Abstract:
A power tool including a braking and control circuit. The braking and control circuit includes a microcontroller-based control means circuit. The microcontroller assures control of switch means, such as triacs, switches and relays, and ensures that braking is effectuated regardless of the phase in the power cycle of the alternating current. Also, the microcontroller is programmable so that the braking and control circuit accommodates different braking conditions for different power tools and accommodates combinations of braking conditions for the same power tool. Further, the microcontroller is programmable to configure the braking and control circuit so that the braking force applied to the motor and the stopping time of the motor are regulated and adjustable. This may be accomplished by outputting a control signal so that the switch means skips cycles in the alternating current or by otherwise adjusting the operation of the switch means.

Description:
RELATED APPLICATIONS 
     This application claims the benefit of prior filed provisional patent application, serial No. 60/088,176, filed on Jun. 5, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to motors for electric power tools. More particularly, the invention relates to a braking and control circuit for such a motor. 
     AC universal motors have commonly been used in electric power tools, such as drills, circular saws and other types of equipment. Generally, such motors provide a high-power, light-weight power source for these power tools. Typically, the universal motor includes a housing, a stator assembly having a run coil, and a rotatable shaft or arbor having an armature mounted thereon. Current flows through the run coil and creates a magnetic field which interacts with the magnetic field of the armature. This interaction rotatably drives the arbor. To drive a tool element, such as a drill bit or a saw blade, the tool element may be mounted directly on the arbor or be coupled to the arbor by a gear transmission or the like. 
     Conventional universal motors tend to coast, i.e., the arbor continues to rotate for some time after the motor is disconnected from the electrical power source. This coasting generally results from the rotational momentum of the arbor, the transmission, and the tool attachments. To prevent or limit coasting, the motor often includes a braking arrangement. 
     A typical braking arrangement includes a dynamic braking circuit which relies on passive generation of free wheeling current in the stator to produce a counter-electromagnetic force (counter-EMF) to stop the rotation of the arbor and to, thereby, brake the motor. One such dynamic braking circuit is shown and described in U.S. Pat. No. 5,294,874. 
     SUMMARY OF THE INVENTION 
     One problem with existing braking arrangements, such as the above-described dynamic braking circuit is that, if the motor is disconnected from the power source and reconnected in a closed loop at a point or phase in the power cycle of the alternating current at which there is little or no voltage, the braking circuit will not generate the necessary counter-EMF to brake the arbor. 
     Another problem with existing braking arrangements is that the conditions in which braking required are different for different power tools. Therefore, for different power tools, the braking arrangement must be configured to accommodate the different braking conditions. For example, in some power tools, such as portable drill presses, braking is required when the drill press accidentally disconnects from the workpiece (“breakaway”) during drilling operations, a safety-related braking condition. In other power tools, such as circular saws, braking is required when the tool element, such as the saw blade, binds on the workpiece and the power tool is jerked or kicks back, another safety-related braking condition. Further, in some power tools, braking may be desired each time the operator releases the trigger so that the blade stops quickly and the operator can move to the next drilling or cutting operation, a productivity-related braking condition. 
     Yet another problem with existing braking arrangements is that, if the motor is braked too quickly, the arcing occurs between the rotor and the commutator brushes, thereby reducing the life of the motor and the brushes. This arcing can be especially problematic if the motor is braked frequently, i.e., productivity-related braking. However, if the motor is not braked quickly enough, the braking can be ineffective, i.e., in a safety-related braking condition. 
     The present invention provides a power tool including a braking and control circuit that alleviates the problems with existing braking arrangements. The present invention provides a braking and control circuit including a microcontroller-based control circuit. The microcontroller assures control of switch means, such as triacs, switches and relays, and ensures that braking is effectuated regardless of the phase in the power cycle of the alternating current. Also, the microcontroller is programmable so that the braking and control circuit accommodates different braking conditions for different power tools and accommodates combinations of braking conditions for the same power tool. Further, the microcontroller is programmable to configure the braking and control circuit so that the braking force applied to the motor and the stopping time of the motor are regulated and adjustable. This may be accomplished by outputting a control signal so that the switch means skips cycles in the alternating current or by otherwise adjusting the operation of the switch means. 
