Abstract:
An electromagnetic clutch ( 10 ) for an air conditioning compressor includes a generally cylindrical pulley ( 11 ) rotatably mounted on a compressor housing ( 16 ) and having an annular friction surface ( 21   b ), a driven member ( 12 ) mounted on a compressor shaft ( 13 ) and having an annular friction surface ( 21   a ) positioned radially adjacent the first friction surface to form an annular space ( 21   c ) therebetween. A quantity of flowable magnetic material ( 27 ) is provided in the annular space ( 21   c ) and a magnetic coil ( 34 ) is fixed on the housing adjacent thereto. A control ( 40,46,46 ′) connected to the magnetic coil ( 34 ) supplies electrical power from a power supply ( 47 ) to energize the magnetic coil and create magnetic flux in the annular space ( 21   c ) polarizing the magnetic material and frictionally coupling the first and second friction surfaces ( 21   a   ,21   b ) to cause the pulley ( 11 ) to rotate the driven member ( 12 ). The control ( 40,46,46 ′) can pulse width modulate or ramp the supplied power for “soft” starting and/or stopping of the compressor.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     This invention relates to the field of clutches for a compressor in an air conditioning system. More particularly, the invention pertains to such a clutch having soft start characteristics. 
     2. Description of the Prior Art 
     A compressor for an air conditioning system of an automobile typically includes a clutch to enable disengagement of the compressor during periods in which no air conditioning is desired. During demand situations, the clutch is engaged so that the compressor compresses the refrigerant in a known manner. Prior art clutches typically are of an electromagnetic friction clutch construction. A compressor typically is driven by a V-groove pulley assembly supported by a bearing pressed onto the body of the compressor. The pulley is driven by the automotive accessory drive belt. 
     Prior art clutches typically include an armature affixed to the splined shaft of the compressor via a bolt. A stationary wire coil assembly is press-fit to the compressor housing. The armature is normally separated by a small air gap from the pulley face by a spring mechanism, so that the clutch is normally disengaged. Automotive voltage is applied to the coil to engage the clutch, normally having a nominal value of 14.4 volts. Upon application of this voltage, the current in the coil increases from zero on a time scale related to the inductive time constant of the clutch, typically  150  milliseconds. The current induces magnetic flux to flow in the pulley, across the air gap, and into the armature. When the magnetic flux density reaches a critical level, the attractive force between the armature and the pulley becomes large enough to overcome the spring force holding the armature away from the pulley. The armature is then rapidly drawn into contact with the pulley, suddenly initiating torque transfer to the compressor and causing the compressor shaft to begin to rotate. When the shaft speed matches the pulley speed, the torque then reaches a steady-state level that is a function of pulley speed, cooling demand, and other vehicle operational characteristics. 
     When such a prior art clutch is engaged, undesirable effects can occur, including stumble, surge, and noise. Stumble is a longitudinal vehicle vibration induced by the sudden change in engine torque demand which occurs upon compressor engagement. Surge is a lurch that occurs when the clutch is disengaged. Noise is generated as the armature of the electromagnetic friction clutch is rapidly driven into contact with the clutch pulley during engagement. These effects of rapid compressor engagement are objectionable to the vehicle occupants and may contribute to premature failure of compressor components. Previous efforts to overcome these concerns include using passive mechanical means, such as mating slots or other structures provided in the pulley and armature, which are said to reduce the rate of increase of magnetic force. An example of this is illustrated in U.S. Pat. No. 4,749,073 to Olden. 
     Another attempt to reduce these concerns includes the insertion of an elastomeric coupling between the armature and compressor shaft to damp transients encountered during engagement and operation, as shown in U.S. Pat. No. 5,219,273 to Chang. Other attempts to reduce these concerns include electronic controls of the clutch current in an attempt to produce soft-start coupling, as described in U.S. Pat. No. 4,509,091 to Booth and U.S. Pat. No. 4,567,975 to Roll. These patents disclose a method of generating a time-varying clutch current. These methods draw the armature initially to the pulley, but allow the pulley to slip. The clutch current is increased smoothly to gradually increase the level of torque transfer and decrease the slip until a state of complete engagement is reached. These methods permit slip, which causes the electromagnetic friction clutch surfaces to become burnished over time, reducing the ability of the clutch to transfer torque. 
     It would therefore be desirable to provide a clutch which softens the engagement of an air conditioning compressor, but which does not cause excessive wear of the clutch friction surfaces. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide, in an air conditioning compressor a clutch having a soft start which does not experience excessive wear. 
