Abstract:
A valve-actuator includes a chamber, a spool that opens and closes connections between ports in the chamber, an actuator including a housing containing a piezoelectric stack having a bore, and a spring contacting the spool through the bore and forcing the stack toward the chamber, the stack expanding and contracting in response to a voltage, the spool moving in the chamber in response to said expansion and contraction.

Description:
This application is a divisional application of pending U.S. application Ser. No. 11/487,661, filed Jul. 17, 2006. 
    
    
     BACKGROUND OF THE INVENTION 
     The preferred embodiment relates generally to an apparatus for actuating a hydraulic valve spool that moves in a chamber to control a hydraulic function produced by the valve. More particularly, it pertains to regulating the magnitude of fluid pressure at an outlet port of the valve using piezoelectric actuation of the valve spool and stabilizing the valve with an electrical feedback circuit. 
     Hydraulic valves include a chamber, a spool formed with control lands, which open and closed inlet and outlet ports that communicate mutually through the chamber as the spool moves. Various techniques are used to develop forces that are applied to the spool to determine its position in the chamber including forces produced by springs, hydraulic pressure on the lands, and forces produced by an electromechanical actuator acting on the spool. 
     In the hydraulic systems of automatic transmissions for motor vehicles, pressure regulating valves typically use magnetic solenoids to control the position of the valve spool in its control chamber. The electronic driver circuit receives an electrical command signal produced by a powertrain control unit. In response to the command signal, electric current is applied to the coil of the solenoid to force the spool to a desired position in its chamber where the desired valve function is produced. But the solenoids of this type require up to 1 Amp of electric current to hold a given hydraulic pressure in the system. This energy consumption causes heating in both the coil and driver circuit. In addition, magnetic solenoids are prone to oscillation, which can adversely affect the control function of the valve. 
     There is a need, therefore, for an actuator that avoids excessive energy consumption, unnecessary heating, and operates with acceptable stability. 
     SUMMARY OF THE INVENTION 
     An actuator for a hydraulic valve that includes a chamber, and a spool that opens and closes connections between ports in the chamber, the actuator including a housing containing a piezoelectric stack having a bore, and a spring contacting the spool through the bore and forcing the stack toward the chamber, the stack expanding and contracting in response to a voltage, the spool moving in the chamber in response to said expansion and contraction. 
     Whenever the flow component moves against the piezoelectric stack, a current is produced opposing the electronic driver&#39;s signal. This induces a feedback voltage in the resistors placed in series with the piezoelectric stack adding to the command signal, thereby providing an automatic damping mechanism for the valve. 
     Temperature variations cause dimensional changes in the piezoelectric stack. These dimensional changes cause a shift in the regulated pressure. A thermistor located in the force motor assembly, in conjunction with the feedback resistors, allows the voltage across the stack to be controlled with temperature, and it helps null the effect of expansion of the piezoelectric stack. The parallel resistor path lowers the input impedance of the system and decreases its susceptibility to electromagnetic interference. 
     Conventional designs use an electromagnetic solenoid to provide force needed to move either the spool or a pilot section poppet valve. These electromagnetic solenoids require continuous current of up to 1 Amp to hold a pressure. This energy causes heating in both the coil and driver circuit. The piezoelectric force motor draws current only to change the position of the spool in the chamber. Thereafter, it resembles a reverse biased diode with the high resistance thermistor in parallel. This arrangement reduces the energy consumption and lessens the need for heat dissipation in the driver. The piezoelectric regulator valve with damping resistors has smaller pressure oscillations than a conventional system, which relies on hydraulic damping. 
