WO1991011339A2

WO1991011339A2 - Digital suspension system

Info

Publication number: WO1991011339A2
Application number: PCT/CA1991/000026
Authority: WO
Inventors: Andrew Bruce Dunwoody
Original assignee: The University Of British Columbia
Priority date: 1990-02-02
Filing date: 1991-01-31
Publication date: 1991-08-08
Also published as: JPH05503903A; EP0513075A1; WO1991011339A3; CA2074514A1; KR920703354A

Abstract

A digital suspension system for a wheeled vehicle incorporates digital hydraulic actuators (42) at each wheel and includes a system for sensing lateral and longitudinal accelerations (18, 24) of a body portion of the vehicle and the position of each wheel (10, 12, 14, 16) relative to the body and provides this information to a computer (28). The computer computes the forces required at each wheel and controls the digital hydraulic actuator at each wheel by adjusting in increments the force applied at each actuator between the wheel and the body to maintain the body portion of the vehicle in a substantially stable position. Preferably the hydraulic actuator used in the suspension system will be formed by an assembly including a first element, a cylinder, a body section and a chamber. The first element is provided with a first set of a plurality of different sized piston cavities and piston elements which cooperate with a second set of piston elements and piston cavities respectively formed on a second element. The second element forms a piston in cylinder. The body sides incorporate, preferably in symmetrical relationship around the axis, of the assembly a plurality of valves, one valve for each of the pairs of cooperating pistons and cavities. A piston divides the chamber into a high pressure reservoir and a low pressure reservoir with the piston being urged toward the high pressure reservoir to maintain a difference in pressure between the high and low pressure reservoirs. Passages connect each of the valves to the high and low pressure reservoir and each of said cavities with its respective valve adapted to selectively connect its respective cavity to either the high pressure reservoir or the low pressure reservoir.

Description

DIGITAL SUSPENSION SYSTEM Field of the Invention

The present invention relates to a hydraulic suspension system and to digital actuators suitable for use therein. More particularly the present invention relates to a digital force adjusting computer controlled hydraulic suspension system and an actuator therefor.

Background of the Present Invention

The use of computers to control hydraulic systems in the suspension of motor vehicles has been discussed and various arrangements for controlling the suspension and for construction of the hydraulic actuator have been described and patented.

U.S. Patent 4,333,668 issued June 8, 1982 to Hendrickson et al discloses a shock absorber with damping orifices that are controlled by solenoids which in turn are activated by a computer control in response to the rate of change of the extension of the absorber. Pitch and roll of the vehicle are also imposed on the control which energizes the solenoid to vary the opening and closing of the valves and maintain the vehicle substantially stable.

U.S. Patent 4,639,013 issued January 27, 1987 to Williams et al describes an active vehicle suspension incorporating a double acting hydraulic actuator in parallel with a gas spring. An actual change in load is sensed and an appropriate adjustment of the actuator is made to compensate for the sensed load change.

Canadian patent 1,230,657 issued December 22, 1987 to Williams et al describes an active vehicle suspension system using hydraulic actuators for each wheel to generate signals in accordance with their displacements and forces applied thereto and controls the displacement of the hydraulic actuator in accordance with the interpretation of these signals to maintain vehicle stability.

U.S. Patent 4,753,328 issued June 28, 1988 to Williams is a further modification of the systems described in the preceding William's U.S. and

Canadian patents and further discloses a damping system to selectively apply positive or negative damping to the movement of the pistons of the actuators.

Digital hydraulic actuators are also known. U.S. Patent 4,602,481 issued July 29_/ 1986 to Robinson describes a particular form of digital actuator utilizing piston areas of different sizes to selectively apply forces of a preselected magnitude. The force applied is controlled by adjusting the ratio of piston area subjected to the source pressure to that subjected to return pressure. The pressures may selectively be applied to force the actuator in opposite directions. By changing the piston area subjected to source (higher) pressure driving the actuator in one direction relative to the area under similar pressure driving the actuator in the opposite direction one can adjust the force to be positive in either direction and to have a selected value depending on the combination of areas subjected to source pressure or return pressure.

Generally linear digital hydraulic actuators are limited in those cases where a double action arrangement is required as the total area or range of pressures that may be applied are limited since the forces must be applied to move in both directions. This limits the variation in pressure that may be applied in any one direction or increases sigmficantly the size of the actuator, see U.S. Patent 4,602,481 issued July 29, 1986 to Robinson which discloses a linear double acting digital system of the kind described.

U.S. patent 3,068,841 issued December 18, 1962 to Robbins et al discloses a system to permit rapid advance of a ram toward a workpiece under low power and low pumping volume requirements and for effecting full force against the work piece after the ram is positioned by utilizing different piston and cylinder sizes to obtain the desired results.

Generally digital actuators are separate elements and are connected. to discrete valves and separate reservoir systems all arranged in positions remote from the actuator itself.

Brief Description of the Present Invention

It is an object of the present invention to provide a suspension system for wheeled vehicles wherein the force between the body of the vehicle and each wheel is adjusted independent of displacement of the actuator to maintain the stability of the vehicle.

Broadly the present invention relates to a suspension system for a wheeled vehicle having a body portion comprising means for sensing lateral and longitudinal acceleration of said body portion, a digital hydraulic actuator supporting said body portion from each of said wheels, each said actuator including a plurality of different effective area actuator sections, a first source of hydraulic fluid at a first pressure, a second source of hydraulic fluid at a second pressure, said second pressure being different from said first pressure, valve means for selectively connecting said first and said second sources to selected of said actuator sections thereby to vary the amount of said effective area of said actuator subjected to said first and said second pressures to vary the force applied by each said actuator independent of the extension of said actuator, computer means for controlling said valves to adjust the number of said actuator sections of each said digital hydraulic actuator subjected to said first and said second pressures based on anticipated forces at each said actuator as determined by said computer means based on conditions sensed by said means for sensing lateral and longitudinal acceleration thereby to maintain said body portion substantially stable.

Preferably means to measure the displacement of each said actuator will provide a signal representing the displacement of each said actuator to said computer means and said computer means will control said valves to tend to maintain said actuator means in a position wherein said each said actuator has a preselected degree of extension.