     An advantage of the use of a clutch according to the present invention is that the air conditioning system will have less objectionable noise and vibration. A second advantage is that, by reducing the objectionable characteristics of such a system, one may cycle the clutch more frequently and thereby maximize fuel efficiency and optimally control the temperature of the passenger compartment. 
     Further advantages include having the ability to operate the compressor at high speeds. With conventional clutches, operation at high engine RPMs causes undesirable noise. With a clutch according to the present invention, the clutch may be slipped, permitting lower RPM operation for a given input speed. 
     The present invention concerns an electromagnetic clutch for an air conditioning compressor housed in a compressor housing having a drive shaft extending from the housing and being rotatable about an axis of rotation. The clutch includes a generally cylindrical driving member having an axis of rotation and a peripheral annular first frictional surface, an annular driven member extending about the driving member and having an axis of rotation coaxial with the driving member axis of rotation, the driven member having an annular second frictional surface positioned radially adjacent the first frictional surface to form an annular space therebetween, a quantity of flowable magnetic material provided in the annular space, a magnetic coil positioned adjacent the annular space and control means connected to the magnetic coil for supplying electrical power from a power supply to energize the magnetic coil. When the driven member is attached to the compressor shaft, the driving member is rotatably mounted on the compressor housing and the magnetic coil is fixed to the compressor housing, the driving member can be rotated without rotating the driven member. When the control means applies electrical power to the magnetic coil, magnetic flux is created in the annular space polarizing the magnetic material and frictionally coupling the first and second frictional surfaces to cause the driving member to rotate the driven member. The control means can pulse width modulate the supplied power for “soft” starting and/or stopping of the compressor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross sectional view of a clutch according the present invention. 
     FIG. 2 is a graph illustrating the torque to current relationship in the clutch of the FIG.  1 . 
     FIGS. 3 through 5 are schematic representations of electronic circuits to operate a clutch as shown in the FIG.  1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring first to FIG. 1, there is shown a clutch  10  for driving an air conditioning compressor (not shown) from an engine (not shown) in a motor vehicle (not shown). The clutch  10  includes a driving member  11  in the form of a pulley and a driven member  12  coupled to a shaft  13  of the compressor. The pulley  11  is driven by the vehicle engine through a belt (not shown) that engages V-grooves  14  formed in an exterior surface of the pulley in a known manner to provide rotation thereof while the vehicle engine is running. The clutch  10  allows the compressor to be selectively engaged with and disengaged from the pulley  11 . 
     An annular bearing  15  is mounted on an axial extension of a housing  16  of the compressor. A ring shaped bearing mount  17  has an axially extending U-shaped cross section with an inner leg  18  supported on the bearing  15  and a generally parallel outer leg  19 . The driven member  12  includes a peripheral flange  20  that extends axially between the legs  18  and  19  and has an axially extending circumferential annular first friction surface  21   a  facing the outer leg  19 . The pulley  11  also is ring shaped and has an axially extending U-shaped cross section with an inner leg  22  and a generally parallel outer leg  23 . The V-grooves  14  are formed on an outwardly facing surface of the outer leg  23  and the inner leg  22  extends toward the outer leg  19  of the bearing mount  17  in a common plane. The legs  19  and  22  form an annular second friction surface  21   b  facing the first friction surface  21   a . The facing friction surfaces  21   a  and  21   b  form an annular space  21   c  therebetween. The facing ends of the legs  19  and  22  are spaced apart to form a gap in which a nonferrous spacer  24  is positioned. The spacer  24  couples the pulley  11  to the bearing mount  17  for co-rotation on the bearing  15 . 
     A front cover  25  has a generally J-shaped cross section and is attached to the pulley  11  at a bottom of the inner leg  22 . The cover  25  extends from the pulley  11  into contact with an outer end of the flange  20  to form an enclosed first reservoir  26  for a quantity of magnetic powder  27  disposed therein. The reservoir  26  can be filled with the magnetic powder  27  through an aperture in the cover  25  sealed with a removable plug  28 . A seal  29  is attached to the driven member  12  and contacts the front cover  25  to protect the sliding seal where the cover contacts the flange  20 . A generally U-shaped seal  30  is attached to the inner leg  18  of the bearing mount  17  and contacts an inner end of the flange  20  to form an enclosed second reservoir  31  for a quantity of the magnetic powder  27  disposed therein. The reservoirs  26  and  31  are in communication through the space  21   c  between the friction surfaces  21   a  and  21   b . The surface  21   a  has an annular central groove  32  formed therein opposite the spacer  24  and a pair of narrower secondary grooves  33  formed therein on either side of the central groove  32 . 
