• Mikes Tips - (Rotary) Hall Position Sensors

    MyCockpit ® Presents "Mikes Tips" by Mike Powell

    (Rotary) Hall Position Sensors

    We use position sensors to convert the positions and movements of flight and engine controls into electrical signals we can interface to our simulation computers. Potentiometers are the most common position sensor, but occasionally we look for something better. That’s when the Hall position sensor comes to mind.

    Unlike potentiometers, Hall position sensors have no sliding electrical contacts to wear out, so they can offer higher reliability, particularly in corrosive environments. They show up in some models of commercial joysticks, as well as in after-market conversion kits for high end gaming hardware, such as the CubPilot Hall upgrade for the Thrustmaster HOTAS Cougar (http://cubpilotshangar.net).

    Hall position sensors are based on magnetic principles. If you take one apart you’ll find a magnet, a few bits of high-permeability, low-remanence alloy (tech-speak for expensive), and a magnetic field measuring chip not too surprisingly called a Hall sensor. A Hall sensor produces a voltage that varies in proportion to the magnetic field passing through it. Turning the shaft of a Hall position sensor changes the strength of the magnetic field hitting its internal Hall sensor, hence changing the output voltage.

    Hall position sensors are made in various shapes and sizes. Some are designed to be “drop in replacements” for potentiometers. On the outside these models appear identical to potentiometers. The picture below could be a potentiometer or a rotary Hall position sensor, and that’s really the point. If you happened to be a manufacturer of a product using potentiometer-style position sensors, you could upgrade your product by substituting drop-in replacement Hall position sensors without a requirement to mechanically alter your product.

    Even better, Hall position sensors can sometimes be substituted with little or no change to the circuitry. These sensors aren’t universal replacements for potentiometers, but when potentiometers are used as position sensors, they are often wired in a standard fashion. One of the pot’s electrical terminals connects to ground while another connects to power. The remaining terminal, the wiper, produces a voltage that changes proportionately as the potentiometer shaft rotates. This terminal generally connects to an analog input on a microcontroller. Both the Vishay Spectrol RHE and 351HE, and the Honeywell Clarostat HRS-100 models of rotary Hall position sensors have three electrical connections. One goes to ground, one to +5 volt power, and one produces a voltage that varies proportionately with shaft rotation. (Sound familiar?)

    These drop-in replacement Hall position sensors work well in typical USB game controllers. In addition to high reliability, Hall position sensors are available in models with rotational ranges that might better match the motion of the controls. While most standard potentiometers have a rotational range of three quarters of a turn, Hall position sensors are available in 45, 60, 90, and 180 degree models.

    You can buy rotary Hall position sensors from industrial electronics suppliers like State Electronics (http://www.potentiometers.com/HRS100.cfm), Newark (http://www.newark.com/honeywell-s-c-...sor/dp/32M7190), Allied Electronics (www.alliedelec.com), Digikey (www.digikey.com), and so on. However, it doesn’t take much catalog browsing to realize that Hall position sensors are expensive.

    If you want to use Hall position sensing on a budget, you can take the do-it-yourself approach. Grab a couple of magnets and buy a linear Hall sensor IC. The Allegro Microsystems A-1301 is a good choice. It costs about a dollar, takes a +5 volt power supply, and provides a 0.2 to 4.7 volt output in response to a -1000 to +1000 Gauss magnetic field.

    Hall sensors not only respond to changes in the strength of a magnetic field, they also respond to changes in the direction of the field. You can use this directional sensitivity to good effect. By rotating a Hall sensor in a constant-strength magnetic field (or rotating the field around the sensor) you get a changing electrical output.

    Strictly speaking the output is horribly non-linear; it varies as the cosine of the rotation angle. However, if you limit the range of rotation, non-linearity becomes a non-problem. When you restrict the rotation to 60 degrees, the output is linear within a few percent which is right up there with a high quality potentiometer. Even a 90 degree rotation can show a not too shabby linearity. Of course, you have to choose the correct portion of the curve.

    An easy source of the magnetic field for a DIY position sensor is a bar or block magnet. Typically, you’ll need a small neodymium magnet, because the Hall sensor is positioned to the side of the magnet rather than at one of the poles. This places the sensor in the flux midway in its path between the poles where the field is more uniform. Rotating the magnet changes the direction of the flux passing through the sensor, and that changes the electrical output.

    A shortcoming of this approach is that the sensor output will change if the sensor and magnet move apart. You’ll want to avoid any play in your mechanical setup so this doesn’t become a problem.

    A second shortcoming is that the Hall sensor and magnet are unshielded. Other magnetic fields can affect the sensor output. If you have several controls in proximity using this approach, they can interact and limit accuracy.

    Another approach is to use a magnet assembly that shields the magnets and Hall sensor. This can be a short length of mild steel pipe with a pair of suitably mounted magnets inside. Because the steel contains the magnetic flux, less powerful ceramic magnets are adequate. An added plus is that the magnet assembly will provide more nearly uniform flux so relative motion, other than rotation, between the magnets and sensor is less likely to cause undesired sensor outputs.

    For both approaches the required magnet strength depends on the rotational range of motion: the smaller the range, the stronger the needed magnet. The electrical output should come close to maxing out at the extremes of rotation. Be prepared for a little experimentation.

    The A-1301 can be connected to many USB game controller interfaces in place of a position-sensing potentiometer. The A-1301 ground lead goes to ground. The power lead goes to +5, and the output goes to the analog or wiper input.

    The Allegro A-1301 Hall sensor is available through the normal electronic distributors. Magnets can be purchased from The SuperMagnetMan, (http://www.supermagnetman.net/index.php), or any of a number of Ebay vendors.

    If you want to know more about the Hall effect and Hall effect sensors see Hall Effect Sensing and Applications by the guys at Honeywell (www.honeywell.com/sensing). For A-1301 Hall sensor spec sheets, visit the Allegro MicroSystems, Inc. website, (http://www.allegromicro.com/en/).

    Mike Powell, author of

    Building Recreational Flight Simulators and

    Building Simulated Aircraft Instrumentation.