Preferable the hydraulic actuator comprises a housing, a rotor extending through said housing, a plurality of axially extending radially projecting circumferentially spaced lugs on said housing and defining a plurality of circumferentially extending spaces therebetween, a pair of circumferentially opposed surfaces on an adjacent pair of said lugs defining opposite circumferentially spaced ends of each of said spaces, each of surfaces of said pairs of opposed surfaces of an adjacent pair lugs having different areas, axially extending radially projecting spaced bosses on said rotor, each said boss being received within a different one of said spaces and dividing its respective said space into a pair of actuator sections one on each side of said boss, each said boss having each of its radial circmferentially spaced sides substantially the same area as its adjacent opposed surface of said lug forming the adjacent circumferential end of said space in which it is received, said lugs having end faces cooperating with said rotor and said bosses having surfaces cooperating with circumferentially extending surfaces of said spaces to seal one said actuator section of said pair of sections in said space in which said boss is received from the other of said pair of actuator sections, said rotor being rotatably mounted within said housing so that each boss may rotate within its respective space through a preselected angle of rotation and means for directing fluid under preselected pressures into each of said actuator sections.

It is a further object of the present invention to provide a compact, easily installed, digital hydraulic actuator assembly wherein the various components are provided in a single assembly having a hydraulic output that may be coupled to a suitable hydraulic actuator.

Broadly the present invention also relates to a digital hydraulic actuator assembly comprising a cylinder, a body portion including a first element and a chamber integrally interconnected, a second element forming a piston in said cylinder, said first and second elements having sets of cooperating pairs of pistons and cavities, a piston dividing said chamber into a high pressure reservoir and a low pressure reservoir, means urging said piston toward said high pressure reservoir thereby to tend to increase the size of the low pressure reservoir and reduce the size of the high pressure reservoir and thereby generate a pressure differential between hydraulic fluid in said high and said low pressure reservoirs, set of valves in said body portion , said set of valves including one valve for each of said cooperating pairs of piston and cavities, first passage means through said body portion connecting each of said valves to said high pressure reservoir, second passage means connecting said low pressure reservoir to each of said valves, individual passages connecting each of said cavities of said cooperating pairs of pistons and cavities with its respective said valve of said set of valves.

Preferably said cylinder will be hydraulically connected to one side of a hydraulic actuator to drive said actuator in accordance with the pressure in said cylinder. Preferably said chamber and said cylinder will be positioned at opposite axial ends of said assembly.

Preferably further valve means will be provided in said body portion, further first passage means will connect said further valve means with said high pressure reservoir, a further second passage means will connect said further valve means with said low pressure reservoir and wherein a hydraulic connector will connect to said hydraulic actuator to said further valve means whereby said further valve means may selectively connect said hydraulic actuator to said high or said low pressure reservoir to drive said actuator in the opposite direction to the pressure applied from said cylinder.

Broadly the present invention also relates to a digital hydraulic actuator comprising a fixed element, a driven piston cooperating with a first cylinder formed by said fixed element, means to digitally vary the pressure acting between said fixed element and said driven piston tending to displace said driven piston in said first cylinder, a second cylinder, a hydraulic coupling hydraulically connecting said first cylinder to said second cylinder, said second cylinder having a cross sectional area different from said first cylinder, a working piston in said second cylinder adapted to apply a force determined by the ratio of the cross sectional areas of said first and second cylinders.

Preferably said means to digitally vary the pressure acting between said fixed element and said driven piston includes a first set of different cross sectional area piston cavities and a first set of different cross section area pistons on said fixed element, a second set of different cross sectional area pistons and a second set of different cross sectional area piston cavities on said driven piston, each piston of said second set of pistons being received within said one of said cavities of said first set of piston cavities and each cavity of said second set of piston cavities receiving a piston of said first set of pistons, and means to selectively apply fluid under selected pressures to each cavity of said first and said second sets of cavities.

Preferably said means to selectively apply fluid pressure will apply fluid under a first pressure or a second pressure different from said first pressure to each cavity in said first and said second sets of cavities.

Preferably said operating piston will be a double acting piston and means will be provided to apply fluid under pressure under the side of said piston remote from said fluid coupling.

Preferably said hydraulic coupling will. comprise a straight tubular passage section interconnecting said first and second cylinders and having a portion changing the cross sectional size of said passage from a cross sectional area equal to that of said first cylinder to a cross sectional area equal to that of said second cylinder.

Preferably said first set of cylinders and said first set of pistons will be concentric and pistons of said first set will separate and form the walls of cavities of said first set.

Preferably said second set of cylinders and said second set of pistons will be concentric and pistons of said second set will separate and form the walls of cavities of said second set. Preferably means will be provided to adjust the amount of fluid in said fluid coupling to maintain the spacing between said driven and said working pistons within a preselected range.

Brief Description of the Drawings Further features, objects and advantages will be evident from the following detailed description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings in which.

Figure 1 is a schematic illustration of a wheeled vehicle illustrating a suspension system constructed in accordance with the present invention. Figure 2 is a schematic illustration of a digital hydraulic actuator in position supporting one of the wheels of the vehicle relative to the chassis.

Figure 3 is a view similar to Figure 2 but illustrating a preferred form of hydraulic actuator.

Figure 4 is a schematic illustration of one form of hydraulic actuator that may be used with the present invention.

Figure 5 is a schematic illustration of a control valve that may be used to control the pressure applied to sections of the actuator.

Figure 6 is a. schematic radial cross sectional illustration of a preferred hydraulic actuator. Figure 7 is a cross-section through a preferred form of digital actuator constructed in accordance with the present invention.

Figure 8 is a cross section through a preferred form of actuator constructed in accordance with the present invention. Figure 9 is a section along the lines 9-9 of Figure 8. Figure 10 is a section along the line 10-10 of Figure 8. Figure 11 is a section along the line 11-11 of Figure 8. Figure 12 is a schematic illustration of a system constructed in accordance with the present invention and further including an actuator throttle.

