Control apparatus for variable displacement type compressor

An improved control apparatus for a vehicle air conditioning system controls a variable displacement compressor. The discharge capacity of the compressor is changed by changing the difference between the pressure in the crank chamber and the suction pressure. The difference between the pressure in the crank chamber and the pressure of the suction chamber is maintained at a set value. A target value is determined based on the temperature of the passenger compartment. The target value is corrected based on the rotational speed of the engine driving the compressor, which is detected. The set value is determined by the corrected target value.

BACKGROUND OF THE INVENTION
 The present invention relates to a variable displacement type compressor
 for use in vehicle air-conditioners. More particularly, this invention
 relates to an apparatus for controlling the discharge capacity of a
 variable displacement type compressor by changing the tilt angle of a cam
 plate with a control valve.
 This type of control apparatus known has a control passage that connects a
 discharge pressure area to the crank chamber, which houses a cam plate,
 and adjusts the difference between the pressure in the crank chamber and
 the pressure in the cylinder bores to change the tilt angle of the cam
 plate, thereby adjusting the discharge capacity. The adjustment of the
 difference between the pressure in the crank chamber and the pressure in
 the cylinder bores is carried out by changing the position of the
 displacement control valve, which is located in the control passage, under
 the control of a computer.
 Japanese Unexamined Patent Publication (KOKAI) No. Hei 6-341378 discloses a
 displacement control valve that has a constant differential pressure valve
 section and an electric driving section. The displacement control valve
 performs control such that the difference between the pressure of the
 intake refrigerant gas (hereinafter referred to as the suction pressure),
 which has a correlation with the pressure in the cylinder bore, and the
 pressure in the crank chamber becomes equal to a preset value. More
 specifically, the constant differential pressure valve section adjusts the
 restriction of the control passage by actuating the valve body to keep the
 difference between the pressure in the crank chamber and the suction
 pressure at the preset value. An electric driving section changes a
 reference set value for the operation of this valve section by adjusting
 the load acting on the valve body under the control of the computer.
 When the suction pressure rises to make the difference between the pressure
 in the crank chamber and the suction pressure fall below the set value,
 the valve section actuates the valve body to open the control passage.
 This increases the amount of the high-pressure refrigerant gas supplied to
 the crank chamber from the discharge pressure area, thus raising the
 pressure in the crank chamber. As a result, the difference between the
 pressure in the crank chamber and the suction pressure is maintained at
 the preset value.
 When the suction pressure falls, making the difference between the pressure
 in the crank chamber and the suction pressure greater than the set value,
 on the other hand, the valve section moves the valve body in the direction
 to close the control passage. This decreases the amount of the
 high-pressure refrigerant gas supplied to the crank chamber from the
 discharge pressure area, thus dropping the pressure in the crank chamber.
 This keeps the difference between the pressure in the crank chamber and
 the suction pressure at the set value.
 The computer compares the temperature detected by a temperature sensor with
 the temperature set by a temperature setting unit to determine a target
 value, and controls the electric driving section in such a way that the
 reference set value for driving the valve section is the target value.
 When the cooling load acting on the compressor is heavy, for example, the
 difference between the temperature detected by the temperature sensor and
 the temperature set by the temperature setting unit is larger. Based on
 this large difference, the computer controls the electric driving section
 to decrease the reference set value for driving the valve section. As a
 result, the tilt angle of the cam plate increases based on a small
 difference between the pressure in the crank chamber and the pressure in
 the cylinder bore, thus increasing the discharge capacity of the
 compressor in accordance with the heavy cooling load.
 When a light cooling load is acting on the compressor, on the other hand,
 the difference between the temperature detected by the temperature sensor
 and the temperature set by the temperature setting unit is smaller. Based
 on this small difference, the computer controls the electric driving
 section to increase the reference set value for driving the valve section.
 As a result, the cam plate decreases the tilt angle based on a large
 difference between the pressure in the crank chamber and the pressure in
 the cylinder bore via the associated piston, thus reducing the discharge
 capacity of the compressor in accordance with the light cooling load.
