Patent Publication Number: US-7587897-B2

Title: Magnetically sequenced pneumatic motor

Description:
BACKGROUND 
   The present invention relates generally to pneumatic devices and, in certain embodiments, to air motors with valves having magnetic detents. 
   Pneumatic motors are often used to convert energy stored in the form of compressed air into kinetic energy. For instance, compressed air may be used to drive a reciprocating rod or rotating shaft. The resulting motion may be used for a variety of applications, including, for example, pumping a liquid to a spray gun. In some spray gun applications, the pneumatic motor may drive a pump, and the pump may convey a coating liquid, such as paint. 
   Conventional pneumatic motors are inadequate in some regards. For example, the mechanical motion produced by the pneumatic motor may not be smooth. Switching devices in pneumatic motors may signal when to re-route pressurized air during a cycle of the motor. When operating, the switching devices may intermittently consume a portion of the kinetic energy that the pneumatic motor would otherwise output. As a result, the output motion or power may vary, and the flow rate of a liquid being pumped may fluctuate. Variations in flow rate may be particularly problematic when pumping a coating liquid to a spray gun. The spray pattern may contract when the flow rate drops and expand when the flow rate rises, which may result in an uneven application of the coating liquid. 
   The switching devices in conventional pneumatic motors can produce other problems as well. For example, some types of switching devices, such as reed valves, may quickly wear out or be damaged by vibrations from the pneumatic motor, thereby potentially increasing maintenance costs. Further, some types of switching devices may be unresponsive at low pressures, e.g., less than 25 psi. Unresponsive switching devices may impede use of the pneumatic motor in applications where low-speed motion is desired or higher pressure air supplies are not available. 
   BRIEF DESCRIPTION 
   The following discussion describes, among other things, a pneumatic motor having a piston and a magnetically actuated valve. The magnetically actuated valve may be adjacent the piston and, in some embodiments, include a spool valve. 

   
     DRAWINGS 
     These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
       FIG. 1  is a perspective view of an exemplary spray system in accordance with an embodiment of the present technique; 
       FIG. 2  is a graph of pressure of the coating liquid versus time for various types of spray systems; 
       FIG. 3  is a perspective view of an exemplary pneumatic motor in accordance with an embodiment of the present technique; 
       FIGS. 4-7  are cross-sectional views of the pneumatic motor of  FIG. 3  during sequential stages of a cycle; 
       FIGS. 8-9  are cross-sectional views of a magnetically actuated pilot valve in two different states; 
       FIG. 10  is a perspective view of another pneumatic motor in accordance with an embodiment of the present technique; 
       FIG. 11  is an elevation view of the pneumatic motor of  FIG. 10 ; 
       FIG. 12  is a cross-sectional view of the pneumatic motor of  FIG. 10 ; 
       FIG. 13  is a top view of the pneumatic motor of  FIG. 10 ; 
       FIG. 14  is another cross-sectional view of the pneumatic motor of  FIG. 10 ; 
       FIG. 15  is a perspective view of a third embodiment of a pneumatic motor in accordance with an embodiment of the present technique; 
       FIG. 16  is a top view of the pneumatic motor of  FIG. 15 ; and 
       FIG. 17  is a cross-sectional view of the pneumatic motor of  FIG. 15 . 
   

   DETAILED DESCRIPTION 
   As discussed in detail below, some of the embodiments of the present technique provide a method and apparatus for coordinating air flow in a pneumatic motor. Of course, such embodiments are merely exemplary of the present technique, and the appended claims should not be viewed as limited to those embodiments. Indeed, the present technique is applicable to a wide variety of systems. 
   As used herein, the words “top,” “bottom,” “upper,” and “lower” indicate relative positions or orientations and not an absolute position or orientation. The term “or” is understood to be inclusive unless otherwise stated. The term “exemplary” is used to indicate that something is merely a representative example and not necessarily definitive or preferred. Herein, references to fluid pressures are gauge pressure (in contrast to absolute pressure) unless otherwise noted. 