     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 
       These and other advantages will become readily apparent to those skilled in the art from the following detailed description of a preferred embodiment when considered in the light of the accompanying drawings in which: 
         FIG. 1  is a schematic diagram showing a regulator valve controlled by a piezoelectric force actuator; 
         FIG. 2  is a schematic diagram showing an alternate embodiment of the regulator valve and piezoelectric force actuator of  FIG. 1 ; 
         FIG. 3  is a schematic diagram showing an alternate embodiment of the regulator valve and piezoelectric force actuator; 
         FIG. 4  is a schematic diagram showing a bleed type valve controlled by a piezoelectric force actuator; 
         FIG. 5  is a schematic diagram showing an alternate embodiment of the valve controlled by a piezoelectric force actuator illustrated in  FIG. 4 ; and 
         FIG. 6  illustrates a piezoelectric valve and resistor system that provides electronic damping and temperature compensation. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIG. 1 , a regulator valve  10  includes a valve body  12 ; a cylindrical chamber  14  formed in the body; and ports  16 ,  17 ,  18  spaced mutually along the wall of the body and opening into the chamber. A valve spool  20 , located in the chamber  14 , is formed with axially spaced control lands  22 ,  24 , which open and close the ports and hydraulic connections among the ports as the spool moves along the axis  26  of the chamber. At least one port is an inlet port, through which supply flow enters chamber  14 . At least one other port is an outlet port  18 , through which flow is exhausted to lower the pressure in chamber  14 . At least one other port is a control port  17 , through which pressure at a regulated magnitude exits the valve through chamber  14 , depending, at least in part, on the degree to which the inlet and outlet ports are fully or partially opened by a control land  22 ,  24 . 
     A compression spring  28 , located in chamber  14 , applies a spring force to the spool  20  at land  24 , where the spring contacts the spool. The force produced by spring  28  continually urges the spool axially toward a piezoelectric variable force actuator  30 , located at the opposite axial end of the chamber  14  from the location of the spring  28 . 
     Actuator  30  includes a housing  32  having axially spaced walls  34 ,  36 , a piezoelectric stack  38  located between the walls, electrodes  40 , and a connector  42 , which extends through the lower wall  36  and contacts the spool  20  at land  22  and a plate  44 . The piezoelectric stack  38  is offset from plate  44  by a spacer  45 . The actuator housing  32  may be made of a metallic material, and there may be preloaded contact between the walls  34 ,  36  and the piezoelectric stack  38  when the stack is installed in the housing. The piezoelectric stack  38  is commonly made of a piezoelectric material such as PZT. 
     The electrodes  40  connect the piezoelectric stack  38  to a power supply (not shown), which is controlled by a transmission controller, a component of a powertrain control unit (not shown). When electrical voltage is applied to the electrodes  40 , the piezoelectric stack  38  contracts and expands depending on certain variables, which include the polarity of the voltage, its amplitude, and a duty cycle. The inner surface of wall  34  is a reference surface, on which the adjacent surface of the piezoelectric stack  38  continually bears to resist axial expansion of the stack. The inner surface of wall  36  is spaced from the piezoelectric stack  38  when the stack is installed in the housing  32  and while the stack is de-energized, thereby providing a clearance space  37 , into which the stack expands when it is energized. Expansion of the piezoelectric stack  38  is transmitted by connector  42  to the valve spool  20 , causing the spool to move along axis  26  away from actuator  30 . Contraction of the piezoelectric stack  38  allows the force of spring  28  to move spool  20  along axis  26  toward actuator  30 . 
     Movement of the spool  20  in response to a duty cycle applied to electrode  40 , causes the control lands to open and close hydraulic interconnections among the ports  16 - 18 , thereby modulating pressure at a control port of the valve. Thus, by controlling the voltage level of the power supply and modulating its frequency through the powertrain control unit, the position of the spool and the magnitude of pressure exiting the valve  10  at a control port  17  can be accurately controlled, achieving the desired magnitude of pressure in at least a portion of the hydraulic system of the transmission. The electric power requirement varies depending on the specification of the piezoelectric stack  38 , operating conditions, and the duty cycle. 
