Abstract:
A system controlling pressure in a transmission including a variable displacement pump, a circuit carrying fluid from the pump to the transmission, a valve using fluid in said circuit to regulate pressure that controls displacement of the pump, a source of control pressure including an accumulator, a first spring acting with said source causing the valve to change the regulated pressure, and a second spring acting with feedback pressure from said circuit to oppose said change.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to control of a variable displacement hydraulic pump and more particularly to control of an eccentric rotor vane pump. 
     2. Description of the Prior Art 
     A variable displacement hydraulic pump provides variable flow rate output depending on the transmission system flow requirement, also known as “flow demand”. Change of the flow output is achieved by changing displacement of the pump or volume of fluid transported from inlet to outlet per one revolution of the pump&#39;s input shaft. Flow output could be maintained constant when pump rotational speed increases. 
     Variable displacement hydraulic pumps are used for many automatic transmissions. Unlike a variable displacement hydraulic pump, another type of transmission pump, a gear pump, uses a fixed displacement that provides hydraulic flow directly proportional to pump&#39;s rotational speed. 
     The power hydraulics and control system for an automatic transmission require that flow rate supply (flow demand) be proportional to pressure required at a given temperature and not affected by pump speed. 
     System hydraulic pressure is regulated by an electronically controlled hydraulic valve and maintained by a hydraulic control system that exhausts an excess of hydraulic flow to transmission sump. 
     A difference between the hydraulic flow required and the delivered flow at pressure and temperature results in excessive hydraulic flow at pressure, thereby producing hydraulic losses that are proportional to pump&#39;s hydraulic output, which is proportional to pump speed. 
     SUMMARY OF THE INVENTION 
     A system controlling pressure in a transmission including a variable displacement pump, a circuit carrying fluid from the pump to the transmission, a valve using fluid in said circuit to regulate pressure that controls displacement of the pump, a source of control pressure including an accumulator, a first spring acting with said source causing the valve to change the regulated pressure, and a second spring acting with feedback pressure from said circuit to oppose said change. 
     No specially sized openings or bleed orifices in the control system&#39;s hydraulic passages are required to reduce hydraulic control system response time or to reduce hydraulic pressure instability. 
     Due to the elimination of control bleed orifices, hydraulic losses that would have required an increase of engine power to maintain hydraulic flow are reduced, thereby leads to increased fuel economy. 
     The scope of applicability of the preferred embodiment will become apparent from the following detailed description, claims and drawings. It should be understood, that the description and specific examples, although indicating preferred embodiments of the invention, are given by way of illustration only. Various changes and modifications to the described embodiments and examples will become apparent to those skilled in the art. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which: 
         FIG. 1  is a cross section of a variable displacement vane pump; and 
         FIG. 2  is schematic diagram of a system for controlling a variable displacement vane pump for an automatic transmission. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, there is illustrated in  FIG. 1  a variable displacement hydraulic pump  10  for an automatic transmission, designated generally as  40  in  FIGS. 1 and 2 , of a motor vehicle. 
     A driven rotor  12  supports vanes  14 , which are supported on the rotor to slide radially into contact with an eccentric moveable bore ring  16  surrounding the rotor. Displacement of the pump is reduced as the moveable bore ring  16  pivots at a pivot pin  18 , decreasing its eccentric position relative to the driven rotor  12 . 
     A force produced by a compression spring  20  and applied to the moveable bore tends to pivot the moveable bore ring  16  clockwise about pivot pin  18  to its maximum eccentricity, thereby tending to produce maximum pump displacement. 
     A hydraulic pressure in a sealed chamber  23 , containing pressurized hydraulic fluid and extending counterclockwise circumferentially between pivot pin  18  and seal  22 , opposes the force of spring  20  on the moveable bore ring  16 , thereby tending to pivoting the moveable bore ring counterclockwise, reducing the eccentricity of the moveable bore ring  16  and thereby reducing the pump&#39;s volumetric displacement. 
     A vane ring  24  limits the inward radial travel of the vanes  14 . Fluid flows from a sump  26  through a strainer  28  to the pump&#39;s inlet. The vanes  14  rotate clockwise about axis  30  drawing fluid from the inlet along an arcuate passage  32  in which the fluid is compressed and delivered though an outlet passage  34  to the transmission hydraulic system. Chamber  36  is volume extending clockwise circumferentially between pivot pin  18  and seal  22 , exhausted through pump housing  38 . 
