Abstract:
An actuator system includes a piezoelectric pump; an accumulator including a cylinder, a piston located in the cylinder, a port located at a first side of the piston and hydraulically connected to the pump outlet and a switching circuit, a relief port hydraulically connected to the reservoir and located at a second side of the piston, and a spring biasing the piston toward the port; and a hydraulically actuated servo including a front port hydraulically connected to the switching circuit, a back port hydraulically connected to the reservoir, and the servo causing the friction element alternately to engage and to disengage in response to a magnitude of hydraulic pressure in the servo.

Description:
BACKGROUND OF THE INVENTION 
   The preferred embodiment relates generally to an apparatus for actuating a friction element. More particularly it pertains to a piezoelectric actuator for use in an automatic transmission. 
   A conventional step-ratio automatic transmission system uses wet friction components, such as band brakes and plate clutches, for automatic shifting. Friction components couple, decouple or ground rotating gear elements to alter a torque path within gear sets, thereby achieving a desired level of torque multiplication between an engine and the driven wheels. When used as a part of a gasoline-electric hybrid powertrain, the transmission system may include a gear set that couples torque from the engine with that from an electrical machine. 
   A typical friction element is actuated hydraulically, requiring an engine-driven oil pump and a complex hydraulic pressure control system that typically includes a number of pressure and flow control valves. A powertrain control unit controls the engine and the hydraulic control system in a coordinated manner for automatic shifting. The engine-driven oil pump draws an appreciable amount of torque from the engine, lowering overall powertrain efficiency. Alternatively, friction components can be actuated mechanically or through other means. However, these means tend to be physically large in order to deliver torque capacity comparable to hydraulically-actuated systems. 
   New applications of automatic transmission technologies, such as a gasoline-electric hybrid powertrain, require small, packageable, highly efficient actuators as an alternative to conventional systems. 
   SUMMARY OF THE INVENTION 
   A self-contained hydraulic actuator system, which includes a piezoelectric pump and an accumulator, is suitable for vehicular powertrain applications and non-vehicular applications. The accumulator provides fluid flow to quickly stroke a hydraulic servo device while the pump accurately controls fluid pressure within the servo. The self-contained actuator system requires neither an engine-driven oil pump nor a conventional hydraulic control system. Thus, it entirely eliminates parasitic torque loss associated with the engine-driven oil pump for improved powertrain efficiency. Its compact, stand-alone design is readily packageable and serviceable. The new device can be used as a part of a new powertrain or to add a friction component to an existing vehicular subsystem that is equipped with neither a pump nor a hydraulic control system. 
   The actuator is suitable for a wide variety of vehicular and non-vehicular applications that require a hydraulically-operated device. For example, it is suitable for actuating a transfer case friction component in an all-wheel drive powertrain application even if an engine-driven pump and a hydraulic control system are not available. This invention is also well-suited for a use in a gasoline-electric hybrid powertrain that can benefit from as few as one additional gear ratio for improved fuel economy. The modular actuator enables the use of a conventional planetary gear train without requiring an inefficient engine-driven oil pump and a complex hydraulic system for providing additional gear ratios in the hybrid powertrain. 
   A self-contained actuator system for actuating a friction element includes a hydraulic pump that includes an inlet and an outlet for pumping fluid from the inlet to the outlet in response to expansion and contraction of a pump element produced by a piezoelectric effect in response to a voltage signal having a variable amplitude and frequency. An accumulator includes a cylinder, a piston located in the cylinder, a port located at a first side of the piston and hydraulically connected to the pump outlet and a switching circuit, a spring biasing the piston toward the port, and a pressure relief port connected to a fluid reservoir and located at a second side of the piston. A hydraulically actuated servo includes a front port hydraulically connected to the switching circuit, a back port hydraulically connected to the reservoir, and the servo causing the friction element alternately to engage and to disengage in response to a magnitude of hydraulic pressure in the servo. 
   Advantages of various embodiments include stand-alone modularity and packageability of the actuator, which can be readily attached to a friction component with a hydraulic servo without requiring an engine-driven oil pump and a conventional hydraulic control system. Efficiency of the actuator is high, partially because the pump can be switched off except during friction component engagement, release, and accumulator pressure maintenance. Pressure applied to a friction element during engagement by the actuator can be controlled accurately due to the use of a piezoelectric diaphragm pump with or without the use of conventional hydraulic pressure regulator valves. The actuator is a self-contained system that is sealed for its entire service life. 