     FIG. 1  depicts an exemplary spray system  10 . The spray system  10  includes a pneumatic motor  12  that may address one or more of the inadequacies of conventional pneumatic motors discussed above. As described below, in some embodiments, the pneumatic motor  12  includes a magnetically actuated pilot valve that may tend to consume less of the energy that would otherwise be output from the pneumatic motor  12 . As a result, the pneumatic motor  12  may facilitate the production of more uniform pumping pressures than conventional devices. Further, in certain embodiments, magnetic actuation of the pilot valve may enable the pneumatic motor  12  to operate even when supplied with low pressure air. It should also be noted that, in some embodiments, the magnetically actuated pilot valve includes a spool valve that is robust to impacts and wear. Relative to conventional devices, these spool valves may tend to have a relatively long operating life. Details of the pneumatic motor  12  are described below after addressing features of the spray system  10 . 
   In addition to the pneumatic motor  12 , the exemplary spray system  10  may include a pump  14 , a coating liquid inlet  16 , a stand  18 , a spray gun  20 , an air conduit  22 , a liquid conduit  24 , and a regulator assembly  26 . The pump  14  may be a reciprocating pump that is mechanically linked to the pneumatic motor  12  in a manner described further below. In other embodiments, the pump  14  may any of a variety of different types of pumps. 
   The intake of the pump  14  may be in fluid communication with the coating liquid inlet  16 , and the outlet of the pump  14  may be in fluid communication with the liquid conduit  24 . The liquid conduit  24  may, in turn, be in fluid communication with a nozzle of the spray gun  20 , which may also be in fluid communication with the air conduit  22 . 
   The regulator assembly  26  may be configured to directly or indirectly regulate air pressure in the air conduit  22 , the pressure of air driving pneumatic motor  12 , and/or the pressure of a coating liquid within the liquid conduit  24 . Additionally, the regulator assembly  26  may include pressure gauges to display one or more of these pressures. 
   In operation, the pneumatic motor  12  may translate air pressure into movement of the pump  14 . Rotating pumps  14  may be driven by a crankshaft connected to the pneumatic motor  12 , and reciprocating pumps  14  may be directly linked to the pneumatic motor  12  by a rod, as explained below. The pump  14  may convey a coating liquid, such as paint, varnish, or stain, through the coating liquid inlet  16 , the liquid conduit  24 , and the nozzle of the spray gun  20 . Pressurized air flowing through the air conduit  22  may help to atomize the coating liquid flowing out of the spray gun  20  and form a spray pattern. As discussed above, the pressure of the coating liquid may affect the spray pattern. Pressure fluctuations may cause the spray pattern to collapse and expand. 
     FIG. 2  is a graph of coating liquid pressure versus time for three types of spray systems: an ideal system  23 , the exemplary spray system  10 , and a conventional spray system  32 . (The conventional spray system  32  is shown with an arbitrarily selected one-half cycle phase shift to highlight differences between the systems.) As illustrated by  FIG. 2 , in the two non-ideal systems  10  and  32 , the coating liquid pressure fluctuates. However, the exemplary spray system  10  has a variation  34  that is smaller than a variation  36  of the conventional spray system. The features of the exemplary spray system  10  that may tend to enable relatively small variation  34  in coating liquid pressure are discussed below. 
     FIGS. 3-9  illustrate details of the pneumatic motor  12 .  FIG. 3  is a perspective view of the pneumatic motor  12  and the pump  14 .  FIGS. 4-7  are cross-sectional views of the pneumatic motor  12  in sequential stages of an energy conversion cycle, and  FIGS. 8 and 9  are cross-sectional views of a switching device in the pneumatic motor  12 .  FIGS. 8 and 9  illustrate two states assumed by the switching device during various portions of the cycle. After describing the components of the pneumatic motor  12 , their operation during the energy conversion cycle will be explained. 
   With reference to  FIGS. 3 and 4 , the pneumatic motor  12  may include an upper-pilot valve  38 , a lower-pilot valve  40 , a cylinder  42 , a bottom head  44 , a top head  46 , an air-motor piston  48 , a piston rod  50 , and a main valve  52 . To pneumatically or fluidly couple these components, the pneumatic motor  12  may include an upper-pilot signal path  54 , an upper-pilot signal path  56 , a lower-pilot signal path  58 , a lower-pilot signal path  60 , an upper primary air passage  62 , and a lower primary air passage  64 . 
