Inductive, capacitive, mechanical, magnetoresistive, Hall-effect and optical, to name just a few, are all viable sensing options, and the list continues to expand. Yet, for a designer, there always remain the same critical elements that need to be addressed, which inevitably link the requirements of the application to the appropriate sensing technology.
Critical requirements such as cost, distance of travel (effective operating air gap), resolution and accuracy all need to be determined to effectively and efficiently select the proper sensing technology. Of course, constructing answers for each of these elements isnot always a straightforward task. Here, though, the flexibility of Hall-effect sensing technology is most advantageous. High reliability, small size, production-viable cost, wide operating voltage ranges, variety of output options, and ease of implementation allow Hall-effect sensing technology to service applications in almost every market.
Hall Effect Technology
The Hall effect, named after Sir Edwin Hall and discovered in 1879, refers to the measurable voltage across a semiconductive material, for example silicon (Si) or gallium arsenide (GaAs), that occurs when an electric current flowing through a conductor is influenced by a magnetic field. This transverse force created by the magnetic field is known as the Lorentz force. A Hall-effect sensor requires a magnetic field in order to activate the device.
Although quite common today, Hall-effect technology did not truly begin to gain mass acceptance until the 1980s. This was because the voltage potential across the Hall element is minuscule, and may easily be influenced by outside forces, such as temperature and package stresses.
Although quite common today, Hall-effect technology did not truly begin to gain mass acceptance until the 1980s. This was because the voltage potential across the Hall element is minuscule, and may easily be influenced by outside forces, such as temperature and package stresses.
More recent devices incorporate advances in the ability to amplify the signal, in addition to the utilisation of on-chip, offset cancellation techniques, which have allowed Hall-effect sensing technology to be employed even under extreme environmental conditions, such as under-hood applications in automobiles. Furthermore, the 'non-contacting' operation of Hall-effect sensors affords the user a nearly infinite life with regard to actuation and switching.
Hall Device Options
Further investigating the elements that require consideration for a position or level sensing application, Hall-effect sensors provide the designer with a multitude of features and variations, including either digital or analogue output. The former option is optimal for sensing discrete positions, while the latter affords the user a relatively infinite number of positions for greater resolution. Some examples of applications requiring discrete position or level sensing are:
• Automotive shift selectors
• Seat belt buckle switches
• Seat position sensors
• Cellular flip phones
• Brushless DC motor commutation
• Windshield wiper fluid reservoirs
• Fuel tanks.
Because of its high reliability, Hall-effect technology is used to replace reed switches and mechanical switches in these applications.
Most Hall-effect switches have output structures that are open-drain and provide low resistance, thus simplifying the interface to most microprocessors and other digital electronics (threshold comparators, multiplexers, basic TTL gates etc.). Typically in open-drain outputs, switching the device 'on' causes the output voltage of the Hall-effect device to switch from high to low. However, there is an abundance of variations for Hall-effect sensors in order to service to the plethora of position and level sensing applications, each one with its own nuances. These variations include features such as:
• Micro-power consumption
• Magnetic pole independent sensing
• User-programmable options
• Two-wire current sourced output devices
• Magnetic bias for sensing ferrous targets
• Inverted outputs.
For the purpose of this article, the focus will be on standard devices, their operation and application uses.
Standard Hall Device Characteristics
There are three common variations of standard digital position and level sensors: unipolar, latching, and bipolar. With unipolar switches, the actuation is caused by a magnetic field of sufficient strength to turn the device 'on'. Once the magnetic field is reduced below the magnetic release point of the device, these devices return to the 'off' state.Latching sensors turn on in a manner similar to that of unipolar switches. However, latching sensors can only be turned off (unlatched) when the device sees sufficient magnetic field strength of the opposite polarity.
Bipolar switches are similar to latching devices in that they use opposing magnetic polarities to turn on and off. But, owing to the high sensitivity of these devices, they cannot be guaranteed to operate as a latch. In some cases, bipolar switches can have switch points that cause them to function as a standard unipolar switch or even as a negative switch (switching only in the presence of sufficient north magnetic polarity).
