- 1 -
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`Pro-Dex v. Intelligent Automation
`U.S. Patent 7,091,683
`Pro-Dex Ex. 1026
`
`

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`U.S. Patent
`
`Nov. 12, 2002
`
`Sheet 1 of6
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`US 6,479,958 B1
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`TRIGGER
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`CURRENT
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`AC
`INPUT
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`POWER
`SUPPLY
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`NEUTRAL
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`LINE
`SWITCH
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`S| DETECT
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`MICRO CONTROLLER
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`AVERAGE
`CURRENT
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`CIRCUIT
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`TRIAC DAIVE
`CIRCUIT
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`CURRENT
`SENSOR
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`- 2 -
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`
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`DOES
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`RATE EXCEED
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`THRESHOLD
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`U.S. Patent
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`Nov. 12, 2002
`
`Sheet 2 of6
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`US 6,479,958 B1
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`RETURN FROM
`PULSE ROUTINE
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`RATE oFCHANGE
`POSITIVE?
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`ON TIMEi
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`bore
`fTTINE PULSE MODE sf
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`DURATION
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`- 3 -
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`

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`U.S. Patent
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`Nov. 12, 2002
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`Sheet 3 of 6
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`US 6,479,958 Bl
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`LN38ENI
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`U.S. Patent
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`Nov. 12, 2002
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`Sheet 4 of6
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`US 6,479,958 B1
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`A/DVALUEOFAVERAGEMOTORCURRENT
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`eon
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`THRESHOLD oF
`SWITCH7&
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`TUAN ON 71
`JRAPID CURRENT
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`INCREASE
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`THRESHOLD
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`as
`—
`ABSOLUTE ___ __.
`THRESHOLD
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`MOTOR CURRENT >
`UPPER THRESHOLD
`et
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`pat
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`U.S. Patent
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`Nov. 12, 2002
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`Sheet 5 of6
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`US 6,479,958 B1
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`A/DVALUEOFAVERAGEMOTORCURRENT
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` ee es Fe ee ee
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`MOTOR CURRENT <
`LOWER THRESHOLD
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` SWITCH
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`TURN ON
`TURN RATCHET OFF-
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`RAPID
`RETURN TO NORMAL
`INCREASE
`OPERATIO
`AND/OR > ABS
`ERATION
`THRESHOLD ai So
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`Fig-8
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`MOTOR CURRENT > LOWER
`2 o60
`THRESHOLD AND < UPPER
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`LINE CYCLES
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`- 6 -
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`Sheet 6 of 6
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`US 6,479,958 B1
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`Nov. 12, 2002
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`U.S. Patent “IWELNAN
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`- 7 -
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`

`

`US 6,479,958 B1
`
`1
`ANTI-KICKBACK AND BREAKTHROUGH
`TORQUE CONTROL FOR POWER TOOL
`
`This is a continuation of U.S. patent application Ser. No.
`08/369,358, filed Jan. 6, 1995 now abandoned.
`
`BACKGROUND AND SUMMARYOF THE
`INVENTION
`
`The present invention relates to electrically driven power
`tools. In particular, the invention relates to a motor control
`circuit and method for detecting and responding to the onset
`of stall conditions (e.g., kickback or breakthrough
`conditions) by pulsing the motor to overcome the stall
`condition, if possible.
`Electrical power tools typically employ a motor that
`imparts torque to the tool through a spindle. In the case of
`an electric drill, the motor spindle is coupled througha series
`of reducing gears to the chuck, which in turn holds the drill
`bit or other cutting or abrading tool, such as a hole saw, a
`grinding wheel, or the like. Power screwdrivers work on a
`similar principle, with the chuck holding a screwdriverbit.
`In both cases, the function of the reducing gears or gear train
`is to reduce the rotational speed of the tool while increasing
`the rotational torque.
`Powerrouters are somewhat different. The cutting tool
`(router bit) of the typical hand-held router is typically
`direct-coupled to the spindle of the motor. In this case, the
`full rotational speed or RPM of the motor is used without
`gear reduction, to rotate the router bit at high speed. Recip-
`rocating saws and jigsaws use yet another type of gear train
`that translates the rotational movementof the motor spindle
`to reciprocating movement.
