Abstract:
Method and apparatus are described for a variable force solenoid for controlling the flow of a fluid in a housing having a bore for receiving the fluid flow, at least one port for exhausting the fluid flow, and a seat extending between the port and the bore. The solenoid includes a valve member disposed in the housing. In a closed position, the valve member is engaged with the seat, preventing fluid flow from the bore to the port. In an open position, the valve member is not engaged with the seat, allowing fluid flow from the bore to the port. A subassembly is disposed in the housing and surrounds a portion of the valve member for moving the valve member to its closed position. The subassembly has a movable armature for engaging the valve member, an annular member for attracting the armature, and a gap defined between the armature and the annular member. The gap is maintained by engagement of the armature by the valve member. A coil is disposed in the housing radially outward from the subassembly. Reception of electric current by the coil produces magnetic flux, thereby attracting the armature to the annular member, and thus moving the valve member to the closed position.

Description:
BACKGROUND 
     This disclosure relates generally to a solenoid in a hydraulic pressure management system, and more particularly, to a normally rising variable force solenoid interfaced between the electronic logic and hydraulic fluid controls of such a system. 
     Solenoids have been used to provide hydraulic pressure management in situations where hydraulic output pressure must be independent of hydraulic supply pressure. Output pressure is proportional to the closing force of the solenoid. If the closing force can be varied, the solenoid is said to be a variable force solenoid, and if output pressure increases proportionally with applied closing force, the solenoid is said to be normally rising. 
     Some previous solenoid designs included springs, either as a closing force or as balance springs in combination with another closing force. However, springs are undesirable for several reasons, for example, decreased performance with wear. Moreover, decreasing the number of components to facilitate manufacture is a major concern in the industry. 
     One type of closing force used presently is magnetic flux applied to actuate an armature. The magnetic flux is produced by application of an electrical input current to a coil. However, the performance of these solenoids are often negatively impacted by hysteresis, a lagging in the values of resulting magnetization in a magnetic material due to a changing magnetizing force. In practical terms, hysteresis makes the solenoid less responsive to opening or closing commands by resisting movement of the armature. Furthermore, solenoids of this type must be calibrated to provide a predetermined output pressure. In the past, calibration has been a relatively arduous undertaking. 
     Therefore, what is needed is a normally rising variable force solenoid that is easily calibrated, and has relatively fewer components while minimizing hysteresis. 
     SUMMARY 
     Accordingly, an embodiment of the present invention provides a variable force solenoid for controlling the flow of a fluid in a housing having a bore for receiving the fluid flow, at least one port for exhausting the fluid flow, and a seat extending between the port and the bore. The solenoid includes a valve member disposed in the housing. In a closed position, the valve member is engaged with the seat, preventing fluid flow from the bore to the port. In an open position, the valve member is not engaged with the seat, allowing fluid flow from the bore to the port. A subassembly is disposed in the housing and surrounds a portion of the valve member for moving the valve member to its closed position. The subassembly has a movable armature for engaging the valve member, an annular member for attracting the armature, and a gap defined between the armature and the annular member. The gap is maintained by engagement of the armature by the valve member. A coil is disposed in the housing radially outward from the subassembly. Reception of electric current by the coil produces magnetic flux, thereby attracting the armature to the annular member, and thus moving the valve member to the closed position. 
     One advantage of the embodiments described herein is that hysteresis is minimized by creating a gap between the armature and the annular member. Another advantage of the embodiments is that the subassembly greatly simplifies calibration, as the subassembly is adjusted as a singular component, providing more consistent and accurate output pressure. Yet another advantage is that fewer components are used when compared to previous designs. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is an isometric view of a normally rising variable force solenoid according to an embodiment of the present invention. 
     FIG. 2 is a cross-sectional view of the solenoid of FIG. 1 with a subassembly removed. 
     FIG. 3 is a cross-sectional view of the subassembly of the solenoid of FIG.  1 . 
     FIG. 4 is a cross-sectional view of the solenoid of FIG.  1 . 
     FIG. 5 is a schematic diagram of the solenoid of FIG. 1 in the open position. 
     FIG. 6 is a cross-sectional view of another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, the reference numeral  10  refers to a normally rising, multi-port, variable force solenoid. The solenoid  10  has a cylindrical housing  12 , and a snout  14  attached to the housing, for example, by a press fit. The snout  14  has a reduced diameter end portion  14   a , which protrudes from the housing  12 , and which has an external groove  14   b  for facilitating attachment to a hydraulic fluid supply by accepting an o-ring (not depicted) for sealing fluid pressure. The end portion  14   a  also has an axial bore  14   c  which is in fluid communication with an interior portion of the housing  12  and two ports,  16   a  and  16   b.    
