Hall Effect Sensors Magneto Resistive Sensors Magneto Resistive Detector

Hall Effect
Introduction The function of a Hall sensor is based on the physical principle of the Hall effect named after its discoverer E. H. Hall: It means that a voltage is generated transversely to the current flow direction in an electric conductor (the Hall voltage), if a magnetic field is applied perpendicularly to the conductor. As the Hall effect is most pronounced in semiconductors, the most suitable Hall element is a small platelet made of semiconductive material. The Hall effect: In a semiconductive platelet, the Hall voltage is generated by the effect of an external magnetic field acting perpendicularly to the direction of the current.

Fig. 1The Hall effect: In a semiconductive platelet, the Hall voltage is generated by the effect of an external magnetic field acting perpendicularly to the direction of the current.

Hall Effect Sensors

Hall Effect - The development of a voltage between the two edges of a current carrying conductor whose faces are perpendicular to a magnetic field. Hall Effect Sensor - A device that converts the energy stored in a magnetic field to an electrical signal by means of Hall effect. Hall Effect Sensor, Digital - A device that converts the energy stored in a magnetic field to an electrical signal which is always in one or two stable states (ON or OFF, High or Low, 1 or 0). Hall Effect Sensor, Linear - A device that converts the energy stored in a magnetic field to a voltage which is proportional to its input. Hall Element - The basic component of a Hall effect sensor that converts magnetic field to a lower level electrical signal. When used alone is often referred to as a Hall effect generator. Hall Sensor - A term sometimes used to describe a SENSOR based upon a Hall effect sensor.

Hall Effect Sensors

Applications
The most commonly used application of this sensor is a trigger alarm system which is installed in big warehouses or stores, where robberies are a common threat. A magnetic strip is attached to the merchandise activates the alarm if the client passes through the exit doors without paying the product. If the product was previously paid the store teller removes the magnetic strip or demagnetizes applying a magnetizing force that reduces the residual magnetism on the strip almost to cero. The Hall effect sensor is also used to show the velocity of a bicycle in a digital display conveniently located over the steering handle.

Hall Effect Sensors

Supposing that a bicycle has a 26 in diameter wheel, the circumference will approximately be 82 inches. In a mile, the number of turns is: (5280 ft)(12 in / 1 ft) (1 turn / 82 in) = 773 turns If the bicycle is moving at 20 mph, an outwards pulse will occur at a relation of 4.29 per second. It is interesting to point out that the wheel is turning a more than 4 turns per second and that the total turns in 20 miles are 15,460.

Fig. 2 Obtaining the velocity indication for a bicycle using the Hall effect: (a) Components mounting (b) Hall effect response

Hall Effect Sensors

Fig.3 Functional principle of a Hall sensor: The output voltage of the sensor and the switching state, respectively, depend on the magnetic flux density through the Hall plate.

Hall Effect Sensors

During recent years, Hall effect sensors became increasingly popular. There are two types of Hall sensors: linear and threshold.

Fig. 4 Circuit diagrams of linear (A) and threshold (B) Hall effect sensor

Hall Effect Sensors

A linear sensor usually incorporates an amplifier for the easier interface with the peripheral circuits. In comparison with a basic sensor, they operate over a broader voltage range and are more stable in a noisy environment. These sensors are not quite linear with respect to magnetic field density and, therefore, the precision measurements require a calibration. In addition to the amplifier, the threshold-type sensor contains a Schmitt trigger detector with a built-in hysteresis. The output signal as a function of magnetic field density is shown in figure 5B.

Hall Effect Sensors

The signal is a two-level and one has clearly pronounced hysteresis with respect to the magnetic field. When the applied magnetic flux density exceeds a certain threshold, the trigger provides a clean transient from the OFF to the ON position.

Fig. 5 Transfer functions of a linear (A) and a threshold (B) Hall effect sensor.

Hall Effect Sensors

The hysteresis eliminates spurious oscillations by introducing a dead-band zone, in which the action is disabled after the threshold value has passed. The hall sensors are usually fabricated as monolithic silicon ships and encapsulated into small epoxy or ceramic packages.

Hall Effect Sensors

The magnetic field has two important characteristics for better responsivity, magnetic field lines must be normal (perpendicular) to the flat face of the sensor and must be at the correct polarity. Before designing a position detector with Hall sensor, an overall analysis should be performed in the following manner. First the field strength on the magnet should be investigated. The strength will be greatest at the pole face and by gaussmeter or a calibrated Hall sensor.

Hall Effect Sensors

The Hall sensors can be used for interrupter switching with a moving object. In this mode, the activating magnet and the Hall sensor are mounted on a single rugged assembly with a small air gap between them.

Fig. 6 The Hall effect sensor in the interrupter switching mode: (A) the magnetic flux turns the sensor on; (B) the magnetic flux is shunted by a vane.

Hall Effect Sensors

The design of a three-dimensional (3-D) coordinate hall effect sensor works by electronically measuring and comparing the magnetic flux from a movable target through four geometrically equal magnetic paths arranged symmetrically around the axis of the probe
Fig.7 Angular Hall sensor bridge (A) and the internal sensor interface (B) (Courtesy of Austria Micro Systems). A cut-away view (C) of the sensor with the target and the probe shows the magnetic flux paths. A cut-away view (D) shows four Hall effect sensors with four flux return paths.