     FIG. 8  is an enlarged view of the upper-pilot valve  38 , which may also be referred to as a switching device, a magnetically actuated switching device, a magnetically actuated pilot valve, a piston position sensor, or a magnetically actuated valve. The upper-pilot valve  38  may include a magnet  66 , a spool valve  68 , an end cap  70 , a sleeve  72 , and a magnet stop  74 . 
   The magnet  66  may be positioned such that an axis from its north pole to its south pole is generally parallel to the direction in which the spool valve  68  moves, as explained below. For example, in the orientation depicted by  FIG. 8 , the north and south poles of the magnet  66  may be oriented one over another. The magnet  66  may be an electromagnet or a permanent magnet, such as a neodymium-iron-boron magnet, a ceramic magnet, or a samarium-cobalt magnet, for instance. 
   The spool valve  68  may include a magnet mount  76 , a lower seal  78 , a middle seal  80 , and an upper seal  82 . The volume generally defined by the upper seal  82  and the middle seal  80  is referred to as an upper chamber  84 , and the volume generally defined by the middle seal  80  and the lower seal  78  is referred to as a lower chamber  86 . The upper chamber  84  may be in fluid communication with the upper-pilot signal path  56 , and the lower chamber  86  may be in fluid communication with the upper-pilot signal path  54 . In some embodiments, these passages may be in fluid communication regardless of the position of the spool valve  68  relative to the sleeve  72 . The spool valve  68  may be generally rotationally symmetric (e.g., circular) and have a central axis  88  about which the various portions  78 ,  80 ,  82 ,  84 , and  86  are generally concentric. The spool valve  68  may be manufactured, for example, machined on a lathe, from hardened metal, such as hardened stainless steel (e.g., 440C grade). The magnet mount  76  may couple, e.g., affix, the magnet  66  to the spool valve  68 . 
   The end cap  70  may include exhaust ports  90  and  92  and a vent  94 . The vent  94  may be in fluid communication with a top  96  of the spool valve  68 , and the exhaust ports  90  and  92  may be selectively in fluid communication with the upper chamber  84  depending on the position of the spool valve  68 , as explained below. 
   The sleeve  72  may have a generally circular-tubular shape sized such that it may form dynamic seals (e.g., slideable seals) with the lower seal  78 , the middle seal  80 , and the upper seal  82 . In some embodiments, the sleeve  72  may be generally concentric about the central axis  88  of the spool valve  68 . The sleeve  72  may have passages through which the upper-pilot signal path  54 , the upper-pilot signal path  56 , and the exhaust ports  90  and  92  may extend. The sleeve  72  may be manufactured from hardened metal, such as those discussed above. In certain embodiments, the sleeve  72  may form a matched set with the spool valve  68 . In other words, the tolerance of the difference between outer diameter of the spool valve  68  and the inner diameter of the sleeve  72  may be configured to form a dynamic seal. In some embodiments, the spool valve  68  and sleeve  72  may form dynamic seals that are generally free of o-rings or other types of seals, e.g., U-cup or lip seals. Advantageously, the spool valve  68  may slide within the sleeve  72  with relatively little friction, which may tend to lower the amount of energy consumed by the spool valve  68  when it moves. 
   The magnet stop  74  may be integrally formed with the top head  46  and may include a pressure inlet  100 . The pressure inlet  100  may place a bottom surface  103  of the magnet  66  in fluid communication with the interior of the cylinder  42 . The pressure inlet  100  may be generally smaller than the magnet  66  to generally constrain movement of the magnet  66  within a range of motion. 
   Returning to  FIG. 4 , the lower-pilot valve  40  may be similar or generally identical to the upper-pilot valve  38 . The lower-pilot valve  40  may be oriented upside down relative to the upper-pilot valve of  38 . Consequently, the magnet  66  of the lower-pilot valve  40  may be proximate the interior of the cylinder  42 . 
   The cylinder  42  may have a generally circular tubular shape with an inner diameter sized to form a dynamic seal with the air-motor piston  48 . Tie rods  102  (see  FIG. 3 ) may compress the walls of the cylinder  42  between the top head  46  and the bottom head  44 . 