     Referring to FIG. 2, the housing  12  has a stepped axial bore which divides the interior of the housing into four sections:  12   a ,  12   b ,  12   c , and  12   d.    
     The snout  14  is substantially “Y” shaped in cross section, and is attached to the housing section  12   a . A stepped axial bore divides the interior of the snout  14  into three sections:  14   c ,  14   d , and  14   e . A protruding interior seat  14 f is provided at the interior end of the section  14   c.    
     The snout section  14   e  receives a diaphragm  18 , which is biased between the snout and the radial wall defining the housing section  12   a , to form a contaminant barrier. An opening  18 a extends through the center of the diaphragm  18 , and a pin  20  is slidably mounted in the opening to allow axial movement by the pin. The diaphragm  18  forms a fluid tight seal around the pin  20  to prevent fluid from reaching the housing sections  12   b ,  12   c , and  12   d . Thus, the diaphragm  18  and snout section  14   d  define a fluid chamber  22  which is disposed between, and in communication with, the snout section  14   c  and the ports  16   a  and  16   b.    
     In an open position of the pin  20 , as illustrated, a flanged end portion  20   a  of the pin is spaced away from the snout seat  14   f . In a closed position of the pin  20 , the flanged end portion  20   a  is engaged with the snout seat  14   f.    
     A nonmagnetic coil housing  24  is disposed in the housing sections  12   c  and  12   d  with a portion  24   a  of the coil housing protruding from the rear of the housing  12  to attach to an external power supply. A cylindrical wire coil  25  is wrapped around an outer portion of the housing  24 . A bore  24   b  is formed through the housing  24  for receiving a subassembly, generally referred to by the reference numeral  26  in FIG.  3 . 
     The subassembly  26  is designed to be pressed into the bore  24   b  of the housing  24 , with the end of the subassembly engaging housing section  12   b  in a tight fit. Thus, the subassembly  26  is disposed radially inwardly relative to the coil housing  24 . The subassembly  26  includes a cylindrical centerpole  30  having an externally tapered end  30   a . A bore  30   b  is formed in the centerpole  30 , and receives a plug  32 . An external stepped-in portion  30   c  is formed on the centerpole  30  adjacent to the tapered end  30   a  for receiving a connection sleeve  34 , which extends beyond the centerpole tapered end to engage a ring  36 , thereby connecting the centerpole to the ring. 
     The ring  36  has an external stepped-in portion  36   a  for receiving the connection sleeve  34 , such that the exterior surface of the ring is flush with the exterior surface of the connection sleeve to facilitate insertion of the subassembly  26  into the bore  24   b  of the housing  24 . The ring  36  defines an axial bore  36   b , one end of which is tapered outwardly, to receive the pin  20  (FIG. 2) in a manner which allows the pin to slide between its above-described open and closed positions. A distal portion of the axial bore  36   b  is also tapered to define a seat  36   c . Opposing surfaces of the centerpole  30 , the connection sleeve  34 , and the ring  36  define an armature chamber  38 . 
     A nonmagnetic inner sleeve  40  is disposed in the centerpole bore  30   b , and extends into the chamber  38  for receiving a magnetically susceptible armature ball  42  in an axially sliding fit, with the sleeve  40  isolating the ball from lateral magnetic flux carried through the centerpole  30 . The ball  42  is kept in the chamber  38  by the plug  32  and the ring  36 . The shape of the ball  42  minimizes its lateral surface area, and therefore reduces possible lateral friction with the sleeve  40 . 
     Referring to FIG. 4, the solenoid  10  is depicted after the subassembly  26  has been inserted into the bore  24   b  of the housing  24 . A washer  41  engages the housing section  12   d  and the coil housing  24 , holding the subassembly  26  in place. The ball  42  contacts and moves with the pin  20  between the pin&#39;s open and closed positions. The ball  42  is held away from the seat  36   c  by the pin  20 , forming a gap  43 , in a manner to be explained. It is understood that the thickness of the gap  43  is exaggerated for the purposes of illustration. 
