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handbook of modern sensors

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7.4 Inductive and Magnetic Sensors 271<br />

(A)<br />

(B)<br />

(D)<br />

(C)<br />

(E)<br />

Fig. 7.19. Magnetoresistive sensor output in the field <strong>of</strong> a permanent magnet as a function <strong>of</strong> its<br />

displacement x parallel to the magnetic axis (A–C). The magnet provides both the axillary and<br />

transverse fields. Reversal <strong>of</strong> the sensor relative to the magnet will reverse the characteristic.<br />

(D and E) Sensor output with a too strong magnetic field.<br />

7.4.5 Magnetoresistive Sensors 5<br />

These <strong>sensors</strong> are similar in application to the Hall effect <strong>sensors</strong>. For functioning,<br />

they require an external magnetic field. Hence, whenever the magnetoresistive sensor<br />

is used as a proximity, position, or rotation detector, it must be combined with a source<br />

<strong>of</strong> a magnetic field. Usually, the field is originated in a permanent magnet which is<br />

attached to the sensor. Figure 7.19 shows a simple arrangement for using a sensor–<br />

permanent-magnet combination to measure linear displacement. It reveals some <strong>of</strong><br />

the problems likely to be encountered if proper account is not taken <strong>of</strong> the effects<br />

described in this subsection. When the sensor is placed in the magnetic field, it is<br />

exposed to the fields in both the x and y directions. If the magnet is oriented with its<br />

axis parallel to the sensor strips (i.e., in the x direction) as shown in Fig. 7.19A, H x<br />

then provides the auxiliary field, and the variation in H y can be used as a measure<br />

<strong>of</strong> x displacement. Figure 7.19B shows how both H x and H y vary with x, and Fig.<br />

7.19C shows the corresponding output signal. In this example, H x never exceeds ±H x<br />

(the field that can cause flipping <strong>of</strong> the sensor), and the sensor characteristics remain<br />

stable and well behaved throughout the measuring range. However, if the magnet is<br />

5 Information on the KZM10 and KM110 <strong>sensors</strong> is courtesy <strong>of</strong> Philips Semiconductors BV<br />

(Eindhoven, The Netherlands).

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