Method and apparatus for controlling a friction device

Abstract: A friction device, such as a clutch or brake, has a fluid-operated piston (52) that is pressurized to cause engagement of the friction device. An accumulator (20) and flow restriction are disposed to assist in controlling engagement time of the friction device. The accumulator (20) and flow restriction are disconnected from each other and from the friction device during disengagement. A shift valve is positioned to provide the desired fluid connections during engagement and disengagement.

Description: This invention relates to a method and apparatus for controlling a fluid-operated piston of a friction torque transmitting device, and more particularly, to a method and apparatus which can rapidly exhaust the fluid-operated piston, and which can separate the piston exhaust fluid from the exhaust fluid of a timing device during disengagement. US-A-2 833 385 discloses a control mechanism for a friction device in which the friction device is engaged and disengaged by pressurised oil. Cooling oil is also supplied to the friction device under the control of an accumulator which is exhausted on disengagement of the friction device. Prior art controllers seeking to separate the fluid of the timing device from the friction device use a valve member, separate from the shift valve, to control fluid flow to and from the friction device and the accumulator. With such controllers, the timing device is exhausted through a feed restriction and/or a ball check valve to the exhaust passage of the friction device. While these controllers may provide for slightly more rapid friction device exhaust, they do not permit rapid accumulator exhaust of the timing device. The present invention seeks to provide an improved method and apparatus for controlling a friction device. Accordingly, an aspect of the present invention provides a control mechanism for controlling a fluid-operated friction device as specified in claim 1. The present invention can incorporate a flow control function for a friction device and accumulator (timing device) into a shift valve, thereby eliminating a separate control valve and the accompanying cost of manufacture and complexity of assembly. Such a flow control function can selectively interconnect the accumulator, any flow restrictions and the friction device to provide for timed engagement of the friction device while permitting rapid exhaustion of the accumulator and timed exhaustion of the friction device during disengagement. By reducing the disengagement time of the off-going friction device, the total heat generated during a downshift can be significantly reduced. A computer simulation of the energy transfer during a downshift of a commercially available transmission indicates that a reduction in the energy density of 252 KJ/m² (220 ftlb/sqin) can be attained. This is a substantial reduction in the heat generating potential and an improvement in the overall operating efficiency. This arrangement can also have the advantage of having the accumulator fully exhausted in preparation for an immediate upshift if such is required by the driver. These same benefits can be attained during an upshift since the accumulator of the off-going friction device is exhausted substantially separately. Advantageously, the friction device and the accumulator are in fluid communication with a common flow restriction during engagement and the accumulator is connected for free flow exhaust during disengagement. Preferably, the friction device is supplied through a flow restriction during engagement and is exhausted through another flow restriction during disengagement. In a practical embodiment, an accumulator is disposed in fluid flow arrangement with a shift valve, a flow restriction and a fluid-operated piston, and further wherein, the shift valve is selectively controlled to provide serial fluid flow between the restriction and the accumulator and between the restriction and the piston during engagement to control the timing thereof and to provide a flow path from the accumulator to exhaust in bypassing relation with the restriction and the friction device during disengagement. According to another aspect of the present invention, there is provided a method of controlling a fluid-operated friction device as specified in claim 7. The invention is not restricted for use with any particular type of fluid-operated torque transmitting device. Some embodiments of the present invention are described below, by way of illustration only, with reference to the accompanying drawing, in which: Figure 1 is a schematic representation of a portion of a first embodiment of control system; Figure 2 is a schematic representation of a portion of a second embodiment of control system; and Figure 3 is a schematic representation of a portion of a third embodiment of control system. Referring to Figure 1, a conventional positive displacement pump 10, which is driven by an engine (not shown) is connected to deliver pressurized fluid to a passage 12. The pressure level in the passage 12 is controlled by a conventional regulator valve 14 which bypasses excess fluid flow to a sump or reservoir 16. The fluid in passage 12 is delivered to a control valve assembly 18 which comprises a plurality of conventional control elements and valve members. The control valve assembly 18 is effective to control the gear ratios in a conventional automatic transmission (not shown). One of the valve members included in the control assembly 18 is a shift valve 20 which includes a movable valve member 22 operable to control fluid flow between a plurality of ports or passages (hereinafter referred to simply as ports) connected to the shift valve 20. These ports include a supply port 24, an accumulator feed port 26, an accumulator return port 28, a clutch port 30 and two exhaust ports 32 and 34 which are, in turn, connected to the reservoir 16. The port 28 is connected through an accumulator passage 60 to a conventional hydraulic accumulator 62. As is well known, such accumulators have an accumulator chamber 64 and a control chamber 66. A spring member 68 urges a floating piston 70 in a direction to decrease the size of the accumulator chamber 64. The clutch port 30 is connected via a passage 40, through a restriction 42 and a