   With continued reference to  FIG. 4 , the top head  46  may be integrally formed with portions of the upper-pilot valve  38  and a portion of the upper primary air passage  62 . The upper primary air passage  62  may extend through the top head  46 , placing the upper primary air passage  62  in fluid communication with an upper interior portion  104  of the cylinder  42 . Similarly, the bottom head  44  may be integrally formed with portions of the lower-pilot valve  40  and a portion of the lower primary air passage  64 . The lower primary air passage  64  may be in fluid communication with a lower interior portion  106  of the cylinder  42 . 
   The air-motor piston  48  may separate the upper interior portion  104  from the lower interior portion  106 . The piston  48  may include a sealing member  108  (e.g., o-ring) that interfaces with the cylinder  42  to form a sliding seal. The air-motor piston  48  may include an upper surface  110  and a lower surface  112 . The piston rod  50  may be affixed or otherwise coupled to the air-motor piston  48  and may extend through the bottom head  44  to the pump  14 . 
   The main valve  52  may be referred to as a primary pneumatic switching device or a pneumatically controlled valve. The main valve  52  may include a housing  114 , a sleeve  116 , and a main spool valve  118 . The housing  114  may include a primary air intake  120  and vents  122  and  124 . The main spool valve  118  may form a number of sliding seals with the sleeve  116 . Together, the main spool valve  118  and sleeve  116  may define an upper chamber  126  and a lower chamber  128 . The upper chamber  126  and lower chamber  128  may be separated by a middle seal  130 . 
   The sleeve  116  and the housing  114  may define a path and direction of travel for the main spool valve  118 . This path and direction of travel can be seen by comparing the position of the main spool valve  118  in  FIGS. 4-7 , which depict the main spool valve  118  translating up and down in the housing  114 . In other embodiments, the main spool valve  118  may travel a different path and/or may rotate, depending on the configuration of the main spool valve  118  and the housing  114 . 
   In some embodiments, the main spool valve  118  may include a magnetic detent formed by static magnets  119  and  121  attached to the housing  114  and moving magnetically responsive materials  123  and  125  (e.g., ferromagnetic materials or other materials with a high magnetic permeability) attached to the main spool valve  118 . The magnetically responsive materials  123  and  125  are illustrated in  FIGS. 4-7  as a separate material from the main spool valve  118 , but in some embodiments, the main spool valve  118  may be made of a magnetically responsive material. The magnets  119  and  121  may hold the main spool valve  118  against opposing ends of the main valve  52  until a threshold force is applied to the main spool valve  118 , as explained below. 
   Depending on the embodiment, the magnetic detents may take a variety of forms. In certain embodiments, the positions of the magnets  119  and  121  and the magnetically responsive materials  123  and  125  may be reversed. That is, the magnets may be coupled to, and move with, the main spool valve  118 , and the housing  114  may include or be coupled to a magnetically responsive material. In other embodiments, both the housing  114  and the main spool valve  118  may include magnets. These magnets may be oriented such that the north pole of the magnets in the housing is facing the south pole of the magnets on the main spool valve  118 , or vice versa. 
   The present embodiment may include a variety of types of magnets. For instance, the illustrated magnets  119  and  121  may be an electromagnet or a permanent magnet, such as a neodymium-iron-boron magnet, a ceramic magnet, or a samarium-cobalt magnet, for instance. 
   The illustrated embodiment includes two magnetic detents, one at each end of the path through which the main spool valve  118  travels. The poles of the magnets  119  and  121  may be generally parallel to this direction of travel and the fields from these magnets may overlap the main spool valve  118  when the main spool valve  118  is positioned at the distal portions of its path. In other embodiments, the main spool valve  118  may include a single magnetic detent disposed at one end of the main spool valve&#39;s path, e.g., at the top of its travel. 
   Certain embodiments may include a single magnetic detent that employs magnetic repulsion instead of, or in addition to, magnetic attraction. For instance, the main spool valve  118  may include a magnet near its middle seal  130  with poles that extend generally perpendicular to the main spool valve&#39;s direction of travel, and the housing may include a repelling magnet positioned near the middle of the main spool valve&#39;s path, such that the repelling magnet pushes the main spool valve  118  either to the top or the bottom of the housing  111 . That is, a single magnet disposed near the mid-section of the housing  111  may bias the main spool valve  118  against the top or the bottom of the housing  111 , depending on where the main spool valve  118  is relative to the mid-point of its path. In some of these embodiments, the poles of the static, repelling magnet may be oriented generally perpendicular to the main spool valve&#39;s direction of travel and generally parallel to the moving magnet on the main spool valve  118 . 
