Abstract:
A retarding system for an electric drive machine is provided. The retarding system includes an electrical retarding system and a hydraulic braking system. A single pedal controls both the electrical retarding system and the hydraulic braking system. The pedal includes a first range of travel that provides input to the electrical retarding system and a second range of travel that additionally controls the hydraulic braking system. The pedal further includes different levels of travel resistance in each of the two ranges of travel that correspond to providing inputs to the electrical retarding system and the hydraulic braking system.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates generally to braking systems, and, more particularly to braking systems and methods that combine electric retarding and friction braking to slow a machine. 
       BACKGROUND 
       [0002]    Braking systems are used in a large variety of machines and vehicles to control, slow and stop the machine. Exemplary machines include passenger vehicles, trains, dump trucks, and mining vehicles. Machines increasingly use electric drive systems to provide propulsion for the machine. For example, passenger vehicles may use a hybrid drive system whereby a traditional gasoline powered engine and an electric motor are both used to provide propulsion for the vehicle. Machines, such as a railway engines and off-road vehicles may use a diesel-powered engine to drive a generator, which provides electric power to a motor. The motor then provides propulsion for the machine. 
         [0003]    Braking systems may take advantage of components in electric drive systems to provide braking for machines. For example, a hybrid passenger vehicle may include a regenerative braking system whereby the vehicle is slowed by the electric drive system while at the same time a battery in the vehicle is recharged. Railway engines use dynamic retarding to slow the train. Although brake systems utilizing electric drive systems have been used, these systems cannot stop a machine traveling at high speed quickly, nor can these systems consistently slow a heavily loaded machine traveling downhill or in slippery conditions. 
         [0004]    Some prior systems include a manual retarder lever that enables the operator to control ground speed by manually selecting the level of retarding or automatic retarder control that automatically controls machine speed based the operator&#39;s machine speed setting. The manual or automatic retarder may control an electric retarding system. Additionally, the operator may control a traditional braking pedal to actuate hydraulic brakes. In this way, the operator can manually control both dynamic retarding and hydraulic brakes. However, this configuration may be difficult for an operator to control effectively. For example, if the speed setting lever is set to high, the operator may have to rely more on the service brakes. In a large, heavily loaded machine, this may lead to the service brakes overheating. In addition, excess service brake wear may occur on a machine if the service brakes are used for continuous retarding. 
         [0005]    U.S. Pat. No. 20090,179,486 to Ikeda et al., issued Jul. 16, 2009, entitled “BRAKE SYSTEM IN ELECTRIC DRIVE DUMP TRUCK,” discloses a brake system in an electric drive dump truck having a hydraulic brake and a generator-type retarder operated by a brake pedal. However, the Ikeda reference does not disclose how to provide feedback to the truck&#39;s operator when the brake system transitions between hydraulic function and retarder function. Nor does the Ikeda reference discuss how the brake system manages the transition between hydraulic braking and retarder operation when the retarder is not available. 
       SUMMARY OF THE INVENTION 
       [0006]    In one aspect of the current disclosure, a retarding system for a machine having an electric drive system powering a set of rear wheels is disclosed. The retarding system comprises an electrical retarding system associated with the electric drive system and configured to supply a retarding torque to the rear wheels in response to a requested retarding torque, a hydraulic brake system configured to supply a braking torque to a set of wheels in response to a requested braking torque, a brake pedal having a total range of travel comprising a first range of travel and a second range of travel, and an encoder configured to provide an output to the retarding system proportional to the total range of travel. The first range of travel is associated with a first level of travel resistance and is configured to provide a requested retarding torque in response to the output and the second range of travel is associated with a second level of travel resistance and is configured to provide a requested braking torque and a requested retarding torque in response to the output. 
         [0007]    In another aspect of the current disclosure, a method for retarding a machine having an electric drive system powering a set of rear wheels, an electrical retarding system associated with the electric drive system and configured to supply a retarding torque to the rear wheels in response to a requested retarding torque, a hydraulic brake system configured to supply a braking torque to a set of wheels, is disclosed. The method comprises receiving an output from a brake pedal having a total range of travel comprising a first range of travel and a second range of travel, supplying a retarding torque in response to an output corresponding to the first range of travel that is associated with a first level of travel resistance and supplying a braking torque and a requested retarding torque in response to an output corresponding to the second range of travel that is associated with a second level of travel resistance. 
