• The Mechanics of Using Rotary Pots and Hall Position Sensors

    MyCockpit ® Presents "Mikes Tips" by Mike Powell

    The Mechanics of Using Rotary Pots and Hall Position Sensors

    So you’ve bought a handful of potentiometers or other position sensors and you’re wondering how to hook them up. I have a few thoughts about that, though they’re not about the wiring part of the hook up. The interface documentation most likely has that covered pretty well. I’m thinking about the mechanical side, because the installation affects the overall system performance. It pays to get it right. It’s pretty easy to stick things together and get adequate results, but hey, we’d still be using $17 discount store joysticks if adequate were good enough, right? So, here are a few things to think about to get the best performance when building ultimate simulator controls.

    The first step is to use the full range of motion of the potentiometer or position sensor. If the potentiometer is capable of turning 270 degrees then it should turn 270 degrees even if the control it’s mechanically connected to only moves 90 degrees. The same thing goes for other types of position sensors. If they turn X degrees to get a full 0 to 5 volt output, their mechanical installation should let them turn through the full X degrees.

    The electronics which interfaces the potentiometer (or other position sensor) generally accepts a 0 to 5 volt signal. The interface converts this voltage to a digital number typically consisting of eight bits. (Some interfaces convert to ten or even 12 bits, but let’s stick with eight bits as a worst case.) The conversion to an 8-bit number means that our 0 to 5 volt analog signal will be quantized into one of 256 possible values.

    If the potentiometer moves through its full range when the simulator control moves through its full range, then the flight simulator application “sees” the control in one of 256 possible positions. But what happens if the potentiometer moves through only a fraction of its range?

    Let’s suppose that an off-the-shelf 270 degree potentiometer is directly connected to a joystick with a +/- 30 degree motion. If we have the same 8-bit interface, the simulator application “sees” the joystick axis as in one of (60/270)*256 or only 35 possible positions. That means you have to move the joystick about 2 degrees for the application to see a change in position. There’ll be no finesse when flying with controls like that.

    If you want fine control, use the full range of the potentiometer or position sensor. (And, if you have a choice, use an interface with better than 8-bit resolution).

    However you choose to accommodate a difference between control movement and potentiometer movement, make certain to minimize backlash. Backlash is the accumulated effect of all the mechanical play or looseness in a system. It might be the clearance space between mating gear teeth, or the difference between the outer diameter of a shaft and the inner diameter of the bushing supporting it, or anything. Its impact is that the control doesn’t move the potentiometer with repeatable accuracy. If you push the control one direction, the motion first takes up the backlash before moving the potentiometer. If you pull the control back, the potentiometer initially does not move. The backlash must first be taken up in this new direction. Because of backlash, placing the control in a given position doesn’t reliably place the potentiometer in the same position or provide the same input to the simulator application. Flying finesse takes another hit.

    A misguided approach to combating backlash is to aggressively tighten everything. Gears are forced against each other. Belts are tensioned. Screws and bolts are ratcheted until they scream. Backlash becomes unnoticeable because of the stiff, binding operation of the setup. Oh, and the mechanical lifetime of the potentiometer/position sensor plummets.

    The reality is that you can reduce backlash, but shouldn’t completely eliminate it. Some clearance is required to prevent binding and minimize wear.

    A quality potentiometer has a robust appearance, but don’t be misled. It’s designed specifically for gentle rotation of the shaft. It’s metal sliding on metal without the benefit of bearings. You can turn it, but there are limits on how much force you should place on the shaft, either sideways, or in or out. The spec sheet lists acceptable forces. Exceed those specs and mechanical wear goes up while lifetime goes down.

    So, how do the commercial game controller designs cope with the need to use the full range of motion of the potentiometer, minimize backlash, and limit abusive shaft forces?

    They cheat.

    They special order custom potentiometers with ranges of motion that match their specific applications. No clearance is required because the shaft is fastened rigidly to the control, be it joystick, throttle, or whatever. And as for limiting shaft forces, well, some don’t, and that’s why we can find $17 joysticks at discount stores.

    We can order custom parts, too. It only takes money. (A lot of it.) However, assuming for whatever reason that we don’t want to spend that kind of cash, we might take another approach. A common approach is based on using gears.

    A pair of gears in the proper ratio will convert the range of motion of the control to that of the potentiometer. For example, suppose we go back to our +/- 30 degree joystick and 270 degree potentiometer. A pair of gears with a 4.5 to 1 ratio would convert the 60 degree joystick movement to 270 degrees which would match the potentiometer.

    Gear ratio is simply the number of teeth on one gear divided by the number of teeth on the second. To get a 4.5 to 1 ratio we might choose one gear with 90 teeth and another with 20, or perhaps 72 and 16.

    Gears are available in various materials and sizes. For this application, acetal plastic (Delrin®) works well. Acetyl is a nylon-like plastic that does not bind. As far as size goes, smaller teeth provide smoother movement than larger teeth, but are more fragile. A 48 pitch tooth size is a good choice. Forty-eight pitch means that there are 48 teeth per inch of gear diameter.

    When a pair of 48 pitch gears is properly meshed together there will be only a small amount of backlash. Depending upon your application this may be acceptable or you may wish to remove the effects of the backlash. A simple approach is to use a spring to place a small torque on the potentiometer shaft so that the same faces of the gears always touch regardless of the direction of motion. You can use a constant torque spring that looks like a spiral clock spring, or clamp a string to the potentiometer shaft, wrap the string around the shaft, and connect the free end to a tension spring.

    In years past suitable gears were hard to find. With most companies now having an Internet presence, that’s no longer the case and we have a range of sources to choose among. Serv-O-Link (www.servolink.com), MSC Industrial Supply (www.mscdirect.com), and Stock Drive Products (www.sdp-si.com) are just a few. MSC also carries springs.

    Mike Powell, author of

    Building Recreational Flight Simulators and

    Building Simulated Aircraft Instrumentation.