     At the vehicle operating state in which pump  10  begins to produce more flow than is required to maintain the operating system&#39;s targeted pressure, the excess flow is redirected to the sealed chamber  23 . As pressure begins to increase within the sealed chamber  23 , the hydraulic force counteracting the spring  20  force begins to pivot the bore ring  16  counterclockwise about pivot pin  18 , thereby reducing the pump&#39;s eccentricity and the flow the pump can deliver. When additional flow is required to maintain the system&#39;s operating pressure, flow to the sealed chamber  23  is reduced by the system pressure regulating valve  38 , thereby reducing the force counteracting the spring  20 . The moveable bore ring  16  changes position to equalize the forces, increasing the pump&#39;s flow to meet the additional flow demand. 
     The pump control system  50 , shown in  FIG. 2 , includes a source  52  of solenoid feed pressure, a limited pressure derived from pump output pressure in line  34 , which is line pressure. 
     A line pressure control solenoid  54 , an electro-hydraulic device, controls pressure in a line pressure control (LPC) circuit  56 , which connects the solenoid output and to the main regulator valve (MRV)  58 . The LPC solenoid  54  is commanded electrically to output a specified pressure, which corresponds to a desired line pressure. A LPC orifice  60  is located in circuit  56 . 
     A command rate limiter  62  is an accumulator, which communicates hydraulically through orifice  60  with the line pressure control solenoid  54  and contains fluid under pressure produced and maintained by a spring  64 . 
     The MRV  58  includes a valve spool  59 , displaceable along a valve chamber  61 . A line pressure offset spring  66  establishes the minimum line pressure that can be achieved using the MRV  58 . Pressure from the line pressure control source in circuit  56  produces a force on the left hand end of the spool  59  of MRV  58  in addition to the force of spring  66 . 
     A decrease-line-pressure circuit  68  carries regulated pressure to chamber  23  of pump  10  for actuating the bore ring  16  in opposition to the force of spring  20 . 
     A noise response reduction spring  72  applies a force to the right-hand end of the MRV  58  spool  59 . Noise response reduction spring  72  has a spring constant sufficiently high to eliminate overtravel of spool, preferably in the range 3-7 N/mm. 
     Line pressure circuit  76  carries fluid at line pressure from the outlet  34  of pump  10  to the transmission&#39;s hydraulic actuation system, which returns fluid to sump  26 . Feedback pressure in circuit  80  produces a force on the right-hand end of the spool  59  of MRV  58  in addition to the force of spring  72 . The LPC pressure and the force of spring  66  are opposed by a pressure force from line pressure circuit  80  and the force of noise response reduction spring  72 , resulting in balanced forces at a position of the MRV spool  59  that meters flow to line pressure decrease circuit  68 . 
     A prioritized oil circuit  70  can be closed to reduce total flow load. 
     The transmission flow load  74  is the flow rate at line pressure required for current transmission operation, except the flow rate in prioritized circuit  70 . 
     No orifice is used in the feedback pressure line  80  to the MRV  58 , because orifice  60  is more effective at providing damping. Lag resulting in delayed response to a line pressure error would result if an orifice in feedback line  80  were present. 
     The LPC circuit orifice  60  provides damping for the MRV  58 . Orifice  60 , in conjunction with the command rate limiter  62 , set the maximum rate at which the spool  59  of MRV  58  will move in response to a change in LPC pressure in circuit  56 . A portion of the fluid flow throttled through the LPC circuit orifice  60  flows into the command rate limiter (CRL)  62 , thereby requiring more flow to actuate MRV  58  and damping MRV movement. 
     Fluid, which is pushed toward LPC solenoid  54  when the spool of MRV  58  is moving leftward toward line pressure offset spring  66  to reduce line pressure, also flows into the CPL accumulator  62 . This flow rate can often be greater than the flow rate that can readily flow through the LPC circuit orifice  60 , or that can be rejected by LPC solenoid  54 , resulting in a pressure buildup on LPC side of MRV  58  and holding line pressure higher than the commanded line pressure. 
     The noise response reduction spring  72  minimizes rightward displacement of the spool of MRV  58  away from line pressure offset spring  66 . Spring  72  opposes excessive valve movement in response to noise in the pressure signals that would change flow load  74 . This type of noise input to MRV  58  could result in large displacements of MRV due to (i) the speed discrepancy between MRV  58  and its displacement control mechanism, and (ii) the fact that MRV controls decrease pressure in circuit  68 , but its feedback signal is line pressure in line  80 . The function of noise response reduction spring  72  is essentially unidirectional. 
     In accordance with the provisions of the patent statutes, the preferred embodiment has been described. However, it should be noted that the alternate embodiments can be practiced otherwise than as specifically illustrated and described.