     The present invention provides a braking and control circuit for an electric motor, the motor including a housing, a stator supported by the housing, and a shaft rotatably supported by the housing, wherein the stator is selectively connected with a power source to rotatably drive the shaft. The braking and control circuit comprises first switch means for selectively disconnecting the motor from the power source, second switch means electrically connected across the motor; and control means electrically connected with at least one of the first switch and the second switch means and operable to output a control signal to control the at least one of the first switch means and the second switch means to brake the motor. 
     The control means is preferably electrically connected with the first switch means and with the second switch means. Preferably, the control means outputs a first control signal to the first switch means so that the first switch means disconnects the motor from the power source. At approximately the same time or shortly thereafter, the control means also preferably outputs a second control signal to the second switch means so that the second switch means connects the motor in a closed loop and generates a counter-electromagnetic force to brake the motor. Also, the control means preferably selectively outputs the first control signal to the first switch means so that the first switch means selectively disconnects and reconnects the motor and the power source and selectively outputs the second control signal to the second switch means so that the second switch means selectively connects and disconnects the motor in a closed loop to regulate a braking force applied to brake the motor. 
     The control means preferably includes a microcontroller operable to output the control signal and programmable to optimize braking of the motor. Preferably, the microcontroller is programmable to change the stopping time of the motor and to change the braking force applied to the motor. Also, the microcontroller is preferably programmable to output the control signal on selected ones of the plurality of cycles of the alternating current to control the first switch means and the second switch means on the selected ones of the plurality of cycles to brake the motor and to output the control signal at a point in the alternating current so that a desired voltage is supplied to brake the motor. 
     The present invention also provides a power tool comprising a housing, an electric motor, and braking and control means for controlling and braking the motor. The braking and control means includes switch means electrically connected with the motor and control means electrically connected with the switch means and operable to output a control signal to control the switch means to brake the motor. 
     Preferably, the control means includes a microcontroller operable to output the control signal. The switch means are preferably operable to selectively disconnect the motor from the power source, and the braking and control means preferably further includes second switch means electrically connected with the motor and operable to selectively connect the motor in a closed loop. The microcontroller preferably outputs a first control signal to the first switch means so that the first switch means disconnects the motor from the power source and outputs a second control signal to the second switch means so that the second switch means connects the motor in a closed loop and generates a counter-electromagnetic force to brake the motor. 
     Preferably, the microcontroller is programmable to configure the braking and control means for a selected power tool and for a selected braking condition. The power tool further preferably comprises trigger means electrically connected with the control means and operable to trigger braking of the motor when a braking condition for the power tool exists. The trigger means outputs a trigger signal to the control means so that the microcontroller outputs the control signal to the switch means to brake the motor. The trigger means may include the on/off switch or may include sensing means for sensing a safety-related braking condition, such as “breakaway” of a drill press from a workpiece or “binding” of a tool element, i.e., a saw blade, on the workpiece. 
     One advantage of the present invention is that the braking and control circuit operates to provide the necessary braking force regardless of the phase of the alternating current being supplied to the motor. 
     Another advantage of the present invention is that the braking and control circuit is programmable to accommodate different braking conditions for different power tools and accommodates combinations of braking conditions in the same power tool. 
     Yet another advantage of the present invention is that the braking force applied to the arbor may be controlled and adjusted so that the arbor may be stopped more or less quickly. 
     Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims and drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a power tool and a braking and control circuit embodying the invention. 
     FIG. 2 is a schematic illustration of an electric motor and the braking and control circuit. 
     FIG. 3 is a schematic illustration of an alternative embodiment of the motor and the braking and control circuit. 
     FIGS. 4A and 4B are schematic diagrams of portions of the motor and the braking and control circuit. 
     FIGS. 5A,  5 B and  5 C are detailed schematic diagrams of the portions of the motor and the braking and control circuit illustrated in FIG.  4 A. 
     FIGS. 6A and 6B are detailed schematic diagrams of the portions of the motor and the braking and control circuit illustrated in FIG.  4 B. 
     FIG. 7 is a perspective view of an alternative power tool and a braking and control circuit embodying the invention. 