`Generally speaking, all of these power tools may sud-
`denly encounter impending stall conditions at which time
`the output torque rapidly rises because of local changes in
`workpiece hardness, workpiece binding or jamming, tool
`obstruction from burrs and so forth. If the cause of the
`condition is not overcome, the tool may jam and the motor
`will stall. When drilling a hole with a power drill, for
`example, some workpieces will develop burrs on the tool
`exit side of the workpiece, and these burrs can engage the
`flutes of the drill bit, causing a rapid increase in torque as the
`drill bit tries to break free. In some instances, especially with
`metal workpieces,
`the burrs may actually stop drill bit
`rotation, causing a strong reaction torque that is imparted to
`the tool operator as the motor turns the tool in the operator’s
`hand (instead of turning the drill bit).
`Arelated phenomenon occurs with power saws. Referred
`to as kickback, the cutting movementof the saw blade may
`become partially or fully arrested by the workpiece, often
`whenthe saw cut is approaching completion and the unsup-
`ported workpiece becomes jammed against the saw blade.
`With the movementof the saw blade impeded, a large motor
`torque is generated and,
`in some case,
`the motor may
`actually stall.
`These conditions are hereinafter generically referred to as
`“kickback”or “stall” conditions, regardless of the particular
`power tool
`involved or the specific circumstance which
`gives rise to the impending kickbackorstall condition.
`In the past,
`the Applicant’s assignee developed anti-
`kickback power tool control
`techniques that sensed an
`impending kickback condition and inhibited the coupling of
`power to the tool, and/or optionally applied a brake to the
`tool, in response to the impending kickback condition. These
`systemsare described more fully in U.S. Pat. No. 4,267,914
`to Saar, entitled “Anti-Kickback Power Tool Control,”
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`2
`issued May 19, 1981; and in U.S. Pat. No. 4,249,117 to
`Leukhardt et al., entitled “Anti-Kickback Power Tool
`Control,” issued Feb. 3, 1981. The anti-kickback control
`systems described in these patents are designed to interrupt
`powerto the motor once the impending kickback condition
`occurs. In order for power to once again be coupled to the
`tool, the trigger switch mustbe fully released and then again
`retracted, or some other signal provided by the operator.
`Although the systems described in these patents are
`effective in detecting and preventing a kickback condition,
`the response of the control circuits disclosed therein to the
`impending kickback condition may hamper the operator’s
`ability to complete the desired task. For example, if power
`has been interrupted due to a drill bit binding on a burr
`formed during breakthrough,
`it may be difficult for the
`operator to break the burr free to complete the hole without
`repeating the stall condition and causing re-interruption of
`power to the motor. Consequently, an improved control
`techniqueis neededthat is not only effective in detecting and
`preventing a kickback condition, but is also effective in
`enabling the operator to possibly overcome the impediment
`and completing the intended task.
`to
`The present
`invention represents an improvement
`Applicant’s prior anti-kickback technology.
`Instead of
`responding to an impending kickback condition by simply
`interrupting power to the tool (and/or applying a brake), the
`present invention effects a motor pulsing operation that, in
`many cases, can actually resolve or clear the breakthrough
`or kickback condition, so that the tool does not need to be
`shut down and restarted. When the invention is used with
`powerdrills, for example, the operator can keep the trigger
`switch actuated while the present controller senses an
`impending stall condition (e.g., kickback or breakthrough
`condition) and responds to that condition by pulsing the
`motor for a predetermined time period to deliver a series of
`torque pulses. These torque pulses each have a peak torque
`that is substantially greater than the average torque delivered
`during the series of pulses. The impactof these torque pulses
`may allow the tool to break through the burrs or workpiece
`restrictions that are causing the impendingstall or kickback
`condition. In one embodiment, the pulses are delivered in a
`sequence designed to be harmonically related to the natural
`frequency of the gear train of the power tool. This provides
`even greater peak torque output by causing the geartrain to
`oscillate between an energized state and a relaxed state at the
`gear train’s natural frequency.
`The present invention can be used with virtually any
`power-driven tool. When incorporated into the motor control
`of a rotary saw,
`the motor pulsing effect may cause the
`workpiece to become unjammedor break through the bind-
`ing condition that originally caused the impendingstall or
`kickback.
`
`Accordingly, the present invention provides a method and
`apparatus for controlling virtually any powertool having a
`motor that imparts torque to an output spindle when actuated
`by an operator actuable switch. The method involves sensing
`a motor parameterindicative of the onsetof a stall condition.
