Hardware Reference
In-Depth Information
Figure 9.14
Interface circuit for an optical proximity sensor
Optical proximity detectors
Optical proximity detectors generally offer increased sensing ranges in com-
parison with both capacitive and inductive types. Optical proximity sensors are
available in two basic forms:
diffuse scan
and
through scan
types. The former
types rely on the target surface returning a proportion of the modulated light
emitted by an optical transmitter which is mounted in the same enclosure as
the receiver. In such an arrangement, a reflective target may be detected by the
presence of a received signal. Through scan types, on the other hand, employ a
separate transmitter and receiver and operate on the principle of the interrupted
light beam (i.e. the target is detected by the absence of received light). Typical
ranges vary from about 100 mm to 300 mm for diffuse scan sensors with plane
white surfaces to up to 15 in (380 mm) for through scan sensors with opaque
targets.
Proprietary sensor units are generally rather slow in operation and, for appli-
cations which involve rapid motion (such as counting shaft speeds) faster
sensors should be employed. Here, a simple optical sensor (comprising an
unmodulated infra-red emitting LED and photodiode) may be employed. Such
devices are readily available in a variety of packages including miniature dif-
fuse scan types and slotted through scan units. Figure 9.14 shows the circuitry
required to interface such a device to a typical digital input port.
Position transducers
Position transducers can be used to provide an accurate indication of the posi-
tion of an object and are available in a variety of forms (including linear and
rotary types). Linear position sensors use linear law potentiometer elements
(of typically 5 k
) and offer strokes of typically 10 or 100 mm. Rotary pos-
ition sensors are also available. These provide indications over typically 105
◦
and use linear law potentiometer elements similar to those found in conven-
tional rotary potentiometer controls. A typical value for the resistive element is
again 5 k
.
The output of linear and rotary position sensors is usually made available
as an analogue voltage and a typical arrangement is shown in Figure 9.15.
Note that the analogue input port should have a high impedance (say 500 k
or more) in order to avoid non-linearity caused by loading of the sensing
potentiometer.
