Patent Publication Number: US-9409450-B2

Title: Tire inflation system and method of control

Description:
TECHNICAL FIELD 
     This patent application relates to a tire inflation system and a method of control. 
     BACKGROUND 
     A tire pressure monitoring method is disclosed in U.S. Reissue Pat. No. RE41,756. 
     SUMMARY 
     In at least one embodiment, a method of controlling a tire inflation system is provided. The method may include checking an inflation pressure of a tire. A type of tire pressure drop may be categorized when the inflation pressure is less than a target tire pressure. The tire pressure sampling interval may be adjusted based on the type of pressure drop. 
     In at least one embodiment, a tire inflation system may be provided. The tire inflation system may include a pressurized gas source, an outlet valve, an inlet valve, and a pressure sensor. The pressurized gas source may be fluidly connected to a tire and may be configured to supply a pressurized gas. The outlet valve may control the flow of pressurized gas to a tire. The inlet valve may control the flow of pressurized gas to the outlet valve. The pressure sensor may detect pressure indicative of an inflation pressure of the tire. The inflation pressure of the tire may be checked when a tire pressure sampling interval has elapsed. The tire pressure sampling interval may be adjusted based on a type of pressure drop when the inflation pressure is less than a target tire pressure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic of an exemplary vehicle having a tire inflation system. 
         FIG. 2  is a flowchart of an exemplary method of control of the tire inflation system. 
         FIG. 3  is a flowchart of an exemplary method of categorizing a tire pressure drop. 
         FIG. 4  is a chart depicting attributes that may be used to modify or override a tire pressure sampling interval. 
     
    
    
     DETAILED DESCRIPTION 
     As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. 
     Referring to  FIG. 1 , an exemplary vehicle  10  is shown. The vehicle  10  may be of any suitable type, such as a motor vehicle like a truck, bus, farm equipment, military transport or weaponry vehicle, or cargo loading equipment for land, air, or marine vessels. 
     The vehicle  10  may include a plurality of axles or axle assemblies  12  that may support and facilitate rotation of at least one wheel assembly  20 . In  FIG. 1 , a fragmentary portion of the axle assemblies  12  is shown for clarity. An axle assembly  12  may or may not be configured as a drive axle that may provide torque to at least one associated wheel assembly. In addition, an axle assembly  12  may or may not be configured to steer the vehicle  10 . Moreover, an axle assembly may be configured as or may include a steering knuckle assembly in a non-drive axle configuration. 
     Each wheel assembly  20  may include at least one inflatable tire  22  that may be mounted on an associated wheel  24 . Each tire  22  may have a tire valve  26  that may facilitate inflation of the tire  22 . A tire valve  26  may extend through a hole in an associated wheel  24  and may be configured to provide gas to a chamber that may be disposed between or at least partially defined by the tire  22  and the wheel  24 . Each tire valve  26  may be normally closed to inhibit pressurized gas from exiting the tire  22  through the tire valve  26 . The tire valve  26  may open when pressurized gas is supplied to the tire valve  26  under sufficient pressure, such as a pressure that is greater than the pressure inside the tire  22 . In  FIG. 1 , the tire valve locations are generalized for illustration purposes and are not intended to be limiting. 
     A wheel speed sensor  28  may be provided to detect or provide data indicative of the speed of the vehicle  10 . The wheel speed sensor  28  may be of any suitable type. For example, the wheel speed sensor  28  may be configured to detect the rotational speed of a wheel  24  or associated wheel axle. Data from the wheel speed sensor  28  may also be used to determine a distance of travel of the vehicle  10 . For instance, the distance of travel may be based on tire size (e.g., tire diameter) and the rotational distance or number of revolutions detected by the wheel speed sensor  28 . In at least one embodiment, a wheel speed sensor  28  may be associated with each wheel assembly  20  or wheel axle, such as may be provided with an anti-lock brake system (ABS) or traction control system. As such, the wheel speed sensor  28  may also detect wheel slip or unexpected rotation of a wheel assembly  20  in a manner known by those skilled in the art. 
