Abstract:
A braking system for an electric motor operated vehicle including a system for providing both regenerative and reverse excitation braking and shifting between the braking modes in response to operator demand.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of our application of the same title, filed Nov. 5, 1997 under Ser. No. 08/964,500. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to an electric motor control and more particularly to an electric motor control for an electric motor powered vehicle for providing improved braking operation therefor. 
     A wide variety of types of vehicles are frequently employ electric motors for their propulsion. These types of vehicles may take many forms such as golf carts and the like. In connection with these vehicles, it is common to utilize a frictional brake system for the vehicle which is of the type used with wheeled vehicles that are powered by gasoline engines or other forms of internal combustion engines. These frictional brakes may be either hydraulically or electrically actuated. This obviously adds to the cost of the total vehicle. 
     There have been proposed systems which regenerative braking is employed for the vehicle. With this type of arrangement, when the operator wishes to decelerate the vehicle, the motor is switched to operate as a generator and either charges the battery or discharges its generated current through a resistive load so as to provide regenerative braking. However, with these systems, conventional frictional brakes also are employed. 
     It is, therefore, a principle object of this invention to provide an improved braking arrangement for an electrically powered vehicle wherein the electric motor can provide substantially all of the braking force for the vehicle. 
     It is a further object of this invention to provide an improved braking system for an electric powered vehicle wherein the electric motor is utilized as a brake in addition to the regenerative braking normally employed in connection with such vehicles. 
     SUMMARY OF THE INVENTION 
     This invention is adapted to be embodied in a vehicle having an electric motor for propelling the vehicle. The electric motor is of the shunt type and is supplied with electric current from a battery in response to an accelerator control for controlling the speed of the vehicle. A regenerative braking arrangement is employed for generating electric power by using the shunt motor as a generator and returning the power to the power source for braking. In addition, a reverse exciting winding control circuit is provided for applying reverse excitation to the winding of the shunt motor for effecting reverse excitation braking. Means are provided for switching the braking control from the regenerative braking to the reverse exciting winding braking based upon the operator input. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partially schematic control circuit for an electric motor powered vehicle and shows certain of the components in electrical diagrams and other components only in block form. 
     FIG. 2 is a graphical view showing the reverse excitation and regenerative braking and shows curves indicating vehicle speed, type of brake operation, rotor current, and exciter current in connection with this embodiment. 
     FIG. 3 is a graphical view showing a portion of the control routine for this embodiment. 
     FIG. 4 is a graphical view showing the control routine during the reverse excitation braking. 
     FIG. 5 is a partially schematic electrical diagram, in part similar to FIG.  1  and shows a second embodiment of the invention. 
     FIG. 6 is a graphical view, in part similar to FIG. 2, and shows the operation modes and curves for this embodiment. 
     FIG. 7 is a graphical view showing the three-dimensional map employed to utilize brake pedal actuation and vehicle speed to determine the brake assist amount for regenerative braking. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now in detail to the drawings and initially to FIG. 1, the control and drive system for an electric motor propulsion system for a vehicle is depicted. The system includes a shunt-type electric motor, indicated generally by the reference numeral  11  having an exciter winding  12 . 
     This shunt motor  11  is energized by an electric battery source  13  through a control system which includes a main switch  14  that operates a main relay switch  15  for energizing the motor  11  in a manner which will be described for powering the associated vehicle. The vehicle per se is not illustrated, but it should be understood that the invention can be used with any of a wide variety of types of vehicles that are powered by electric motors. A golf cart is just a typical example of such a vehicle. 
     Although the vehicle is not shown, it includes an accelerator switch S 2  which is operated by the operator accelerator pedal or other speed control so as to provide a speed control signal to the electric motor and specifically to its control circuit, indicated generally by the reference numeral  16 . A potentiometer type of sensor  17  is also associated with the accelerator pedal so as to provide a signal indicative of the operator demand for speed. 
     The system is also provided with a braking arrangement that is actuated by a switch S 3  which operates to provide either selective regenerative or reverse excitation braking of the motor  11  in manners which will be described. 