Description of the Preferred Embodiments

In the illustration of Figure 1 the system of the present invention is applied to a four wheeled vehicle having wheels 10, 12, 14 and 16 each of which is supported from chassis (not shown in Figure 1.) The system includes a plurality of accelerometers measuring the verticle, lateral and fore and aft acceleration of the vehicle as well as the pitch and roll. In the illustrated system five accelerometers indicated at 18, 20, 22, 24 and 26 have been provided. The accelerometer 18 measures fore and aft acceleration, the accelerometer 20 measures vertical acceleration at one end of the vehicle, accelerometers 22 and 26 measure vertical acceleration at opposite sides of the vehicle and the accelerometer 24 measures horizontal lateral acceleration. Each of the accelerometers 18 to 26 inclusive as above described transmit signals representing measured accelerations to the control computer or controller 28. The controller 28 in turn sends signals to each of the control valves systems 30, 32 and 34 and 36 (each of which is composed of a plurality of valves as will be described below) used to control the hydraulic actuator 42 (57 and 200) at each of the wheels 10, 12, 14 and 16 respectively. The hydraulic circuits for each of the actuators 42 in the illustrated arrangement include a high pressure tank 38 and a low pressure tank 4 _.0 which are connected to the valve systems 30, 32, 34, 36 which control the forces generated by the digital hydraulic actuator 42 at each of the wheels 10, 12, 14 and 16 respectively. The actuators 42 will be described in more detail hereinbelow with respect to actuators 57 and 200 shown in Figures 4 and 6 respectively.

In the illustrated arrangement the hydraulic pump 44 supplies high pressure P_H fluid to the high pressure cylinders 38 and maintains the low pressure cylinders 40 at a significantly lower pressure P_L. Any suitable type of hydraulic actuator provided it is digital hydraulic actuator permitting adjustment of applied force independently of extension may be used. Obviously the computer control 28 will operate the valves for actuators 42 in accordance with the type of digital hydraulic actuator employed, in particular, the control will be different if a single acting actuator (force applied in a single direction only) is used than when a double acting actuator (force applied in two opposite directions) is used. Examples of the two different types are shown in figures 4 and 6.

An example of the mounting of an axially acting hydraulic actuator is shown in Figures 2. In Figure 2 a piston and cylinder type actuator 42 (Figure 4) which changes its axial or longitudinal extension between the body 46 and the suspension 48 to which the wheel (10, 12, 14 or 16) is mounted has been shown, In Figure 3 a torque type digital actuator 42A (eg. the double acting torque type actuator 200 in Figure 6) has been shown interposed between the body 46 and the suspension system 48 for each wheel (10, 12, 14 and 16.) The actuator 42 (or 42A) may be a single or a double acting actuator, a single acting axial extension actuator is shown in Figure 4 (actuator 57) and a double acting axial extension actuator is shown in said U.S. patent 4,602,481.

Generally when a double acting actuator is used as is preferred a suitable suspension spring such as that illustrated at 43 will be used in parallel with the digital hydraulic actuator to support the body 46 at each of the wheels (10, 12, 14 and 16.)

A single acting piston and cylinder type actuator 57 has been, illustrated in Figure 4. In this system two high pressure sources 38 have been shown; one for the actuator sections of the upper portion 59 of the actuator 57 and one for the actuator sections in the lower portion 76 of the actuator. The description below will only refer to a high pressure source 38 and a low pressure source 40 as separate sources need not be provided for each section. The source 38.is at a pressure p_H whereas the low pressure reservoir or source 40 is at a lower pressure p,.

The high pressure source 38 is connected via a manifold 50 with valves 52, 54 and 56 which will form part of each the valve system 30, 32, 34 or 36. These valves are also connected via a low pressure manifold 58 with the low pressure reservoir 40. Each of valve 52, 54 and 56 is connected respectively to actuator sections in this case formed by annular, concentric chambers 60, 62 and 64 respectively in the upper portion 59 of the actuator 57. The chambers or sections 60, 62 and 64 have different effective cross sectional areas measured in a plane radial to the axis 66 around which the chambers, 60, 62 and 64 are concentric.

Similarly the annular ring dividers separating the chambers or sections 60, 62 and 64 are received within a second set of annular chambers 68, 70, 72 and 74 concentric with the axis 66 formed in second or bottom section 76 of the actuator 57. The effective cross sectional areas of the sections 60, 62, 64, 68, 70, 72 and 74 are related such that for example the effective area of chamber 60 is preferably twice that of chamber 62 and chamber 62 is twice that of chamber 64 etc., however it is not essential that the effective areas of these chambers be stepped in this manner but it is important that the areas be different.

Each of these chambers 60, 62 and 64 formed in the upper portion 59 of the actuator 57 is connected respectively to the valve 50, 52 and 56 while the chamber 68, 70, 72 and 74 in the lower section 76 are connected to valves, 78, 80, and 82 respectively (there are valves 78, 80, 82 in each of the valve systems 30, 32, 34 or 36) which valves are connected via a high pressure manifold 86 to the high pressure reservoir 38 and via mamfold 88 and the low pressure source or reservoir 40.

The controller 28 actuates each of the valves 52, 54, 56, 78, 80, 82 and 84 to connect the various sections 60, 62, 64, 68, 70, 72 and 74 either to the high pressure source 38 or the low pressure source 40 so that the total force tending to separate the two units 59 and 76 is governed by the sum of the forces in the cylinders 60, 62, 64, 68, 70, 72 and 74. The sum of these forces is dependent directly on the area of each chamber multiplied by the pressure in each chamber which pressures are selected as P_H or P_L depending on which source of fluid is connected thereto.

The valves 52, 54, 56, 78, 80, 82 and .84 may be any suitable valve connecting the actuator section to one pressure source or the other, however the operation of the system will be described with respect to the valve shown in Figure 5. In this system the valve has a control solenoid indicated at 90 which may be any suitable solenoid or the like to move the body of the valve, formed by two interconnected cylinders 92^"and 94, back and forth to open and close the two inlet ports 96 and 98, one for high pressure (96) and one for low pressure (98). Intermediate port 100 connects the valve to its respective section or chamber 60, 62, 64, 68, 70, 72 or 74. It will be apparent that as the valve moves to close off say port 96 and open port 98 it opens port 98 before it totally closes off port 96 as indicated by the dimension x. Obviously a similar phenomena occurs when the valve is moved in the opposite direction to close off port 98 and open port 96. The rate of movement of the cylinders 92 and 94 to open and close the ports 96 and 98 is relatively rapid, thus the time period in which both ports are simultaneously cracked open is small and is provides to ensure that there is no significant build up of pressure during the transition between the high and low pressure sources. It will be apparent that any of the cylinders 60, 62, 64, 68, 70, 72, and 74 may be connected either to the low pressure source 40 or the high pressure source 38. If the ratio of effective areas in the various sections or chambers, i.e. the effective cross sectional areas of the chambers are in multiples of two a convenient stepping of the pressures applied tending to force the two sections 59 and 76 apart can be established by appropriately connecting the various chambers to the high pressure source p_H or the low pressure source p,.

