Page 202 - Robot Builders Source Book - Gordon McComb

!90 Feedback Sensors

FIGURE 5.22 Electromagnetic
device for measuring speed of
rotation (car speedometer).

proportional to the speed of rotation. Therefore, the torque angle of spring 3 and the
rotational angle of indicator 4 are also proportional to the speed of rotation.
The method of length measurement mentioned at the end of the previous section
(5.1) is easily transformed into speed.
Flow-rate sensors are also a kind of speed-measuring device which provide infor-
mation about the flow rates of gases or liquids. Here we consider some ideas used for
this purpose. Figure 5.23 shows an example of a design for an impeller-type sensor.
Impeller 1 (a miniature turbine) rotates in agate bearings 2, installed in supports 3
inside a pipe. Axial displacement is limited by agate thrust bearings 4. The pipe section
and the impeller are made from a nonmagnetic material. A small piece of a magnetic
material 5 is set into one side of the impeller shaft. Outside the pipe a permanent
magnet 6 with coil 7 wound around it is installed. During rotation the impeller's shaft
turns piece 5, and thus the magnetic flux changes depending upon its position rela-
tive to the permanent magnet. This induces an alternate electromotive force of a fre-
quency double that of the impeller's speed of rotation. The rotation speed of the

FIGURE 5.23 Gas or liquid flowmeter with an impeller.
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