Hall effect sensors have changed from the error prone devices of the past. Now they are precise, intelligent, and affordable.
Hall Effect sensors have been around for a long time. While the early bipolar versions were great at detecting magnetism, they had several drawbacks. For one, they couldn't control their response because they lacked error correction circuitry. Another drawback was that they were effected by temperature and stress, which often changed output voltage readings. They were also expensive to make and there were no economies of scale to help lower cost.
That was about twenty years ago. New manufacturing methods and error correction techniques have turned Hall sensors into sleek, accurate switches that have a lot to offer motion designers. And, the cost has come down to the point where the technology is economical for practically any application requiring the sensing of speed, direction, position, and current.
A change in direction
A typical Hall effect sensor consists of a sensing cell (Hall plate) and an operational amplifier. In the presence of a magnetic field, the Hall cell produces a small voltage that the op amp increases. Ideally, when the field is removed, the output voltage goes to zero. However, both the Hall cell and the op amp can produce substantial offset voltages that will vary the actual response.
Early manufacturing and assembly procedures often failed to remove these offset errors, and in many cases played a role in creating them. Wafer fabrication processes - such as heating and cooling, thin film deposition, sawing, die mount, encapsulation, and lead trim - contributed piezoresistive effects and resistive changes, which in turn, resulted in errors that were costly to correct later in production.
Trimming was the most common method of offset correction. Unfortunately, trimming techniques and the extensive testing needed to verify correction, plus low chip yield, totaled about 50% of the cost of Hall effect sensors.
But that was before CMOS. The main benefit of CMOS technology is that it shrinks the size of the sensors. Depending on the design, the die can be as small as 1 mm2.
CMOS also makes it easier to build switches. This was an important development because most error-correction circuitry is based on switching technology. The combination of smaller die and CMOS switch also meant that Hall sensors were more stable over a wider temperature range.
Today, thanks to CMOS, Hall sensors incorporate chopper stabilization and quadrature switching to decrease offset errors. Chopper stabilization is used to reduce input offset errors at the op amp, and is a benefit for both digital and linear (analog) Hall sensors. The quadrature scheme involves actively switching the direction of current through the Hall elements. The combined effect of both techniques is an order of magnitude improvement in switch point drift and gain and offset errors.
As with other electronic devices, newer digital design techniques are also helping Hall sensors. Circuit techniques reduce the number of external components needed to implement certain functions. Recently, engineers took advantage of this capability to develop programmable Hall sensors. Now it's possible to have sensors with user defined sensitivity and offset.
More recently, engineers developed 3-V logic systems, which make Hall sensors compatible with the newest Pentium- class processors.
Hall switches typically have a Hall integrated circuit, a magnet, and a means of moving the magnet or the magnetic field. Operation is simple. The switch is ON in the presence of the field and OFF when the field is removed. Both the operate point and release point, as well as the differential can be precisely set by the designers.
The operate point is where the magnetic flux density turns the sensor ON, allowing current to flow from the output to ground. Conversely, the release point is where the magnetic flux density turns the sensor OFF. The absolute difference between them is referred to as hysteresis. Its purpose is to eliminate false triggering, which can be caused by minor variations in input, electrical noise, and mechanical vibration.
Depending on how designers employ these characteristics, they can solve a wide range of motion sensing problems.
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In a slide-by design, for example, a magnet changes the field from high to low magnitude within a small range of motion. A proximity switch works in a similar fashion but requires more movement so that the magnetic field is placed close to the sensing face. Field intensity is greatest when the magnet is against the switch's front side and decreases exponentially as the magnet is moved away.
An interrupt switch consists of a fixed position Hall chip facing a fixed magnet. A vane of ferrous material moves between them, shunting or reducing the magnetic field to turn the switch OFF.
A rotary interrupt switch has a similar set up. A toothed ring, though, serves to interrupt the magnetic field. At a gap in the ring, the flux intensifies, which turns the switch ON. You can detect the speed or position of rotating objects with this design.
Rotary slide-by versions measure rotary speed, synchronizing it with position. The Hall chip is activated by a rotating magnet with alternating North and South poles. When a South pole passes the chip, it turns ON. As the North pole passes, the chip turns OFF.
Putting it to work
Hall sensors can meet a wide range of needs. In motion, they can be used to detect level, position, acceleration, and vibration.
One method is to suspend a magnet from a pendulum so that it's free to move in the X-Y plane. In a level, stable condition, the magnet's North pole faces the switch, putting it in the OFF state. Any motion moves the magnet in such a way that the South pole goes over the sensor face, turning it ON.
When used in conjunction with an electromagnet, Hall switches make efficient, isolated current-sensing devices that can protect components from damage resulting from overheating. The chip is placed between a slotted ferrite toroid core and drives an indicator, relay, or logic-level fault signal. By varying the number of windings and the programming codes, you can detect from 100 mA to 500 A. Programmable linear sensors are well suited for this application.
One complete turn of coil around the core with a current of 1 A will produce a flux density of about 0.6 mT on the Hall switch. Based on this characteristic, you can turn a Hall switch into a simple current indicator or limiter by adjusting the number of coil turns around the core and adding a simple circuit.
The flux variation sensing capability is useful in determining the speed and position of rotating objects, such as ferrous gear teeth. There, you'd place a sensor near a gear and position it so it faces the magnet. As the teeth pass in front of the sensor, flux density changes in the air gap, and turns the sensor ON and OFF. Signal processing converts the signal from the Hall element to a digital value for output.
Although Hall sensors can switch states quickly, there are limits as to how rapidly they can respond to speed changes. There may be a loss of accuracy in extremely demanding timing requirements, like those found in crank position applications.
Hall sensors are also widely used in brushless dc motors. They eliminate the friction, electrical noise, and power loss common to other types of mechanical commutation. Because the newer versions are essentially digital, they easily interface with digital circuits.
A brief history of Halls
The first generation of Hall integrated circuits in the 1970s suffered from offset errors primarily in the Hall plate. Second generation designs assumed that piezoresistive errors were symmetrical in closely adjacent sections of silicon, and that an error in one plate could be canceled by an equal and opposite error from an adjacent plate.
Most Hall switches produced from 1985 to the present connect 4 to 16 plates in parallel. However, active trimming at probe is still required to remove residual piezoresistive errors and op amp offset. Resistive trim, however, does not address the large offset errors caused by poorly defined temperature coefficients resulting from a mismatch between the on-chip resistors and the stress causing the offset.
Auto-nulling techniques used in other electrical components were too expensive for Hall designs. So designers turned to other techniques.
Quadrature switching, either electronically or interchanging the power and signal contacts through CMOS transmission gates, proved that it could eliminate the Hall plate voltage offset. With chopper stabilization, input and output contacts are switched synchronously, so that the op amp offset essentially becomes an ac signal riding on the amplified input voltage. As modified, the offset is easily removed using a low pass filter.
Quadrature switching, despite its advantages, requires a large number of circuit components, which in prior art was not economically feasible. It took the development of small feature CMOS to make it practical to squeeze logic gates, switches, and other active components in a small chip area at low cost.
Chad Pepin is senior sensor applications engineer at Melexis Inc., Concord, N.H.