Abstract:
A system for managing braking torque in a machine uses a first braking system, a second braking system that is independent of the first braking system and a controller that monitors activity in both braking systems. The controller selectively reduces torque in the first braking system as torque in the second braking system increases to limit undesired effects of possible over-braking.

Description:
TECHNICAL FIELD 
     The present disclosure is generally directed to braking in a machine and more particularly to controlling braking via retarding in the presence of mechanical braking initiated by an operator. 
     BACKGROUND 
     Some large machines, particularly those associated with mining and other earthmoving operations, use dual braking systems. One configuration of such systems involves the use of an electric retarder, for example, an AC motor configured to generate electricity to slow rotation of an armature of the motor and thereby an axle coupled to the armature. As a backup to the electric motor, a conventional hydraulic braking system may also be installed in the machine. Because braking using a retarding mode in a traction motor provides a high retarding capability and reduces wear in mechanical brakes, the retarder may be the preferred system to use for ordinary braking. In the event of an electrical failure, the hydraulic braking system may also be used to slow or stop the machine. 
     It follows that both the electric retarder and the hydraulic braking system each have the braking capacity to bring the machine to a full stop in a worst-case operating situation, such as a mining truck with a full payload operating on a downhill grade. However, in a situation where the electric retarder is already supplying some measure of braking force and the hydraulic braking system is activated by an operator, up to double the braking force required to bring the machine to a safe stop can be applied, sometimes with several negative consequences. 
     First, because the electric retarder is generally mounted on an inboard portion of an axle and the hydraulic brake is generally mounted on an outboard portion of the axle or a wheel, the difference in brake torque between the electric retarder and the hydraulic brake can cause a sudden and excessive torsional shock to the driveline and axle. Second, the brake force can be so strong that the machine&#39;s pitch and bounce modes are excited and the rear wheels of the machine may actually jump off the ground and bounce causing at least stress if not damage to tires, wheels, axles, and other drive train components. 
     With respect to braking system performance management, U.S. Patent Publication 2012/0175200 to Ford Global Technologies discloses a system that allows a user to configure a preferred braking profile and, responsive to a brake pedal position signal, provide brake torque according to the profile. Such a system does not, however, disclose parallel braking systems or the use of a control scheme to manage over-braking. 
     SUMMARY OF THE DISCLOSURE 
     In an aspect of the disclosure, a method of adjusting braking torque in a machine having two separate braking systems includes identifying a first condition and activating a first braking system to produce a first brake torque in response to the first condition. The method may include determining that a second braking system is producing a second brake torque and automatically adjusting the first brake torque corresponding to the second brake torque. In an embodiment, the first brake torque is reduced as the second brake torque increases. 
     In another aspect of the disclosure, a system for managing braking torque in a machine includes a first braking system and a second braking system independent of the first braking system. The system includes a controller that monitors activity in both the first braking system and the second braking system and alters a first braking torque of the first braking system responsive to a change in a second braking torque of the second braking system. 
     In yet another aspect of the disclosure, a system for managing braking in a machine with independent braking systems includes an electric retarding system coupled to a first location on an axle of the machine, the electric retarder supplying a first brake torque when activated. The system also includes a hydraulic brake coupled to the axle at a second location remote from the first location, the hydraulic brake supplying a second brake torque directly proportional to a position of a foot pedal. The system further includes a controller that controls an amount of braking torque applied by the electric retarder responsive to either recognizing a predetermined condition or a manual setting. The controller is configured to monitor a position of the foot pedal of the hydraulic brake and to reduce the first brake torque of the electric retarder relative to an increase in the second brake torque of the hydraulic brake to reduce axle stress when both the electric retarder and the hydraulic brake are active. 
