Abstract:
A circuit for providing a braking force to a power tool is provided comprising a motor adapted to rotate a drive shaft in a power tool, a power supply electronically connected to the motor, and a braking module, located between the motor and the power supply, for applying a current limited braking force to the motor when the power supply is disconnected to the motor.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    At least one embodiment of the present invention generally relates to variable speed power tools. More particularly, at least one embodiment of the present invention relates to controlling dynamic braking of electric motors in power tools.  
           [0002]    Hand held power tools, such as electric drills and screw drivers, use electric motors to power a chuck holding a tool. Such power tools usually include a trigger which is manually operated by a user with the motor being controlled by the user pressing the trigger. Power tools in which the motor and chuck speed are controlled based on the amount that the trigger is depressed are known as variable speed power tools. In other power tools the trigger turns the motor on and off but does not vary the speed of the motor.  
           [0003]    Typically, power tools include motors that are powered by an AC or DC power source that delivers current through commutators and brushes to temporary magnets on the motor rotor shaft. As the user squeezes the trigger, more power is delivered through the brushes and commutator to the temporary magnets to cause the shaft to rotate faster. Once the trigger is released, current is no longer delivered to the motor. However, when the trigger is released, the motor begins to operate as a generator so long as the motor rotates (free wheels). While free wheeling, the motor creates current that passes in the opposite direction through the commutators and brushes.  
           [0004]    Conventional power tools typically provide braking of the motor once it is turned off. In many power tools it is desirable to brake the tool in a very short period of time. Variable speed motors may be stopped by electrical means such as by creating an electrical short circuit across the motor when the trigger is released. Actively engaging an electrical means to stop the motor, is known as a dynamic brake. Although dynamic brakes have been provided for braking power tools, such dynamic brakes do so at the expense of the life of the motor.  
           [0005]    In conventional dynamic braking circuits, when the direct short circuit is applied across the motor leads, a high current surge is experienced through the motor. The high current surge is caused since the motor attempts to produce current while free wheeling yet the current has no where to flow but through the short circuit. The high current surge causes the magnetic fields of the temporary magnets on the rotor shaft to interfere with permanent magnets surrounding the rotor shaft. The interfering magnetic fields cause the motor to brake rapidly.  
           [0006]    However, the current spikes cause a significant amount of wear on the brushes of the motor. The life of the motor is related to the amount of wear that the brushes experience.  
           [0007]    A need, therefore, exists for an improved dynamic brake that provides the necessary braking force and reduces damage to and extends the life of the motor.  
         BRIEF SUMMARY OF THE INVENTION  
         [0008]    In accordance with at least one embodiment of the present invention, a circuit for providing a braking force to a power tool is provided comprising a motor adapted to rotate a drive shaft in a power tool, a power supply electronically connected to the motor, and a braking module, located between the motor and the power supply, for applying a current limited braking force to the motor when the power supply is disconnected to the motor.  
           [0009]    One aspect of an embodiment of the present invention is the use of a current limiter for dampening the current induced by the motor while the motor is decelerating to a stopped position. Another aspect of one embodiment of the present invention is the use of a current limit for preventing the current passing through the motor from exceeding a predefined threshold while decelerating the motor to a stopped position. Another aspect of an embodiment of the present invention is the use of a resistive load connected between the terminals of the motor when the power supply is disconnected to induce a braking force on the motor.  
           [0010]    One aspect of an embodiment of the present invention is the use of a resistor connected between the terminals of the motor when the power supply is disconnected to induce a braking force on the motor. Optionally, a resistive bank may be connected between the terminals of the motor when the power supply is disconnected to induce a braking force on the motor. Alternatively, a dynamic brake contact may be connected in series with a resistor brake.  
           [0011]    Another aspect of an embodiment of the present invention is the use of a power MOSFET connected between the terminals of the motor when the power supply is disconnected to induce a braking force on the motor. Alternatively, a resistive material may be formed of at least one of brass or polymer connected between the terminals of the motor when the power supply is disconnected to induce a braking force on the motor. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    The foregoing summary, as well as the following detailed description of the embodiments of the present invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there is shown in the drawings, embodiments which are presently preferred. It should be understood, however, that the present invention is not limited to the precise arrangements and instrumentality shown in the attached drawings.  
         [0013]    [0013]FIG. 1 illustrates a power tool formed according to one embodiment of the present invention.  
         [0014]    [0014]FIG. 2 illustrates a schematic diagram of a control circuit according to one embodiment of the present invention.  
         [0015]    [0015]FIG. 3 is a graph of resistance values of a current dampening resistor of one embodiment of the present invention versus the time required to stop the motor.  
         [0016]    [0016]FIG. 4 is a graph of resistance values of a current dampening resistor of one embodiment of the present invention versus the percentage increase in tool life.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]    [0017]FIG. 1 illustrates an electric power tool  10  with a body  70 , a trigger  80 , a variable speed motor  20 , a chuck  30  for holding a tool, a DC battery  40 , a drive shaft  60  and a circuit  50  for braking the motor according to one embodiment of the present invention. The motor  50  of the drill  10  is adapted to drive the chuck  30  through the shaft  60 . The trigger  80  allows the user to vary the speed of the chuck  30  by controlling the current from the battery  50  to the motor  20  based on how much the user squeezes the trigger  80 .  
