GB2372794A

GB2372794A - An actively controlled fluid damper utilising a magnetorheological fluid

Info

Publication number: GB2372794A
Application number: GB0102679A
Authority: GB
Inventors: Patrick Lowe
Original assignee: Trysome Ltd
Current assignee: Trysome Ltd
Priority date: 2001-02-02
Filing date: 2001-02-02
Publication date: 2002-09-04
Anticipated expiration: 2021-02-02
Also published as: GB0102679D0; GB2372794B

Abstract

A damper system comprising: a damper having a cylinder 50 defining a chamber 52; a magnetorheological fluid located in the chamber; a piston 56 moveable within the chamber against resistance of the fluid; and an electromagnet 64 capable of applying a magnetic field across the fluid to alter the viscosity of the fluid and thereby the response of the damper. Position 80 and load 81 sensors, respectively produce analogue position 82 and load 83 output signals. The load output signal is converted to a digital signal by A-D converter 108, and the position output signal is differentiated by analogue differentiation unit 109 into an analogue signal representative of the velocity of the piston and cylinder and is then converted into a digital signal by A-D converter 110. These digital signals are then processed by the digital micro-controller 111 which generates a digital control signal 105 which is then converted to an analogue signal by D-A converter 107 to produce analogue control signal 106 which controls the response of the damper.

Description

ADJUSTABLE DAMPERS This invention relates to adjustable dampers, for example for vehicle suspension arrangements.

Figure 1 shows a typical damper usable in a vehicle suspension. The damper comprises a cylinder 1 enclosing a chamber 2 of fluid. A piston having a head 3 and a piston rod 4 can slide longitudinally in the chamber 2. Passages 5 through the piston head allow fluid to pass against resistance from one side of the piston head to the other. Fixing points 6 and 7 for the vehicle's body and the vehicle's wheel structure are provided on the cylinder and the piston rod. A chamber 7 in the cylinder contains a gas and is separated from the chamber 2 by a diaphragm 9.

The force required to move the piston in the cylinder is dependant on the relative velocity of the piston and the cylinder. Figure 2 illustrates a typical force/velocity response curve for a vehicle damper. In figure 2 the curve 10 shows the force required to move the piston in the bump (compression) direction. Curve 11 shows the force required to move the piston in the rebound (extension) direction.

In designing vehicle suspension the response of the damper is engineered to approach a desired force/velocity response. In a damper of the type shown in figure 1 this is generally done by means of mechanical adjustments to valves 8 set across the orifices 5. However, this arrangement has a number of disadvantages: 1. Since the valves are located inside the sealed chamber 2 it is difficult to arrange for mechanical adjustment of the valves.

2. Access to the damper is often hindered by other elements of the vehicle suspension, so it is difficult for an engineer to gain access to the damper to adjust it. For this reason, the damper cannot in normal circumstances be adjusted whilst the vehicle is running. 3. The valves only allow a finite number of adjustments to be made to the damper, so only an approximation to the desired response can normally be achieved; and it is difficult to determine the correct set of valve adjustments to be made.

4. It is normally desired to have different forms of response in bump and rebound and one-way valving therefore has to be provided and adjusted separately for each direction of response.

5. The response of the damper is additionally dependant on factors such as temperature, which alters the viscosity of the fluid in the chamber 2. Since it is difficult to adjust the damper, viscosity changes cannot easily be compensated for, and the characteristics of the damper are therefore likely to vary during use.

These problems are especially acute in suspension dampers for motor racing, where it is often necessary to make rapid adjustments to dampers in an attempt to achieve an ideal car set-up.

Figure 3 shows another form of damper that has been proposed. The damper of figure 3 is a magnetorheological damper. The fluid in chamber 20 contains particles of a magnetically susceptible material such as iron. The piston head 21 contains a coil 22 which can be caused to induce a magnetic field by application of an alternating current electrical signal from drive unit 23 through wires 24,25 that lead through the piston rod 26. The field from coil 22 extends across orifices 27 through the piston head 21. When a field is applied by the coil the arrangement of the particles in the fluid changes and the viscosity of the fluid in the region of the field (including in the orifices 27) increases.

Figure 4 shows typical force/velocity response curves for the bump direction for a damper of the type shown in figure 3 at a range of applied voltages across the coil 22. The applied voltage can be selected by selector switch 28 in figure 3. By selecting one of the applied voltages a desired one of the response curves can be chosen. This provides a convenient way of selecting the response of the damper from one of the available curves. However, other response curves are not available unless mechanical adjustments of the orifice dimensions are also made, for example by means of additional mechanically adjustable valves set across the orifices 27, this arrangement does not allow for precise variation of the form of the response curve in order to achieve a desired set-up.

