Patent Publication Number: US-9840240-B2

Title: Trailer backup aid speed limiting via braking

Description:
CROSS-REFERENCE TO THE RELATED APPLICATION 
     This application is a continuation-in-part of U.S. patent application Ser. No. 14/682,204, filed Apr. 9, 2015, and entitled “TRAILER BACKUP AID SPEED LIMITING VIA BRAKING,” now U.S. Pat. No. 9,744,972, the entire disclosure of which is hereby incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to systems for controlling vehicle parameters during vehicle guidance of a trailer, such as in a trailer backup assist system. In particular, various systems are disclosed for controlling the speed or a vehicle during use of a trailer backup assist system. 
     BACKGROUND OF THE INVENTION 
     Reversing a vehicle while towing a trailer can be challenging for many drivers, particularly for drivers that drive with a trailer on an infrequent basis or with various types of trailers. Systems used to assist a driver with backing a trailer can control various vehicle systems to attempt to keep the speed of the vehicle below a limit where such systems become unreliable, particularly at preventing the trailer from converging toward a jackknife angle or the like. Further advances in such systems may be desired. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, a trailer backup assist system for a vehicle reversing a trailer is provided. The system includes a brake system and a throttle sensor module outputting a throttle application signal. A controller outputs a brake torque request to the brake system based at least in part on a trailer mass and the throttle application signal. 
     According to another aspect of the present invention, a trailer backup assist system for a vehicle reversing a trailer is provided. The system includes a brake system and a steering system of the vehicle. A controller is configured to output a brake torque request to the brake system and a steering command to the steering system, wherein the brake torque request and the steering command are each based at least in part on a trailer mass. 
     According to another aspect of the present invention, a method of reversing a trailer towed by a vehicle is provided. The method includes the steps of determining a trailer mass, outputting a brake torque request to a brake system of the vehicle, and outputting a steering command to a steering system of the vehicle if it&#39;s determined that the trailer is being reversed along a straight path. The brake torque request and the steering command are each based at least in part on the trailer mass. 
     These and other aspects, objects, and features of the present invention will be understood and appreciated by those skilled in the art upon studying the following specification, claims, and appended drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1  is a schematic depiction of an example vehicle and trailer; 
         FIG. 2  is a schematic depiction of the vehicle and the trailer of  FIG. 1  after reversing; 
         FIG. 3  is a schematic block diagram of a portion of a system for assisting the vehicle in reversing the trailer and including functionality for limiting the speed of the vehicle; 
         FIG. 4  is a flowchart showing a method for limiting the speed of the vehicle, including by determining a road grade beneath the trailer of  FIGS. 1 and 2 ; 
         FIG. 5  is a schematic block diagram of a portion of an alternative system for assisting the vehicle in reversing the trailer and including functionality for limiting the speed of the vehicle; 
         FIG. 6  is a flowchart showing an alternative method for limiting the speed of the vehicle, including by dynamically adjusting a target speed of the system. 
         FIG. 7  is a schematic block diagram of a system for limiting the speed of a vehicle reversing a trailer based on a trailer mass, according to one embodiment; 
         FIG. 8  is a schematic block diagram of an alternative embodiment of the system shown in  FIG. 7 ; 
         FIG. 9  is a schematic block diagram of a trailer backup assist system according to one embodiment; and 
         FIG. 10  is a flowchart showing a method of reversing a trailer towed by a vehicle according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     For purposes of description herein, the terms “upper,” “lower,” “right,” “left,” “rear,” “front,” “vertical,” “horizontal,” and derivatives thereof shall relate to the invention as oriented in  FIG. 1 . However, it is to be understood that the invention may assume various alternative orientations, except where expressly specified to the contrary. It is also to be understood that the specific devices and processes illustrated in the attached drawings, and described in the following specification are simply exemplary embodiments of the inventive concepts defined in the appended claims. Hence, specific dimensions and other physical characteristics relating to the embodiments disclosed herein are not to be considered as limiting, unless the claims expressly state otherwise. 
     Referring to  FIGS. 1-3 , reference numeral  10  generally designates a vehicle that includes a system  12  for assisting vehicle  10  in backing up a trailer  14  coupled therewith. System  12  includes a brake system  16  ( FIG. 3 ) and a throttle sensor  22  ( FIG. 3 ) that outputs a signal  24  relating to an amount of throttle being applied. System  12  further includes a controller  30  that estimates a road grade RG t  beneath the trailer  14  and outputs a brake torque request  34  to the brake system  16  based on the estimated road grade RG t  and the throttle application signal  24 . 