       FIG. 2  illustrates an alternate embodiment, in which connector  42  contacts a plate  46 , and a coupling spring  48 , supported on plate  46 , contacts the lower surface of the piezoelectric stack  38 , thereby maintaining a resilient connection between spool  20  and stack  38 . 
       FIG. 3  illustrates an alternate embodiment, in which a piezoelectric stack  50  is formed with a central, axial bore  52 . A stem formed on the spool  20 , or a connector  54  contacting spool  20  at land  22 , extends through the wall  36  and bore  52  and contacts a spring plate  60 , which is held in contact with the piezoelectric stack  50  by the force produced by a compression spring  58 , fitted between wall  34  and plate  60 . The spring force, transmitted to the valve spool  20  through connector  54 , continually urges the spool downward away from actuator  30 ″. Spring plate  60  is spaced from the piezoelectric stack  50  by a spacer  62 , which is held in contact against the lower surface of the piezoelectric stack by the force of spring  58 . 
     When electrical voltage is applied to the electrodes  40 , the piezoelectric stack  50  contracts and expands. The inner surface of wall  36  is a reference surface, on which the adjacent surface of the piezoelectric stack  50  continually bears to resist axial expansion of the stack. The inner surface of wall  34  is spaced from the piezoelectric stack  38  when the stack is installed in the housing  32  and while the stack is deenergized, thereby providing a clearance space  56 , into which the stack expands when it is energized. 
     Expansion of the piezoelectric stack  50  compresses spring  58  and takes up its force, which is transmitted by connector  54  to the valve spool  20 , causing the spool to move along axis  26  toward the actuator  30 ″. Contraction of the piezoelectric stack  50  allows spring  58  to expand, transferring its force to the spool  20  to move along axis  26  away from actuator  30 ″. By controlling the voltage level of the power supply and modulating its frequency, the position of the spool and the magnitude of pressure of the flow exiting the valve  10  at the control port  17  are controlled. 
     The coupling spring  58  interposed between the valve body  12  and spool  20  maintains contact between the spool  20  and stack  50  through the connector  54 . 
       FIG. 4  illustrates a bleed type valve  90 , which includes a valve body  12 ; a cylindrical chamber  94  formed in the body; a fluid supply orifice  96 ; control port  97 , and an exhaust port  98 , spaced mutually along the wall of the body and opening into the chamber. A valve spool  100 , located in the chamber  14  may be guided for axial movement along axis  26  by a cylindrical bore  102  formed in the valve body  92 . Spool  100  extends through wall  36  into housing  32  of actuator  30  and is formed with a land  104 . Located within chamber  94  and formed on the valve body  92 , is a seat  106 . When land  104  contacts seat  106 , hydraulic connection between the lower volume of chamber  94  where the supply orifice  96  and the control port  97  are located and the upper volume of the chamber where the exhaust port  98  is located is closed. Those hydraulic connections are opened and closed as spool  100  moves in response to expansion and contraction of the piezoelectric stack  38 . 
     The piezoelectric stack  38  is offset from spool  100  by spacer  45 , a plate  44  between the lower surface of the stack  38 , the coupling spring  107 , a second plate  46 , and the end of spool  100 . When land  104  is seated, a pressure differential can develop across the land  104  due to there being a greater pressure in chamber  94  below land  104  than above the land. The pressure differential tends to open the valve  90  by unseating the land. When the spool is unseated, fluid entering chamber  94  through supply orifice  96  or port  97  can exit the valve through the exhaust port  98 . The pressure differential is regulated by the pressure drops across the supply orifice  96  and the seat  106  to land  104  opening. When voltage is applied to the electrodes  40 , the stack  38  expands against the inner surface of wall  34 , which expansion applies a force to spool  100  tending to reseat the land  104 . 
