I hadn’t heard of a stringpot until I received a product press release from Celesco, a company that manufactures a variety of stringpot devices. At its most basic, a stringpot, also called a cable-extension sensor, comprises a multi-turn potentiometer attached to a spool with a flexible cable wrapped around it. The spool, much like that in a metal extension ruler, has a return spring that puts tension on the cable. The cable wraps directly onto the spool, so each complete rotation of the potentiometer corresponds to the same length of cable. If the cable wrapped in layers on the spool, external cable layers would produce one potentiometer turn for a longer extended cable length than would inner cable layers.
Image courtesy of Celesco. For a larger image and a Q&A section about stringpots, go to: www.celesco.com/faq/cet.htm.
You can use a stringpot in equipment that requires an absolute distance measurement and where you might have a long measurement length, a wet environment (outdoors, for example) or a varying alignment of components. Manufacturers offer a wide variety of devices, some of which can operate in dirty environments or in equipment that require a “wash down” after use.
Stringpots can measure ranges from a few inches to many feet and provide a linear output of resistance, voltage, or current for controllers or measurement equipment. And you can use distance measurements over time to obtain velocity and acceleration information. The flexible cable lets equipment designers use pulleys to route the cable into tight spaces where another type of distance sensor would not fit. Unlike a long linear variable differential transformer (LVDT) used to measure distance, the spool-and-cable arrangement requires less space and doesn’t require co-axial placement with respect to movement.
Information on the Firstmark Controls Web site lists the following recent applications, among others, for stringpots:
- Delta IV missile thrust-vectoring system
- Diesel-engine fuel-index measurement
- International Space Station environmental control systems
- Excavator hydraulic-cylinder control
- Medical table actuation-feedback system
- Global Hawk UAV landing-gear stroke measurement.
Firstmark has helpful design information, diagrams, and photos at: www.firstmarkcontrols.com/s054b.htm. –Jon Titus