    
    
     Before one embodiment of the invention is explained in detail, it is to be understood that the invention is not limited in its application to the details of the construction and the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or carried out in various ways. Also, it is understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIGS. 1-3 illustrate a power tool including a braking and control circuit  10  (schematically illustrated in FIGS. 2-3) embodying the invention and for braking and controlling an electric motor  14  (schematically illustrated in FIGS.  2 - 3 ). In the illustrated construction, the power tool (see FIG. 1) is a portable drill press  18  including a housing  22  supported by a base  26 . The base  26  includes a force applying element  30  (partially shown) for connecting the base  26  to the surface of a workpiece W. In the illustrated construction, the force applying element  30  is an electromagnet assembly  34  (partially shown) for attaching the drill press  18  to a ferro-magnetic workpiece W. In other constructions (not shown), the force applying element  30  may be a permanent magnet, a vacuum pad, or a clamp mechanism. 
     The electric motor  14  is supported by the housing  22  and is operable to rotatably drive a spindle assembly  38 . The spindle assembly  38  is connected to a tool element, such as a drill bit  42 , to drill through or cut the workpiece W. An on/off switch  46  is operated by a trigger or button  48  and selectively connects the electric motor  14  to a power source  50 . 
     The motor  14  is generally conventional and, as shown in FIGS. 2-3, includes a rotating arbor or shaft  54  on which an armature (not shown) is mounted. The armature includes armature windings (not shown) for generating an armature field. Electricity is conducted to the rotating armature by a pair of commutator brushes  58 . The motor  14  also includes a stator assembly (partially shown) including run winding means  62  for generating a magnetic field for rotating the armature and the arbor  54 . 
     The braking and control circuit  10  includes (see FIGS. 2-3) first switch means  66  connected in series with the motor  14  and operable to selectively disconnect the motor  14  from the power source  50  to brake the motor  14 . In the illustrated construction, the first switch means  66  includes a triac  70  which is turned off to disconnect the motor  14  from the power source  50 . However, in other constructions (not shown), the first switch means  66  may be any type of switch means which is operable to disconnect the motor  14  from the power source  50 . 
     The braking and control circuit  10  also includes (see FIGS. 2-3) second switch means  74  connected in parallel with the motor  14  across the armature. The second switch means is operable to connect the motor  14  in a closed loop and to generate counter-electromagnetic force (counter-EMF) to brake the motor  14 . In the construction illustrated in FIG. 2, the second switch means  74  includes a mechanical switch  78  which is normally open and which is closed to connect the motor  14  in the closed loop. In the alternative construction illustrated in FIG. 3, the second switch means includes a solid state switch, such as a triac  82  or other solid state device (not shown). The triac  82  is normally turned off and is turned on to connect the motor in the closed loop. 
     In addition, the braking and control circuit  10  includes (see FIGS. 2-3) a control circuit or control means  86  electrically connected to at least one of the first switch means  66  and the second switch means  74  and operable to control at least one of the first switch means  66  on the second switch means  74  to brake the motor  14 . In the preferred embodiment, the control means  86  is electrically connected to and controls both the first switch means  66  and the second switch means  74  to brake the motor  14 . To brake and control the motor  14 , the control means  86  selectively outputs a first control signal to control the first switch means  66 , to disconnect the motor  14  from the power source  50 , and selectively outputs a second control signal to control the second switch means  74 , to connect the motor  14  in a closed loop and to generate counter-EMF. 
     The braking and control circuit  10  further includes (see FIGS. 2-3) trigger means  90  electrically connected to the control means  86  and operable to trigger braking of the motor  14 . When a braking condition occurs, the trigger means  90  outputs a trigger signal to the control means  86  to trigger braking of the motor  14 . 
     There are two general categories of braking conditions, i.e., conditions in which braking of the motor  14  is required or desired. The first category includes safety-related braking conditions. In this category, braking of the motor  14  is required if an unsafe operating condition for the power tool arises. For example, such a safety-related braking condition occurs if the force applying element  30  of the drill press  18  accidentally disconnects from the workpiece W during drilling operations (“breakaway”). Another safety-related braking condition occurs when the tool element, such as a drill bit or a saw blade, binds on the workpiece W causing the power tool to jerk or kick back. In either of these safety-related braking conditions, braking of the motor  14  is required to prevent injury to the operator or damage to the equipment or workpiece W. Further, in such safety-related braking conditions, braking of the motor  14  is accomplished as quickly as possible without damaging the components of the motor  14  (i.e., the motor  14  is braked in 1 sec.). 