     FIG. 5 depicts the solenoid  10  connected to a current input device  44 , which is operably connected to the solenoid for providing an electrical current to the coil  25  (FIG. 4) of the solenoid. A hydraulic control chamber  46  is formed between the solenoid  10  and an orifice  48  which connects the control chamber  46  to a hydraulic supply chamber  50 . The fluid pressure in the control chamber  46  is monitored by a pressure sensing device  52 . Hydraulic fluid (not shown) is supplied from the hydraulic supply chamber  50 , and flows through the orifice  48 , which acts as a flow restrictor so that fluid flowing into the control chamber  46  from the supply chamber  50  is impeded, whereas flow into the section  14   c , and, if the pin  20  is in its open position, out the ports  16   a  and  16   b  is relatively unimpeded. If used in an automatic transmission system, the ports  16   a  and  16   b  are connected to an exhaust reservoir  53 . 
     In operation, referring to FIG. 4, if no electrical current is supplied to the coil housing  24  from the current input device  44  (FIG.  5 ), hydraulic fluid flows through the bore  14   c  of the snout  14 , and contacts the pin  20 . The resulting hydraulic opening force keeps the pin  20  in its open position, away from the snout seat  14   f . The fluid thus flows virtually unimpeded into the chamber  22 , out the ports  16   a  and  16   b , and to the exhaust reservoir  53  (FIG.  5 ). Thus, as inflow from the supply chamber  50  (FIG. 5) into the chamber  46  (FIG. 5) is impeded by the orifice  48  (FIG.  5 ), and outflow from the chamber  46  (FIG. 5) is relatively unimpeded, the hydraulic pressure in the control chamber  46  (FIG.  5 ), which is monitored to indicate the output pressure, is essentially zero. 
     To close the solenoid  10 , electrical current is supplied from the input current device  44  (FIG.  5 ), and as a result, the coil  25  develops a magnetic flux. The centerpole  30  and ring  36  have shapes which enhance and focus the magnetic flux, which travels in a circular pattern, for example through the housing  12 , into the washer  41 , to the centerpole  30 , into the ball  42 , and across the gap  43  to the ring  36 , such that the ball is attracted to the ring, providing a magnetic closing force for the pin  20 , as will be explained. The flux path then returns to the housing  12  via the ring  36  and begins the cycle again. 
     Thus, as current levels are increased, magnetic flux levels increase, and the attraction between the ring  36  and the ball  42  grows stronger. Since the ball  42  is in contact with the pin  20 , the ball does not move towards the ring  36  until the magnetic closing force is greater than the counteracting hydraulic opening force acting on the pin. During this operation, the inner sleeve  40  isolates the ball  42  from lateral pull from the magnetic flux field, and subsequent undesirable frictional effects. 
     Once an electrical force sufficient to produce a net magnetic closing force is provided, the ball  42  moves toward the ring  36 , and therefore the pin  20  is moved toward the snout seat  14   f , i.e., downwards as viewed in FIG. 4, towards its closed position. The pin  20  thereby restricts hydraulic flow between the snout section  14   c  and the ports  16   a  and  16   b , thus causing a corresponding increase in the output pressure. As this output pressure increases, it results in an increase of the hydraulic opening force acting on the pin  20 , thereby requiring a greater threshold magnetic closing force to continue movement of the ball  42  and, therefore, the pin. If a sufficient level of electrical current is present, or is supplied, the ball  42  continues to urge the pin  20  towards the snout seat  14   f  until the pin reaches its closed position in which it contacts the snout seat, thus preventing flow of the hydraulic fluid. Pressure in the control chamber  46  (FIG. 5) is at its peak when the pin  20  is in this closed position, and is equal to the supply pressure from the supply chamber  50  (FIG.  5 ). 
     When the electrical current is decreased, the hydraulic opening force forces the pin  20 , and therefore the ball  42 , away from the snout seat  14   f , until the magnetic closing force and opposing hydraulic opening force are in equilibrium, whereupon the pin takes an equilibrium position, and produces a corresponding output pressure. Thus, by applying different electrical current levels, the solenoid  10  may be operated along a continuum of positions of the pin  20  and associated output pressures ranging between the fully closed position, where output pressure is equal to the supply pressure, and the fully open position, where the pin is pushed as far back as possible by the hydraulic opening force, and the output pressure is essentially zero. 