 In the above-described compressor, however, moment M in the direction of
 increasing the tilt angle is acting on a cam plate 102 based on inertial
 force F of a piston 101 which reciprocates, as shown in FIG. 4. That is,
 in addition to the difference between the pressure in the crank chamber
 and the pressure in the cylinder bore via the piston 101, the moment M
 that acts on the cam plate 102 based on inertial force F of the piston 101
 greatly affects the determination of the tilt angle of the cam plate 102.
 The magnitude of moment M is not always constant. As the rotational speed
 of the engine increases, the rotational speed of a drive shaft 103 rises
 too. When the piston 101 reciprocates at a high speed accordingly, the
 inertial force F of the piston 101 that acts on the cam plate 102
 increases, thus making the moment M greater.
 FIG. 5 shows a state where the cooling load is constant and the set value
 for driving the displacement control valve is also constant. In other
 words, FIG. 5 shows the state where the difference between the pressure in
 the crank chamber and the pressure in the inertial force F of the piston
 101 cylinder bore via the piston 101 is maintained at a certain value.
 Even in such state, an increase in the rotational speed of the engine
 driving the compressor and an increase in the inertial force F of the
 piston 101 will increase the tilt angle of the cam plate 102 or the
 discharge capacity of the compressor. Such a low-precision control on the
 discharge capacity of a compressor without considering variation in the
 inertial force F of the piston 101 is apt to degrade the cooling
 performance of the air-conditioning system of a vehicle.
 SUMMARY OF THE INVENTION
 Accordingly, it is an object of the present invention to provide a control
 apparatus for a variable displacement type compressor that can perform
 high-precision control of the discharge capacity of the variable
 displacement type compressor.
 To achieve the above object, according to this invention, there is provided
 a control apparatus for controlling the discharge capacity of a variable
 displacement type compressor, which has a cam plate supported on a drive
 shaft to rotate together with the drive shaft in a crank chamber. The
 apparatus changes the discharge capacity by changing the difference
 between the pressure in the crank chamber and a suction pressure. The cam
 plate converts a rotational movement of the drive shaft, which is driven
 by an engine, to reciprocal movement of the pistons, which compresses gas.
 The control apparatus includes: a valve for keeping the difference between
 the pressure in the crank chamber and the suction pressure at a set value;
 an electric driving mechanism that changes a reference set value, wherein
 the reference set value is used to operate the valve; an external
 information detector for outputting information about the temperature of a
 passenger compartment of the vehicle; a rotational speed sensor for
 detecting the rotational speed of the engine or a rotational speed related
 to the rotational speed of the engine; and a computer for determining a
 target value based on the temperature information and for controlling the
 electric driving mechanism such that the target value determines the set
 value, wherein the computer corrects the target value based on rotational
 speed information from the rotational speed sensor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
 One embodiment of the present invention will now be described referring to
 the accompanying drawings.
 To begin with, the structure of a variable displacement type compressor
 will be described.
 As shown in FIG. 1, a front housing member 11 is connected to the front end
 of a cylinder block 12. A rear housing member 13 is connected to the rear
 end of the cylinder block 12 via a valve plate 14. The front housing
 member 11, the cylinder block 12 and the rear housing member 13 constitute
 the housing of the compressor. A crank chamber 15 is defined by the front
 housing member 11 and the cylinder block 12.
 A drive shaft 16 is supported between the front housing member 11 and the
 cylinder block 12 and passes through the crank chamber 15. The drive shaft
 16 is coupled to an engine Eg via a clutch mechanism C such as an
 electromagnetic clutch. When the engine Eg runs, therefore, the drive
 shaft 16 is rotated when the clutch mechanism C is engaged.
 A lug plate 17 is fixed to the drive shaft 16 in the crank chamber 15. A
 swash plate 18 is supported to slide along the surface of the drive shaft
 16 and to incline with respect to the drive shaft 16.
