Abstract:
A mechanical braking system is provided for a vehicle having an electric propulsion system which is utilized for retard speed regulation. While the retard speed regulation is normally performed via electric braking, in some conditions the electric braking is not able to maintain a desired speed for the vehicle. In this condition, upon receipt of a signal from the electric propulsion system, the mechanical braking system is automatically activated so as to maintain the vehicle at the desired speed.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a mechanical braking system for use on a vehicle. More particularly, the present invention relates to a mechanical braking system for use on a vehicle having an electric propulsion system and automatic retard speed regulation.  
         [0003]     2. Related Art  
         [0004]     An electric propulsion system for a vehicle, such as a traction vehicle, includes two electric traction motors coupled to a pair of rear wheels on opposite sides of the vehicle. The two electric motors are generally adjustable speed, reversible direct current (dc) motors and can operate so as to either propel or dynamically retard the traction vehicle.  
         [0005]     In the propel mode, the two motors operate so as to adjust the speed of the vehicle. In the retard mode, or electric braking mode, the two motors behave as generators. Dynamic braking resistor grids are connected across the armatures of the respective motors in order to dissipate the electric power output of the motors during electric braking.  
         [0006]     While in the retard mode, the goal of the two motors is to maintain a reference desired speed of the vehicle. For example, when the vehicle goes downhill, gravity will tend to accelerate the vehicle. In such a condition, the two motors will enter the electric braking mode to slow down the vehicle. Depending on the grade of road, however, it is possible that the electric motors will not be able to maintain the desired speed of vehicle. In addition, when the road condition is icy or wet, traction effort is reduced, and if one or both of the rear wheels becomes locked, the vehicle may enter a sliding mode. In this case, retard effort is reduced in order to avoid the sliding condition. Consequently, in some particular conditions of the road, the vehicle speed may increase, and the electric propulsion system cannot maintain the desired speed.  
         [0007]     Under any such condition in which the electric propulsion system alone is not able to maintain the desired speed, an alarm signal may be output which alerts the driver of the vehicle that mechanical braking must be performed in order to maintain the desired speed of the vehicle. An example of an electric propulsion system for a traction vehicle which outputs such an alarm signal is disclosed in U.S. Pat. No. 4,495,449, the disclosure of which is incorporated herein by reference in its entirety.  
         [0008]     Manually applying a mechanical braking system after such an alarm is received, however, requires that the driver of the vehicle be aware of the desired vehicle speed, and that the driver be able to apply the braking in an appropriate manner. Therefore, what is needed is a mechanical braking system that can be automatically applied so as to maintain a desired speed of a vehicle when the electric propulsion system of the vehicle is not able to maintain the desired speed by itself.  
       SUMMARY OF THE INVENTION  
       [0009]     Accordingly, it is a general objective of the present invention to provide a mechanical braking system that is automatically applied to help maintain the desired speed of a vehicle when the electric propulsion system fails to achieve the goal of maintaining the desired speed. The electric propulsion system operates by outputting an alarm signal, which, when active, indicates that mechanical braking must be used to maintain the desired speed.  
         [0010]     In accordance with one aspect of the present invention, the alarm signal output by the electric propulsion system is used to automatically active the mechanical braking system, the mechanical braking system including four brake controllers, two for two front wheels of the vehicle, and two for two rear wheels of the vehicle. The mechanical braking system according to the present invention may be, for example, a hydraulic braking system.  
         [0011]     In accordance with another aspect of the present invention, a gain control circuit is provided which is responsible for outputting a gain signal to each of the four brake controllers, whereby the brake signals generated by the brake controllers for the front wheels are controlled so as to always be equal to or less than the brake signals generated by the brake controllers for the rear wheels of the vehicle.  
