Abstract:
A control system for driving a power tool is provided comprising a power source, a motor adapted to drive a shaft, a power switching unit interconnecting the power source and the motor, and a controller. The power switching unit applying a pulse width modulated (PWM) drive signal from the power source to the motor. The controller monitoring at least one electrical characteristic of at least one of the power source, motor and power switching unit and adjusting the operating duty cycle of the PWM drive signal based on the electrical characteristics.

Description:
BACKGROUND OF THE INVENTION 
     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 the speed and frequency of electric motors in power tools. 
     Hand held power tools, such as electric drills screw drivers and the like, 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 varied based on the amount that the trigger is depressed are known as variable speed power tools. Power tools include motors that are powered by an AC or DC power source that delivers current to the motor. As the user squeezes the trigger, more power is delivered to the motor to cause the shaft to rotate faster. Once the trigger is released, current is no longer delivered to the motor. 
     Typically, power tools include speed control circuits that use pulse width modulation (PWM) to control the voltage applied to the motor. More specifically, the PWM control circuit rapidly cycles power on and off to the motor. The PWM control circuit controls the duty cycles based on the trigger position. The more the trigger is squeezed the larger the on-time duty cycle is and the faster the shaft rotates. 
     Power tools often experience high current or stalled conditions when a work load exceeds the capability of the motor or the battery. These conditions create extreme loads on the battery, motor and other electric components of the tool. These conditions also reduce the effectiveness of the tool by damaging the battery, motor and other electric components of the tool. 
     Conventional power tools exaggerate the negative effects of stalled conditions by including a by-pass contact that, when closed, by-passes the variable speed control. The by-pass contact is closed when the desired power output exceeds a certain point. When the by-pass contact closes, the tool directly connects the motor and battery to deliver all available power to the motor. Under certain conditions the use of a by-pass contact is undesirable because it may damage the battery, motor or other electrical components in the tool. The use of a by-pass contact therefore may lead to a reduced tool life and may also lead to a stalled motor condition. 
     A need exists for a control circuit that more effectively monitors the electrical condition of the power tool in determining the duty cycle. A need also exists for a control circuit that monitors the electrical conditions of the power tool in determining the frequency of the duty cycle. A need further exists for a power tool controller that provides a maximum amount of power to the motor without damaging the battery and that eliminates or reduces stalled motor conditions. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with at least one embodiment of the present invention, a control system is provided for driving a power tool, comprising a power source, a motor adapted to drive a shaft, and a power switching unit interconnecting the power source and the motor. The power switching unit applies a pulse width modulated (PWM) drive signal from the power source to the motor. A controller monitors at least one electrical characteristic of at least one of the power source, motor and power switching unit, and adjusts an operating duty cycle of the PWM drive signal based on the electrical characteristic. 
     One aspect of another embodiment of the present invention is monitoring the voltage of the power source, the motor or the power switching unit. Optionally, the system may monitor the current of the power source, the motor or the power switching unit. 
     Another aspect of an embodiment of the present invention is the use of a controller that detects a voltage drop across the power source. Optionally, the controller detects a voltage drop across said power source and the motor. 
     In one embodiment of the present invention, the power switching unit comprises a power MOSFET connected in series between the power source and the motor. The power MOSFET switches between ON and OFF states to vary the pulse width of said PWM drive signal. Optionally, an input lead connected to the controller provides a user trigger signal indicative of a trigger position or a motor speed. Alternatively, the PWM drive signal adjusting the motor speed. 
     Another aspect of an embodiment of the present invention is the use of a voltage sensor to monitor a voltage drop across at least one of the power source, the motor and the controller. Optionally, the controller determines a target duty cycle representative of a target motor condition selected by a user and sets the operating duty cycle below the target duty cycle or at a value not equal to the target duty cycle. Optionally, the target motor condition may constitute the motor speed or torque. Alternatively, the operating duty cycle may be set from the peak current and time period over which the power source delivers a current at or near the peak current. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing summary, as well as the following detailed description of the preferred 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 present preferred. It should be understood, however, that the present invention is not limited to the precise arrangements and instrumentality shown in the attached drawings. 
     FIG. 1 illustrates a power tool formed according to one embodiment of the present invention. 
     FIG. 2 illustrates a schematic diagram of a control circuit according to one embodiment of the present invention. 
     FIG. 3 is a graph of applied voltage versus time for different power tools and preferred embodiments of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates an electric power tool  10  with a body  70 , a trigger  80 , a forward/reverse control  90 , a variable speed motor  20 , a chuck  30  for holding a tool, a DC battery  40 , a drive shaft  60  and a control system  50  for driving the motor  20 . The motor  20  of the tool  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 delivered from the battery  40  to the motor  20  based on how much the user squeezes the trigger  80 . 
     FIG. 2 illustrates the control system  50  formed in accordance with one embodiment of the present invention. The control system  50  is connected in series with DC battery  40  and motor  20 . The DC battery  40  has a positive terminal  41  and a negative terminal  42  electronically connected to the first set of contacts  100 . The terminals  41  and  42  of the battery  40  are connected to a first set of contacts  100  and arranged in a series with an on-off switch  110 . A brake switch  115  and a power MOSFET  130  are arranged in series with one another and are connected across the contacts  100  and the battery  40 . 
