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`Pro-Dex v. Intelligent Automation
`U.S. Patent 7,091,683
`Pro-Dex Ex. 1001
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`US 7,091,683 BI
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`Page 2
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`U.S. PATENT DOCUMENTS
`5,731,673 A *
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`3/1999 Bufe et al. ceccecceee 388/804
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`4/1999 Becker
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`6,836,614 BL* 12/2004 Gilmore 0... 388/811
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`i
`2002/0050364 AI
`5/2002 Suzukiet al
`2002/0153856 Al* 10/2002 Gilmore wioccccssene 318/599
`2003/0037423 Al
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`* cited by examiner
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`U.S. Patent
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`Aug. 15,2006
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`Sheet 1 of 2
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`US 7,091,683 B1
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`U.S. Patent
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`Aug. 15,2006
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`Sheet 2 of 2
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`US 7,091,683 B1
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`US 7,091,683 Bl
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`1
`METHOD OF MONITORING AND
`CONTROLLING THE SEATING OF SCREWS
`TO THE OPTIMUM POINT OF GRIP
`INDEPENDENT OF SCREWSIZE AND
`MATERIAL DENSITY
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`REFERENCE TO RELATED APPLICATION
`
`The present application claims the benefit of U.S. Provi-
`sional Patent Application No. 60/513,544, filed Oct. 24,
`2003, whose disclosure is hereby incorporated by reference
`in its entirety into the present disclosure.
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`FIELD OF THE INVENTION
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`The present invention is directed to a method of moni-
`toring and controlling the seating of screws and more
`particularly to such a method which minimizes or avoids
`stripping.
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`DESCRIPTION OF RELATED ART
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`Whenusing screws (107) to fasten two materials together,
`the objective is to stop turning the screw (107) when the
`optimum point of grip is achieved between the two materials
`and the screw (107). The torque required to seat the screw
`(107) increases as the screw (107) passes through the
`various zones of the seating process. Once the optimum
`point of grip is reached, the torque required to continue
`turning the screw (107) decreases rapidly as the screw (107)
`enters the strip or material yield zone.
`Traditional approaches have used clutches configured
`with a prefixed torque limit to disengage the motor (106)
`turning the screwdriver bit (105) once the preset torque
`value has been reached. There are a number of problems
`associated with the use of clutches in screw (107) seating. As
`mechanical devices, clutches wear over time and their
`accuracy and repeatability suffer as a result. In addition, a
`device that stops the turning of the screw (107) when a
`specific torque threshold has been reached does not stop the
`screws(107) at the optimum pointofgrip. Torque thresholds
`do not account for slight variations in screw (107) size and
`shape as well as slight variations in the density of the
`material
`the screw (107) is being driven into. Finally,
`devices that use torque thresholds as the determining factor
`have to be specifically configured for each screw (107) and
`material combination.
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`To overcome these problems, some patents monitor the
`various torque zones in the screw (107) seating process and
`stop the motor (106) when an increase in the rate of change
`of torque is recognized. This rate of change increase corre-
`spondsto the head ofthe screw (107) coming in contact with
`the material the screw (107) is being driven into. However,
`by stopping the screw (107) when a rate of change of
`increase in torque is recognized, the screw(107) is stopped
`prior to reaching the optimum point of grip. Others have
`improvedon this approach by movinga fixed distance after
`the rate of change is recognized. This moves the seating of
`the screw (107) closer to optimum pointof grip but will stop
`the screw (107) prior to or after the optimum pointof grip.
`Noneof these approaches eliminates the need to configure
`the tool for the specific screw (107) and material density
`being used.
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`SUMMARY OF THE INVENTION
`
`the present
`To avoid the above and other problems,
`invention is directed to a method of seating screws (107)to
`the optimum point of grip by stopping the motor (106) when
`a decrease in the rate of change of torque is realized. By
`averaging torque readings, the controller (100) smoothes the
`torquerise andfalls that occur due to inconsistencies in the
`material and screw (107). This in turn, eliminates the pos-
`sibility of a false decrease in the rate of change stopping the
`motor (106) whenit is not fully seated at the optimum point
`of grip. The speed of the motor (106)is controlled directly
`by the controller (100). As the screw (107) passes through its
`various seating zones, this zone change is detected by the
`controller (100) and the speed of the motor (106) lowered to
`insure the motor (106) is stoppedatthe instant the controller
`(100) detects the optimum pointof grip. This method makes
`the tool completely independent of the screw (107) size,
`shape and material, eliminating the need to configure the
`tool.
`The control system in one embodiment utilizes pulse
`width modulation (PWM)to control motor (106) speed and
`torque. The PWM signal controls the duty cycle of the
`transistors that supply current
`to the motor (106). The
`amount of current flowing through the motor (106) coils is
`proportional to the amount of torque the motor (106) is
`producing.
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`BRIEF DESCRIPTION OF THE DRAWINGS
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`A preferred embodiment will be set forth with reference
`to the drawings, in which:
`FIG. 1 shows a motor controller;
`FIG. 2 showsa surgical screwdriver;
`FIG. 3 shows a miniature motor;
`FIG. 4 showssurgical screws; and
`FIG. 5 showsa seating torque profile.
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`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENT
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`In the preferred embodiment, the torque is controlled in
`the following manner, which will be explained with refer-
`ence to FIG. 5. The seating torque profile shown in FIG. 5
`can be divided into the following zones.
