Abstract:
A method of inflating a tire, or vehicle tire, that minimizes the amount of time needed for same is disclosed. The method of inflating a tire with a tire pressure management system includes introducing continuous fluid flow of a fluid into the tire, ascertaining the dynamic pressure of the fluid during said introducing continuous fluid flow, terminating continuous fluid flow when the dynamic pressure exceeds or equals a predetermined amount, and introducing pulsed fluid flow of the fluid into the tire.

Description:
BACKGROUND OF THE INVENTION 
     Conventional tire pressure management systems typically have central tire inflation systems (CTI systems), also known as on-board inflation systems and traction systems. These tire pressure management systems are well known, as may be seen by reference to the following U.S. Pat. Nos.: 5,516,379; 5,313,995; 5,273,064; 5,253,687; 5,180,456; 5,179,981; 5,174,839; 5,121,774; 4,924,926; 4,922,946; 4,917,163; 4,893,664; 4,883,106; 4,883,105; 4,825,925; 4,782,879; 4,754,792; 4,724,879; 4,678,017; 4,640,331; and 4,619,303. The entire disclosure of each of these patents is incorporated herein. 
     Generally, tire pressure management, systems employ a pneumatically controlled wheel valve that is affixed to each vehicle wheel assembly for controlling tire pressure in response to pressure signals from a fluid control circuit. The fluid control circuit is connected to each wheel valve via a rotary seal assembly associated with each wheel valve. In some systems, tire pressure is monitored by means of a sensor that is positioned in a conduit assembly in the fluid control circuit. When the wheel valve and certain control valves are opened, the pressure in the conduit assembly equalizes to tire pressure which can be sensed by the sensor. An electronic control unit receives electrical pressure signals generated by the sensor and appropriately controls the fluid control circuit in response thereto for inflating or deflating a selected tire. 
     Over time, the energy costs for operating a tire inflation management system can grow. Also, although not continuous, tire inflation management systems chronically draw compressed fluid from, thus have potential for compromising, a vehicle compressed fluid supply that services higher priority vehicle systems, such as a vehicle braking system. Reducing the amount of time a tire inflation management system draws compressed fluid from the vehicle compressed fluid supply and inflating or deflating vehicle tires ensures the availability of compressed fluid for other, perhaps higher-priority, vehicle systems and reduces the amount of energy needed to maintain vehicle tires. Reducing inflation time also enables vehicles to quickly adapt to changed surface conditions, for example, when surface conditions change from uneven or soft to level and hard, which may require significant inflation of all vehicle tires. What is needed is a method of inflating vehicle tires that minimizes the amount of time needed for same. 
     SUMMARY OF THE INVENTION 
     The invention provides a method of inflating a tire, or vehicle tire, that minimizes the amount of time needed for same. The method of inflating a tire with a tire pressure management system includes introducing continuous fluid flow of a fluid into the tire, ascertaining the dynamic pressure of the fluid during said introducing continuous fluid flow, terminating continuous fluid flow when the dynamic pressure exceeds or equals a predetermined amount, and introducing pulsed fluid flow of the fluid into the tire. 
     The invention provides improved elements and arrangements thereof, for the purposes described, which are inexpensive, dependable and effective in accomplishing intended purposes of the invention. Other features and advantages of the present invention will become apparent from the following description of the preferred embodiments, which refers to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is described in detail below with reference to the following figures, throughout which similar reference characters denote corresponding features consistently, wherein: 
     FIG. 1 is a diagrammatic view of a tire pressure management system for a vehicle, a vehicle incorporating same being shown in dotted line; 
     FIG. 2 is a cross-sectional detail view of a conventional vehicle wheel assembly; 
     FIG. 3 is a schematic view of components of the system of FIG. 1; and 
     FIG. 4 is a schematic view of a flow chart for a method configured according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The invention is a method of inflating vehicle tires that minimizes the amount of time needed for same. The method may be achieved with known tire pressure management systems, such as the exemplary tire pressure management system described below. 
     FIG. 1 shows a tire pressure management system  10  for a vehicle  12  for describing, but not limiting applicability of the invention. Vehicle  12  may be, but is not limited to being a tractor-trailer. The system may be used in connection with a wide variety of vehicles, including automobiles. 