         [0008]    In another aspect of the current disclosure, a pedal for providing inputs to two different machine retarding systems is disclosed. The pedal comprises a base, a pedal portion pivotally attached to the base and having a total range of travel, an encoder configured to provide an electrical signal corresponding to an angle between the base and the pedal portion, a first spring operably connected between the base and the pedal portion and having a first spring constant, and a second spring operably connected between the base and the pedal during a portion of the total range of travel and having a second spring constant. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a diagrammatic view of an electric drive system for use with the current disclosure; 
           [0010]      FIG. 2  is a diagrammatic view of a retarding system for use with the current disclosure; 
           [0011]      FIG. 3  is a diagrammatic view of a retarding pedal for use with the current disclosure; 
           [0012]      FIG. 4  is a plot illustrating the function of the retarding system according to the current disclosure; 
           [0013]      FIG. 5  is a plot illustrating a faulty encoder output according to the current disclosure; 
           [0014]      FIG. 6  is a plot illustrating a torque gap according to the current disclosure; 
           [0015]      FIG. 7  is a plot illustrating blended braking torque according to the current disclosure; and 
           [0016]      FIG. 8  is a flow chart depicting a process for implementing blended braking torque according to the current disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    Referring to the drawings,  FIG. 1  illustrates a schematic view of an exemplary electric drive system including an electric retarding system for a machine. The exemplary electric drive system includes an engine  100 . Suitable engines include gasoline powered and diesel powered internal combustion engines. When in a drive configuration, the engine  100  powers a generator  102 . The generator  102  produces three-phase alternating current. The three-phase alternating current passes through a rectifier  104 , which converts the alternating current to direct current. An inverter or inverters  106  convert the direct current to variable frequency back to alternating current which feeds a motor  108 . By controlling the frequency of the current produced by the inverters  106 , the speed of the motor  108  is controlled. The motor  108  produces torque which powers the drive wheels  110 . 
         [0018]    In an alternative example of the current disclosure, an engine is not needed and the motor  108  is driven directly from an electric power source, such as a battery. In some example of the current disclosures, one motor powers all drive wheels. In alternative example of the current disclosure, various numbers of motors are used to power drive wheels. For example, each drive wheel may have an individual motor associated with the wheel. 
         [0019]    When operating in an electric braking, also known as electric retarding, configuration, the drive wheels  110  power the motor  108 . Driving the motor  108  places a torque on the drive wheels  110  and causes them to slow, thus braking the machine. The motors  108  generate alternating current. The inverters  106  convert the alternating current to direct current and feed the current to a chopper  112 , which acts as a direct current to direct current converter, and resistor grid  114 . The power generated by the motors  108  is thus dissipated thru heat by the resistor grid  114 . However, in an alternative example of the current disclosure, the power generated by the motors  108  is stored for later use. In one example of the current disclosure, the power generated by the motors  108  is stored in an electric battery. The energy in the electric battery can then be used in drive mode to power the motors  108  and propel the machine. 
         [0020]    The braking system operates in two modes. In first mode, the electric retarding system  130  supplies retarding torque  240 . In a second mode, the electric retarding system supplies maximum retarding torque  240 , while the hydraulic brake system  142  provides braking torque  250 . 
         [0021]    Turning to  FIG. 2 , a block diagram illustrating a braking system for a machine including a hydraulic brake system  142  and an electric retarding system  130  is illustrated. In one example of the current disclosures, a user interface  116  allows the operator of the machine to view status information relating to the braking system on a display  118 . Displayed information may include whether the electric retarding capacity has been exceeded. Additionally, status information regarding whether a front brake enable selection is set, automatic retarding settings and manual retarding settings may be shown on the display  118 . 
         [0022]    A speed sensor  122  is operably connected to receive information regarding the ground speed of the machine  10 . The speed sensor  122  may be connected to a motor  180 , front wheels  138 , or rear wheels  140 . The speed sensor  122  maybe connected to either drivetrain ECM  126  or the brake ECM  132 . 
         [0023]    The user interface  116  includes a manual and automatic retarder interface  120 . The user interface  116  interacts with a controller  124 . The controller  124  may include one or more control modules. In the illustrated example of the current disclosure, two electronic control modules (ECM) are used to implement the controller  124 . The drivetrain ECM  126  controls elements in the drivetrain  128 . The drivetrain  128  includes the engine  100 , generator  102 , rectifier  104 , inverters  106 , motor  108 , and chopper  112 . When braking the machine, the electric retarding system  130  includes the rectifier  104 , inverters  106 , motor  108 , and chopper  112  and the resistor grid  114 . In electric retarding mode, the drivetrain ECM  126  commands the electric retarding system  130  to provide a requested desired machine retarding torque and a ratio of retarding torque splits between sets of wheels. Thus, the drivetrain ECM  126  may command the machine to apply the proper ratio of torque splits between, for example a set of front wheels and a set of rear wheels. 
