Patent Publication Number: US-2023149231-A1

Title: Hydraulic valve and system

Description:
This application claims the benefit of U.S. patent application Ser. No. 17/081,593 filed on Oct. 27, 2020, entitled HYDRAULIC VALVE AND SYSTEM, by inventors Chad C. Souke and Ross T. Lucas, which claims the benefit of U.S. provisional patent application Ser. No. 62/926,711 filed Oct. 28, 2019, entitled HYDRAULIC VALVE AND SYSTEM by inventors Chad C. Souke and Ross T. Lucas, which are hereby incorporated by reference in their entireties. 
    
    
     TECHNICAL FIELD AND BACKGROUND 
     The present disclosure relates to a hydraulic valve and control system that can be used, for example, in a patient handling apparatus, such as emergency cot, medical bed, stretcher, stair chair, or other apparatuses that support a patient where increased speed of a component, such as a hydraulic cylinder used to move the base of a patient handling apparatus, is desired. 
     For example, when a patient handling apparatus, such as an emergency cot, is to be loaded into an emergency vehicle, such as an ambulance, the patient handling apparatus is moved to the rear of the emergency vehicle where it is then at least partially inserted into the compartment so that it is initially supported on one end, for example, by its head end wheels resting on the compartment floor. Alternately, the cot may be moved onto a loading arm or arms, which extend from the emergency vehicle into the cot and fully support the cot, but do not interfere with the lifting mechanism. In any case, once the cot is supported (either by the head end wheels or a loading arm or loading arm(s)), the base can be raised to allow the cot to then be fully loaded in to the emergency vehicle. The faster the base can be raised, the faster the patient handling apparatus can be loaded into the vehicle, and the quicker the patient weight can be unloaded from a caregiver and transferred to the emergency vehicle, which significantly reduces the stress and strain on a caregiver. The increase speed also increases the speed at which the patient can be handled and delivered to the medical facility, typically an emergency room. Therefore, quick retraction of the base can be significant to the caregiver in all cases and even more significant to the patient in some cases. 
     Accordingly, there is a need to provide a patient handling apparatus with a hydraulic valve and control system that can quickly move one component relative to another component, such as an emergency cot&#39;s base relative to the cot&#39;s frame without inducing an unacceptable increase in pressure in the hydraulic cylinder that is doing the work. 
     SUMMARY 
     Accordingly, a hydraulic valve and control system is disclosed that can move one hydraulic component relative to another hydraulic component more quickly when needed. 
     In one embodiment, an apparatus includes a hydraulic circuit. The hydraulic circuit is configured to selectively open fluid communication between one portion of the hydraulic circuit and another portion of the hydraulic circuit based on the flow of the hydraulic fluid in the one portion. When the flow of hydraulic fluid exceeds a selected threshold in the one portion of the hydraulic circuit, the flow of fluid urges the opening of a hydraulic component of the hydraulic circuit to allow communication between the one portion and the other portion of the hydraulic circuit. 
     For example, the one portion of the hydraulic component comprises a pilot operated control valve. The pilot operated control valve has a first chamber with a first inlet, a second inlet, an outlet, and a pilot piston assembly mounted for movement in the first chamber. The pilot piston assembly includes a pilot piston with a piston side facing the first inlet and a pilot rod that extends from the first chamber into a second chamber, which is sealed from the first chamber. The second inlet is in fluid communication with the outlet of the first chamber so that fluid flows from the outlet of the first chamber during all fluid flow conditions. The second chamber includes an inlet, an outlet, and a valve poppet movably mounted in the second chamber between a closed position wherein the inlet of the second chamber is not in fluid communication with the outlet of the second chamber and one or more open positions wherein the inlet of the second chamber is in fluid communication with the outlet of the second chamber. When the fluid flow to the first inlet of the first chamber exceeds a preselected flow rate, back pressure at the inlet of the first chamber will move the pilot piston and cause the pilot rod to move the valve poppet from its closed position to one of its open positions to allow fluid flow from the inlet of the second chamber to the outlet of the second chamber. 
     In another embodiment, a pilot operated control valve includes a first chamber with a first inlet, a second inlet, an outlet, and a pilot piston assembly mounted for movement in the first chamber. The pilot piston assembly includes a pilot piston with a piston side facing the first inlet and a pilot rod that extends from the first chamber into a second chamber, which is sealed from the first chamber. The second inlet is in fluid communication with the outlet of the first chamber so that fluid flows from the outlet of the first chamber during all fluid flow conditions. The second chamber includes an inlet, an outlet, and a valve poppet movably mounted in said chamber between a closed position wherein the inlet of the second chamber is not in fluid communication with the outlet of the second chamber and one or more open positions wherein the inlet of the second chamber is in fluid communication with the outlet of the second chamber. When the fluid flow to the first inlet of the first chamber exceeds a preselected flow rate, back pressure at the inlet of the first chamber will move the pilot piston and cause the pilot rod to move the valve poppet from its closed position to one of its open positions to allow fluid flow from the inlet of the second chamber to the outlet of the second chamber. 
     For example, in one aspect, the pilot operated control valve includes a valve body, such as a cylindrical valve body, with the first and second chambers located in the valve body. 
     In a further aspect, the second inlet is formed in the valve body. For example, the second inlet may be formed by two or more orifices formed in the valve body wall. 
     In another embodiment, the second inlet is formed by a passageway through the pilot piston. 
     In yet another embodiment, an apparatus includes a hydraulic circuit and a hydraulic cylinder. The hydraulic cylinder has a rod, a cap end chamber, and a rod end chamber. The hydraulic circuit is operable to direct the flow of hydraulic fluid between a pump, the hydraulic cylinder, and a reservoir. Further, the hydraulic circuit is configured to selectively open fluid communication between one chamber of the hydraulic cylinder and the reservoir based on the flow condition of the hydraulic fluid flowing to the other chamber of the hydraulic cylinder to thereby allow faster evacuation of the hydraulic fluid from the one chamber of the hydraulic cylinder. 
     In one aspect, the hydraulic circuit is configured to selectively open fluid communication between the cap end chamber of the hydraulic cylinder and the reservoir to allow the hydraulic fluid to be quickly exhausted from the cap end chamber based on the flow condition of the hydraulic fluid flowing to the rod end chamber of the hydraulic cylinder. 
     According to yet another form of the disclosure, a patient handling apparatus includes a hydraulic circuit and a hydraulic cylinder to raise or lower a component of the patient handling apparatus. The hydraulic cylinder has a rod, a cap end chamber, and a rod end chamber. The hydraulic circuit is operable to direct the flow of hydraulic fluid between a pump, the hydraulic cylinder, and a reservoir. Further, the hydraulic circuit is configured to selectively open fluid communication between one chamber of the hydraulic cylinder and the reservoir based on the flow condition of the hydraulic fluid flowing to the other chamber of the hydraulic cylinder to thereby allow faster evacuation of the hydraulic fluid from the one chamber of the hydraulic cylinder. 
     In one aspect, the patient handling apparatus includes a frame, a base, and a lift assembly supporting the frame relative to the base. The hydraulic cylinder is configured to extend or retract the lift assembly to thereby raise or lower the base or the frame with respect to the other. 
     In yet another aspect, the hydraulic circuit includes a control valve to control the fluid communication between the cap end chamber and the reservoir, and the hydraulic circuit is configured to selectively open the control valve to allow fluid to evacuate at least some of the hydraulic fluid from the cap end chamber to the reservoir based on the flow condition of the hydraulic fluid flowing to the rod end chamber. For example, the hydraulic circuit is configured to selectively open the control valve when there is a high flow condition to the rod end chamber of the hydraulic cylinder to thereby allow faster evacuation of the hydraulic fluid from the cap end chamber of the hydraulic cylinder. 
