These ultra-flat devices simplify position sensing, coming close to the motion area of the device without requiring a lot of installation space.
Membrane potentiometers have changed the way engineers think about sensing. With some measuring only 0.5mm [.05cm] thick, ultra-flat membrane potentiometers have characteristics comparable to conventional mechanical potentiometers, but they can also fit into space-constrained areas.
Due to its flat design, a membrane potentiometer can be brought close to the motion area of the device that it is measuring, without requiring a lot of installation space.
In general, a membrane potentiometer is a voltage divider. It is constructed as an open circuit that only gives output when a wiper connects the top and bottom circuits by applying pressure to the membrane surface.
The wiper for a membrane potentiometer can be as simple as a small plastic knob sliding across the surface; it requires no external electrical contact. Most membrane potentiometers can also be operated by sliding a finger over the surface, and some are even available in a contactless magnetic version.
The spacing between the top and bottom circuit is constructed mainly of sealant adhesive, allowing the device to be sealed against environmental factors such as moisture or dust. Most can be sealed at up to an IP65 (NEMA 4x) rating and beyond.
This spacing adhesive requires a minimum width of 2-3 mm [0.2 0.3 cm] on all sides of the membrane potentiometer. The ideal width of the resistive active area should be between 3 mm and 6 mm, but can also be extended to 10 mm or even 12 mm. Linear elements can reach up to 760 mm in active length, while rotary elements with a central hole can measure from 20 mm in diameter to approximately 450 mm in diameter. The life cycle and operating temperature correspond to those of mechanical potentiometers and, depending on the design, up to 100 million operations are possible. The operating temperature can range from -40°C (104°F) to 85°C (185°F), and there are even systems in development that can withstand temperatures up to 125°C (257°F).
Different surface pressures are defined depending on the application, but in general, a range of 1 Newton (N) to 6 N is recommended for most applications, depending on whether the membrane potentiometer is configured in a standard foil layout or in a hybrid version with additional metal bonds to strengthen the structure. A standard off-the-shelf wiper is usually in the 1-3N or 3-6N range.
Electrical characteristics will vary somewhat with the design, but are generally standard, with similar output to that of a voltage divider. When compared to a conventional precision potentiometer, the most significant difference is the method of achieving linearity. While the conventional potentiometer may use laser-trimming to achieve linearity, membrane potentiometers are based on PET-Material and therefore rely on production improvements to improve linearity. Hence, the typical linearity offered is 2% for standard linear potentiometers, although FR4-based membrane potentiometers can reach as little as 0.5% linearity, as one option.
More significant than linearity are repeatability and hysteresis; the accuracy of membrane potentiometers can be as good as 0.01 mm over a length of 500 mm, but most standard membrane potentiometers range anywhere from 0.05 mm to 0.1 mm. The main impact on all of the electrical outputs is found in the mechanical motion and stability of the wiper. As a benchmark, energy dissipation of up to 1 Watt and a dielectric strength of 500 V are standard, with isolation strength of 100 V dc also possible.
Due to the small height of membrane potentiometers, a typical application is the linear measuring of actuators. Control valves are another possible application, although currently the standard requires gears and a turning sensor to measure position. Other applications include string-pots, wire sensors, magnetic tape, keypads for hand controls. In other applications, like seat-level adjustment, both rotary and linear membrane potentiometers can be used in the same device. for space constrained applications, printed circuit boards (PCBs) themselves can be a base for a membrane potentiometer. A few manufacturers can even print a conductive ink directly onto the FR4 and combine this with the PCB and its components, directly assembling the components onto the PCB.
Guido Woska is president and CEO of Hoffmann + Krippner Inc., Peachtree City, GA.
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