Hardware Reference
In-Depth Information
Figure 9.12
Interfacing inductive proximity sensors: (a) NPN output types;
(b) PNP output types
Figure 9.13
Interface circuit for a typical capacitive proximity sensor
Inductive proximity detectors are available with either NPN or PNP outputs
(as shown in Figures 9.12(a) and (b)). An NPN type will return a logic 0 (low)
when a target is detected whilst a PNP type will return logic 1 (high) in similar
circumstances. When selecting a transducer for use with conventional I/O cards,
it is advisable to choose a device which operates from a
+
5 V supply as this
obviates the need for level shifting within the interface. A further consideration
with such devices is the maximum speed at which they can operate. This is
typically 2 kHz (i.e. 2000 pulses per second) but not that some devices are very
much slower.
Capacitive proximity detectors
Capacitive proximity detectors provide an alternative solution to the use of
inductive sensors. Unfortunately, such devices are also limited in their speed
of response (typically 250 Hz maximum) and often require supply voltages in
excess of the conventional
+
5 V associated with TTL signals. Capacitive prox-
imity sensors will, however, detect the presence of materials such as cardboard,
wood and plastics, as well as certain powders and liquids. Typical sensing dis-
tances range from 20 mm for metals to 4 mm for cardboard. As with inductive
proximity sensors, the sensitivity of the detector is proportional to target size.
A typical interface circuit for a DC-powered capacitive proximity detector is
shown in Figure 9.13. This circuit provides a logic 0 (low) when a target is
detected.