     Vehicle  12  may include a plurality of axles, including a steer axle  14 , a tandem axle assembly having drive axles  16 ,  18  and another tandem axle assembly having trailer axles  20 ,  22 . As shown in greater detail in FIG. 2, each axle, such as drive axle  14 , may include wheels  24  affixed to wheel hubs  26  disposed at each outboard end of the axle and rotationally supported on axle  14 . Each wheel  24  may include one or more inflatable tires  28  mounted thereon. 
     System  10  monitors and controls pressure within each tire  28  of vehicle  12 . System  10  may include wheel valve assemblies  30 , a fluid source  32 , a vacuum source  34 , and a fluid control circuit  36 . System  10  may further include at least a sensor  200 , one or more electronic control units  42 , one or more load sensors  44 , a speed sensor  46 , and an operator control device  48 . 
     Wheel valve assemblies  30  are provided to control the flow of pressurized fluid into and out of tires  28 . Valve assembly  30  is mounted to each end of each axle and is connected to the remainder of system  10  through a rotary seal connection  50 . Wheel valve assembly  30  is conventional in the art and may include the wheel valve assembly described and illustrated in U.S. Pat. No. 5,253,687 or U.S. Pat. No. 6,250,327, the entire disclosures of which are incorporated herein. 
     Rotary seal assembly  50  also is conventional in the art and may include the rotary seal assembly described and illustrated in U.S. Pat. No. 5,174,839, the entire disclosure of which also is incorporated herein. 
     Referring again to FIG. 2, wheel valve assembly  30  may include an inlet port  30   a  coupled to a rotatable port  50   b  of rotary seal assembly  50 , an outlet port  30   b  in fluid communication with the interior of tire  28 , and an exhaust port  30   c , best shown in FIG.  1 . Rotary seal assembly  50  may further include a non-rotatable port  50   a  connected to a conduit  52  of fluid control circuit  36 . Valve assembly  30  assumes a closed position, as illustrated in FIG. 1, when the fluid pressure at inlet port  30   a  is substantially atmospheric, an open position connecting inlet port  30   a  and outlet port  30   b  when the fluid pressure at inlet port  30   a  is a positive pressure, and an exhaust position connecting outlet port  30   b  and exhaust port  30   c  when the fluid pressure at inlet port  30   a  is a negative pressure. 
     Fluid source  32  provides positive pressurized fluid to system  10  and tires  28 . Fluid source  32  is conventional in the art and may include a pressure source, such as a pump  54 , an air dryer  56 , and a first fluid tank  58  connected via a conduit  60  to the brake system fluid tanks  62 ,  64  and to the fluid control circuit  36  via a branch conduit  60   a . Check valves  66  prevent sudden loss of fluid pressure in brake tanks  62 ,  64  in the event of upstream pressure loss. A pressure sensor  68  monitors pressure within tank  58  and provides a pressure indicative signal to electronic control unit  42 . 
     Vacuum source  34  produces a negative pressure in system  10  to decrease fluid pressure in tires  28  of vehicle  12 . Vacuum source  34  also is conventional in the art and may include a vacuum generator  70  controlled through a solenoid valve  72 . A low pressure zone is produced by passing fluid through a venturi like portion of vacuum generator  70 . Upon urging solenoid valve  72  into an open position via a control signal from electronic control unit  42 , a vacuum or negative fluid pressure, relative to atmospheric pressure, is introduced in a conduit  74 , which has a small orifice  76  disposed proximate the low pressure zone produced by generator  70 . Conduit  74  also is connected to a one-way vent valve  78  for rapid venting of positive fluid pressure from conduit  74 . Vent valve  78  includes a valving member  80  that is drawn into a closed position in response to negative fluid pressure in conduit  74  and is urged into an open position in response to positive pressure fluid in conduit  74 . 
     Fluid control circuit  36  directs the flow of pressurized fluid within system  10  for controlling pressure in tires  28  of vehicle  12 . Control circuit  36  may include a pair of pressure control valves  82 ,  84  and a plurality of axle distribution valves  86 ,  88 ,  90 . As shown, a single fluid control circuit  36  controls pressure in all of the tires  28  of vehicle  12 . However, control circuit  36 , and other portions of system  10 , may be replicated so that, for example, one control circuit  36  may control tire pressures in the tractor portion of vehicle  12  and another control circuit  36  may control.tire pressure in the trailer portion of vehicle  12 . 