     These and other aspects and features will be more readily understood when reading the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a machine having multiple independent braking systems in accordance with the current disclosure; 
         FIG. 2  is a chart illustrating torque at an axle of the machine of  FIG. 1 ; 
         FIG. 3  is a chart of electric retarder torque to hydraulic brake torque in accordance with the current disclosure; 
         FIG. 4  is a chart of total brake torque to hydraulic brake torque in accordance with the current disclosure; and 
         FIG. 5  is a flowchart of an exemplary method of adjusting brake force in a machine having two separate braking systems. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1  a machine  100  having two or more independent braking systems is illustrated. The machine  100  may be used in any number of applications including construction, mining, or earthmoving, to name a few. The machine  100  may use wheels  108 ,  110  as illustrated in  FIG. 1 , may use tracks, or may use a combination of wheels and tracks. For the sake of clarity and without limitation, the following discussion of machine  100  will reference an off-road truck such as one that may be used in the mining industry. 
     The simplified illustration of the machine  100  shows an engine  101  that transmits power to a generator  102 . The generator  102  creates alternating current (AC) electrical energy that is converted to direct current (DC) electrical energy by a rectifier  103 . The DC electrical energy is transmitted via a high voltage bus  104  to an inverter  105 . The inverter  105  powers an electric motor  120  in a drive mode and captures electric energy generated by the electric motor  120  in retard mode. The energy created in retard mode may be dissipated or stored in a retard arrangement  106 . The machine  100  may include a first braking system  116  and a second braking system  118 . 
     The first braking system  116  may use the electric motor  120  in retarding mode coupled to the axle  114 . In some embodiments, another electric motor  125  (shown in dashed lines) may be disposed on the other side of the machine  100 . The electric motor  120 , and electric motor  125  when present, may be an AC motor/generator that converts electrical energy to mechanical energy to drive the wheels  110  in a traction mode and converts mechanical energy in the axle  114  to electrical energy in a retarding mode. The electric motor  120  in retarding mode may be used to slow down the machine  100  or bring it to a complete stop. The electric motor  120  may be controlled by a signal from a controller  126 . The first braking system  116  may also include a manual retarder input  142  and an input for automatic retarder settings  144 . The second braking system  118  may include a brake pedal  146  and one or more hydraulic cylinders  128  that pressurize fluid in hydraulic brake lines  124 . 
     In an embodiment, the electric motor  120  may be used for braking the machine  100  for several reasons including energy recovery, reduced wear on mechanical brakes, an ability to contour the braking profile, and automatically triggered braking such as entry into a speed zone area. A rate of application of the electric motor  120  may depend on a payload and slope of the machine  100 . For example, an empty machine  100  on level ground may require less brake torque than the machine  100  with a payload of 150 tons operating on a steep downhill incline. The use of the electric motor  120  allows setting a given level of brake torque needed, for example, to maintain speed on a downhill incline. As opposed to a hydraulic brake  122  that generates heat with use, the electric motor  120  may, in some embodiments, generate electricity that can be used to charge a battery (not depicted). This stored power can be used to operate fans or other electrical equipment. 
     The second braking system  118  may include hydraulic brakes  122  that may also be used to slow or stop the machine  100 . In an embodiment, the hydraulic brakes  122  are preferably used for emergency stopping when either the electric motor  120  fails or if an operator identifies a hazard requiring immediate action. As discussed more below, the controller  126  may monitor a position of the brake pedal  146  or may optionally receive a hydraulic pressure from a hydraulic brake line  124 . 
