PUTTING SENSORS   TO WORK

Position Monitoring with
Hall Effect Sensors

Nearly every manufacturing operation today requires some form of automated position monitoring. While the particulars of each application differ from one another, most can be satisfied by a single technology—Hall effect sensors.

Christine Graham and John Feltus,
Allegro Microsystems, Inc.

Figure 1. The Hall effect produces a signal proportional to the intensity of the normal magnetic flux density. Advances in this technology have made wide operating voltage ranges, robust radiated and conducted transient protection, and high ESD levels possible in a single-chip solution.

A Hall element is simply a sheet of semiconductor material through which a constant voltage source forces a constant bias current to flow. The output, a voltage measured across the width of the sheet, theoretically reads near zero if a magnetic field is not present (see Figure 1). If the biased Hall sensor is placed in a magnetic field oriented at right angles to the Hall current, the voltage output is in direct proportion to the strength of the magnetic field (see Figure 2). This is the Hall effect [1].

Figure 2. A magnetic field oriented at right angles to the Hall current will create a voltage output directly proportional to the strength of the magnetic field. Gain amplifiers, offset-adjust circuits, and digital processing of the analog signal lets the Hall effect sensor self-calibrate every time the sensor powers on.

Semiconductor companies have improved Hall effect technology by adding gain amplifiers, offset-adjust circuits, and other circuit blocks, such as regulators and low-pass filters. These sensors can handle supply-voltage variations from 3 to 26.5 V, radiated and conducted transients characteristic of harsh automotive environments, and ESD levels from 8000 to 10,000 V. Some Hall effect sensors can calibrate themselves on the job, and a number of programmable switches, gear tooth sensors, and linear devices reduce end-of-line stack-ups and improve productivity.

Hall effect sensors, still considered a relatively new technology, are not universally understood, even by their users. To remedy this, many sensor suppliers offer seminars to educate both system designers and the industry in general.

Figure 3. Hall effect position sensors relay seat location to the controller when the vehicle is in operation. Hall effect technology is a contact-less solution that can reliably assist the air bag controller make life-or-death decisions in the event of a crash.

Applications
Seat Position Sensor. Some of the most complex systems incorporate the simplest Hall effect sensors. A perfect example is the vehicle seat-position sensing system, which determines the driver’s proximity to the steering wheel and—in the event of a collision—adjusts the air bag deployment force accordingly (see Figure 3). The simplest way to do this is to use a 2-wire unipolar switch that can sense two seat position zones—near and far. The sensor relays the correct position in a digital output to the controller. Because the information must be correct when the vehicle starts up, the sensor output must decode automatically.

In this application, the mechanism that attaches the seat to the positioning track is equipped with a plate made of a ferrous metal capable of interrupting the magnetic field between the Hall effect sensor and the magnet (see Figure 4).

Figure 4. Digital output Hall effect sensors allow easy interfacing with most types of controllers. The controller decodes the output of the sensor to determine in which zone the seat is positioned.

When the seat passes between the switch and the magnet, a change in the sensor’s output tells the controller that the seat track has passed into a particular zone.

You can have any number of zones, depending on how many sensors you use. For example, if you place two sensors on each seat track, four

TABLE 1 Decoded Digital Logic State of Hall Effect Output Zone Hall 2 Output Hall 1 Output 1 0 0 2 0 1 3 1 1 4 1 0

zones would be possible. A seat in one of the zones closer to the steering wheel would tell the controller that the air bag should be deployed with reduced force. Seats in one of the rear zones, farthest from the steering wheel, would call for greater deployment force. The controller decodes the outputs of the two Hall effect sensors to determine in which of the four zones the seat is positioned. Two sensors provide a convenient gray-code output (see Table 1 and Figure 4).

The diversity of Hall effect sensors makes it possible for you to come up with a variety of solutions for any given application. For example, you may need high resolution to determine where the seat is at all times. You can meet this requirement by using an analog linear Hall effect sensor, which produces an output proportional to the strength of the magnetic field. A dual-pole magnet in a slide-by configuration with a linear sensor will produce an output ranging from 0 to 5 V with the proper design.

Occupant safety systems in vehicles must be auto-adjusting. This eliminates the need for the driver to ensure that he or she is a safe distance from the steering wheel. Instead, the system will sense the driver’s position and adjust the deployment appropriately.