   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 of a vehicular powertrain system; 
       FIG. 2  is a schematic diagram of a self-contained friction component actuator; 
       FIGS. 3A and 3B  are schematic diagrams of a piezoelectric diaphragm pump for use with the actuator of  FIG. 2 ; 
       FIG. 4A  is a schematic diagram showing the state of the friction component actuator before engagement of the friction component; 
       FIG. 4B  is a schematic diagram showing oil flow during engagement of the friction component; 
       FIG. 4C  is a schematic diagram showing oil flow during release of the friction component while independently charging the accumulator; 
       FIGS. 5A ,  5 B and  5 C illustrate examples of a control sequence, hydraulic response and mechanical response, respectively, of the friction component actuator shown in the embodiment of  FIGS. 4A ,  4 B and  4 C; 
       FIG. 6  shows an alternative embodiment in which the back side of the accumulator and the back side of the actuator are used as reservoirs; 
       FIG. 7  shows an alternative embodiment that in which a low rate accumulator is a reservoir; 
       FIG. 8  shows an alternative embodiment that includes a switching circuit having a pressure regulator valve; 
       FIG. 9  shows an alternative embodiment that includes a switching circuit having a pulse-width modulated (PWM) valve; and 
       FIGS. 10A ,  10 B and  10 C are partial cross sections showing packaging examples of the actuator for a band brake application. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   A conventional step-ratio automatic transmission system  1 , as illustrated in  FIG. 1 , includes wet friction components  2  such as band brakes and plate clutches for automatic shifting among the speed ratios produced by the transmission. Friction components couple, decouple or ground rotating gear elements to alter a torque path within gear sets  3 , to achieve a desired level of torque multiplication between an engine  4  and the driven wheels  5 . When used as a part of a gasoline-electric hybrid powertrain, the system  1  may include a gear set  6 , which couples torque from the engine  4  with torque from an electrical machine  7 . A typical friction element  2  is actuated hydraulically, requiring an engine-driven oil pump  8  and a complex hydraulic pressure control system  9 , which typically includes a number of pressure and flow control valves. A powertrain control unit  10  controls the engine  4 , the hydraulic control system  9  and the electrical motor  7  in a coordinated manner. The engine-driven oil pump  8  draws an appreciable amount of torque from the engine  4 , lowering overall powertrain efficiency. Alternatively, friction components  2  can be actuated mechanically or through other means. However, these means tend to be physically large in order to deliver torque capacity comparable to hydraulically-driven systems. 
   Referring now to  FIG. 2 , the modular friction component actuator  11  is illustrated with a band brake system  12 , as an example. The actuator  11  includes a unique arrangement of sub-components, including a piezoelectric diaphragm pump  13 , an accumulator  14 , an apply line  15 , a flow control orifice  16 , a switching circuit  17 , a return line  18  and an oil reservoir  19 . The accumulator  14  may be a piston-type or a bladder-type and may include a pressure relief port  21  or a pressure relief valve. The actuator  11  can be readily attached to a friction component, in this example, a band brake servo  20 . The actuator system  11  is commanded through a vehicular powertrain control unit  10 , shown in  FIG. 1 . The actuator  11  strokes and de-strokes the servo  20  in order to engage or release, respectively, the band brake  12 , whereby the gear ratios produced by the transmission gear sets  3  are automatically changed. Although the actuator is described for used with a band brake system  12 , as readily recognized, the friction component actuator  11  can be used to actuate a hydraulic servo of any other friction component device in a vehicular system, such as a multiple plate clutch. 
     FIG. 3  illustrates an example of a piezoelectric diaphragm pump  13 . It consists of a piezoelectric stack  32 , an electrode  22 , a diaphragm  23 , an inlet port  24 , an inlet chamber  25 , an inlet valve  26 , a pumping chamber  27 , an outlet valve  28 , an outlet chamber  29 , an outlet port  30  and an enclosure  31 . A one-way flow valve, such as a reed-type valve, may be employed for the inlet valve  26  and the outlet valve  28 . The diaphragm  23  may be made of a metallic material and may be preloaded when installed adjacent to the piezoelectric stack  32 . The piezoelectric stack  32  is commonly made of a piezoelectric material such as PZT. The electrode  22  connects the piezoelectric stack  32  to a power supply (not shown), which is controlled by a powertrain control unit (not shown). When electrical voltage is applied, the piezoelectric stack  32  either contracts or expands. As illustrated in  FIG. 3A , when the stack  32  retracts the diaphragm  23 , oil flows into the pumping chamber  27  through the inlet valve  26 . As shown in  FIG. 3B , when the piezoelectric stack  32  expands and deflects the diaphragm  23 , oil flows out from the pumping chamber  27  into the outlet port  30  through the outlet valve  28 . Thus, by controlling the voltage level of the power supply and modulating its frequency through the powertrain control unit, the diaphragm motion can be accurately controlled, achieving the desired level of oil flow rate and pressure. The power requirement varies depending on the specification of the piezoelectric stack  32 , operating conditions and duty cycles. 