 A hinge mechanism 19 is located between the lug plate 17 and the swash
 plate 18. The hinge mechanism 19 permits the swash plate 18 to incline
 with respect to the axial line L of the drive shaft 16 and to rotate
 together with the drive shaft 16. As the center portion of the swash plate
 18 moves toward the lug plate 17, the tilt angle of the swash plate 18
 increases. As the center portion of the swash plate 18 moves toward the
 cylinder block 12, on the other hand, the tilt angle of the swash plate 18
 decreases. A stop 20, which defines the minimum inclination of the swash
 plate 18, is provided on the drive shaft 16 between the swash plate 18 and
 the cylinder block 12. The maximum tilt angle of the swash plate 18 is
 defined by the abutment of the swash plate 18 against the lug plate 17.
 A plurality of cylinder bores (only one shown in FIG. 1) 21 are formed
 through the cylinder block 12. A one-headed piston 22 is retained in each
 cylinder bore 21. Each piston 22 is attached to the periphery of the swash
 plate 18 via a shoe 23 and reciprocates forward and backward in the
 cylinder bore 21 as the swash plate 18 rotates.
 A suction chamber 24, which forms a suction pressure area, and a discharge
 chamber 25, which forms a discharge pressure area, are defined in the rear
 housing member 13. A suction port 26, suction valve 27, discharge port 28
 and discharge valve 29 are formed in the valve plate 14. As the piston 22
 moves from the top dead center to the bottom dead center, refrigerant gas
 in the suction chamber 24 is drawn into the cylinder bore 21 via the
 suction port 26 and the suction valve 27. As the piston 22 moves from the
 bottom dead center to the top dead center, the refrigerant gas drawn into
 the cylinder bore 21 is compressed to a predetermined pressure and is then
 discharged to the discharge chamber 25 via the discharge port 28 and the
 discharge valve 29.
 In the above-described compressor, the suction chamber 24 and the discharge
 chamber 25 are connected by an external refrigeration circuit 61, which
 has a condenser 62, an expansion valve 63 and an evaporator 64.
 A control apparatus for controlling the discharge capacity of the
 compressor will now be discussed.
 As shown in FIG. 1, a control passage 30 connects the discharge chamber 25
 to the crank chamber 15. A displacement control valve 31 is located in the
 control passage 30. A bleed passage 32 connects the crank chamber 15 to
 the suction chamber 24. A pressure transmitting passage 38 extends between
 the suction chamber 24 and the displacement control valve 31.
 A temperature setting unit 33 for setting the temperature of the vehicle
 passenger compartment, a temperature sensor 34 for detecting the
 temperature in the passenger compartment (the sensor 34 is located in the
 passenger compartment in this embodiment) or a temperature that reflects
 the passenger compartment temperature (in which case, the sensor 34 may be
 located, for example, near the evaporator 64), a rotational speed sensor
 35 for detecting the rotational speed of the output shaft (not shown) of
 the engine Eg, and the clutch mechanism C are connected to a computer 37.
 The computer 37 is connected to the displacement control valve 31 via a
 drive circuit 36.
 The structure of the displacement control valve 31 will be discussed below.
 In the displacement control valve 31, as shown in FIG. 2, a valve housing
 41, which has a valve section 46, is connected to a solenoid section 42,
 which is also referred to as an electric driving section. A valve chamber
 43 is formed in a distal end of the valve housing 41. A spherical valve
 body 44 is retained in the valve chamber 43 and is movable in the axial
 direction (in the vertical direction in FIG. 2) of the valve housing 41. A
 valve hole 45 faces the valve body 44 in the valve chamber 43. A spring
 47, which is housed in the valve chamber 43, urges the valve body 44 in
 the direction to close the valve hole 45. The valve hole 45 extends in the
 axial direction of the valve housing 41. The valve chamber 43 communicates
 with the discharge chamber 25 via an upstream section of the control
 passage 30.
