Page 386 - The Mechatronics Handbook

Another type of resistive sensor is the flexible bend sensor. Conductive ink between two electrical
contacts on a flexible material changes resistance as the material bends and stretches. Used in a voltage
divider with a fixed resistor, the analog voltage may be used as a measure of the bend. Such a sensor
could be used to detect contact (like a whisker) or as a rough measure of the deformation of a surface
to which it is attached.

Tilt (Gravity)
A mercury switch can be used to provide one bit of information about orientation relative to the gravity
vector. A small drop of mercury enclosed in a glass bulb opens or closes the electrical connection between
two leads depending on the orientation of the sensor. Several mercury switches at different orientations
may be used to get a rough estimate of tilt. The signal from a mercury switch may “bounce” much like
the signal from a mechanical contact switch (Fig. 19.2).
An inclinometer can be used to measure the amount of tilt. One example is the electrolytic tilt sensor.
Manufacturers include The Fredericks Company and Spectron Glass. Two-axis models have five parallel
rod-like electrodes in a sealed capsule, partially filled with a conductive liquid. Four of the electrodes are
at the corners of a square, with one in the middle. Tilting the sensor changes the distribution of current
injected via the center electrode in favor of the electrodes which are more deeply immersed.
Tilt sensors may be obtained with liquids of varying viscosity, to minimize sloshing. Because a DC
current through the liquid would cause electrolysis and eventually destroy the sensor, AC measurements
of conductivity are used. As a result, the support electronics are not trivial.
The liquid conductivity is highly temperature dependent. The support electronics for the tilt sensor
must rely on a ratio of conductivity between pairs of rods. Also, although the electrolytic tilt sensor
operates over a wide temperature range, it is greatly disturbed by nonuniformities of temperature across
the cell.
Another kind of simple inclinometer can be constructed from a rotary potentiometer with a pendulum
bob attached. A problem with this solution is that friction may stop the bob’s motion when it is not
vertical. A related idea is to use an absolute optical encoder with a pendulum bob. Complete sensors
operating on this principle can be purchased with advanced options, such as magnetic damping to reduce
overshoot and oscillation. An example is US Digital’s 12-bit A2I absolute inclinometer.
Of course, gravity acting on a device is indistinguishable from acceleration. If the steady-state tilt of
a device is the measurement of interest, simple signal conditioning should be used to ensure that the
readings have settled.
Other more sophisticated tilt sensors include gyroscopes and microelectromechanical (MEMS) devices,
which are not discussed here.

Capacitive
Capacitance can be used to measure proximity or linear motions on the order of millimeters. The
capacitance C of a parallel plate capacitor is given by C = ε r ε o A/d, where ε r is the relative permittivity
of the dielectric between the plates, ε o is the permittivity of free space, A is the area of overlap of the two
plates, and d is the plate separation. As the plates translate in the direction normal to their planes, C is
a nonlinear function of the distance d. As the plates translate relative to each other in their planes, C is
a linear function of the area of overlap A. Used as proximity sensors, capacitive sensors can detect metallic
or nonmetallic objects, liquids, or any object with a dielectric constant greater than air.
One common sensing configuration has one plate of the capacitor inside a probe, sealed in an insulator.
The external target object forms the other plate of the capacitor, and it must be grounded to the proximity
sensor ground. As the sensor approaches the target, the capacitance increases, modifying the oscillation
of a detector circuit including the capacitor. This altered oscillation may be used to signal proximity or
to obtain a distance measurement.
Manufacturers of capacitive sensors include Cutler-Hammer and RDP Electronics.

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