     The operator can also select reverse operation by a reverse switch S 4  which reverses the direction of rotation of the motor  11  so as to drive the vehicle in a rearward direction. When reverse operation is selected, a buzzer, indicated generally by the reference numeral  18 , is activated so as to provide a warning signal. 
     For the vehicle control, there is also provided a vehicle speed sensor  19  which may be of any known type and which can cooperate, for example, with a driven wheel or a portion of the driving transmission to provide a speed signal to the motor control  16 . 
     Referring now in more detail to the motor control, this includes a power control circuit  21  that powers the motor controller  16  and which is energized when the main switch  14  is closed. There is also provided a switch input circuit  22  that senses the conditions of the switches S 2 , S 3 , and S 4  so as to provide the motor controls, as will be described. 
     The switch input circuit  22  inputs a signal to a CPU  23  which includes several sections, as will be described. This CPU  23  controls the operation of the motor  11  and the excitation of the winding  12  in the manner which will also be described. 
     The speed sensor  19  inputs its signal to a speed input circuit  23  which in turn provides a signal to the CPU  24  which provides indication of the speed of travel of the vehicle to the CPU  23  for control of the various system. In addition, the potentiometer  17  outputs data to a potentiometer input circuit  25 , which, in turn, inputs its input data to the CPU  23 . 
     The CPU  23  includes three sections associated with the braking. These are comprised of a regenerative braking circuit  26 , a reverse excitation braking circuit  27  and a shift control circuit  28  which shifts between the regenerative braking provided by the control circuit  26  and the reverse excitation braking provided by the circuit  27 . 
     The CPU  23  also includes controls a main relay drive circuit  29  which energizes the main relay  15  when the system is operational. 
     The armature windings of the electric motor  11  are connected to the poles of the battery through the main relay  15  and through a first field effect transistor (FET  1 ). The FET  1  is switched by a FET drive circuit, indicated schematically at  31 . This controls the speed or power applied to the shunt type motor  11  from an electric current input circuit, indicated at  32 . This circuit communicates with a motor control pulse width modulation circuit (PWM)  33  which functions so as to provide switching of the FET drive circuit under the control of the CPU  23  so as to provide the amount of speed required as demanded by the operator due to the position of the accelerator switch S 2  and the associated potentiometer  17 . 
     The armature current is sensed by a current sensor CT 1  in the circuit between FET  1  and the motor  11 . 
     The exciter winding  12  of the shunt motor  11  is controlled by an FET drive circuit, indicated generally by the reference numeral  34  and which operates so as to determine the polarity of the winding  12  by switching FETs, FET  2 , FET  3 , FET  4  and FET  5  in an obvious manner. 
     The FET drive circuit  34  is controlled by a field direction control circuit  35  which receives input signals from the CPU  23  indicative of whether the shunt motor  11  is in a forward drive mode or is shifted into a reverse drive mode by switching the reverse drive switch S 4  or to provide regenerative braking in the manner which will be described shortly. 
     The exciter winding current is sensed by a sensor CT 2  which outputs its signal to both a protection circuit  36  and magnetic field current input circuit  37  each of which input data to the CPU  23 . The amount of excitation is controlled by a magnetic field direction pulse width modulating circuit  38  which is also driven by the CPU  23  to control the amount of current flow through the exciter winding  12  to achieve the desired performance. 
     This control methodology will now be described by reference to first FIG.  2  and later associated with it FIGS. 3 and 4 which show the braking circuit. Referring first to FIG. 2, this shows the vehicle speed, rotor or armature current and exciter current through the shunt motor  11  and exciter winding  12 , respectively. 
     As may be seen in the left-hand side of the vertical broken line that indicates the demarcation point between regenerative braking control and reverse excitation braking control, which occurs when the operator depresses the brake pedal. The condition illustrated is when braking after traveling at a constant speed. As such, the exciter and rotor currents are constant and the vehicle speed will be constant until the brake pedal is operated. 
     During this phase, if the accelerator pedal is released, there will be regenerative braking. However, as shown by the broken line view of FIG. 1, the vehicle speed will decay relatively slowly and, therefore, the amount of braking with conventional vehicles employing regenerative braking is frequently supplemented by a more conventional frictional type brake. 