     The machine  100  may also include an operator station  140 . The operator station  140  may include a manual retarder input  142 . In some embodiments, the manual retarder input  142  may be a lever mounted on a steering column near a steering wheel. The operator station  140  may also include automatic retarder settings  144 , such as incline-based braking or speed zone braking. The brake pedal  146  may operate in a conventional manner to apply the hydraulic brakes  122  by using a foot to depress the pedal. For the purpose of illustration, hydraulic brake torque will develop to a measureable extent when the brake pedal  146  is at 30% of its range of motion and will reach 100% when the brake pedal  146  is at 100% of its range of motion or fully depressed. An inertial measurement unit  130  (IMU) may be used to detect instantaneous values for acceleration of the machine  100 . The use of the IMU  130  is discussed further below. 
     As discussed above, both the electric motor  120  and the hydraulic brakes  122  are capable of fully stopping the machine  100  within its specified stopping distance at various payloads and angles of operation (inclination). In the illustrated embodiment, the machine  100  may be a dump truck with a payload of about 150 tons. The braking force at a wheel  110  supplied in this exemplary embodiment by either the electric motor  120  in retarding mode or the hydraulic brake  122  may be in a range around 750,000 Newton-meters (Nm). However, in a situation such as a panic stop on a downhill incline, where the electric motor  120  may already be set at 100% braking and an operator fully applies the hydraulic brakes  122  nearly double the braking force may be applied at each wheel. In this situation, the wheels  108  and  110  may stop so quickly that the rear wheels  110  of the vehicle may create an exceedingly high torque spike, causing the machine  100  to pitch or bounce and may cause the tires to momentarily leave the ground, which may result in a lessened amount of control of the machine  100 . This action may also potentially damage the wheels  110  and tires of the machine  100  as well as upstream drivetrain components, not to mention alarming the machine operator. 
     In addition to the vehicle impact discussed above, another impact area of double braking is illustrated in  FIG. 2 , where a chart  150  shows exemplary measurements of torque on the axle  114 . A first graph  151  illustrates torque on the axle  114  when only the hydraulic brake  122  is applied. A second graph  152  illustrates torque on the axle  114  when both the hydraulic brake  122  and the electric motor  120  are both applied at 100%. Because the hydraulic brake  122  is located at an outboard end of the axle  114  and the electric motor  120  is located at an inboard end of the axle  114 , a rate and timing of torque application may be uneven between the axle ends. This can result a torque shock on the axle  114  shown in graph  152 . This large wrapup of the axle  114  not only stresses the axle  114  but transmits the shock through to other drivetrain components. 
     To avoid this torque shock, the controller  126  may monitor a setting of retarding supplied by the electric motor  120  as well as a status of the hydraulic brake  122 . When the electric motor  120  is operating in retarding mode, especially at very high levels, and the hydraulic brake  122  is applied, the controller  126  may automatically reduce the brake torque supplied by the electric motor  120 . 
     Table 1 illustrates two different mappings to reduce retarder braking torque as a function of hydraulic braking. In these illustrations, the hydraulic brake (sometimes called the service brake) pedal position as a percent is used as a surrogate for hydraulic brake torque. That is, a 30% depression of the brake pedal  146  is considered as a transition from zero to a positive hydraulic brake torque and 100% brake pedal depression is considered full hydraulic brake torque. 
     Looking at Table 1 below, two different mappings for electric retarding reduction as a function of hydraulic brake torque are illustrated. To begin in both Map  1  and Map  2 , the brake pedal position is at 30% or zero hydraulic brake torque and the electric motor retarding torque level is at 100%, for example, due to either an automatically detected condition or a manual setting of the retarder input  142  in the operator station  140 . As the brake pedal position percentage increases, a setting of the level of the electric motor retarding torque is progressively reduced to a final value of 60% for Map  1 . 
     Map  2  of Table 1 illustrates a steeper reduction in electric motor torque as a function of brake pedal position. In this example, the final electric motor retarding torque level is set to only 20% when the brake pedal position is at 100%. 
     