     In other aspects, the control valve is a pilot operated control valve that includes a first chamber with a first inlet, a second inlet, an outlet, and a pilot piston assembly mounted for movement in the first chamber. The pilot piston assembly includes a pilot piston with a piston side facing the first inlet and a pilot rod that extends from the first chamber into a second chamber, which is sealed from the first chamber. The second inlet is in fluid communication with the outlet of the first chamber so that fluid flows from the outlet of the first chamber during all fluid flow conditions. The second chamber includes an inlet, an outlet, and a valve poppet movably mounted in the second chamber between a closed position wherein the inlet of the second chamber is not in fluid communication with the outlet of the second chamber and one or more open positions wherein the inlet of the second chamber is in fluid communication with the outlet of the second chamber. When the fluid flow to the first inlet of the first chamber exceeds a preselected flow rate, back pressure at the inlet of the first chamber will move the pilot piston and cause the pilot rod to move the valve poppet from its closed position to one of its open positions to allow fluid flow from the inlet of the second chamber to the outlet of the second chamber. 
     In another embodiment, a method of loading a patient handling apparatus from a cargo area of an emergency vehicle includes moving the patient handling apparatus adjacent an opening to the cargo area of an ambulance and supporting the litter frame of the patient handling apparatus in a manner such that the base is free to be raised relative to the litter frame (and hence deck). The method further includes directing hydraulic fluid at a high flow rate to the rod end of the lift assembly hydraulic cylinder and, based on that high flow rate, directing at least some of the hydraulic fluid from the cap end of the hydraulic cylinder to a reservoir, to thereby allow faster discharge or evacuation of the hydraulic fluid from the cap end chamber of the hydraulic cylinder. 
     Accordingly, the present disclosure provides a hydraulic valve and hydraulic circuit that can improve the control over the movement of a component of an apparatus, such as a patient handling apparatus, and further allows the component to be moved quickly while maintaining acceptable pressure in the hydraulic circuit. 
     These and other objects, advantages, purposes and features of the disclosure will become more apparent from the study of the following description taken in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a perspective view of a patient handling apparatus (with the patient support surface removed) with the lift assembly in its fully raised configuration; 
         FIG.  1 A  is an enlarged view of a foot-end upper pivot connection between the lift assembly and the frame; 
         FIG.  2    is a second perspective view of the patient handling apparatus of  FIG.  1   ; 
         FIG.  3    is a side elevation view of the patient handling apparatus in its fully lowered configuration; 
         FIG.  4    is a top plan view of the patient handling apparatus of  FIG.  3   ; 
         FIG.  5    is a bottom plan view of the patient handling apparatus of  FIG.  3   ; 
         FIG.  6    is a hydraulic circuit diagram of the hydraulic system and control system in one embodiment of the patient handling apparatus illustrating the flow of hydraulic fluid in the lifting or raising mode of the frame relative to the base of the patient handling apparatus when the base is supported on a ground surface or lowering the base in a low flow condition when the frame is supported; 
         FIG.  7    is the hydraulic circuit diagram of  FIG.  6    illustrating the flow of hydraulic fluid in the raising or retracting mode of the base of the patient handling apparatus when the frame is raised and supported by an emergency vehicle or when lowering the frame in a low flow condition when the base is supported; 
         FIG.  8    is the hydraulic circuit diagram of  FIG.  6    illustrating the flow of hydraulic fluid in the lowering mode of the base of the patient handling apparatus in a faster mode, for example, when the patient handling apparatus is in a compact configuration and the frame is supported by an emergency vehicle or lowering the frame in a faster mode when the base is supported; 
         FIG.  9    is a hydraulic circuit diagram of another hydraulic circuit that may be used in the hydraulic circuit shown in  FIG.  6    illustrating the flow of fluid during lowering of the frame relative to the base in a low flow condition when the base is supported on a ground surface or raising of the base when the frame is supported in a low flow condition; 
         FIG.  10    is similar view to  FIG.  9    illustrating the flow of fluid during a rapid raising or retracting of the base when the base is unsupported, for example, during loading of the patient handling apparatus or rapid lowering of the frame when the base is supported; 
         FIG.  11    is an exploded perspective view of a pilot operated control valve of the hydraulic circuit of  FIG.  9   ; 
         FIG.  12    is an exploded cross-sectional view of the pilot operated control valve of  FIG.  11   ; 
         FIG.  13    is a cross-sectional view of the pilot operated control valve; 
         FIG.  14    is a schematic cross-sectional view of the pilot operated control valve of  FIGS.  11 - 13    illustrating the flow through the pilot valve in a low flow condition; and 
         FIG.  15    is a schematic cross-sectional view of the pilot operated control valve of  FIGS.  11 - 13    illustrating the flow through the pilot valve in a high flow condition. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG.  1   , the numeral  10  generally designates a patient handling apparatus. The term “patient handling apparatus” is used broadly to mean an apparatus that can support a patient, such as a medical bed, including an apparatus that can transport a patient, such as an emergency cot, a stretcher, a stair chair, or other apparatuses that support and/or transport a patient. Further, the term “patient” is used broadly to include persons that are under medical treatment or an invalid, or persons who just need assistance. Although the patient handling apparatus  10  is illustrated herein as an emergency cot, the term “patient handling apparatus” should not be so limited. 
     Referring again to  FIGS.  1 - 3   , patient handling apparatus  10  includes a frame  12 , which in the illustrated embodiment comprises a litter frame that supports a litter deck (shown in phantom in  FIG.  3   ), which provides a patient support surface, and a base  18 . As will be more fully described below, patient handling apparatus  10  includes a lift assembly  20  that raises or lowers the base  18  or the frame  12  with respect to the other so that the patient handling apparatus  10  can be rearranged between a more compact configuration, for example, for loading into an emergency vehicle, such as an ambulance, and a configuration for use in transporting a patient across a ground surface. Further, as will be more fully described below, the mounting of lift assembly  20  to the frame  12  is optionally configured to allow the frame  12  to be tilted relative to the lift assembly  20  so that one end (e.g. head-end or foot-end) of the frame  12  can be raised beyond the fully raised height of the lift assembly to allow the patient handling apparatus to be inserted more easily into the compartment of an emergency vehicle. 
     Referring again to  FIG.  1   , frame  12  is mounted to base  18  by lift assembly  20 , which includes load bearing members  22  pivotally coupled to the frame  12  and to the base  18 . In the illustrated embodiment, load bearing members  22  are pivotally coupled to the frame  12  by head-end upper pivot connections  24   a  and foot-end upper pivot connections  24   b . Further, as will be more fully described below, head-end upper pivot connections  24   a  are fixed to the frame  12  along the longitudinal axis  12   b  of frame  12  and foot-end upper pivot connections  24   b  are movable so that the head-end of frame  12  can be tilted upwardly, as more fully described below. 
     In the illustrated embodiment, each load bearing member  22  comprises a telescoping compression/tension member  42 . Compression/tension members  42  may be pivotally joined at their medial portions about a pivot axis to thereby form a pair of X-frames  44  ( FIG.  2   ). The upper ends of each X-frame  44  are, therefore, pivotally mounted to the frame  12  by head-end upper pivot connections  24   a  and foot-end upper pivot connections  24   b . The lower ends of each X-frame  44  are pivotally mounted to the base  18  by head-end lower pivot connections  26   a  and foot-end lower pivot connections  26   b . However, it should be understood that load bearing members  22  may comprise fixed length members, for example such of the type shown in U.S. Pat. No. 6,701,545, which is commonly owned by Stryker Corp. of Kalamazoo, Mich. and incorporated herein by reference in its entirety. For another example of suitable lift assemblies reference is made to U.S. Pat. Nos. 7,398,571 and 9,486,373, which are commonly owned by Stryker Corp. of Kalamazoo, Mich. and incorporated herein by reference in their entireties. 