     Calibration, at a given electrical current level, involves balancing the magnetic closing force against the hydraulic opening force acting on the pin  20  to produce a predetermined output pressure. To calibrate the solenoid  10 , an electrical current level sufficient to fully engage the ball  42  with the seat  36   c  is applied, as is a predetermined fluid supply pressure, while axially inserting the subassembly  26  into the housing  12 . As the subassembly  26  is inserted, in a downward direction in FIG. 4, the ball  42  engages and moves the pin  20 , causing the pin to move toward the snout seat  14   f , thereby restricting hydraulic fluid flow between the snout section  14   c  and the ports  16   a  and  16   b . This restriction creates a measurable rise in the output pressure, as monitored by the pressure in the control chamber  46  (FIG.  5 ). 
     A peak control pressure in the chamber  46  (FIG. 5) is attained while the ball  42  is still fully engaged with the seat  36   c , and the pin  20  is in contact with the snout seat  14   f , preventing fluid flow between the snout section  14   c  and the ports  16   a  and  16   b . After the control pressure in the chamber  46  (FIG. 5) reaches a peak, continued axial insertion of the subassembly  26  will cause the pin  20 , which can advance no further because of the snout seat  14   f , to push the ball  42  away from the seat  36   c , creating the gap  43  between the ball and the seat, as shown in FIG.  4 . 
     The gap  43  reduces the magnetic closing force between the ball  42  and the ring  36 , which produces a lower output pressure. Thus, after the gap  43  is formed, more electrical current will be required to produce the same magnetic closing force. However, the gap  43  is beneficial, as it allows the solenoid  10  to avoid gross hysteresis both initially and as normal wear occurs. The gap  43  not only prevents metal to metal contact in the magnetic circuit, it compensates for some wear on the pin end  20   a  and snout seat  14   f , before the consequent lengthening of the distance the pin  20  travels causes the ball  42  to “bottom out” on the seat  36   c . Repair is necessary once the ball  42  starts to bottom out as a result of such wear, as the ball can provide no further closing force on the pin  20  when bottomed out, and thus the pin  20  will not tightly engage the seat  14   f.    
     Returning to calibration, the subassembly  26  is further inserted until a predetermined output pressure is produced for the given electrical input current. In practice, the size of the gap  43  produced to obtain this predetermined output pressure may vary slightly between solenoids of the present embodiment, as a result of minor dimensional differences resulting from manufacture. However, once calibrated by the above-described method, all such solenoids will produce the predetermined output pressure at the given current level, with consistency and accuracy. 
     One advantage of this embodiment is that it minimizes hysteresis with its friction reducing inner sleeve and gap. Another advantage of this embodiment is that the subassembly greatly simplifies calibration, as the subassembly is adjusted as a singular component, providing more consistent and accurate output pressure. Yet another advantage is that the embodiment uses fewer components than previous designs. 
     Referring to FIG. 6, the reference numeral  54  refers to an alternative embodiment of a normally rising, multi-port, variable force solenoid. It is understood that the embodiment of FIG. 6 is connected to the same fluid controls as shown in FIG.  5 . 
     The solenoid  54  has a cylindrical housing  56 , which has a stepped axial bore that divides the interior of the housing into four sections:  56   a ,  56   b ,  56   c , and  56   d . A substantially “Y” shaped snout  58  is attached to the housing section  56   a . The snout  58  has a reduced diameter end portion  58   a , which protrudes from the housing  56 , and which has an external groove  58   b  for facilitating attachment to a hydraulic fluid supply by accepting an o-ring (not depicted) for sealing fluid pressure. A stepped axial bore divides the interior of the snout  58  into three sections:  58   c ,  58   d , and  58   e . The snout section  58   c  is in fluid communication with two ports,  60   a  and  60   b . A protruding interior seat  58   f  is provided at the interior end of the snout section  58   c.    
     A diaphragm  62  is disposed in the snout section  58   e , and is biased between the snout  58  and the radial wall defining the housing section  56   a , to form a contaminant barrier. An opening  62   a  extends through the center of the diaphragm  62 , and a pin  64  is slidably mounted in the opening. The diaphragm  62  forms a fluid tight seal around the pin  64  to prevent fluid from reaching the housing sections  56   b ,  56   c , and  56   d , and thus, the diaphragm and snout section  58   d  define a fluid chamber  66  which is disposed between, and in communication with, the snout section  58   c  and the ports  60   a  and  60   b.    
     The pin  64  has a pin cap  64   a  and a pin shaft  64   b . In a open position of the pin  64 , the pin cap  64   a  is spaced away from the snout seat  58   f . In a closed position, as shown in FIG. 6, the pin cap  64 a is engaged with the snout seat  58   f.    
