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100
Resistance (k ohms) 10

1

0.1
10 100 1000 10000
Force (g)

FIGURE 19.41 Resistance as a function of force for a typical force sensing resistor.

Magnetoelastic Force Sensors
Magnetoelastic transducer devices operate based on the Joule effect, that is, a ferromagnetic material is
dimensionally altered when subjected to a magnetic field. The principle of operation is as follows: Initially,
a current pulse is applied to the conductor within the waveguide. This sets up a magnetic field circumference-
wise around the waveguide over its entire length. There is another magnetic field generated by the
permanent magnet that exists only where the magnet is located. This field has a longitudinal component.
These two fields join vectorally to form a helical field near the magnet which, in turn, causes the waveguide
to experience a minute torsional strain or twist only at the location of the magnet. This twist effect is
known as the Wiedemann effect [8].
Magnetoelastic force transducers have a high frequency response (on the order of 20 kHz). Some of
the materials that exhibit magnetoelastic include Monel metal, Permalloy, Cekas, Alfer, and a number of
nickel–iron alloys. Disadvantages of these transducers include: (1) the fact that excessive stress and aging
may cause permanent changes, (2) zero drift and sensitivity changes due to temperature sensitivity, and
(3) hysteresis errors.
Torsional Balances
Balancing devices that utilize the deflection of a spring may also be used to determine forces. Torsional
balances are equal arm-scale-force measuring devices. They are comprised of horizontal steel bands instead
of pivots and bearings. The principle of operation is based on force application on one of the arms that
will deflect the torsional spring (within its design limits) in proportion to the applied force. This type
of instrument is susceptible to hysteresis and temperature errors and, therefore, is not used for precise
measurements.

Tactile Sensors
Tactile sensors are usually interpreted as a touch sensing technique. Tactile sensors cannot be considered
as simple touch sensors, where very few discrete force measurements are made. In tactile sensing, a force
“distribution” is measured using a closely spaced array of force sensors.
Tactile sensing is important in both grasping and object identification operations. Grasping an object
must be done in a stable manner so that the object is not allowed to slip or get damaged. Object
identification includes recognizing the shape, location, and orientation of a product, as well as identifying
surface properties and defects. Ideally, these tasks would require two types of sensing [9]:

1. continuous sensing of contact forces,
2. sensing of the surface deformation profile.
These two types of data are generally related through stress–strain relations of the tactile sensor. As a
result, almost continuous variable sensing of tactile forces (the sensing of the tactile deflection profile)
is achieved.

©2002 CRC Press LLC