     Pressure control valve  82  directs positive pressurized fluid from fluid source  32  to tires  28  of vehicle  12 . Valve  82  may include a conventional two position-two way, solenoid controlled and pilot fluid operated valve. Valve  82  includes a valving member  92  that is spring biased toward a closed position, as shown in. FIG.  1 . Valving member  92  assumes an open position in response to energizing of a solenoid operatively associated therewith via control signals from electronic control unit  42 . Valve  82  has a first port  82   a  coupled to a conduit  94  leading to fluid source  32 . Valve  82  has a second port  82   b  coupled to another conduit  96  leading to axle distribution valves  86 ,  88 ,  90 . 
     Pressure control valve  84  vents control circuit  36 . Valve  84  is conventional in the art and may also include a two position-two way, solenoid controlled and pilot fluid operated valve. Valve  84  includes a valving member  98  that is spring biased toward an open position, as shown in FIG.  1 . Valving member  98  assumes a closed position in response to energizing a solenoid operatively associated therewith via control signals from electronic control unit  42 . Valve  84  has a first port  84   a  coupled to conduit  74  leading to orifice  76 . Valve  84  has a second port  84   b  coupled to conduit  96  leading to axle distribution valves  86 ,  88 ,  90 . 
     Axle distribution valves  86 ,  88 ,  90  limit the supply of positive pressurized fluid to, or the release of fluid from, the tires  28  of one or more axles  14 ,  16 ,  18 ,  20 ,  22  of vehicle  12 . Valves  86 ,  88 ,  90  are conventional in the art and may include two position-two way, solenoid controlled and pilot fluid operated valves. Valves  86 ,  88 ,  90  direct the flow of fluid to and from the tires  28  of axles  14 ,  16  and  18 , and  20  and  22 , respectively. Each of valves  86 ,  88 ,  90  includes a valving member  100 ,  102 ,  104 , respectively, that is spring-biased toward an open position, as shown in FIG. 1, and which assumes a closed position in response to energizing a solenoid operatively associated therewith via electrical signals from electronic control unit  42 . Each of valves  86 ,  88 ,  90  respectively has first ports  86   a ,  88   a ,  90   a  coupled to conduit  96 . Each of valves  86 ,  88 ,  90  respectively has second ports  86   b ,  88   b ,  90   b  leading to respective corresponding conduits  52 ,  106 ,  108  for each axle or tandem axle of vehicle  12 . 
     Although axle distribution valves  86 ,  88 ,  90  are shown, individual tire distribution valves could be used in conjunction with axle distribution valves  86 ,  88 ,  90  or as an alternative to axle distribution valves  86 ,  88 ,  90  to further control the flow of fluid to and from individual tires  28  of vehicle  12 . Further, although only three axle distribution valves  86 ,  88 ,  90  are shown, the number of axle distribution valves may be varied depending upon the number of axles of vehicle  12  and to allow for greater individual control of the tires  28  of vehicle  12 . 
     Sensor  200  may be electrically integrated with electronic control unit  42 . Sensor  200  is disposed in fluid communication with conduit assemblies for conducting fluid to and/or from tires  28 . Sensor  200  may transmit a parameter signal indicative of a measured parameter associated with a corresponding tire  28  of vehicle  12 . The parameter may be fluid pressure or another value, such as tire temperature, that may be indicative of tire pressure. 
     Referring to FIG. 3, electronic control unit  42  controls fluid control circuit  36 . Control unit  42  may include a microprocessor operating under the control of a set of programming instructions commonly referred to as software. Electronic control unit  42  may include a memory  114  in which the programming instructions are stored. Memory  114  also may contain identification codes for each tire  28  of vehicle  12  to uniquely identify the particular tire  28  to which a particular parameter signal corresponds. Memory  114  also may be used to record tire pressure values or user inputs over a period of time to assist in evaluating tire pressure management. 
