CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application relates to, and claims the benefit of the filing date of, co-pending U.S. provisional patent application Ser. No. 61/767,755 entitled HYBRID BRAKE SYSTEM FOR A MODEL VEHICLE, filed Feb. 21, 2013, the entire contents of which are incorporated herein by reference for all purposes. 
     
    
     TECHNICAL FIELD 
       [0002]    This application relates to model vehicles and, more particularly, to braking systems for model vehicles. 
       BACKGROUND 
       [0003]    In conventional two-wheel drive electric model vehicles, the vehicle is braked exclusively by the driven wheels. By short-circuiting its motor windings, the primary electric motor brakes the driven wheels through the drivetrain. The two driven wheels are usually the rear wheels in order to provide optimum driving traction when the vehicle accelerates forward. 
         [0004]    However, braking a forward-moving vehicle with only the rear wheels causes increased forward weight transfer, a shifting of the vehicle&#39;s weight to the front tires. This forward weight transfer limits the maximum deceleration which braking the rear wheels can provide. If too much braking force is applied to the rear wheels, the vehicle&#39;s rear tires may begin to slide, causing a loss of control of the vehicle. Consequently, a conventional two-wheel drive electric model vehicle may only apply very little brake force if the driver is to maintain control of the vehicle. 
         [0005]    Thus, the braking capabilities of conventional two-wheel drive electric model vehicles are limited. Because electric motor brakes are applied through the drivetrain, it is infeasible to simply add the braking system used for the drive wheels to the non-drive wheels. Such an addition would effectively introduce the same expense and other considerations as producing a four-wheel drive vehicle. It would be desirable if a braking system permitted the application of greater braking force without an accompanying loss of control. 
         [0006]    Additionally, a conventional radio transmit controller only provides two channels of information to the model vehicle receiver: the position of the throttle trigger and the position of the steering wheel. The position of the throttle trigger controls three operations: forward acceleration, reverse acceleration, and braking. However, a conventional throttle trigger can be moved in only two directions: pulling the throttle trigger toward the user and pushing the throttle trigger away from the user. 
         [0007]    Typically, the model vehicle&#39;s Electronic Speed Control (ESC) uses the speed and direction of the vehicle motor to determine if the throttle trigger controls forward and reverse acceleration, forward acceleration and braking, or reverse acceleration and braking. When the vehicle motor speed is at or below a speed threshold, the ESC permits the user to select between forward and reverse acceleration. Forward acceleration is performed by pulling the throttle trigger from a neutral position toward the user and reverse acceleration is performed by pushing the throttle trigger from the neutral position away from the user. When the motor is moving at above the speed threshold, the ESC permits the user to select between further acceleration in the same direction and braking. Further acceleration is performed by continuing to pull or push the throttle trigger, depending on which direction the vehicle motor is moving. Braking is performed by moving the throttle trigger to the opposite position, a position which would have caused acceleration in the other direction if the motor speed were at or below the speed threshold. 
         [0008]    A disadvantage of this approach is that the trigger position used to brake varies depending on the direction the vehicle is currently accelerating. An alternative approach is a “one direction only” mode for the ESC. The one direction only mode may also be called a “forward only” mode when it permits acceleration in the forward direction, and may also be called a “reverse only mode” when it permits acceleration in the reverse direction. 
         [0009]    When set to a one direction only mode, the ESC may consistently interpret all pulling on the throttle trigger from the neutral position as an instruction to accelerate the electric motor in a particular direction. The ESC may consistently interpret all pushing on the throttle trigger from the neutral position as an instruction to brake the electric motor. In both cases, the magnitude of the acceleration or braking force typically increases with the distance from the neutral position. A one direction only mode may permit consistent throttle positions to accelerate or brake the vehicle, but has the disadvantage of only permitting the vehicle to move in one direction. 
         [0010]    It would be desirable if a transmit controller could permit the same positions of a throttle trigger to consistently accelerate and brake a model vehicle, regardless of the speed and direction of the vehicle motor, while still permitting both forward and reverse acceleration. 
         [0011]    It would further be desirable if both the desired enhanced braking and transmit controller features described above could be implemented in an existing model vehicle, with a minimum of replacement of conventional vehicle components. 