     Referring to  FIG. 1 , vehicle  10  is shown in an example scenario towing trailer  14 . An arm  18  of trailer  14  extends toward and couples trailer  14  with vehicle  10  via a hitch (not shown) on the rear of vehicle  10 . In this example, the vehicle  10  is reversing to move the trailer  14  from the position of  FIG. 1  to the position of  FIG. 2 . In the example of  FIGS. 1 and 2 , the vehicle  10  is a truck and the trailer  14  is a boat trailer and the reversing of vehicle  10  may be so as to move trailer  14  into in a body of water at a boat lift, for example. The reversing may be carried out using system  12 , which is generally configured to assist a driver of vehicle  10  in various ways in reversing vehicle  10  and trailer  14 . In one example, such a trailer backup assist system  12  can includes both actions carried out by the driver of vehicle  10  as well as by system  12 . In particular, the driver may initiate system  12  after driving vehicle  10  along a path to a desired location at which the reversing is to begin and placing vehicle  10  in reverse. Once system  12  is activated, the driver may, for example, select a desired vehicle curvature using an input device (such as a dedicated knob or, in some examples, the steering wheel (not shown) of vehicle  10 ), while simultaneously controlling the longitudinal motion (i.e., speed) of vehicle  10  using the throttle and brakes. In general, system  12  executes an operating routine to determine if the desired curvature can be safely executed, which may mean that the desired curvature will maintain the hitch angle (i.e., an angle defined between the vehicle  10  and the trailer  14  along a lateral plane at the point of coupling therebetween) below a “jackknife angle.” In general, a jackknife angle is described as an angle at which a maximum steering input in either direction will fail to decrease the hitch angle. System  12  causes vehicle  10  to steer automatically, such as by control of an electronic power assisted steering (“EPAS”) system, to implement either the desired curvature or a modified curvature determined to be appropriate for preventing a jackknife condition, which may be determined by controller  30 . 
     As mentioned, while system  12  is causing vehicle  10  to automatically steer to maintain an appropriate curvature, the driver may maintain the general responsibility for controlling the longitudinal motion of vehicle  10  using the throttle and brakes. Initially, such control should cause vehicle  10  to begin rearward motion. As vehicle  10  accelerates, it may be generally the responsibility of the driver to maintain sufficient vehicle speed until a desired position is reached based on the curvature along which system  12  steers vehicle  10 . Upon vehicle  10  reaching the desired location, the driver may slow vehicle  10  by reducing throttle position and applying brake torque before placing vehicle  10  in park and deactivating system  12 , at which point system  12  relinquishes control of the steering system. 
     The speed at which vehicle  10  travels while system  12  steers, however, can affect the ability of system  12  to avoid a jackknife condition or other adverse conditions. In particular, at higher vehicle speeds, the dynamics of the yaw rate of trailer  14  with respect to that of vehicle  10  and, accordingly, the hitch angle may occur at a rate that is too fast for system  12  to react to avoid a hitch angle increase to or beyond a jackknife angle, as explained above. Accordingly, it may be desirable for system  12  to be able to determine if the speed of vehicle  10  is at or is approaching a threshold at which system  12  may be unable to reliably control the hitch angle and to act to slow vehicle  10 , if necessary. Further, it is noted that an EPAS system may only function to control the steering of vehicle  10  while vehicle  10  is traveling below a cutoff speed. Conversely, it may also be useful for system  12  to allow the driver to utilize as much of the speed band as possible for purposes of flexibility and sense of control. 
     Accordingly, systems such as system  12  can include the ability within controller  30  to limit the speed of vehicle  10  by automatically applying the brakes, via an input to the vehicle brake system  16 . A controller  30  can be configured for speed limiting by the incorporation of a proportional-integral-derivative (“PID”) controller  42  to monitor the difference between the vehicle speed and the target speed (such difference being designated a speed error) to request a brake torque request that will be sent to the brake system  16 . This brake system  16  in turn applies the brakes appropriately, which alters the vehicle speed and the speed error  40 . For the purposes of speed limiting within a system such as system  12 , the desired response is a system that quickly limits the vehicle speed to the target speed with very little overshoot. It is noted that minimizing overshoot overall, as opposed to simply reducing overshoot quickly is desired, as the vehicle speed is desirably maintained below the EPAS cutoff speed, for example, at all times, but flexibility through increased speed availability may also be desired. Accordingly, system  12  is configured to adjust to the outside disturbances of road grade and throttle apply input, which may be the most likely disturbances to significantly affect system  12  and the overall speed of vehicle  10 . 