US 5,652, 704 describes a damper system in which a magnetorheological damper of the type shown in figure 3 is controlled by a microprocessor in order to provide more scope for adjustment. The displacement of the piston of the damper is sensed and input to the microprocessor. The software then periodically calculates a control signal for the damper by a process that includes subjecting the displacement to a rate control filter in software to estimate the rate of change of displacement. In a high performance damper system it is desirable to calculate a control signal for the damper at very short intervals, for example every 2ms or less. To do this a high level of computing power is needed in the microprocessor.

It would be desirable to reduce the amount of computing power needed by the microprocessor, so as to reduce its cost, size and weight and to lessen the risk of faults.

According to the present invention there is provided a damper system comprising: a damper having: a cylinder defining a chamber, a fluid located in the chamber whose viscosity is dependant on magnetic field across it, a piston moveable within the chamber against resistance of the fluid, and an electromagnet capable of applying a magnetic field across the fluid to alter its viscosity and thereby alter the response of the damper; a sensor for sensing the relative position of the cylinder and the piston and generating a analogue position signal representative thereof; analogue differentiation circuitry for differentiating the position signal to generate an analogue velocity signal representative of the relative velocity of the cylinder and the piston; and a digital processing unit responsive to an input signal derived from the analogue velocity signal for generating a control signal for the electromagnet in order to control the response of the damper. Suitably, the input signal is a digital signal. The damper system then preferably comprises an analogue-to-digital converter for generating the input signal in dependence on the analogue velocity signal. The sensor may suitably be a potentiometer.

The digital processing unit is suitably arranged to periodically perform processing of the input signal derived from the analogue velocity signal to generate the control signal. The interval between successive cycles is suitably less than 5ms, and preferably less than 3ms.

The digital processing unit may be operable to control the response of the damper in response to the input signal to cause the damper to exhibit a selected damping force versus velocity profile. The system may comprise a memory connected to the digital processing unit, the memory storing at least one desired damping force versus velocity profile. Most preferably, the memory unit stores a plurality of such profiles and the system is equipped with means for selecting one of those profiles for use.

Preferably the digital processing unit does not receive a signal directly representative of the relative position of the piston and the cylinder.

The system may comprise a sensor for sensing the load between the cylinder and the piston and generating a load signal representative thereof. The digital processing unit may then be responsive to an input signal derived from the load signal for generating the control signal for the electromagnet in order to control the response of the damper.

The piston suitably comprises a piston head located in the chamber and a piston rod fast with the head and extending out of the chamber, and the damper comprises a first attachment point fast with the cylinder and a second attachment point fast with the piston rod, and the piston is constrained to move linearly relative to the cylinder. According to a second aspect of the invention there is provided a vehicle comprising a damper system as set out above. Suitably one of the mounting points is attached to the body of the vehicle and the other of the mounting points is attached to move with a wheel of the vehicle. There may suitably be a spring means connected between the body of the vehicle and the wheel.

The present invention will now be described by way of example with reference to the accompanying drawings, in which: figure 1 shows a conventional fluid damper; figure 2 shows an example of a response curve for a damper of the type shown in figure 1; figure 3 shows a magnetorheological damper arrangement; figure 4 shows examples of response curves for a damper of the type shown in figure 3; figure 5 shows a magnetorheological damper; figure 6 shows response curves of the damper of figure 5; figure 7 illustrates a control process for the damper of figure 5; figure 8 illustrates a damper of the type shown in figure 5 installed in a vehicle; and figure 9 illustrates apparatus for implementing the control process of figure 7 for a pair of vehicle dampers.

Figure 5 shows a damper suitable for use in accordance with the present invention. The damper of figure 5 is a magnetorheological damper and comprises a cylinder 50 enclosing a space which is divided into two chambers 51,52 by a separating piston 53. Chamber 51 is filled with a gas, which can be pressurised by means of valve 54. Chamber 52 is filled with a magnetorheological fluid, whose viscosity is dependant on the magnetic field across it. The fluid may be a suspension of particles of a magnetically susceptible material in oil.

A piston is slidably moveable relative to the cylinder. The piston comprises a piston head 56 located in chamber 52 and a piston rod 59 which is fixed to the piston head and passes through a hole in an end cover 60 of the cylinder. The end cover has seals 61 which bear against the piston rod 59 to prevent the magnetorheological fluid from escaping from the chamber 52. The piston head has seals 62 which bear against the inner surface of the cylinder to prevent the flow of magnetorheological fluid around the outside of the piston head from one side of the piston head to the other. The piston head has fluid passages 63 passing through it from one side of the piston head to the other by means of which the magnetorheological fluid can communicate between the regions of the chamber 52 on either side of the piston head. Sliding movement of the piston in the cylinder is thus resisted by the viscosity of the fluid in the chamber 52 as it is forced to flow through the restrictive passages 63 from one side of the piston to the other.