     The vehicle  10  may also include a tire inflation system  30  that may monitor or determine tire pressure and that may inflate one or more tires  22 . More specifically, the tire inflation system  30  may be configured to provide a pressurized gas or pressurized gas mixture to one or more tires  22  via a corresponding tire valve  26 . For clarity, the term “pressurized gas” may refer to a pressurized gas or a pressurized gas mixture in this application. The tire inflation system  30  may include a pressurized gas source  32 , a gas supply subsystem  34 , and a control system  36 . 
     The pressurized gas source  32  may be configured to supply and/or store a volume of a pressurized gas or pressurized gas mixture, such as air and/or nitrogen. For example, the pressurized gas source  32  may include a tank and/or a pump like a compressor that may be driven by a vehicle engine or vehicle power source. The pressurized gas source  32  may be disposed on the vehicle  10  and may be configured to provide a pressurized gas or pressurized gas mixture at a pressure that is greater than or equal to a target tire pressure or a target inflation pressure of a tire  22 . Each tire  22  may or may not have the same target tire pressure in one or more embodiments. 
     The gas supply subsystem  34  may fluidly connect the pressurized gas source  32  to one or more tires  22 . The gas supply subsystem  34  may include one or more conduits  40 , such as a hose, tubing, pipe, or combinations thereof, which may provide pressurized gas to at least one tire  22  via a corresponding tire valve  26 . The conduit configuration in  FIG. 1  is merely exemplary. For instance, a single conduit  40  may be associated with each tire  22  rather than multiple tires as is shown in the bottom half of  FIG. 1 . In at least one embodiment, the gas supply subsystem  34  may include an inlet valve  42 , at least one outlet valve  44 , a first pressure sensor  46 , and a second pressure sensor  48 . 
     The inlet valve  42  may enable or disable the flow of pressurized gas from an outlet of the pressurized gas source  32  to at least one outlet valve  44 . Operation of the inlet valve  42  may be controlled by the control system  36 . For instance, the inlet valve  42  may include or may be controlled by an actuator, such as solenoid, that may actuate the inlet valve  42  between an open position and a closed position. In the open position, pressurized gas may flow from the pressurized gas source  32  to a manifold  50 . The manifold  50  may distribute pressurized gas to multiple conduits  40  and may be disposed between the inlet valve  42  and one or more outlet valves  44 . In the closed position, pressurized gas may be inhibited from flowing from the pressurized gas source  32  to the manifold  50 . In at least one embodiment, the inlet valve  42  may be normally closed under predetermined operating conditions, such as when the vehicle  10  is not operational or turned off or when the vehicle engine is not running. As such, the inlet valve  42  may inhibit depressurization of the pressurized gas source  32  in the event of a downstream leak. 
     The outlet valve  44  may enable or disable the flow of pressurized gas from the manifold  50  to a tire  22  or tire valve  26 . In  FIG. 1 , six outlet valves  44  are shown, although it is contemplated that a greater or lesser number of outlet valves  44  may be provided. Each outlet valve  44  may be associated with a different tire  22  and a different conduit  40 . Moreover, each outlet valve  44  may be actuated independently of the inlet valve  42  and independently of each other. As such, the inflation and pressure assessment of different tires  22  or sets of tires  22  may be independently controlled. Operation of the outlet valve  44  may be controlled by the control system  36 . For instance, the outlet valve  44  may include or may be controlled by an actuator, such as solenoid, that may actuate the outlet valve  44  between an open position and a closed position. In the open position, pressurized gas may flow from the manifold  50  to at least one corresponding tire valve  26 . In the closed position, pressurized gas may be inhibited from flowing from the manifold  50  to at least one corresponding tire valve  26 . As such, pressurized gas may not be constantly provided to one or more tires  22 , which may facilitate the use of pressure pulses to determine tire pressure as will be discussed in more detail below. In addition, the outlet valve  44  may allow a conduit  40  to be vented to the surrounding environment between the outlet valve  44  and a corresponding tire valve  26 . In at least one embodiment, the outlet valve  44  may be normally closed under predetermined operating conditions, such as when the vehicle  10  is not operational or turned off or when the vehicle engine is not running. 
