Membrane potentiometers are mature components with more than twenty years in the industry, and can be used in the same applications as traditional potentiometers. Over the years, the technology has made some significant advances making them a competitive alternative to traditional linear and rotary position sensors.
Key technical parameters that designers should consider when selecting a potentiometer include life cycle, repeat accuracy, hysteresis, resolution, repeatability, and errors due to linearity in addition to operational temperatures.
What is a membrane potentiometer? Fundamentally, a membrane potentiometer consists of a resistive path that is printed onto a membrane base, and a collector with a printed short-circuit path that is applied on top of this base. Both paths are separated by a circumferential spacer.
When pressure is applied to the collector foil, electrical contact is simultaneously applied to the resistive path and a voltage can be tapped into movement via the collector foil. Once the pressure being applied to the collector foil ceases, the voltage can no longer be tapped. This characteristic must be kept in mind when operating the membrane potentiometer manually; however, when a wiper or magnet is used, the voltage remains constant.
The membrane is comprised of polyethylene terephthalate (PET), which is a thermoplastic material (plastomer) that can be molded within a specific temperature range. Since this shaping process is reversible, the procedure can be repeated in an unlimited number of times by cooling and reheating the material, as long as the thermal decomposition of the PET material is not damaged due to overheating. The critical temperature for materials used in membrane potentiometers is approximately 73°C.
PET’s ability to be molded has implications for membrane potentiometers. When a wiper is pressed statically onto the collector foil that has been heated beyond the critical temperature subsequent to its cooling, a permanent deformation could occur at the pressure point. Depending on the circumstances, this deformation could cause anything from a slight change in linearity to permanent contact between the resistance and collector path. This effect is known as the denting effect.
However, there are membrane potentiometers available that operate beyond the thermoplastic point of temperature. Here are the characteristics of several membrane potentiometers.
PET membrane potentiometer
In this class of membrane potentiometers, the resistance layer, spacer, and collector foil are made of various thicknesses of PET material, depending on the application and the manufacturer. The thickness of the film is usually between 60µm and 190µm, while the overall height is less than 600µm.
The PET membrane potentiometer is usually operated by using a mechanical wiper with a surface pressure ranging from 0.7N to 3N, but the potentiometer can also be operated by hand. Due to the material’s physical characteristics, the operation temperature ranges between -40°C and 60°C.
Kapton membrane potentiometer
Kapton is a thermosetting polyimide (PI) that can be operated continuously under temperatures up to 230°C, or even as high as 400°C (temporarily). A PI is much more dimensionally stable than PET and can be treated in a reflow oven. The potentiometer layout matches the PET potentiometer’s layout; however, its production is more expensive. In terms of the denting effect, Kapton potentiometers only show a slight improvement when compared to PET potentiometers.
Membrane potentiometer with SET function
A membrane potentiometer equipped with the SET function allows the last set value to be stored—even when someone’s finger or wiper is no longer activating the collector foil. These potentiometers are mainly used in applications requiring manual operation (input systems with an integrated membrane potentiometer). A value is set by applying slight pressure. The set function can be realized electronically or via a special doping, which allows for large areas up to DIN A4/US letter.
Hybrid membrane potentiometer
Hybrid membrane potentiometers are made of a combination of different materials, and they exhibit differences depending on the manufacturer.
- Base and spacer made of PET, collector foil made of a thicker, foil-like material: Very inexpensive with a slightly delayed denting effect. The operating temperature falls into a range between -40°C and 65°C. Limited use for static applications.
- PET base, collector foil made of FR4 (usually between 80µm and 150µm thick): Considerably delayed and reduced denting effect, but higher wiper pressure possible (3N to 5N). Operating temperature range between -25°C and 75°C.
- Base and collector foil made of FR4 with a thickness of up to 150µm: The operation temperature can reach up to 85°C, but a wiper surface pressure of 3 to 5N is required. The denting effect is significantly delayed.
- PET base, collector foil made of a flexible metal: This new alternative exhibits a high level of resistance to the denting effect; however, a wiper surface pressure of 2 to 6N is required. The temperature ranges between -25°C and 85°C.
Magnetic membrane potentiometer
The magnetic membrane potentiometer utilizes a magnetic field and therefore contactless connection. Different designs have evolved and are listed below, but for almost all variants, the magnet can provoke a tap either from the top through the collector foil, or from the bottom through the enclosure.
- PET base, ferrite band on top of the collector: Pulled by the magnet, the ferrite band presses down onto the collector foil. The upper operation temperature can be specified to be a little higher (about 70°C); otherwise the same characteristics as for the PET potentiometer apply. This technology was introduced to the market in 2004.
- PET base with a wiper between the foils; the wiper is moved by a magnet: The magnetic retention force can be less than what is required for a potentiometer with a ferrite band, and the distance between the magnet and potentiometer can be greater. The internal wiper is abrasive, resulting in the potentiometer’s limited lifetime. Operating the potentiometer without hysteresis is not possible because the internal wiper is pulled by the magnet.
- PET base with inserted flexible magnetic metal strip: Only minimal information exists on this new technology; however, its properties are similar to the potentiometers with a PET base and a ferrite band on top of the collector.
- PET base with conductive ferrite band: This variant is under development, but the magnetic force is presumed to be as small as necessary to trigger a reed contact.
FR4 membrane potentiometer
Almost all models mentioned above can also be produced with a FR4 base instead of PET. Some of the advantages of FR4 include better overall operation, an improved resistor paste (manufacturer-specific), and a simplified attachment of connectors or wires with the ability to integrate the complete circuitry into the PCB.
The respective printing of the potentiometer directly onto the PCB, the populating of the components, the soldering, and the applying of the collector foil with integrated spacer are further benefits. The extent of temperature resistance depends on the material used for the collector. Different collector materials exist in combination with FR4 PCB’s, often made of very thin PET or FR4, among them the before mentioned flexible metal foil.
Membrane potentiometers can be used in a variety of applications, but in general, a membrane potentiometer with a length of up to 700 mm is an economical alternative to a traditional potentiometer. Rotary membrane potentiometers with diameters up to 400 mm are also relatively inexpensive to manufacture.
The membrane potentiometer has opened up sensing possibilities which were not possible before thanks to its ultra-flat design and flexible usage combined with cost advantages. With new and innovative materials like magnetic operations or hybrid membrane potentiometers (such as the patented “Sensofoil Hybrid” by Hoffmann + Krippner), designers and engineers can use this potentiometer in many applications. Complex sensor constructions can be reduced, simplified and integrated less expensively with modern membrane potentiometers.
Jens Kautzor is CEO at Hoffmann + Krippner Inc.