`The sensed parameter can be motor current, for example,
`and the onsetof stall can be inferred by monitoring the rate
`of change in motor current with respect to time. While the
`trigger switch remains actuated, the motor is pulsed for a
`predetermined period to deliver a series of torque pulses
`each having a peak torque substantially greater than the
`average torque delivered during the series of torque pulses.
`In many cases, these torque pulses will clear the condition
`that caused the impendingstall. If so, then normal motor
`operation is resumed. If not, then the power to the motoris
`removed after a predetermined time to avoid burnoutof the
`motor.
`
`- 8 -
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`

`

`US 6,479,958 B1
`
`3
`For a more complete understanding of the invention, its
`objects and advantages, reference is made to the following
`specification and to the accompanying drawings.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is an elevational side view of a typical variable
`speed power drill with which the control circuit of the
`invention may be used;
`FIG. 2 is a simplified block diagram of a preferred
`embodiment of the control circuit of the present invention;
`FIG. 3 is a flowchart illustrating one embodimentof the
`invention;
`FIG. 4 is a pulse mode timing diagram useful in under-
`standing the invention;
`illustrating another preferred
`FIG. 5 is a flowchart
`embodiment of the invention;
`FIG. 6 is a graph of motor current as a function of time,
`illustrating how the motoris controlled in normal operation
`and in pulse mode operation;
`FIGS. 7, 8 and 9 are graphs of motor current (digital
`values) as a function of time (line cycles), useful in under-
`standing the invention in operation; and
`FIG. 10 is a detailed schematic diagram of a motor control
`circuit for a powerdrill incorporating the present invention.
`
`DESCRIPTION OF THE PREFERRED
`EMBODIMENT
`
`The present invention is useful with a wide range of
`different power tools. To explain the principles of the
`invention, a power drill will be illustrated and described. It
`will be readily apparent that the invention can be incorpo-
`rated into other types of electrical power tools, as well.
`Referring to FIG. 1, a powerdrill is depicted generally at
`10. The drill incorporates a control circuit 12 in accordance
`with preferred embodiments of the present invention. The
`drill 10 includes, in conventional fashion, a motor 16, a gear
`train 20, and a tool bit-receiving chuck 22. A trigger switch
`24 controls the application of current to the motor and may
`also be used to vary the speed of the motor to suit various
`work needs. A drill bit 26 may be installed in the chuck, as
`illustrated. The drill bit has a fluted shank 28 and a cutting
`tip 30 of conventional configuration.
`Referring to FIG. 2, a block diagram of a preferred
`embodiment of the control circuit 12 is illustrated. The
`control circuit 12 supplies current to motor 16 for both
`normal (continuous) operation and also for pulse mode
`operation in accordance with the invention.
`Control circuit 12 includes a microcontroller 30 in the
`
`form of a microprocessor or microcomputer. A power supply
`circuit 32 is coupled to the AC powerline input and supplies
`suitable DC voltage to operate microcontroller 30. As
`illustrated, the trigger switch 24 suppliesa trigger signal to
`microcontroller 30. This trigger signal indicates the position
`or setting of the trigger switch as it is manually operated by
`the tool operator. If desired, microcontroller 30 can include
`a reset circuit 34 which, when activated, causes the micro-
`controller to be re-initialized.
`
`Drive current for operating motor 16 is controlled by a
`triac drive circuit 36. The triac drive circuit is, in turn,
`controlled by a signal supplied by microcontroller 30. A
`current sensor 38 is connected in series with motor 16 and
`
`triac drive circuit 36. Current sensor 38 may be, for example,
`a low resistance, high wattage resistor. The voltage drop
`across current sensor 38 is measured as an indication of
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`actual instantaneous motor current. The instantaneous motor
`current, so measured,
`is supplied to an average current
`measuring circuit 40, which in turn supplies the average
`current of value to microcontroller 30. Microcontroller 30
`uses the average current to evaluate whether to switch into
`and out of pulse mode operation. In addition to average
`current, microcontroller 30 is also supplied with a signal
`from current detector circuit 42. This circuit is coupled to the
`triac drive circuit 36 and supplies a signal indicative of the
`conductive state of the triac drive circuit. If for some reason
`
`the triac does not turn on in response to the control signal
`from microcontroller 30, circuit 42 detects this and notifies
`the microcontroller so another control signal pulse can be
`sent by the microcontroller. Specifically, if the triac gate
`voltage is zero (triac not conducting), circuit 42 supplies an
`indication of this condition to the microcontroller.