Low Resolution Applications
An excellent example of an application that uses discrete position sensing is an automobile gear-shift selector. In shift selectors, there are commonly as few as five discrete positions (park, reverse, neutral, drive and low). With a unipolar switch placed at each individual position (P, R, N, D and L), each switch only turns on when the magnet in the shifter is moved directly adjacent to the switch.Should the designer require additional positions, the spacing between the sensors can be reduced to create 'crosstalk' between the sensors. In this manner, additional positions are obtained when the magnet is close enough to two devices for them both to be turned on, thereby increasing the number of positions from, for example, five to nine. Simple binary coded decimal (BCD) systems, or more advanced systems such as Gray code or densely packed decimal (DPD), can be used to decode the logic and acquire positional information.
Similarly this tactic could be used to sense fluid levels in a tank by means of a flotation device with a magnet inside. As the magnet floats up and down with the changes in the level of the fluid, discrete levels are determined by which sensor is in the 'on' state.
High Resolution Applications
It can be seen very quickly from the shift selector example that discrete position or level sensing is ideal when only a few positions are required. However, this method of adding a sensor for each position very quickly becomes cost prohibitive and spatially challenging when the application requires finer resolution.For such applications, the linear Hall-effect sensor with an analogue output is used. There is an abundance of features available in linear devices, including ratiometric outputs, user programmability, digital outputs (such as PWM), and unidirectional or bidirectional sensing.
Most standard linear Hall-effect sensors have ratiometric outputs that respond proportionately to magnetic field strength. These devices generally require a regulated 5.0 V supply, and the quiescent voltage output is 2.5 V when there is no significant magnetic field present. The output voltage increases when sensing an increasing magnetic field from the south pole of a magnet, approaching 5.0 V. Conversely, the output voltage will decrease when sensing an increasing magnetic field from the north pole of the magnet, approaching 0 V.
There are two common configurations for applications of linear devices, which form the foundation for most designs. These techniques are termed 'slide by' and 'head on'.
Slide By Configurations
In a standard slide-by application, a magnet moves across the face of the sensor in such a way that the Hall element senses one or both magnetic poles. There can be effectively three positions at which the voltage output is zero:(a) Before the magnet is close enough for the field to be sensed by the device
(b) Once the zero crossing (B = 0) between the poles is directly adjacent to the Hall element, and
(c) Once the magnet has moved past the device far enough that there is no longer sufficient field detectable at the element.
Effectively, the change in output voltage is from 2.5 to 0 V (assuming that VDD is 5 V) as the north pole of the magnetic field passes the face of the sensor, and from 2.5 to 5.0 V as the south pole passes the face of the sensor. This is typically labelled 'bidirectional sensing'.
It is also possible to sense the change of only one pole across the device, although this could limit the available range. This configuration is known as 'unidirectional sensing', and the change in the output is then limited to only 2.5 V for standard linear devices. To obtain the full range of operation, one would have to employ a user-programmable linear sensor with this feature. The change in voltage output from the Hall-effect sensor as the field changes across the face can then be used to determine the relative position of the moving magnet. An A/D convertor on a standard microprocessor and a simple lookup table can then be employed to convey the actual position.
In this situation, the resolution (the number of positions that can be detected) is predicated on the resolving capabilities of the A/D convertor, but the analogue signal provides a relatively infinite number of positions.
An example of an application that can use slide-by sensing is valve positioning. In this application, the magnet is often a two-pole ring magnet that rotates in front ('slides by' the face) of the Hall-effect sensor. As the opposing magnetic fields pass in front of the sensor, the voltage output changes proportionately to the change in field strength. By means of precise sensing, the position of the valve can be controlled to dictate more accurately the flow of a substance through a carrier.