     The first pressure sensor  46  may be configured to detect the pressure of the pressurized gas provided by the pressurized gas source  32 . The first pressure sensor  46  may be of any suitable type and may be fluidly connected to the pressurized gas source  32 . For example, the first pressure sensor  46  may be fluidly connected to the pressurized gas source  32  between the pressurized gas source  32  and the inlet valve  42 . 
     The second pressure sensor  48  may be configured to detect the pressure of the pressurized gas provided to a tire  22  or tire valve  26 . The second pressure sensor  48  may be of any suitable type and may be disposed between the inlet valve  42  and the tire valve  26  and may be fluidly connected to the manifold  50 . As such, the second pressure sensor  48  may be isolated from the pressurized gas source  32  by closing the inlet valve  42 . In at least one embodiment, the second pressure sensor  48  may be disposed between the inlet valve  42  and one or more outlet valves  44  so that the second pressure sensor  48  may be used to detect the pressure of pressurized gas supplied to different tires. Alternatively, multiple second pressure sensors  48  may be provided that may detect the pressure supplied to a particular conduit  40  or particular tire  22 . 
     Optionally, a tire pressure sensor  52  that may be disposed inside the tire  22  or inside a tire chamber that receives the pressurized gas. Such a tire pressure sensor  52  may provide a signal or data that is indicative of the inflation pressure of the tire to the control system  36 . The tire pressure sensor  52  may wirelessly communicate with the control system  36  in one or more embodiments. 
     The control system  36  may monitor and control operation of the tire inflation system  30 . The control system  36  may include one or more electronic controllers or control modules that may monitor and/or control various components of the tire inflation system  30 . For example, the control system  36  may be configured to control actuation of the inlet valve  42  and the outlet valve  44  to control the flow of pressurized gas. In addition, the control system  36  may be configured to receive data from the first pressure sensor  46  and the second pressure sensor  48  that may be indicative of pressure. In  FIG. 1 , communication between the control system  36  and these components is represented by the double arrowed line that is located adjacent to the control system  36 . 
     The control system  36  may also communicate with and/or receive data from other devices, such as sensors like the wheel speed sensors  28 , an ambient temperature sensor  60 , an engine temperature sensor  62 , and a wheel end temperature sensor  64 . 
     Communication between the control system  36  and each wheel speed sensor  28  is represented by connection nodes W 1  through W 6  in  FIG. 1 . 
     The ambient temperature sensor  60  may detect or provide data indicative of a temperature of ambient air in the environment surrounding the vehicle  10  or proximate the vehicle  10 . The ambient temperature sensor  60  may be a physical sensor that may be disposed on the vehicle  10  or a virtual sensor. For example, a virtual ambient air temperature sensor may be based on temperature data that may be wirelessly transmitted to the vehicle  10 . Such temperature data may be based on the location of the vehicle  10 , which may be provided by a global positioning system (GPS) or other telemetry or location data. 
     The engine temperature sensor  62  may detect or provide data indicative of an engine temperature. For example, the engine temperature sensor  62  may detect the temperature of a fluid associated with the engine of the vehicle  10 , such as a coolant temperature or oil temperature as these fluids receive thermal energy when the engine is running. 
     The wheel end temperature sensor  64  may detect or provide data indicative of the temperature of one or more components that may be associated with a wheel end assembly that may rotatably support a wheel  24 . For instance, the wheel end temperature sensor  64  may detect or provide data indicative of a tire temperature and/or a brake temperature. 
     Referring to  FIG. 2 , a flowchart of an exemplary method of control of the tire inflation system  30  is shown. As will be appreciated by one of ordinary skill in the art, the flowchart represents control logic which may be implemented or affected in hardware, software, or a combination of hardware and software. For example, the various functions may be affected by a programmed microprocessor. The control logic may be implemented using any of a number of known programming and processing techniques or strategies and is not limited to the order or sequence illustrated. For instance, interrupt or event-driven processing may be employed in real-time control applications rather than a purely sequential strategy as illustrated. Likewise, parallel processing, multitasking, or multi-threaded systems and methods may be used. 