Each of the sources p_H and p_L are substantially constant pressure sources in that they maintain a substantially constant pressure eg. by a pneumatic pressure at the top of the fluid so that the amount of flow fluid does not significantly affect the pressure. The low pressure p_L may simply be atmospheric pressure.

The pressure sources 38 and 40 are of the type that maintain essentially the same pressure to the valves substantially irrespective of the flow to or extension of the actuator eg. separation of the portions 59 and 76 of actuator 57 or as will be described below relative rotation of the rotor and housing of Figure 6, for example by maintaining substantially constant pneumatic pressures in the reservoirs 38 and 40.

Control of the valves 52, 54, 56, 78, 80, 82 or 84 to direct the appropriate pressure into the various chambers 60, 62, 64, 68, 70, 72 or 74 is by the computer controller 28 and is based on the anticipated movement of the chassis as determined by calculations based on the signals from the accelerometers 18, 20, 22, 24 and 26 to define the expected movement of the chassis relative to the wheels and using the weight and centre of gravity (assumed or determined) calculate the force required to resist movement of the body 46 at each of the wheels.

In order to calculate the forces involved so that the forces can be matched in the actuator, it is necessary not. only to know the acceleration but also the weight and centre of gravity of the vehicle must be assumed or determined. Thus appropriate means may be provided in each of the suspension systems to determine the weight of the vehicle. This weight of the vehicle may easily be obtained based on the measured displacement of the suspension system eg. the actuators 42 or 42A at each of the wheels 10, 12, 14 and 16 measured by a suitable measuring device such as that indicated at 104 in Figure 2 or the angular position of the wheel by the measuring device as indicated at 106 in Figure 3.

The signals from the accelerometers 18, 20, 22, 24 and 26, the signals from the actuator extension measurement devices on each wheel 104 or 106 and signals from such other measuring devices as may be used are presented to the controller or control computer 28. The control computer 28 calculates the force to be applied by each actuator and the positions of each of the valves 52, 54, 56, 78, 80, 82 and 84 of each of the valve blocks 34.

In the preferred embodiment of the present invention, the force to be applied by each actuator is calculated as a weighted sum of the most recent, and preferably some preceding-values of each of the accelerometer and extension signals. The weighting coefficients are determined using known modern linear control theory, preferably the Ricatti equation of optimal linear quadratic control theory, and a knowledge of the vehicle parameters. The parameters describing the vehicle include mass, mass moments of inertia and dimensions are programmed into the computer when the system is installed.

In a preferred case the vehicle parameters are identified from the accelerometer and extension signals and the actuator forces, for example a maximum likelihood method of parameter estimation from statistical theory may be applied to generate appropriate calculation procedures. In this case, the control computer would, in addition to calculating actuator forces, periodically perform the calculations to identify the parameters of the vehicle and perform the calculations to determine the weighting coefficients mentioned previously and in the manner compensate for different loading conditions of the vehicle. Normally the calculations of actuator forces will be repeated at least every 0.1 seconds while the calculations to determine weighting coefficients may be done less frequently preferable at least every 10 seconds.

Calculating the actuator force at least every 0.1 seconds is sufficiently rapid to respond to turning, acceleration and braking of the vehicle as well as to variations in the road surface over which the vehicle is travelling.

The prime function of the suspension system is to respond to variations in road surface, thereby to substantially isolate the body 46 from the road surface. However if the wheel is displaced too far from its nominal position (generally in about the middle of the wheel well) the suspension may reach the limit of its travel and a strong shock will be transmitted to the body 46. To avoid this the suspension must move the body 46 sufficiently to maintain suspension (wheel relative to the body) in its normal position. A stiff suspension hold the wheel near the normal position, but at the expense of a lot of road roughness being transmitted to the body 46, a soft suspension on the other hand permits freer movement of the wheel relative to the body 46 and better isolates the body from the road but at the expense of more likelihood of the suspension reaching the limits of its travel and transmitting shocks to the body. The computer control must control the active suspension system to provide the kind of suspension system selected and in any event must react at least as well as a conventional suspension system to prevent the suspension from reaching its limits and transmitting shocks to the body 46.

In the preferred embodiment of the present invention hydraulic actuator will be a torsional actuator as indicated at 200 in Figures 3 and 6. The torsional actuator has an outer housing 202 and a rotor 204 contained therein. Rotor 204 has an axially extending shaft 208 that is clamped to the suspension arm 48 for a wheel so that the angular position of the arm 48 relative to the body 46 is adjusted by rotating the rotor 204 relative to the housing 202. The torsional actuator 200 shown in Figure 6 also includes the same valve systems as with the axial actuator 57 and like parts have been indicated with like numerals in the Figures 4 and 6 embodiments. In the torsional actuator 200 a plurality of circumferentially extending spaces 208, 210, 212 and 214 are provided. The space 208 is formed between lugs 216 and 218, the space 210 between the lugs 218 and 220 and space 212 between the lugs 220 and 222. A further space 214 is formed between the lug 222 and a face 224 formed on the rotor 204.

The rotor 204 is provided with circumferentially spaced bosses 242, 244 and 246 which are circumferentially shorter than and are received within the spaces 208, 210 and 212 respectively"and divide the spaces into actuator sections or chambers 208A, 208B; 210A, 210B; and 212A, 212B respectively.

In the illustrated arrangement the ports 228, 230 are provided leading to the sections 208A and 208B adjacent the lugs.218 and 216 respectively and are connected to the valves 80 and 56 respectively. Similarly the sections 210B and 210A are connected by ports 232 and 234 positioned adjacent the lugs 218 and 220 respectively to the valves 54 and 82 respectively. The sections 212B and 212A are connected by ports 236 and 238 positioned adjacent the lugs 220 and 222 to the valves 52 and 84 respectively. The space (section) 214 is connected via port 240 with the valve 78.

It will be noted that the face 248 of the boss 242 facing the face 250 on the abutment 216 have essentially the same effective area measured radially to the axis of rotation of the rotor 204. Similarly the faces 252 and 254 on the boss 242 and lug 218 are of the same radial area as are the adjacent faces on each of the respective adjacent lugs and bosses.

