Page 179 - MEMS Mechanical Sensors

168 Force and Torque Sensors

which produces a drain current proportional to the charge induced in its gate by the
local electric field. Based upon the pattern of outputs of the field-detecting ele-
ments, the position and orientation of the field emitters relative to the field detector
array can be inferred, with some redundancy. The compliance properties of the
elastomer material separating the two chips determine the sensitivities to forces and
torques. Resolutions of 75 nm translational in the x-y plane and of 0.01° rotation
about the z-axis seem possible. A packaged device would have dimensions of the
order 5 × 5 × 5 mm.

7.9 Future Devices

It will be evident from the review undertaken in this chapter that MEMS technolo-
gies are already playing a significant role in force and torque measurements.
Besides industrial weighing, power-assisted steering, shaft power, force/torque
drives, and fastener fixing, sensors are used in a number of advanced applica-
tions such as force microscopy, dexterous and dynamic manipulation of fragile
or irregular objects with microgrippers [67] and robotic hands [69]. “Smart”
structures in aerospace [72] need low-cost integrated force/torque sensor systems,
and microsystems technologies will be at the forefront of new sensor developments.
New approaches [73] and new applications are being established all the time [74].
Miniaturization and integration of several technologies, such as silicon
micromaching, microelectronics, optical fibers, and thin/thick-films, will contribute
to these developments. MEMS actuation will be combined with the MEMS sensors
to develop miniature MEMS feedback control systems to control small forces and
torques [75]. Arrays of devices will provide two- and three-dimensional capabili-
ties. Resonant force/torque sensors requiring only very low levels of power will
continue to be developed for noncontact/remote applications, and batteryless/wire-
less/autonomous devices should become available. Small electrical energy genera-
tors operating on available local vibration will also be used.

References

[1] Schicker, R., and G. Wegener, Measuring Torque Correctly, Darmstadt, Germany:
Hottinger Baldwin Messtechnik (HBM) GmbH, 2002.
[2] Garshelis, I. J., “Torque and Power Measurement,” in The Mechatronics Handbook,R.H.
Bishop, (ed.), Boca Raton, FL: CRC Press, 2002, pp. 19.48–19.61.
[3] Guide to the Measurement of Force, London, England: The Institute of Measurement and
Control, 1998.
[4] Resistance Strain Gauge Load Cells NMP7, Watford, England: Training Publications
Ltd., 2002.
[5] Kanda, Y., “Graphic Representation of the Piezoresistance Coefficients in Silicon,” IEEE
Trans. on Electron Devices, Vol. 29, No. 1, 1982, pp. 64–70.
[6] Jones, B. E., Instrumentation, Measurement, and Feedback, New York: McGraw-Hill,
1977.
[7] Elwenspoek, M., and R. Wiegerink, Mechanical Microsensors, Berlin, Germany: Springer-
Verlag, 2001, pp. 97–106.