     Control logic may be independent of the particular programming language, operating system, processor, or circuitry used to develop and/or implement the control logic illustrated. Likewise, depending upon the particular programming language and processing strategy, various functions may be performed in the sequence illustrated, at substantially the same time, or in a different sequence while accomplishing the method of control. The illustrated functions may be modified, or in some cases omitted, without departing from the scope intended. 
     In at least one embodiment, the method may be executed by the control system  36  and may be implemented as a closed loop control system. Moreover, the method may be enabled or disabled based on the operating state of the vehicle  10 . For example, the method or control logic may be enabled when the vehicle ignition is turned on, when the engine is running, or when the vehicle is in motion in one or more embodiments. In addition, the method may be manually activated. 
     The method will be primarily described in the context of evaluating the pressure of a single tire, but it is to be understood that the method may be applied to evaluate and/or adjust the pressure of multiple tires or sets of tires. 
     As an overview, the method may determine a tire pressure sampling frequency or determine when to check the pressure of a tire. The method may also trigger or execute a tire pressure check or adjust the tire pressure sampling frequency based on tire pressure or various factors that may be indicative of changes in tire pressure, such as tire pressure leakage, wheel rotational speed, and/or temperature attributes. These factors may override a previously established tire pressure sampling frequency and may cause the tire pressure to be checked sooner. Moreover, a different tire pressure sampling frequency may be set for individual tires or sets of tires. As such, tire pressure sampling may be tailored to an individual tire and/or changes in operational characteristics of the vehicle or vehicle environment. 
     Tailoring or adjusting tire pressure sampling may help inhibit tire overinflation in a tire inflation system that checks tire pressure by opening a tire valve. For example, tire pressure may be determined by opening a tire valve with a pulse of pressurized gas and then measuring the pressure upstream or in a supply conduit. The pressurized gas pulse forces an additional volume of pressurized gas into the tire, thereby increasing the tire pressure. The cumulative effect of multiple pressure pulses may result in overinflation of the tire. Such tire overinflation may be exasperated in a system that checks tire pressure every vehicle ignition cycle (e.g., each time the vehicle is turned on). For instance, use of such a system in a vehicle that undergoes frequent stops and/or frequent vehicle ignition cycles (e.g., a delivery truck or cargo loading equipment) may result in frequent tire pressure checks and oversample the tire pressure, which may cause tire overinflation, increase pressurization and wear in associated seals, and increase the demand for pressurized gas from a pressurized gas source. 
     At block  100 , the method may determine whether a tire pressure check or tire pressure sampling is desired. Tire pressure sampling or sampling of the inflation pressure of a tire may be desired when a tire pressure sampling interval has elapsed. Initially, a tire pressure sampling interval may be set at a default value or default period of time for one or more tires. The tire pressure sampling interval may be expressed in terms of time or distance. 
     A tire pressure sampling interval expressed in terms of time may be measured with a timer, clock, or counter (which may be collectively referred to as a timer herein) that may be associated with the control system  36 . The timer may stop when the vehicle is not operational (e.g., when the ignition is turned off) or may be reset during each ignition cycle (e.g., time measurement may cease in a configuration in which power is not supplied to the timer when the ignition is turned off). Alternatively, a timer that is not reset during each ignition cycle (e.g., a real-time clock that continuously keeps track of time even when the ignition is turned off) or that is based on time data that may be provided by an external source and/or wirelessly transmitted to the vehicle (such as by a global positioning system or wireless network) may be employed for time measurement. 