Hot Swapping. Not every application has safety or reliability as its main focus; sometimes it’s an effort to save money. One example of this would be the proactive use of Hall effect sensors for safe hot swapping of electronic modules and subsystems.

You’ll find that certain facets of system design are commonly applied throughout the industry. These include the management of power, the need to signal an operator to perform an action, and the ability of the system to be made aware of an operator’s action and to recognize completion of that operation. Hot swapping is one type of operator function common to many segments of industry. An array of hard drives in an Internet server or module bays in test equipment are such examples.

Currently, many techniques that allow hot swapping of circuit cards or modules are reactive (i.e., the system is not aware of the hot-swap process until the target module is already removed). If a module is active when it is removed, hot swapping not only destroys the processes in motion but also potentially destroys hardware.

By using Hall effect sensors and a simple controller, the effort becomes proactive, initiating a system response that allows for a clean shutdown or reassignment of tasking. Concerns of voltage stress and ESD decrease, and ultimately the reliability and profitability of the end system improves.

When using hot-swapping techinques, system developers worry about the ability to power sequence the subsystem being removed and replaced during both power-down and power-up. The most common technique has been to physically sequence the contacts on the interfacing connector so that the ground or common is the last-broken or first-made sequence. However, as systems and subsystems become more complex—with the size of ICs becoming smaller and the inherent increase in ESD sensitivity—this method becomes less desirable.

An alternative solution uses features found in Allegro’s Hall effect sensors—such as an ultrasensitive, low-power, sampling algorithm used in the A3212-type device, and LEDs, which allow for a more proactive approach to hot swapping of subsystems. These devices respond to changes in the presence of magnetic fields and have an open-drain logic-level output. This Hall effect device has extremely low active-state power requirements and is appropriate for today’s power-conscious workplace environment.

The Hall effect sensor allows the host system to be notified of the intent to perform an extraction and/or insertion before any activity takes place. The sensor detects the intent to activate the process and the actual completion. Further, by using variations in Hall effect devices, you can even determine some personality before the host system responds to the new subsystem.

A circuit board containing a Hall effect device for each slot or bay is assembled onto the chassis. The chassis can be a server full of controllers, hard drives, and network interface cards. Each slot in the chassis has a hinged cover with a magnet attached, which covers the Hall effect device. When the cover is opened, the Hall effect device senses the change in the magnetic field, which changes the logic state of its output, signaling the host of pending activity.

When the cover is lifted and the actual withdrawal of the target subsystem occurs, the system controller takes the device off line and redirects its activity. If this were a line card, the associated slot in the channel bank would be taken off line. If it were a hard drive in a server, all programs and data files on that drive would be locked to prevent access and corruption. When the replacement module is installed and the cover is returned to its closed position, the Hall effect device senses the change in the magnetic state and signals the system host of the procedure’s completion.

In a similar process, the magnet is contained in the handle of a jackscrew, which is used to secure the module or subsystem in its slot in the host chassis. When the handle is turned to allow the module to be unscrewed, the Hall effect device senses the change in the magnetic field and signals the host controller of the intent to remove a module or subsystem.

Using this method, there’s sufficient time for the functions of the module to be reassigned to other modules within the system or for the service to be closed and the power to the module to be sequenced off. Because of the close proximity of the magnet in the handle to the associated Hall effect device, the intensity of the magnet can be small, ensuring that there’s no risk of corruption to magnetic-media recording systems (e.g., hard drives).

When the new module has been installed in the slot, the screw is tightened to secure the module, and the handle is flipped back to its secure position. When the handle is stowed, there’s another change to the magnetic field, which signals the chassis host of the completion of the process, and the slot can be powered up and initialized. In both scenarios, single- or dual-colored LEDs can be used to signal each step in the process.

Conclusion
Hall effect technology is highly reliable and relatively inexpensive. Sensing objects may not always be as critical as the seat-position sensor, but if automatic sensing is required, the solution must be dependable.

Reference
1. Allegro MicroSystems Data Book, AMS-502, copyright 1995.

For further reading on this and related topics, see these Sensors articles. "A Self-Calibrating Miniature Hall Effect Solution to Gear Tooth Speed Sensing," September 2001 "Expanding the Operative Range of Open-Loop Hall Effect Current Sensing," September 1999 "A Low-Power Hall Effect Switch," June 1999

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