     Control unit  42  may receive input signals from sensor  200 , one or more load sensors  44 , speed sensor  46 , and operator control device  48 . Control unit  42  outputs a plurality of control signals to control valves  82 ,  84 ,  86 ,  88 ,  90  of fluid control circuit  36  and solenoid valve  72  of vacuum source  34 . Control unit  42  also may generate a plurality of output signals to a display device which may include a part of operator control device  48  or a freestanding device. The latter signals may be used to trigger the display pressure readings and/or deflection levels for each vehicle tire  28 , the load on vehicle  12  or a portion of it, and the speed of vehicle  12 . The signals may also be used to trigger warnings to the operator of vehicle  12  in the event that pressure cannot be maintained in one of the vehicle tires  28 , the pressure exceeds or falls below predetermined maximum and minimum tire pressure values, or the pressure differs from a target pressure value by more than a predetermined amount. 
     Load sensors  44  provide an indication as to the load on vehicle  12  and, consequently, tires  28  of vehicle  12 , or the load on some portion of vehicle  12  and, consequently, select tires  28  of vehicle  12 . Load sensors  44  are conventional in the art and load sensing may be provided in a variety of known ways, including through analysis of pneumatic pressure in the suspension of vehicle  12 , analysis of powertrain parameters, the use of displacement transducers, or the implementation of load beams and strain gauges. Each load sensor  44  may provide one or more signals to electronic control unit  42  indicative of the load bearing on vehicle  12  or a portion thereof. 
     Electronic control unit  42  may initiate pressure adjustment in tires  28  of vehicle  12  in response to signals from load sensors  44  in a variety of ways. For example, electronic control unit may cause an increase or decrease in the pressure in one or more tires  28  responsive to a corresponding increase or decrease in vehicle load based on a variety of linear or non-linear functions. One or more tire deflection tables may be stored in a memory, such as memory  114 , and accessed by electronic control unit  42  responsive to the signals from load sensors  44 . 
     Speed sensor  46  measures the speed of vehicle  12  to further control deflection levels for tires  28 . High deflection levels can create safety concerns and reduce tire life if maintained while vehicle  12  is operating at relatively high speeds. Speed sensor  46  is conventional in the art and provides a signal to electronic control unit  42  corresponding to speed. 
     Operator control device  48  may allow the operator of vehicle  12  to exert at least some level of control over system  10 . Device  48  is conventional in the art and may include a plurality of input/output devices, such as a keypad, touch screen, switches or similar input devices, and a display screen, sound generator, lights or similar output devices. Thus, device  48  permits an operator of vehicle  12  to transmit control signals to electronic control unit  42  to adjust pressure levels within the tires  28  of vehicle  12 . The control signals may, for example, correspond to deflection levels for tires  28  of vehicle  12 . As a result, the operator is able to adjust the deflection level of the tires  28  to correspond to the terrain over which vehicle  12  is traveling. Such control is desirable to provide improved floatation and traction on certain terrain. 
     The sequencing and interaction of components of system  10  may be appreciated more readily in the context of the following description of the present method. 
     FIG. 4 diagrammatically shows a flow chart of an embodiment of the present method. This embodiment could be called within the execution of a master tire pressure maintenance program (not shown). For example, control unit  42  would pass control to step SO when a routine of the master tire pressure maintenance program informed control unit  42  that a current pressure value corresponding to the pressure measured in at least one of tires  28  is less than an operator-configured, or operator designated, target pressure. Thereafter, control unit  42  passes control to step S 10 . 
     At step S 10 , the invention provides for determining whether a line leak fault exists. Control unit  42  evaluates whether a line leak flag was set by a line leak testing routine (not shown) which determines whether the conduit assemblies exhibit an incapacity to maintain fluid pressure, for example, due to a rupture. If control unit  42  determines that a line leak flag has been set, control unit  42  passes control along branch B 10  to step S 25 , described below. If control unit  42  determines that a line leak flag has not been set, control unit  42  passes control along branch B 15  to step S 15 . 
     At step S 15 , the invention provides for determining whether the current pressure value is within a tolerance range, such as within 0-10% of a target pressure. If the current pressure value is within the tolerance range, or less than 10% below the target pressure, control unit  42  passes control along branch B 20  to step S 25 , described below. If current pressure value is not within the tolerance range, or greater than 10% below the target pressure, in consideration of the regular frequent master tire pressure maintenance program cycles, such is indicative of a problem. Consequently, control unit  42  passes control along branch B 25  to step S 20 . 
     At step S 20 , the invention provides for illuminating a lamp as an alarm to indicate that a problem may exist with respect to system  10  or tires  28 . Alternatively, step S 20  may provide for for otherwise alerting an operator that immediate servicing may be required. Control unit  42  thereafter passes control to step S 25 . 
