User systems often require the detection of position for operating in a specific switch mode. Such type of On or Off functionality is a straightforward requirement and many devices implement it with Hall-effect switches, including power tools, light switches, safety harnesses, and laptop lids.
The output of the sensor toggles its state as soon as the input magnetic field crosses the operating threshold. Likewise, the output reverts to the idle state when the magnitude of the magnetic field reduces below the release threshold. Hysteresis built into the device prevents the output from toggling rapidly where the magnitude of the magnetic field is close to the operating threshold.
Many applications use this functionality. For typical cases, two output states are adequate, thereby helping to reduce mechanical wear and preventing interference from grease and dust.
Although two positions may be adequate for detection in many applications, others require the detection of additional states. For instance, a tool may require a three-position power switch, denoting Off, Low, and High power modes. Detecting all three states is difficult using a single sensor. Initially, it may seem possible by adding a sensor for every switch position in the system.
A unipolar switch is well-suited for such an application. The designer places the magnet very close—so the air gap is small—thereby ensuring the pole of the magnet facing the sensor will always exceed the worst-case operating point. When the magnet is above the sensor, it results in an upwardly directed field vector. When the magnet has traveled greater than its own width, the sensor will not activate, as the direction of the field is now downwardly directed. Therefore, there can be an array of sensors representing any number of positions, provided the sensor spacing exceeds the full width of the magnet.
While the above arrangement is convenient for a low number of positions, the number of components required gets more difficult to manage as the number of positions increases. For such arrangements, dual-unipolar switches are more convenient.
Texas Instruments offers a dual-unipolar switch, DRV5032DU. It has two independently operating outputs. Each output is sensitive to an opposite polarity of the magnetic field. Where one sensor responds as it nears a North pole, the other will respond as it nears a South pole. This functionality allows the detection of three positions with a single magnet.
With the magnet mid-way between the two sensors, there is no component of the magnetic field available to activate the sensors, and therefore, both sensors remain deactivated. When the magnet moves to the left, it activates the N pole-sensitive output. Likewise, when the magnet moves to the right, the S pole-sensitive output activates. However, for this arrangement to function correctly, the magnet must have a length two times the distance of travel between the switch positions. When the magnet moves by one-half its length, one of its poles is directly above the sensor, thereby activating it.
Extending this format makes it possible to sense more than three positions. It requires an array of sensors spaced appropriately for creating additional unique positions.