     It is for this reason that system  12  uses feed forward tables based on a road grade estimate  32  and the amount of throttle applied (“throttle apply”) to increase the robustness of the speed limiting controller for use in system  12 , as shown in  FIG. 3 . In particular, system  12  is configured such that controller  30  receives a vehicle speed input  28  from speed detector  26 , which is compared with a vehicle target speed  38 , which may be stored in memory  36 , to arrive at a speed error signal  40 , which is input into PID controller  42  to arrive at an initial brake torque request signal  43 . Simultaneously, system  12  can, using sensor assembly  20  (and possibly various other inputs, as described further below) estimate the road grade RG t  below trailer  14  to determine if additional brake torque is desirable. In general, such additional torque can be added to the initial brake torque demand signal  43  to compensate for an additional load on vehicle  10  by trailer  14  being on an increased road grade (i.e. an additional disturbance). An additional torque demand can be correlated with variation in road grade RG t  in feed forward tables stored in memory within controller  30  and can vary with trailer  14  weight, brake system  16  parameters, desired response characteristics of system  12  and the like. 
     Still further, controller  30  can receive a throttle apply input  24  from throttle sensor  22  and can determine a desired additional brake torque demand corresponding to an amount of disturbance (if any) affecting system  12  due to an increased throttle application by the driver. An additional brake torque demand can be correlated with variation in throttle in another feed forward table stored in memory  36  within controller  30  and can vary with engine characteristics, engine control settings, desired response characteristics of system  12 , and the like. The feed forward gain added to the initial brake torque demand  43  can result in a modified brake torque request  34  that can be output from controller  30  to brake system  16  to slow vehicle  10  appropriately. 
     Referring to  FIG. 4 , a method  50  for controlling the speed of vehicle  10  using the system  12 , is described, along with example steps by which controller  30  can estimate the road grade RG t  beneath trailer  14 . For a given vehicle towed, its associated road grade is, generally, a grade (or slope) of an area beneath the vehicle. Road grade can be expressed as a percentage of variation from a horizontal (zero) grade Hg. A road grade RG v  beneath vehicle  10  vehicle is a grade of an area of the road beneath vehicle  10 . A trailer road grade RG t  is a grade of a road beneath the trailer  14 . In the example of  FIGS. 1 and 2 , the trailer road grade RG t  and the vehicle road grade RG v  are the same in  FIG. 1  and different in  FIG. 2 . In  FIG. 2 , the trailer road grade RG t  is greater than the vehicle road grade RG v . 
     As discussed above, vehicle  10  includes a sensor assembly  20  that monitors the vehicle road grade RG v . The assembly may include accelerometers, wheel speed sensors, and the like, that may monitor the vehicle road grade RG v  according to known methods. The vehicle  10  further includes controller  30  coupled to the sensor assembly  20 . The controller  30  is a specialized controller and includes programming to estimate the trailer road grade RG t  based, in part, on the vehicle road grade RG v . The controller  30  and sensor assembly  20  together provide a trailer road grade assembly or system  12  for controlling the trailer  14 . Although described as road grade, it is to be understood that a traditional road is not required for there to be a road grade. Road grade refers generally to the area underneath a vehicle  10  whether that area is a road or that area is an off-road. 
     With continuing reference to  FIG. 4 , an example trailer road grade estimating and speed control method  50  includes the step  52  of estimating a first total mass of vehicle  10  and trailer  14 . The step  52 , thus, establishes a reference total mass. Notably, road grade RG t  beneath the trailer  14  is one of the variables used to estimate the total mass. In this example, changes in the total mass are attributed to changes in the road grade RG t  beneath the trailer  14  relative to the road grade RG v  beneath vehicle  10 . Subsequently, at a step  54 , the method  50  estimates a second total mass of vehicle  10  and trailer  14 . The step  54  occurs after movement in a reverse direction of the trailer  14  such as from a first position (e.g. as shown in  FIG. 1 ) to a second, different position (e.g. as shown in  FIG. 2 ). At a step  56 , the method  50  calculates whether the first total mass is different than the second total mass. If not, the method  50  returns to the step  54  and estimates another second total mass after more movement of the trailer  14 . 
     If the second total mass is different than the first total mass, the method  50  moves to step  58 . At step  58  the difference between the first total mass and the second total mass are used to calculate the road grade beneath the trailer  14 . The method  50  may then correlate the road grade RG t  beneath trailer  14  with an additional brake torque demand in step  60  before feeding such additional brake torque demand to an initial brake torque demand  43  from PID controller  42  in step  62 , which may be effective to adjust the response of system  12  to an overspeed condition, such as by automatically applying additional braking force that that which would otherwise be demanded by controller  30 . As discussed above, the method  50  can also feed forward values based on torque apply to a brake torque demand from PID controller  42  to further adjust the final brake torque demanded by controller  30 . 
     In a more specific example of the method  50 , the step  52  includes estimating the total mass of the vehicle  10  and the trailer  14  using the equation: 
     