     At step S 25 , the invention provides for inflating any or all of tires  28 . Control unit  42  instructs solenoid  82  and any of solenoids  86 ,  88  and/or  90  to open, and solenoid  84  to close, thereby opening fluid communication between fluid source  32  and tires  28 , as described above. Control unit  42  then passes control to step S 30 . 
     At step S 30 , the invention provides for determining whether the inflating time exceeds a manufacturer-configurable, or manufacturer determined, inflating time limit. For example, if a line leak exists or fluid source  32  is supplying fluid at an inadequate pressure, inflating may extend indefinitely without tires  28  ever attaining the target pressure. If allowed to attempt to inflate tires  28  indefinitely, pressurized fluid in the conduit assemblies would act against the rotary seal assembly  50 , which would suffer excessive wear and fail prematurely. Accordingly, if the inflating time is greater than the inflating time limit, control unit  42  passes control along branch B 35  to step S 40 , described below. If the inflating time is not greater than the inflating time limit, control unit  42  passes control along branch B 30  to step S 35 . 
     At step S 35 , the invention provides for delaying further action for a predetermined time. This provides sufficient time for the fluid flow to stabilize. Control unit  42  then passes control to step S 50 , described below. 
     At step S 40 , the invention provides for establishing that a fault exists. Control unit  42 , for example, may set a fault flag then pass control to step S 45 . 
     At step S 45 , the invention provides for returning control to the master tire pressure maintenance program. 
     At step S 50 , the invention provides for determining whether the dynamic gage pressure in the conduit assemblies is greater than an operator-configurable, or operator determined, target tire pressure. During steps S 25 , S 30 , S 35 , S 50  and S 60 , tires  28  undergo dynamic inflation whereby the pressure thereof rapidly approaches the target pressure. However, the dynamic pressure measured necessarily will be higher than a static pressure measured after fluid flow is terminated and pressure among the conduit assemblies and tires  28  equalizes. Thus, dynamically inflating tires  28  until the dynamic pressure measured is greater than the target will not result in a static pressure that is greater than the target, rather close to, but less than the target pressure. Thereafter, if needed, according to the invention, subsequent inflation to close the gap between the equalized static pressure and the target pressure occurs by supplying one or more short pulses of compressed fluid from fluid supply  32  to tires  28 , each pulse being followed by an equalization delay and static pressure measurement, as described below. The additional pressurized fluid required for realizing a static pressure equal to the target pressure may be discretized, or determined, for example, based on inflating and settling characteristics of system  10  stored in and interpolated from a look up table retained in a memory of control unit  42 . Accordingly, if measured dynamic pressure is not greater than the target pressure, control unit  42  passes control along branch B 45  to step S 60 , described below. If measured dynamic pressure is greater than the target pressure, control unit  42  passes control along branch B 40  to step S 55 . 
     At step S 55 , the invention provides for determining whether a line leak fault exists. Step S 55  is similar to step S 10 , described above, therefore described no further herein. If control unit  42  determines that a line leak flag has been set, control unit  42  passes control along branch B 60  to step S 70 , described below. If control unit  42  determines that a line leak flag has not been set, control unit  42  passes control along branch B 65  to step S 75 , described below. 
     At step S 60 , the invention provides for determining whether the dynamic inflating time exceeds a dynamic inflating time limit. The dynamic inflating time limit is manufacturer-configurable or manufacturer determined. This ensures that fluid source  32  is capable of supplying fluid at an adequate pressure to increase tire pressure. Accordingly, if the dynamic inflating time is not greater than the dynamic inflating time limit, control unit  42  passes control along branch B 50  back to step S 25 , described above. If the dynamic inflating time is greater than the dynamic inflating time limit, therefore control unit  42  passes control along branch B 55  to step S 65  to determine whether sufficient fluid source pressure exists to continue dynamic inflation. 
     At step S 65 , the invention provides for measuring the gage pressure in the conduit assemblies and storing the value as a dynamic pressure variable. This establishes a reference pressure value which, following a fluid source pressure check, in step S 80 , is compared against a pressure value offset therefrom, as discussed below. Control unit  42  then passes control to step S 80 , described below. 