`Zone 1: The screw (107) tip makes contact with the
`material and the required torque level ramps up until the
`barrel of the screw (107) is reached.
`Zone 2: The barrel of the screw (107), which is of a
`relatively constant diameter, requires a constant torque to
`continue driving the screw (107) into the material.
`Zone 3: Once the barrel is seated into the material, the
`torque required to continue driving the screw (107) rises
`significantly as the head of the screw (107) comesin contact
`with the material. The required torque to turn the screw
`(107) continues to rise until the optimum point of grip is
`reached, which occurs at the transition point from zone 3 to
`zone 4.
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`Zone 4: In zone 4, the torque required to continue turning
`the screw (107) drops significantly as the threads that were
`created in the material begin to strip. This dramatic decrease
`in torque continues until the screw (107) completes one full
`revolution beyond the optimum point of grip.
`Zone 5: After one full revolution beyond the optimum
`point of grip has been completed, all the threads previously
`created in the material will be stripped. The torque required
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`US 7,091,683 Bl
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`3
`to continue turning the screw (107) is the same asthe torque
`level before the screw (107) seating process started.
`Thus, by measuring the torque, the controller can deter-
`mine whether the optimum point in the seating torque profile
`has been reached and can control the motor accordingly.
`A motor (106) used to drive a screwdriver bit (105) is
`controlled such that screws (107) are seated to the optimum
`point of grip between the screw (107) and the work piece
`material. An electronic control circuit (108) controls the
`speed and output torque of the motor (106). The contro!
`system utilizes pulse width modulation (PWM)to control
`motor (106) speed and torque. The PWMsignal controls the
`duty cycle of the transistors that supply current to the motor
`(106). The amount of current flowing through the motor
`(106) coils is proportional to the amountof torque the motor
`(106) is producing. Motor (106) current
`is measured as
`voltage produced across a precision resistor that is in series
`with the motor (106) coils. As the current in the motor (106)
`increases, the voltage across the resistor increases (V=IR).
`To accurately measure the torque, as measured by the
`voltage across the precision resistor, a capacitor with a
`resistor and/or a diode is used to average the analog signal
`that is measured by the microprocessor. The microprocessor
`further filters the analog signal using an averaging formula
`to produce a stable value corresponding to motor (106)
`torque/current. When a decrease in current, correspondingto
`a drop in torque,is detected at the optimum pointof grip, the
`controller (100) stops the motor (106).
`Motor (106) current
`is measured as voltage produced
`across a precision resistor that is in series with the motor
`(106) coils. As the current in the motor (106) increases, the
`voltage across the resistor increases (V=IR). To accurately
`measure the torque, as measured by the voltage across the
`precision resistor, a capacitor with a resistor and/or a diode
`is used to average the analog signalthat is measured by the
`microprocessor. The microprocessor further filters the ana-
`log signal using an averaging formula to produce a stable
`value corresponding to motor (106) torque/current.
`Sample averaging formula:
`(Current valuex9.9+New Value*0.1)/10=New Cur-
`rent Value
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`Averaging the analog signal prevents random peaks/
`spikes and valleys/drops from creating false triggers. Varia-
`tions to the averaging formula can be used to adjust the
`weighting applied to the current and new value.
`The motor (106) speed is determined by monitoring the
`frequency of the Hall-effect signals produced as the motor
`(106) turns. External devices such as an encoderor resolver
`can also be used as feedback devices. The motor (106)
`current and speed are measured by the microprocessor,
`which determines how the motor (106) is controlled using
`the PWMsignal.
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`While a preferred embodiment has been set forth in detail,
`it will be appreciated that other embodiments can berealized
`within the scope ofthe invention. For example, the invention
`is not limited to surgical screws, but can instead be applied
`to any system in which it is desired to prevent stripping.
`Therefore,
`the present invention should be construed as
`limited only by the appended claims.
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`What is claimedis:
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`1. A method of controlling a motor (106) used to drive a
`screwdriverbit (105) such that screws (107) are seated to the
`optimum point of grip between the screw (107) and the work
`piece material, the method comprising:
`(a) detecting a torque of the motor;
`(b) determining a time when the torque reaches a maxi-
`mumby an average meansfor determining an average
`value as a function of a current value and a new value,
`thereby determining the optimum point of grip; and
`(c) stopping the motor at the optimum point ofgrip.
`2. A method according to claim 1, wherein step (b)
`comprises determining a time when a rate of change of the
`torque becomes negative.
`3. A method according to claim 1, wherein step (a)
`comprises averaging detected values of the torque overtime.
`4. A method according to claim 3, wherein step (b)
`comprises determining a time when a rate of change of the
`torque becomes negative.
`5. A method according to claim 4, further comprising
`controlling a speed of the motor (106) in accordance with the
`detected torque.
`6. A speed/torque controller (100) for controlling the
`rotation speed and output torque of the motor (106) with
`either sensor feedback or back EMF used to monitor motor
`(106) speed and current used to monitor motor (106) torque,
`the controller comprising:
`a detector for detecting the output torque of the motor; and
`a control circuit for determining a time when the torque
`reaches a maximum by an average means for deter-
`mining an average value as a function of'a current value
`and a new value, thereby determining the optimum
`point of grip, and stopping the motor at the optimum
`point of grip.
`7. A controller according to claim 6, wherein said control
`circuit removes power to the motor (106) when the optimum
`grip threshold is detected.
`8. A controller according to claim 7, wherein the control
`circuit stops the motor (106) by dynamically braking the
`motor (106).
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