     In addition to load bearing members  22 , patient handling apparatus  10  includes a pair of linkage members  50  and  52  ( FIG.  1   ), which are pivotally mounted on one end to transverse frame members  18   b  of base  18  and on their other ends to brackets  54 ,  56  ( FIG.  1   ), which mount to the X-frames and also provide a mount for a linear actuator  30  ( FIG.  1   ), which extends or contracts the lift assembly to raise or lower frame  14  relative to the base  18  (or raise or lower base relative to the frame  12 ) described below. Brackets  54  and  56 , therefore, pivotally mount linkage members  50  and  52 , as well as actuator  30  (described below), to the X-frames  44  ( FIG.  2   ) so that members  50  and  52  provide a timing link function as well as a moment coupling function. It should be understood that multiple actuators may be used to raise or lower frame  12 . 
     As best seen in  FIG.  1   , base  18  is formed by longitudinal frame members  18   a  and transverse frame members  18   b , which are joined together to form a frame for base  18 . Mounted to the longitudinal frame members  18   a  are bearings  18   c , such as wheels or castors. Transverse frame members  18   b  provide a mount for the lower pivot connections  26   a ,  26   b  ( FIGS.  3  and  5   ) of load bearing members  22  and also for the rod end of the actuator  30 . As described above, the upper end of actuator  30  is mounted between the X-frames (formed by load bearing members  22 ) by a transverse member  30   a  ( FIG.  1 A ) that is mounted to brackets  54 ,  56 . 
     As noted above, lift assembly  20  is extended or contracted by actuator  30 . In the illustrated embodiment actuator  30  comprises a hydraulic cylinder  80 , which is controlled by a control system  82 . Although one actuator is illustrated, it should be understood that more than one actuator or cylinder may be used. As will be more fully described below, control system  82  includes a hydraulic circuit  90  and a controller  120 , which is in communication with hydraulic circuit  90  and a user interface  120   a  that allows an operator to select between the lifting, lowering, and raising functions described herein. For example, user interface controls  120   a  may have a touch screen with touch screen areas or may comprise a key pad with push buttons, such as directional buttons, or switches, such as key switches, that correspond to the lifting, lowering, raising, and retracting functions described herein to allow the user to select the mode of operation and generate input signals to controller  120 . As will be more fully described below, the controller  120  may also automatically control the mode of operation. 
     Referring again to  FIGS.  6 - 8   , cylinder  80  includes cylinder housing  84  with a reciprocal rod  86 . Mounted at one end of rod  86  is a piston  88 , which is located within the cylinder housing  84 . The distal end of the reciprocal rod  86  is extended from housing  84  and connected in a conventional manner to transverse member  18   b  of base  18 . And as described above, the other end or fixed end (or cap end) of cylinder  80  is mounted between brackets  54 ,  56 . 
     Cylinder  80  is extended or retracted by control system  82  to extend or contract lift assembly  20  and generally operates in four modes, namely (first mode) to raise the frame  12  when base  18  is supported on, for example, a ground surface ( FIG.  6   ), (second mode) to lower the frame  12  when base  18  is supported on, for example, a ground surface ( FIG.  7   ), (third mode) to lower or extend base  18  when apparatus  10  is in its loading (compact) configuration and when the frame  12  is supported, for example, by an attendant or a loading and unloading apparatus ( FIG.  8   ), or (fourth mode) to raise base  18  when the frame  12  is supported, for example, by an attendant or a loading and unloading apparatus ( FIG.  7   ) and when apparatus  10  is its transport (raised) configuration to reconfigure the apparatus into its loading (compact) configuration. As will be more fully described below, when lowering base  18  relative to frame  12  (when frame  12  is supported) control system  82  is configured to automatically lower or extend base  18  at a faster speed unless certain conditions exist. 
     Referring to  FIGS.  6 - 8   , hydraulic circuit  90  includes a pump  92 , which is in fluid communication with a fluid reservoir or reservoir R, to pump fluid from the reservoir R to the cylinder  80 . As best seen in  FIG.  6   , when a user selects the first mode of operation (e.g. via the user interface) to raise or lift the frame  12 , controller  120  powers motor  94 , which operates pump  92  to pump fluid from the reservoir R, through filters  92   b  and check valves  92   a , into the hydraulic circuit  90  to direct the flow of fluid to cylinder  80 . To avoid over pressurization, for example, when a heavy patient is supported on frame  12 , fluid may be discharged from the hydraulic circuit  90 . For example, when the pressure in the hydraulic circuit  90  exceeds a designated pressure (e.g. 3200 psi on the cap side of the hydraulic circuit, and 700 psi on the rod side of the hydraulic circuit) through pressure relief valves  90   a  and  90   b . It is to be understood that the pump  92 , cylinder  80 , and the various conduits carrying hydraulic fluid to the cylinder are typically always filled with hydraulic fluid. Pump  92  is driven by an electric motor  94  (both of which are optionally reversible), which motor is controlled by controller  120  to thereby control pump  92 . 
     Referring again to  FIG.  6   , when an operator wishes to raise frame  12  relative to base  18  (first mode), and base  18  is supported on a support surface, the operator, using interface controls  120   a  ( FIG.  6   ), generates input signals that are communicated to controller  120 . When operating in the first mode, the output of the pump  92  (in the direction indicated by the arrows in  FIG.  6   ), will supply hydraulic fluid through a hydraulic conduit  96  to the cap end chamber  84   a  of the cylinder housing  84 , which is on the piston side of rod  86 . Hydraulic circuit  96  includes a pilot operated check valve  98  that is opened when fluid flows to the cap end chamber  84   a  and closed when fluid to the cap end chamber  84   a  stops to retain the pressure in the cap end chamber  84   b  until it is opened by the pilot signal received from the other side of the hydraulic circuit (check valve  108 , described below) to allow the flow fluid from the cap end chamber  84   a  of cylinder  84  in the reverse direction when the rod is being retracted. 
     When fluid is directed to cap end chamber  84   a , the rod  86  will extend to raise the frame  12  relative to base  18  at a first speed. This mode of operation is used when base  18  is supported on a support surface, such as the ground, which can be detected by a controller  120  in various ways described below. It should be understood, that the first mode may also be used to lower or extend base  18  when the faster speed of the third mode described below is not appropriate or desired. 
     Referring to  FIG.  7   , when an operator user wishes to select the second mode or the fourth mode—that is lower the frame  12  relative to base  18  (when base  18  is supported on a support surface) or raise base  18  relative to frame  12  (when frame  12  is supported), using interface controls  120   a , the operator will generate an input signal to controller  120  that will cause controller  120  to operate in the second mode or the fourth mode. In the second mode or the fourth mode, the direction of pump  92  is reversed, so that fluid will flow in an opposite direction (see arrows in  FIG.  7   ) to cylinder  80  through a second hydraulic conduit  100 , which is in fluid communication and connected to the rod end chamber  84   b  of the cylinder housing  84 . Conduit  100  includes a check valve assembly  102 , with an orifice or fluid throttle  104  and a poppet or check valve  106  in parallel, to control the flow of fluid through conduit  100 . Fluid flow in this direction will cause the rod  86  to retract and raise the base  12  when the frame  12  is supported or lower the frame  12  relative to base  18  when the base  18  is supported. 
     Also provided is a second pilot operated check valve  108  connected between the valve assembly  102  and pump  92 . Optionally, valves  98  and  108  are provided as a dual pilot operated check valve assembly  110 , which includes both valves ( 98  and  108 ) and allows fluid flow through each respect conduit in either direction. The valves  98  and  108  of the dual pilot check valve assembly  110  are operated by the fluid pressure of the respective branch of fluid conduit ( 96  or  100 ) as well as the fluid pressure of the opposing branch of fluid conduit ( 96  or  100 ), as schematically shown by the dotted lines in  FIGS.  6 - 8   . 