     At step S 70 , the invention provides for determining whether the gage pressure of the conduit assemblies is greater than the sum of a manufacturer-configured, or manufacturer designated, target pressure plus a manufacturer-configured, or manufacturer designated, dynamic inflation overshoot amount. The overshoot amount added to the target pressure amount is configured to counteract typical losses occasioned by line leaks. In other words, since, at step S 55 , existence of a line leak is presumed, the method compensates for fluid, hence fluid pressure, lost due to the line leak by executing an overinflation routine. The overinflation routine does not terminate until the conduit assemblies exhibit a pressure value that, absent a line leak, would be likely to realize a pressure in tires  28  in excess of the target, but, as a consequence of the line leak, may be likely to realize a pressure in tires  28  that equals or approaches the target. Accordingly, if gage pressure is greater than the target, overinflation may not be needed to compensate for line leaks, therefore control unit  42  passes control along branch B 80  to step S 45 , as described above. However, if gage pressure is not greater than the target, overinflation may be required to compensate for line leaks, therefore control unit  42  passes control along branch B 85  to step S 100 , described below. 
     At step S 75 , the invention provides for allowing the conduit assemblies and tires  28  to reach equilibrium. Control unit  42  instructs supply solenoid  82  to close, but allows control solenoids  86 ,  88  and/or  90  to remain open, thereby maintaining fluid communication among the conduit assemblies and tires  28  without having additional pressurized fluid introduced thereto. Control unit  42  then passes control to step S 90 , described below. 
     At step S 80 , the invention provides for determining the pressure of fluid available from fluid source  32 . Such may be achieved, for example, by calling a subroutine (not shown) which returns a supply fluid pressure value. Control unit  42  then passes control to step S 85 . 
     At step S 85 , the invention provides for determining whether the dynamic pressure variable, as described with respect to step S 65 , is less than the difference of the supply fluid pressure value, described with respect to step S 80 , less a manufacturer-configured, or manufacturer designated, dynamic inflation offset amount. The offset ensures that fluid source  32  is greater than, thus able to increase, the pressure of tires  28 , as opposed to merely statically pressurizing rotary seal assembly  50 . Accordingly, if the dynamic pressure variable is less than the offset supply fluid pressure value, control unit  42  passes control along branch B 70  to branch B 25  and then to step S 20 , as described above. If the dynamic pressure variable is not less than the offset supply fluid pressure value, system  10  exhibits normal functioning characteristics, therefore control unit  42  passes control along branch B 75  to branch B 50  and then to step S 25 , described above. 
     At step S 90 , the invention provides for measuring the equilibrium gage pressure in the conduit assemblies and storing the value as a static pressure variable for use in subsequent step S 95 . Control unit  42  then passes control to step S 95 . 
     At step S 95 , the invention provides for determining whether the static pressure variable is greater than or equal to the target pressure. If the static pressure variable is greater than or equal to the target pressure, system  10  exhibits normal functioning characteristics, therefore control unit  42  passes control along branch B 90  to step S 110 , described below. If the static pressure variable is not greater than or equal to the target pressure, incremental inflation is required to bring tires  28  up to target pressure, as described above with respect to step S 50 , therefore control unit  42  passes control along branch B 95  to step S 100 . 
     At step S 100 , the invention provides for inflating for a discrete, manufacturer-configured, or manufacturer designated, inflation time. Control unit  42  opens solenoid  82  and any of solenoids  86 ,  88  and/or  90 , and closes solenoid  84 , thereby fostering fluid communication between fluid source  32  and tires  28 . After the inflation time, control unit  42  passes control to step S 105 . 
     At step S 105 , the invention provides for determining tire pressure. Like step S 80 , described above, such may be achieved, for example, by calling a subroutine (not shown) which returns a tire pressure value. The invention then provides for cycling through the method once again to bring tire pressure up to the target pressure, therefore control unit  42  passes control to step S 110 , described above. 
     At step S 110 , the invention provides for extinguishing the lamp illuminated in step S 20 . Thus conditions identified steps preceding step S 20  which suggested a problem with respect to system  10  or tires  28  are deemed to have been corrected, thus not regarded as symptomatic of failures in system  10  or tires  28 . Control unit  42  then passes control to step S 45 , described above. 
     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it is well understood by those skilled in the art that various changes and modifications can be made in the invention without departing from the spirit and scope of the invention.