`
`In operation, the trigger switch 24 supplies the trigger
`signal to microcontroller 30 that varies in proportion to the
`switch setting. Based on this trigger signal, microcontroller
`30 generates a control signal which causes the triac drive
`circuit
`to conduct,
`thereby allowing motor 16 to draw
`current. This motor current causes the motor to turn, the
`motor current being approximately proportional to the motor
`torque. The average current circuit 40 and current sensor 38
`monitor the motor current and supply an average current
`signal to microcontroller 30. This average current signal is
`processed by microcontroller 30, as will be described below,
`to sense when an impending stall condition has developed.
`When an impendingstall condition is detected, microcon-
`troller 30 switches to a pulse mode operation, causing the
`triac drive circuit to supply rapid ON/OFFcurrent pulses to
`motor 16. These current pulses deliver a series of torque
`pulses each having a peak torque substantially greater than
`the average torque delivered during pulse mode operation.
`In many cases, these torque pulses will clear the condition
`that caused the impendingstall. If so, then microcontroller
`30 senses this, by monitoring the average current signal, and
`reverts automatically to normal motor operation.
`The pulse mode operation of the invention can be imple-
`mented by a variety of different microprocessor-
`implemented procedures. Two procedures are described
`here: a first procedure illustrated in FIG. 3 and a second
`procedure illustrated in FIG. 5. A pseudocode routine illus-
`trating the details of one possible pulse mode routine
`appears in the Appendix. By way of further illustration, a
`detailed schematic of a motor control circuit for an AC
`
`powerdrill is illustrated in FIG. 10 and described below.
`Referring to FIG. 3, a first motor control routine is
`illustrated. Beginning at start step 100,
`the routine first
`checks to see whether the operator switch is closed (step
`102). The operator switch may be, for example, the manu-
`ally actuated trigger switch 24 of a powerdrill. If the switch
`is not closed, then power to the motor is switched off (step
`104) and control branches back to step 102. If the operator
`switch is closed, then power is supplied to the motor(step
`106) and the motor will then draw current in proportion to
`the load placed on the motor by the tool. Motor current is
`monitored (step 108) and the rate of change in motor current
`is measured (step 110). If the rate of change in motor current
`is positive (step 112) and if the rate of change in motor
`current also exceeds a predetermined threshold (step 114),
`then the pulse routine is called (step.116). If the rate of
`change in motor current is not positive, or if the rate of
`change does not exceed the predetermined threshold, then
`control simply branches back to step 102.
`The pulse mode routine can be implemented by a variety
`of different microprocessor-implemented procedures. One
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`- 9 -
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`

`

`US 6,479,958 B1
`
`5
`such procedure is described in the pseudocodelisting that
`appears in the Appendix. Referring to the pseudocode
`listing,
`the procedure defines a current ON time and a
`current OFF time. These are used to generate the motor
`current pulse waveform illustrated in FIG. 4. If desired, the
`ON time and OFFtime can be of equal duration (a 50% duty
`cycle) or they may be of different durations.
`In some
`embodiments, the ON time and OFF time durations can be
`derived from the AC powerline frequency. Alternatively, the
`ONtime and OFF time durations can be generated by the
`onboard clock of the microcontroller 30. By suitably choos-
`ing the ON time and OFFtime values, the frequency of the
`pulsating current signal can be established during system
`design.
`Although the invention can be implemented with a variety
`of different ON time and OFF time combinations, sometool
`applications will benefit by selecting the ON time and OFF
`time to correspond to the natural resonant frequency of the
`tool gear train. Typically, there is a certain degree of play in
`a powertool geartrain that allows the motorspindle to rotate
`between an energized position and a relaxed position when
`power is applied and removed, respectively. Like many
`physical systems, the motor and its gear train will exhibit a
`natural resonant frequency, namely the frequency at which
`oscillation between the energized state and the relaxed state
`most naturally occurs. If pulsating current
`is applied in
`synchronism with this natural frequency, then oscillation
`energy is enhanced. To illustrate the principle, consider how
`a basketball is dribbled by manually applying energy at or
`near the peak of each bounce in synchronism with the
`bouncing frequency. Energy is added at the proper time so
`that oscillation (bouncing) is enhanced. Energy can be added
`to the oscillating gear train system in much the samefashion.