parallel ball check valve 44, to a passage 46 which, in turn, is connected to a selectively engageable clutch assembly 48. The ball check valve 44 is open when fluid is flowing towards the port 30 so as to allow the fluid to by-pass the restriction 42 during disengagement of the clutch assembly 48. The passage 46 also communicates with an accumulator passage 50, which in turn, is connected to the accumulator feed port 26. The clutch 48 is a conventional fluid-operated friction clutch incorporating a fluid-operated piston 52, an input hub 56, an output hub 58, and a plurality of interleaved friction discs 54 which are alternately splined between the hubs 56 and 58. The input hub 56 is connected to a transmission input shaft, and the hub 58 is connected with a planetary gear member (neither of which is shown). Of course, the arrangement could be reversed. When fluid pressure is applied via passage 46 to act on the piston 52, the friction discs 54 are urged into frictional engagement to provide a torque transmitting connection between the hubs 56 and 58. The movable valve member 22 is urged by a spring 36 to the spring set position shown. A shift signal passage 38 is operable to supply fluid pressure to the movable member 22 against the spring 36 to a pressure set position. The shift signal is generated in a conventional manner by the control assembly 18 and in general will be determined by the engine torque level and vehicle speed. In the spring set position shown, the supply port 24 and accumulator feed port 26 are both closed by the valve 20, while the accumulator return port 28 is open in a manner as to allow free flow through exhaust port 32, and the clutch port 30 is open in a manner as to allow restricted flow through exhaust port 34, such that fluid can flow through both these ports 30, 34 into the reservoir 16. In the pressure set position, that is, when passage 38 is pressurized, the supply port 24 becomes connected to the clutch port 30 and the accumulator feed port 26 becomes connected to the accumulator return port 28. When the shift valve 20 is actuated by pressure in the passage 38, that is, when the shift valve 20 is in the pressure set position, fluid passing through the restriction 42 is directed to both the piston 52 and, via passages 50 and 60, to the accumulator chamber 64. As is well known, the pressure rise in the fluid acting on the piston 52 is dependent upon the restriction 42 and the accumulator 62. The restriction 42 provides a flow control and the accumulator 62 provides a variable volume which controls pressure rise. In order to provide a wide range of control for the pressure change characteristics at the piston 52, an accumulator valve 72 is provided. The valve 72 receives a torque signal via a passage 74 from the control assembly 18. The accumulator valve 72 modulates the signal in passage 74 and delivers the modulated signal through a passage 76 to the chamber 66. As the pressure in chamber 66 increases, the pressure level at which the piston 70 begins to move will also increase, thereby affecting the initial pressure level directed to the piston 52. The fluid passing through the restriction 42 is all directed to the piston 52, until the piston 70 begins to move. While it is advantageous to provide timed clutch engagement which is dependent upon the torque being transmitted, it is equally desirable to permit rapid disengagement of the clutch independent of the torque being transmitted. To accomplish this, the shift valve 20, upon removal of the signal in passage 38, will be urged to the spring set position. In this position, the accumulator chamber 64 is freely exhausted to the reservoir 16, while the fluid in clutch 48 is exhausted through the ball check 44 and passage 40 to the reservoir 16. In most arrangements, it is desirable to provide a slight restriction in the exhaust passage of the clutch piston 52 to ensure against engine flare. In the prior art control devices, the accumulator fluid and the clutch fluid are exhausted in unison through a single flow path, such that a longer clutch disengagement time is present. Also in the event that an immediate upshift is demanded by the valve 20, the accumulator chamber 64, in a conventional system, may have some residual pressure which would affect the clutch engagement time. Such an event is not likely to take place with the above-described embodiment due to the free fluid flow path between the accumulator and the reservoir which is provided by the shift valve 20. The embodiment shown in Figure 2 includes a shift valve 100, which has associated therewith a supply port 102, an accumulator feed port 104, a clutch apply port 106, a clutch return port 108, an accumulator port 110 and a pair of exhaust ports 112 and 114. In the spring set position shown, the ports 102, 104 and 106 are blocked, while port 110 is connected to the reservoir at port 112, and the port 108 is connected through a restricted passage to the reservoir. The port 106 is connected via a passage 116 through a restriction 118 to a passage 120 which is connected to an apply piston 122 of a disc brake 124. The passage 120 is also in fluid communication with a passage 126 which is connected to the port 108 and a passage 128 which is connected to the port 104. The disc brake 124 is a conventional torque transmitting device in which a plurality of friction discs are urged into engagement by the piston 122 to establish a torque transmitting connection between a hub 130 and a housing or other stationary member 132. The shift valve 100 is also usable with a disc clutch, such as that described above for clutch 48. The port 110 is connected via a passage 134 to a conventional accumulator 62. The accumulator 62 has associated therewith an accumulator valve 72 which functions in a manner identical to that described for the accumulator valve in Figure 1. When the shift valve 100 receives a shift signal through the passage 38, shift valve 100 is effective to provide fluid communication between the supply port 102 and the port 106 and also between the port 104 and the port 110. In this pressure set position, the shift valve 100 is also effective to close the ports 108, 112 and 114. With the valve 100 in the pressure set position, fluid is directed from the supply port 102 to the port 106 and through passages 