Biomedical Engineering Reference
In-Depth Information
can operate with a maximum velocity of 25 m/s for sinusoidal outputs, whereas digital
models provide resolutions of 100, 50, and 10
μ
m at 25 m/s, improving to 5
μ
mat
20 m/s, and 1
m at 4 m/s. Resistant to shock, vibration, and pressure, they operate over
a temperature range of -20 to 85
◦
C.
μ
2.4.4 Capacitive Displacement Sensors
The capacitance of a parallel plate capacitor is directly proportional to the area of the two
plates and inversely proportional to the distance between them. Capacitance is also directly
proportional to the dielectric constant of the material between the plates. A simple, albeit
very short range sensor could be made by having one fixed plate and one plate that moves.
However, more sophisticated capacitance-based sensors use a pair of coaxial capacitors
with a movable dielectric sleeve that fits between the outer and inner electrode of the
one capacitor. The proportion of the area between the two electrodes that is filled with
dielectric determines the capacitance (Fraden, 1996).
These sensors are ratiometric because they measure the ratio of two capacitors main-
tained under the same conditions of temperature and humidity to minimize drift. This ratio
can be determined using an AC equivalent of the Wheatstone bridge described earlier or
by incorporating the capacitors into a pair of radio frequency (RF) LC oscillators and
measuring the frequency difference between the two.
2.4.5 Optical Displacement Sensors
2.4.5.1 Digital Optical Encoders
Digital optical encoders are probably the most common of all the sensors used for angle
and angular rate measurement. They consist of a light source and a number of photo
transistors separated by a rotating mask made from alternating opaque and transparent
radial bands. As with magnetic encoders, rotary optical encoders come in two types: (1)
absolute encoders, in which a unique digital word corresponds to every rotational position
(within the quantization level); and (2) incremental encoders, which produce a sequence
of digital pulses as the shaft rotates, allowing measurement of the relative displacement
of the shaft.
Rotary encoder discs can be made from glass or plastic onto which has been deposited
a radial pattern organized into tracks or, for more robust applications, a metal disk through
which holes have been cut. Astride this disk is placed the light-emitting diode (LED)
source on one side and a number carefully aligned photo transistors on the other. As the
disk rotates, it will periodically interrupt the beam between the emitter-receiver pair to
produce a change in the digital level as shown in Figure 2-28.
FIGURE 2-28
Schematic diagram
of a generic rotary
optical encoder.
