Method of monitoring and controlling the seating of screws to the optimum point of grip independent of screw size and material density

• US 7,091,683 B1
• Filed: 10/22/2004
• Issued: 08/15/2006
• Est. Priority Date: 10/24/2003
• Status: Active Grant

• Current Assignee: Intelligent Automation Design, LLC
• Original Assignee: Intelligent Automation Design, LLC
• Inventors: Smith, Mark, Whyte, Joseph
• Primary Examiner(s): IP, SHIK LUEN PAUL
• Application Number:
  US10/970,682
• Time in Patent Office: 662 Days
• Field of Search: 318430-434, 318/272, 318/599, 318/283, 318/284, 173/176, 173/217, 388/804, 388/811, 700/168, 737/61
• US Class Current: 318/432
• CPC Class Codes: A61B 17/1626 Control means; Display units
• CPC Class Codes: A61B 17/8875 Screwdrivers, spanners or wrenches
• CPC Class Codes: A61B 2090/031 torque limiting
• CPC Class Codes: A61B 2090/064 for measuring force, pressure or mechanical tension
• CPC Class Codes: B25B 23/14 Arrangement of torque limiters or torque indicators in wrenches or screwdrivers couplings for transmitting rotation or clutches F16D; devices for measuring torque per se G01L
• CPC Class Codes: B25B 23/147 specially adapted for electrically operated wrenches or screwdrivers

Abstract

A method of controlling a motor (106) used to drive a screwdriver bit (105) 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 controls the speed and output torque of the motor (106). The control system 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. 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, corresponding to a drop in torque, is detected at the optimum point of grip, the controller (100) stops the motor (106).

Claims

1. A method of controlling a motor (106) used to drive a screwdriver bit (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 maximum by an average means for 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 of grip.

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 over time.

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 determining 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).