Despite the fact that Edwin Hall discovered the Hall effect in 1879, readily available and cost effective Hall-effect sensors would not surface in the marketplace until the later part of the 20th century. The rapid development and expansion of silicon integrated circuit (IC) manufacturing technologies in the 1970s essentially enabled the modern day Hall-effect sensor. And in recent years, as Hall sensor technology evolves to an even higher level of integration, the popularity of Hall effect sensors is on the rise in appliance design applications.

Advertisement:

The Hall effect refers to the measurable voltage that appears across a conductive material, for example silicon (Si), when an electric current flowing through the conductor is influenced by a magnetic field. Under these conditions a transverse voltage is generated due to the balancing of the Lorentz and electromagnetic forces. In simple terms, a Hall-effect transducer creates a differential Hall voltage that is proportional to the perpendicular component of an applied magnetic field.

The fact that silicon can be used to create an accurate and effective Hall-effect transducer yields substantial benefits. First and foremost, the use of silicon wafers allows for economical and effective integration of various signal-processing circuits on a single silicon die or substrate. (See Fig. 1.) The integration of the Hall-effect transducer, chopper-stabilized amplifiers, filters, comparator circuits, and complex logic functions allows for the creation of a vast array of highly accurate, reliable, application-specific Hall-sensor solutions.

Advantages

Enlarge this picture Fig. 1. Typical micro-power, Hall-effect sensor switch. In the block diagram, the X symbol depicts the Hall transducer.

The contactless operation of Hall sensors improves system reliability and durability by virtually eliminating mechanical wear, mechanical shock, and electrical contact oxidation as failure modes in the sensor system. For example, wear or shock-related bending of the moving components internal to reed switches, and potential oxidation of the electrical contacts in these switches, present reliability concerns that are simply not present in similar Hall-effect switches. Additionally, while Reed switches can malfunction after exposure to large magnetic fields, Hall sensors are essentially impervious to exposure to large magnetic fields.

Hall-effect devices also have the ability to sense magnetic fields that are physically obstructed by non-ferromagnetic materials (any material that does not attract a magnet). However, optical switches require a clear, non-obstructed path between an optical source and an optical receiver. Therefore, the use of Hall-effect sensors results in potentially simplified mechanical designs when compared to optical sensor solutions.

Hall-effect sensors will also operate reliably in the presence of, or when fully covered with, dust particles. In desktop printers Hall-effect sensors are replacing optical proximity sensors in lid open/close and print-head proximity sensing applications. Over the life of a printer, as toner particles inevitably accumulate in undesirable areas, optical sensor performance can suffer from light-path obstruction in the area of the optical sensor transmitter or receiver. However, the Hall sensor is completely immune to the presence of toner particles.

Enlarge this picture Table 1. Pros and cons of commonly available proximity sensor and switch technologies.

The integration of power management capabilities into Hall-effect switches has allowed for drastic reductions in the power consumed by these sensors. Many new Hall-effect sensors employ internal power cycling circuits that reduce average power consumption of the sensor to a typical value of 1 uA or less. As a result, the power consumed by a Hall-effect sensor is often lower than the power consumed by similar optical or reed-sensor devices.

The recent inclusion of push-pull outputs of opposite polarity in Hall-effect switches allows for use of these sensors with a single external bypass capacitor. The elimination of output pull-up resistors reduces bill-of-material costs and saves PC board area in end-user applications.

Finally, Hall sensors are sold in a wide variety of very small and lightweight packages, including leaded packages, surface-mount packages, and wafer-level chip-scale packages (WLCSP). The size and overall weight of these packages generally meet the needs of even the most discerning appliance design engineer.

Table 1 shows a comparison of commonly available proximity sensor and switch technologies as a function of common demands in the appliance market.

Sensor design

Enlarge this picture Fig. 2. Transfer function for typical Hall-effect sensor switches and latches.

At a fundamental level, Hall-effect sensors can be classified based on the transfer function between an input magnetic field and the output signal of the sensor. Table 2 segregates various types of Hall-effect sensors by output type, while also discussing typical applications for these types of sensors.

The output transfer function of a linear Hall-effect proximity sensor is extremely simple to understand. In short, the output voltage (or duty cycle in the case of a PWM output) of a linear Hall sensor is directly proportional to an applied magnetic field.