     However, when the brake switch S 3  is turned on in this embodiment, then an exciter current is applied in a negative direction with the amount limited by the limit value of the protective circuit  36  so that there will be generated a reverse strong current flow through the rotor which will provide a rapid decrease in vehicle speed. Thus, when the brake switch S 3  is operated with this embodiment, the system switches from regenerative braking to reverse excitation braking by the shift controller  28  of the ECU  23  switching from the one braking control  26  to the other braking control  27 . 
     The logic by which this operates will now be described by reference to FIGS. 3 and 4. FIG. 3 shows the basic control routine for determining braking. The program starts and moves to the step S 1  to determine if the brake switch S 3  is on. If it is not, the program moves to the step S 2  so as to provide regenerative braking when required and then returns. 
     If, however, at the step S 1  it is determined that the brake switch S 3  is on, then the program moves to the step S 3  so as to initiate reverse excitation braking in the manner which will be described by reference to FIG.  4 . The program then returns. 
     Referring now to FIG. 4, when operating in the reverse excitation braking mode, the program moves to the step S 3 . 1  wherein the magnetic field direction instruction to reverse is set. This is done by the section  35  of the motor controller  16  previously referred to and occurs at the time when the brake switch S 3  is applied, as aforenoted. 
     The program then moves to the step S 3 . 2  so as to read the exciter current flow from the CT 2  and to determine if it has reached the maximum limited value set by the protection circuit  36 . It should be noted and as shown by the lower portion of FIG. 2, that the exciter current is not immediately placed at its maximum value, but is gradually built up so as to avoid abrupt braking condition. Therefore, if at the step S 3 . 2  maximum current is not experienced, the program moves to the step S 3 . 3  to incrementally increase the exciter current. 
     If the maximum value is found at the step S 3 . 2 , the program jumps ahead. 
     From either the step S 3 . 2 , if the value is “yes,” or from the step S 3 . 3  after completion of the increase, the program moves to the step S 3 . 4  to determine if the rotor current has fallen to zero. This current is sensed so as to determine if the rotation of the motor has, in fact, stopped. If the rotor current as sensed by the CT 1  is not zero, the program skips ahead and returns. 
     If, however, the rotor current has fallen to zero, then the program moves to the step S 3 . 5  so as to terminate the exciter current supplied to the exciter coil  12 . The program then moves to the step S 3 . 6  to set the run condition flag to indicate that the vehicle has stopped. 
     In the embodiment of the invention thus far described, the brake operation has been such that, when the brake switch is switched on, the device operates so as to apply a regenerative braking in gradually increasing amounts by increasing the exciter current up to a maximum value. This is then held until the vehicle comes to a halt or the brake pedal is released. 
     Obviously, it may be desirable to provide more accurate and finer braking control for the operator. FIGS. 5-7 show another embodiment of the invention wherein this is achieved. In this embodiment, the basic control circuit is the same as that previously described and, therefore, this description will not be repeated. 
     This device differs from that previously described in that there is associated with the brake switch S 3  a further potentiometer, indicated by the reference numeral  51  which, like the accelerator potentiometer  17 , outputs its signal indicating the degree of braking called for by the operator to a potentiometer input circuit  52 . This outputs its signal to the CPU  23  so as to provide a control signal. 
     In this embodiment, it will be seen that the exciter current will gradually build up and will then decrease, not because of any reason other than the operator will release the pressure on the brake pedal as the speed of the vehicle decreases. Thus, it is possible to provide a much more modulated braking effect. Of course, this system is somewhat more expensive than the previously described embodiment. 
     As may be seen from FIG. 7, the CPU regulates the braking force based on a combination of brake pedal depression and vehicle speed, in accordance with a suitable mapped relationship. 
     Thus, from the foregoing description, it should be readily apparent that the described embodiments of the invention provide extremely good braking operation for an electric motor-driven vehicle without necessitating use of a frictional brake or with substantially reducing the nature of the frictional brake that must be employed. Of course, the foregoing description is that of preferred embodiments of the invention and various changes and modifications may be made without departing from the spirit and scope of the invention, as defined by the appended claims.