     A voltage regulator  140  include an input connected through one contact  100  to one terminal of the battery  40 . An output of the voltage regulator  140  is connected to a power input terminal VCC on a microprocessor  120 . The voltage regulator  140  regulates the voltage delivered to the microprocessor  120 . The voltage regulator  140  also includes a ground terminal GND that is connected to one end of a potentiometer  150 . An opposite end of the potentiometer  150  is connected to the input terminal VCC of the microprocessor  120 . A center tap on the potentiometer  150  is connected to a reference input terminal GP  4  on the microprocessor  120  to monitor the output voltage of the voltage regulator  140 . The microprocessor  120  is connected to an audio output  160 . 
     A first voltage divider  170 ,  171  is provided between the terminals of the battery  40 . A center tap  172  of the voltage divider  170 ,  171  is connected to an input terminal GP1 of the microprocessor  120  to monitor the voltage potential across the battery  40 . A second voltage divider  180 ,  181  is provided across the terminals of the power MOSFET  130 . A center tap  182  of the voltage divider  180 ,  181  is connected to an input terminal GP2 of the microprocessor  120  to monitor the voltage potential across the power MOSFET  130 . 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. 
     The control system  50  determines the duty cycle and/or frequency of the motor  20  when the user squeezes the trigger  80 . The on-off switch  110  is controlled by the trigger  80  and is opened when the trigger  80  is released and closed when the trigger  80  is squeezed. Optionally, the on-off switch  110  may also be opened and closed based on a button located proximate the trigger  80  to afford added sofets. The DC battery  40  is attached to, and disconnected from the motor  20  by the on-off switch  110 . The inductance and resistance of the motor  20  are schematically modeled in FIG. 2 as coil inductance  21  and coil resistance  22 . The motor  20  also includes a forward/reverse switch  25  that allows the user to switch the direction of the tool through a forward reverse control  90 . When the user completely releases the trigger  80  the on-off switch  110  is opened and the brake switch  115  is closed. When the brake switch  115  closes, it creates a short circuit across the terminals of the motor  20 . When on-off switch  110  is opened, power is no longer delivered to the motor  20 . However, the motor  20  continues to rotate and thus function as a generator. While the motor  20  operates as a generator, it produces current that is short circuited by the brake switch  115 . The short circuit inhibits current flow from the motor  20  which in turn causes the magnetic fields created by the windings to interfere with the magnetic fields of the surrounding permanent magnets, thereby inducing a braking force onto the drive shaft  60  and chuck  30 . 
     The control system also includes a fly wheel diode  200  which is electrically connected to the motor  20 . When current passes through the inductor  21 , yet the power MOSFET  130  is turned off, the current is dissipated through the flywheel diode  200 . Two diodes  210 ,  220  may also be provided that prevent the power MOSFET  130  from turning off too quickly. 
     The power MOSFET  130  and microprocessor  120  are electrically connected to the control system. The microprocessor  120  cycles the power MOSFET  130  on and off to generate a PWM current/voltage to the motor  20 . The microprocessor  120  may be a commercially available microprocessor such as an eight pin microprocessor. The microprocessor  120  may be larger or smaller depending on the number of components or features of the tool  10 . 
     The control system  50  contains two voltage divider networks  170 - 172  and  180 - 182  that sense the voltage of electrical components of the tool. One voltage divider network  180 - 182  is electrically connected to the battery  40 , senses the voltage across the battery  40  and provides the battery voltage to the microprocessor  120 . Another voltage divider network  170 - 172  is electrically connected to the power MOSFET  130 , senses the voltage across the MOSFET  130  and provides the voltage across the power MOSFET  130  to the microprocessor  120 . 
     The control system  50  contains two voltage divider networks  170 - 172  and  180 - 182  that sense the voltage of electrical components of the tool. One voltage divider network  170 - 172  is electrically connected to the battery  40 , senses the voltage across the battery  40  and provides the battery voltage to the microprocessor  120 . Another voltage divider network  180 - 182  is electrically connected to the power MOSFET  130 , senses the voltage across the MOSFET  130  and provides the voltage across the power MOSFET  130  to the microprocessor  120 . 
     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 and in the direction of path A). The microprocessor  120  determines the desired duty cycle based on the trigger  80  position. The microprocessor  120  monitors the voltage across the battery  40  and the power MOSFET  130  and determines if the desired duty cycle (based on the user input) exceeds a maximum safe output. 