`The natural resonant frequency of the gear train of a
`power tool can be determined experimentally using the
`procedure described in the pseudocode of the Appendix. To
`find the natural resonant frequency, the microcomputer is
`programmedto iteratively select a range of different ON
`times and OFFtimes while the torque output of the tool is
`measured. Those ON time and OFFtime valuesthat corre-
`spond to the maximum torque output correspond to the
`natural resonant frequency of the gear train system (or in
`some cases, a harmonic thereof).
`Referring to the pseudocodelisting in the Appendix, the
`procedure also defines a pulse mode duration. Also shown in
`FIG. 4, the pulse mode duration is a predetermined time
`during which the pulse mode routine will operate if called
`upon. The presently preferred embodiment will terminate
`the pulse mode routine under certain conditions after the
`pulse mode duration has elapsed. Thus,if the stall condition
`is not relieved within the predefined pulse mode duration,
`the motor may be shut down to avoid burnout.
`The procedure of the pseudocode Appendix also defines a
`current threshold at which the pulse mode routine may be
`terminated because the impendingstall condition has been
`cleared.
`
`The procedure begins by initializing the above values.
`Next,
`the pulse mode duration counter is started. The
`remainderof the routine is performed while the pulse mode
`duration counter has not elapsed.
`The motor current is turned on for an interval dictated by
`the current ON time. This is accomplished by starting a
`current ON time counter and then turning on the motor
`current while the ON time counter is not elapsed. While-the
`motor current is on, the current sensing system measures the
`average motorcurrent. Preferably, the measure is made on a
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`cycle-by-cycle basis, using the circuitry described above. If
`the average motorcurrentis less than the initialized current
`threshold, then the motor pulse routine exits early. Also, if
`the operator opens the switch while the current ON timeis
`not elapsed, the motor pulse routine exits.
`After the ON timehas elapsed, the OFF time is measured.
`The motorcurrent is turned off and the OFF time counteris
`
`started. Until the OFF time elapses, the routine monitors to
`see if the operator has opened the switch. If so, the motor
`pulse routine exits. After the motor OFF time has elapsed,
`control branches back to the ON time segment of the
`procedure where by the cycle repeats until the pulse mode
`duration has elapsed.
`When the pulse mode duration counter has elapsed, a
`condition that is reached only if the stall condition is not
`cleared, powerto the motor is switched off, a brake may be
`applied, if desired, and the trigger switch is locked out until
`physically reset by the operator.
`Referring to FIG. 5, a second, presently preferred embodi-
`mentof the motor control routineis illustrated. Beginning at
`start step 130, the routine first measures the trigger travel in
`step 132 to determinethe presentsetting of the trigger switch
`24. Next (step 134), the average motor current is measured
`and the motor voltage is set proportional to the trigger travel
`(step 136). In this way, the motor is set to operate at a speed
`dictated by the trigger setting.
`In the embodiment of FIG. 5, pulse mode operation is
`performed only when the change in trigger travel or move-
`mentis greater than a predetermined threshold. Thus,in step
`138 the change in trigger travel is compared with a delta
`trigger threshold. If the changein triggertravel is not greater
`than the threshold, control simply loops back to step 132.
`Otherwise, control proceeds to step 140 where the absolute
`current is compared with an absolute current threshold. If the
`absolute current is above the threshold, pulse mode opera-
`tion is begun (step 142). If not, then control proceeds to step
`144 where the time rate of change in average current is
`analyzed. If the change in average current (di/dt) is greater
`than an upper threshold, or if the change in average current
`is greater than an intermediate threshold over two consecu-
`tive cycles, then pulse mode operation is commenced (step
`142). If neither of these tests (step 144)are satisfied, control
`loops back to step 132.
`The motor will continue in pulse mode operation until
`certain events occur. As illustrated in steps 146 and 148, if
`the average currentis greater than an upper threshold during
`pulse mode operation, then the motor is shut off. Also, as
`illustrated in step 150, if the average current is less than a
`lower threshold, pulse mode operation is terminated and
`control loops back to step 132.