     A tire pressure sampling interval expressed in terms of time may also be measured based on data from the ambient temperature sensor  60  and the engine temperature sensor  62 . Data from the engine temperature sensor  62  may be indicative of an amount of time that the engine has been turned off. More specifically, the temperature of a previously running engine may decrease over time after the engine is turned off. The rate of change in engine temperature may be affected by the ambient temperature. For example, the engine temperature may decrease more rapidly at lower ambient temperatures than at higher ambient temperatures. As such, engine temperature and ambient temperature may be used to calculate or estimate the amount of time that the engine has been turned off. For instance, a lookup table may be populated with time values or “time since engine off” values that may be referenced with engine temperature and ambient temperature values. A time value may be calculated or referenced when the vehicle is turned on and this time value may be compared to the tire pressure sampling interval to determine how much time, if any, remains in the tire pressure sampling interval. For example, if the total elapsed time (e.g., sum of the time elapsed since the last tire pressure check and the time elapsed while the engine was off) is greater than the tire pressure sampling interval, then the tire pressure sampling interval has been exceeded and a pressure check may be performed. If the total elapsed time is not greater than the tire pressure sampling interval, then a pressure check may not be performed until the tire pressure sampling interval has elapsed. Under this approach, engine temperature and ambient temperature may be used in place of a real-time clock or continuous time measurement that may require that power be continuously provided to a timer or microprocessor clock to provide a continuous time measurements that are not reset or lost when the vehicle is turned off. In addition, this approach may reduce tire pressure checks and potential tire overinflation as compared to automatically checking tire pressure each ignition cycle or each time the vehicle is turned on. 
     Vehicle travel distance may also be employed as the basis for the tire pressure sampling interval. For instance, the tire pressure sampling interval may be expressed in terms of distance (e.g., feet, miles, kilometers, etc.) such that a tire pressure check may be desired when the vehicle has travelled a predetermined distance. As such, the tire pressure sampling interval may have elapsed when the travel distance of the vehicle exceeds a threshold vehicle travel distance. The threshold vehicle travel distance may be used such that tire pressure is checked each time the vehicle travels the threshold vehicle travel distance. The threshold vehicle travel distance may be based on vehicle development testing. Vehicle travel distance may be based on data from the wheel speed sensor  28 , an onboard vehicle navigation system, and/or data from an external data source like a global positioning system (GPS) that may be indicative of vehicle speed and/or vehicle location. 
     It is also contemplated that time and distance may be used in conjunction to measure a tire pressure sampling interval. For instance, the tire pressure sampling interval may elapse when a threshold period of time has passed and the travel distance of the vehicle exceeds a threshold vehicle travel distance. As such, the method may be configured to reduce tire pressure checks when the vehicle has generally been stationary or has not travelled much distance. 
     A tire pressure check may be triggered or executed or the tire pressure sampling interval may be modified based on various factors as will be discussed in more detail below. If a tire pressure check or tire pressure sampling is not desired, then the method may end at block  102  without checking or sampling the tire pressure. If a tire pressure check is desired, the method may continue at block  104 . 
     At block  104 , tire functionality may be assessed. Tire functionality may be abnormal when a tire is not installed or when a tire blowout or major tire pressure drop has occurred. For example, a tire blowout or major pressure drop may exist when the tire cannot be inflated to a target tire pressure and/or when the target tire pressure cannot be maintained for a threshold period of time. Such tire functionality may be stored in memory and may be based on a previous iteration of the method. If tire functionality is not normal, then the method may end at block  102  without checking or sampling the tire pressure, thereby avoiding or reducing use of pressurized gas from the pressurized gas system. If tire functionality is normal or not abnormal, then the method may continue at block  106 . 
     At block  106 , the tire pressure may be checked or sampled. Tire pressure may be checked or sampled in various ways depending on the configuration of the tire inflation system. For example, tire pressure may be directly detected or measured with a tire pressure sensor that may be disposed inside the tire or inside a tire chamber that receives the pressurized gas. Such a tire pressure sensor may wirelessly transmit tire pressure data to the control system  36 . Tire pressure may also be directly detected with a pressure sensor that is disposed outside the tire. For example, tire pressure may be detected or measured by opening the tire valve  26  with a pressurized gas pulse and detecting pressure of a feedback pulse with the second pressure sensor  48 . Tire pressure may also be indirectly detected with a pressure sensor that is disposed outside the tire as is described in U.S. Patent Publication No. 2015/0075672, the disclosure of which is hereby incorporated by reference in its entirety. 
     At block  108 , the detected or measured tire pressure may be compared to a target tire pressure. The target tire pressure may be indicative of a desired tire pressure and may account for design tolerances of the tire valve  26 , such as the tolerance range associated with opening the tire valve  26 . The target tire pressure may be a predetermined value that may be based on the make and model of the tire and/or vehicle development testing. If the detected or measured tire pressure is not less than the target tire pressure, then the tire is not underinflated (i.e., the tire is inflated to at least the target tire pressure) and the method may end at block  110  and the tire pressure sampling interval may be maintained. If the detected or measured tire pressure is less than the target tire pressure, then the tire may be underinflated (e.g., a tire leak may have occurred) and the method may continue at block  112 . 