       
         
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Representative braking maps for electric 
               
               
                 motor retarding torque reduction 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Map 1 
                 Brake Pedal 
                 30 
                 44 
                 58 
                 72 
                 86 
                 100 
               
               
                   
                 Position % Inputs 
               
               
                   
                 Electric motor 
                 100 
                 92 
                 84 
                 76 
                 68 
                 60 
               
               
                   
                 retarding Lever 
               
               
                   
                 % Level 
               
               
                 Map 2 
                 Brake Pedal 
                 30 
                 44 
                 58 
                 72 
                 86 
                 100 
               
               
                   
                 Position % Inputs 
               
               
                   
                 Electric motor 
                 100 
                 84 
                 68 
                 52 
                 36 
                 20 
               
               
                   
                 retarding Lever 
               
               
                   
                 % Level 
               
               
                   
               
             
          
         
       
     
     Map  1  of Table 1 can be reduced to a simple equation of electric motor torque as a function of brake pedal position. A similar equation could be developed for electric motor torque as a function of hydraulic brake fluid pressure.
 
(Electric motor retarding torque %)=lesser of 100 or 117−0.57*(brake pedal position %)
 
     Similarly, Map  2  of Table 1 can be reduced to a similar equation.
 
(Electric motor retarding torque %)=lesser of 100 or 134−1.1*(brake pedal position %)
 
     The information in Table 1 is illustrated graphically in  FIG. 3 , where chart  160  has electric motor retarding torque on the Y axis  162  and hydraulic brake torque on the x-axis  164 . A second x-axis  166  is also labeled to illustrate the relationship of hydraulic brake pedal position to hydraulic brake torque, that is 100% brake pedal position is roughly equivalent to 720,000 Nm for the exemplary machine  100 . Line  168  illustrates a baseline of electric motor retarding torque at a constant 100%. Line  170  illustrates electric motor retarding torque as a function of hydraulic brake torque for Map  1  and line  172  illustrates electric motor retarding torque as a function of hydraulic brake torque for Map  2 . Line  174  plots the increasing hydraulic brake torque. 
     Table 2 below illustrates the effect on total braking torque at a wheel  110  for the baseline condition and those of the electric motor retarding torque reductions shown in Map  1  and Map  2  above. As can be seen, a 100% condition for both hydraulic braking and electric motor retarding braking is nearly 1.5 million Nm of braking torque, which has been shown to have several undesired consequences. In contrast, Map  2  illustrates that at a 100% brake pedal position the total braking torque at the wheel  110  is only slightly more than either braking system used separately and is unlikely to cause undesirable reactions in the machine  100  or high axle torque. Map  1  illustrates the less dramatic reduction of brake torque from the electric motor  120  but still much less than the baseline. 
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Brake force capacity 
               
             
          
           
               
                   
                   
                 Hydraulic 
                   
                 Electrical 
                 Total 
               
               
                   
                   
                 Brake 
                 Electrical 
                 Brake 
                 Braking 
               
               
                   
                   
                 Torque 
                 Brake 
                 Torque 
                 Torque 
               
               
                   
                 Brake pedal 
                 at wheel 
                 Torque 
                 at wheel 
                 at wheel 
               
               
                   
                 position 
                 (Nm) 
                 Percentage 
                 (Nm) 
                 (Nm) 
               
               
                   
                   
               
             
          
           
               
                 Baseline 
                  30% 
                 0 
                 100% 
                 748000 
                 748000 
               
               
                   
                 100% 
                 720000 
                 100% 
                 748000 
                 1468000 
               
               
                 Map1 
                  30% 
                 0 
                 100% 
                 748000 
                 748000 
               
               
                   
                 100% 
                 720000 
                  60% 
                 448800 
                 1168800 
               
               
                 Map2 
                  30% 
                 0 
                 100% 
                 748000 
                 748000 
               
               
                   
                 100% 
                 720000 
                  20% 
                 149600 
                 869600 
               
               
                   
               
             
          
         
       
     
