Abstract:
A vehicle speed determining arrangement for a vehicle driven by an electric motor wherein an actual speed sensor is not required and the vehicle speed is determined by measuring the actual current flow through the motor.

Description:
BACKGROUND OF INVENTION  
       [0001]     This invention relates to an electrically driven vehicle and more particularly to an improved and simplified vehicle speed control and particularly to one powered by a shunt motor.  
         [0002]     A wide variety of vehicles are provided with electric motors as a power source as a prime mover therefore. For example, as shown in Japanese Published Patent Application Hei 10-309005 (A) it has been proposed to employ a DC shunt motor for the driving of electric powered vehicles such as golf carts or other vehicles. There the armature coil and the field coil are connected in parallel to a common electric power source. As is known, it is possible to energize the armature coil and the field coil independently from each other. The amount of current supplied to the armature coil is controlled based on the position of a operator controlled vehicle speed control such as, for example, an accelerator pedal. Then a specific current is supplied to the field coil depending on the armature current value. This is generally done by reference to a field map that is constant or pre-designed for each motor. This produces a specific torque from the electric motor to control the operation required by the various operating conditions of the electric vehicle.  
         [0003]     One type of control employed is effective to limit the speed of the vehicle so that a specific speed will not be exceeded, regardless of the operator demand. Alternately or additionally torque is generated in the motor, and the motor can be driven and controlled depending on the operating conditions.  
         [0004]     In an electric vehicle such as a golf car, the vehicle speed is detected, and the motor driving current is controlled to regeneratively brake the motor when the vehicle speed exceeds a speed limit and/or the current to the field coil relative to the current to the armature coil is changed depending on the vehicle speed for controlling the motor driving current so that the motor can be optimally controlled depending on the vehicle speed.  
         [0005]     With arrangements of this type, it is therefore necessary to determine the actual vehicle speed. This is generally done by calculating the vehicle speed by measuring pulses from an encoder provided in the motor or from pulses generated by a projection or a magnet, provided on a driven component of the vehicle such as a drive or axle shaft. This adds to the cost and complicates construction.  
         [0006]     It is therefore a principal object of the invention to provide an improved and simplified manner for detecting the traveling speed of an electric motor driven vehicle.  
       SUMMARY OF THE INVENTION  
       [0007]     A first feature of the invention is adapted to be embodied in a speed sensor for a vehicle driven by an electrical motor. A control outputs a driving current to the electric motor. A current sensor detects the current flow through the electric motor and a calculator determined the vehicle speed from the measured current.  
         [0008]     In accordance with a further characteristic of this first feature, the electric motor is a shunt motor having field and armature coils and the current to these coils is controlled by the control.  
         [0009]     A second feature of the invention is adapted to be embodied in determining the speed of a vehicle driven by an electrical motor. In accordance with this method, the driving current to the motor is controlled. The current flow through the coil detected and the vehicle speed determined from the measured current.  
         [0010]     In accordance with a further characteristic of this second feature, the electric motor is a shunt motor having field and armature coils and the current to these coils is controlled. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a partially schematic top elevational view of an electric powered vehicle constructed and operated in accordance with the invention.  
         [0012]      FIG. 2  is a schematic electrical diagram of the vehicle and its control.  
         [0013]      FIG. 3  is a graphical view of the relation of the current in the coils. 
     
