Hydraulic Control System for Electronic Transmission Control

1. Field of the Invention

This invention relates to a hydraulic control system for pressurizing clutches in an automatic transmission. The invention pertains more particularly to a hydraulic control system for an automatic transmission which, upon sensing an unacceptable combination of pressurized clutches, reverts to the combination that produces a predetermined speed ratio.

2. Description of the Prior Art

An automatic transmission for automotive use includes planetary gearsets whose elements are hydraulically connected or held in order to produce selected ratios of the engine crankshaft speed to the speed of the output of the transmission. Automatic gear change transmissions may be controlled by electronic means which produce analog current signals that operate solenoid valves. These valves connect hydraulic line pressure to selected clutches and brakes, or vent the hydraulic cylinder of these elements to atmosphere. The control system should operate such that, in the event of a control system malfunction, the transmission may operate in at least one forward speed ratio and in reverse drive in order to allow the vehicle to be driven to a service station for repair.

Line pressure in an automatic transmission is usually established by connecting the output of a hydraulic pump to a pressure regulator valve through which the regulated fluid is directed to selectively operated shift valves, which control the engagement of clutches and brake servos whose selective engagement produces the various speed ratios of which the transmission is capable. If the pressure applied to the various clutches differs beyond a certain limit or if the clutches that are concurrently pressurized is inconsistent with the schedule or pressurized clutches required to produce the several drive ratios of the transmission, then incorrect operation will result. It is preferred, in the event of such unscheduled clutch engagement or malfunction of the electronic control that operates solenoid valves, that the hydraulic control system automatically operate to engage those clutches required to produce a predetermined forward speed ratio. Furthermore, reverse drive operation should result without reference to the electronic control.

In the hydraulic control system, according to this invention line pressure is applied through selective operation of a manual valve to a feedback valve on which are developed pressure forces produced by clutch pressure applied to the clutches whose engagement is necessary for the higher forward drive ratios. A feedback valve applies a force to the feedline valve in oppostion to the pressure forces produced by the forward drive ratio clutches. These valves sense the application of an unscheduled pressurized clutch combination and sense if a scheduled combination has clutch pressure below a critical magnitude. If either of these conditions is present, the feedline through which the clutches are pressurized by operation of the electronic control are vented and a second feedline is connected to line pressure. Those clutches whose engagement is necessary to produce a predetermined forward speed ratio are pressurized from the second feedline.

A solenoid operated clutch actuator valve selectively connects the first feedline to the various clutches so that the clutches are quickly filled and quickly vented. The pressure drop across an orifice of a given size determines the state of the actuator valve in accordance with an electrical signal applied to the solenoid winding.

In the event of a tie-up during shifting or an electronic system malfunction, the solenoid feed valve cooperates with a solenoid feedback valve to produce a snap action that prevents the feed valve from hunting between positions. Pressure forces developed on differential pressure areas of the feed valve are balanced in part by an opposing pressure force and by the force of a spring which maintains clutch pressure in the unpressurized clutch within limits. The feed valve and feedback valve sense if clutch pressure exceeds certain limits during shifting and sense if one clutch pressure exceeds a certain limit when the other two are at normal clutch regulated pressures.

FIGS. 1 and 2 show a schematic diagram of the hydraulic control system according to this invention.

FIG. 3 shows a schedule for the forward drive ratios and reverse drive ratio of the transmission and the various clutches and servos that are pressurized to produce each of these drive ratios.

Refering to FIG. 1, the transmission is supplied with enough fluid to keep a torque converter 10 completely filled, operate the hydraulic controls, lubricate the working parts and provide a reverse of fluid in the sump 12, which is a storage space in the oil pan at the bottom of the transmission case. Atmospheric pressure in the sump forces fluid through the sump screen 14 and into the evacuated inlet of the pump 16. Fluid delivered by the pump is displaced at a volumetric rate which is in proportion to the speed at which the pump is driven. The pump is sized to deliver more fluid than the transmission needs and excess hydraulic fluid if recirculated to the sump by the pressure regulator system and other portions of the hydraulic control system.

The output of the pump connected by hydraulic line 18 supplies fluid regulated to line pressure to the manual valve 20. The pump outlet is connected through lines 22 and 28 to the control chamber 23 of a main regulator valve 24. The orifice 26 in line 21 has an appreciable pressure drop across it only when flow in line 21 reverses direction at high frequency.