       SUMMARY OF THE INVENTION 
       [0012]    The present invention provides a model vehicle having a motor and electronic speed control for driving and braking driven wheels, combined with a separate braking system for non-driven wheels. A controller and a method of operation are provided. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following Detailed Description taken in conjunction with the accompanying drawing, in which: 
           [0014]      FIG. 1  depicts a model vehicle and transmit controller in accordance with an exemplary embodiment of the present invention; 
           [0015]      FIGS. 2A-2C  depict a transmit controller in accordance with an exemplary embodiment of the present invention; 
           [0016]      FIG. 3  depicts the operation of a brake bias user interface in accordance with an exemplary embodiment of the present invention; 
           [0017]      FIG. 4  depicts a rear-wheel drive model vehicle in accordance with an exemplary embodiment of the present invention; and 
           [0018]      FIG. 5  depicts a front-wheel drive model vehicle in accordance with an exemplary embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    In the following discussion, numerous specific details are set forth to provide a thorough explanation. However, such specific details are not essential. In other instances, well-known elements have been illustrated in schematic or block diagram form. Additionally, for the most part, specific details within the understanding of persons of ordinary skill in the relevant art have been omitted. 
         [0020]    With reference to  FIG. 1 , depicted is an exemplary combination  100  of a transmit controller  102  and a model vehicle  104 . Transmit controller processor  105  on transmit controller  102  may communicate with receiver  106  on model vehicle  104  through radio link  108 . Radio link  108  may be bidirectional and transmit controller  102  and receiver  106  may be transceivers, each capable of both transmitting and receiving radio signals through radio link  108 . By convention, however, transmit controller  102  may be called a “transmitter” and receiver  106  may be called a “receiver.” 
         [0021]    Transmit controller  102  may have throttle trigger  110 , which may have a range of movement from acceleration range  110 A to neutral range  110 B to braking range  110 C. Acceleration range  110 A may be a range of positions nearer to a user holding transmit controller  102 , braking range  110 C may be a range of positions farther from a user holding transmit controller  102 , and neutral range  110 B may be a middle range of positions between acceleration range  110 A and braking range  110 C. 
         [0022]    Transmit controller  102  may also have direction switch  112 . Direction switch  112  may allow the user to switch between forward and reverse acceleration for model vehicle  104 . Direction switch  112  may have two positions, forward position  112 A and reverse position  112 B. When direction switch  112  is in forward position  112 A, placing throttle trigger  110  in acceleration range  110 A causes transmit controller processor  105  to instruct vehicle  104  to accelerate forward. Similarly, when direction switch  112  is in reverse position  112 B, placing throttle trigger  110  in acceleration range  110 A causes transmit controller processor  105  to instruct vehicle  104  to accelerate in reverse. 
         [0023]    Model vehicle  104  may be a four-wheeled rear-wheel drive electric model vehicle. Model vehicle  104  may have two rear wheels  105 A and two front wheels  105 B. Model vehicle  104  may have receiver  106 , ESC  114 , and primary electric motor  116 . ESC  114  may be a conventional ESC in one direction only mode. However, the direction ESC  114  accelerates the vehicle in is dependent on information received from receiver  106 , as will be discussed further. Primary electric motor  116  may drive and brake rear wheels  105 A conventionally as instructed by ESC  114 . 
         [0024]    Model vehicle  104  may also have front brake servo  118  and front brake system  120 . When instructed to by receiver  106 , front brake servo  118  may brake front wheels  105 B using front brake system  120 . Front brake system  120  may include mechanical or hydraulic brakes. Front brake system  120  preferably does not include an electric motor brake, as including an electric motor brake would defeat many of the reasons to create a two-wheel drive electric model vehicle. 
         [0025]    Instead of the conventional two channels of information, transmit controller processor  104  may provide four channels of information to receiver  106  through radio link  108 . Some existing receivers may be capable of receiving more than two channels of information, so existing receivers may be configured to utilize the four channels of information. 
         [0026]    The four channels may include steering channel  108 A, throttle/rear brake channel  108 B, acceleration direction channel  108 C, and front brake channel  108 D. Steering channel  108 A may control the steering servos of model vehicle  104  as is known in the art, and need not be discussed further. 