         [0018]    [0018]FIG. 2 illustrates the braking circuit  50  formed in accordance with one embodiment of the present invention. The braking circuit  50  includes a DC battery  40 , a first set of contacts  100 , an on-off switch  110 , a second set of contacts  120 , a dynamic brake switch  130 , a resistor  150  and a motor  20 .  
         [0019]    The circuit  50  is used to brake the motor  20  after the user has completely released the trigger  80 . The DC battery  40  has a positive lead  160  and a negative lead  170  electronically connected to the first set of contacts  100 . The on-off switch  110  is controlled by the trigger  80  and is opened when the trigger is released and closed when the trigger is squeezed. The DC battery  40  is attached to, and disconnected from, the motor  20  by the on-off switch  110 . The second set of contacts  120  are electrically connected to a dynamic brake switch  130  and a resistor  140  connected in series. Finally, the motor  20  is electrically connected in parallel to the dynamic brake switch  130  and the resistor  140 .  
         [0020]    In operation, when the user presses the trigger  80  the on-off switch  110  is closed and current flows from the battery  40  to the motor  20  (along path A). The DC battery  40  may include a voltage regulator to regulate the power supplied across the circuit  50  and to the motor  20 . Optionally, an AC power source may be used with an AC-to-DC converter to deliver a DC power to the first set of contacts  100 . The motor  20  drives the drive shaft  60  which turns the chuck  30  at a speed dependant on the trigger  80  position. When the user completely releases the trigger  80  the on-off switch  110  is opened and the dynamic brake switch  130  is closed. This creates a short circuit across the path between the resistor  140 , switch  130  and the motor  20  (along path B), thereby stopping the drive shaft  60  and chuck  30 .  
         [0021]    When the user presses the trigger  80 , the circuit  50  delivers current to the motor  20  which spins the shaft  60 . When the user releases the trigger  80  the dynamic brake switch  130  is closed creating a short across the motor  20 . This short creates a large current across the motor  20  which damages the motor  20  and can reduce the life of the motor  20 .  
         [0022]    The resistor  140  reduces and dampens the current applied across the motor  20  when this short circuit is induced to stop the motor  20 . By dampening the current across the motor  20 , damage to the motor  20  is reduced and the life of the motor  20  is increased.  
         [0023]    The resistance value resistor  140  and desired stop time may vary from tool to tool depending on the design of the tool and manufacturer&#39;s preference. Different power tools or tools made by different manufacturers may have different current generating characteristics and thus may preferably use different resistance values and/or resistor banks to obtain a desired dampening effect. The resistance value for a specific tool may be determined by first inducing a short circuit across the motor  20  to stop the motor  20 , and determining a resistance value that reduces the current on the motor  20  and/or increases the motor  20  life and stops the motor  20  in sufficient time. It will be appreciated that a resistor  140  need not be used. Instead, other devices or components may be used that reduce the peak current on the motor  20  when a short circuit is induced to stop the motor thereby increasing the life of the motor  20 .  
         [0024]    Optionally, the resistor  140  may be a resistor bank with any number of resistors in series or parallel. Alternatively, a power MOSFET or a resistive material may be used in place of the resistor  140 . The resistive material may be either brass, a polymer and the like. Resistor banks and MOSFETs may be adjusted (manually or electrically) to vary the dampening effect on the current pikes experienced during dynamic braking. Alternatively, the MOSFET may be controlled during a dynamic braking operation to yield a first resistance value at the beginning of a braking operation and a different resistance value at the end of the braking operation. The dampening characteristics of the MOSFET may be varied in two or more discrete steps or continuously throughout a braking operation.  
         [0025]    [0025]FIG. 3 illustrates the resistance value versus the time to stop the motor  20  (in milliseconds). FIG. 3 demonstrates that a resistance value sufficient to reduce significant loads on the motor can be selected without significantly changing the time to stop the motor  20 . The resistor  140  therefore reduces wear on the motor  20  and only increases the time to stop the motor  20  by a negligible amount which is unnoticeable to the user. By way of example only, 0.200 ohm and 0.500 ohm resistors in at least one type of power tool stop the chuck in approximately 250 ms and 350 ms, respectively.  
         [0026]    [0026]FIG. 4 illustrates the resistance value versus the increase in tool life. FIG. 4 demonstrates that as the resistance value increases, thereby decreasing the load on the motor, tool life is greatly increased. By way of example only, 0.200 ohm and 0.500 ohm resistors in at least one type of power tool increase the tool life by approximately 32% and 36%, respectively.  
         [0027]    Finally, the invention is not limited to drills. Instead, in other embodiments, the circuit of FIG. 2 is adaptable to various other types of commercial and residential power tools, such as cordless or AC power screw drivers, saws, and others. Also, the tool need not be variable in speed, but instead merely may have ON and OFF states.  
         [0028]    While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its scope. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.