The pump outlet supplies hydraulic fluid through line 28 to a central chamber of valve 24. The torque converter circuit is charged or pressurized through line 30, which leads from regulator valve 24. The converter circuit includes a relief valve 32, which is adapted to open the circuit to the pump inlet as necessary to protect the converter from excessive pressure build-up. Upon leaving the converter, hydraulic fluid passes through a check valve 34 which has a light spring that opens with low pressure to maintain a continuous fluid flow to the cooler 36. When the engine is shut down, the spring will reseat the valve, which prevents the converter oil from draining through the cooler and lubrication system back to sump. Therefore, the converter is maintained full of fluid and can begin to transmit torque through the turbine shaft as soon as the engine is started.

The pump outlet is also connected through orifice 38 in hydraulic line 40 to inlet port 42 of the boost valve 58 and through hydraulic line 44 to the inlet port of a line pressure regulator solenoid valve 46. The solenoid of valve 46 has a winding to which is applied a pulse width modulated signal produced by an electronic control system. Valve 46, which is normally closed, opens line 44 to a vent port through an orifice 48 when the solenoid winding is energized. Regulator valve 24 has a central chamber within which valve spool 50 moves due to the effect of a pressure force developed on land 52, the force of a spring 54 and pressure force developed on the land 56 of boost valve 58. The end of the valve chamber is blocked by a plug against which spring 54 bears. The pressure regulator and boost valve operates to control the pressure in line 18 between 60 p.s.i. and 260 p.s.i.

In order to fill the lines of the hydraulic system, the spool 50 of regulator valve 24 is moved by spring 54 to the lower end of the valve chamber, the pump is driven by the engine and the lines are filled at low pressure. As pressure begins to rise, spool 50 is forced upward against spring 54 to approximately the position shown in FIG. 1 permitting land 60 to open communication between lines 28 and 30 through which the torque converter circuit and hydraulic control system are fully charged.

Regulation of line pressure begins by opening and closing orifice 48 of line pressure regulator solenoid valve 46 as the windings of that solenoid are energized and de-energized in accordance with a pulse width modulated signal applied to the solenoid. Normally valve 46 is closed and its solenoid coil de-energized. Through operation of valve 46 line pressure is controlled by the pressure acting on land 56 of the boost valve 58. Maximum line pressure occurs when no pulse width modulated signal is applied to the line pressure solenoid winding. Then no hydraulic fluid flows through the orifices 38, 48 but pressure acts on the boost valve land 56, which applies a force through contact with valve spool 50 and the effect of spring 54 tending to close communication between lines 18, 22, 28 and vent port 62. Maximum line pressure results when the solenoid of valve 46 is de-energized and when the pressure force on land 56, plus the spring force are balanced against the pressure force on land 52 such that line 18 is closed to vent port 62. Minimum line pressure, which is determined with solenoid valve 46 energized, is equal to the sum of the pressure drops across orifices 38 and 48. When the duty cycle from which the signal applied to the solenoid of valve 46 is less than 100 percent and more than 0 percent, i.e., when the solenoid winding is energized for a period whose duration is proportional to the difference between the actual line pressure and the commanded line pressure, orifice 48 opens and closes cyclically. In this way line pressure is regulated between the maximum and minimum valves.

The manual valve 20 directs line pressure to the valves, clutches and servos whose actuation produces the forward and reverse drive ratios. Valve 20, whose position is controlled by the vehicle operator, receives line pressure through inlet port 64 and directs line pressure to line 66 if valve element 68 is moved to the Overdrive, 3 or 1 positions. If the valve is moved to the Park, Reverse or 1 positions, hydraulic line 67 receives line pressure. Hydraulic line 70 is pressurized with line pressure when the manual valve is moved to the Reverse position, but line 70 is open to vent port 72 when the manual valve is set other than in the Reverse position. When the manual valve is in the Overdrive, 3 or 1 positions, an electronic control system selectively energizes the windings of solenoid-operated valves 74, 76 and 78 by means of which an intermediate overdrive clutch 80, forward clutch 82 and direct clutch 84, respectively, are engaged and disengaged.

FIG. 3 shows the schedule of clutch and servo actuation required to produce the various speed ratios. While manual valve 20 is at the O/D position, any of the four forward speed ratios are produced by actuation of valves 74, 76, 78 and through operation of control valve 92. When the manual valve is at the 3 position, only the three lowest forward speed ratios can be produced. When the manual valve is at the 1 position, only the lowest forward speed ratio operation can result.

In the event of an electronic system malfunction, the transmission is shifted hydraulically to second gear through operation of a solenoid feed valve 86. When manual valve 20 is set at the Reverse position, solenoid feed valve 86 and the reverse clutch 88 are pressurized directly from the manual valve 20 with line pressure, and the low/reverse servo 90 is pressurized through operation of coast control valve 92.