   A variety of fluid conduits may connect to the main valve  52 . The upper-pilot signal path  56  may extend through the housing  114 , placing it in fluid communication with a top surface  132  of the main spool valve  118 . Similarly, the lower-pilot signal path  60  may be in fluid communication with a bottom surface  134  of the main spool valve  118 . Depending on the position of the middle seal  130 , the primary air intake  120  may be in fluid communication with either the upper primary air passage  62  via the upper chamber  126  or the lower primary air passage  64  via the lower chamber  128 . 
   The pneumatic motor  12  may be connected to a source of a pressurized fluid, such as compressed air or steam. For instance, the pneumatic motor  12  may be connected to a central air compressor (e.g., factory air) via the primary air intake  120  and the pilot signal paths  54  and  58 . 
   In operation, the pneumatic motor  12  may receive pneumatic power through the primary air intake  120  and output power through movement of the piston rod  50 . To this end, the pneumatic motor  12  may repeat a cycle depicted by  FIGS. 4-7 . To signal the appropriate point at which to transition between the stages of this cycle, the pilot valves  38  and  40  may sense the position of the air motor piston  48  and switch between the states depicted by  FIGS. 8 and 9 . Consequently, in some embodiments, the pilot valves  38  and  40  may function as sensors that signal the main valve  52  when to redirect air flow from the primary air intake  120 , as explained below. 
   Starting at an arbitrarily selected point in the cycle,  FIG. 4  depicts the middle of an upstroke of the air-motor piston  48 , which is depicted by arrow  136 . At this stage, a primary air in-flow  138  is flowing in through the primary air intake  120  and is being directed to the lower primary air passage  64  by the main spool valve  118 . To reach the lower primary air passage  64 , the primary air in-flow  138  passes through the lower chamber  128 . Once in the lower primary air passage  64 , the primary air in-flow  138  passes into the lower interior portion  106  of the cylinder  42 . As the lower interior portion  106  is pressurized by the primary air in-flow  138 , a force is applied to the lower surface  112  of the air-motor piston  48 , and the air-motor piston  48  translates upwards, pulling the piston rod  50  with it, as indicated by arrow  136 . 
   The upper interior portion  104 , above the air-motor piston  48 , may be evacuated by a primary air out-flow  140  during an upstroke. The primary air out-flow  140  may pass through the upper primary air passage  62  into the upper chamber  126  of the main valve  52  and out through the vent  122 , to atmosphere. In the illustrated embodiment, the primary air in-flow  138  and the primary air out-flow  140  may continue to follow this path until the air-motor piston  48  approaches the top head  46 , at which point the pneumatic motor  12  may transition to the state depicted by  FIG. 5 . 
   In  FIG. 5 , the air-motor piston  48  is at the top of its stroke, and the main valve  52  has reversed the primary air flows  138  and  140 . As explained below, in the present embodiment, the upper-pilot valve  38  magnetically senses that the air-motor piston  48  is near the top of its stroke and directs a burst of air into the top of the main valve  52 , thereby shifting the position of the main spool valve  118 . 
   The upper-pilot valve  38  may transition between the states depicted by  FIGS. 8 and 9  when the air-motor piston  48  reaches the top of its stroke. Initially, the upper-pilot valve  38  may be in the state depicted by  FIG. 8 , with the spool valve  68  in an elevated, or recessed, position within the sleeve  72  (hereinafter “the first position”). When the spool valve  68  is in the first position, the upper-pilot signal path  56  may be in fluid communication with the exhaust ports  90  and  92  via the upper chamber  84 , and the upper-pilot signal path  54  may be isolated from the upper-pilot signal path  56  by the middle seal  80  of the spool valve  68 . In other words, the upper-pilot signal path  56  may be vented, and the upper-pilot signal path  54  may be sealed. The spool valve  68  may be held in the first position by magnetic attraction between the sleeve  72  and the magnet  66 . 