The periphery of the bosses 242, 244 and 246 are in wiping relationship with the circumferences of the spaces 208, 210 and 212 respectively and form a seal between the sections 208A and 208B: 210A and 210B; and 212a and 212B respectively. Similarly the lugs 216, 218, 220, 222 are in sealing relationship with the circumference of the rotor 204 to further seal sides of the various sections 208A and 208B: 210A and 210B; and 212A and 212B and 214.

It will be apparent that the area of the face 224 of the space 214 is substantially equal to the area of the facing surface of the lug 222. It can be seen that a plurality of different radial effective cross sectional area actuator sections are provided circumferentially spaced around the rotor. Preferably the radial area of the corresponding pairs of adjacent faces will be in a specific ratio to the areas of other corresponding pairs of radial areas so that the cross sectional areas subject to different pressures may be combined to produce the desired resultant force by selectively connecting the sections 208A, 208B, 210A, 210B, 212A, 212B and 214 to P_H or P_L via the valves 80, 56, 82, 54, 84, 52 and 78 respectively.

The torsional actuator 200 is a double acting actuator in that as viewed in Figure 4 it may be used to apply pressure to tend to rotate the suspension 48 either in a clockwise or a counterclockwise under control of the computer 28 to maintain the suspension in the desired position relative to the body 46. A single acting actuator can only be manipulated to keep the suspension in its normal position by changing the magnitude but not the direction of the forces applied between each wheel and the body 46.

The digital hydraulic actuator 310 illustrated in Figure 8 includes a first or fixed element 312 having a plurality of annular piston cavities 314, 316, 318 and 320 forming a first set of piston cavities separated (surrounded) by a plurality of annular pistons 322, 324, 326 and 328 forming a first set of pistons. A driven piston or second element 330 is formed with a plurality of annular pistons 332, 334 and 336 forming a second set of pistons and a second set of discrete annular piston cavities 338, 340, 342 and 344. The second set of pistons 332, 334 and 336 are received within the first set of piston cavities 314, 316 and 318 while the first set of pistpns 322, 324, 326 and 328 are received within the second set of piston cavities 338, 340, 342 and 344 respectively. Each of the cavities 314, 316, 318, 334, 340, 342, and 344 are connected via lines 346, 348, 350, 352, 354, 356 and 358 respectively to their respective valves 360, 366, 368, 370 and 372 respectively. Each of these valves are essentially the same and are essentially the same as the valve described above and illustrated in Figure 5. Each is adapted to be moved from first position connecting its respective cylinder to the high pressure line 374 (equivalent to line 50 or 86 described above) leading from a high pressure tank 376 (tank 38 above) and designated by the symbol P_H or to the low pressure line 378 (line 58 or 88 above) connected to the low pressure source of hydraulic fluid 380 (tnak 40 above) as symbolized by the symbol P_L. These valves 360, 362, 364, 366, 368, 370 and 372 are controlled via a controller 382 as indicated by the dot/dash lines 384 to apply either a high pressure or a low pressure to each of the cavities and thereby vary the total pressure tending to separate the two elements 312 and 330.

It is preferred that there by a direct relationship between the cross sectional areas of the various cavities of the first and second sets of cavities forming the actuator 310 to obtain a digital effect by properly connecting the various cavities to either the source of high or low pressure to increase the pressure in selected steps, i.e. each cavity will have a cross sectional area that is a direct ratio to the cross sectional areas of the other cavities, e.g. multiples of two.

The actuator 310 incorporates an annular extension to the element 312 which forms a first cylinder 386 in which the driven piston or second element 330 mates to form a hydraulic piston 330 and cylinder 386. The axial length of the cylinder 386 is sufficient to accommodate movement of the piston 330 for the full extension of the pistons of the first and second sets of pistons in the second and first sets of piston cavities respectively. The cooperation of the piston 330 in the cylinder 386 g ides and better ensures that the small piston elements ie. the pistons of the first and second sets of pistons extending between the driven piston 330 and the first element 312 are not broken.

The cylinder 386 is connected via a hydraulic coupling section 388 to a second cylinder 392 accommodating a second or working piston 390. In the illustrated arrangement this coupling is a straight tubular passage as is preferred, however it could if desired be bent or formed by a hydraulic hose connection or the like. The cross sectional area of the piston 390 in the illustrated arrangement is significantly less than the cross sectional area of the piston 330 thus a the pressure applied by the piston 330 through the fluid coupling 388 will result in a corresponding pressure applied to the piston 390 and the piston rod 394 connected thereto. If the ratio of the areas of the piston 330 and 390 are 10 to 1 then the force applied to the piston 390 will be 1/10 times that of piston 330 (the total pressures will be essentially the same) and the movement, if the piston 390 is free to move will be such that a 1/10 of an inch movement of the piston 330 will result in a full inch of travel of the piston 390. Thus a significant increase in the travel of the working piston 390 can be obtained through the use of different cross sectional areas of the pistons 330 and 390.

To accommodate the differences in cross sectional areas of the first and second cylinders 386 and 392 the hydraulic coupling 388 has a tapered section 387 gradually changing the cross sectional area from the larger area of cylinder 386 to the smaller area of cylinder 392

To move the element 330 toward the element 312 the side of the piston 390 remote from the coupling 388 is connected via line 396 to a further two position valve 398 that may be connected to the high pressure source while a sufficient number of the cavities of the first and second sets of cavities between the elements 312 and 330 are connected to a low pressure source whereby the pressure acting on the piston 390 in the direction toward the piston 330 is sufficient to force the piston 390 upward in Figure 7 and force the element or piston 330 to approach the element 312.

As above indicated the valves 360, 362, 365, 366, 368, 370, 372 and 398 may be any suitable valve to connect the actuator to one pressure source or another such as that described hereinabόve with respect to Figure 5.

The hydraulic connector 388 may be provided with a system for adding or removing fluid via vent 408 which is controlled via the computer 82 to add or subtract fluid as indicated by arrows 412 and 414 respectively in accordance with the relative positions of the pistons 330 and 390 as measured by the sensorw 400 and 402 respectively.

The description has dealt with only two hydraulic pressures, it will be apparent that more than two could be used with appropriate valve changes so that any one pressure of a number of different pressures could be applied selectively to the cavities.

Referring now to Figure 8 which illustrates a preferred form of the present invention wherein all of the valves, high and low pressure reservoirs and the output cylinder are neatly combined and arranged in an assembly 450.