         [0024]    In one example of the current disclosure, the drivetrain ECM  126  receives signals indicating the front brake retarding enable switch  122  status, the manual retarder torque setting and the auto retarder torque setting from a brake ECM  132 . Based on these signals, the drivetrain ECM  126  calculates the desired machine retarding torque to be applied to the machine. The drivetrain ECM  126  provides signals indicating the desired machine retarding torque and the requested electric retarding torque to the brake ECM  132 . The brake ECM, based on these signals, determines whether the requested electric retarding torque is sufficient to provide the full desired machine retarding torque. If additional braking is necessary to meet the desired machine retarding torque, the brake ECM requests a ratio of additional braking torque from the front friction brake system  134  and the rear friction brake system  136 . The front friction brake system  134  connects to a front set of wheels  138  and the rear friction brake system  136  connects to a set of rear wheels  140 . In one example of the current disclosure the front friction brake system  134  and the rear friction brake system  136  are part of a hydraulic brake system  142 . In one example of the current disclosure, the hydraulic brake system includes a front brake solenoid valve  144  for controlling the flow of hydraulic fluid to the front friction brake system  134 . Likewise, a rear brake solenoid valve  146  controls the pressure of hydraulic fluid to the rear friction brake system  136 . The front friction brake system  134  and rear friction brake system  136  each include a hydraulic brake piston  148  that applies hydraulic force to actuate said brakes. 
         [0025]    In another example of the current disclosure, a single brake solenoid valve may control both the front and rear friction brake systems  134 ,  136 . In yet another example of the current disclosure, only the rear brake solenoid valve  146  may be present. 
         [0026]    Turning to  FIG. 3 , a brake pedal  150  includes a pedal portion  170  pivotally attached to a base  160 . The pedal portion  170  is designed to pivot when depressed by an operator&#39;s foot. The degree of depression is measured by an encoder  180 . The encoder  180  is configured to indicate angular position by sending an electrical signal to an ECM, such as a brake ECM  132 . The brake ECM  132  measures the electrical signal, and compares it to certain parameters to determine its validity, then to other parameters to calculate the measured angular position. The encoder  180  may be of the optical type. Various such encoders  180  are known in the art. The output of the encoder  180  may be a pulse-width modulated (PWM) signal as is known in the art. The output may be expressed as a percentage of the duty cycle of the output signal, from 0 to 100%. In one alternative of the current disclosure, a second encoder  182  may be included for redundancy. The second encoder  182  may identical to the first encoder  180 . The second encoder  182  is also electrically connected to an ECM, such as a brake ECM  132 . 
         [0027]    The pedal portion  170  has a total travel range  210  as it pivots on base  160 . The total travel range  210  is divided into a first travel range  220  and the second travel range  230 . Pivoting between the pedal portion  170  and the base  160  is resisted by force from a first spring  190  when pivoting within the first travel range  220 , and an additional second spring  200  when pivoting within the second travel range  230 . The first spring  190  is operably located between the pedal portion  170  and the base  160 . The change in force provided by the first spring  190  is characterized by its spring constant k 1  and follows Hooke&#39;s law ΔF 1 =k 1 *Δx, where Δx is the change in distance between base  160  and pedal portion  170  as it pivots. The change in force provided by the second spring  200  is likewise characterized by ΔF 2 =k 2 *Δx. Therefore, the change in resisting force when the pedal portion  170  is pivoting within the second travel range  230  is given by ΔF 1 +ΔF 2 . 
         [0028]    Turning to  FIG. 4 , it is seen that a preload may be applied to either first spring  190  or second spring  200 . The preload may be achieved by using a device that provides a degree of compression of the springs  190 ,  200  before they are compressed by pivoting. For instance, the length of the first spring  190  could be greater than the distance between the pedal portion  170  and the base  160 . The pivot angle between the pedal portion  170  and the base  160  may be limited in order to achieve a preload. Preload on second spring  200  could be achieved by employing a mechanical stop  202  on the second spring  200 . The stop  202  is configured to stop movement of a plunger  204  thereby providing a preload on second spring  200 . 