     The other category of braking conditions includes productivity-related braking conditions. In this category, braking of the motor  14  is desired to stop the associated tool element so that the operator can move to the next drilling or cutting operation more quickly. The operator does not have to wait for the tool element to coast to a stop before continuing operations. Such productivity-related braking can be accomplished more slowly than the safety-related braking to reduce the wear on the motor (i.e., the motor  14  is braked in 2 sec.). This is important because productivity-related braking occurs more frequently than safety-related braking. Generally, a productivity-related braking condition results when the operator releases the trigger and on/off switch to disconnect the motor from the power source. 
     In the construction illustrated in FIG. 2, the trigger means  90  includes sensing means  92  for sensing a safety-related braking condition. Specifically, the drill press  18  includes a breakaway sensor  94  for sensing breakaway of the base  26  and the electromagnet assembly  34  from the workpiece W. Such a breakaway sensor  94  may be any type of sensing means such as a mechanical sensor, i.e., a depressible plunger (not shown), an electrical sensor, or a magnetic sensor, i.e., a Hall Effect sensor, capable of sensing relative movement of the drill press  18  and the workpiece W or “breakaway” of the base  26  from the workpiece W. In this construction, if the drill press  18  breaks away from the workpiece W, the breakaway sensor  94  outputs the trigger signal, a “breakaway” signal, to the control means  86  to trigger braking of the motor  14 . 
     In the construction illustrated in FIG. 3, the trigger means  90  triggers braking for a productivity-related braking condition. In the illustrated construction, the trigger means  90  includes the on/off switch  46 . When the operator releases the trigger  48 , so that the on/off switch  46  disconnects the motor  14  from the power source  50 , the trigger signal, an “off” signal, is output to the control means  86  to trigger braking of the motor  14 . 
     It should be understood that, in other constructions, the trigger means  90  may trigger braking of the motor  14  for both a safety-related braking condition and a productivity-related braking condition and may, therefore, include combinations of components to trigger braking in both categories of braking conditions. Further, it should be understood that, in yet other constructions, the trigger means  90  may include different types of sensing means  92  for sensing different types of safety-related braking conditions. 
     As explained below in more detail, the control means  86  receives an electrical signal representing the alternating current provided to the motor  14  by the power source  50 . The electrical signal may be a current or a voltage waveform, though, in the preferred embodiment, the electrical signal is a current signal. The current signal is used to determine the present state of the alternating current provided to the motor  14  by the power source  50 . After the trigger means  90  has output the trigger signal to the control means  86 , the control means  86  outputs the control signals at a selected brake starting point or phase angle of the alternating current provided by the power source  50 . As a result, braking is initiated when there is the desired voltage to generate the necessary counter-EMF to brake the motor  14 , and braking is not generally initiated at points in the power cycle when there is little or no voltage from the power source  50 , e.g., at a “zero-crossing” point of the alternating current. 
     As explained below in more detail, the control means  86  includes components which are programmable to optimize the braking of the motor  14 . The components of the control means  86  are programmable so that the control means  86  outputs the control signals on selected power cycles and at selected phase angles and voltages of the alternating current from the power source  50 . In this manner, the control means  86  can vary the braking force applied to the motor  14 . Further, in this manner, the control means  86  can vary the stopping time of the motor  14  during braking. 
     FIGS. 4A-B,  5 A-C and  6 A-B are schematic diagrams of portions of the motor  14  and the braking and control circuit  10  for use with the drill press  18 . As shown in FIGS. 4A and 5A, the motor  14  includes a power supply  98  which is connected with the power source  50 . In the construction illustrated in FIG. 5A, the power supply  98  is in a non-isolated fly-back configuration. The power supply  98  creates a 12 V DC and a 5 V DC output from a 90 V AC to a 255 V AC input. U 4  is the controller for the power supply  98  and is a three-terminal, off-line PWM switch. Capacitor C 4  charges to the peak of the AC mains voltage of the power source  50 . Half-wave rectification by diode D 5  converts the AC voltage to DC voltage but generates a ripple voltage on capacitor C 4 . Zener diode VR 1  and diode D 2  clamp voltage spikes and reduce drain voltage ringing when field-effect transistor (“FET”) U 4 , a TopSwitch device turns off. Diode D 6  and capacitor C 2  rectify and filter the secondary of coupling transformer T 1 . The output voltage is directly sensed by Zener diode VR 2 . Diode D 1  is a blocking diode that prevents loading of FET U 4  control pan period. Capacitor C 14  on the control pin of FET U 4  determines the auto-restart frequency during startup and output short circuit conditions, filters internal MOSFET gate charge currents flowing into the control pin, and provides loop compensation. Regulator U 1  is a basic fixed 5 V DC regulator with C 3  filtering the output. C 1  and L 1  are all EMI filters. 
