Page 108 - Sensors and Control Systems in Manufacturing

Classification and Types of Sensors

FIGURE 2.52 Sensing distance tolerances. 69

range can be as much as 120 percent or as little as 81 percent of the
nominal stated range (Fig. 2.52).

S = 0.9 < S < 1.1
r n
S = 0.9 < S < 1.1
n r
where S = nominal sensing range
n
S = effective sensing range
r
S = usable sensing range
u
2.6.4 Target Material and Size
As a target approaches the sensing field, eddy currents are induced in
the target. In order to ensure that the target has the desired damping
affect on the sensor, the target must be of the appropriate size and
material. Metallic targets can be defined as:

• Ferrous. Containing iron, nickel, or cobalt
• Nonferrous. All other metallic materials, such as aluminum,
copper, and brass.

Eddy currents induced in ferrous targets are stronger than in non-
ferrous targets, as illustrated in Table 1.5.
An increase in target size will not produce an increase in sensing
range. However, a decrease in target size will produce a decrease in
sensing range, and may also increase response time. Figure 2.53 illus-
trates the relationship of target size and target material to the sensing
range of a limit-switch-type sensor with a nominal 13-mm sensing
range.
Table 1.6 shows correction factors by which the rated nominal
sensing range of most inductive proximity sensors can be multiplied.