       
         
           
             
               
                 M 
                 c 
               
               = 
               
                 
                   
                     T 
                     pt 
                   
                   - 
                   
                     T 
                     brk 
                   
                 
                 
                   
                     R 
                     w 
                   
                   ⁢ 
                   
                     a 
                     x 
                     s 
                   
                 
               
             
             , 
           
         
       
         
         
           
             where: 
             M c  represents the total unit mass of the vehicle  10  added to the total mass of the trailer  14 ; 
             R w  represents the wheel radius; 
             a x   s  represents an acceleration output from an accelerometer; 
             T pt  represents a torque output from a powertrain of the vehicle  10 ; and 
             T brk  represents a braking force output from a frictional brake of the vehicle  10 , the trailer  14 , or both. 
           
         
       
    
     The above equation may be utilized to calculate total mass when, for example, the vehicle  10  and trailer  14  are moving forward. If the vehicle  10  and the trailer  14  stray from forward movements and, for example, begin to reverse, the example method  50  uses an alternative formula to instantaneously estimate mass the of the vehicle  10  and the trailer  14 . The equation below shows an example formula that demonstrates relationships between variables when the vehicle  10  and the trailer  14  are reversing: 
     
       
         
           
             
               
                 
                   M 
                   ⋓ 
                 
                 c 
               
               = 
               
                 
                   
                     M 
                     c 
                   
                   + 
                   
                     
                       m 
                       tlr 
                     
                     ⁢ 
                     g 
                     ⁢ 
                     
                       
                         ( 
                         
                           
                             sin 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               α 
                               
                                 r 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 2 
                               
                             
                           
                           - 
                           
                             sin 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               α 
                               
                                 r 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 1 
                               
                             
                           
                         
                         ) 
                       
                       
                         a 
                         x 
                         s 
                       
                     
                   
                 
                 = 
                 
                   
                     
                       T 
                       pt 
                     
                     - 
                     
                       T 
                       brk 
                     
                   
                   
                     
                       R 
                       w 
                     
                     ⁢ 
                     
                       a 
                       x 
                       s 
                     
                   
                 
               
             
             , 
           
         
       
         
         
           
             where: 
             {hacek over (M)} c  is the instantaneously estimated mass for the vehicle  10  plus the trailer  14 ; 
             m tlr  is a mass of the trailer  14 ; 
             α r1  is a road grade under the vehicle  10 ; 
             g represents the gravity of earth; and 
             α r2  is the road grade under the trailer  14 . 
           
         
       
    
     Changes in the instantaneously estimated mass {hacek over (M)} c  as the vehicle  10  and the trailer  14  reverse are used to determine the grade α r2  under the trailer  14 . To derive the grade α r2  under the trailer  14 , the changes in the instantaneously estimated mass {hacek over (M)} c  are determined using the equation:
 
 RG   load   +   =m   tlr   g (sin α r2 −sin α r1 )=({hacek over (M)} c   −M   c ) a   x   s ,
         where RG load   +  represents changes in load due to changes in road grade beneath the vehicle  10  relative to the trailer  14 .
 
