Page 126 - Sensors and Control Systems in Manufacturing

Classification and Types of Sensors
• AC or DC control voltage. Use of AC control may seem to 87
require the use of an AC-configured sensor. However, inter-
face circuitry can allow for DC sensors even if the main con-
trol voltage source is AC.
• Control circuit current requirements. Usually control circuits
operating in the 200- to 300-mA range can use either AC or
DC sensors. Circuits with 0.5-A and higher current will dic-
tate the type of sensor to be used.
• Application output requirements. NO output is the most com-
monly used output type. Controlled circuit configurations may
dictate use of NC or complementary-type configured sensors.
• Switching speed requirements. AC circuits are limited in their
operations per second. DC circuits may be required for appli-
cations involving counting or high speed.
• Connecting logic device. The device to which the sensor is
connected—such as programmable controller, relay, solenoid,
or timer/counter—is usually the most important factor in
sensor circuit and output configuration.

2.9.5 Accessories for Sensor Circuits
Sensor circuits and their output configurations must have various
types of indicators and protection devices, such as:

• Light-emitting diode (LED) indicators
• Short-circuit protectors
• Reverse-polarity protectors—DC three-wire
• Wire terminators—color-coded wire
• Pin connector type and pin-out designator

2.9.5.1 LED Indicators
LED indicators provide diagnostic information on the status of
sensors (e.g., operated or not operated) that is vital in computer-
integrated manufacturing. Two LEDs also indicate the status of
complementary-type sensor switches and power ON/OFF status, as
well as short-circuit condition.
2.9.5.2 Short-Circuit Protection
Short-circuit protection is intended to protect the switch circuit from
excessive current caused by wiring short circuits, line power spikes
from high inrush sources, or lightning strikes. This option involves
special circuitry that either limits the current through the output
device or turns the switch OFF. The turn off–type switch remains
inoperative until the short circuit has been cleared—with power dis-
connected. Afterwards, power is reapplied to the sensor. A second