     Referring to  FIG.  8   , when an operator selects the base  18  lowering function and the litter is supported (and the base is unsupported), controller  120  will automatically increase the speed of the cylinder  80  over the first speed (the third mode). As would be understood by those skilled in the art, the speed of the cylinder or cylinders may be increased by increasing the flow of hydraulic fluid and/or pressure of the hydraulic fluid flowing to the cylinder(s)) unless certain conditions exist. Optionally, user interface  120   a  may allow an operator to generate an input signal to select the third mode and/or to disable the third mode. 
     In order to speed up the extension of rod  86  when operating in the third mode, hydraulic circuit  90  includes a third hydraulic conduit  112 , which is in fluid communication with conduits  96  and  100  via a check valve  114 , to thereby allow fluid communication between the cap end chamber  84   a  and the rod end chamber  84   b  and to allow at least a portion of the fluid output from the rod end chamber  84   b  to be redirected to the cap end chamber  84   a , which increases the speed of the rod  86  (i.e. by increasing the pressure and/or fluid flow of the fluid delivered to the end cap chamber  84   a ). 
     To control (e.g. open and close) fluid communication between the cap end chamber  84   a  and rod end chamber  84   b  via conduit  112 , conduit  112  includes a valve  116 , such as a solenoid valve or a proportional control valve, which is normally closed but selectively controlled (e.g. opened) to open fluid communication between the rod end chamber  84   b  and the cap end chamber  84   a  as described below. As noted, this will allow at least a portion of the fluid output from the rod end chamber  84   b  to be redirected to the end cap chamber  84   a  to thereby increase the speed of rod  86 . Optionally, an additional valve, (not shown) such as a solenoid valve, may be included in conduit  100 , for example, between conduit  112  and pump  92 , which is normally open but can be selectively controlled (e.g. closed), so that the amount of fluid (and hence fluid pressure and/or fluid flow) that is redirected from the rod end chamber  84   b  may be varied. For example, all the fluid output from rod end chamber  84   b  may be redirected to the cap end chamber  84   a . In another embodiment, an additional electrically operated proportional control valve may be used in any of the branches of the conduit (e.g.  96 ,  100 , or  112 ) to control the rate of fluid flow through the respective conduits and thereby control and vary the speed of the extension of rod  86 . 
     As noted above, control system  82  includes controller  120 , which is also schematically represented in  FIG.  6   . Controller  120  may be powered by the battery (not shown) on board the patient handling apparatus  10 . A hydraulic fluid pressure monitoring device (not shown) may be connected to the hydraulic circuit  90  to provide a signal to controller  120  indicative of the magnitude of the fluid pressure, which may be used as input when controlling the hydraulic cylinder  80 . 
     Referring again to  FIG.  6   , controller  120  may be in communication with one or more sensors, which generate input signals to controller  120  (or controller  120  may detect the state of the sensor) to allow controller  120  to adjust the hydraulic circuit based on an input signal or signals from or the status of the sensors, described more fully below. Suitable sensors may include Hall Effect sensors, proximity sensors, reed switches, optical sensors, ultrasonic sensors, liquid level sensors (such as available from MTS under the brand name TEMPOSONIC) linear variable displacement transformer (LV DT) sensors, or other transducers or the like. 
     For example, controller  120  may control (e.g. open or close) the valve  116  to increase or stop the increased speed of cylinder  80  and/or slow or stop the pump to slow or stop the cylinder, or any combination thereof based on an input signal or signals from or the status of the sensor(s). Further, controller  120  may control (e.g. close) the valve  116  before, after, or at the same time as slowing or stopping the pump based on an input signal or signals from or the status of the sensor(s). Alternately, controller  120  may slow, increase the speed of, or stop the pump P in lieu of controlling (e.g. closing) the valve  116  based on an input signal or signals from or the status of the sensor(s). For example, when there is no weight is sensed on the base, the motor may be configured to drive the pump at a higher speed (e.g. by increasing the motor pulse width modulation (PWM)) to generate higher fluid flow and this pressure. 
     As described in copending application, entitled, HYDRAULIC CIRCUIT FOR A PATIENT HANDLING APPARATUS, attorney docket no. 143667.185016 (P-619), filed on even date herewith, in one embodiment, control system  82  may include one or more sensors to detect when the base  18  of the patient handling apparatus  10  is contacting the ground or other surface, such as a bumper or another obstruction, which, as noted, may be used as an input signal or signals to the controller  120  to control the hydraulic circuit  90 . Suitable sensors may include Hall Effect sensors, proximity sensors, reed switches, optical sensors, ultrasonic sensors, liquid level sensors (such as available from MTS under the brand name TEMPOSONIC), linear variable displacement transformer (LVDT) sensors, or other transducers or the like. For further details reference is made to the copending application, which is incorporated by reference herein in its entirety. 
     As described in the referenced application, controller  120  may control (e.g. open or close) the valve  116  to increase or stop the increased speed of cylinder  80  and/or slow or stop the pump to slow or stop the cylinder, or any combination thereof based on an input signal or signals from or the status of the sensor(s). Further, controller  120  may control (e.g. close) the valve  116  before, after, or at the same time as slowing or stopping the pump based on an input signal or signals from or the status of the sensor(s). Alternately, controller  120  may slow, increase the speed of, or stop the pump P in lieu of control (e.g. close) the valve  116  based on an input signal or signals from or the status of the sensor(s). For example, when there is no weight is sensed on the base, the motor may be configured to drive the pump at a higher speed (e.g. by increasing the motor pulse width modulation (PWM)) to generate higher fluid flow and this pressure. 
     Further, in addition, or alternately, control system  82  may include one or more sensors  124  ( FIG.  6   ) that detect the height of the patient handling apparatus  10 . Similarly, suitable sensors may include Hall Effect sensors, proximity sensors, reed switches, optical sensors, ultrasonic sensors, liquid level sensors (such as available from MTS under the brand name TEMPOSONIC), linear variable displacement transformer (LVDT) sensors, or the like. 
     For example, in one embodiment, referring to  FIG.  1 A , an array of transducers T may be attached to the frame  12 , and a magnet M mounted, for example, to the foot-end upper pivot connections  24   b , including for example, to transverse member  60  forming or supporting the foot-end upper pivot connections  24   b  (e.g.  FIGS.  2  and  4   ). The array of transducers T may be mounted to frame  12  adjacent to or incorporated in guide  32  along path P, as partially shown ( FIG.  2   ). In this manner, as the foot-end upper pivot connections  24   b  move along path P ( FIG.  2   ) magnet M ( FIG.  1 A ) will also move along the array of transducers ( FIG.  1 A ), and the magnetic field of the magnet will be detected by one or more of transducers T to create an input signal or signals to the controller  120  that is indicative of the height position of the patient handling apparatus  10 . For additional details reference is made to U.S. patent application Ser. No. 15/949,648, entitled. PATIENT HANDLING APPARATUS WITH HYDRAULIC CONTROL SYSTEM filed on Apr. 10, 2018 (P-567A), which is incorporated by reference in its entirety herein. For examples of other suitable sensors that may be used, reference is made to U.S. patent application Ser. No. 16/271,117, which is entitled TECHNIQUES FOR DETERMINING A POSE OF A PATIENT SUPPORT TRANSPORT APPARATUS, filed Feb. 8, 2019, Attorney Docket 060252.00426 (P-1095), which is incorporated by reference herein in its entirety. 