The estimated road grade α r2  beneath the trailer  14 , is then determined using the equation:
       

     
       
         
           
             
               
                 α 
                 
                   r 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   2 
                 
               
               = 
               
                   
               
               ⁢ 
               
                 arcsin 
                 ⁡ 
                 
                   [ 
                   
                     
                       
                         
                           ( 
                           
                             
                               
                                 M 
                                 ⋓ 
                               
                               c 
                             
                             - 
                             
                               M 
                               c 
                             
                           
                           ) 
                         
                         ⁢ 
                         
                           a 
                           x 
                           s 
                         
                       
                       
                         
                           ( 
                           
                             
                               M 
                               c 
                             
                             - 
                             
                               m 
                               trk 
                               * 
                             
                           
                           ) 
                         
                         ⁢ 
                         g 
                       
                     
                     + 
                     
                       sin 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         α 
                         
                           r 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           1 
                         
                       
                     
                   
                   ] 
                 
               
             
             , 
           
         
       
         
         
           
             where m* trk  is an estimated mass of the vehicle  10 .
 
The mass of the vehicle  10  may be determined by weighing the vehicle  10  or through some other technique, for example. A first technique uses a constant value of the truck curb weight as m* trk . Such a nominal value may be evaluated during or after assembling the vehicle  10  at a factory and may be based on the standard truck loading condition. In such an example, m* trk  would remain constant. In a second example technique, m* trk  may be an estimated mass based on the vehicle  10  mass for a specific trip. This is useful when, for example, the vehicle  10  is periodically heavily loaded with cargo. This second technique may provide a better estimate than the constant of the first technique. The mass of the vehicle  10  for the second example technique may be obtained using many different methods. An example is to estimate mass of the vehicle  10  using active suspension sensor. Deflection of the active suspension sensor at a steady state tells the load variation on truck unit. The estimated truck unit mass m* trk  in such an example will be the truck curb weight plus the indicated load weight from the suspension deflection.
 
           
         
       
    
     In some examples, the estimated road grade can be used to calculate a total road grade torque exerted on the vehicle  10 . This total road grade torque, represented as τ rgl , can be calculated using the equation:
 
τ rgl =( M   c   −m*   trk ) g  sin α r2   +m*   trk   g  sin α r1 .
 
The total road grade torque can, as described above, be used as a feedforward to derive a compensating torque to control the backup speed of the trailer  14  during an automatic backup procedure. As also mentioned previously, additional feedforward compensating torque for backup speed control can also be provided by the torque apply signal  24 .
 