     At block  112 , the tire may be inflated. The tire may be inflated by providing pressurized gas from the pressurized gas source  32  to the tire  22  by opening the inlet valve  42  and the outlet valve  44  associated with the tire  22 . Pressurized gas may be provided for a predetermined period of time and/or until the tire is inflated to the target tire pressure. 
     At block  114 , a pressure drop associated with the tire may be categorized and the tire pressure sampling interval may be updated or revised if appropriate. An exemplary method of categorizing a tire pressure drop or categorizing of the ability of a tire to maintain pressure is shown in  FIG. 3 . In the example shown in  FIG. 3 , pressure drops are categorized as being major (Type I), moderate (Type II), or minor (Type III); however, it is contemplated that a greater or lesser number of pressure drop categories may be employed. A major (Type I) pressure drop may be associated with a greater tire pressure drop than a moderate (Type II) pressure drop. A moderate (Type II) pressure drop may be associated with a greater tire pressure drop than a minor (Type III) pressure drop. 
     At block  120 , the method may determine whether the tire can be inflated to the target tire pressure. The tire pressure may be detected or measured as previously discussed with reference to block  112 . If the tire cannot be inflated to the target tire pressure or cannot adequately maintain the target tire pressure, then the method may continue at block  122 . If the tire can be inflated to the target tire pressure or can adequately maintain the target tire pressure, then the method may continue at block  124 . 
     At block  122 , a Type I (major) pressure drop may exist since the tire cannot be inflated to the target tire pressure or adequately maintain the target tire pressure. As such, pressurized gas that is provided to the tire may escape to the surrounding environment and not inflate the tire. For instance, a major pressure drop may exist when the tire pressure does not reach the target tire pressure when the tire is inflated and may be indicative of a tire blowout or major tire leak or may be indicative of leakage or a pressure drop elsewhere in the tire inflation system, such as a wheel seal or a damaged or missing pneumatic line. The inflation time may be limited to limit depletion of the pressurized gas source. For instance, the supply of pressurized gas to inflate a tire may be terminated when the tire cannot be inflated to the target tire pressure within a predetermined inflation period of time or within a predetermined vehicle travel distance. The predetermined inflation period of time or predetermined vehicle travel distance may be default values that may be set to provide sufficient time or distance is provided to inflate a tire that may be flat but able to maintain the target tire pressure or that may have a moderate or minor pressure drop. The predetermined inflation period of time may be greater than the threshold inflation time that may be used to distinguish a minor pressure drop from a moderate pressure drop as discussed below. In response, the method may stop providing pressurized gas to the tire by closing a valve, such as the inlet valve  42  and/or the associated outlet valve  44 . In addition, an alert, alarm, or error message may be provided to the driver. The alert, alarm, or error message may be of any suitable type, such as an audible signal, visual signal, tactile (haptic) signal, or combinations thereof, and may identify a tire and warn the driver that the target tire pressure cannot be obtained or maintained and that a major pressure drop or blowout may have occurred. In addition, a memory flag may be set to indicate that the functionality of the tire is abnormal for use in conjunction with block  104 . As such, block  104  may prevent additional tire pressure sampling of the associated tire until the tire is serviced or replaced or the memory flag has been reset. For example, the memory flag may be reset manually by the driver with an input device (e.g., button, switch, touchscreen sensor, or the like). 
     At block  124 , the method may determine whether the tire inflates to the target tire pressure within a threshold inflation time (i.e., whether the time to inflate the tire to the target tire pressure does not exceed a threshold inflation time). The tire inflation time may be measured with a timer, counter, or clock which may be started when tire inflation begins, such as when the inlet valve  42  and outlet valve  44  are opened. The threshold inflation time may be a predetermined value that may be based on the attributes of the tire and/or tire inflation system  30  (e.g., tire size, flow rate of gas from the pressurized gas source, conduit size, valve actuation speed, etc.) and/or vehicle development testing. If the tire does not inflate to the target tire pressure within the threshold inflation time, then the method may continue at block  126 . If the tire is inflated to the target tire pressure within the threshold inflation time, then the method may continue at block  128 . 