In Figure 8 like parts to those illustrated in Figure 7 have been designated by the same reference numerals followed by the letter A and thus a detailed description of the digital hydraulic system will not be repeated (however, it should be noted that there is one less piston and cavity in the two elements 312A and 330A than with the arrangement shown in Figure 7, i.e. the piston 328 and its corresponding cavity 344 are not provided in the arrangement shown in Figure 8). Some of the valves have been generally designated with different numerals than those used in Figure 7.

The compact digital actuator assembly 450 as shown in Figure 8 is provided at one axial end with a cylinder section 452 which defines the pressure cylinder 386A which is substantially equivalent to this cylinder 386 of the Figure

7 embodiment. At the opposite axial end of the unit 450 chamber 454 is provided.

Interposed between the cylinder 452 and chamber 454 is a body portion 455 made up of the first elements 312A (which cooperates with the moveable element 330A (second element) or piston) and a plurality of disc shaped elements 456, 458 and 460 which combine to define the various passages and to hold or contain the valves as will be described hereinbelow.

The assembly 450 has a longitudinal axis 462 about which various elements of the assembly are substantially symmetrically positioned.

The chamber 454 is divided into a high pressure chamber P_H as indicated at 376A and a low pressure chamber P_L 380A by a piston 464. A spring or other suitable means 466 biases the piston 464 toward the high pressure chamber 376A to maintain a pressure differential between the high pressure chamber 376A and the low pressure chamber 380A, i.e. the spring 466 biases the piston 464 to or to tend to reduce the size of the high pressure chamber 376 and increase the size of the low pressure chamber 380A and thereby maintain the pressure differential between the high pressure P_H and the low pressure P_L.

In the illustrated arrangement the disc 456 has an axial extension 468 through which a longitudinal passage 470 extends. The piston 464 slides in sealing relationship the axial extension to 368.

The various valves (described hereinabove as valves 360, 362, 364, 366, etc. generally indicated by the reference numeral 472 (all of the valves are indicated with the same reference numeral) are mounted in the body portions 455 in the elements 312A, 460, 458 and 456 and as well be apparent from Figures 9,

10 and 11 are symmetrically positioned about the axis 462 in the cavities 474. A first passage system connecting the valves 472 to the high pressure chamber 376A is provided by the longitudinal passage 470 which connects to cavities 474 via radial passages 476, 478, 480, 482, 484 and 486.

In reviewing Figure 9, it will be noted that there is an extra cavity 474 which is adapted to contain a further valve. In this case the valve substantially equivalent to the valve 398 of the previous embodiment, i.e. the further first passage 490 connects the high pressure reservoir or chamber 376A with the valve 472 substantially equivalent to the valve 396.

Each of the valves 472 is connected via a separate passage namely the passages 492, 494, 496, 500 and 502 which connect with their respective axially extending passages 504, 506, 508, 510, 512 and 514 respectively, to their respective cavity of the digital actuator formed by the elements 312A and 330A (see Figure

11).

Referring again to Figure 10, the cavity 474 housing the valve equivalent to the valve 396 is provided with a radial passage 516 which connects to an axial passage 518 which in turn connects to fitting 520 for coupling to a hydraulic hose or the like as will be described below.

Low pressure chamber 380A is connected via axial passage 522 (see Figures 9 and 10) to a circumferentially extending channel 524 which interconnects the cavities 474 (see Figure 11).

To ensure proper functioning, piston 330A has been provided with a system for limiting free piston travel to ensure that the piston 330A does not bottom out against the element 312A or against the cylinder end of the cylinder section 452. This travel limiter is formed by an annular groove 526 (see Figure 8) which connects via axial passage 528, radial extension 530, radial passage 582, passage 470 and thus the high pressure chamber 376A (see Figures 8 and 9).

A second annular groove 532 is also provided in the cylinder wall

452 spaced axially from the groove 526 and is connected to a axial passage 534 which connects to a radial passage 536, the interconnecting passage 524, the axial passage 522 and thus to the low pressure reservoir 380A (see Figures 8, 9 and 11).

The piston 330A is provided on its outer surface, i.e. around its peripheral with a circumferential groove 538 which is connected via an L shaped passage 540 with the inside of the cylinder 386A.

SUBSTITUTE SHEET The travel limiter operates as follows: if the piston 330A moves too close to the cylinder end, i.e. extended too far from the fixed element 312A the groove 538 aligns with the groove 526 thereby connecting inside of the cylinder 386A to the high pressure chamber 376A via the passage 528, etc. thereby forcing the piston 330A towards the element 312A. On the other hand if the piston 330A approaches too closely the element 312A, the groove 538 will align with the groove 532 and connect the cylinder 386A via the line 534, 536, and 538, etc. to the low pressure reservoir 380A so that if there is any of the piston and cavities of the digital actuator system at high pressure the piston 330A will be forced into the cylinder 386A away from the- fixed element 312A.

If the present invention were to be incorporated in an active suspension system wherein each wheel of a vehicle would be supported by an actuator 399 formed by, for example, a piston and cylinder arrangement such as the piston and cylinder 390A, 392A with for example, shaft of the piston as indicated at 394A connected to the wheel and the body of the cylinder 392A connected to the body of the vehicle. Regardless of the use to which the system may be applied, the high pressure fluid from the cylinder 386A may be connected to one side of a double piston and cylinder 390A, 392A by a line 388A providing a unit pressure to the piston 390A essentially equal to the unit pressure within the cylinder 386A.

On the opposite end of the piston and cylinder 390A, 392A, i.e. the chamber 392A is connected by a suitable coupling such as a hose coupling as indicated at 396A to the connectors 520 and thus equivalent to valve 396 which will normally be connected to the low pressure reservoir 380A. To drive the piston 390A in the opposite direction valve equivalent to the valve 396 will be shifted to direct high pressure fluid into the chamber 392A as above described.

Figure 12 provides a schematic illustration somewhat similar to

Figure 8 further incorporating a pump 600 that is connected via a valve 602 to the high and low pressure lines 374 (374A) 378 (378A) respectively which in turn are connected to the high and low pressure reservoir 376 (376A) and 380 (380A) respectively.

The system illustrated in Figure 12 further includes an actuator throttle 604 the purpose of which is to limit the speed of the working piston 390

SUBSTITUTE SHEET (390A), 394 (394A) when there is no load on the working piston.

The actuator throttle 604 includes a throttle valve 606 which is adapted to connects the pressure line from the cylinder 386, 386A through a throttling orifice schematically illustrated at 608 or in the position illustrated to bypass this throttling orifice 608 and be connected directly with the cylinder 386, 386A_ Similarly the return line 396, 396A may be connected through to the low pressure reservoir 380, 380A via an orifice 610 as illustrated by shifting the valve 606 directly to the low pressure reservoir 380, 380A.