     As shown in FIG. 4A,  5 A and  5 B, the power supply  98  is electrically connected to a magnet/auto demag circuit  102  (node A to node B). The magnet/auto demag circuit  102  controls the electromagnet assembly  34  so that the drill press  18  is selectively connected to the surface of the workpiece W. 
     As shown in FIGS. 4A,  5 A and  5 C, the power supply  98  is also electrically connected to a motor control circuit  106  (partially illustrated) (node C to node D). The motor control circuit  106  controls the operation of the motor  14 . The motor control circuit  106  includes the second switch means  74 , in the illustrated construction, relays CR 3  and CR 4 . The operation of the motor control circuit  106  and the second switch means is explained below in more detail. 
     As shown in FIGS. 4A and 5C, the motor control circuit  106  is electrically connected to the phase delay feedback circuit  110 . The phase delay feedback circuit  110  monitors the speed of the motor  14  in an attempt to hold the speed of the motor  14  constant. In the illustrated construction, the phase delay feedback circuit  110  does not provide true “classical” speed feedback, i.e., does not directly monitor the speed of the motor  14 . Instead, in the phase delay feedback circuit  110 , the load point of the motor  14  is sensed via resistor R 23 , capacitor C 8 , resistor R 24 , transistor Q 3  (FIG. 6A) and resistor R 8  (FIG.  6 A). When an increase in the loading of the motor  14  is detected, the conduction angle of the triac  70  is increased to compensate for the additional loading of the motor  14 . 
     As shown in FIGS. 4A and 5C, the phase delay feedback circuit  110  is electrically connected to a fire circuit  114 . The fire circuit  114  includes the first switch means  66 , in the illustrated construction, triac T 1 , and is operable to selectively disconnect the motor  14  from the power source  50 . The operation of the fire circuit  114  and the first switch means  66  is explained below in more detail. 
     As shown in FIGS. 4A and 5C, the fire circuit  114  is electrically connected to a fault detector circuit  118 . The fault detector circuit  118  generates a signal in both the operating and non-operating state of the motor  14 . The fault detection circuit  118  includes the trigger means  90 , detects whether a braking condition exists for the drill press  18 , and provides the trigger signal to trigger braking of the motor  14 . The fault detection circuit  118  is explained in more detail below. 
     As shown in FIGS. 4B and 6A, the control means  86  includes a microprocessor or microcontroller  122 . The control means  86  and the microcontroller  122  are connected to the power supply  98  (connectors R 22 , see FIGS.  5 A and  6 A), the magnet/auto demag circuit  102  (see FIG.  6 A and FIG. 5A (connectors R 37 )), the motor control circuit  106  (FIG.  6 A), the phase delay feedback circuit  110  (FIG.  6 A), the fire circuit  114  (connectors R 36 , see FIGS. 5C and 6A) and the fault detector circuit  118  (FIG.  6 A). It should be understood that, in other constructions (not shown), the control means  86  may include different and separate components performing the functions of the microcontroller  122 , as described below. 
     The microcontroller  122  is operable and programmable to control braking of the motor  14 . The microcontroller  122  outputs the control signal to at least one of the first switch means  66  and the second switch means  74  to brake the motor  14 . Preferably, the microcontroller  122  is electrically connected with the first switch means  66  and with the second switch means  74 . Also, to brake the motor  14 , the microcontroller  122  is preferably operable to output the first control signal to the first switch means  66 , to disconnect the motor  14  from the power source  50 , and the second control signal to the second switch means  74 , to connect the motor  14  in a closed loop and to generate counter-EMF. The operation of the control means  86  and the microcontroller  122  is explained below in more detail. 