     A nonmagnetic coil housing  68  is disposed in the housing section  56   c  with a portion  68   a  of the coil housing protruding from the side of the housing  56  to attach to an external power supply. A cylindrical wire coil  69  is wrapped around an outer portion of the housing  68 . A bore  68   b  is formed through the housing  68  for receiving a subassembly, generally referred to by the reference numeral  70 . 
     The subassembly  70  is designed to be pressed into the bore  68   b  of the housing  68 , with the end of the subassembly engaging housing section  56   b  in a tight fit. Thus, the subassembly  70  is disposed radially inwardly relative to the coil housing  68 . An annular spacer  72  engages the housing section  56   d  and the subassembly  70 , holding the subassembly in place. 
     The subassembly  70  includes a cylindrical centerpole  74  having an externally tapered end  74   a . A bore  74   b  is formed in the centerpole  74 , and receives a plug  78 . An external stepped-in portion  74   c  is formed on the centerpole  74  adjacent to the tapered end  74   a  for receiving a connection sleeve  76 , which connects the centerpole to a cylinder  80  in a spaced relationship. 
     An external portion  80   a  of the cylinder  80  is stepped-in to receive the connection sleeve  76 , such that the exterior surface of the cylinder is flush with the exterior surface of the connection sleeve to facilitate insertion of the subassembly  70  into the bore  68   b  of the housing  68 . The cylinder  80  also has a tapered external end  80   b , and defines an axial bore  80   c . A distal portion of the axial bore  80 c is tapered to define a seat  80   d . A pin support cylinder  80   e  is disposed in the axial bore  80   c  for receiving the pin shaft  64   b  in a manner which allows the pin  64  to slide between its open and closed positions. Opposing surfaces of the centerpole  74 , the connection sleeve  76 , and the cylinder  80  define an armature chamber  82 . 
     A nonmagnetic inner sleeve  84  is disposed in the centerpole bore  74   b , and extends into the chamber  82  for receiving a magnetically susceptible armature ball  86  in an axially sliding fit, with the sleeve  84  isolating the ball from lateral magnetic flux carried through the centerpole  74 . The ball  86  is kept from exiting the rear of the centerpole bore  74   b  by the plug  78 . The shape of the ball  86  minimizes its lateral surface area, and therefore reduces possible lateral friction with the sleeve  84 . 
     The ball  86  contacts and moves with the pin  64  between the pin&#39;s above-described open and closed positions. The ball  86  is held away from the seat  80   d  by the pin  64 , forming a gap  87 , in a manner to be explained. It is understood that the thickness of the gap  87  is exaggerated for the purposes of illustration. 
     In operation, if no electrical current is supplied to the coil housing  68  from the current input device  44  (FIG.  5 ), hydraulic fluid flows through the snout bore  58   c , and the resulting hydraulic opening force pushes the pin  64  away from the snout seat  58   f . The fluid thus flows into the chamber  66 , out the ports  60   a  and  60   b , and to the exhaust reservoir  53  (FIG.  5 ). In this fully open position of the pin, output pressure is essentially equal to zero. 
     To close the solenoid  54 , electrical current is supplied from the input current device  44  (FIG.  5 ), and as a result, the coil  69  develops a magnetic flux. The centerpole  74  and the cylinder  80  have shapes which enhance and focus the magnetic flux, which travels in a circular pattern, for example through the housing  56 , into the washer  72 , to the centerpole  74 , into the ball  86 , and across the gap  87  to the cylinder  80 , such that the ball is attracted to the ring, providing a magnetic closing force for the pin  64 , as will be explained. The flux path then returns to the housing  56  via the cylinder  80  and begins the cycle again. 
     As current levels are increased, magnetic flux levels increase, and the attraction between the cylinder  80  and the ball  86  grows stronger. Since the ball  86  is in contact with the pin  64 , the ball does not move towards the cylinder  80  until the magnetic closing force is greater than the counteracting hydraulic opening force acting on the pin. During operation, the inner sleeve  84  isolates the ball  86  from lateral pull from the magnetic flux field, and subsequent undesirable frictional effects. 
