Page 114 - Sensors and Control Systems in Manufacturing

Classification and Types of Sensors
• Distance between RFI source and the sensor. Typically, induc- 75
tive proximity switches are not affected by RFI when a
transceiver is 1 ft away from the inductive switch. How-
ever, if closer than 1 ft, the switch may operate without a
target present.
• Signal frequency. The signal frequency may be the determin-
ing factor that will cause a particular device to false-operate
• Signal intensity. Radio-frequency transceivers usually are
portable devices with a power rating of 5 W maximum.
• Inductive proximity package. The sensor package construc-
tion may determine how well the device resists RFI.
• Approach to the sensor. A transceiver approaching the connect-
ing cable of a switch may affect it at a greater distance than if
it was brought closer to the sensing face. As RFI protection
varies from device to device and manufacturer to manufac-
turer, most manufacturers have taken steps to provide the
maximum protection against false operation due to RFI.
• Showering arc. Showering arc is the term applied to induced line
current/voltage spikes. The spike is produced by the electrical
arc on an electromechanical switch or contactor closure. The cur-
rent spike is induced from lines connected to the electromechan-
ical switch to the lines connected to the inductive proximity
switch, if the lines are adjacent and parallel to one another. The
result can be false operation of the inductive proximity switch.
The spike intensity is determined by the level of induced voltage
and the duration of the spike. Avoiding running cables for con-
trol devices in the same wiring channel as those for the contactor
or similar leads may eliminate spikes. Most electrical code spec-
ifications require separation of control device leads from electro-
mechanical switch and contractor leads.

2.7 Understanding Capacitive Proximity Sensors

2.7.1 Principles of Operation
A capacitive proximity sensor operates much like an inductive prox-
imity sensor. However, the means of sensing is considerably differ-
ent. Capacitive sensing is based on dielectric capacitance. Capacitance
is the property of insulators to store an electric charge. A capacitor
consists of two plates separated by an insulator, usually called a
dielectric. When the switch is closed (Fig. 2.61) a charge is stored on
the two plates.
The distance between the plates determines the ability of a capac-
itor to store a charge and can be calibrated as a function of stored
charge to determine discrete ON and OFF switching status.