Head On Configuration
Head-on position sensing is very similar to the unidirectional sensing of the slide-by configuration. In essence, the linear sensor only differentiates the change in magnetic field strength for one magnetic pole, which can be of either north or south polarity. The detection pattern is straightforward. As the magnet approaches the device, the field detected by the sensor increases, and the field strength decreases as the magnet is removed.Determining Field Specifications
As with any technology, there are some specific considerations when designing an application using a Hall-effect sensor. Careful selection of the magnet is of the utmost importance, including its shape and placement.Magnetic field strength decreases exponentially over distance. Furthermore, magnets have temperature coefficients that need to be considered. Therefore, for discrete position sensing, it is always good practice to determine the effective air gap, from the face of the sensor to the magnet, at the required switching position, and then determine the maximum and minimum field strengths, over the rated temperature range, at that distance. This value should then be compared to the maximum rated operating switchpoint for each alternative device.
A chart and formula for estimating field degradation by effective air gap.
A good rule of thumb for a designer is to make certain that, at the required position for the device to switch, there is at least 10% more field strength than is required at the maximum rated switch point. For example, if it is required for a unipolar switch with a maximum field strength of 50 G to turn on at a certain distance, then the field strength at that distance should be no less than 55 G under all conditions.
Designing Linear Applications
Unlike digital Hall-effect switches, which require only a certain strength and polarity of field in order to actuate, linear devices require a little more application specification in order to achieve satisfactory results. The gain of a sensor device determines the resolution at a given distance. Therefore, regardless of whether the application is slide-by or head-on, one must select the appropriate gain.In order to do this, two known end points and the required resolution (number of data points) must be established. The following is a brief example for determining the appropriate gain.
Assuming that the requirements for the application, the usable linear range would be 3 V. The full range as the magnet travels across the device would be 200 G. Dividing the change in output voltage by the change in applied field provides the appropriate gain of the linear Hall-effect device for this application.
Of course, in real-world applications the transfer functions are not perfectly linear, and there can be an inherent offset in the system. For this reason, further consideration must be given to the accuracy required by the application, as well as the resolution capabilities of the A/D convertor or similar device that must read the output, and the temperature coefficient of the magnet.
It is helpful in these situations to consider:
• The change in the quiescent output voltage as a function of temperature
• The change in sensitivity (gain) as a function of temperature
• The linearity of the device over a given range of magnetic field strength.
Linear Hall-effect sensors can be back-biased with a magnetic field in order to sense ferrous targets. For example, Hall sensors are widely accepted in the automotive industry to accurately sense the position of cam lobes and the speed of crankshafts in engines, in order to improve timing and thereby grant more efficient consumption of fuel. The high bandwidth capability of many Hall-effect linear sensors allows them to be used to sense changes in current for DC/DC convertors and battery management systems in hybrid vehicules.
Other Applications
Other interesting examples of important Hall sensor options include:• The current source outputs of two-wire devices are ideal for safety-critical applications, such as seat position and seat belt buckle sensors. This is because these devices output two distinct current levels to indicate the 'on' and 'off' states. Any output that deviates from these levels is a fault condition, giving the user an inherent diagnostic capability.
• The extremely low power consumption (less than 5 W) permits Hall-effect sensors to act as open/closed circuit sensors. This is particularly valuable in battery-operated applications that are sensitive to power loss, for example: cellular flip phones, laptop computers and pagers.
• The flexibility of these sensors is further enhanced by the assortment of package options. Some micro-leaded packages (MLP, also known as leadless DFN or QFN packages) are as small as 2.0 × 2.0 × 0.5 mm, while others are large enough to include a samarium-cobalt magnet to back-bias the sensor.
It is the myriad of applications that can be served by Hall-effect technology that drives the ever-increasing diversity of these devices. As a result, the technology continues to evolve. The ongoing reductions in size and continual increase in capabilities mean that Hall-effect technology is a viable solution to almost any position or level sensing application.
Fuente: http://www.electronicscomponentsworld.com/articleView~idArticle~71786_2303944122472007.html
Nombre: Rodriguez B. Joiver I.
Asignatura: CRF
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