   As the air-motor piston  48  reaches the top of its stroke, the upper-pilot valve  38  may transition from the first position, depicted by  FIG. 8 , to a second position, which is depicted by  FIG. 9 . The magnet  66  may be attracted to the air-motor piston  48  and, as a result, the spool valve  68  may be pulled downward. In some embodiments, the air-motor piston  48  may include a magnet  146  to increase the attractive force. Alternatively, or additionally, the air motor piston  48  may include a material having a high magnetic permeability, e.g., a material with a magnetic permeability greater than 500 μN/A 2 . The magnet  66  may be pulled downward until it hits the magnet stop  74 , at which point the spool valve  68  may be in the second position. 
   When the spool valve  68  is in the second position, the upper-pilot signal path  54  may be in fluid communication with the upper-pilot signal path  56  via the upper chamber  84 . As a result, a pneumatic signal  142 , for example an airflow and/or pressure wave, may be transmitted through the upper-pilot signal path  56  to the main valve  52 . 
   Returning briefly to  FIGS. 4 and 5 , the pneumatic signal  142  may drive the main spool valve  118  from a first position depicted by  FIG. 4  to a second position depicted by  FIG. 5 . The pneumatic signal  142  may elevate the air pressure acting upon the top surface  132  of the main spool valve  118 , and overcome a magnetic attraction between the magnet  119  and the magnetically responsive material  123 . As this force is overcome, the main spool valve  118  may translate through the sleeve  116  to the second position depicted in  FIG. 5 . The main spool valve  118  may be held in this position by magnetic attraction between the magnet  121  and the magnetically responsive material  125 . In the present embodiment, moving the main spool valve  118  from the first position to the second position reverses the primary air flows  138  and  140 . At this point, the air-motor piston  48  may begin its downstroke, as illustrated by arrow  146  in  FIG. 5 . 
   As the air-motor piston  48  translates downward, away from the top head  46 , the upper-pilot valve  38  may transition back from the second position, depicted by  FIG. 9 , to the first position, depicted by  FIG. 8 . The primary air in-flow  138  into the upper interior portion  104  of the cylinder  42  may elevate the pressure of the upper interior portion  104 . In addition to driving the air motor piston  48  downwards, this increased pressure may propagate through the pressure inlet  100  of the upper-pilot valve  38 , and, as a result, the spool valve  68  may be driven upwards, back into the first position, depicted by  FIG. 8 . Magnetic attraction between the magnet  66  and the sleeve  72  may retain the spool valve  68  in the first position until the next time the air motor piston  48  arrives. 
   Advantageously, in the illustrated embodiment, the pilot valves  38  and  40  are returned to their original, closed position by air pressure rather than a mechanical coupling, which could wear and increase mechanical stresses in the motor  12 . In some embodiments, the pilot valves  38  and  40  may be referred to as pneumatically-reset pilot valves. Notably, the pilot valves  38  and  40  are reset in this embodiment with the air pressure that they modulate via the main valve  52  (i.e., the pressure inside the cylinder  42 ). As a result, the illustrated pilot valves  38  and  40  self-regulate their position. That is, the pilot valves  38  and  40 , in the present embodiment, are returned by the air pressure they were initially moved to increase, so pressure in the cylinder  42  acts as a pneumatic feedback control signal to the pilot valves  38  and  40 . In other words, the pilot valves  38  and  40  are configured to terminate the pneumatic signal they send to the main valve  52  in response to a change (e.g., increase) in pressure in the portion of the cylinder  42  that they sense. 
   In some embodiments, the magnet  66  may seal against the top head  46 , so the pressure in the cylinder  42  acts against the larger, bottom surface  103  of the magnet. In other embodiments, the bottom seal  78  may define the surface area over which the pressure in the cylinder acts. Some designs may include a separate piston to reset the pilot valves  38  and  40 . 
   In some embodiments, the pilot valves  38  and  40  may not necessarily be both magnetically actuated and pneumatically returned. In some embodiments, the pilot valves  38  and  40  may be initially displaced by a force other than magnetic attraction or repulsion. For instance, they may be driven toward the piston  48  by a cam or other device and returned by air pressure in the cylinder  42 . Conversely, in another example, the pilot valves  38  and  40  may be drawn toward the piston  48  by magnetic attraction and returned by a member extending from the piston  48 , rather than being pneumatically returned. In some embodiments, a magnetic force may return the pilot valves  38  and  40 , e.g., a magnetic force weaker than the one which pulls them toward the air-motor piston  48 . 