     The control means  86  and the microcontroller  122  receive a current signal (node E to node F) representing the power cycle of the alternating current supplied by the power source  50 . With this current signal, the microcontroller  122  is operable to begin braking operations at the selected brake start point on the power cycle, skip the selected number of power cycles during braking operations and ramp the voltage provided to regulate the braking force applied to the motor  14  and the stopping time of the motor  14 , and stop braking operations at the selected brake end point on the power cycle (after a selected number of power cycles). 
     The control means  86  requires an input frequency of 45-70 Hz and works with stepped and square-wave waveforms that are commonly seen on inverters and alternators, alternate sources of power. The microcontroller  122  senses the frequency and internally self-adjusts by looking at the current signal generated from resistors R 22 , R 7 , R 21 , and transistor Q 1 . Pin  3  on the microcontroller  122  becomes active only if an inverter is used as power. If DC power is applied to the control means  86 , a fault condition will occur. If power to the control means  86  is lost for less than approximately 0.300 seconds, the electromagnet assembly  34  will stay in the state it was in before the power loss, and the motor  14  will turn off, if it was running. If power is lost for greater than approximately 0.300 seconds, the motor  14  and electromagnet assembly  34  will turn off. At no point will the motor  14  ever operate while the electromagnet assembly  34  is not energized. 
     As shown in FIGS. 4B,  6 A and  6 B, the microcontroller  122  is also connected to a low/no current detector circuit  126  (node G to node H and node I to node J). As shown in FIGS. 5B and 6B, the low/no current detector circuit  126  is also connected to the magnet/auto demag circuit  102  (connectors R 45 ). The low/no current detector circuit  126  includes the trigger means  90  to trigger braking of the motor  14 . 
     As shown in FIGS. 4B,  6 A and  6 B, the microcontroller  122  is also electrically connected with a dial speed control circuit  130  (node K to node L and node M to node N). The dial speed control circuit  130  operates to control the speed of the motor  14 . In the illustrated construction, to control the speed of the motor  14 , the dial speed control circuit  130  includes a potentiometer R 33  and a divider network including R 34  and R 15 . The resistor divider network develops and supplies a speed control signal to the microcontroller  122 . The microcontroller  122  manipulates the supplied speed control signal and then controls the triac firing delay, which, in turn, varies the rotational speed of the motor  14 . The speed control resulting from the dial speed control circuit  130  is a digital implementation and is controlled by the microcontroller  122 . The digital implementation is self-calibrating and is thus less susceptible to tolerance stackups in the potentiometer R 33  and the resistor divider network. 
     In operation, the drill press  18  is connected to the AC power source  50 . Referring now to the magnet/auto demag circuit  102 , when switch SW 3  is closed, the red light emitting diode (“LED”) illuminates and relay CR 2  closes; this powers up the full wave bridge (consisting of diodes D 7 , D 8 , D 9 , and D 10 ). When switch SW 3  is closed while the electromagnet assembly  34  is energized and the motor  14  is not operating, the LED turns off, and the microcontroller  122  goes through the following demag sequence: 
     1. Relay CR 2  opens (0.400 seconds). 
     2. Relay CR 1  closes to the demag position. 
     3. Relay CR 2  closes. 
     4. Triac T 3  is fired starting on a negative AC half cycle, then two half cycles are skipped and the triac T 3  is then fired again. The triac T 3  fires twelve times (0.300 seconds) with decreasing amplitude on each pulse, creating a ringing situation which causes the demag function to operate more efficiently. 
     5. Relay CR 2  opens. 
     6. Relay CR 1  opens to the magnet position. 
     Referring now to the fire circuit  114 , this fire circuit  114  is the firing circuit for the triac T 1  which is controlled by the microcontroller  122 . Firing pulses from the microcontroller cause the logic triac T 2  to conduct, which in turn causes the power triac T 1  to control current flow through the motor  14 . The speed of the motor  14  increases as the microcontroller  122  delivers more firing pulses to triac T 2 . Electronic “Pre Burners” are generated by the microcontroller  122  at the maximum dial speed. This provides the maximum motor speed achievable from the triac control circuitry. 