       FIG. 5  illustrates an alternate embodiment of the bleed type valve  90  illustrated in  FIG. 4 . Actuator  30 ″ includes the piezoelectric stack  50 , which is formed with a central, axial bore  52 . Spool  100  or a connector extending through the bore  52  and wall  36  contacts a spring plate  60 , which is held in contact with the piezoelectric stack  50  by the force produced by a compression spring  58 , fitted between wall  34  and plate  60 . The force of spring  58 , transmitted to the valve spool  100 , continually urges the spool  100  and land  104  downward away from actuator  30 ″ toward seat  106 . Spring plate  60  is spaced from the piezoelectric stack  38  by a spacer  62 , which is held in contact with a surface of the piezoelectric stack by the force of spring  58 . 
     When spool  100  is seated, a pressure differential can develop across the land  104  due to there being greater pressure in chamber  94  below land  104  than above the land. The pressure differential tends to open the valve  90  by unseating the land  104 . When land  104  is unseated, fluid entering chamber  94  through supply orifice  96  or control port  97  can exit the valve through the exhaust port  98 . When a voltage signal is applied to electrode  40 , the stack  50  expands thereby compressing spring  58 . This action increases the force applied by spring  58  to spool  100  tending to resist the force due to the pressure differential and to reseat land  104  on seat  106 . 
     The current flows until the piezoelectric material  50  expands to a length related to the applied voltage, producing a force on the plate  60  that adds to the hydraulic feedback force. The stack  50  pushes plate  60  away from the spool  100  allowing it to move away from the exhaust seat  106 , allowing more oil to flow back through the control port  97  to the exhaust seat  106  lowering the pressure in the lower chamber  94  until the pressure on the larger exhaust end of the spool  100  balances the forces of the stack  50  and spring  58 . 
       FIG. 6  illustrates the regulator valve  10  controlled by a piezoelectric force motor, such as any of the actuators  30 ,  30 ′,  30 ″. The position of the spool  20 , in the valve is damped electronically and includes temperature compensation. The terminals of a voltage driver  70  are electrically connected by damper resistors  72 ,  74  to the electrodes  76 ,  78  of the piezoelectric stack  38 . A thermistor  80  is arranged in parallel to the stack  38 , between the electrodes  76 ,  78 . 
     Resistors placed in series with the piezoelectric stack  38  produce a feedback voltage whenever the flow component moves against the piezoelectric stack, thereby providing an automatic damping mechanism for the valve  10 . Placing the thermistor  80  in the force motor assembly  30 ,  30 ′,  30 ″ allows the voltage across the stack to be controlled with temperature and helps null the effect of expansion of the piezoelectric stack. The parallel resistor path lowers the input impedance of the system and decreases its susceptibility to electromagnetic interference. 
     The piezoelectric force motor  30 ,  30 ′,  30 ″ draws current only to change the position of the spool  20  in the chamber  14 . Thereafter, it resembles a reverse biased diode with the thermistor in parallel. This arrangement reduces the energy consumption and lessens the need for heat dissipation in the driver. The piezoelectric regulator valve with damping resistors  72 ,  74  has smaller pressure oscillations than a conventional system, which relies solely on hydraulic damping. 
     The piezoelectric valve of  FIG. 6  uses a stack of piezoelectric elements  38  to produce force and displacement for the spool  20 . Conventional designs use an electromagnetic solenoid to translate an electrical command into force on the hydraulic regulator valve. The stack of piezoelectric elements  38  expands or contracts when a voltage is applied by the voltage driver circuit  70 . A coupling spring  48  can be interposed between the stack  38  and the spool to allow extra freedom of motion in case the stack&#39;s spring rate is excessive. Resistors  72 ,  74  placed in series with the stack  38  produce a feedback voltage whenever the spool  20  moves against the stack  38 , providing an automatic damping mechanism for the valve. 
     Placing a thermistor  80  on or in the housing  32  will allow the voltage across the stack  38  to be controlled with temperature and help null out the effect of expansion of the stack  38 . The parallel resistor path will lower the input impedance of the system, decreasing the susceptibility of the system to electromagnetic interference. 
     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.