magnetoresistive sensor measurements

Magnetoresistive Sensors

These sensors are similar in application to the Hall effect sensors. For functioning, they require an external magnetic field. Hence, whenever the magnetoresistive sensor is used as a proximity, position, or rotation detector, it must be combined with a source of a magnetic field. Usually, the field is originated in a permanent magnet which is attached to the sensor. Figure 7.19 shows a simple arrangement for using a sensor– permanent-magnet combination to measure linear displacement.

It reveals some of the problems likely to be encountered if proper account is not taken of the effects described in this subsection. When the sensor is placed in the magnetic field, it is exposed to the fields in both the x and y directions. If the magnet is oriented with its axis parallel to the sensor strips (i.e., in the x direction) as shown in Fig. 7.19A, Hx then provides the auxiliary field, and the variation in Hy can be used as a measure of x displacement. Figure 7.19B shows how both Hx and Hy vary with x, and Fig. 7.19C shows the corresponding output signal. In this example,Hx never exceeds±Hx (the field that can cause flipping of the sensor), and the sensor characteristics remain stable and well behaved throughout the measuring range. However, if the magnet is too powerful or the sensor passes too close to the magnet, the output signal will be drastically different.

Suppose the sensor is initially on the transverse axis of the magnet (x =0). Hy will be zero and Hx will be at its maximum value (>Hx ). Thus, the sensor will be oriented in the +x direction and the output voltage will vary as in Fig. 7.19E. With the sensor’s movement in the +x direction, Hy and V0 increase, and Hx falls to zero and then increases negatively until Hy exceeds −Hx . At this point, the sensor characteristic flips and the output voltage reverses, moving fromAto B in Fig. 7.19E. A further increase in x causes the sensor voltage to move along BE. If the sensor is moved in the opposite direction, however, Hx increases until it exceeds +Hx and V0 moves from B to C. At this point, the sensor characteristic again flips and V0 moves from C to D. Then, under these conditions, the sensor characteristic will trace the hysteresis loop ABCD and a similar loop in the −x direction. Figure 7.19E is an idealized case, because the reversals are never as abrupt as shown.

Figure 7.20A shows how KMZ10B and KM110B magnetoresistive sensors may be used to make position measurements of a metal object. The sensor is located between the plate and a permanent magnet, which is oriented with its magnetic axis

magnetoresistive sensor measurements

Fig. 7.20. One point measurement with the KMZ10. (A) The sensor is located between the permanent magnet and the metal plate; (B) Output signals for two distances between the magnet and the plate.

Optimum operating position of a magnetoresistive module.

Fig. 7.22. (A) Optimum operating position of a magnetoresistive module. Note a permanent magnet positioned behind the sensor. (B) Block diagram of the module circuit.


normal to the axis of the metal plate.Adiscontinuity in the plate’s structure, such as a hole or a region of nonmagnetic material, will disturb the magnetic field and produce a variation in the output signal from the sensor. Figure 7.20B shows the output signal for two values of spacing d. At the point where the hole and the sensor are precisely aligned, the output is zero regardless of the distance d or surrounding temperature.


Figure 7.21 shows another setup which is useful for measuring angular displacement. The sensor itself is located in the magnetic field produced by two RES190 permanent magnets fixed to a rotable frame. The output of the sensor will then be a measure of the rotation of the frame.


Figure 7.22Adepicts the use of a single KM110 sensor for detecting rotation and direction of a toothed wheel. The method of direction detection is based on a separate signal processing for the sensor’s two half-bridge outputs.

The sensor operates like a magnetic Wheatstone bridge measuring nonsymmetrical magnetic conditions such as when the teeth or pins move in front of the sensor. The mounting of the sensor and the magnet is critical, so the angle between the sensor’s symmetry axis and that of the toothed wheel must be kept near zero. Further, both axes (the sensor’s and the wheel’s) must coincide. The circuit (Fig. 7.22B) connects both bridge outputs to the corresponding amplifiers and, subsequently, to the low-pass filters and Schmitt triggers to form the rectangular output signals.Aphase difference between both outputs (Figs. 7.23Aand 7.23B) is an indication of a rotation direction.

A magnetostrictive detector uses ultrasonic waves

Fig. 7.24. A magnetostrictive detector uses ultrasonic waves to detect position of a permanent magnet.