• Mikes Tips - Choosing a Potentiometer

    MyCockpit ® Presents "Mikes Tips" by Mike Powell

    Choosing a Potentiometer for Sim Controls

    We use potentiometers (“pots”) to convert the mechanical motion of our simulator controls into electrical signals our computers can use. A pot is a three terminal variable resistor. Two of the terminals connect to the ends of the resistance element. The third connects to a moveable contact called the wiper. Moving the wiper changes the resistance between the wiper contact and the other terminals. Interface circuitry uses this changing resistance to create a changing voltage which is converted to a digital number which is passed to the simulator application which does really neat things (hopefully). While there are other devices that can be used to sense control movement, the potentiometer is the most common.

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    Selecting a pot may appear to be a guessing game. The large variety of technical parameters that catalogs use to characterize pots don’t make for an easy choice. Fortunately there are only a few key parameters when it comes to choosing a pot for flight and engine controls. Make good choices on physical type, resistive linearity, lifetime, and resistance value and you’ll get a pot that works well in your simulator.

    An early choice is deciding between a rotary pot and a linear-motion slide pot. You’ll get higher reliability from a rotary pot for the same cost, but sometimes space constraints or the mechanical realities of your control’s motion speak strongly in favor of a slide pot. The most common rotary pots turn about three quarters of a turn. Slide pots are available in a variety of sizes with 20, 30, 45, and 60 millimeter wiper movement being common.

    Rotary motion pots and slide (linear motion) pots.

    Resistive linearity (also known as “taper”) is how the resistance varies as the wiper is moved. The wiper to end terminal resistance of a linear pot changes in proportion to the wiper motion. When the wiper is moved through 30% of its range, the resistance has moved through 30% of its range. Put the wiper at the 75% position and the resistance is at 75%, and so on. We use linear pots in simulators.

    There is occasional confusion in the use of the term “linear”. A slide pot has a wiper with a linear motion. However, a slide pot is not always a linear pot. That is to say, any pot can have a resistance that varies in a non-linear fashion as the wiper moves. For example, some pots are designed so that the resistance varies as the logarithm of wiper movement. This non-linearity is useful because it complements the non-linearity in human senses. It’s just not particularly useful for simulator controls.

    Potentiometer lifetime is specified as how many rotations the pot is expected to last if not abused. Distributor catalogs typically don’t show a lifetime rating for most pots, because most pots are designed for “general service” and have a lifetime of 30,000 to 50,000 rotations. This sounds like a lot and, in fact, is fine for controls that get adjusted a few times a day. But, if you use a general service pot in a joystick or yoke that is continually moved, you’re looking at only several dozen hours of service. Not so good!

    When shopping, look specifically for pots listed as long life. You want one with a rating of at least 1,000,000 rotations. The distributor catalog may list the lifetime or simply list the pot under a “long lifetime” heading. In either case, verify the lifetime rating by going to the manufacturer’s website and searching for the product spec sheet.

    Resistance value is usually the easiest choice, because it’s frequently made for you. If you’re connecting the pots in a DIY throttle quad to an interface board, the interface documentation tells you what value to use. If you’re using the electronics stripped out of a game controller, you can measure the resistance of the pots that were originally connected. If you’re doing something else, well, 20,000 ohms (20K) is a very useful value. Most USB game controllers and interfaces accept a range of pot resistances, and 20,000 ohms is usually in the middle of that range.

    Both the Bourns series 6639S and Vishay Spectrol model 357 are excellent rotary pot choices. The Bourns PTE series is a good choice for a long life slide pot. A long-life slide pot will not have the same lifetime rating as a true long-life rotary pot, but will still offer good service if treated well. All are available through major distributors like Digikey and Mouser.

    Other potentiometer performance factors are not unimportant; it’s just that you rarely need to check them when selecting pots for simulator control applications. Interface circuitry operates at such low voltages that the standard specifications like voltage, power, and temperature are never challenged. Choosing for a long lifetime also means you get a well engineered device with a low friction, smoothly moving wiper.

    There is a great deal more information about potentiometers available online. The Bourns website, in particular, is an excellent resource. Go to www.bourns.com and click on the “Library” tab. There’s an assortment of product documentation along with a sequence of online tutorials. There is also a pdf version of the book Potentiometer Handbook. The page order was somewhat scrambled when the book was scanned, but if you can see past that, it’s got a lot to offer.

    Mike Powell, author of

    Building Recreational Flight Simulators and

    Building Simulated Aircraft Instrumentation.