     At block  126 , a Type II (moderate) pressure drop may exist since the tire can be inflated to the target tire pressure but takes more time to be inflated than expected. As such, the tire may adequately maintain tire pressure for a period of time, but may be experiencing a pressure drop or may be leaking pressurized gas such that more frequent tire pressure sampling is desired. In response, the method may reduce the tire pressure sampling interval for that tire by a predetermined moderate pressure drop adjustment amount or to a predetermined value that may be associated with a moderate pressure drop. In addition, an alert, alarm, or error message may be provided to the driver. The alert, alarm, or error message may be of any suitable type, such as an audible and/or visual signal, and may identify a tire and warn the driver that a moderate pressure drop may be in progress and may indicate that the tire should be checked or serviced. A memory flag associated with functionality of the tire may continue to indicate that tire functionality is normal since the target tire pressure can be obtained and maintained. As such, additional tire pressure sampling may be permitted at block  104  during a future iteration of the method. 
     At block  128 , a Type III (minor) pressure drop may exist since the tire can be inflated to the target tire pressure and can be inflated within the threshold inflation time. As such, the tire may adequately maintain tire pressure for a longer period of time than a tire with a Type II (moderate) pressure drop, but may be experiencing a pressure drop or may be leaking pressurized gas such that more frequent tire pressure sampling is desired as compared with a tire that does not experience a pressure drop. In response, the method may reduce the tire pressure sampling interval for that tire by a predetermined minor pressure drop adjustment amount or to a predetermined value associated with a minor pressure drop. The predetermined moderate pressure drop adjustment amount or predetermined value associated with a Type III pressure drop may differ from the predetermined minor pressure drop adjustment amount or predetermined value associated with a Type II pressure drop. As such, tire pressure may be sampled more frequently when a Type II (moderate) pressure drop is detected than for a Type III (minor) pressure drop. As one example, a pressure drop adjustment amount for a Type II (moderate) pressure drop may reduce the tire pressure sampling interval by 30 minutes and a minor pressure drop adjustment amount for a Type III (minor) pressure drop may reduce the tire pressure sampling interval by 15 minutes. An alert, alarm, or error message may not be provided to the driver. A memory flag associated with functionality of the tire may continue to indicate that tire functionality is normal since the target tire pressure can be obtained and maintained. 
     Referring to  FIG. 4 , a chart is shown that depicts attributes that may be used to trigger, override, modify, or interrupt a previously established tire pressure sampling interval. These attributes may be used independently or in combination to trigger an immediate tire pressure check at block  100  or shorten the tire pressure sampling interval such that a tire pressure check may be executed sooner than may otherwise occur if the tire pressure sampling interval was not modified. Attributes may include or may be based on pressure, wheel speed, and temperature. 
     As discussed above, tire pressure data may be employed to categorize a tire pressure drop and the type of tire pressure drop may be used to modify the tire pressure sampling interval. Thus, the tire pressure sampling interval may be reduced or overridden as the ability of a tire to maintain pressure decreases. 
     Wheel speed may be used to infer a change in tire pressure. For example, wheel speed may be detected for a first wheel and a second wheel using corresponding wheel speed sensors  28 . The first wheel and the second wheel may be provided on the same axle assembly or may be provided on different axle assemblies in one or more embodiments. If the detected wheel speeds are sufficiently close to each other or within a threshold wheel speed amount (e.g., the difference between the first wheel speed and the second wheel speed is less than the threshold wheel speed amount), then a tire may not be underinflated. If the detected wheel speeds are not sufficiently close to each other (e.g., the difference between the first wheel speed and the second wheel speed is not less than the threshold wheel speed amount), then the wheel having the smaller effective radius or greater rotational speed may have an underinflated tire and the tire pressure sampling interval may be decreased and/or a tire pressure check may be triggered or executed. The wheel speed may also be compared to the wheel speeds of additional tires to provide additional confidence that a wheel may have an underinflated tire. In addition, a wheel speed may be compared to a reference wheel speed or reference vehicle speed rather than to the speed of another wheel to infer a change in tire pressure. Such reference speed values may be based on sensors on the vehicle or may be virtual speed data, such as may be provided by a GPS system. 