     With respect to  FIG. 5 , another embodiment of a system  112  including a controller  130  for assisting in maintaining the speed of vehicle  10  below at maximum level when reversing a trailer  14 , including under various forms of automated assistance from system  112 , is described. In particular, controller  130  operates using a PID controller  142  in a manner similar to that described above with respect to  FIG. 3 , where PID controller  142  provides a brake torque request  134  to brake system  116  to attempt to slow vehicle  10  to reduce an error signal  140  between a detected speed  128  and a target speed  150 . However, controller  130  can employ a dynamic adjustment of the vehicle speed error to adjust the controller for steady state error or variation in overshoot. Such a controller  130  can be used in a vehicle  10  that is not configured for providing an estimate for the road grade below trailer  14  or can be incorporated into the above described system  12  to provide for robust overshoot control in a condition where a road grade estimate is not available (such as when the associated system has not yet accumulated enough data to implement the above equations or the like). 
     System  112 , in particular, dynamically adjusts the target vehicle speed to force the controller to come back to the desired steady state speed based on the effect the adjustment on the target speed has on the speed error  140 . In particular controller  130  includes the ability, illustrated in module  144  to receive as input the vehicle speed  128  from speed detector  126 , which can be compared against a predetermined condition in the form of an initial (non-adjusted) target speed plus a predetermined maximum allowable error (which may be referred to as a “threshold speed”). Module  144  can then determine if the vehicle speed plus the maximum error is less than the non-adjusted target speed plus the maximum error. If such a condition is present, module  144  can maintain a “NoAdj” mode, in which the non-adjusted target speed is output from memory  136  for use in the error calculation for output of the error  140  to PID controller  142 . If module  144  determines that the current speed  128  plus the maximum error is greater than the target speed plus the maximum error, an adjusted (lowered) target speed can be substituted for the predetermined target speed in determining the error  140  provided to PID controller  142 . A dynamically lowered error  140  increases the brake torque request  134  output by controller  142 , which forces system  112  to lower the speed of vehicle  10  faster than it would using the non-adjusted target speed. 
       FIG. 6  illustrates a method  166  by which system  112  can operate to attempt to regulate the speed of vehicle  10  during an assisted backup operation. In particular, once initiated in step  168 , system  112  operates with controller  130  utilizing the actual, non-adjusted target speed to send to PID controller  142  (step  170  ). If, however, in step  172  module  144  determines that the detected speed  128  of vehicle  10  is greater than the target speed plus the maximum error, module  144  can cause controller  130  to transition to a “timer” state. As the maximum error may be the maximum amount of error that is desired for the steady state behavior of the controller, the timer state is used as a timeout period to ensure that the overshoot of the controller does not affect the steady state behavior by lowering the target speed in response to controllable overshoot. Accordingly the delay in step  174  may correlate with the response time of PID controller  142  or other, related parameters of system  112 . If the detected speed  128  is brought back down such that the speed  128  plus the maximum error is lower than the non-adjusted target speed plus the maximum error before the delay in step  174  is over, then the system transitions back to the “NoAdj” state (step  170 ). 
     If the speed  128  is still such that speed  128  plus the maximum error is greater than the non-adjusted target speed plus the maximum error after the delay  174  is over, then the system  112  in step  176  transitions to an “Adjustment” state. In such a state, the target speed  150  that is fed into the PID controller  142  is substituted with a downward adjusted target speed  148  to pull the steady state speed back towards the actual target speed. If after another delay period (step  180 ) the speed is still high (as determined in step  182 ), the adjusted target speed  148  will be adjusted downward again (step  178 ). This will continue until the speed  128  is within the determined range, as determined in step  182 . 
     If the speed  128  drops such that the speed plus the maximum error is below the non-adjusted target speed plus the maximum, such as when the driver is applying the brakes to slow down or the trailer  14  is no longer on a higher road grade area than vehicle  10 , the system  112  transitions into a “slowrise” state  184 . This state is designed to slowly raise the adjusted target speed  148  back up to the non-adjusted target speed at a controlled rate. The slow raising of the adjusted target speed  148  can help prevent undesirable behavior in the controller  130 . Finally, once the adjusted target speed  148  reaches the non-adjusted target speed again, the system  112  reenters the “NoAdj” state (step  170 ) until system  112  is deactivated. 
     Referring to  FIG. 7 , another embodiment of a system  212  for limiting the speed of a vehicle  10  based on a mass of trailer  14  is described. The system  212  includes controller  230 , which receives a vehicle speed input  228  from speed detector  226 . Vehicle speed input  228  is compared with a vehicle target speed  238  stored in memory  236  to arrive at a speed signal error  240 . Speed signal error  240  is inputted into PID controller  242  to ultimately arrive at brake torque request  234 , which is outputted from controller  230  to brake system  216  to slow vehicle  10  appropriately. In the illustrated embodiment, PID controller  242  may be modified directly based on the mass of trailer  14 . According to one embodiment, PID controller  242  is tuned for different trailer masses and the gains of PID controller  242  are determined using lookup tables, which may be stored to a memory (e.g.,  236 ) of controller  230 . 