     In yet another embodiment, control system  82  may include one or more sensors  126  ( FIG.  6   ) that detect the configuration of the ambulance patient handling apparatus  10 . For example, similar to sensor  124  noted above, transducers (see above for list of suitable transducers or sensors) may be placed at different locations about the patient handling apparatus  10  that detect magnets also placed at different locations about the patient handling apparatus  10 . In this manner, when a magnet is aligned with the transducer (or one of the transducers), the magnet field will be detected by that transducer, which transducer then generates a signal or signals that indicate that the patient handling apparatus  10  is in a defined configuration or height (associated with the location of that transducer) of the patient handling apparatus  10 . The number of configurations may be varied—for example, a single sensor may be provided to detect a single configuration (e.g. fully raised configuration or a fully lowered configuration) or multiple sensors may be used to detect multiple configurations, with each transducer detecting a specific configuration. Again, the sensors can create an appropriate input signal to the controller  120  that is indicative of the configuration of the patient handling apparatus  10 . For example of a suitable control system that senses a safe transport height, reference is made to copending U.S. patent application Ser. No. 16/271,114, which is entitled PATIENT TRANSPORT APPARATUS WITH DEFINED TRANSPORT HEIGHT, filed on Feb. 9, 2019, Attorney Docket 060252.00425 (P-1067), which is incorporated by reference herein in its entirety. 
     Further, when multiple configurations are detected, controller  120  may compare the detected configuration of patient handling apparatus  10  to a prescribed configuration and, in response, control the hydraulic circuit  90  based on whether the patient handling apparatus  10  is in or near a prescribed configuration or not. Or when only a single configuration is detected, controller  120  may simple use the signal from the sensor as an input signal and control hydraulic circuit  90  based on the input signal. 
     When the patient handling apparatus  10  is no longer in the prescribed configuration (e.g. by comparing the detected configuration to a prescribed configuration stored in memory or detecting that it is not in a prescribed configuration), controller  120  may be configured to open or reopen the valve  116  to allow cylinder  80  to operate at its increased speed but then close valve  116  when controller  120  detects that patient handling apparatus  10  is in a prescribed configuration and/or, further, may slow or stop the motor to stop the pump or reverse the motor. 
     For example, one of the prescribed configurations may be when the lift assembly is in its transport or fully raised configuration. In this manner, similar to the previous embodiment, when controller  120  detects that patient handling apparatus  10  is near or in its fully raised configuration, controller  120  may be configured to close valve  116  so that cylinder  80  can no longer be driven at the increased speed, and further may also stop motor  94  to stop pump  92 . As noted above, controller  120  may open or close the valve  116  before, after, or at the same time as stopping the pump (or reversing the motor) based on the input signal or signals from or the status of the sensor(s). Alternately, controller  120  may stop the pump  92  in lieu of closing the valve  116  based on an input signal or signals from or the status of the sensor(s). 
     In yet another embodiment, the control system  82  may include a sensor  128  ( FIG.  6   ), which is in communication with controller  120 , to detect when a load on the motor (or on the pump) occurs. For example, sensor  128  may detect current drawn by the motor. In this manner, using sensor  128 , controller  12  can detect when the base is supported on a surface, such as the ground or the deck of the emergency vehicle, by detecting when the motor or pump encounter increased resistance, for example, by detecting the current in the motor. As would be understood, this increase resistance would occur when the base  18  is either supported or encounters an obstruction. Further, controller  120  may be configured to detect when the load has exceeded a prescribed value (e.g. by comparing the detected load to a store load value in memory), and optionally close valve  116  to no longer allow fluid communication between the rod end chamber  84   b  and the cap end chamber  84   a  via conduit  112  when the load has exceeded the prescribed value. As noted above, controller  120  may open or close the valve  116  before the load reaches the prescribed value and further before, after, or at the same time as slowing or stopping the pump based on an input signal or signals from or the status of the sensor(s). As noted above, controller may also reverse the motor before, after or at the same time it closes valve  116 . Alternately, controller  120  may slow or stop the pump  92  in lieu of closing the valve  116  based on an input signal or signals from or the status of the sensor(s). 
     So for example, if an attendant is removing patient handling apparatus from an emergency vehicle and has selected the base lowering function, and while the base is being lowered at the increased speed, controller  120  detects that the motor or pump is under an increase in load (e.g. detects an increase in current) (which, as noted, would occur when the base  18  is supported, either by a support surface or an obstruction) controller  120  may close valve  116  so that cylinder  80  will no longer be driven at the increased speed. Optionally, controller  120  may also or instead slow or stop the pump and/or stop the pump before closing the valve. Alternately, controller  120  may simultaneously close the valve  116  and slow or stop the pump. As described above, in yet another embodiment, controller  120  may close the valve  116  prior to base  18  being supported (for example, when the frame  12  or base  18  reaches a prescribed height or when apparatus  10  has a prescribed configuration) and only after controller  120  detects that base  18  has contacted the ground surface and/or the base  18  is fully lowered, controller  120  will stop pump  92  so that cylinder  80  will no longer extend. Or the controller  120  may be configured to stop the pump  92  before the base reaches the ground to avoid overshoot. 
     The controller  120  may also receive signals indicative of the presence of the patient handling apparatus  10  near an emergency vehicle. For example, a transducer may be mounted to the patient handling apparatus  10 , and a magnet may be mounted to the emergency vehicle and located so that when the patient handling apparatus is near the emergency vehicle, the transducer will detect the magnet and generate a signal based on its detection. In this manner, when an operator has selected the base extending (e.g. lowering) function and controller  120  detects that patient handling apparatus  10  is near an emergency vehicle and, further, detects one or more of the other conditions above (e.g. that the base is not contacting a support surface or there is no load on the motor or pump or the patient handling apparatus  10  is not in a prescribed configuration), controller  120  may open valve  116  to allow the cylinder to be driven at the increased speed. In this manner, these additional input signals may confirm that the situation is consistent with a third mode operation. 
     Alternately, controller  120  may also receive signals indicative of the presence of the patient handling apparatus  10  in an emergency vehicle. For example, a transducer may be mounted to the patient handling apparatus  10 , and a magnet may be mounted to the emergency vehicle and located so that when the patient handling apparatus is in the emergency vehicle, the transducer will detect the magnet and generate a signal based on its detection. In this manner, when an operator has selected the base lowering function and controller  12  detects that patient handling apparatus  10  is in the emergency vehicle and detects one or more of the other conditions above (e.g. that the base is not contacting a support surface or there is no load on the motor or pump or the patient handling apparatus  10  is not in a prescribed configuration), the signal indicating that patient handling apparatus  10  is in the emergency vehicle will override the detection of the other conditions and the controller  120  may maintain valve  116  closed to prevent the cylinder from being driven at the increased speed and, further, override the input signal generated by the operator. For further details of sensing the proximity to or location in an emergency vehicle, reference is made to U.S. patent application Ser. No. 14/998,028, entitled PATIENT SUPPORT, filed on Jul. 7, 2014 (P-433A), which is incorporated by reference in its entirety herein. 
     In yet another embodiment, the patient handling apparatus  10  may include a patient handling apparatus-based communication system  130  ( FIG.  6   ) for communicating with a loading and unloading based communication system  132  ( FIG.  6   ) on a loading and unloading apparatus. For example, the communication systems  130 ,  132  may be wireless, such as RF communication systems (including near-field communication systems). For example, the control system  82  may be operable to open or close the valve  116  based on a signal received from the loading and unloading based communication system  132 . In this manner, the deployment of the base of the patient handling apparatus  10  may be controlled by someone at the loading and unloading apparatus or someone controlling the loading and unloading apparatus. 
     In one embodiment, rather than allowing controller  120  to start in the third mode (when all the conditions are satisfied), controller  120  may be configured initially start the base lowering function in the first mode, where the base is lowered at the slower, first speed. Only after controller  120  has checked that there is a change in the load (e.g. by checking a sensor, for example a load cell or current sensing sensor) on the motor or cot to confirm that the motor or pump are now under a load (which would occur once the apparatus is pulled from the emergency vehicle and the base is being lowered), does controller  120  then switch to the third mode to operate the cylinder at the faster, second speed. Again, once operating in the third mode, should controller  120  detect one or more of the conditions noted above (base  18  is supported or encounters an obstruction, the height exceeds a prescribed height, the configuration is in a prescribed configuration, the load on the motor or pump exceeds a prescribed value) controller  120  will close valve  116  and optionally further slow or stop pump. As noted above, the valve  116  may be closed by controller  120  after the pump  92  is slowed or stopped or simultaneously. 
