Page 118 - Sensors and Control Systems in Manufacturing

Classification and Types of Sensors
sensing-range derating factors that apply to capacitive proximity sen- 79
sors. Capacitive sensors can be used to detect a target material through
a nonmetallic interposing material like glass or plastic. This is benefi-
cial in detecting a liquid through the wall of a plastic tank or through
a glass sight tube. The transparent interposing material has no effect
on sensing. For all practical purposes, the target size can be deter-
mined in the same manner as for inductive proximity sensors.

2.7.5 Surrounding Conditions
Capacitive proximity devices are affected by component tolerances
and temperature variations. As with inductive devices, capacitive prox-
imity devices are affected by the following surrounding conditions:
• Embeddable mounting. Capacitive sensors are generally treated
as nonshielded nonembeddable devices.
• Flying chips. Capacitive devices are more sensitive to metallic
and nonmetallic chips.
• Adjacent sensors. Allow more space than with inductive prox-
imity devices because of the greater sensing range of capaci-
tive devices.
• Target background. Relative humidity may cause a capacitive
device to operate even when a target is not present. Also, the
greater sensing range and ability to sense nonmetallic target
materials dictate greater care in applying capacitive devices
with target background conditions.
• Magnetic fields. Capacitive devices are not usually applied in
welding environment.
• Radio-frequency interference. Capacitive sensor circuitry can be
affected by RFI in the same way that an inductive device can.
• Showering arc. An induced electrical noise will affect the cir-
cuitry of a capacitive device in the same way it does an induc-
tive device.

2.8 Understanding Limit Switches
A limit switch is constructed much like the ordinary light switch used
in home and office. It has the same ON/OFF characteristics. The limit
switch usually has a pressure-sensitive mechanical arm. When an
object applies pressure on the mechanical arm, the switch circuit is
energized. An object might have a magnet attached that causes a con-
tact to rise and close when the object passes over the arm.
Limit switches can be either normally open (NO) or normally closed
(NC) and may have multiple poles (Fig. 2.65). A normally open switch
has continuity when pressure is applied and a contact is made, while