       INDUSTRIAL APPLICABILITY 
       [0029]    The brake pedal  150  is designed to indicate to an operator  20  of a machine  10  when the retarding system  50  is transitioning between a mode providing retarding torque  240 , and a mode providing braking torque  250 . The brake pedal  150  provides a greater travel resistance during the second travel range  230  than in the first travel range. The higher travel resistance is great enough to be noticed by the operator  20 . The higher travel resistance is provided by the addition of the spring rate of second spring  200 .  FIG. 4  shows angular displacement, or pivoting, between the pedal portion  170  and the base  160  on the horizontal axis. The vertical axis on the top plot shows the force provided by the operator  20  to pivot the pedal portion  170 . The vertical axis on the bottom plot shows the encoder  180  output as a percentage. When the pedal portion  170  is pivoting in the first travel range  220 , the travel resistance follows the slope of k 1 . In the first travel range  220 , the retarding system  50  is commanded to provide a retarding torque  240 . When the pedal portion  170  is pivoting in the second travel range  230 , the travel resistance follows the slope of k 1 +k 2 . In the second travel range  230 , the retarding system  50  is commanded to provide a braking torque  250 . The difference between the two spring rates during the transition between the first travel range  220  and second travel range  230  is sufficient to be noticed by an operator  20  using the brake pedal  150 . 
         [0030]    In one example of the current disclosure, the transition between the first travel range  220  and second travel range  230  may also include a preload that must be overcome before the pedal portion  170  will pivot in the second travel range  230 . Refer to  FIG. 4 . The force needed to overcome the preload may be between 10 and 50% of the total force needed to pivot the pedal portion  170  through the second travel range  230 . The force needed to overcome the preload may provide an additional indication to the operator  20  that the pedal portion  170  is transitioning to the second travel range  230  and that the retarding system  50  is entering a mode to provide braking torque  250 . A preload of between 50 and 100 N may be used. 
         [0031]    In another example of the current disclosure, the first travel range  220  may include a preload that must be overcome before the pedal portion  170  will pivot in the first travel range  220 . Refer to  FIG. 4 . The force needed to overcome the preload may be between 10 and 50% of the total force needed to pivot the pedal portion  170  through the first travel range  220 . 
         [0032]    In another example of the current disclosure, there may be provided a dead band region  300  at the beginning of the second travel range  230  in which no braking torque  250  is commanded. The dead band region  300  prevents the operator  20  from inadvertently commanding a braking torque  250  when the pedal portion  170  is at the transition between the first travel range  220  and second travel range  230 . 
         [0033]    In one example of the current disclosure, the encoder  180  and second encoder  182  are both connected to an ECM, such as a brake ECM  132 . The second encoder  182  provides signal redundancy such that the brake ECM  132  always receives at least one valid signal indicating the sensed angle position between the pedal portion  170  and the base  160 . The brake ECM  132  may use both signals to indicate the sensed angle position. Alternatively, it may use the signal from encoder  180  under normal conditions. The brake ECM  132  may then switch to the signal from second encoder  182  if the signal from  180  is determined to be faulty. The fault can be determined as is known in the art. As shown in  FIG. 5 , if a signal is determined to be faulty the brake ECM  132  will use the other, valid, signal to indicate the sensed angle position. Brake ECM  132  may send a fault signal to the drivetrain ECM  126  if a faulty encoder output is detected. The drivetrain ECM  126  may sound an audible alarm and/or display a fault message or icon on the display  118 . Similarly, if the signals from encoders  180  and  182  are not determined to be faulty, but rather disagree, then a number of actions may be taken. For instance, the brake ECM  132  may switch to the minimum signal or the maximum signal depending on the application. 
         [0034]    In another aspect of the current disclosure, the brake ECM  132  is configured to use the hydraulic brake system  142  provide braking torque  250  throughout the total travel range  210  if a failure is detected in the electric retarding system  130 . For instance, if a failure such as a ground fault or open circuit is detected in the resistor grid  114 , motors  108 , or drivetrain  128  is detected, the drivetrain ECM  126  may send a fault signal to the brake ECM  132 . The brake ECM  132  can then use the hydraulic brake system  142  to provide braking torque  250  for any pedal position within the total travel range  210 . Upon detection of a failure in the electric retarding system  130 , the drivetrain ECM  126  may sound an audible alarm and/or display a fault message or icon on the display  118 . 