     In any of the above embodiments, it should be understood that control system  82  can control hydraulic circuit  90  to slow or stop the extension of rod  86  of cylinder, using any of the methods described above, before the conditions noted above, such as before reaching a predetermined height, before reaching a predetermined configuration, before making contact with the ground or an obstruction, or before reaching a prescribed load on the motor etc. Further, control of the fluid through the hydraulic circuit may be achieved by controlling the flow rate or opening or closing the flow using the various valves noted above that are shown and/or described. Further, as noted to avoid excess pressure in the hydraulic circuit, controller  120  may reverse the motor when controlling the valves described herein or may slow or stop the motor and pump before reaching the target (e.g. maximum height). Additionally, also as noted, controller  120  may control the hydraulic circuit by (1) adjusting the flow control valves or valves (e.g. valve  116 ), (2) adjusting the pump  92  (slow down or stop) or 3) adjusting both the flow control valves or valves (e.g. valve  116 ) and the pump, in any sequence. 
     Further, it should be understood, in each instance above, where it is described that the controller or sensor or other components are in communication, it should be understand that the communication may be achieved through hard wiring or via wireless communication. Further, although illustrated as discrete separate components, the various components may be assembled or integrated together into a single unit or multiple units. 
     Optionally, described more fully below, hydraulic circuit  90  may instead, or in addition, incorporate a hydraulic based logic component that is configured to generate a pilot signal to control another hydraulic component, such as a valve, based on flow through the hydraulic based logic component. For example, as will be described, when the flow of fluid through the hydraulic based logic component reaches a threshold value, the hydraulic based logic component will generate a pilot signal to open a pilot operated valve to divert the flow of fluid away from another hydraulic component to avoid over pressurizing or simply change the logic of the hydraulic circuit. 
     As noted above, the frame  12  is optionally configured to allow the frame  12  to be tilted relative to the lift assembly  20  so that one end (e.g. head-end or foot-end) of the frame  12  can be raised beyond the fully raised height of the lift assembly to allow the patient handling apparatus to be inserted more easily into the compartment of an emergency vehicle. In addition, the frame  12  can be tilted without decoupling the frame  12  from the lift assembly  20 . 
     In the illustrated embodiment, movable foot-end upper pivot connections  24   b  are configured so that they can move in a direction angled (e.g. oblique (acute or obtuse) or even perpendicular) relative to the longitudinal axis  12   b  of the frame  12  and optionally along or relative to the longitudinal axis  12   b  ( FIG.  1   ) of the frame  12 . In this manner, the movable foot-end upper pivot connections  24   b  follow a non-linear path P that takes them toward or away from the longitudinal axis  12   b  of the frame  12  over at least a portion of the range of motion of the movable foot-end upper pivot connections  24   b  to cause the frame  12  to tilt relative to the lift assembly  20  (as opposed to being tilted by the lift assembly). 
     Referring to  FIGS.  1  and  2   , this range of motion where the frame  12  tilts may be at one end of the range of motion of the foot-end upper pivot connections  24   b  and, for example, where lift assembly  20  is raised to its maximum height or may be intermediate the ends of path P. Further, after lift assembly  20  has raised frame  12  to its maximum raised height (see  FIG.  2   ), frame  12  may be tilted further to raise the head-end of the frame  12  so that head-end wheel  12   a  can be raised sufficiently to rest on the litter frame of an emergence vehicle compartment. 
     Referring again to  FIG.  1   , movable foot-end upper pivot connections  24   b  are mounted to frame  12  by guides  32 . Guides  32  form a non-linear guide path P ( FIGS.  1 - 5   ) (“non-linear path” means a path that does not form a straight line) for the movable foot-end upper pivot connections  24   b . While guide path P is non-linear, path P may include one or more linear sections and one or more non-linear sections, such as arcuate sections. In the illustrated embodiment, guides  32  provide a non-linear guide path P with one linear section that corresponds to the lowered height ( FIG.  3   ) of the lift assembly  20  where movable foot-end upper pivot connections  24   b  are at their lowest height and lift assembly  20  is in its folded, most compact configuration. The path P of each guide  32  also includes an arcuate section, which is the adjacent linear section and may have a single radius of curvature or two or more radii of curvatures. Further, the arcuate section may have two portions, with a first portion corresponding to the fully raised height of lift assembly  20  and a second portion corresponding to the fully raised height of lift assembly  20 , but with the frame  12  tilted further ( FIG.  2   ). 
     Thus, when lift assembly  20  starts in its lowermost position and is extended, movable foot-end upper pivot connections  24   b  move along guide path P from one end (which corresponds to the lowermost position of lift assembly  20 ) where the movement of movable foot-end upper pivot connections  24   b  is generally linear (and parallel to longitudinal axis  12   b  of frame  12 ) to a non-linear portion of path P, which corresponds to a raised position of lift assembly. As lift assembly  20  continues to extend and raise frame  12  further, movable foot-end upper pivot connections  24   b  continue to move along non-linear path P ( FIG.  2   ) and initially move further away from longitudinal axis  12   b  (while still moving relative or along longitudinal axis  12   b ). During this movement, frame  12  remains substantially horizontal. As lift assembly  20  continues to extend to its fully raised position, movable foot-end upper pivot connections  24   b  continue to move along the non-linear portion of path P and, further, continue to move away from longitudinal axis  12   b . This movement is then followed by movable foot-end upper pivot connections  24   b  moving toward longitudinal axis  12   b  where frame  12  tilts upwardly ( FIG.  1   ). It should be understood that the positions of load bearing members  22  and movable foot-end upper pivot connections  24   b  are controlled and “locked” in their positions by the hydraulic cylinder. In order to further tilt frame  12  upwardly from its position shown in  FIG.  1    to its position shown in  FIG.  2   , a downward force is applied to the foot-end of the litter, which causes movable foot-end upper pivot connections  24   b  to move toward the end of path P and move further towards longitudinal axis  12   b , which causes frame  12  to further tilt upwardly. Because the position of foot-end upper pivot connections  24   b  is essentially locked in its position shown in  FIG.  1   , only an external force will cause upper pivot connections  24   b  to move to the end of path P as shown in  FIG.  2   . As noted this external force may simply be manually applied by an attendant (e.g. an EMS person) at the foot-end of the litter- or it may be applied by an actuator. 
     As best seen in  FIG.  6   , foot-end upper pivot connections  24   b  are supported on or formed by a transverse member  60 , which is mounted to the upper ends of telescoping members  42  by a rigid connection. In the illustrated embodiment, foot-end upper pivot connections  24   b  are formed by the ends of transverse member  60 . For example, transverse member  60  may comprise a tubular member or solid bar with a circular cross-section. To accommodate the rotation of each telescoping member  42  (as lift assembly is extended or retracted) and allow each telescoping member  42  at the foot-end to pivot and translate along guide path P, foot-end upper pivot connections  24   b  optionally each include a roller. The rollers are mounted about the respective ends of transverse member  60  and guided along guide paths P of guides  32 . For example, the rollers may each comprise a low friction collar, such as a high density polyethylene collar, or a bearing assembly, which is free to rotate about the end of tubular member and further, as noted, roll along guide path P. Alternately, foot-end upper pivot connections  24   b  may be configured to slide along path P. 
     In the illustrated embodiment, guides  32  are each formed from a low friction member or plate, such as a high density polyethylene plate, mounted to frame  12 . Each low friction member or plate  72  includes a recess formed therein, which forms guide path P. Alternately, guide  32  may be formed from a metal member or plate with the recess formed therein lined with a low friction material, such as high density polyethylene. 