         [0027]    Receiver  106  may provide information from throttle/rear brake channel  108 B and acceleration direction channel  108 C to ESC  114 . Throttle/rear brake channel  108 B may specify either an amount of acceleration or an amount of braking to apply to rear wheels  105 A. Acceleration direction channel  108 C may specify the direction for ESC  114  to accelerate the electric motor in. ESC  114  may use acceleration direction channel  108 C to determine a direction for its one direction mode, and may use information from throttle/rear brake channel  108 B to determine whether to accelerate or brake primary electric motor  116 . 
         [0028]    ESC  114  using the speed and direction of primary electric motor  116  to determine whether to accelerate or brake is unnecessary. Acceleration direction channel  108 C alone may specify whether acceleration should be in the forward or reverse direction. Throttle/rear brake channel  108 B alone may specify an amount of acceleration or braking. 
         [0029]    Receiver  106  may provide information from front brake channel  108 D to front brake servo  118 . Front brake channel  108 D may specify how much braking force should be applied to front wheels  105 B, and front brake servo  118  may actuate front brake system  120  accordingly. 
         [0030]    The user may select a brake bias, a ratio identifying an allocation of braking force between front brake system  120  and the rear brakes. For example, a brake bias of 70% front and 30% rear means 70% of the braking force specified by throttle trigger  110  will be applied by front brake system  120  and 30% of the braking force will be applied by the rear brakes. 
         [0031]    The brake bias may be selected using brake bias user interface  122 . The brake bias may be classified as a vehicle operational parameter and brake bias user interface  122  may be a user interface also used for other vehicle operational parameters. Brake bias user interface  122  may be a user interface built into the transmit controller, such as a dial or knob. Alternately, brake bias user interface  122  may be an auxiliary user interface device described in published patent application Ser. No. 12/850,453. 
         [0032]    Transmit controller processor  105  may translate the position of throttle trigger  110  and the brake bias into information sent through throttle/rear brake channel  108 B and front brake channel  108 D. When throttle trigger  110  is pushed away from the user, an amount of braking force is specified by the distance of throttle trigger  110  from neutral range  110 B. Using this specified amount of braking and the brake bias, transmit controller processor  105  may calculate an amount of rear braking force and an amount of front braking force. The amount of rear braking force may then be provided to ESC  114  through throttle/rear brake channel  108 B, and the amount of front braking force may be provided to front brake servo  118  through front brake channel  108 D. 
         [0033]    When throttle trigger  110  is pulled toward the user, an amount of acceleration is specified by the distance of throttle trigger  110  from neutral range  110 B. During acceleration, front brake channel  108 D may be unnecessary. Transmit controller processor  105  may simply provide the amount of acceleration to ESC  114  through throttle/rear brake channel  108 B. 
         [0034]    When model vehicle  104  brakes while moving forward, its weight is transferred towards the front tires. Therefore, for forwards movement the brake bias should preferably be toward the front brakes. However, when model vehicle  104  brakes while moving in reverse, its weight is transferred towards the rear tires, so the brake bias should preferably be toward the rear brakes. In an embodiment, when the direction of acceleration is changed by direction switch  112 , transmit controller processor  105  may automatically reverse the brake bias. A brake bias of 70% front and 30% rear may become a brake bias of 30% front and 70% rear, and vice versa. Model vehicle  104  may thereby retain approximately the same braking behavior regardless of whether it is driven forwards or in reverse. 
         [0035]    Alternately, rather than reversing the brake bias, transmit controller processor  105  may set the brake bias to a predetermined amount which favors or strongly favors the wheels in the direction the vehicle is moving. A brake bias favoring the front wheels would be used for forward movement, and a brake bias favoring the rear wheels would be used for reverse movement. 
         [0036]    While the above discussion was with reference to a rear-wheel drive electric model vehicle, it may also be applied to a front-wheel drive electric model vehicle. For a front-wheel drive electric model vehicle, brake servo  118  and brake system  120  may be placed on rear non-drive wheels  105 A. The electric motor brake would then be for front drive wheels  105 B and brake servo  118  would control the rear wheel brake system  120 . 
