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Rubber makes a better input device

Rubber makes a better input device

A well-chosen material can indeed make familiar devices smaller, cheaper, and better. Just take a look at the tiny joysticks that Varatouch Technology Inc. can create from a single bit of rubber measuring as little as 7 mm across.

Of course, it's not just any rubber that Varatouch uses. The company instead developed a patented "resistive rubber" compound, a carbon-filled silicone elastomer that performs both mechanical and electrical functions. "Resistive rubber allowed us to reduce the mechanical complexity of our input devices to just the essential electronic elements," says Michael Rogers, Varatouch's president.

In MiniPoint joysticks, which are just one of several input devices based on Varatouch materials technology, the resistive rubber component and a plastic cap join to form a one-piece actuator button. The user's finger pushes the button to move a cursor around a display screen. Nothing too unusual there. What's different is how the lone resistive rubber component works in conjunction with an underlying printed-circuit board (PCB) and a tracking algorithm to determine cursor position. As a user deflects the actuator, the resistive rubber touch-es an electrical contact area on the PCB, triggering voltage changes. The tracking algorithm then translates those voltage changes into cursor position and speed.

On its tiny pointing devices -- like this 13 mm diameter joystick -- Varatouch senses the cursor location from the five electrical contact points on the PCB. Electrically, the sensor functions as two variable-rate resistors, but physically it is just a single rubber component.

Comparing Varatouch's technology to traditional pointing devices, Rogers says that the latter typically cost at least twice as much, take up twice as much space, and have more than a half-dozen different parts. Varatouch's miniature joystick can be boiled down to just one component in its simplest embodiment: When integrated with an existing OEM circuit board and enclosure, the rubber piece is the only extra part.

Varatouch's innovation began with an observation about a familiar family of elastomers-the so-called conductive rubbers used in electronics applications such as bridge contacts. These materials don't conduct as well as the name might suggest, Rogers says. "So our breakthrough was understanding that material considered to be a poor conductor is a good resistor and then building a product around that realization," he notes.

Turning this bit of inspiration into a useful pointing device took about three years of work by five mechanical and electrical engineers. Despite the apparent simplicity of the device, the design team had to master what Rogers calls the "hideously complex" relationship between material properties, mechanical design, and electronic functions.

The mechanical design centered on the tricky geometry of the rubber component, which is slightly curved on the bottom where it meets the PCB. Every aspect of the geometry combines to produce a characteristic "swing" under deflection. "And swing relates to the electrical signal," Ro-gers says. Getting a reliable signal also occupied the electrical designers too. According to electrical engineer Kevin Bodily, they grappled for a long time with electrical properties of the material: A low-resistance compound makes for a stable pointer but it is more difficult to interface with the electronics. "Sensor-to-sensor reliability was also a big problem at first," he adds, explaining that the design team had to develop its own testing routines to account for the imprecise electrical properties of the rubber compound. Other inconsistencies in the signal were addressed by the firmware for the tracking algorithm, says firmware engineer Brian St. Jacques.

Variable-rate input devices' small size, simplicity, and low cost may open doors for their use in places that bulkier, costlier devices don't fit. "We've had a lot of interest from the makers of cell phones, PDAs, digital cameras, and other devices that haven't traditionally had this kind of input device," Rogers says.

Additional Details
Contact Michael Rogers, Varatouch Technology Inc., 7325 Roseville Rd., Sacramento, CA 95842-1600; Tel: (916) 331-6300, x302; Fax: (916) 338-8255; E-mail: [email protected] ; or Enter 546
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