Browse Prior Art Database

Fine Motor Torque Controller

IP.com Disclosure Number: IPCOM000101232D
Original Publication Date: 1990-Jul-01
Included in the Prior Art Database: 2005-Mar-16
Document File: 3 page(s) / 96K

Publishing Venue

IBM

Related People

Tamatani, H: AUTHOR

Abstract

Disclosed is an automated device to tighten a screw within specified torque with DC servo motor in low torque range. This automated device has three functions: 1. motor current monitor function 2. motor current limiter function 3. motor rotation speed control function

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Fine Motor Torque Controller

       Disclosed is an automated device to tighten a screw
within specified torque with DC servo motor in low torque range. This
automated device has three functions:
      1.   motor current monitor function
      2.   motor current limiter function
      3.   motor rotation speed control function

      Fig. 1 schematically shows a motor current monitor function to
detect motor torque, motor current limiter function to compare motor
torque with specified torque, motor rotation speed control function
to shorten process time and reduce friction and reactive force
effects.

      Motor torque is lineally proportional to motor current as shown
in the following formula: T = K x I where T = torque, K =
torque constant, and I = motor current.  But current limitation only
is not enough to properly control torque because rotation energy
makes inertia force which makes friction between the screw bearing
surface and tapped hole and also makes reactive torque when the
mechanical system does the force task.  This friction and reactive
force disturbs to tighten the screw properly.

      As shown in Fig. 2, depicting the relation between friction and
motor rotation speed, there are static friction and dynamic friction
in accordance with motor rotation speed.  So it is necessary to
reduce friction by slowing down the motor rotation speed when
specified torque is achieved.  Motor rotation speed is controlled by
pulse width modulation method.  This pulse duty is commanded by a
digital-to-analog converter which has feedback from pulse encoder.
Motor rotation speed has to be controlled as in the following three
phases.
Phase 1:  while no load, rotation, speed should be high to shorten
process time.
 Phase 2:  until the bearing surface of screw fits the tapped hole,
rotation speed should be low to reduce friction effect.
 Phase 3:  until the specified torque is achieved, rotation speed
should be zero to neglect friction, reactive force effect.

      Current limitation threshold commanded by the microprocessor is
variable and initially set under specified current...