    
     DETAILED DESCRIPTION  
       [0014]     Referring now in detail to the drawings and initially to  FIG. 1 , an electrically powered vehicle such as a golf cart, as an example of vehicle with which the invention may be practiced is identified generally by the reference numeral  21 . This golf cart  21  is provided with a body, frame  22  that rotatably supports in any desired manner paired front wheels  23  and rear wheels  24 . In the illustrated embodiment, the rear wheels  24  are driven by a shunt type electric motor  25  through a transmission  26 . Associated with some or all of the wheels  23  and  24  (only the front wheels  23  in the illustrated embodiment) are brakes  27  of any desired type.  
         [0015]     An operator may be seated on a suitable seat (neither of which are shown) behind an accelerator pedal  28 , for controlling the speed of the electric motor  25 , a brake pedal  29 , for operating the wheel brakes  27 , and a steering wheel  31 , for steering the front wheels  23  in any desired manner.  
         [0016]     Also juxtaposed to the operator&#39;s position is a main switch  32 , and a direction control switch  33 , for controlling the direction of travel of the golf cart  21  by controlling the direction of rotation of the motor  25 . The main switch  32  and the direction control switch  33  are connected to a controller  34 . Operation of the accelerator pedal  28  is transmitted to an on off pedal switch  35  and an accelerator opening degree sensor  36  connected to the controller  34 , to send on or off state of the accelerator  28  and its degree of opening to the controller  34 .  
         [0017]     A plurality of batteries  37  (48 V in total, for example) as power sources are mounted suitably on the body frame  22  and are connected through a relay  38  to the controller  34 .  
         [0018]     Referring now to  FIG. 2  this is a circuit block diagram of the electric vehicle  21  using the shunt motor  25  and embodying the invention. The supply voltage is supplied to the shunt motor  25  for driving the vehicle  21  and the controller  34  for driving and controlling the shunt motor  25  from the batteries  37 . The supply voltage (48 V) from the batteries  37  is supplied to the shunt motor  25  via the relay  38  and to the controller  34  via a fuse  39  and a towing switch  41 . The towing switch  41  is used to stop the supply of power to the controller  34  when necessary such as when the vehicle is towed and the operation of an automatic brake circuit is stopped. The supply voltage from the battery  37  is converted to 5 V by a voltage regulator  42  and a 5V power source circuit  43  in the controller  34  and then supplied to calculation circuits and drive circuits in the controller  34  which will be described shortly.  
         [0019]     Signals from the main switch  11 , the accelerator pedal switch  12 , the direction change shift switch  13  and the accelerator opening sensor  14  are inputted into a CPU  44 . The CPU  44  controls to drive the shunt motor  25  based on these signals.  
         [0020]     The shunt motor  25  has an armature coil  45  and a field coil  46 . A command current is calculated in an armature PWM calculation circuit  47  of the CPU  44  is applied to the armature coil  45  via an armature drive circuit  48 . In this case, the command current is a PWM signal indicating the percentage (%) of the drive pulse width and a driving current is supplied to the armature coil  45  according to the PWM (%) command.  
         [0021]     The armature drive circuit  48  consists of, by way of example, a bipolar circuit containing two arrays of eight FETs, and the arrays of FETs are alternately switched on and off to apply the driving current to the armature coil  45 .  
         [0022]     A field drive circuit  49  consisting of, by way of example, an H-bridge circuit having four FETs effects changes in the direction of current by switching obliquely paired two FETs on or off simultaneously.  
         [0023]     A command current calculated in a field PWM calculation circuit  51  of the CPU  44  is applied to the field coil  46  via a field drive circuit  49 . The command signal for the field coil  46  is calculated based on an Ia-If map, as shown in  FIG. 3 , is stored in the memory  52 . The Ia-If map is a map showing the field current (If) to the armature current (Ia) at which the motor is driven at the maximum degree of efficiency based upon the motor characteristics.  
         [0024]     As with the armature current, the command signal for the field current is a PWM signal indicating the percentage (%) of the drive pulse width. A driving current is supplied to the field coil  46  according to the PWM (%) command.  
         [0025]     The current which actually flows through the armature coil  45  is detected by a current sensor  53  and the command signal to the armature coil  45  is feedback-controlled. The current which actually flows through the field coil  46  is detected by a current sensor  54  and the command signal to the field coil  46  is feedback-controlled.  
         [0026]     The CPU  44  has a vehicle speed calculation circuit  55 . The vehicle speed calculation circuit  55  calculates the vehicle speed based on a detection value from the armature current sensor  53  for the armature coil  45  in the manner now to be described.  
         [0027]     This methodology is based on the principle that the motor shaft angular speed is proportional to the counter electromotive voltage. The proportionality constant is a counter electromotive voltage constant. The motor angular speed w is calculated based on the following equations: 
 
 V   r   =K   v ×ω
 
 e   o   =V   B   ×DUTY  
 
  e   o   =R   m   ×I   m   +K   v ×ω. 
 
Thus, 
 
ω= V   r   /K   v =( V   B   ×DUTY−R   m   ×I   m )/ K   v  
        where 
            V r : counter electromotive voltage (V),     e o : voltage applied to the motor (V),     R m : equivalent resistance of the motor (Ω),     I m : motor current (armature coil current) (A),     K v : counter electromotive voltage constant (V/rad/s) (K v  is proportional to the field current If.),     ω: motor angular speed (rad/s),     V B : battery voltage (V), and     DUTY: PWM command value (%).    
               
 
         [0037]     Once the motor angular speed is obtained, the vehicle speed is can be calculated based on the mechanical transmission characteristics from the motor to the axle and tire which can be calculated in advance and is constant.  
         [0038]     Thus from the foregoing description it should be readily apparent to those skilled in the art that a simplified method and structure for determining the vehicle speed for a variety of purposes is possible without requiring separate sensors or detectors and thus simplifying the construction and reducing costs. Of course those skilled in the art will readily understand that the described embodiments are only exemplary of forms that the invention may take and that various changes and modifications may be made without departing from the spirit and scope of the invention, as defined by the appended claims.