     A wire wound coil  34  is positioned between the legs  22  and  23  of the pulley  11 . The coil  34  is attached to a coil mounting bracket  35  mounted on the air compressor housing  16 . Thus, the coil  34  and the mounting bracket  35  remain stationary as the pulley  11  is rotated by the vehicle engine. When no current is flowing in the coil  34 , the driven member  12  and the shaft  13  are not rotating. As described below, when electrical power is applied to the coil  34 , a magnetic field is created with lines of force being concentrated in the air gap  21   c  between the surfaces  21   a  and  21   b . The magnetic field polarizes the magnetic particles  27  such that they are attracted to one another to the point that the pulley  11  becomes coupled to the driven member  12  thereby rotating the shaft  13  and operating the compressor. 
     Such magnetic powders  27  are well known to one skilled in the art and are not described in great detail here. However, such powders have good magnetic properties and are resistant to corrosion and wear. A preferred embodiment utilizes 400-level stainless powders, which contain iron, chromium, and other elements at low concentration. The powder is readily flowable so as to fill the space  21   c  between driving and driven members and should not form an irreversible compaction. The particle size and shape are chosen to obtain these desired characteristics. Particle sizes are preferably between 1 to 100 micrometers and are spherical or spheroidal. Alternatively, irregular shapes may be desired. A dispersing agent, typically small quantities of silica, graphite, zinc oxide, or other particulate materials, are preferably added to improve the flow properties of the powder. Alternatively, one skilled in the art could provide a magnetorheological fluid (MR fluid) in the space therebetween, in which case, one must include the appropriate seals to retain the MR fluid. 
     The magnetic powder material  27  provides a coupling between the friction surfaces  21   a  and  21   b  when a magnetic field is applied thereto. The shear yield stress (τ y ) of the material is related to the intensity of the magnetic field. Thus, when a low intensity magnetic field is applied, the clutch  10  may be permitted to slip. The torque transfer characteristics of the clutch can be widely controlled as described below. The clutch  10  is capable of operating under slipping conditions, as is further described below. 
     The magnetic field generated by the coil  34  magnetizes the powder  27  particles, causing them to attract each other, forming chains or complex structures, along the magnetic field lines that span the space  21   c  and link the driving and driven members mechanically in a manner known to one skilled in the art. The strength of this attractive force, and consequently the torque transfer capacity of the clutch, is a continuous (but nonlinear) function of the applied field, as shown graphically in FIG. 2 as a curve  36  of torque versus current. The torque is related to the magnetic field strength, and therefore to the applied D.C. input current. Output torque is controlled by varying the D.C. input current as described below. 
     The nonferrous spacer  24  creates a nonferrous portion of the pulley  11  at approximately the center of the coil  34 . The nonferrous spacer  24  is provided to force most of the magnetic flux to flow through the powder  27  to the driven member  12  by minimizing leakage around the spacer. The spacer  24  should be strong and possess good dimensional stability and thus is preferably constructed from aluminum, brass, or nonmagnetic stainless steels. Alternatively, the spacer  24  could be omitted or replaced by ferrous metal in an application where it is determined that the device efficiency is not critical. 
     A variety of metals could be used in the ferrous portions of the clutch  10 , including the pulley  11 , the driven member  12  and the bearing mount  17 . These members are preferably made from low-carbon steel such as 1008 or 1010, cast irons, 400-level stainless steels, powder-metal processed materials or any other suitable material known to one skilled in the art. 
     The coil  34  is preferably a multistrand copper wire coil wound around a nonferrous bobbin, or can be wound and potted as a freestanding entity (not shown). The portion of the mounting bracket  35  that is inserted into the pulley  11  is constructed of a ferrous material to enable magnetic flux to flow. In an alternative embodiment, the coil  34  can be aluminum wire. 
     In the preferred embodiment, the coil  34  is fixed to avoid the problems of embedding a coil in a rotating member. A rotating coil requires the use of slip rings to make the electrical connection, which are less reliable than the fixed coil illustrated in the FIG.  1 . 
     Because the facing friction surfaces  21   a  and  21   b  are cylindrical, during rotation the magnetic powder  27  is forced outwardly against the surface  21   b , thereby reducing drag of the powder  27  when the magnetic field is removed. When a magnetic field is applied, the level of torque transfer (T) of the clutch for this cylindrical arrangement is expressed approximately as: 
     
       
         T=2πLR 2 τ Y   
       
     
     wherein “L” is the effective axial length of the friction surface  21   a  of the driven member  12 , “R” is the radius of surface  21   a  from the longitudinal axis of the shaft  13 , and “τ Y ” is the shear yield stress of the powder  27 . 
