Page 190 - Sensors and Control Systems in Manufacturing

Fiber Optics in Sensors and Contr ol Systems
They will not be affected by electromagnetic interference (EMI), and 151
there are no premature responses in the face of EMI. However, because
they are mechanical devices, limit switches face physical limitations
that can shorten their service life even though they are capable of
several million operations. Also, heavy sludge, chips, or coolant can
interfere with their operation.

3.6 Factors Affecting the Selection of Position Sensors
In selecting a position sensor, several key factors should be considered:
• Cost. Both initial purchase price and life-cycle cost must be
considered.
• Sensing distance. Photoelectric sensors are often the best selec-
tion when sensing distances are longer than 25 mm. Photo-
electric sensors can have sensing ranges as long as 300,000 mm
for outdoor or extremely dirty applications, down to 25 mm
for extremely small parts or for ignoring background. Induc-
tive proximity sensors and limit switches, on the other hand,
have short sensing distances. The inductive proximity sen-
sors are limited by the distance of the electromagnetic field—
less than 25 mm for most models—and limit switches can
sense only as far as the lever operator reaches.
• Type of material. Inductive proximity sensors can sense only
ferrous and nonferrous materials, whereas photoelectric and
limit switches can detect the presence of any solid material.
Photoelectric sensors, however, may require a polarizer if the
target’s surface is shiny.
• Speed. Electronic devices using DC power are the fastest—as
fast as 2000 cycles per second for inductive proximity models.
The fastest-acting limit switches can sense and reset in 4 ms
or about 300 times per second.
• Environment. Proximity sensors can best handle dirty, gritty
environments, but they can be fooled by metal chips and
other metallic debris. Photoelectric sensors will also be fooled
or left inoperable if they are fogged or blinded by debris.
• Types of voltages, connections, and requirements of the device’s
housing. All three types can accommodate varying require-
ments, but the proper selection must be made in light of the
power supplies, wiring schemes, and environments.
• Third-party certification. Underwriters Laboratories (UL),
National Electrical Manufacturers Association (NEMA),
International Electrotechnical Commission (IEC), Factory
Mutual, Canadian Standards Association (CSA), and other
organizations impose requirements for safety, often based on