     The motor control circuit  106  also includes a soft-start feature to increase the life of the motor  14  and to decrease stress on the overall system by ramping the motor  14  to full-on. This soft-start feature ramps the motor speed from zero to full-on over a time period of 0.400 seconds and is facilitated by the microcontroller  122 . The direction of rotation of the armature (and the associated spindle assembly  38  and drill bit  42 ) is switched from forward to reverse with relays CR 3  and CR 4 . When switch SW 4  is closed, the microcontroller  122  closes relay CR 4 , controls the triac, and soft-starts the motor  14  in the forward direction. When switch SW 1  is closed the microcontroller  122  closes relay CR 3 , controls the triac, and soft-starts the motor  14  in the reverse direction. If the motor  14  is already operating in one direction and a change in the direction of armature rotation is requested, a delay of 0.320 seconds is implemented to allow the motor speed to decrease before changing the direction of rotation of the armature. 
     The fault detection circuit consists of transistor Q 9  and resistors R 16 , R 38  and R 20  and generates a signal to the microcontroller  122  in both the running and non-running state of the motor. Before powering the motor  14 , the microcontroller  122  verifies the integrity of relays CR 3  and CR 4  (welded contacts or non-functional contacts). This integrity check assures proper operation of the motor braking and control feature in the system. If, when the motor  14  is in a non-operating state, a signal is present at input P 20  of the microcontroller  122 , the microcontroller  122  will assume a fault condition due to shorted relay contacts of either relays CR 3  or CR 4 . If, when the motor  14  is in an operating state, a signal is present at input P 20  of the microcontroller  122 , the microcontroller  122  will assume a fault condition due to open relay contacts of either relays CR 3  or CR 4 , or a shorted triac condition. The fault detection circuit  118  also recognizes proper connection of the motor  14  to the control panel. 
     Referring to the low/no current detection circuit  126 , the electromagnetic holding force of the electromagnet assembly  34  decreases as the current through the magnet coil decreases. The minimum voltage required to adequately secure the drill press  18  to a properly sized workpiece W during drilling operations is 90 V AC. If the power source  50  does not reach at least 90 V AC within five seconds of application, operation of the motor  14  will be disabled and the control panel will signal a fault condition. The five second window accounts for the time required for alternate sources of power to switch from an idle condition to a stabilized power source. If, while the control panel is operational, the input voltage drops below approximately 90 V AC, the motor  14  will not operate, and the panel will signal a fault condition until proper voltage levels are re-established. If the motor  14  is operating and the input voltage level drops below 90 V AC, motor operation will cease and the panel will signal a fault condition. The fault condition will exist until proper voltage levels are re-established. The electromagnet assembly  34  will continue to operate during this condition unless the user presses the magnet on/off button. If current through the electromagnet assembly  34  is interrupted (i.e., broken magnet wire) while the motor  14  is operating, the control panel will trigger braking of the motor  14  and assume a fault condition, as explained below in more detail. 
     In the illustrated construction, resistor R 45  senses the current flowing through the electromagnet assembly  34 . The voltage across the sensing resistor R 45  is rectified by diode D 4  and filtered by capacitor C 11 . This voltage is then presented to the U 3  comparators, which are referenced to different voltage levels derived from the +5 V DC bus. Resistors R 49  and R 50  establish the reference for the low voltage condition, and resistors R 51  and R 52  establish the reference for the no voltage condition. 
     If any of the microswitches SW 1 -SW 4  remain shorted for more than two seconds, the control panel will assume a fault condition. The motor  14  will not operate or will shut off and be braked if it was operating. The electromagnet assembly  34  will remain in its current state if this fault occurs. In order to minimize the effects of vibration, the microcontroller  122  repetitively samples the microswitches SW 1 -SW 4  to confirm an intended actuation. 
     There are two types of flashes that occur when a system fault is detected, a “blink” and a “flash”. The flash is a 50% duty cycle of the LED and a blink is a less than 50% duty cycle of the LED. The following is a list of the conditions that cause system faults: 
     Flash: 
     bad electromagnet assembly  34   
     bad motor  14   
     bad electromagnet assembly connection 
     bad motor connection 
     failed or stuck switch 
     Blink: 
     low electromagnet assembly current 
     DC power applied 
     power frequency too high or too low 
     If a system fault occurs, a more detailed explanation of the failure can be seen from the output signal on pin  1  of the microcontroller  122 . 