Hall Effect Sensors

It is a magnetic equivalent of a Wheatstone bridge. The targets symmetrical magnetic field, generated by a permanent magnet, travels from the central pole through the air to the outer rim, when it is not in the vicinity of the probe. Because the flux from the target will take the path of least resistance (reluctance), the flux will go through the probe when the target is sufficiently close to it. The probe has a central pole face divided into four equal sections. The values of flux in the A, B, C, and D paths are measured by the respective Hall effect sensors.

Hall Effect Sensors

There are two ways to fabricate a target. One is active and the other is passive. An active target uses a permanent magnet to generate a magnetic field, which is sensed by the probe when it is within the operating range. A passive target does not generate a magnetic; instead, the field is generated by the probe and returned by the target.

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.

Magnetoresistive Sensors
Figure 9 shows a simple arrangement for using a sensorpermanent-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.
Figure 9. Magnetoresistive sensors output in the field of a permanent magnet as a function of its displacement x parallel to the magnetic axis (A-C). The magnet provides both axillary and transverse fields. Reversal of the sensor relative to the magnet will reverse the characteristic. (D and E) Sensor output with a too strong magnetic field.

Magnetoresistive Sensors
Figure 10 A 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 orientated with its magnetic axis normal to the axis of the metal plate. A discontinuity in the plates 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 10 B 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.

Magnetoresistive Sensors
Figure 11 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 11. Angular measurement with the KMZ10 sensor.

Magnetoresistive Sensors
Figure12A depicts the use of a single KM110 sensor for detecting rotation and direction of a toothed wheel. The output of the sensor will then be a measure of the rotation of the frame.

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 sensors symmetry axis and that of the toothed wheel must be kept near zero. Further, both axes (the sensors and the wheels) must coincide. The circuit (Fig. 12B) connects both bridge outputs of the corresponding amplifiers and, subsequently, to the low pass filters and Schmitt triggers to from the rectangular output signals. A phase difference between both outputs (Fig.12A & 12B) is an indication of a rotation direction.

MAGNETORESISTIVE EFFECT VS. HALL-EFFECT

The following compares the Hall-effect in silicon to the Magnetoresistive effect in a Permalloy thin film. Both technologies are compatible with integrated circuit processing and may be used to make totally integrated single-chip sensors. Both effects occur for time-invariant fields and may be used to construct zero-speed sensors. However, MR is roughly 100 times more sensitive than the Hall-effect in silicon. Furthermore, its sensitivity is adjustable through selection of film thickness and line width. Another advantage when replacing Hall-effect with MR sensors in applications that count revolutions using ring magnets is that the resolution doubles since MR sensors are omnipolar (operates with North or South pole). Although, Hall-effect has advantages as it is highly linear with no saturation effects out to extremely high fields. Hall-effect films respond to fields perpendicular to the sensor, and AMR bridges respond to parallel fields, as illustrated in Figure 13. These are the main application differences between Hall-effect and Magnetoresistive sensors.

Fig.13 Hall-effect Versus Magneto resistive

Magnetoresistive Applications
Cylinder position sensing in pneumatic cylinders Elevator sensor Lid sensor for laptop computers Digital current sensing for: overload circuit protection, traffic light burnout detection, motor overload sensor, power loss detection, and industrial process monitoring Position sensor for materials handling equipment (lift trucks) Geartooth sensor for industrial applications Handicapped lift for van / bus Low-cost industrial proximity sensors for ferromagnetic targets Blood analyzer Magnetic encoders

Magnetostrictive Detector
A transducer which can measure displacement with high resolution across long distances can be built by using magnetostrictive and ultrasonic technologies. The transducer is comprised of two major parts: a long waveguide (up to 7 m long) and a permanent ring magnet.
Fig. 14 A magnetostrictive detector uses ultrasonic waves to detect position of a permanent magnet.

Magnetostrictive Detector
The magnet can move freely along the waveguide without touching it. A position of that magnet is the stimulus which is converted by the sensor into an electrical output signal. A wave guide contains a conductor which, upon applying an electrical pulse, sets up a magnetic field over its entire length. Another magnetic field produced by the permanent magnet exists only in its vicinity. Thus, the two magnetic fields may be setup at the point where the permanent magnet is located. A superposition of two fields results in the net magnetic field, which can be found from the vector summation. This net field, although helically formed around the waveguide, causes it to experience a minute torsional strain, or twist at the location of the magnet. This twist is known as the Wiedemann effect.

Fig. 15 A magnetostrictive detector uses ultrasonic waves to detect position of a permanent magnet.

Magnetostrictive Detector
The advantage of using this sensor is in its high linearity (on the order of 0.05% of full scale), good repeatability (on the order of 3m), and long-term stability. The sensor can withstand aggressive environments, such as high pressure, high temperature, and strong radiation. Another advantage of this sensor is its low-temperature sensitivity which by careful design can be achieved on the order of 20 ppm/C.

Magnetostrictive Detector
Applications of this sensor include hydraulic cylinders, injection-molding machines (to measure linear displacement for mold clamp position, injection of molding material, and ejection of the molded part), mining (for detection of rocks movements as small as 25m), rolling mills, presses, forges, elevators, and other devices where fine resolution along large dimensions is a requirement.