         [0012]     The above and other features of the invention including various and novel details of construction and combination of parts will now be more fully described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular features embodying the invention are shown by way of illustration only and not as a limitation of the invention. The principles and features of this invention may be employed in varied and numerous embodiments without departing from the scope of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     Aspects of illustrative, non-limiting embodiments of the present invention will become more apparent by describing in detail embodiments thereof with reference to the attached drawings in which:  
         [0014]      FIG. 1  is a block diagram of a gain control circuit for the brake controllers according to the present invention;  
         [0015]      FIG. 2  is a block diagram of the two brake controllers for the two rear wheels of a vehicle according to the present invention; and  
         [0016]      FIG. 3  is the block diagram of the two brake controllers for the two front wheels of the vehicle according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]     The following description of the invention discloses specific configurations, features, and operations. However, the description is merely of an example of the present invention, and thus, the specific features described below are merely used to more easily describe the invention and to provide an overall understanding of the present invention.  
         [0018]     Accordingly, one skilled in the art will readily recognize that the present invention is not limited to the specific embodiments described below. Furthermore, the description of various configurations, features, and operations of the present invention that are known to one skilled in the art are omitted for the sake of clarity and brevity. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting.  
         [0019]      FIG. 1  is a block diagram of a gain control circuit according to the present invention, in which it is shown how the alarm signal output by the electric propulsion system is used to obtain the gain for each of the brake controllers used in the mechanical braking system. In particular, as shown in  FIG. 1 , the alarm signal  1  enters the plus (+) input of a comparator  3 , and a bias signal  2  enters the minus (−) input of the comparator  3 . The bias value to be used is, for example, half of the voltage value of the alarm signal  1 .  
         [0020]     Therefore, an output signal  4  of the comparator is either equal to V, the supply voltage of the comparator if the alarm signal is non-zero (e.g., logical 1), or equal to zero if the alarm signal  1  is zero (e.g., logical 0). The output signal  4  enters a functional block  5 , which produces an output signal  6  as a constant scalar either equal to “1” if the voltage level of the input signal  4  is equal to V, or equal to “0” if the voltage level of the input  4  is equal to zero.  
         [0021]     The output signal  6  will be used as the gain for two front brake controllers as described below with reference to  FIG. 3 . As shown in  FIG. 1 , the output signal  6  of the functional block  5  is also input to a Maximum block  7 . Accordingly, due to the configuration of the gain control circuit, an output signal  8  of the Maximum block  7  is the greater of the two input signals  6  and  10 .  
         [0022]     As shown in  FIG. 1 , the input signal  10  is the feedback from the output  8  via a gain block  9  of gain equal to 0.5. Accordingly, if the input signal  6  is one, the output signal  8  will also be one. However, if the input signal  6  changes from one to zero, then the input signal  10  of 0.5 will be the greater of the two inputs to the Maximum block  7 , and therefore, the output signal  8  becomes 0.5.  
         [0023]     At the next sampling, if the output signal  6  is still zero, the input signal  10  will reduce to 0.25, and therefore, the output signal  8  will be 0.25. During the next samplings, if the output signal  6  maintains to be zero, the output signal will be gradually reduced to zero. Therefore, the purpose of the functional block  7  is to reduce the output  8  gradually to zero if the input  6  is changed from one to zero.  
         [0024]     In summary, the block diagram in  FIG. 1  shows how the gain to the two front brake controllers and the gain to the two rear brake controllers is calculated. In short, the gain to the two front brake controllers will be one or zero depending on whether the alarm signal is at logical 1 or logical 0, respectively. Similarly, the gain to the two rear brake controllers will be 1 if the alarm signal is at logical 1, and will reduce gradually to zero if the alarm signal changes from logical 1 to logical 0.  
         [0025]     According to an illustrative embodiment of the present invention, the mechanical braking system includes two brake controllers for the rear wheels of the vehicle and two brake controllers for the front wheels of the vehicle. The four brake controllers may each be embodied as a separate unit, or alternatively, two or more of the brake controllers may be integrated in a single unit.  
         [0026]     As shown in  FIG. 2 , for the left rear wheel of the vehicle, a summer  22  takes the difference between a Left Rear Speed (LRS) signal  21  and a Desired Speed (DS) signal  24 . The LRS signal  21  can be generated by a conventional speed sensor, and the DS signal  24  can be manually set by an individual. The summer  21  outputs the difference between these two signals as output signal  23  to a Proportional-Derivative (PD) controller  28 .  