`By way offurtherillustration, FIG. 6 graphically depicts
`the motor current of a power drill equipped with the inven-
`tion. Motor torque is proportional to motor current, hence
`the graphical depiction of FIG. 6 also illustrates how motor
`torque is controlled. When the tool is switched on, motor
`current rapidly rises, as at A, as the tool accelerates to full
`speed. Thereafter, the motor current settles down to a normal
`operating current, as at B. While the workpiece is being
`drilled, motor current may fluctuate up and downslightly
`due to changes in workpiece hardness and changes in
`operator-applied pressure. These minor variations aside,
`motor current is essentially constant as the drill bit works its
`way through the workpiece, as depicted in inset I of FIG. 6.
`When breakthrough occurs,as illustrated in inset II, motor
`current rises rapidly, as at C. Specifically, the rate of change
`in motor current
`is more rapid than during initial
`
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`US 6,479,958 B1
`
`7
`acceleration, as at A. The reason for this rapid increase in
`motor currentis that the holeat the exit side of the workpiece
`is not yet perfectly round and may have burrs or unremoved
`portions of material that become lodged in the flute portion
`of the drill bit.
`
`When the rapid rise in motor current is sensed by the
`invention, the pulse mode routine is called and the motor
`current is thus pulsed, as depicted at D. Note that the peak
`motor current (and motor torque) achieved during this pulse
`mode operation is substantially greater than the average
`motor current (and average torque) during this interval. The
`peak motor current and peak torque is also substantially
`greater than the average torque during the normaldrilling, as
`at B. Also note that the peak current attains a maximum
`value at E and thereafter diminishes with each successive
`
`the peak current drops to the current
`peak. Ultimately,
`threshold at F, whereupon normal (nonpulsed) operation is
`resumed.
`
`During each pulse of the pulse mode operation, the drill
`bit cutting edge is driven to impact the burr or unremoved
`workpiece material in chisel-like fashion. After each impact,
`the gear train is allowed to relax, permitting the drill bit
`cutting edge to back off slightly for the next impact. In this
`way, the drill bit cutting edge effects a chopping motion and
`the peak torque developed during each chopis substantially
`greater than the average torque the motor is otherwise
`capable of providing. This pulsing action may allow the tool
`to break through any remaining burrs or other material
`obstructions. After breakthrough has occurred,
`the tool
`resumes normal operation so that the hole can be further
`cleaned out, as depicted byinsert III. The pulsing also warns
`the operator that the above condition has occurred.
`the
`Notably, during this entire pulse mode operation,
`trigger switch remains on,
`that
`is, activated by the tool
`operator. The system enters pulse mode operation automati-
`cally if a rapid rise in motor currentdictates, and the operator
`can simply continue to hold the trigger switch closed while
`concentrating on the drilling or cutting operation.
`If the pulse mode operation is not successful in overcom-
`ing the stall condition, then the motor will shut down after
`the predetermined pulse mode duration has elapsed.
`Although the inset views J-III of FIG. 6 have illustrated
`a drill bit, the invention is also useful with other types of
`powertools, as discussed above. In the case of a powersaw,
`a potential stall condition can occur after the cut has been
`completed and a portion of the removed material becomes
`jammed between the saw blade and rip fence or other guide.
`The pulsating action of pulse mode operation will frequently
`vibrate the workpiece free of the jam and will also warn the
`operator of the condition. Thus, the present invention can be
`incorporated into a wide variety of power tools, including
`drills and saws.
`
`To further understand the motor control routine of FIG. 5,
`refer to FIGS. 7, 8 and 9. These Figures show average motor
`current (the A/D value generated by the analog-to-digital
`conversion circuitry of microcontroller 30) as a function of
`time (measured in line cycles).
`The preferred embodimentof FIG. 5 performs pulse mode
`operation in synchronism with the AC line frequency.In the
`pseudocodelisting in the Appendix, a more generalized case
`is illustrated in which pulse mode operation is not neces-
`sarily synchronizedto the line frequency. In the pseudocode
`listing, the ON and OFF times may be any value measured
`by the microprocessor’s clock circuitry. In the embodiment
`illustrated in FIGS. 7-9, the ON and OFF times are mea-
`sured by counting a predetermined numberof ACline cycles
`
`10
`
`15
`
`20
`
`25
`
`30
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`8
`instead of using the clock circuitry of the microprocessor.