   To summarize before returning to  FIGS. 4-7 , at the top of a stroke of the air-motor piston  48 , the upper-pilot valve  38  may magnetically sense the position of the air-motor piston  48  and pneumatically switch the main valve  52  to begin a downstroke. 
     FIG. 5  illustrates the beginning of a downstroke, and  FIG. 6  illustrates the middle of a downstroke. In  FIG. 5 , the air-motor piston  48  is still near the top head  46 , and the pneumatic signal  142  is still being applied to the main valve  52  via the upper-pilot signal path  56 . In  FIG. 6 , the air-motor piston  48  has translated away from the upper-pilot valve  38 , and the pneumatic signal  142  is no longer applied to the main valve  52 . At this point, the upper-pilot signal path  56  may be vented, as previously discussed with reference to  FIG. 8 . 
   Throughout the downstroke, the primary air in-flow  138  may pass through the primary air intake  120 , into the upper chamber  126 , and through the upper primary air passage  62  to the upper interior portion  104 . The primary air out-flow  140  may flow from the lower interior portion  106 , through the lower primary air passage  64 , and out the vent  124  via the lower chamber  128 . The resulting pressure difference across the air-motor piston  48  may drive the piston rod  50  downward, as depicted by arrow  146 . 
     FIG. 7  illustrates the bottom of a downstroke. During the transition from a downstroke to an upstroke, the lower-pilot valve  40  may transition between the states depicted by  FIGS. 8 and 9 . Like the upper-pilot valve  38 , the lower-pilot valve  40  may magnetically sense the position of the air-motor piston  48  and assert pneumatic signal  142  through the lower-pilot signal path  60 . The pneumatic signal  142  may drive the main spool valve  118  from the second position back to the first position, thereby reversing the primary air flows  138  and  140  and initiating an upstroke. 
   The air-motor piston  48  may move upwards through the state depicted by  FIG. 4 , and the cycle illustrated by  FIGS. 4-7  may repeat indefinitely. At the end of each stroke, the pilot valves  38  and  40  may signal the main valve  52  to reverse the direction of primary air flows  138  and  140  with the pneumatic signal  142 . The resulting up and down oscillations of the piston rod  50  may be harnessed by the pump  14  to convey the coating liquid through the spray system  10  and out the spray gun  20 . The speed of the pneumatic motor  12  may be regulated, in part, by adjusting the pressure and/or flow rate through the primary air intake  120 , e.g., via the regulator assembly  26 . 
   Advantageously, in the present embodiment, the pilot valves  38  and  40  sense the position of the air-motor piston  48  without contacting other moving parts. Further, the spool valves  68  may slide within the sleeves  72  with very little friction. As a result, in some embodiments, very little energy may be wasted when sequencing the primary air flows  138  and  140 . Moreover, in certain embodiments, the pilot valves  38  and  40  may tend to have a long useful life due to the low friction and contactless actuation with no seals to wear. Less contact and friction may tend to reduce wear and fatigue. Additionally, in some embodiments, the pilot valves  38  and  40  may be actuated without biasing a resilient member, e.g., a reed or spring, which might otherwise fatigue and shorten the useful life of the pilot valve. Providing yet another advantage, some embodiments may operate even when relatively low pressure air is supplied to the primary air intake  120 . For instance, some embodiments may be capable of operating at pressures less than 25 psi, 15 psi, 5 psi, or 2 psi. 
   Further, in certain embodiments, the pilot valves  38  and  40  may be more reliable than conventional designs when exposed to dirty air. Air with particulates or vapors may form deposits on valve parts, and in certain types of valves, for instance, some reed valves, the deposits may prevent the valves from operating. 