 A pressure-receiving member 52 is retained in the valve housing 41 and is
 axially reciprocated. A lower end of an actuator rod 55 is secured to the
 pressure-receiving member 52, and an upper end of the actuator rod 55 is
 inserted in the valve hole 45, so that the upper end abuts against the
 valve body 44. The upper and lower directions being referred to are the
 upper and lower directions of FIG. 1. The space between the top surface of
 the pressure-receiving member 52 and the valve hole 45 is connected to the
 crank chamber 15 via a downstream section of the control passage 30. The
 space between the bottom surface of the pressure-receiving member 52 and
 the solenoid section 42 (more specifically, a fixed attracting member 49
 to be discussed later) is connected to the suction chamber 24 via the
 pressure transmitting passage 38. The valve chamber 43, the valve hole 45,
 the spring 47, the pressure transmitting passage 38, the valve body 44,
 the pressure-receiving member 52, the actuator rod 55 are included the
 valve section 46.
 A plunger chamber 48 is formed in the solenoid section 42, and the fixed
 attracting member 49 is securely fitted in its upper opening. A plunger 50
 is accommodated in the plunger chamber 48 and is axially reciprocate. A
 cylindrical coil 51 is located around the plunger chamber 48, the fixed
 attracting member 49, and the plunger 50. The drive circuit 36 is
 connected to the coil 51.
 A rod guide hole 53 passes through the fixed attracting member 49. A drive
 rod 54 is fitted in the rod guide hole 53 in a slidable manner. The lower
 end of the drive rod 54 is fixed to the plunger 50, and the upper end of
 the drive rod 54 abuts against the bottom surface of the
 pressure-receiving member 52. Therefore, the plunger 50 and the valve body
 44 are functionally coupled by the drive rod 54, the pressure-receiving
 member 52 and the actuator rod 55.
 The operation of the above-described control apparatus will now be
 discussed.
 When the engine Eg is activated, when the temperature detected by the
 temperature sensor 34 is equal to or higher than the temperature set by
 the temperature setting unit 33, and when an unillustrated activation
 switch of the air-conditioning system is on, the computer 37 engages the
 clutch mechanism C, thus causing the compressor to be driven by the engine
 Eg.
 When the compressor is activated, the pressure Pc in the crank chamber 15
 acts on the upper surface of the pressure-receiving member 52 and the
 pressure Ps of the intake refrigerant gas (hereinafter referred to as the
 suction pressure) acts on the bottom surface of the member 52. Downward
 force is applied to the plunger 50 of the solenoid section 42 by the
 pressure-receiving member 52 and the drive rod 54 in accordance with the
 difference between the pressure Pc in the crank chamber 15 and the suction
 pressure Ps (Pc.gtoreq.Ps).
 When the clutch mechanism C is engaged, the computer 37 determines the
 input current value based on external information, such as the temperature
 set by the temperature setting unit 33 and the temperature detected by the
 temperature sensor 34, and instructs that the determined input current be
 sent to the drive circuit 36.
 The drive circuit 36 sends the specified input current value to the coil 51
 of the displacement control valve 31. As the current is supplied to the
 coil 51 from the drive circuit 36, a force of attraction (electromagnetic
 force) resulting from the input current is produced between the fixed
 attracting member 49 and the plunger 50. This attracting force acts on the
 valve body 44 via the drive rod 54, the pressure-receiving member 52 and
 the actuator rod 55 in the direction to open the valve hole 45 (the upward
 direction).
 Therefore, the opening size of the valve hole 45 of the displacement
 control valve 31 is determined essentially by a balance between the
 downward force based on the difference between the pressure Pc in the
 crank chamber 15 and the suction pressure Ps, which acts on the
 pressure-receiving member 52, and the upward force based on the attracting
 force between the fixed attracting member 49 and the plunger 50.
 Suppose that the attracting force between the fixed attracting member 49
 and the plunger 50 is constant. In this case, as the suction pressure Ps
 rises, which reduces the difference between the suction pressure Ps and
 the pressure Pc in the crank chamber 15, the downward force from the
 pressure-receiving member 52 that acts on the plunger 50 becomes weaker.
 This makes the urging force of the solenoid section 42 that acts on the
 valve body 44 in the upward direction and opens the valve hole 45
 relatively stronger. As a result, the valve body 44 is moved in the
 direction of opening the valve hole 45 against the force of the spring 47.
