pressure for each one command corresponding to the
different flow demands. For example, at a certain motor speed and
temperature, the least mean square method can be applied to
determine the mean pressure variation with respect to a single flow
demand (FP, PP, PM, and FM).
[0026] With reference to FIG. 3, the algorithm 80 utilizes the data
on motor speed 82 to determine the period (T, ms) related to the
mean pressure variation, since the motor speed 82 can be changed
according to the flow demand for each cylinder 28. The algorithm 80
utilizes the database 76 to look up the mean pressure variation 78
corresponding to the particular combination of motor speed 82,
strut temperature 84 and the flow demand 64. The algorithm
determines the period T as indicated by block 88. As indicated at
block 90, the mean pressure rate is determined according to the
equation:
MPR=MPV/T (1)
[0027] where MPR is mean pressure rate, MPV is mean pressure
variation and T is the period.
[0028] When the flow demand is IDLE, the mean pressure rate=0. The
mean pressure rate is computed for each cylinder 28 and each flow
demand thereon in order to update the strut mean pressure (SMP). As
indicated at step 92, the strut mean pressure is updated every
millisecond for a length of time equal to the period T according to
the equation:
SMP.sub.c=SMP.sub.p+.lambda.*MPR. (2)
[0029] where SMP.sub.c is the current strut mean pressure and
SMP.sub.p is the prior strut mean pressure.
[0030] The .lambda. represents a variable which is set to
approximate the efficiency of the digital displacement pump motor
22 (including the combined electric and hydraulic sub-systems), and
hence .lambda. usually falls between 0.9 and 1.1. In most cases,
.lambda.=1. The efficiency for different flow demand combinations
can be decided by using the testing data through an optimization
process to reduce the estimation error.
[0031] A time delay is calculated as indicated at block 94, the
time delay being predetermined to represent the travel time of the
flow demand execution through the pipe lines from the motor 22 to
the compressible fluid strut 40. Finally, the algorithm 80 sends a
current estimate 96 of the strut mean pressure, which is utilized
by the mean pressure estimator 70 and the device controller 60 in
order to generate future flow demands as previously discussed with
reference to FIG. 2.
[0032] In accordance with another embodiment of the present
invention, the strut mean pressure can be updated according to the
following equations:
SMP.sub.c=SMP.sub.p+.lambda.*MPR*(1+a). (3)
SMP.sub.c=SMP.sub.p+.lambda.*MPR*(1-a). (4)
[0033] In this case, a is a value between 0 and 1, in the mean
pressure estimator 70 will utilize equation 3 for the first half of
the period (T), and then use the equation 4 for the second half of
the period (T). Accordingly, based on the testing data, the
equations for determining the strut mean pressure may be adjusted
between the first half of the period and the second half of the
period to more accurately reflect the change in pressure within the
compressible fluid strut 40.
[0034] FIG. 4 depicts a graph showing the change in pressure (shown
on the Y axis) over time (shown on X axis). The first line 100
represents actual testing data that was directly detected for a
single strut 40 being controlled by a first cylinder having a flow
demand of full pumping (FP) and a flow demand of IDLE for the other
cylinder 28. The second line 102 represents the current estimation
of strut mean pressure (SMP.sub.c) estimated by the device
controller 60 and the mean pressure estimator 70 as previously
discussed. It can be seen that the estimation of mean pressure in
accordance with the present invention eliminates much of the
undesired fluctuations in the detected pressure 100.
[0035] Similarly, FIG. 5 depicts a graph of pressure versus time
for one cylinder having a flow demand of full motoring (FM) and the
other cylinder having a flow demand of partial pumping (PP). As in
the previous figure, the line 104 represents the testing data,
While line 106 represents the data generated from the device
controller 60 and the mean pressure estimator 70 of the present
invention.
[0036] Accordingly, the present invention provides a method to
continuously update the mean pressure in a compressible fluid strut
in correspondence with the flow demands executed by a digital
displacement pump motor. The present invention excludes pressure
sensors in the pulsations induced by executing the discrete flow
demands. In the estimation, motor speed and strut temperature are
included to improve the accuracy in all environments and operating
conditions.
[0037] The foregoing description of various embodiments of the
invention has been presented for purposes of illustration and
description. It is not intended to be exhaustive or to limit the
invention to the precise embodiments disclosed. Numerous
modifications or variations are possible in light of the above
teachings. The embodiments discussed were chosen and described to
provide the best illustration of the principles of the invention
and its practical application to thereby enable one of ordinary
skill in the art to utilize the invention in various embodiments
and with various modifications as are suited to the particular use
contemplated. All such modifications and variations are within the
scope of the invention as determined by the appended claims when
interpreted in accordance with the breadth to which they are
fairly, legally, and equitably entitled.
* * * * *