   The presently discussed techniques are applicable to a wide variety of embodiments. For example, as mentioned above, the air-motor piston  48  may include a magnet  146  (see  FIG. 9 ) to increase the attractive force pulling on the magnet  66  in the pilot valves  38  and  40 . In such embodiments, the poles of the magnet  66  and the upper-pilot valve  38  may be oriented the same as the pole of the magnet  66  in the lower-pilot valve  40 . That is, if the north pole of the magnet  66  in the upper-pilot valve  38  is facing downwards, the south pole of the magnet  66  in the lower-pilot valve  40  may be facing upwards, and vice versa. Alternatively, or additionally, a high magnetic permeability material (e.g., a ferrous material) may be coupled to the spool valve  68  to draw the spool valve  68  towards the magnet  146  on the air-motor piston  48 . In some embodiments, the magnet  66  may be omitted, and an attraction between a high magnetic permeability material coupled to the spool valve  68  and the magnet  146  may actuate the spool valve  68 , which is not to suggest that other features discussed herein may not also be omitted. 
   In some embodiments, other types of pilot valves  38  and/or  40  may be employed. In one example, the pilot valves  38  and/or  40  may include seals, such as a lip seal to reduce machining costs. In another example, the dynamic seal may be formed between a rotating sealing member and a generally static cylinder, or vice versa. The rotating member may be coupled to a magnet  66  to apply a torque when the air motor piston  48  is proximate. In another embodiment, instead of, or in addition to, returning to the state illustrated by  FIG. 8  the pilot valves with air pressure, the pilot valves  38  and  40  may be biased away from the air motor piston  48  by static magnets or springs. 
     FIGS. 10-14  illustrate another pneumatic motor  148 . In the pneumatic motor  148 , a variety of the previously discussed features may be integrated into shared housings or components. For example, the pneumatic motor  148  may include a top integrated manifold  150  and a bottom integrated manifold  152 . The integrated manifold  150  and  152  may be integrally formed, e.g., machined and/or cast from a single piece of material, with the top head  46  and the bottom head  44 , respectively. As illustrated by the cross-sectional view of  FIG. 14 , the upper primary air passage  62  may be routed directly from the main valve  52  through the top integrated manifold  150 . The bottom integrated manifold  152  may be similarly configured with respect to the lower primary air passage  64 . Additionally, the upper-pilot signal path  56  and upper-pilot signal path  54  may be, at least in part, integrally formed with the top integrated manifold  150 , and the lower-pilot signal path  58  and lower-pilot signal path  60  may be integrally formed with the bottom integrated manifold  152 . As illustrated by  FIG. 11 , in some embodiments, the top integrated manifold  150  may be rotationally symmetric with the bottom integrated manifold  152  but not reflectively symmetric with the bottom integrated manifold  152 . That is, the manifolds  150  and  152  may be generally equally and oppositely askew. Additionally, in the illustrated embodiment, the pilot signal paths  54  and  58  are in fluid communication with the primary air intake  120  via a manifold  154  integrally formed with the main valve  52 . 
     FIGS. 15-17  illustrate a third embodiment of a pneumatic motor  156 . The illustrated pneumatic motor  156  includes mechanically-actuated pilot valves  158  and  160 , an exhaust silencer  162 , and a main valve  52  with a magnetic detent, which is formed by magnets  170  and  172  and a ferromagnetic spindle  164 . The magnets  170  and  172  may magnetically retain the spindle  164  at opposing ends of the sleeve  116  in which the spindle  164  slides until a burst of air pressure from the mechanically-actuated pilot valves  158  or  160  overcomes this magnetic detent. The mechanically-actuated pilot valves  158  and  160  may selectively apply air pressure to the top or bottom of the spindle  164  when the air-motor piston  48  mechanically contacts a valve member  174 . The main valve  52  may also include shock absorbing pads  166  and  168  configured to cushion the impact when the spindle  164  reaches the top or bottom of the sleeve  116 . The shock absorbing pads  166  and  168  may be made of polyurethane, rubber, or other appropriate materials. In the present embodiment, the shock absorbing pads in  166  and  168  are disposed between the magnets  170  and  172  and the spindle  164 . The thickness of the shock absorbing pads  166  and  168  may be selected with the strength of the magnets  170  in  172  in mind, so that the magnets  170  and  172  retain the spindle  164  until a pneumatic signal is received from the mechanically-actuated pilot valves  158  or  160 . 
   While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.