 This increases the amount of high-pressure refrigerant gas (Pd) supplied
 to the crank chamber 15 from the discharge chamber 25, which raises the
 pressure Pc in the crank chamber 15; therefore, the difference between the
 pressure Pc in the crank chamber 15 and the suction pressure Ps is
 maintained at the set value.
 When the difference between the pressure Pc in the crank chamber 15 and the
 suction pressure Ps increases, on the other hand, the downward force from
 the pressure-receiving member 52 that acts on the plunger 50 increases.
 This relatively reduces the urging force of the solenoid section 42 that
 acts on the valve body 44 in the direction of opening the valve hole 45.
 As a result, the valve body 44 is moved in the direction of closing the
 valve hole 45 due to the force of the spring 47. This decreases the amount
 of high-pressure refrigerant gas (Pd) supplied to the crank chamber 15
 from the discharge chamber 25, which lowers the pressure Pc in the crank
 chamber 15, and the difference between the pressure Pc in the crank
 chamber 15 and the suction pressure Ps is maintained at the set value.
 Thus, the valve section 46 actuates the valve body 44 such that the
 difference between the pressure Pc in the crank chamber 15 and the suction
 pressure Ps, which correlates with the pressure in the cylinder bores 21,
 is kept at the set value. That is, the valve section 46 operates to keep
 constant the difference between the pressure Pc in the crank chamber 15,
 which determines the tilt angle of the swash plate 18, and the pressure in
 the cylinder bores 21, thus making the tilt angle of the swash plate 18
 (the discharge capacity of the compressor) constant.
 As described above, the reference set value for the operation of the valve
 section 46 or the discharge capacity of the compressor can be adjusted
 externally by changing the attracting force between the fixed attracting
 member 49 and the plunger 50.
 When the cooling load is heavy the difference between the detected
 temperature and the set temperature becomes large. Based on the large
 difference between the detected temperature and the set temperature, the
 computer 37 controls the input current value to the coil 51 of the
 displacement control valve 31 to reduce the set value, which increases the
 discharge capacity of the compressor. That is, the computer 37 instructs
 the drive circuit 36 to reduce the input current value to the coil 51 as
 the temperature difference increases, which increases the attracting force
 between the fixed attracting member 49 and the plunger 50. Therefore, the
 solenoid section 42 reduces the force acting on the pressure-receiving
 member 52 in the direction of opening the valve hole 45. Consequently, the
 valve section 46 moves the valve body 44 to reduce the opening size of the
 valve hole 45, which reduces the difference between the pressure Pc in the
 crank chamber 15 and the suction pressure Ps and increases the discharge
 capacity of the compressor.
 When the cooling load is light, on the other hand, the difference between
 the detected temperature and the set temperature is relatively small.
 Based on the small difference between the detected temperature and the set
 temperature, the computer 37 increases the input current value to the coil
 51 of the displacement control valve 31, which reduces the discharge
 capacity of the compressor. That is, the computer 37 instructs the drive
 circuit 36 to increase the input current to the coil 51 as the temperature
 difference decreases, which increases the attracting force between the
 fixed attracting member 49 and the plunger 50. The solenoid section 42
 therefore increases the force acting on the pressure-receiving member 52
 in the direction of opening the valve hole 45. As a result, the valve
 section 46 moves the valve body 44 to increase the opening size of the
 valve hole 45, which increases the difference between the pressure Pc in
 the crank chamber 15 and the suction pressure Ps and reduces the discharge
 capacity of the compressor.
 According to the prior art, with reference to FIG. 4, the factors that
 determine the discharge capacity of the compressor include the moment that
 acts on the swash plate 18 based on the reciprocation of the piston 22 in
 addition to the difference between the pressure Pc in the crank chamber 15
 and the pressure in the cylinder bore 21. Therefore, determining the input
 current value (target set value) to the coil 51 of the displacement
 control valve 31, which does not reflect the variation in the moment F due
 to the pistons 22, i.e., the changes caused by variation in the rotational
 speed of the engine Eg, reduces the precision of the discharge capacity
 control, thus deteriorating the air-conditioning performance.