   The actuator  11  of  FIG. 2  consumes electrical energy only during a gear ratio change, called a shift event, which typically lasts for 1 sec. and intermittently thereafter, if required, to make up the effects of oil leakage, if any. Its peak power consumption is expected to be below 100 watts during the short engagement interval. In contrast, an engine-driven oil pump runs constantly in a typical automatic transmission system, continuously consuming from 700 watts (1 hp) to over 2000 watts (3 hp) of engine output, depending on pump capacity and operating conditions. The piezoelectric pump  13  has no rotating elements, unlike a conventional electrical pump, attaining a superior hydraulic response time required for directly controlling pressure applied to a friction component during its engagement. However, the pumping capacity of the piezoelectric pump is generally insufficient by itself for stroking a friction component servo within 0.5 second or less when friction component engagement is commanded. 
     FIGS. 4A ,  4 B and  4 C illustrate the unique operating principles of the actuator  11  based on the arrangement of sub-systems shown in  FIG. 2 . The actuator  11  is illustrated with a band brake  12 , as an example.  FIG. 4A  shows the state of the modular device  11  before band brake engagement. The piezoelectric pump  13  remains off with no electrical power consumption. The apply line  15  is fully filled with oil and pressurized. The accumulator  14  is either fully or partly stroked, depending on device applications. The spring rate of the accumulator  14  is designed to maintain the apply line  15  at a pre-specified pressure level prior to engagement. The switching circuit  17  disconnects the apply line  15  from the servo  20 . The outlet valve  28  of the pump  13  is closed, isolating the apply line  15 . The volume of the accumulator  14  and the apply line  15  is designed to retain the right amount of oil to stroke the servo  20  in a timely manner when the band engagement is commanded. 
     FIG. 4B  shows the oil flow during band engagement. When the engagement is commanded, the switching circuit  17  opens a flow channel from the apply line  15  to the band servo  20 . As the accumulator  14  de-strokes, oil flows from the apply line  15  into the servo  20  in a controlled manner through the flow control orifice  16  in order to stroke the servo piston within a desired time interval. At the same time, oil may flow from the back side of the servo  20  into the reservoir  19  through the back port  48  and from the reservoir  19  to the back side of the accumulator  14  though the relief port  21 . The flow control orifice  16  may be placed within the switching circuit  17 . After a calibrated time delay, the pump  13  is turned on to actively control band servo pressure for smooth band engagement. The electrical voltage and frequency applied to the pump  13  may be controlled either through an open loop process or a closed loop process based on measured or estimated slip speed of the band brake  12 . Closed loop feedback signals may be augmented with pressure measurements at the apply line  15  or the servo  20 . After completing band engagement, the pump  13  continues to run for a short time until the servo pressure reaches a holding level of the band brake  12 . This may be readily accomplished by installing a pressure relief port  21  or a valve as a part of the accumulator  14  and running the pump  13  for a pre-calibrated period. Specifically, as the accumulator  14  is stroked, the relief port  21  opens and returns oil to the reservoir  19  to maintain pressure at the holding level. Alternatively, apply line pressure can be directly monitored and controlled by electrical voltage and frequency applied to the pump  13 . After the servo pressure reaches the holding level, the pump  13  is shut off, no longer consuming any electrical power. The outlet valve  28  of the pump  13  is closed, maintaining the constant apply line pressure and servo pressure. Alternately, a three-way valve may be employed in the switching circuit  17  in order to disconnect the servo  20  from both the apply line  15  and the return line  18  at the same time and to maintain the servo pressure. A back port  48  may be attached to the servo  20  and connected to the return line  18 , capturing leakage flow, if required, across the servo piston back to the reservoir  19 . The reservoir  19  may be equipped with a fill-level sensor and a refill port for an improved serviceability in the case of an unexpected loss of oil. 
     FIG. 4C  illustrates the oil flow during band release. When the release of the band brake  12  is commanded, the switching circuit  17  opens a flow channel from the servo  20  to the return line  18 , which feeds into the oil reservoir  19 . Some oil may flow into the back side of the servo  20  through the back port  48 . The apply line  15  and the accumulator  14  remain pressurized at the friction component holding level during and after the release event. This enables the actuator  11  to respond to the next engagement command without any delay. Although it is not necessary, the pump  13  can be turned on during the release to ensure that the apply line  15  and the accumulator  14  remain pressurized at a desired level. In this case, the pump control may be based on an open loop process with pre-calibrated control parameters for a fixed duration. The accumulator  14  may include a pressure relief port or valve  21  to accurately limit the charge pressure at a pre-calibrated level. The relief port  21  may also be used to purge trapped air to prevent air accumulation. The pump control may be based on a closed loop process based on measurements of apply line pressure to achieve a desired pressure level. After running the pump  13  for a pre-calibrated period through an open loop process or after the apply line pressure reaches a desired level by means of a closed loop control, the pump  13  can be shut off until the next engagement event is commanded. 