         [0027]     Similarly, for the right rear wheel of the vehicle, a summer  26  takes the difference between a Right Rear Speed (RRS) signal  25 , which can be generated by a conventional speed sensor, and the Desired Speed signal  24 . The summer  26  outputs the difference between these two signals as output signal  27  to a Proportional-Derivative (PD) controller  29 .  
         [0028]     As shown in  FIG. 2 , an output signal  30  of the PD controller  28  enters a multiplier  33  along with the gain signal  8  from the gain control circuit as shown in  FIG. 1 . The output  34  of the multiplier  33 , which is the product of the output signal  30  of the PD controller  28  and the gain signal  8 , is the value of a Left Rear Brake (LRB) signal  34 .  
         [0029]     Similarly, an output signal  31  of the PD controller  29  enters a multiplier  32  along with the gain signal  8  from the gain control circuit as shown in  FIG. 1 . The output  35  of the multiplier  32 , which is the product of the output signal  31  of the PD controller  29  and the gain signal  8 , is the value of the Right Rear Brake (RRB) signal  35 .  
         [0030]     As shown in  FIG. 2 , the output signal  34  (i.e., the LRB signal) and the output signal  35  (i.e., the RRB signal) are also input to a Minimum block  36  which outputs the lesser value of the LRB signal  34  and the RRB signal  35  as output signal  37 . As will be described below with reference to  FIG. 3 , the output signal  37  is used to control the two front brake signals.  
         [0031]     As shown in  FIG. 3 , for the left front wheel, a summer  42  takes the difference between a Left Front Speed (LFS) signal  41 , which can be generated by a conventional speed sensor, and the Desired Speed (DS) signal  24 . The summer  42  outputs the difference between these two signals as output signal  43  to a Proportional-Derivative (PD) controller  48 .  
         [0032]     Similarly, for the right front wheel, a summer  46  takes the difference between a Right Front Speed (RFS) signal  45 , which can be generated by a conventional speed sensor, and the Desired Speed (DS) signal  24 . The summer  46  outputs the difference between these two signals as output signal  47  to a Proportional-Derivative (PD) controller  49 .  
         [0033]     As shown in  FIG. 3 , an output signal  50  of the PD controller  48  enters a multiplier  53  along with the gain signal  6  from the gain control circuit as shown in  FIG. 1 . The output  54  of the multiplier  53 , which is the product of the output signal  50  of the PD controller  48  and the gain signal  6 , enters the Minimum block  57 . In addition, as shown in  FIG. 3 , the output  37  of the Minimum block  36  (see  FIG. 2 ) also enters the Minimum block  57 . Thus, in the Minimum block  57 , the lesser of output signal  54  of the multiplier  53  and the output signal  37  of the Minimum block  36  is output as Left Front Brake (LFB) signal  58 .  
         [0034]     Similarly, the output signal  51  of the PD controller  49  enters a multiplier  52  along with the gain signal  6  from the gain control circuit as shown in  FIG. 1 . The output  55  of the multiplier  52 , which is the product of the output signal  51  of the PD controller  49  and the gain signal  6 , enters the Minimum block  56 . In addition, as shown in  FIG. 3 , the output  37  of the Minimum block  36  (see  FIG. 2 ) also enters the Minimum block  56 . Thus, in the Minimum block  56 , the lesser of output signal  55  of the multiplier  52  and the output signal  37  of the Minimum block  36  is output as Right Front Brake (RFB) signal  59 .  
         [0035]     Thus, as is evident from the above description, and as shown in  FIGS. 2 and 3 , while the brake signals at the front wheels (i.e., LFB and RFB) are calculated in a similar manner as the brake signals for the two rear wheels (i.e., LRB and RRB), the brake signals at the front wheels will always be less than or equal to the brake signals at the rear wheels. By generating the brake signals in this manner, the mechanical braking system according to the present invention will provide a smooth braking operation while maintaining the desired speed of the vehicle.  
         [0036]     The previous description is provided to enable a person skilled in the art to make and use the present invention. Moreover, various modifications to the illustrative embodiments above will be readily apparent to those skilled in the art, and the generic principles and specific examples defined herein may be applied to other embodiments without the use of inventive faculty. Therefore, the present invention is not intended to be limited to the embodiments described herein but is to be accorded the widest scope as defined by the limitations of the claims and equivalents.