     There is shown in the FIG. 3 a schematic circuit diagram of a control circuit for the clutch  10 . A control module  40  has a plurality of inputs connected to signal sources such as a “Climate Control Switch Settings” source  41 , a “Compressor Low Pressure Switch” source  42 , an “Engine Speed” source  43 , an “Engine Torque Demand” source  44 , and “Other Inputs” sources  45  to receive information relevant to the operation of the clutch  10 . The control module  40  utilizes these input signals to determine when to engage the clutch  10 . An output of the control module  40  is connected to an input of a driver electronics circuit  46 . The circuit  46  is connected in series with the coil  34  between a power supply terminal  47  and a circuit ground potential terminal  48 . The control module  40  generates a control signal to the driver electronics circuit  46  to control the flow of current through the coil  34 . 
     The driver electronics circuit  46  is shown in more detail in the FIG. 4. A pulse-width modulation driver  49  has an input for receiving the control signal from the control module  40 , another input connected to the power supply terminal  47  and an output connected to a base of a bipolar (or FET) power transistor  50 . The transistor  50  has a collector connected to the coil  34  and an emitter connected to the ground terminal  48 . A diode  51  is connected in parallel with the coil  34  with an anode connected to the transistor collector and a cathode connected to the power supply terminal  47 . The driver  49  generates a high fixed frequency signal of variable pulse width to turn on and off the transistor  50 . The driver  49  can be programmed to gradually increase and decrease the current for engagement and disengagement of the clutch  10 . The driver  49  also can respond to the control signal to reduce the current flowing in the coil to conserve electrical power during low clutch torque demand conditions. 
     An alternate embodiment driver electronics circuit  46 ′ is shown in the FIG.  5 . The control signal line from the control module  40  is connected through a first resistor  52  to the ground terminal  48 . The line is connected through a second resistor  53  to an anode of a first diode  54 . The first diode  54  has a cathode connected to a gate of a FET power transistor  55 . The line is further connected through a third resistor  56  to a cathode of a second diode  57 . The second diode  57  has an anode connected to the cathode of the first diode  54 . The anode of the second diode  57  also is connected through a fourth resistor  58  to one side of a capacitor  59 . Another side of the capacitor  59  is connected through the coil  34  to the power supply terminal  47 . The transistor  55  is connected between the ground terminal  48  and the junction of the coil  34  and the capacitor  59 . The circuit  46 ′ is responsive to the control signal to generate a ramp signal increase and decrease in coil voltage for clutch engagement and disengagement. The turn-on characteristic time is controlled by the product of the value of the second resistor  53  and the value of the capacitor  59 , while the turn-off characteristic time is controlled by the product of the value of the third resistor  56  and the value of the capacitor  59 . 
     The control circuits shown in the FIGS. 3-5 control the torque transfer capacity of the magnetic powder clutch  10 . These control means enable the clutch  10  to be softly engaged and disengaged to minimize or eliminate the generation of stumble and noise. The torque capacity of the clutch  10  is a function of the instantaneous value of the current in the coil  34 . The rise and fall times for the torque are comparable to the inductive time constant of the clutch coil  34 . The inductive time constant is in general so short that if the coil  34  is excited by the stepwise application of battery voltage, the engagement and disengagement events are quite perceptible. The soft-start character of the clutch  10  is therefore preferably obtained by engaging and disengaging the clutch using a time-dependent voltage with a characteristic duration of approximately 2.5 seconds. One skilled in the art recognizes that longer duration may be desirable in various applications. Substantially shorter rise and fall times result in perceptible stumble, while times that are substantially longer may cause increased mechanical power loss due to prolonged clutch slip. 
     One skilled in the art appreciates that various means are available to control clutch torque capacity, and the preferred embodiments described herein are merely illustrative. A number of alternatives each involving different levels of complexity and expense are possible for soft-start operation. The above-described preferred embodiments retain the same basic mode of operation as the conventional clutch/compressor system, including an increase in the system pressure caused by vaporization of the refrigerant upon increased cooling demand, which causes a pressure switch to close, whereafter the a/c mode signal rapidly approaches the battery voltage. 
     The form of the invention shown and described herein constitutes preferred embodiments of the invention; it is not intended to illustrate all possible forms thereof. The words used are words of description rather than of limitation, and various changes may be made from that which is described here without departing from the spirit and scope of the invention.