 According to this invention, therefore, the determination of the input
 current value (target set value) to the coil 51 of the displacement
 control valve 31 is allowed to reflect variation in the rotational speed
 information of the engine Eg, which is detected by the rotational speed
 sensor 35. The computer 37 instructs the drive circuit 36 to output a
 corrected input current value (target value), which is acquired by adding
 a correction amount that is determined in accordance with the rotational
 speed detected by the rotational speed sensor 35 to the uncorrected input
 current value, which has been computed based on the difference between the
 detected temperature from the temperature sensor 34 and the temperature
 set by the temperature setting unit 33. The correction amount is prestored
 in the computer 37 as correction map data, which has the rotational speed
 of the engine Eg as a parameter.
 That is, the higher the rotational speed of the engine Eg becomes, the
 greater the moment F of the piston 22 that acts on the swash plate 18
 becomes. If the uncorrected input current value is sent to the coil 51,
 the discharge capacity of the compressor is changed by a greater amount
 than when the rotational speed of the engine Eg is low (see FIG. 5).
 Therefore, the computer 37 corrects the uncorrected input current value to
 increase as the rotational speed of the engine Eg increases. Accordingly,
 the computer 37 instructs the drive circuit 36 to send the corrected input
 current value, which adds to the uncorrected set value, to the coil 51.
 Particularly, as shown in FIG. 3A, the correction map data is such that the
 discharge capacity decreases as the rotational speed of the engine Eg
 increases on the assumption that the cooling load (uncorrected input
 current value) is constant.
 Further, the correction map data is set in such a way that the discharge
 amount of refrigerant gas from the compressor to the external
 refrigeration circuit 61 per unit time, or the amount of work of the
 compressor per unit time, becomes substantially constant, regardless of
 the rotational speed of the engine Eg on the assumption that the cooling
 load is constant, as shown in FIG. 3B. Note that the amount of work done
 by the compressor per unit time can be expressed by the amount of work of
 the compressor (which is determined by the discharge capacity of the
 compressor) per unit rotation of the drive shaft 16 multiplied by the
 rotational speed of the drive shaft 16 (which is determined by the
 rotational speed of the engine Eg).
 According to this invention, the rotational speed of the engine Eg is
 reflected in determining the target value for the displacement control
 valve 31. It is therefore possible to control the discharge capacity of
 the compressor with a high precision by accounting for a variation in the
 moment F of the pistons 22.
 Given that the cooling load is constant, the correction map data is set
 such that, as the rotational speed of the engine Eg increases, the
 discharge capacity decreases. Thus, the amount of work done by the
 compressor per unit time is not changed much by a variation in the
 rotational speed of the engine Eg, which improves the cooling performance
 of the air-conditioning system.
 The rotational speed sensor 35 detects the rotational speed of the engine
 Eg, which is essential information in the general control of the engine
 Eg. Since a this rotational speed sensor 35 is provided in nearly all
 vehicles, it is unnecessary to provide a special unit to implement this
 control.
 Although a single embodiment of the present invention has been described
 herein, it should be apparent to those skilled in the art that the present
 invention may be embodied in many other specific forms without departing
 from the spirit or scope of the invention. Particularly, it should be
 understood that the invention may be embodied in the following forms.
 A sensor that detects the rotational speed of the drive shaft 16 may be
 used instead of an engine speed sensor.
 This invention may be embodied into a control apparatus for a variable
 displacement type compressor that alters the discharge capacity as the
 size of the bleed passage alone is adjusted by the displacement control
 valve.
 This invention may be embodied into a control apparatus for a variable
 displacement type compressor that changes the discharge capacity as the
 opening sizes of both the control passage and the bleed passage are
 adjusted by the displacement control valve.
 The set value may decrease as the input current value to the coil increases
 and may increase as this input current value decreases.
 In the above-described embodiment, the control of energization of the coil
 51 is analog current-based control. This control may be changed to duty
 control.
 Therefore, the present examples and embodiments are to be considered as
 illustrative and not restrictive and the invention is not to be limited to
 the details given herein, but may be modified within the scope and
 equivalence of the appended claims.