     In this manner, pivot connections  26   b  allows telescoping members  42  to pivot about a moving horizontal axis (i.e. moving horizontal axis of transverse member  60 ) (moving both in the longitudinal direction and/or vertical direction, as noted above, namely along longitudinal axis  12   a  or toward or away from longitudinal axis  12   a ) and, further, allow lift assembly  20  to adjust the height of frame  12  relative to base  18 . 
     In addition, referring again to  FIG.  2   , frame  12  includes a pair of side frame members  14   a  and  14   b , which are interconnected by cross or transverse frame members  36   a  (only one shown). Cross-frame member  36   a  provides a mounting point for the head-end load bearing members  22  of lift assembly  20 . In addition, side frame members  14   a  and  14   b  may provide a mounting surface for collapsible side rails (not shown). 
     For further details of frame  12 , telescoping members  44 , base  18 , brackets  54  and  56 , linkage members  50  and  52 , and a gatch mechanism, and other components not specifically mentioned or described herein, or for alternate embodiments of components described herein, reference is made to U.S. Pat. Nos. 5,537,700 and 7,398,571, and published Application No. WO 2007/123571, commonly owned by Stryker Corporation, which are herein incorporated by reference in their entireties. 
     Thus, when the ambulance patient handling apparatus is in the fully collapsed position or loading configuration, and referring to  FIG.  4   , an extension of the linear actuator  30  will cause a clockwise ( FIG.  4   ) rotation of the brackets  54 ,  56  about the axis of fasteners  55 . Fasteners  55  secure the upper end of linkage members  50 ,  52  to X-frames  44 . As a result of this geometry, the force in the direction of the extension of linear actuator  30  effects a rapid lifting of the frame  12  to the transport configuration or the full height position of the lift assembly illustrated in  FIGS.  1  and  2   . 
     For further optional details on how lift assembly  20  is mounted to frame  12 , reference is made to copending U.S. application Ser. No. 15/949,624, entitled EMERGENCY COT WITH A LITTER HEIGHT ADJUSTMENT MECHANISM, attorney docket no. 143667.173860 (P-566A) and filed on Apr. 10, 2018, which is incorporated herein by reference in its entirety. For other examples of suitable lift assemblies, including their mounting arrangements, reference is made to U.S. Pat. Nos. 7,398,571 and 9,486,373, which are commonly owned by Stryker Corp. of Kalamazoo, Mich. and incorporated herein by reference in their entireties. 
     Optionally, as noted above, hydraulic circuit  90  may incorporate a hydraulic based logic component that is configured to generate a pilot signal to control another hydraulic component, such as a valve, based on flow through the hydraulic based logic component. In one embodiment, a single hydraulic based logic component, in the form of a flow based pilot valve ( 194 , described below), is employed, but in other embodiments, two or more hydraulic based logic components may be used in series or parallel or a combination of both to achieve the desired logic. 
     In the illustrated embodiment, hydraulic circuit  90  includes hydraulic circuit  190  that is configured to assist in reducing the pressure on the cap side of the hydraulic cylinder when the hydraulic cylinder is being retracted rapidly, especially when the hydraulic cylinder is not loaded, e.g. when the base is not supporting the patient handling apparatus on a floor or ground surface. 
     Referring to  FIGS.  9 - 10   , hydraulic circuit  190  is configured to evacuate the hydraulic fluid from and thereby reduce the pressure on, for example, the cap side of the hydraulic cylinder  80  during a rapid retract of the hydraulic cylinder  80 , especially when the hydraulic cylinder  80  is not loaded. Or stated another way, hydraulic circuit  190  may be used for redirecting some of the fluid discharged from the cap end of the hydraulic cylinder at a faster rate (to speed up the discharge) to allow the retraction speed of the rod to be increased. Further, hydraulic circuit  90  may incorporate hydraulic circuit  190  without the use of the redirect hydraulic circuit (e.g. conduit  112 , check valve  114 , solenoid valve  116 ) that redirects some of the output from the rod end chamber  84   b  to the end cap chamber  84   a.    
     Referring again to  FIGS.  9  and  10   , hydraulic circuit  190  may be located between the check valves  98 ,  108  and pump  92  and includes a pilot operated flow control valve assembly  192  (note in this figure some components of circuit  90  have been omitted, for example, the redirect hydraulic circuit) with a flow based pilot valve  194  to selectively open a pilot operated valve  190   a , which is normally closed to the cap side of the cylinder and the reservoir but when open allows fluid from the cap side of the cylinder  84  to be in fluid communication with the reservoir. Flow control valve assembly  192  is always open and in fluid communication with pilot operated check valve  108 . When the flow rate of hydraulic fluid flowing through valve  194  (fluid flowing to the rod end chamber  84   b  of hydraulic cylinder  84 ) reaches a threshold value, pilot valve  194  is configured to open pilot operated valve  190   a  to allow at least some of the fluid evacuated or discharged from cap end chamber  84   a  to flow to reservoir R. 
     For example, hydraulic cylinder  80  typically has a  2 : 1  rod differential, so that the flow out of the cap end chamber is about twice that of the rate of fluid flow into the rod end chamber. Therefore, when you want to retract the rod at a rapid rate, the rate of fluid flow out of the cap end, which is double the rod end rate, can create some back pressure on the cylinder and/or increased pressure on the pump, which can limit the speed of the rod. Therefore, by redirecting some of the fluid to reservoir R, the pressure on the cylinder and/or the pump can be reduced and, hence, the speed of the rod can be increased without the stress normally associated with such a rapid retract. Thus, instead of being cycled through the pump, the fluid may be able to go straight to the reservoir, which provides a faster path (path of lower resistance) to achieve the circuit completion. In the illustrated embodiment, the switching is “hydraulic switching” and may be all internal to the hydraulics using the flow in the hydraulic circuit and through the hydraulic based logic component, such as the flow based pilot valve described herein. Optionally, this hydraulic switching may be combined with control system logic based on input from a sensor, such as a weight sensor or position sensor (either internal or external), such as described in copending application entitled HYDRAULIC CIRCUIT FOR A PATIENT HANDLING APPARATUS, attorney docket no. 143667.185016 (P-619), filed on even date herewith by Stryker Corp., which is incorporated by reference herein in its entirety. 
     Referring again to  FIGS.  11 - 15   , pilot operated control valve assembly  192  includes a valve body  196 , such as a cylindrical body, which forms or includes a first chamber  198  and a second chamber  200  ( FIGS.  12 - 15   ). First chamber  198  forms the flow based pilot valve  194 . Second chamber  200  forms pilot operated valve  190   a.    
     First chamber  198  includes a first inlet  202 , a second inlet  204 , an outlet  206 , and a pilot piston assembly  208  mounted for movement in the first chamber  198  between a closed position (e.g.  FIGS.  13  and  14   ) and an open position ( FIG.  15   —shown in dashed lines). First inlet  202  of first chamber  198  is in fluid communication with the conduit  96   b  that directs hydraulic fluid to or from the pump  92  (on the rod side of the cylinder). Second inlet  204  is also in fluid communication with the conduit  96   b  that directs hydraulic fluid to or from the pump  92 . Outlet  206  is in fluid communication with the conduit that directs the flow of hydraulic fluid to or from the rod end chamber  84   b  of hydraulic cylinder  80  via pilot operate check valve  108 . 
     The pilot piston assembly  208  includes a pilot piston  210  with a piston side  210   a  ( FIG.  13   ) facing the first inlet  202  and a pilot rod  212  that extends from the first chamber  198  into second chamber  200  and is sealed against the inner wall of first chamber  198  by an annular seal  210   b . Pilot rod  212  extends into second chamber  200  through a passageway  214   a  formed in a wall  214 , which may be formed by a shoulder in valve body  196 , that separates the first chamber  198  from the second chamber  200 . Located in passageway  214   a  is an annular seal  214   b , which seals against rod  212  to seal the second chamber  200  from the first chamber  198 . Piston assembly  208  is biased to its closed position (such as shown in  FIGS.  13  and  14   ) by a spring  216 , such as a coil spring, which extends between wall  214  and piston  210 . 