Enlarge this picture Fig. 3. Head-on Hall sensor actuation. TEAG is the total effective air gap, or the distance between the sensor and the magnet.

In order to understand the output transfer function for Hall-effect switches and latches, consider a simple description of unipolar Hall-effect sensor switches. (See the red transfer function in Fig. 2.) Here we see that the digital output of the sensor is in the high state until a pre-determined level of magnetic field (BOP) is exceeded. When the applied magnetic field exceeds the BOP level, the sensor output transitions to the low state and will remain in this state until the field level falls below the BRP level. The fact that BOP and BRP are both greater than or equal to zero Gauss (where Gauss is a unit of magnetic field) denotes one of the defining characteristics of a unipolar Hall sensor switch.

The defining characteristics of a Hall-effect latch can also be seen in Fig. 2, which shows the same references to the operate and release points of the sensor (BOP and BRP respectively). However, in this case BOP occurs at a positive field level while the BRP transition occurs at a negative magnetic field level. This is what is commonly referred to as Hall-effect latching behavior.

Magnetic design

Enlarge this picture Fig. 4. Slide-by Hall sensor actuation.

Some system designers are unnecessarily intimidated by the need to design a magnetic circuit for use with an appropriate Hall-effect sensor. In most cases, very simple magnetic circuits can be developed for use with a Hall-effect sensor. For example, the “head-on” mode of operation refers to a system where a single-pole magnet moves perpendicular to the active face of the Hall device, as shown in Fig 3. Here we see a change in the incident field through the Hall sensor (y-axis) as a function of the distance between the magnet and the sensor (x-axis). In systems requiring high accuracy, this non-linear transfer function can be compensated for through the use of a look-up table (or other translational mathematic formulae) in the system microcontroller.

Alternatively, the “slide-by” mode of operation refers to movement of a magnet in a plane that is parallel to the active face of the Hall device, as seen in Fig. 4. The slide-by method typically results in better sensing precision than the head on method as a result of the enhanced linearity in the central portion of the field versus distance transfer function. The large magnetic slope between the poles makes it possible to obtain very precise switch-point locations when Hall-effect switches or latches are used.

Fig. 5. Vane-interrupt switching Hall sensor.

Another method for actuating a Hall device is known as vane-interrupt switching. A vane is a ferromagnetic object that has a unique configuration of notches cut into it. With vane-interrupt switching, a magnet and Hall device are mounted in a stationary position such that the Hall device is forced into the “on” state by the activating magnet. When the ferromagnetic material of the vane passes between the Hall sensor and activating magnet, the vane forms a magnetic shunt to divert the field away from the Hall device. (See Fig. 5). Therefore, as one or more vanes pass by the sensor, the digital sensor-output signal can be used to determine the speed of rotation, or the crude position of a rotating shaft or knob.

A variation to vane-interrupt switching involves the use of specialized, back-biased Hall-effect devices (commonly referred to as gear-tooth sensors). With these devices, the magnet and Hall sensor are integrated into a single package, reducing placement and alignment concerns and simplifying the design-in process in appliance applications. When a ferrous material passes in front of the integrated sensor and magnet package, the magnetic field is drawn through the active face of the Hall device, actuating the digital output of the sensor. These sensors are primarily used to detect the speed of rotation of the drum in washing machine and clothes dryer applications. In some appliances, the gear-tooth sensor output signal can be used to diagnose drum overload. The number of broken drive belts in the field can be reduced through the use of these sensors, potentially generating substantial savings in warranty costs for appliance manufacturers.

Enlarge this picture Table 2. Hall-effect sensor selection guide based on device output type.

Those are just a few examples of how Hall-effect technology can improve the performance and reliability of modern appliances. The appropriate Hall sensor for a given application, and associated device datasheets, can be found using the Allegro Selection Guide.

For more information, visit: www.allegromicro.com/en/Products

References:
G. Pepka. “Position and Level Sensing Using Hall-Effect Sensing Technology.” Allegro MicroSystems technical paper. Sensor Review Journal SR27-1, February 2007. M. Hopkins. “The State of the Art in Hall-Effect Technology and Its Implications for Appliance Design and Development.” Allegro MicroSystems technical paper. IATC, March, 2004.