Shifting of the valve 606 is attained by the difference in hydraulic pressure applied to opposite ends of the valve 606 via the valve control pressure lines 612 and 614 connected to the lines 396 (396A) and 388 (388A) respectively. The throttle 604 operates as follows. With no load applied to the working piston 390 (390A), there is no pressure differential across the working piston and therefore the entire pressure differential is applied across the throttle. The flow rate through the throttle, hence the speed of the piston is a function of the size of the throttle orifice 608 or 610 and the pressure differential applied across it. In the extreme case, with no motion of the piston there can be no pressure differential across the throttle and the full pressure differential is applied across the working piston. The valve 606 insures that the throttle is always applied across the flow line 388 or 396 from the actuator 399 which is at the higher pressure. This throttle also helps to prevent cavitation.

It will be evident that to incorporate the assembly into a system four connections are required, namely one to cylinder 386A (i.e. line 388A), one to fitting 520, a low pressure line to the reservoir 380A and a high pressure line to reservoir 376A.

Having described the invention modifications will be evident to those skilled in the art without departing from the spirit of the invention as defined in the appended claims.

Claims

1. A suspension system for a wheeled vehicle having a body portion comprising means for sensing lateral and longitudinal acceleration of said body portion, a digital hydraulic actuator supporting said body portion from each of said wheels, each said actuator including a plurality of different effective area actuator sections, a first source of hydraulic fluid at a first pressure, a second source of hydraulic fluid at a second pressure, said second pressure being significantly different from said first pressure, valve means for selectively connecting either one or the other of said first and said second sources to selected of said actuator sections thereby to vary the number of said actuator sections and thereby the amount of said effective area of each said actuator subjected to said first and said second pressures to selectively vary the force applied by each said actuator independent of the extension of said actuator, computer means for controlling said valves to adjust the number of said actuator sections of each said digital hydraulic actuator subjected to said first and said second pressures based on anticipated forces at each said actuator as determined by said computer means based on conditions sensed by said means for sensing lateral and longitudinal acceleration thereby to maintain said body portion substantially stable.

2. A suspension system as defined in claim 1 further comprising means to measure the displacement of each said actuator to provide a signal representing the displacement of each said actuator to said computer means and said computer means controlling said valves to tend to maintain said actuator means in a position wherein said each said actuator has a preselected degree of extension.

3. A suspension system as defined in claim 2 wherein each said actuator is a double acting actuator.

4. A suspension system as defined in claim 1 wherein hydraulic actuator comprises a housing, a rotor extending through said housing, a plurality of axially extending radially projecting circumferentially spaced lugs on said housing and defining a plurality of circumferentially extending spaces therebetween, a pair of circumferentially opposed surfaces on an adjacent pair of said lugs defining opposite circumferentially spaced ends of each of said spaces, each of surfaces of said pairs of opposed surfaces of an adjacent pair lugs having different areas, axially extending radially projecting spaced bosses on said rotor, each said boss being received within a different one of said spaces and dividing its respective said space into a pair of said actuator sections one on each side of said boss, each said boss having each of its radial circmferentially spaced sides substantially the same area as its adjacent opposed surface of said lug forming the adjacent circumferential end of said space in which it is received, said lugs having end faces cooperating with said rotor and said bosses having surfaces cooperating with circumferentially extending surfaces of said spaces to seal one said actuator section of said pair of sections in said space in which said boss is received from the other of said pair of actuator sections, said rotor being rotatably mounted within said housing so that each boss may rotate within its respective space through a preselected angle of rotation and means for directing fluid under said first or said second pressure into each of said actuator sections.

5. A suspension system as defined in claim 2 wherein hydraulic actuator comprises a housing, a rotor extending through said housing, a plurality of axially extending radially projecting circumferentially spaced lugs on said housing and defining a plurality of circumferentially extending spaces therebetween, a pair of circumferentially opposed surfaces on an adjacent pair of said lugs defining opposite circumferentially spaced ends of each of said spaces, each of surfaces of said pairs of opposed surfaces of an adjacent pair lugs having different areas, axially extending radially projecting spaced bosses on said rotor, each said boss being received within a different one of said spaces and dividing its respective said space into a pair of said actuator sections one on each side of said boss, each said boss having each of its radial circmferentially spaced sides substantially the same area as its adjacent opposed surface of said lug forming the adjacent circumferential end of said space in which it is received, said lugs having end faces cooperating with said rotor and said bosses having surfaces cooperating with circumferentially extending surfaces of said spaces to seal one said actuator section of said pair of sections in said space in which said boss is received from the other of said pair of actuator sections, said rotor being rotatably mounted within said, housing so that each boss may rotate within its respective space through a preselected angle of rotation and means for directing fluid under said first or said second pressure into each of said actuator sections.

6. A suspension system as defined in claim 3 wherein hydraulic actuator comprises a housing, a rotor extending through said housing, a plurality of axially extending radially projecting circumferentially spaced lugs on said housing and defining a plurality of circumferentially extending spaces therebetween, a pair of circumferentially opposed surfaces on an adjacent pair of said lugs defining opposite circumferentially spaced ends of each of said spaces, each of surfaces of said pairs of opposed surfaces of an adjacent pair lugs having different areas, axially extending radially projecting spaced bosses on said rotor, each said boss being received within a different one of said spaces and dividing its respective said space into a pair of said actuator sections one on each side of said boss, each said boss having each of its radial circmferentially spaced sides substantially the same area as its adjacent opposed surface of said lug forming the adjacent circumferential end of said space in which it is received, said lugs having end faces cooperating with said rotor and said bosses having surfaces cooperating with circumferentially extending surfaces of said spaces to seal one said actuator section of said pair of sections in said space in which said boss is received from the other of said pair of actuator sections, said rotor being rotatably mounted within said housing so that each boss may rotate within its respective space through a preselected angle of rotation and means for directing fluid under said first or said second pressure into each of said actuator sections.