     If the microprocessor  120  determines that the desired output is within a safe range then the actual duty cycle will be the desired duty cycle selected by the user. The control system  50  sends a PWM current/voltage signal to the motor  20  in accordance with the user selected duty cycle and the motor  20  drives the drive shaft  60  which turns the chuck  30 . If, however, the microprocessor  120  determines that the desired duty cycle is outside safe operating parameters, the microprocessor  120  will adjust the duty cycle to limit or eliminate damage to the battery  40 , motor  20  or power MOSFET  130 . After the microprocessor  120  determines a duty cycle within a safe operating range, the microprocessor  120  supplies a PWM current/voltage to the motor  20  by cycling the power MOSFET  130  on and off. When the user completely releases the trigger  80  the on-off switch  110  is opened. 
     By way of example only, the user may squeeze the trigger  80  to indicate a desire that the drive shaft  60  spin at 75% of its maximum rotation capacity. However, the microprocessor  120  may determine that a duty cycle associated with a drive shaft  60  rotational speed of 75% of the maximum speed is either not attainable or not desirable given the present condition of the battery  40 , present forces being induced on the drive shaft  60 , demands presently being placed on the motor  20  and power MOSFET  130 , and other considerations. Based upon these inputs, the microprocessor  120  may determine that a lower duty cycle associated with a rotation speed of less than 75% may be preferable. Accordingly, the microprocessor  120  may, by way of example only, drive the power MOSFET  130  to deliver a PWM current/voltage to the motor  20  only affording a rotation speed of approximately 50% of the maximum rotation speed for the drive shaft  60 . 
     The control system monitors and limits excessive currents being applied to the motor  20 , battery  40 , and other electrical components of the tool. The control system may also monitor a decreasing charge on the battery  40  and prevent discharging of the battery  40  below a certain level. 
     The microprocessor  120  monitors the battery  40  voltage and the voltage across the power MOSFET  130  through the voltage dividers  170 - 172  and  180 - 182 . These inputs allow the microprocessor  120  to determine the condition of the battery  40  and the current applied to the motor  20 . For example, the microprocessor  120  can detect excessive currents across the motor  20 , the battery  40  and other electrical components. When the microprocessor  120  detects an excessive current across the motor  20 , power MOSFET  130  or other electrical component, the duty cycle can be lower and thereby lowering the current to an acceptable level. The microprocessor  120  can also detect decreasing voltage in the battery  40 . When the microprocessor  120  detects a low voltage situation across the battery  40 , the microprocessor  120  can lower the duty cycle to reduce the voltage drain on the battery  40 . 
     The control system  50  also monitors the electrical conditions of the tool to determine if the tool  10  has stalled. Once the microprocessor  120  determines that the tool  10  is stalled it switches to a “ratchet mode” and changes the frequency at which the drive signal is supplied to the motor  20 . By changing the frequency of the drive signal, the control system maximizes the available current and increases the tools ability to eliminate the stalled condition. 
     For example, if the user squeezes the trigger  80  and the microprocessor  120  determines that a 50% duty cycle should be applied. The power MOSFET  130  is turning on and off in a pulse with modulation and is supplying current to the motor  20  50% of the time. When the power MOSFET  130  is on it is drawing a high current from the battery  40 , therefore the voltage across the MOSFET  130  is increasing and the voltage across the battery  40  is decreasing. This situation indicates that the tool is pulling a high current. Then in the next half cycle, the MOSFET  130  is turned off. If the motor  20  is not rotating, that is if there is no voltage generated across the motor  20 , then this situation indicates the motor  20  is in a stalled condition. When the microprocessor  120  detects a stalled motor  20  condition the microprocessor  120  will switch the tool  10  into a ratchet mode. In the ratchet mode, the microprocessor  120  changes the frequency at which current is supplied to the motor  20 . Stated another way, when in the ratchet mode, the microprocessor  120  lengthens the period or duty cycle. For example, during normal operation, the frequency may be 10 kHz which corresponds to a period of 0.1 milliseconds. During the ratchet mode, the frequency may be lowered to 1 Hz which corresponds to a period or duty cycle of 1 second. 
     By changing the frequency of the current to the motor  20  to a lower frequency, short high current bursts are delivered to the motor  20 . The ratchet mode operation reduces the amount of voltage drained from the battery  40 . The ratchet mode also increases the ability of the tool to eliminate the stalled motor  20  condition. In one embodiment, the microprocessor can be set to wait a predetermined number of cycles after the microprocessor  120  senses a stalled condition before the microprocessor  120  will switch into the ratchet mode. 
     FIG. 3 illustrates a series of graphs of voltages versus time for a) a tool only having on/off states, b) a tool controlled with PWM, and c) a tool controlled in a ratchet mode. As shown in FIG. 3, a tool operated in a ratchet mode delivers longer pulses at the on voltage to the motor, followed by longer periods of a zero or low voltage state. 
     Optionally, other components or measurements can be monitored to determine if the duty cycle or frequency (i.e., ratchet mode) of the tool should be adjusted by the microprocessor  120 . For example, the speed of the motor  20  could be monitored by the microprocessor  120 . Additionally, the speaker  160  may be used to indicate when either the duty cycle has been adjusted, when the tool is in ratchet mode, or when the battery  40  or another component needs to be changed. 
     Optionally, control system  50  may be used in other types of power tools, such as screw drivers, saws, and others. 
     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.