     Alternatively, with reference to  FIG. 8 , controller  230  may be configured as a feed forward controller where PID controller  242  outputs an initial brake torque request signal  243  that is added to a feed forward trailer mass gain  250  that may be supplied from trailer mass estimation module  252  and helps compensate for variations in trailer mass. The PID controller  242  may be nominally tuned and the value of the feed forward trailer mass gain  250  will depend on the mass of trailer  14 . While not shown in  FIG. 7 or 8 , it should be appreciated that brake torque request  234  may include contributions from a road grade estimate, a throttle apply input, and/or other considerations described previously herein. 
     Referring to  FIG. 9 , controller  230  is shown in one embodiment of a system  312  for assisting a vehicle  320  backing a trailer  325 , also referred to herein as a trailer backup assist system. Controller  230  may be configured similarly to that shown in either  FIG. 7 or 8  and includes a memory  332  having instructions  334  stored thereon. The instructions  334  may be tangibly embodied as non-transitory computer readable medium and are executable by a processor  336 . The instructions are configured to cause the processor  336  to carry out operations for generating a steering command  338  for a steering system  340  of the vehicle  320 , which may include an electric power assisted steering (EPAS) system. Additionally, the instructions  334  are configured to cause the processor  336  to carry out operations for generating a brake torque request  234  to a brake system  216  of the vehicle, as described previously herein. 
     The steering command  338  may be generated in part based on a hitch angle and a kinematic relationship determined between the vehicle  320  and the trailer  325 . In turn, a power-steering system controls steered wheels of the vehicle  320  based on the steering command  338 . The steering of the vehicle  320  may be performed autonomously by the system  312  or manually via an input device such as a rotatable knob or steering wheel of the vehicle  320 . Additional information regarding trailer backup assist systems and the generation of a steering command is found in U.S. Patent Publication No. 2014/0379219 to Rhode et al., entitled “TRAILER BACKUP ASSIST CURVATURE CONTROL,” filed Sep. 10, 2014, the entire disclosure of which is incorporated herein by reference. 
     According to one embodiment, the steering command  338  and the brake torque request  234  are each generated, at least in part, as a function of a mass of the trailer  325 . This may be achieved by modifying the gains of the controller  230  to compensate for variation in the mass of the trailer  325 . Compensating for variation in trailer mass is particularly advantageous because trailers of different masses will behave differently while being reversed. For example, if backed along a curved trajectory, a lighter trailer will generally turn more quickly than a heavier trailer. Thus, if trailer mass is not compensated for, a driver along with other vehicle occupants will encounter inconsistent experiences when reversing trailers of different masses. To provide a more consistent experience, the gains of controller  230  may be increased or decreased based on how heavy or light the trailer  325 . In one embodiment, the controller  230  may be tuned for a particular trailer mass and the gains of the controller  230  may be increased if the trailer  325  is heavier or decreased if the trailer  325  is lighter. In so doing, the driver and any other vehicle occupants are provided a more consistent experience whenever the vehicle  320  reverses a trailer  325 , regardless of what the trailer mass is. As described herein, the gains of the controller  230  may be determined using lookup tables. 
     In operation, the mass of the trailer  325  may be determined in a variety of manners. According to one embodiment, a sensor system  345  operatively coupled to the trailer  325  determines the mass of the trailer  325  and sends the corresponding trailer mass information  350  to the controller  230 . Additionally or alternatively, the mass of the trailer  325  may be determined by first weighing the trailer  325  (e.g., via a weigh scale) and then using a user-input device  352  to send the corresponding trailer mass information  354  to the controller  230 . The trailer mass information  354  may be supplied to the controller  230  using a user-input device  352  located within the vehicle  320  such as a touchscreen display of a center console. It is also contemplated that the mass of the trailer  325  may also be supplied to the controller  230  using a portable electronic device configured to wirelessly communicate with the controller  230 . Such electronic devices may include smartphones, tablets, and the like. Additionally or alternatively still, the mass of the trailer  325  may be determined based on trailer dynamics while the vehicle  320  and trailer  325  are in motion, as described in U.S. Pat. No. 8,793,035 to Yu et al., entitled “DYNAMIC ROAD GRADIENT ESTIMATION,” filed Jan. 7, 2013 the entire disclosure of which is incorporated herein by reference. 