116 and restriction 118 to the fluid-operated piston 122. Fluid is also directed via passages 128 and 134 to the accumulator 62. The restriction 118 provides a flow rate limit to the fluid entering the brake 124 and the accumulator 62, such that the restriction 118 and the fill time of the accumulator 62 are operable to provide a controlled engagement time for the brake 124. As described above for the embodiment of Figure 1, the initiation of engagement can be varied by the accumulator valve 72 in response to various torque signals supplied by the passage 74. When the shift signal is removed from passage 38, the valve 100 will be urged to the spring set position, such that the accumulator 62 is connected directly to the reservoir and the apply piston 122 is connected via passage 126 and a restriction to the reservoir 16. The exhaust fluid from piston 122 bypasses the restriction 118 such that the exhaust time is substantially different from and considerably shorter than the engagement timing. As described above with Figure 1, the accumulator fluid is exhausted directly to the reservoir so that it does not affect the disengaging time of the piston 122. A further embodiment is shown in Figure 3 in which a shift valve 200 is utilized to control the engagement and disengagement of a torque transmitting device, such as a band brake 202 as depicted, or a clutch as shown in Figure 1, or a disc brake as shown in Figure 2. The shift valve 200 has a supply port 204, a first outlet port 206, a second outlet port 208 and a pair of exhaust ports 210 and 212. The supply port 204 is connected to the supply passage from control assembly 18 through a restriction 214. In the spring set position shown, the supply port 204 is closed while the ports 206 and 208 are connected to exhaust ports 212 and 210, respectively. The port 206 is connected via a passage 216 with a servo piston 218 which is operable when pressurized to engage the band brake 202. The port 208 is connected via passage 220 with the accumulator 62. The accumulator 62 and its associated accumulator valve 72 function as previously described for the embodiments of Figures 1 and 2. The shift valve 200, upon the imposition of a shift signal in passage 38, is moved to the pressure set position, wherein the ports 210 and 212 are closed and the supply port 204 is connected with ports 206 and 208 simultaneously. Fluid pressure admitted through the port 204 is directed to both the servo piston 218 and the accumulator 62. However, as previously mentioned, all of the in-coming fluid must pass through restriction 214 which, in cooperation with the accumulator 62, provides an engagement time characteristic for the torque transmitting device 202. When the shift signal is removed from passage 38 and the shift valve 200 has returned to the spring set position, the accumulator 62 is connected directly with the exhaust through ports 208 and 210, while the servo piston 218 is connected to a restricted exhaust via ports 206 and 212. The restriction in the exhaust port for the servo piston 218 presents a minimum flow restriction to the fluid. The use of such exhaust restriction is not mandatory and in some instances, may not be desirable. In some control systems, the size of the exhaust passage may be sufficient to provide the desired control for exhaust flow. As with the embodiments described in Figures 1 and 2, the accumulator 62 is freely exhausted during disengagement of the torque transmitting device 202. Along with being exhausted, the fluid from the accumulator 62 is separated from the flow path of the fluid leaving the servo piston 218.

Claims: A control mechanism for controlling a fluid-operated friction device comprising a fluid chamber and a piston (52) disposed in the fluid chamber for engaging the friction device; the control mechanism comprising a control valve (20) for directing fluid in a path (SUPPLY) to the fluid chamber; characterised by an accumulator (62) coupled to control the rate of pressure increase in the chamber during engagement of the friction device; in that the control valve (20) is effective to connect the accumulator (62) to the path during engagement of the friction device and, on disengagement of the friction device, to exhaust the fluid chamber and accumulator (62) substantially simultaneously and separately during a disengagement period, the accumulator being exhaustable through an unrestricted path during the disengagement period. A control mechanism according to claim 1, wherein the fluid chamber is connected to the control valve by a second path (40, 116) for supplying fluid from the path (SUPPLY) to the fluid chamber, the accumulator being connectable to the second path (40, 116) by a third path (50, 128) passing through the control valve. A control mechanism according to claim 2, wherein the second path (116) has a restriction formed therein so as to restrict fluid flow to the friction device. A control mechanism according to claim 1, wherein the control valve (200) is adapted to allow the passage of fluid from the path (SUPPLY) to both the friction device and the accumulator. A control mechanism according to any preceding claim, wherein the control mechanism is adapted to engage the friction device during an engagement period, the accumulator being adapted to control the rate of increase in pressure in the fluid chamber during a portion of the engagement period. A method of controlling a fluid-operated friction device, which device comprises a fluid chamber and a piston (52) disposed in the fluid chamber for engaging the friction device; the method comprising the steps of (a) during engagement of the friction device directing fluid from a supply path (SUPPLY) into both the fluid chamber and an accumulator (62) provided for controlling the rise in pressure increase in the chamber, (b) during disengagement of the friction device exhausting the fluid chamber and the accumulator (62) substantially simultaneously and separately, and (c) exhausting the accumulator through an unrestricted path during disengagement of the friction device.

Inventors: KOENIG MELISSA MEI, VUKOVICH WILLIAM JOSEPH, KOENIG, MELISSA MEI, VUKOVICH, WILLIAM JOSEPH
Applicants: GEN MOTORS CORP, GENERAL MOTORS CORPORATION
Application year: 1991
Main classification: F16D48
IPC codes: F16D48, F16H61