The piston head also comprises a coil 64 wound around the interior of the piston head, which constitutes an electromagnet. Wires 65 connected to either end of the coil pass to the coil through the piston rod to allow current to be applied to the coil from outside the cylinder. The piston head 56 and the piston 50 are formed of steel or another magnetically susceptible material, and the coil 64 is arranged in the piston head so that when activated it can apply a magnetic flux across the fluid passages 63 to alter the viscosity of the magnetorheological fluid in the passages. The viscosity of the fluid, and therefore its resistance to movement of the piston relative to the cylinder, is dependant on the intensity of the magnetic field applied.

Figure 6 shows a set of response curves 70-75 of the damper for a range of voltages applied to the coil. Each curve shows the response of the damper as a damping force versus velocity curve for a single applied voltage. The greater the voltage of the signal applied to the coil, the greater the damping force. Therefore, the damper can be controlled to have the response indicated by one of the curves 70-75 by applying the appropriate voltage to the coil. In practice, the response desired from the damper may not be that of any of the curves 70-75. For example, the desired response may be as indicated by curve 76. To allow the damper to be controlled to have a desired response of that form the closed-loop control process illustrated by the block diagram in figure 7 is employed.

Referring to figure 7, sensors are provided for sensing the position of the piston relative to the cylinder (as illustrated at 80 in figure 7) and the load on the piston rod in the displacement direction (as illustrated at 81 in figure 7). These sensors produce analogue output signals illustrated at 82 and 83 indicating the measured relative position and measured load respectively. The analogue load signal at 83 is converted to a corresponding digital signal by A-D converter 108. The analogue position signal is differentiated by analogue differentiation unit 109, and the resulting analogue output, which is representative of the relative velocity of the piston and the cylinder, is converted to a corresponding digital signal by A-D converter 110.

A response processing unit as illustrated at 84 is pre-programmed with the desired force/velocity response. The response processing unit receives the measured velocity, and from the pre-programmed desired response determines the currently required force which is output as a signal illustrated at 85. A modulus unit as illustrated at 86 determines the absolute value of the currently required force as an absolute load signal illustrated at 87.

A preload sampling unit as illustrated at 88 receives the measured load and velocity and samples the measured load to generate a signal illustrated at 89 representing the measured static load. This signal is received by a subtraction unit as illustrated at 90 which subtracts from the measured static load a signal illustrated at 91 representing the load due to the steady state force on the damper when it is at rest (e. g. due to the mass of an object supported by the damper and the force due to gas precharge acting on the piston rod), to give a signal at 92 representing the offset of the measured load from the steady state. A modulus unit as illustrated at 93 forms a signal at 94 representing the absolute value of the signal at 92.

A comparator as illustrated at 95 compares the measured absolute load at 92 with the required absolute load at 87 to generate an error signal which is split to give proportional (P) and integral (I) signals at 96 and 97 respectively. These signals are amplified by respective amplifiers as illustrated at 98 and 99 respectively to generate scaled P and I signals 100 and 101 respectively. In this example the P signal is scaled by a factor of 0.003 and the I signal is scaled by a factor of 0.2, but other suitable values could be used. The scaled I signal is integrated in a discrete time integration step as illustrated at 102 to generate an integrated I signal at 103. The scaled P signal and the integrated I signal are summed as illustrated at 104 to generate a current demand signal at 105 from which a drive signal 106 to the damper is generated by drive unit 107.

Most of the components used to implement the control system of figure 7 may be implemented on a single digital microcontroller, as illustrated at 111.

The closed-loop control signal shown in figure 7 allows for accurate real-time control of the damper response to match a desired force/velocity curve. The use of inputs of both damper load (force) and velocity (derived in this case from differentiation of a position measurement) is especially preferred in order to allow the desired curve to be matched precisely. The relative velocity of the damper's cylinder and piston could be sensed directly by means of a velocity sensor.

However, it has been found that the use of an analogue position sensor, whose output is then differentiated (preferably but not necessarily by means of an analogue circuit) is highly preferable because it is less expensive and significantly easier to implement. The differentiation of the position signal may be performed in digital or analogue electronics, but analogue differentiation is preferred since it gives superior high frequency performance. Figure 8 illustrates apparatus installed in a vehicle to implement the process of figure 7 in the damping of the vehicle's suspension. A wheel 120 of the vehicle is movable relative to the vehicle's body 121. The wheel is coupled to the body by a spring 122 and a magnetorheological damper 123 as described above. The damper is fitted with a load cell 124 in its piston rod 125 and a position sensor 126 implemented as a potentiometer connected between the piston rod and the damper's cylinder 127. The potentiometer outputs a voltage which varies with the relative position of the piston and the cylinder. The output 128 from the position sensor is differentiated by analogue differentiation unit 129 to give an analogue signal at 130 representative of the relative velocity of the piston and the cylinder. The analogue outputs of the differentiation unit 129 and the load cell 124 are converted to digital signals by respective A-D converters 131, whose outputs are fed to a digital control unit 132. The digital control unit implements the remaining steps illustrated in figure 7 and generates a digital output at 133 indicating the current to be applied to the coil 134 of the damper. The signal at 133 is converted to analogue by a D-A converter 135 whose output passes to a drive unit 136 which generates a corresponding drive signal for the coil 134.