Solenoid feed valve 86 has a spool element 96 on which several lands are formed. When forward clutch 82 is pressurized, hydraulic line 98 carries fluid at line pressure to a port 100 through which one face of lands 102 and 104 are pressurized. The cross sectional area of land 102 is one unit of area larger than the cross section area of land 104; therefore, the net differential area of the lands pressurized when the forward clutch is applied is one area unit. When the intermediate/overdrive clutch 80 is pressurized, hydraulic line 106 carries fluid at line pressure to port 108 through which a second differential pressure area comprising the opposite force of land 104 and one face of land 110 are pressurized. The cross sectional area of land 104 is one unit of an area larger than the cross sectional area of land 110. When either the direct clutch 84 is pressurized, or the manual valve is moved to the Reverse position, hydraulic line 112 carries fluid at line pressure to chamber 114 of the solenoid feed valve by means of which land 116 is pressurized. The cross sectional area of land 116 is substantially the same as that of lands 110 and 118, specifically one unit of area. Spool 96 has a headed end 120 which can be brought into contact with a headed end on the valve element 122 of the feedback valve 94. A spring 130 is preloaded in compression between the surfaces of land 102 and element 122. A second compression spring 132 biases element 122 leftward.

Hydraulic line 66 connects an outlet port of manual valve 20 to an inlet port of feed valve 86. Depending upon the position of spool 96, either a first solenoid valve feed line 124 or a second feed line 126 is brought into communication with inlet port 128. Solenoid feed valve 86 in cooperation with feedback valve 94 directs line pressure from port 128 to actuator valves 74, 76 and 78 if spool 96 is at the left-hand end of the valve chamber. Alternatively, if spool 96 is moved to the right-hand end of the valve chamber, valves 86 and 94 connect inlet 128 to the second feedline 126, which movement opens the first feed line 124 to vent port 134. Similarly, if line 124 is pressurized, line 126 is open to vent port 136. The first feed line 124 supplies line pressure to feedback line 138 through which the end face of element 122 is pressurized. The cross sectional area of element 122 is two unit areas.

Valves 86 and 94 operating together sense if the magnitude of solenoid feed pressure exceeds certain limits during shifting and sense if the pressure to either the direct clutch, the forward clutch or the intermediate/overdrive clutch exceeds a certain limit when the other two clutches are at line pressure. When spool 96 is at the left end of the valve chamber, direct clutch actuator valve 78, forward clutch actuator valve 76 and intermediate/overdrive clutch actuator valve 78 have solenoid feed pressure applied to inlet ports 144, 142, 140. The output from these valves carried in lines 146, 148, 150 is regulated between zero p.s.i. and line pressure, which regulation eliminates the need for accumulators to control clutch pressure during shifting. A pulse width modulated signal is selectively applied to the windings of solenoid valve 74, 76, 78, in accordance with the schedule of FIG. 3, in order to produce the second, third and fourth forward speed ratios. The solenoid of valve 76 is energized in order to engage forward clutch 82, which acts in combination with the engagement of low/reverse servo 90 to produce the first forward speed ratio.

Each of the actuating valves 74, 76, 78 includes a valve spool 152 having an orifice 154 connecting the valve chamber through an axially directed bore 156 leading to the end of valve land 158 which is formed integrally with the spool. The valve body includes a first vent port 160 and a second vent port 162 to which the valve chamber is communicated when the solenoid winding is energized. Two valve lands of equal size 164, 166 are formed integrally on the spool 152.

In operation, the intermediate/overdrive clutch 80 is filled through valve 74 when its spool 152 is in the position shown in FIG. 3 and solenoid feed line 124 is pressurized. A feedback line 170 develops a pressure force on land 164 through orifice 172, which functions to prevent cyclic movement of spool 152. The maximum pressure that can be applied to any of the clutches or servos is the line pressure. However, clutch and servo pressure can be regulated down to zero p.s.i. through operation of the actuating valve. In order to regulate clutch pressure, a pulse width modulated signal is applied to the winding of the solenoid which causes orifice 174 to open and close cyclically for variable and usually unequal periods, thereby communicating vent port 162 to the valve chamber. When orifice 174 is open, differential pressure across orifice 154 causes an unbalanced pressure force to be developed on spool 152, moving the spool downward and opening line 146 to vent port 160. During the period when the solenoid winding is de-energized, spool 152 moves upward within the chamber to the position shown in FIG. 2, and clutch pressure rises toward line pressure. It can be seen from this that the duration of the period during which the solenoid winding is energized controls the magnitude of the associated clutch pressure and in this way the clutch is vented through vent port 160 when the solenoid winding is energized for an extended period.