         [0035]    At low speed, it can be difficult for the electric retarding system  130  to provide sufficient retarding torque  240  to slow or stop the machine  10 .  FIG. 6  shows how retarding torque  240  drops to zero below a retarding threshold  270 . In previous systems, the retarding system  50  might then engage the hydraulic brake system  142  that would provide braking torque  250  in order to slow or stop the machine  10 .  FIG. 6  shows a plot of vehicle speed on the horizontal axis (decreasing from left to right) and torque on the vertical axis. As shown in  FIG. 6 , the previous systems would cause a torque gap between the retarding torque  240  and the braking torque  250 . The gap exists in part due to a time delay between when the hydraulic brake system  142  activates the front brake solenoid valve  144  and the rear brake solenoid valve  146 , to when enough pressure is built up in the hydraulic brake pistons  148  to provide braking torque  250 . The previous systems therefore produce at least two negative results. First, the torque gap allows the machine  10  to free-wheel for a period of time even though the brake pedal  150  has commanded retarding. Second, when the braking torque  250  becomes available it may be higher in magnitude than the retarding torque  240  at low speeds. The higher braking torque  250  leads to abrupt retarding of the machine  10 . 
         [0036]    One aspect of the current disclosure provides for a solution to smoothly transition between retarding torque  240  and braking torque  250  at low speeds. The system defines a retarding threshold  270 , a pre-pressurize threshold  280 , and a pre-pressurize deactivation threshold  290 . The plot in  FIG. 7  shows a plot of vehicle speed on the horizontal axis (decreasing from left to right) and torque on the vertical axis. When speed of the machine  10  drops below the pre-pressurize threshold  280 , the brake ECM  132  activates front brake solenoid valve  144  and rear brake solenoid valve  146  in order to pre-pressurize the hydraulic brake pistons  148  in the front and rear friction brake system  134 ,  136 . When the speed of the machine  10  drops below the retarding threshold  270 , retarding torque  240  quickly drops to zero, while the braking torque  250  increases. There is little or no torque gap in this instance because the time delay to engage the hydraulic brake system  142  has been minimized by pre-pressurizing the front and rear friction brake systems  134 ,  136 . If the speed of the machine  10  increases above a pre-pressurize deactivate threshold  290 , pressure in the hydraulic brake system  142  is returned to normal. The pre-pressurize deactivate threshold  290  may include hysteresis with respect to the speed of the machine  10  as is known in the art. 
         [0037]    When the speed of the machine  10  drops below the retarding threshold  270 , the brake ECM  132  matches the braking torque  250  to the level previously provided by the retarding torque  240 . Therefore, the retarding system  50  may provide braking torque  250  when the brake pedal  150  is in the first travel range  220  in order to replace lost retarding torque  240  capability. The brake ECM  132  uses the commanded torque from the motors  108  multiplied by the final drive ratio connected to the rear wheels  140  in order to determine the desired machine retarding torque. The brake ECM  132  then calculates the pressure that the hydraulic brake system  142  needs to supply in order to match the retarding torque  240 . The pressure in the hydraulic pressure is related to braking torque  250  by an expression given by N*m/kPa * Final Drive Ratio. For example, the front friction brake system  134  may require about 35/200 N*m/kPa while the rear friction brake system  136  may require 35/200 N*m/kPa. The braking torque  250  is matched to the retarding torque  240  for a predetermined duration before command of the braking torque  250  is set equal to the desired machine retarding torque requested by brake pedal  150 . 
         [0038]    The flow chart in  FIG. 8  shows how a method of blending retarding torque  240  into braking torque  250  when the machine  10  is at low speed may be implemented. First, the method starts at box  400  and proceeds to decision box  410 . The method then checks to see if the desired machine retarding torque is greater than zero, i.e. the brake pedal  150  has been depressed by some degree. The method then checks to see if the speed of the machine  10  is below the pre-pressurize threshold  280 . If the answer to both is YES, then the method proceeds to action box  420 . Otherwise the method returns to the start box  400 . At action box  420 , the hydraulic brake system  142  pre-pressurizes the front friction brake system  134  and/or the rear friction brake system  136 . The method then proceeds to decision box  430 , where the method checks to see if the speed of machine  10  is less than the retarding threshold  270 . If the answer is YES, the method proceeds to action box  440 . If the answer is NO, the method returns to action box  420 . At action box  440 , the method sets the braking torque  250  equal to the retarding torque  240 . From action box  440 , the method proceeds to action box  450 , where retarding torque is decreased to zero. The method then proceeds to action box  460 , where braking torque  250  is set equal to the desired machine braking torque.