Page 458 - Sensors and Control Systems in Manufacturing

Sensors in Flexible Manufacturing Systems
8.10.3 Measurement at High Temperatures 411
in Manufacturing
The LVDT sensor has been developed for measurements involving
very high temperatures. It is capable of operating continuously at
1100°F (600°C) and surviving temperatures as high as 1200°F (650°C)
for several hours. Typical uses include position feedback from jet
engine controls located close to exhaust gases and measurement of
roller position and material thickness in hot strip or slabbing mills. In
scientific research, it can be used to directly measure dimensional
changes in heated test specimens without requiring thermal isola-
tion, which could induce measurement errors. The sensor is the cul-
mination of the development of sophisticated construction techniques
coupled with careful selection of materials that can survive sustained
operation at high temperatures. Because magnetic properties of a
metal vanish above its magnetic transformation temperature (Curie
point), the core material must be made from one of the few magnetic
materials having Curie temperatures above 1100°F (600°C). Another
problem is that, at high temperature, the resistance of windings made
of common magnet wire materials increases so much that an LVDT
sensor using ordinary conductor materials would become virtually
useless. Thus, the winding uses a wire of specially formulated high-
conductivity alloy. The sensors are made with careful attention to
internal mechanical configuration and with materials having com-
patible coefficients of expansion to minimize null shifts due to
unequal expansion or unsymmetrical construction. Hermetic sealing
allows the sensor to be subjected to hostile environments such as
fluid pressure up to 2500 psi (175 bars) at 650°F (350°C). Units can be
factory calibrated in a special autoclave that permits operation at
high temperature while they are hydrostatically pressurized.

8.11 Robot Control through Sensors
In order to pick up an object, a robot must be able to sense the strength
of the object being gripped so as not to crush the object. Accordingly,
the robot gripper is equipped with sensing devices to regulate the
amount of pressure applied to the object being retrieved.
Several industrial sensing devices enable the robot to place objects
at desired locations or perform various manufacturing processes:
• Transducers. Sensors that convert nonelectrical signals into
electrical energy
• Contact sensors (limit switches). Switches designed to be
turned ON or OFF by an object exerting pressure on a lever or
roller that operates the switch
• Noncontact sensors. Devices that sense through changes in
pressure, temperature, or electromagnetic field