     FIGS. 5A ,  5 B and  5 C show the operating behavior of the actuator  11 . Specifically, an example of a control sequence, hydraulic response and mechanical response are presented in  FIGS. 5A ,  5 B and  5 C, respectively, with reference to the embodiment of  FIG. 4 . In  FIG. 5A , the pump control signal indirectly represents a level of electrical voltage and frequency. In the same figure, the solenoid valve control signal indirectly reflects a level of electrical current. Before an engagement event, the pump power is off  301  with no input signal, thereby consuming no electrical power. The solenoid valve in the switching circuit is also turned off  302 . The accumulator  14  and the apply line  15  remain pressurized at a holding level of 150 psi  303 . The accumulator  14  is fully stroked, indicated at the zero position  304 . The servo pressure is zero  305  and its piston stays de-stroked as indicated at 0.13 inch position  306 . 
   When engagement of the brake band or another friction element  12  is commanded, the solenoid valve is turned on  307 , opening the flow switching circuit  17 . As the accumulator  14  de-strokes  308 , oil flows into the servo  20  for stroking  309 . The accumulator  14  and apply line  15  pressure drops  310  as the servo pressure increases  311 . The pump  13  is turned on  312  in order to replenish the apply line  15  with oil. Once the piston of servo  20  is stroked  313 , the pump control signal is adjusted  314  based on a conventional automatic transmission shift control methodology to achieve a desired servo pressure profile  315  for smooth gear ratio shifting. Note that the servo pressure  315  and the apply line pressure  316  stay the same during the engagement. 
   After completing the engagement, the pump control signal is increased  317  to raise the servo pressure  318  to a holding level of 150 psi. The pump continues to run  319  for about 1 sec to stroke the accumulator piston  320  in order to prepare for the next engagement. The pump powered is off  321  after the piston of accumulator  14  is stroked to a desired position  322 . When the release event is commanded, the solenoid valve is turned off  323  to open the flow path from the servo  20  to the release line  18 . The servo de-strokes  324  and its pressure quickly drops  325 . Note that the accumulator  14  remains stroked  326  and its pressure stays unchanged  327 . The pump remains powered off  328  until the next engagement is commanded. 
     FIGS. 6-9  show alternative embodiments of the invention. In the  FIG. 6  embodiment, the release line  68 , the back side  120  of the accumulator  14 , and the back side  70  of the band servo  20  are designed to hold enough oil to function as an oil reservoir. This embodiment offers packaging flexibility depending on the application of the invented device. 
   In  FIG. 7 , an accumulator  94  having a low spring rate replaces oil reservoir  19 . This feature excludes trapped air and reduces the effects of cavitation during pumping operation. A bladder-type accumulator may be employed instead of the piston-type illustrated in  FIG. 7 . 
   In  FIG. 8 , switching circuit  17  is replaced by a flow control circuit  107  that includes a pressure regulator valve  109 . The circuit  107  complements the pump  13  to adjust oil flow and pressure applied to the servo  20  through closed loop control based on measured or estimated slip speed during engagement. In the  FIG. 8  embodiment, a flow control orifice  16  may not be required. 
     FIG. 9  shows the alternative embodiment that employs a flow control circuit  117  having a pulse-width modulated (PWM) valve  119 . The circuit  117  complements the pump  13  to adjust flow and pressure applied to the servo  20  based on slip speed measurements similarly to the embodiment of  FIG. 8 . 
     FIGS. 10A ,  10 B and  10 C show packaging examples of the actuator  11  for a band brake application. The embodiment of  FIG. 10A  corresponds to the embodiment shown in  FIG. 7 . Key components include a piezoelectric diaphragm pump  13 , an accumulator  14 , a low rate accumulator  94 , which functions as a fluid reservoir, and a switching circuit  17  having an on-off solenoid valve. 
   The embodiment illustrated in  FIG. 10B  corresponds to the embodiment shown in  FIG. 8 . It includes a switching circuit  107  having a pressure regulator valve  109 , and a low rate accumulator  94 , which functions as a reservoir. 
   The embodiment illustrated in  FIG. 10C , corresponds to the embodiment shown in  FIG. 9 . It includes a switching circuit  117  having a PWM valve  119 , and a low rate accumulator  94 , which functions as a reservoir. 
   The embodiments comprise a unique architecture of sub-systems, primarily including a piezoelectric pumping device, an accumulator, a pressure relief port, a control orifice, a switching circuit, and a reservoir, which enable use as a modular friction component actuator based on unique operating principles. While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, words used in this disclosure are words of description rather than limitation. It is understood that various changes, including a design of sub-components such as a piezoelectric pump, may be made without departing from the spirit and scope of the invention. It is also understood that the applicability of the invention is not limited to automotive friction components, but includes any other vehicular and non-vehicular applications that include a hydraulically-driven servo. 
   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.