         [0037]    In a front-wheel drive electric model vehicle, the above front brake servo  118  may be configured as and function as a rear brake servo  118 , and the above front brake system  120  may be configured as and function as a rear brake system  120 . The above throttle/rear brake channel  108 B may be configured as and function as a throttle/front brake channel  108 B. The above front brake channel  108 D may be configured as and function as a rear brake channel  108 D. 
         [0038]    In more general terms, the above front brake servo  118  may be configured as and function as a non-drive wheel brake servo  118 , and the above front brake system  120  may be configured as and function as a non-drive wheel brake system  120 . The above throttle/rear brake channel  108 B may be configured as and function as a throttle/drive wheel brake channel  108 B. The above front brake channel  108 D may be configured as and function as a non-drive wheel brake channel  108 D. 
         [0039]    With reference to  FIGS. 2A-2C , depicted is an exemplary transmit controller  102  shown with more detail. In each of  FIGS. 2A-2C , brake bias user interface  122  is a dial. Direction switch  112  is in forward position  112 A. In  FIG. 2A , throttle trigger  110  is in neutral range  110 B. In  FIG. 2B , throttle trigger  110  is in acceleration range  110 A. In  FIG. 2C , throttle trigger  110  is in braking range  110 C. 
         [0040]    With reference to  FIG. 3 , depicted is the operation of an exemplary brake bias user interface  122 . In  FIG. 3 , brake bias user interface  122  is a dial. Brake bias user interface  122  has three positions  122 A,  122 B, and  122 C illustrated. In position  122 A, brake bias user interface  122  is rotated counterclockwise as much as possible. In position  122 C, brake bias user interface  122  is rotated clockwise as much as possible. In position  122 B, brake bias user interface  122  is halfway between positions  122 A and  122 C. 
         [0041]    Graph  300  shows the relationship between the position of brake bias user interface  122 , shown on the horizontal axis, and the resulting brake bias, shown on the vertical axis. In normal operation, the brake bias is shown by line  302 . The brake bias changes from rear to forward as brake bias user interface  122  is turned from position  122 A to position  122 C. As mentioned above, in some embodiments the brake bias is reversed when the vehicle is set to accelerate in reverse. The resulting reversed brake bias is shown by line  304 . The brake bias changes from forward to rear as brake bias user interface  122  is turned from position  122 A to position  122 C. 
         [0042]    Also as mentioned above, the brake bias can also be set to a predetermined amount which favors the wheels in the direction the vehicle is moving. Line  306  shows the brake bias when set to a predetermined amount favoring the rear wheels. Line  308  shows the brake bias when set to a predetermined amount favoring the front wheels. When the brake bias is set to a predetermined amount, the position of brake bias user interface  122  is irrelevant. 
         [0043]    Referring to  FIG. 4 , depicted is model vehicle  104  in more detail. Vehicle chassis  302  and brakes  304  are shown in addition to the parts of vehicle  104  shown in  FIG. 1 . Front brake system  120  includes brakes  304 . 
         [0044]    Referring to  FIG. 5 , depicted is an exemplary front-wheel drive vehicle  500 . Front-wheel drive vehicle  500  is identical to model vehicle  104 , but with parts rearranged for front-wheel drive. Primary electric motor  116  may drive and brake front wheels  105 B conventionally as instructed by ESC  114 . Front wheel brake servo  118  and front brake system  120  on vehicle  104  are rear wheel brake servo  118  and rear wheel brake system  120  on vehicle  500 . Rear brake system  120  includes brakes  304 . When instructed to by receiver  106 , rear brake servo  118  may brake rear wheels  105 A using rear brake system  120 . 
         [0045]    Having thus described the present invention by reference to certain of its preferred embodiments, it is noted that the embodiments disclosed are illustrative rather than limiting in nature and that a wide range of variations, modifications, changes, and substitutions are contemplated in the foregoing disclosure and, in some instances, some features of the present invention may be employed without a corresponding use of the other features. Many such variations and modifications may be considered desirable by those skilled in the art based upon a review of the foregoing description of preferred embodiments. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.
The present invention provides a model vehicle having a motor and electronic speed control for driving and braking driven wheels, combined with a separate braking system for non-driven wheels. A controller and a method of operation are provided.