     In an embodiment, the controller  126  may select Map  1 , Map  2 , or a different map based on conditions at the machine  100 , such as, but not limited to, payload, incline angle, and environment information when available, such as ground condition. For example, a rate of adjustment of the first braking torque may decrease as the payload increases, so that additional braking force is available in that situation. 
     In other embodiments, the controller  126  may use realtime calculations to determine braking adjustments rather than lookup tables. For example, an IMU  130  may sense instantaneous changes in speed of the machine  100 . A current value of the electric retarding torque may be determined through monitoring a speed of the electric motor  120  and/or monitoring current and rotor angle values in the electric motor  120 . Torque supplied by the second braking system  118 , or by another external force, may be calculated by comparing a rate of deceleration vs. that expected due to the only retarding of the electric motor  120 . As above, the braking torque supplied by the first brake system  116  may be adjusted to prevent over-braking. 
     A chart  180  shown in  FIG. 4  illustrates total brake torque  182  as a function of hydraulic brake torque  164  or hydraulic brake pedal position  166 . A baseline line  184  represents brake torque with no abatement of the electric motor retarding. Line  186  and line  188  represent the total brake torque when the electric motor retarding is scaled according to Map  1  and Map  2 , respectively. 
     Note that the increase in brake force shown in both lines  186  and  188  need not be linear as depicted and could follow a nonlinear transfer function. It is, however, desirable that the total brake torque increase monotonically, that is, each successive value of total torque is higher than the previous value for each increase in brake pedal position. In this way an operator always feels that the increased depression of the brake pedal results in an increase in brake torque. For example, as shown by line  190 , if the electric motor retarding were to be cut from 100% to 0% when the brake pedal position reached 50%, a total brake torque would be dramatically reduced as the hydraulic brake becomes the only source of brake torque. An operator may view this reduction in torque as a brake system failure which might cause some level of panic in the operator, a service call, or both. 
     INDUSTRIAL APPLICABILITY 
     In general, the present disclosure can find industrial applicability in a number of different settings. For example, the present disclosure may be employed in braking systems deployed in any machine with a dual braking system. Such machines may be used in a variety of applications, such as, but not limited to those use in the earth-moving, construction, mining, agriculture, transportation, and marine industries. 
     A method  200  of adjusting braking force in a machine  100  having two separate braking systems  116 ,  118  is depicted in  FIG. 5 . At block  202 , a first condition may be identified. In an embodiment, the first condition may be a signal to activate the first braking system  116  received from a manual retarder input  142  such as a hand lever in an operator station  140 . In another embodiment, the first condition may be identification of an operating state that triggers automatic application of the first braking system  116 , using, for example, an electric motor  120  operated in a retarding mode. The first condition may be activation of a rule corresponding to a current operating state such as a combination of payload and incline or may be related to a speed limit in a zone in which the machine  100  is currently operating. 
     At block  204 , the first braking system  116  may be activated to produce a first brake torque in response to the first condition. 
     A determination that a second braking system  118 , such as the hydraulic brakes  122 , is active and producing a second brake torque may be made at block  206 . The determination that the second braking system  118  is active and a magnitude of the braking torque supplied by the second braking system  118  may be made by sensing a position of a brake pedal  146 , a change in brake fluid pressure, or monitoring solenoid current in a brake actuator (not depicted). 
     The first brake torque of the first brake system  116  may be automatically adjusted at block  208  corresponding to the second brake torque supplied by the second braking system  118 . The torque of the first braking system  116  may be automatically changed in an inverse relationship to an increase in the torque of the second braking system  118 . That is, as the second braking system torque increases the torque in the first braking system  116  is reduced according to a formula or look up table of first braking system torque to second braking system torque. The first braking system torque may be reduced in relation to increases in the second braking system torque to ensure that an overall brake torque applied supplied by both the first and second braking systems  116 ,  118  monotonically increases. When the first braking system  116 , such as electric motor  120 , is not at full capacity at the time the hydraulic brake  122  is applied, the controller  126  may determine whether the braking torque provided by the electric motor  120  needs to be reduced or may be maintained at its current level. For example, when the electric motor  120  is at 50% and the application of the hydraulic brake  122  remains at or below 50%, the controller  126  may determine that an appropriate amount of braking force is being applied. However, in other situations such as operating empty (i.e., with no payload) the controller  126  may determine that even with both braking systems at 50%, a risk of axle twist may exist and the retarding torque produced by the electric motor  120  may be reduced accordingly. However, even in this situation, it is desirable to monitor total brake force and ensure that a monotonic increase in total brake force is applied whenever the operator is further depressing the brake pedal  146 . 
     Use of an electric motor  120  to provide braking capacity to a machine  100  provides a more energy-efficient and operator friendly mechanism for providing braking by energy recovery and automated activation in some predetermined circumstances. However, the need to supply a backup hydraulic brake system  118  means that almost double the braking force provided by either braking system  116 ,  118  may available, such as in a panic stop situation. The ability to automatically reduce the brake torque supplied by an electric motor  120  during a panic stop or other identified condition benefits both the machine operator and the equipment owner. The machine operator is not subjected to the torque shock of both braking systems or the physical pounding caused by such a large vehicle stopping unexpectedly quickly to the point of partially lifting off the ground. The owner of the machine benefits by reduced stress on components and an overall reduction in wear on tires and other drivetrain components, such as axle  114 , caused by the torque shock of both braking systems operating at full capacity. 
     While the above discussion has been directed to a particular type of vehicle, the techniques described above have application to many other machines which is a combination of electrical and mechanical braking.