     Once an electrical force sufficient to produce a net magnetic closing force is provided, the ball  86  moves toward the cylinder  80 , and therefore the pin  64  is moved toward the snout seat  58   f . The pin  64  thereby restricts hydraulic flow between the snout section  58   c  and the ports  60   a  and  60   b , thus causing a corresponding increase in the output pressure. As this output pressure increases, it results in an increase of the hydraulic opening force acting on the pin  64 , thereby requiring a greater threshold magnetic closing force to continue movement of the ball  86  and, therefore, the pin. If a sufficient level of electrical current is present, or is supplied, the ball  86  continues to urge the pin  64  towards the snout seat  58   f  until the pin reaches its closed position in which it contacts the snout seat, as shown in FIG. 6, thus preventing flow of the hydraulic fluid. Control chamber pressure is at its peak when the pin  64  is in this closed position, and is equal to the supply pressure from the supply chamber  50  (FIG.  5 ). 
     When the electrical current is decreased, the hydraulic opening force forces the pin  64 , and therefore the ball  86 , away from the snout seat  58   f , until the magnetic closing force and opposing hydraulic opening force are in equilibrium, whereupon the pin takes an equilibrium position, and produces a corresponding output pressure. Thus, by applying different electrical current levels, the solenoid  54  may be operated along a continuum of positions of the pin  64  and associated output pressures ranging between the fully closed position, where output pressure is equal to the supply pressure, and the fully open position, where the pin is pushed as far back as possible by the hydraulic opening force, and the output pressure is equal to zero. 
     Calibration, at a given electrical current level, involves balancing the magnetic closing force against the hydraulic opening force acting on the pin  64  to produce a predetermined output pressure. To calibrate the solenoid  54 , an electrical current level sufficient to fully engage the ball  86  with the seat  80   d  is applied, as is a predetermined fluid supply pressure, while axially inserting the subassembly  70  into the housing  56 . As the subassembly  70  is inserted, in a downward direction in FIG. 6, the ball  86  engages and moves the pin  64 , causing the pin to move toward the snout seat  58   f , thereby restricting hydraulic fluid flow between the snout section  58   c  and the chamber  66  and the ports  60   a  and  60   b . This restriction creates a measurable rise in the output pressure, as monitored by the pressure in the control chamber  46  (FIG.  5 ). 
     A peak control pressure in the chamber  46  (FIG. 5) is attained while the ball  86  is still fully engaged with the seat  80   d , and the pin  64  is in contact with the snout seat  58   f , preventing fluid flow between the snout section  58   c  and the ports  60   a  and  60   b . After the control pressure reaches a peak, continued axial insertion of the subassembly  70  will cause the pin  64 , which can advance no further because of the snout seat  58   f , to push the ball  86  away from the seat  80   d , creating the gap  87  between the ball and the seat, as shown in FIG.  6 . 
     The gap  87  reduces the magnetic closing force between the ball  86  and the cylinder  80 , which produces a lower output pressure. Thus, after the gap  87  is formed, more electrical current will be required to produce the same magnetic closing force. However, the gap  87  is beneficial, as it allows the solenoid  54  to avoid gross hysteresis both initially and as normal wear occurs. The gap  87  not only prevents metal to metal contact in the magnetic circuit, it compensates for some wear on the pin end  64   a  and snout seat  58   f  before the consequent lengthening of the distance the pin  64  travels causes the ball  87  to bottom out on the seat  80   d , requiring repair. 
     Returning to calibration, the subassembly  70  is further inserted until a predetermined output pressure is produced for the given electrical input current. In practice, the size of the gap  87  produced to obtain this predetermined output pressure may vary slightly between solenoids of the present embodiment, as a result of minor dimensional differences resulting from manufacture. However, once calibrated by the above-described method, all such solenoids will produce the predetermined output pressure at the given current level, with consistency and accuracy. 
     One advantage of this embodiment is that it minimizes hysteresis with its friction reducing inner sleeve and gap. Another advantage of this embodiment is that the subassembly greatly simplifies calibration, as the subassembly is adjusted as a singular component, providing more consistent and accurate output pressure. Yet another advantage is that the embodiment uses fewer components than previous designs. 
     It is understood that all spatial references, such as front and rear, are only for the purposes of explanation of the drawings. This disclosure shows and describes illustrative embodiments, however, the disclosure contemplates a wide range of modifications, changes, and substitutions. Such variations may employ only some features of the embodiments without departing from the scope of the underlying invention. For example, two ports are shown, but the present invention embodies achieving the proper exhaust area, and thus encompasses using both more and fewer ports. Accordingly, any appropriate construction of the appended claims will reflect the broad scope of the underlying invention.