Referring to FIG. 3, it can be seen that the second, third and fourth speed ratios are produced when only two of the clutches in the group consisting of the forward clutch 82, direct clutch 84 and intermediate overdrive clutch 80 are pressurized. The solenoid feed valve 86 in cooperation with the feedback valve 94 operate to disconnect line pressure from solenoid feedline 124 and to connect line pressure to the second feeding 126, which directly pressurizes the forward clutch 82 and intermediate/overdrive clutch 80, thereby discounting the operation of the solenoid feed valves 74, 76, 78, when the clutch pressure in one of the lines 146, 148, 150 exceeds the predetermined limit value and the other two clutch pressure lines are within the regulated range. If the second feed line is pressurized, the transmission shifts to the second forward speed ratio where it remains until manual valve 20 is moved to the neutral position. Clutch pressure in lines 146, 148, 150 is fed back in lines 98, 106, 112 to valve 86 where each is applied to the differential areas of spool 96. The cross sectional area of element 122 of valve 94 is equal to two times each of these differential areas; therefore, the pressure in feedback line 138 opposes the force on two of the differential areas of spool 96. The third clutch pressure, therefore, acts only against the force of spring 132, which can be used to set the pressure limit on the third clutch. If this limit is exceeded, spool 96 and element 122 move to the right-hand end of their respective valve chambers and solenoid feed pressure line 124 is vented. After this occurs, spool 96 and element 122 are maintained at the right-hand end of the valve chambers, thereby snapping valve 86 to its hydraulic shift position where feed line 126 is connected to line pressure.

This position is maintained until the manual valve is moved to the Neutral position. The preloaded compression spring 130 located between spool 96 and element 122 forces spool 96 back to the left-hand end of its chamber, but when the manual valve is moved to the Reverse position, reverse pressure in chamber 114 tends to move spool 96 against this spring. The force of spring 130 assures that the spool 96 is in position to allow solenoid feedline 124 to be connected to line pressure when the manual valve is moved from the Reverse position to the Overdrive position.

The coast control valve 92 operates to regulate low/reverse servo pressure when the manual valve is in the Park and 1 positions, to connect line pressure to the low/reverse servo 90 when the manual valve is moved to the Reverse position and to prevent pressurization of the low/reverse servo when direct clutch pressure or intermediate/overdrive clutch pressure exceeds a certain limit.

When the manual valve is in the 1 position, the low/reverse servo is pressurized in order to apply the low/reverse brake band by means of which engine braking of the vehicle wheels results. However, a lower magnitude of low/reverse servo pressure is required in low gear than in reverse drive in order to prevent the brake band from slipping. For this reason the coast control valve 92 regulates low/reverse servo pressure to 25 p.s.i. when the manual valve is set at the 1 position. When the manual valve is in the Reverse position, valve 92 permits line pressure, which may vary between 60 p.s.i. and 260 p.s.i., to be applied to the low/reverse servo.

Coast control valve 92 includes a spool 176, biased by a compression spring 178 to the upper end of the valve chamber in which a vent port 180 is formed.

When manual valve 20 is moved to the 1 position, hydraulic line 67 carries fluid at line pressure to valve 92 whose spindle 176 is biased by spring 178 to the top of the valve chamber. In this position valve 92 pressurizes hydraulic line 182, through which the low/reverse servo 90 and feedback line 184 are pressurized. The pressure developed on land 186 operates against the force of spring 178 and regulates the magnitude of pressure in the low/reverse servo.

If the manual valve is moved to the Reverse position, line pressure carried in hydraulic lines 70 and 188 develops a force on the end of land 190 that adds to the force of spring 178 and acts to maintain valve spool 176 at the top end of the valve chamber. Therefore, line pressure carried to valve 92 in line 67 is applied directly to the low/reverse servo 90 through line 182.

In order to operate in the first forward speed ratio, only the forward clutch 82 and the low/reverse servo 90 can be pressurized. Therefore, when the manual valve is moved directly from the Overdrive position to the 1 position, coast control valve 92 operates to prevent the low/reverse servo from being pressurized while the transmission operates in the third or second gear ratios. Check valve 192 directs fluid at clutch pressure through line 194 to the coast control valve 92 when either the direct clutch or the intermediate/overdrive clutch is pressurized. This develops a pressure force on land 196 which adds to the force on the feedback land 186 in opposition to the spring force 178. Spool 176 is forced to the bottom of the valve chamber, which action opens the low/reverse servo line 182 to vent port 180. The magnitude of the intermediate/overdrive clutch pressure and direct clutch pressure are controlled by the electronic control system. When either of these clutch pressures decrease during a 2-1 coastdown shift, spring 178 moves spool 176 upward and the low/reverse servo pressure is again regulated to approximately 25 p.s.i.