`For AC powered tools, the embodiment of FIGS. 7-9 is
`preferred. For DC powered tools, such as cordless battery
`powered tools, a microprocessor clock-based embodiment
`may be employed.
`FIG. 7 gives a first example of the system of FIG. 5 in
`operation. After the switch is turned on, as at A, the average
`motor current settles down to a stable operating value, as at
`B. This currentis indicative of normaltool operation with no
`impending stall condition. At C, the current rises rapidly,
`indicating an impending stall condition. As described in
`FIG. 5, the impending stall condition occurs when one of
`two conditions are satisfied.
`Condition 1: The average motor currentrise as a function of
`time is greater than a predetermined delta threshold. FIG.
`7 illustratesthis delta threshold as a rate of changeorfirst
`derivative value corresponding to a predeterminedrate of
`current rise or slope.
`Condition 2: The average motor current exceeds an absolute
`threshold. FIG. 7 illustrates the absolute threshold as any
`current above the lower threshold shown. Once one of
`these two conditions has been met(in this case, the first
`condition), pulse mode operation is commenced. If the
`cause of the impendingstall condition is not cleared after
`a predetermined time (after a predetermined number of
`line cycles), the motor is shut off, as at G. Shutoff occurs
`in this case because the average current peaks exceed the
`upper threshold.
`FIG. 8 illustrates the case where motor current enters the
`pulse mode operation, as at C, and then returns to normal
`operation, as at F. Normal operation is resumed because the
`motor current has fallen below the lower threshold.
`FIG. 9 illustrates the case in which pulse mode operation
`is begun, as at C, and continues in this mode without
`invoking motor shutoff. The peak motor current in pulse
`mode operation falls below the upper current
`threshold.
`Because the upper threshold is not reached, pulse mode
`operation will continue. Comparing FIG. 9 with FIG. 7, note
`that in FIG. 7 the average current peaks exceed the upper
`threshold during pulse mode operation. Thus, in FIG. 7 the
`pulse mode operation terminates and the motor is shutoff at
`G, whereas in FIG. 9 pulse mode does not terminate.
`FIG. 10 shows howthe invention may be implemented in
`an AC powered drill. Groups of components in the embodi-
`ment of FIG. 10 correspond to the block diagram elements
`of FIG. 2. Accordingly, refer to FIG. 2 for an overview of
`these circuit component functions. Referring to FIG. 10, the
`presently preferred power supply circuit 32 uses zener
`diodes to supply low voltage power supply voltages to
`variousparts of the circuit, including a 5 volt DC supply and
`a 12 volt DC supply.
`The circuit of FIG. 10 can be constructed using an
`ST6210A microcontroller circuit connected according to the
`pin-out illustrated. A resonator 200 maintains the proper
`clock frequency of the microcontroller. The trigger switch
`circuit 24 is connected to A/D input pin 10 of microcon-
`troller 30. Microcontroller 30 is thus able to monitor the
`
`setting of the trigger switch. The AC line frequency is
`supplied to microcontroller 30 through sensing resistor 202
`coupled to high impedance input pin 8. This provides
`microcontroller 30 with a signal from which the AC line
`frequency can be obtained for synchronizing and for gen-
`erating the time base that controls the rate of pulse mode
`operation.
`Undercertain conditions, microcontroller 30 will shut off
`the power to the LED for purposes of optical indication.
`Microcontroller 30 supplies a signal on pin 12 that is applied
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`10
`claims. For example, while the invention has been disclosed
`primarily in an AC-powered drill application, the invention
`is equally suitable to battery powered tools.
`
`APPENDIX
`
`Pulse Mode Routine
`
`9
`to the base of transistor 204 for this purpose. So that
`microcontroller 30 can monitor the state of the triac drive
`circuit, pin 15 is coupled through pull-up resistor 206 to the
`5 volt supply. Pin 15 is also coupled throughresistor 208 to
`the trigger terminal of triac 36. Depending onthestate of the
`triac, pin 15 of microcontroller 30 may be pulled low,
`thereby signaling a changein state of the triac drive circuit.
`Initialize current ON time
`Microcontroller 30 controls the operation of triac drive
`Initialize current OFF time
`circuit 36 through pin 19. Pin 19 is coupled to the base of
`Initialize pulse mode duration
`triac drive circuit
`transistor 212. A variable duty