     To brake the motor  14 , in the illustrated construction, when the trigger means  90  outputs the trigger signal to the control means  86 , the microcontroller  122  outputs the first signal to the first switch means  66  to open the triac  70  (forcing the triac  70  into a non-conducting state), to disconnect the motor  14  from the power source  50 . The microcontroller  122  then outputs the second signal to the second switch means  74  (by closing both relays CR 4  and CR 3 ), to connect the motor  14  in a closed loop and to generate counter-EMF. In the construction illustrated in FIG. 2, the switch  78  is closed to allow current through the closed loop. In the construction illustrated in FIG. 3, the triac  82  is closed (forced into a conducting state) to allow current through the closed loop. 
     As discussed above, the control means  86  begins the braking operation at the selected brake start point in the power cycle of the alternating current from the power source  50 . The microcontroller  122  then outputs the first control signal and the second control signal on desired power cycles to pulse the first switch means  66  (the triac  70 ) and the second switch means  74  (the relay  78  or the triac  82 ) for the desired number of power cycles and at the desired voltage and phase angle depending on how quickly the motor  14  needs to stop (based on the type of braking condition). 
     In the preferred embodiment, two AC half-cycles are skipped between successive motor braking cycles. Also, the braking operation is preferably conducted for generally 16 power cycles to the selected braking end point. Further, in the preferred embodiment, the microcontroller  122  controls the braking function so that the voltage supplied to the closed loop is ramped, i.e., the supplied voltage increases on subsequent power cycles to the maximum voltage near the end of the braking operation. In this manner, the braking force applied to the motor  14  and the stopping time of the motor  14  is optimized to provide the necessary braking while minimizing any damage to the motor  14  and its components. 
     Once braking is completed, the microcontroller  122  stops outputting the first control signal and the second control signal so that the first switch means  66  and the second switch means  74  reset and return to the normal motor operating state. Specifically, the triac  70  is turned off, and the short across the armature is removed (the relay  78  is opened or the triac  82  turned on) before the motor  14  is again connected to the power source  50 . 
     In an alternative construction illustrated in FIG. 7, the power tool is a circular saw  18 ′ including the electric motor  14  and the braking and control circuit  10  embodying the invention. In this construction, the circular saw  18 ′ includes a housing  22 ′ supported on a workpiece W by a shoe plate  26 ′. The electric motor  14  is connected to a spindle assembly  38 ′ to rotatably drive a tool element, such as a saw blade  42 ′, to cut the workpiece W. An on/off switch  46 ′ is operated by a trigger  48 ′ and selectively connects the motor  14  to the power source  50 . 
     The circular saw  18 ′ includes sensing means  92 ′ for sensing a safety-related braking condition. The sensing means  92 ′ is a “kick-back” or “binding” sensing means  94 ′. Such a binding sensing means  94 ′ senses a change in the position, velocity or acceleration of the power tool, such as the circular saw  18 ′, resulting from the tool element, such as the saw blade  42 ′, binding on the workpiece W. Such binding causes the circular saw to jerk or kick-back. If this occurs, the binding sensing means  94 ′ outputs the trigger signal, a “binding” signal, to trigger braking of the motor  14 . 
     The circular saw  18 ′ also includes trigger means  90 ′ to trigger braking for a productivity-related braking condition. In the illustrated construction, the trigger means  90 ′ includes the on/off switch  46 ′. When the operator releases the trigger  48 ′, so that the on/off switch  46 ′ disconnects the motor  14  from the power source  50 , the trigger signal, an “off” signal, is output to the control means  86 ′ to trigger braking of the motor  14 . 
     Generally, the microcontroller  122  operates as described above to brake the motor  14  in the circular saw  18 ′. When the trigger means  90 ′, binding sensing means  94 ′ or on/off switch  46 ′, outputs the trigger signal to the control means  86 , the control means  86  outputs the first control signal to the first switch means  66  and the second control signal to the second switch means  74  to brake the motor  14 . 
     The microcontroller  122  is programmed to brake the motor  14  more quickly (i.e., the motor  14  is braked in approximately 1 sec.) when the “binding” signal is received—a safety-related braking condition. When the “off” signal is received—a productivity-related braking condition, the motor  14  is braked more slowly (relative to the safety-related braking condition, i.e., the motor  14  is braked in approximately 2 sec.) because this condition occurs more frequently, i.e., each time the operator releases the trigger  48 ′. 
     Various features of the invention are set forth in the following claims.