The digital control unit receives an input at 136 (see also figure 7) which is used to indicate the desired response curve. The control unit could be pre-programmed with a number of response curves, one of which may be selected by means of the input 136, and/or the input 136 could be used to program the control unit with a response curve.

Each wheel of the vehicle may have a similar arrangement to that shown in figure 8. One control unit could be used for each wheel or the control unit could be common to all wheels, receiving velocity and force measurements from each wheel and generating current signals for the coil of the damper of each wheel. The desired response curves for each wheel could be the same or different. The dampers of the front wheels of the vehicle could be controlled to have the same response. The dampers of the front and/or rear and/or left and/or right wheels could be controlled to have the same response as each other.

Figure 9 illustrates the apparatus for installation to one pair of left and right dampers 150,151 respectively, for the front or rear wheels of a vehicle. The potentiometers for sensing the relative location of each cylinder 152,153 and piston 154,155 of the dampers are fitted in sensor cylinders 156,157 mounted on the side of the damper cylinders 152,153 and having sliding pistons 158,159 attached to the damper pistons 154,155. The damper cylinders and the damper pistons have mounting bushes 162-165 at their distal ends to allow them to be mounted to the vehicle. The load cells 166,167 are located in the piston rods between their mounting bushes and the cylinder. Flexible cables 168-171 lead from the dampers to a sealed housing 172 which contains the circuitry of the control system. The housing is connected by a cable 173 to the vehicle's battery and to means such as an on-board controller for controlling and/or monitoring its operation. The apparatus of figure 9 could be duplicated for the other set of the wheels.

In the application illustrated in figure 9 the control sample rate of the control system may suitably be 2ms (500Hz) or thereabouts.

The present invention may include any feature or combination of features disclosed herein either implicitly or explicitly or any generalisation thereof, irrespective of whether it relates to the presently claimed invention. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.

Claims

CLAIMS 1. A damper system comprising: a damper having: a cylinder defining a chamber, a fluid located in the chamber whose viscosity is dependant on magnetic field across it, a piston moveable within the chamber against resistance of the fluid, and an electromagnet capable of applying a magnetic field across the fluid to alter its viscosity and thereby alter the response of the damper; a sensor for sensing the relative position of the cylinder and the piston and generating a analogue position signal representative thereof; analogue differentiation circuitry for differentiating the position signal to generate an analogue velocity signal representative of the relative velocity of the cylinder and the piston; and a digital processing unit responsive to an input signal derived from the analogue velocity signal for generating a control signal for the electromagnet in order to control the response of the damper.
2. A damper system as claimed in claim 1, wherein the input signal is a digital signal and the damper system comprises an analogue-to-digital converter for generating the input signal in dependence on the analogue velocity signal.
3. A damper system as claimed in claim 1 or 2, wherein the sensor is a potentiometer.
4. A damper system as claimed in any preceding claim, wherein the digital processing unit is arranged to periodically perform processing of the input signal derived from the analogue velocity signal to generate the control signal.
5. A damper system as claimed in any preceding claim, wherein the digital processing unit is operable to control the response of the damper in response to the input signal to cause the damper to exhibit a selected damping force versus velocity profile.
6. A damper system as claimed In claim 5, comprising a memory connected to the digital processing unit, the memory storing at least one desired damping force versus velocity profile.
7. A damper system as claimed in any preceding claim, wherein the digital processing unit does not receive a signal directly representative of the relative position of the piston and the cylinder.
8. A damper system as claimed in any preceding claim, comprising a sensor for sensing the load between the cylinder and the piston and generating a load signal representative thereof, and wherein the digital processing unit is responsive to an input signal derived from the load signal for generating the control signal for the electromagnet in order to control the response of the damper.
9. A damper system as claimed in any preceding claim, wherein the piston comprises a piston head located in the chamber and a piston rod fast with the head and extending out of the chamber, and the damper comprises a first attachment point fast with the cylinder and a second attachment point fast with the piston rod, and the piston is constrained to move linearly relative to the cylinder.
10. A vehicle comprising a damper system as claimed in claim 9, wherein one of the mounting points is attached to the body of the vehicle and the other of the mounting points is attached to move with a wheel of the vehicle.
11. A vehicle as claimed in claim 10, comprising a spring means connected between the body of the vehicle and the wheel.
12. A damper system substantially as herein described with reference to the accompanying drawings.
13. A vehicle substantially as herein described with reference to the accompanying drawings.