     Referring to  FIG. 10 , a method  400  for assisting a vehicle  320  in reversing a trailer  325  is described with continued reference to the system  312  disclosed in  FIG. 9 . The method  400  may be embodied as instructions  334  stored in memory  332  and executable by processor  336  of controller  230 . In describing the method  400 , it is assumed that the vehicle  320  and trailer  325  are about to engage in a reversing maneuver. For example, the method  400  may be initiated at step  410  when a driver of the vehicle  320  shifts into reverse or otherwise communicates his or her intent to perform a reversing of the trailer  325 . At step  420 , the mass of the trailer  325  is determined. As described herein, there are several ways in which the trailer mass can be computed such as, but not limited to, using sensor system  345 , weighing the trailer mass and inputting the trailer mass to the system  312  via user-input device  352 , or via calculations related trailer dynamics while the trailer  325  is in motion. Next, at step  430 , the controller  230  checks whether a straight backup maneuver is being performed, that is, whether the trailer  325  is being reversed in a substantially straight line. If so, the controller  230  generates a brake torque request as a function of the trailer mass at step  440 . As described herein, the brake torque request is sent to the brake system  216  to control the speed in which the vehicle  320  and the trailer  325  are being reversed. The brake torque request may additionally or alternatively be a function of other considerations described herein such as, but not limited to, a road grade estimate and/or a throttle application signal. If it is determined at step  430  that a straight backup maneuver is not being performed, the controller  230  generates a brake torque request along with a steering command, each being a function of trailer mass, as illustrated in steps  450  and  460 , respectively. As described herein, the steering command is sent to the steering system  340  to control steered wheels of the vehicle  320  while the trailer  325  is being reversed. As further described herein, the steering command may also be a function of a hitch angle and a kinematic relationship determined between the vehicle  320  and the trailer  325 . It should be appreciated that the controller  230  may modify the brake torque request and the steering command (when applicable) as needed so long as the trailer  325  is being reversed. Once the vehicle  320  and the trailer  325  are parked (e.g., the driver places the vehicle  320  in park), trailer backup assist functionality may come to an end at step  470 . 
     Accordingly, trailer backup assist system has been described herein that is configured to generate a brake torque request for limiting the speed of a vehicle and a steering command for controlling a steering system responsible for automatically steering the vehicle while the vehicle reverses the trailer. The brake torque request and the steering command may each be based at least in part on a mass of the trailer. In this manner, a more consistent driving experience is achieved by virtue of the trailer backup assist system being able to compensating for trailer mass during a backing maneuver. 
     It will be understood by one having ordinary skill in the art that construction of the described invention and other components is not limited to any specific material. Other exemplary embodiments of the invention disclosed herein may be formed from a wide variety of materials, unless described otherwise herein. 
     For purposes of this disclosure, the term “coupled” (in all of its forms, couple, coupling, coupled, etc.) generally means the joining of two components (electrical or mechanical) directly or indirectly to one another. Such joining may be stationary in nature or movable in nature. Such joining may be achieved with the two components (electrical or mechanical) and any additional intermediate members being integrally formed as a single unitary body with one another or with the two components. Such joining may be permanent in nature or may be removable or releasable in nature unless otherwise stated. 
     It is also important to note that the construction and arrangement of the elements of the invention as shown in the exemplary embodiments is illustrative only. Although only a few embodiments of the present innovations have been described in detail in this disclosure, those skilled in the art who review this disclosure will readily appreciate that many modifications are possible (e.g., variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters, mounting arrangements, use of materials, colors, orientations, etc.) without materially departing from the novel teachings and advantages of the subject matter recited. For example, elements shown as integrally formed may be constructed of multiple parts or elements shown as multiple parts may be integrally formed, the operation of the interfaces may be reversed or otherwise varied, the length or width of the structures and/or members or connector or other elements of the system may be varied, the nature or number of adjustment positions provided between the elements may be varied. It should be noted that the elements and/or assemblies of the system may be constructed from any of a wide variety of materials that provide sufficient strength or durability, in any of a wide variety of colors, textures, and combinations. Accordingly, all such modifications are intended to be included within the scope of the present innovations. Other substitutions, modifications, changes, and omissions may be made in the design, operating conditions, and arrangement of the desired and other exemplary embodiments without departing from the spirit of the present innovations. 
     It will be understood that any described processes or steps within described processes may be combined with other disclosed processes or steps to form structures within the scope of the present invention. The exemplary structures and processes disclosed herein are for illustrative purposes and are not to be construed as limiting. 
     It is also to be understood that variations and modifications can be made on the aforementioned structure without departing from the concepts of the present invention, and further it is to be understood that such concepts are intended to be covered by the following claims unless these claims by their language expressly state otherwise.