Page 116 - Sensors and Control Systems in Manufacturing

Classification and Types of Sensors

FIGURE 2.63 Oscillator damping of inductive and capacitive sensors. 77

feedback capacitance is detected. In an inductive proximity switch,
the oscillation is damped when the target is present (Fig. 2.63).
In both capacitive and inductive switch types, the difference
between the operate and the release amplitude in the oscillator and
corresponding detector circuit is referred to as hysteresis of the sensor.
It corresponds to the difference between target detection and release
distances from the sensor face.

2.7.2 Features of Capacitive Sensors
The major characteristics of capacitive proximity sensors are:
• They can detect nonmetallic targets.
• They can detect lightweight or small objects that cannot be
detected by mechanical limit switches.
• They provide a high switching rate for rapid response in
object counting applications.
• They can detect liquid targets through nonmetallic barriers
(glass, plastic, etc.).
• They have a long operational life with a virtually unlimited
number of operating cycles.
• The solid-state output provides a bounce-free contact signal.
Capacitive proximity sensors have two major limitations.

• They are affected by moisture and humidity.
• They must have an extended range for effective sensing.

2.7.3 Sensing Range
Capacitive proximity sensors have a greater sensing range than
inductive proximity sensors as shown in the following table.