     As another example, wheel speed may be detected with a wheel speed sensor  28  and may be compared to a threshold wheel speed value. The threshold wheel speed value may be indicative of an expected wheel rotational speed for a wheel having a tire that is not underinflated. If the detected or measured wheel speed or wheel speed over a predetermined sample time is less than the threshold wheel speed value, then the tire pressure may be less than the target tire pressure and the tire pressure sampling interval may be decreased and/or a tire pressure check may be triggered or executed. If the detected or measured wheel speed or wheel speed over a predetermined sample time is not less than the threshold wheel speed value, then the tire may not be underinflated and the current tire pressure sampling interval may be maintained. 
     Temperature data may be used to infer a change in tire pressure. Such temperature data may be provided by the ambient temperature sensor  60  and/or the wheel end temperature sensor  64 . 
     Tire pressure may change in response to environmental or ambient temperature conditions. Such ambient temperature changes may occur rapidly, such as when a vehicle rapidly ascends to a higher elevation or descends to a lower elevation, such as may occur when driving in mountainous terrain. Tire pressure may decrease as ambient temperature decreases. As such, tire pressure may drop below the target tire pressure due to ambient temperature changes before a tire pressure sampling interval lapses. As such, the rate of change of ambient temperature may be used to revise the tire pressure sampling interval. For example, a rate of change in the ambient temperature (such as may be measured in ° C./min) may be detected or calculated based on data from the ambient temperature sensor  60 . The rate of change in ambient temperature may be compared to a threshold ambient temperature rate of change value. If the rate of change of the ambient temperature exceeds the threshold ambient temperature rate of change value, then the tire pressure may be less than the target tire pressure and the tire pressure sampling interval may be decreased and/or a tire pressure check may be triggered or executed. If the rate of change of the ambient temperature does not exceed the threshold ambient temperature rate of change value, then the tire may not be underinflated and the current tire pressure sampling interval may be maintained. 
     Wheel end temperature may also be used to adjust the tire pressure sampling interval. Low tire pressure or pressure that is substantially below the target tire temperature may result in an increase in temperature of components associated with a wheel end assembly. For example, low tire pressure may result in elevated wheel bearing temperatures or wheel bearing failure, which in turn may increase the temperature proximate the wheel bearings that rotatably support the wheel of the underinflated tire. This additional thermal energy from the wheel bearings may be conducted to one or more associated components, such as the wheel or brake assembly components for the underinflated tire. As such, the wheel end temperature sensor  64  may detect the associated increase in temperature. The temperature detected by the wheel end temperature sensor  64  may be compared to a threshold wheel end temperature value. If the wheel end temperature exceeds the threshold wheel end temperature value, then the tire pressure may be less than the target tire pressure and the tire pressure sampling interval may be decreased and/or a tire pressure check may be triggered or executed. If the threshold wheel end temperature value does not exceed the threshold wheel end temperature value, then the tire may not be underinflated and the current tire pressure sampling interval may be maintained. 
     Alternatively, a tire may be underinflated when the temperature detected by the wheel end temperature sensor  64  exceeds the temperature detected by another wheel end temperature sensor  64  that may be associated with another tire. If the detected wheel end temperatures are sufficiently close to each other or within a threshold wheel end temperature amount (e.g., the difference between a first wheel end temperature and the second wheel end temperature is less than the threshold wheel end temperature amount), then a tire may not be underinflated. If the detected wheel end temperatures are not sufficiently close to each other, then the wheel having the higher wheel end temperature may have an underinflated tire and the tire pressure sampling interval may be decreased and/or a tire pressure check may be triggered or executed. A wheel end temperature may also be compared to the wheel end temperature of additional tires to provide additional confidence that a wheel may have an underinflated tire. The threshold wheel end temperature amount may be established by or based on vehicle development testing. 
     While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention.