Browse Prior Art Database

Measurement of Tape Drive Reel Motor Torque Constants

IP.com Disclosure Number: IPCOM000111746D
Original Publication Date: 1994-Mar-01
Included in the Prior Art Database: 2005-Mar-26
Document File: 4 page(s) / 127K

Publishing Venue

IBM

Related People

Bahr, AA: AUTHOR [+3]

Abstract

This disclosure presents a method to improve tape tension control of a reel-to-reel tape drive by measuring reel motor torque constants in situ. Simultaneous measurement of motor no-load speed, armature current, and applied voltage determines motor torque constant and viscous drag coefficient.

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Measurement of Tape Drive Reel Motor Torque Constants

      This disclosure presents a method to improve tape tension
control of a reel-to-reel tape drive by measuring reel motor torque
constants in situ.  Simultaneous measurement of motor no-load speed,
armature current, and applied voltage determines motor torque
constant and viscous drag coefficient.

      The reel motors of a new tape drive are powered by
Pulse-Width-Modulated (PWM) current-mode power amplifiers which are
capable of operating at 100% duty-cycle.  Actual motor armature
current is sensed and fed back in the current feedback control loop
of each amplifier.  Each motor carries an incremental encoder that is
used to accurately measure position and velocity.

      The tape drive can measure the torque constants and viscous
drag coefficients of its own reel motors when no tape cartridge is
present.  This is done by commanding the reel motor amplifier to its
maximum full-scale armature current.  This drives the PWM amplifier
to 100% duty cycle.  The PWM amplifier FETs have negligible "on
resistance", so the motor has the full power supply voltage applied
across its terminals, and it will accelerate to a no-load speed
determined by the applied voltage divided by the motor torque
constant.

      The torque constant, K[t], of a motor (measured in
Newton-Meters/Ampere) is theoretically identical to the back-EMF
constant, K[e], of the same motor (measured in volts/radian/second).
The units are identical in the metric MKS (meter-kilogram-second)
system.

      Motor shaft speed is measured by comparing the output pulses of
the incremental encoder on each motor to the microprocessor's
crystal-controlled clock.  This means the rotational velocity of each
motor (radians/second) is accurately known at all times.

      The reel motor servo amplifiers convert actual motor armature
current into a scaled analog signal as part of their current-control
feedback network.  This signal is required to control motor shaft
torque, but it can also be read by an analog-to-digital (A/D)
converter and read by the microprocessor.

      A rough estimate of motor torque constant can be obtained by
applying a known voltage to the motor armature and measuring the
no-load speed of the motor.  Divide this voltage by motor angular
velocity in radians/second to obtain an estimate of k[e]  in
volts/radian/second.

      An improved estimate of K[e]  from no-load speed is obtained by
subtracting the I-R drop through the motor from the applied voltage,
and using the result in the computation of K[e].  The resistance of
the motor armature is always part of the motor specification.

      Motor armature resistance (ohms) times the motor's no-load
current (amperes) at no-load speed (radians/second) must be a very
small fraction of the motor's back-EMF (Volts) at its no-load speed.
Armature resistance is known to vary 20% from its room temperature
nominal value and cl...