     Second inlet  204  is in fluid communication with the first chamber  198  and outlet  206  of the first chamber  198  regardless of the position of the piston assembly  208  so that fluid flows from the outlet  206  of the first chamber  198  during all fluid flow conditions-during low flow conditions and high flow conditions. In this manner, hydraulic fluid flows through the pilot operated control valve assembly  192  during all flow conditions. 
     Rapid retraction of rod  212  may be desirable as noted above, for example, when the base of patient handling apparatus  10  is unsupported, and when the patient handling apparatus  10  is being loaded into an emergency vehicle. 
     Second chamber  200  includes an inlet  220 , an outlet  222 , for example, formed by a low pressure bypass (LB) orifice, which is in fluid communication with reservoir R, and a valve poppet  224 . Inlet  220  of second chamber  200  is in fluid communication with a conduit  230  ( FIGS.  9  and  10   ) that is in fluid communication with conduit  96 , which directs hydraulic fluid to or from cap end chamber  84   a  of hydraulic cylinder  80 . 
     Valve poppet  224  is movably mounted in the second chamber between a closed position (shown in  FIGS.  13  and  14   ) wherein valve poppet  224  is seated on a valve seat  226  ( FIG.  14   ) and one or more open positions (shown in dashed lines in  FIG.  15   ) wherein valve poppet  224  moves off seat  226 . Valve seat  226  includes an annular seal  226   a  to seal the inlet  220  of the second chamber from fluid communication with the outlet  222  of the second chamber when valve poppet  224  is in its closed position. When valve poppet  224  is moved off the seat  226 , inlet  220  of the second chamber  200  is in fluid communication with the outlet  222  of the second chamber  200  (thereby opening pilot operated valve  190   a ), which as described below allows at least some of the fluid discharged or evacuated from the cap end of hydraulic cylinder  80  to be discharged to reservoir R from conduits  96  and  230 . 
     When the fluid flow to the first inlet  202  of the first chamber  198  exceeds a preselected flow rate, back pressure at the inlet  202  of the first chamber  198  will move the pilot piston  210  from its closed position to an open position (shown in dashed lines in  FIG.  15   ). This will cause the pilot rod  212  to move the valve poppet  224  from its closed position to one of its open positions to allow fluid flow from the conduit  230  into inlet  220  of the second chamber  200  and to the outlet  222  of the second chamber  200 , which is in fluid communication with reservoir R. In this manner, when the flow of hydraulic fluid pumped from pump  92  to rod end chamber  84   b  of hydraulic cylinder  80  exceeds a threshold flow rate, at least some of the fluid flow from cap end chamber  84   a  of hydraulic cylinder  80  can be discharged to reservoir R, which reduces the pressure in the hydraulic cylinder, especially in the cap side of cylinder  80  where pressure may exceed the normal operating pressure during retraction when the cylinder is unloaded. It also allows, more importantly, the cylinder to contract at a faster rate by reducing the resistant on the cap side of the hydraulic cylinder. 
     In the illustrated embodiment, referring again to  FIG.  12   , the second inlet  204  of first chamber  198  is formed in the valve body wall  196   a  of valve body  196  so that the fluid flow bypasses the pilot piston assembly. For example, the second inlet may be formed by two or more orifices formed in the valve body wall  196   a . Thus, when hydraulic fluid is flowing through circuit  90  from pump  92  to rod end chamber  84   b  of hydraulic cylinder  80  during a low flow condition, such as when the hydraulic cylinder  80  is being retracted while the patient handling apparatus base is supported on a floor or ground surface (when the cylinder lowers the frame (and deck) of the patient handling apparatus), pilot valve  194  will allow the hydraulic fluid to flow through the pilot valve but without moving the pilot valve piston. Although illustrated in the side wall section of the valve body wall, the second inlet may be formed in a bottom wall section of the valve body wall  196   a . Additionally or alternately, another inlet or the second inlet may be formed by a passageway  240  ( FIGS.  14  and  15   ) through pilot piston  210 . 
     Thus, pilot valve  194  provides a flow base pilot valve to control the opening or closing of pilot operated valve  190   a , which redirects some of the fluid from the cap end of the hydraulic cylinder  80  to the reservoir and thereby reduces the pressure in the hydraulic circuit (e.g. and its components) and the pump and, further, enables the speed of the rod to be increased. 
     For example, when the cot is not loaded and the operator wishes to speed up the retraction of the base (for example, when the frame is supported by the deck of an emergency vehicle as described in the above), the higher flow of the fluid through flow based pilot valve, may cause flow based pilot valve  194  to generate the pilot signal  194   a  (caused by movement of the valve poppet  224  described above) will open pilot operated valve  190   a  of pilot operated control valve assembly  192  to allow fluid from the cap side of cylinder  84  to go straight to the reservoir ( FIG.  10   ). 
     In another example, when the cot is loaded with a patient and the operator wishes to lift the frame, the motor will have to drive the pump at a higher speed than when unloaded due to the force need to lift the frame. Flow based pilot valve  194  may be configured to generate the pilot signal when simply raising the frame when it is loaded with a patient (or when a patient has a certain threshold weight) and thereby open the pilot operate valve  190   a  to reduce pressure on the cap side of the cylinder and the pump due to the increase discharge from the cap side of the cylinder. It should be understood that the threshold valve that triggers the pilot signal may be varied and is controlled by the spring constant of spring  216 . 
     As noted, the pilot signal may be generated at high flow conditions, for example, flow rates that are used for a rapid retraction of rod  212 , or for increased rates associated with raising the frame when the apparatus is loaded with a patient. For an ambulance cot designed to carry an adult person and that uses a single cylinder to raise or lower the frame (or retract or lower the base), an example of a low flow rate on the rod side may include a rate in a range of about 0.5 to 1.3 liters/min (1/m) or in a range of about 0.70 to 1.0 liters/minute or in a range of about 0.80 to 0.90 liters/minute. Similarly for an ambulance cot designed to carry an adult person and that uses a single cylinder to raise or lower the frame (or retract or lower the base), an example of a high flow rate on the rod side may include a rate in a range of about 2.0 to 3.0 liters/min (1/m) or in a range of about 2.3 to 2.7 liters/minute or in a range of about 2.40 to 2.6 liters/minute. For the cap side, the flow rate ranges would be approximately double that of the rod side. 
     Although described in the context of a hydraulic control system for a patient support apparatus, the hydraulic based logic component may be used in other applications and to provide logic for other pilot controlled hydraulic devices to achieve the desired logic in a variety of hydraulic systems. 
     The terms “head-end” and “foot-end” used herein are location reference terms and are used broadly to refer to the location of the cot that is closer to the portion of the cot that supports a head of a person and the portion of the cot that supports the feet of a person, respectively, and should not be construed to mean the very ends or distal ends of the cot. 
     While several forms of the cot and hydraulic circuit have been shown and described, other forms will now be apparent to those skilled in the art. For example, one or more of the features of the cot  10  may be incorporated into other cots. Similarly, other features form other cots may be incorporated into cot  10 . Examples of other cots that may incorporate one or more of the features described herein or which have features that may be incorporated herein are described in U.S. Pat. Nos. 7,100,224; 5,537,700; 6,701,545; 6,526,611; 6,389,623; and 4,767,148, and U.S. Publication Nos. 2005/0241063 and 2006/0075558, which are all incorporated by reference herein in their entireties. Therefore, it will be understood that the embodiments shown in the drawings and described above are merely for illustrative purposes, and are not intended to limit the scope of the invention, which is defined by the claims which follow as interpreted under the principles of patent law including the doctrine of equivalents.