Page 171 - MEMS Mechanical Sensors

160 Force and Torque Sensors

specifications, and optical methods are largely immune to such interference effects.
Also, a noncontacting sensor is desirable to improve reliability.
One commercial system [40, 41] employs an optical sensor to measure the rela-
tive angular movement between the ends of a torsion bar as torque is applied to the
shaft. The torsion bar is a compliant portion of shaft designed to increase the angu-
lar displacement for a given torque—for example, approximately 2.5° of twist for
10 Nm of applied torque. Two rotating discs are fitted to the shaft; one mechanically
links to each end of the torsion bar. Each rotating disc has a pattern of slots forming
two tracks of alternating transparent and opaque zones. The optical arrangement is
such that the light intensity received relates to torque, the direction of torque can be
determined, and the variation in light source intensity is eliminated from the torque
sensor output signal. The device has a 5% accuracy level, which is adequate for this
application.
A sensitive optical torque sensor based on the optical measure of the torsion
angle by using a sensitive polarimetric method has been demonstrated [42]; the tor-
sion angle has been measured with 0.001° accuracy. A birefringent torque sensor
[43] uses a photoelastic polymer illuminated by polarized light and experiencing tor-
sional strain. A two-dimensional fringe pattern is viewed through an optical polar-
izer. The strain that causes this observed image pattern is a complex function of the
torque applied to the shaft. A neural network can be trained with the fringe patterns
to activate the applied torque for both static shafts (accuracy level 1%) and rotating
shafts up to 1,500 rev/min (accuracy level 4%).
An optical torque sensor has been demonstrated that uses a modified moiré
fringe method for measuring torque in a rotating shaft [44]. The system utilizes two
circular gratings mounted concentrically at either end of the torsion shaft; one grat-
ing is ruled radially, and the other has rulings that are tangential to a small central
generating circle. Circular fringes are formed that move radially as the angular dis-
placement between the gratings charges; the fringe movement may be read out using
a CCD array.
Two 11-bit absolute position optical encoders have been used for torque
measurement with the torsion bar [45]. Seven additional bits provide interpolation
(relative) position determination between adjacent codes. An ASIC computes
the two optically encoded positions for speeds up to 1,600 rev/min. A tool-
holder torque sensor (spindle-mounted dynamometer) using an optical method has
been developed [46]. Torque is detected by monitoring the change of clearance
of a V-notch between two flanges, with a focused light ray from the side of the
cluck.
A miniaturized optoelectronic torque sensor with hysteresis and nonlinearity
less than 1% for maximum torque measurement of 0.15 Ncm in both directions has
been developed [47]. The torsion of a cross-spring-bearing moves a precision slit in
front of twin-photo diodes, transforming the torque into an electric signal. An inte-
grated optical torque measurement microsystem has been reported [48].

7.5 Capacitive Devices

Noncontact torque measurement on a rotating shaft can be achieved by the use of a
capacitive sensor, although susceptibility of the telemetry circuits to radio frequency