7. A hydraulic actuator comprising a housing, a rotor extending through said housing, a plurality of axially extending radially projecting circumferentially spaced lugs on said housing and defining a plurality of circumferentially extending spaces therebetween, a pair of circumferentially opposed surfaces on an adjacent pair of said lugs defining opposite circumferentially spaced ends of each of said spaces, each of surfaces of said pairs of opposed surfaces of an adjacent pair lugs having different areas, axially extending radially projecting spaced bosses on said rotor, each said boss being received within a different one of said spaces and dividing its respective said space into a pair of actuator sections one on each side of said boss, each said boss having each of its radial circmferentially spaced sides substantially the same area as its adjacent opposed surface of said lug forming the adjacent circumferential end of said space in which it is received, said lugs having end faces cooperating with said rotor and said bosses having surfaces cooperating with circumferentially extending surfaces of said spaces to seal one said actuator section of said pair of sections in said space in which said boss is received from the other of said pair of actuator sections, said rotor being rotatably mounted within said housing so that each boss may rotate within its respective space through a preselected angle of rotation and means for directing fluid under preselected pressures into each of said actuator sections.

8. A digital hydraulic actuator comprising a fixed element, a driven piston cooperating with a first cylinder formed by said fixed element, means to digitally vary the pressure acting between said fixed element and said driven piston tending to displace said driven piston in said first cylinder, a second cylinder, a hydraulic coupling hydraulically connecting said first cylinder to said second cylinder, said second cylinder having a cross sectional area different from said first cylinder, a working piston in said second cylinder adapted to apply a force determined by the ratio of the cross sectional areas of said first and second cylinders.

9 A digital hydraulic actuator as defined in claim 8 wherein said means to digitally vary the pressure acting between said fixed element and said driven piston includes a first set of different cross sectional area piston cavities and a first set of different cross section area pistons on said fixed element, a second set of different cross sectional area pistons and a second set of different cross sectional area piston cavities on said driven piston, each piston of said second set of pistons being received within said one of said cavities of said first set of piston cavities and each cavity of said second set of piston cavities receiving a piston of said first set of pistons, and means to selectively apply fluid under selected pressures to each cavity of said first and said second sets of cavities.

10. A digital hydraulic actuator as defined in claim 9 wherein said means to selectively apply pressure applies one of a first or a second pressure to each of said cavities, said first and second pressures being significantly different.

11. A digital hydraulic actuator as defined in claim 10 wherein said working piston is a double acting piston and further comprising means to apply fluid under pressure under the side of said working piston remote from said fluid coupling.

12. A digital hydraulic actuator as defined in claim 10 wherein said hydraulic coupling comprises a straight tubular passage section interconnecting said first and said second cylinders and includes a portion changing the cross sectional size of said passage from a cross sectional area equal to that of said first cylinder to a cross sectional area equal to that of said second cylinder.

13. A digital hydraulic actuator as defined in claim 10 wherein said first set of cylinders and said first set of pistons are concentric and pistons of said first set separate and form the walls of cavities of said first set of cavities.

14. A digital hydraulic actuator as defined in claim 10 wherein said second set of cylinders and said second set of pistons are concentric and pistons of said second set separate and form the walls of cavities of said second set of cavities.

15. A digital hydraulic actuator as defined in claim 11 wherein said hydraulic coupling comprises a straight tubular passage section interconnecting said first and said second cylinders and includes a portion changing the cross sectional size of said passage from a cross sectional area equal to that of said first cylinder to a cross sectional area equal to that of said second cylinder.

16. A digital hydraulic actuator as defined in claim 11 wherein said first set of cylinders and said first set of pistons are concentric and pistons of said first set separate and form the walls of cavities of said first set of cavities.

17. A digital hydraulic actuator as defined in claim 11 wherein said second set of cylinders and said second set of pistons are concentric and pistons of said second set separate and form the walls of cavities of said second set of cavities.

18. A digital hydraulic actuator as defined in claim 12 wherein said first set of cylinders and said first set of pistons are concentric and pistons of said first set separate and form the walls of cavities of said first set of cavities.

19. A digital hydraulic actuator as defined in claim 12 wherein said second set of cylinders and said second set of pistons are concentric and pistons of said second set separate and form the walls of cavities of said second set of cavities.

20. A digital hydraulic actuator as defined in claim 8 wherein said working piston is a double acting piston and further comprising means to apply fluid under pressure under the side of said working piston remote from said fluid coupling.

21. A digital hydraulic actuator as defined in claim 9 wherein said working piston is a double acting piston and further comprising means to apply fluid under pressure under the side of said working piston remote from said fluid coupling.

22. A digital hydraulic actuator as defined in claim 8 wherein said hydraulic coupling comprises a straight tubular passage section interconnecting said first and said second cylinders and includes a portion changing the cross sectional size of said passage from a cross sectional area equal to that of said first cylinder to a cross sectional area equal to that of said second cylinder.

23. A digital hydraulic actuator as defined in claim 9 wherein said hydraulic coupling comprises a straight tubular passage section interconnecting said first and said second cylinders and includes a portion changing the cross sectional size of said passage from a cross sectional area equal to that of said first cylinder to a cross sectional area equal to that of said second cylinder.

24. A digital hydraulic actuator assembly comprising a cylinder, a body portion having a first element, and a chamber integrally interconnected, said first element cooperating with a second element that forms a piston in said cylinder, said first and second elements having sets of cooperating pairs of pistons and cavities adapted to cooperate to form a digital hydraulic actuator, a piston dividing said chamber into a high pressure reservoir and a low pressure reservoir, means for urging said piston toward said high pressure reservoir to generate a differential in pressure in hydraulic fluid filling said high and said low pressure reservoirs, set of valves in said body portion, said set of valves including one valve for each of said cavities, a first passage means through said body portion connecting each of said valves to said high pressure reservoir, a second passage means in said body portion connecting said low pressure reservoir to each of said valves, individual passages through said body portion connecting each of said cavities with its respective valve of said set of valves.

25. Ap actuator assembly as defined in claim 24 wherein said valves of said set of valves are symmetrically positioned around the longitudinal axis of said assembly.

26. An actuator assembly as defined in claim 25 further comprising means for coupling said cylinder to a hydraulic actuator to deliver hydraulic fluid under pressure from said cylinder to one side of said hydraulic actuator.

27. An assembly as defined in claim 25 wherein said chamber and said cylinder are at axial opposite ends of said body portion.

28. An assembly as defined in claim 25 wherein said first passage means includes a passage extending substantially along the longitudinal axis of said assembly through a shaft projecting into said chamber and wherein said piston surrounds and slides axially along said shaft in sealed relationship thereto.