Philips Magnetoresistive Sensor

2000 Sep 06 3

Philips Semiconductors

Magnetoresistive sensors for

magnetic ﬁeld measurement General

The KMZ range of magnetoresistive sensors is

characterized by high sensitivity in the detection of

magneticfields, a wide operating temperature range, a low

and stable offsetand low sensitivity to mechanical stress.

They therefore provide an excellent means of measuring

both linear and angular displacement under extreme

environmental conditions, because their very high

sensitivitymeans that a fairly small movement of actuating

components in, for example, cars or machinery (gear

wheels, metal rods, cogs, cams, etc.) can create

measurablechanges in magnetic field. Other applications

for magnetoresistive sensors include rotational speed

measurement and current measurement.

Exampleswhere their properties can be put to good effect

can be found in automotive applications, such as wheel

speed sensors for ABS and motor management systems

and position sensors for chassis position, throttle and

pedal position measurement. Other examples include

instrumentation and control equipment, which often

require position sensors capable of detecting

displacements in the region of tenths of a millimetre (or

even less), and in electronic ignition systems, whichmust

be able to determine the angular position of an internal

combustion engine with great accuracy.

Finally,because of their high sensitivity, magnetoresistive

sensors can measure very weak magnetic fields and are

thus ideal for application in electronic compasses, earth

field correction and traffic detection.

Ifthe KMZ sensors are to be used to maximum advantage,

however, it is important to have a clear understanding of

their operating principles and characteristics, and how

theirbehaviour may be affected by external influences and

by their magnetic history.

Operating principles

Magnetoresistive (MR) sensors make use of the

magnetoresistiveeffect, the property of a current-carrying

magnetic materialto change its resistivity in the presence

of an external magnetic field (the common units used for

magnetic fields are given in Table 1).

Table 1 Common magnetic units

Thebasic operating principle of an MR sensor is shown in

Fig.2.

Figure 2 shows a strip of ferromagnetic material, called

permalloy (19%Fe, 81%Ni). Assume that, when no

external magnetic field is present, the permalloy has an

internal magnetization vector parallel to the current flow

(shown to flow through the permalloy from left to right).

If an external magnetic field H is applied, parallel to the

plane of the permalloy but perpendicular to the current

flow,the internal magnetization vector of the permalloy will

rotate around an angle α. As a result, the resistance of R

of the permalloy will change as a function of the rotation

angle α, as given by:

(1)

Ro and ∆Ro are material parameters and to achieve

optimum sensor characteristics Philips use Fe19Ni81,

whichhas a high Rovalue and low magnetostriction. With

this material, ∆Ro is of the order of 3%. For more

information on materials, see Appendix 1.

It is obvious from this quadratic equation, that the

resistance/magneticfield characteristic is non-linear and in

addition, each value of R is not necessarily associated

with a unique value of H (see Fig.3). For more details on

the essentials of the magnetoresistive effect, please refer

to the Section “Further information for advanced users”

laterin this chapter or Appendix 1, which examines the MR

effect in detail.

1 kA/m = 1.25 mTesla (in air)

1 mT = 10 Gauss

Fig.2 The magnetoresistive effect in permalloy.

handbook, halfpage

MLC127

Magnetization

Permalloy

Current

R = R ∆ R cos α

O∆R

cos2α+=