Switches, pushbuttons and similar devices represent the more familiar choice for actuation, but as performance requirements become more demanding in product segments ranging from consumer electronics to medical equipment, alternative actuation methods are increasingly sought. Space constraints are an issue in electronics, reliability is an imperative in medical devices, and environmental sealing is a concern across the board. A force-sensing technology developed by Interlink Electronics, Camarillo, Calif., offers advantages in all three areas.

Using Interlink’s Force Sensing Resistor technology as an actuation device provides greater longevity and reliability, since it has no moving parts. And because it permits a sealed interface, it decreases the chance of failure due to permeation of moisture, dirt, and other contaminants. Furthermore, it creates the opportunity for an extremely thin interface, a huge plus in world of shrinking devices.

An FSR is a variable pressure-sensing resistor that incorporates a grounding layer, substrate layer and silver trace layer. With a proprietary ink, Interlink prints microprotrusions inside the FSRs, which have controlled microsurfaces that, when pressed against electrodes, put a variable resistance between the electrodes. This resistance is dependent on the amount of pressure exerted. With greater pressure, the resistance between the two terminals goes down, and vice versa. The sensor is controlled by a microprocessor that communicates to a central computer.

FSRs have been incorporated as solutions in cell phones, MP3 players, remote controls and cursor controllers, as well as a wide range of other devices. MicroNav products, which use FSR, also increase sensing precision in the touch control devices.

Advantages

The MicroNav Ring allows for fingertip menu scrolling and item selection in personal entertainment devices.

FSRs provide three basic advantages over the traditional electromechanical technologies. First, electromechanical devices have multiple parts and multiple points of contact, which, with wear and tear, fail more quickly than FSRs. The FSR, instead, has one part with only a microsurface coming into contact with an alternate material. This significantly reduces the chance for failure.

Second, electromechanical devices require holes in housings that make the device vulnerable to liquids and other substances that can permeate through the hole and damage the circuit board. The FSR, on the other hand is perfectly sealed and has no requirement for holes in its housing, eliminating the possibility for this form of failure.

Lastly, because electromechanical devices have multiple parts, the footprint of the overall device is larger. With an FSR there is only one part that can be pasted to a very thin material. This allows design engineers to incorporate the technology into significantly smaller devices.

Early Applications

One of Interlink’s early applications for this technology was a sensor solution to sense malfunction in medical equipment. Supplied to Baxter for critical care applications, the solution sensed clogged or empty IV drip lines and, through an integrated microcontroller, alerted critical care personnel when a problem arose.

Another early medical application of Interlink’s FSR was a solution developed for Varian. The use of an FSR helped create a variable-speed foot pedal control that gave surgeons who perform bone replacement operations precise control of high-speed bone drills and other medical equipment.

Starting in the mid-1990s, Interlink began packaging its FSR sensor in a series of cursor control devices for personal computers. Using a FSR beneath a rubber actuator, the device enables pressure-sensitive cursor direction and speed control. The layers of this FSR are patterned into four zones, resulting in five contacts to the sensor — one for each of the zones and one to the ground.

The FSR was also packaged as a Microjoystick for laptop keyboard applications among the “G,” “B,” and “H” keys, a MicroModule mouse button for rugged PCs, and a series of remote controls for portable projectors and presentation devices.

Energy Saving

Microjoystick is used in laptop keyboard applications among the “G,” “B,” and “H” keys, a MicroModule mouse button for rugged PCs, and a series of remote controls for portable projectors and presentation devices.

In the late 1990s, Interlink incorporated FSRs into touchpads designed for laptop computers. Using FSR technology, these touchpads had passive components that require much less power than capacitive components. Unlike a capacitive touchpad, which actively uses batteries for operation, the passive components allow the laptop to run on very little battery power by waiting for the user to turn on the microcontroller. The components were also pressure-sensitive with X, (horizontal) Y (vertical) and Z (downward) dimensions and could be used with any stylus or fingertip.

The FSRs from Interlink’s Micronav line of products in many cases do not require their own dedicated microcontroller. Instead, the FSRs can be configured to operate on the power supplied by the device’s main microcontroller.

“Typically, a MicroNav sensor will not require its own dedicated microcontroller, but rather use space on the products main microcontroller,” says Jeff Baker, director of Research and Development for Interlink Electronics. “So if the electronic product already has some type of processor in it, which is very likely, the processor will also be able to measure the sensor and the device can operate with very low incremental power requirements. In this configuration, the Micronav sensor wakes on touch and the microcontroller is set to sleep until the user touches the sensor.”

In 1999, Interlink’s FSR touchpad technology was used by the company to create the ePad Electronic Signature Capture device. Used by financial institutions such as Prudential Financial and Charles Schwab, ePad captures biometric information such as the pressure exerted and the time taken to sign from the signer. In conjunction with the Intelrink’s IntegriSign Signature Software Suite, the ePad binds the secure e-signatures into digital documents, forms and transactions, allowing companies to automate documents and transactions requiring personal signatures.

Micronav 360 is only 1/2 mm thick and has a 10 sq. mm footprint.

Two years ago, Interlink introduced its smallest FSR to the market. Called the MicroNav 360, this component enables pressure-sensitive cursor control in handheld personal electronic devices, such as cell phones or MP3 players.

And just last year, Interlink added the MicroNav Ring, MicroNav Strip and MicroNav Array sensors to its product lineup. These products allow fingertip menu scrolling and item selection in personal entertainment devices. The MicroNav Ring, a circular multi-zone version of the Micronav sensor, was integrated into a remote control for the Sonos Home Music System, an automated home music system that allows users to listen to their music in any room of their house. The MicroNav Strip, a smaller rectangular sensor component, was used by iRiver as the main user interface for their H10 MP3 player.

When designing MicroNav from its FSR technology, Interlink utilized high temperature-resistant material such as Kapton polyimide and FR4 in lieu of polyester to make Micronav components compatible with automated assembly equipment, such as reflow soldering and pick-and-place robotics.

Used by financial institutions, the ePad signature pad captures biometric information such as the pressure exerted and the time taken to sign from the signer.

“The manufacturer can treat MicroNav solutions like any other electronic component,” Baker says. “Previously you had to worry about high temperatures and you could not solder the sensor directly to the board. Now MicroNav products are compatible with automated assembly equipment.”

Keith M. Roberts, director of corporate communications for Interlink, says Micronav products will increasingly be integrated into home automation control devices as the technology evolves with an ever-shrinking footprint.

“As electronics become more mobile, they are also becoming smaller and more personal,” Roberts says. “Devices such as cell phones, PDAs with e-mail and Internet capabilities and personal media players that play MP3 and Mpeg content, require a user interface or a menuing system that is intuitive, easy-to-use, durable and inexpensive to integrate. Micronav sensor products were created to meet these requirements and offer a form factor tiny enough for the most personal compact device.”