May 17, 2010      

For many robotics applications, the systems must be able to interact tactically with humans and their environment. This requirement has driven research in touch sensing for years. Pressure Profile Systems (PPS) Inc., a Harvard University robotics lab spin-off, has spent more than a decade developing its capacitive touch sensing technology into flexible, versatile sensor products for integration on a variety of platforms.

Unfortunately, high prices had limited touch sensing technology to research projects and prototypes. Competition among producers of consumer electronics, however, has resulted in capacitive touch sensing being added to mobile phones, computers, and other devices. This has brought capacitive touch sensor manufacturing to a high production volume, dramatically reducing the price point, and allowing the technology to be integrated into other types of systems.

Technology Overview
Los Angeles-based PPS grew out of a Harvard University robotics laboratory in 1996 where researchers, including PPS founder and CEO Jae S. Son, were investigating tactile sensors for robotic hands. The core technology initially developed in the lab was improved over four years with funding from the Defense Advanced Research Projects Agency (DARPA), the National Institutes of Health (NIH), and the U.S. Army Research Lab. Subsequent development identified key high-volume markets for application of the technology-including ergonomics, medical devices, and mobile phones-which later helped to commercialize the technology. Now that the distributed pressure measurement technology is available at a reasonable price point, PPS has been working to become an OEM supplier of touch sensors for robot manufacturers to include on end effectors.

At a fundamental level, a capacitor is no more than two electrode plates separated by a dielectric material. The simplest capacitive sensor is a capacitor for which capacitance changes when the thickness of the dielectric changes-in this case, when the capacitor is pressed and the distance between the two electrodes decreases. The measured capacitance is related to the pressure on the sensor. By creating a crisscross pattern of electrode plates, an array of these sensors can be built. This is the underlying technology that makes touch screens possible.

Pressure Profile Systems has focused on developing capacitive touch sensors that can be integrated into moving parts. This requires physical flexibility in the sensor array. Conformable Tactile Technology, PPS’s core technology, can be manufactured as distributed arrays or discrete sensors, which can flexibly conform to the size and shape of a given mechanical component.

Robot Integration
This is the capability that has enabled development of PPS’ RoboTouch product. The RoboTouch sensors each contain 12 to 24 sensing elements that use standard digital communication interfaces. The RoboTouch technology has been integrated onto Barrett Technology’s BH8-series BarrettHand, which Barrett offers as a standalone product. Similarly, Willow Garage has added the sensors to its PR2 research robot’s grippers, and Waseda University’s robotics lab uses them on the humanoid service robot prototype Twendy-One.

Tactile sensing in each of these robots is motivated by a desire for fine-tuned grasping control and, fundamentally, safety. Without tactile sensing, a robot may only know that it has successfully grasped an object through visual inspection-a slow, imprecise method-or by measuring a current spike in the hand grasping motor, which means a robotic hand may close too hard before the system recognizes a successful grasp. Discrete “bump” sensors that switch when physically depressed have also been used in the past, but they are unable to provide a pressure profile that indicates the strength and quality of a grasp.

Capacitive sensors solve these problems, enabling robots to pick up lightweight, soft objects like a loaf of bread; pick up or gently crack an egg; or safely grasp a human’s hand or wrist. Indeed, PPS’ Chief Technology Officer David Ables believes safe tactile grasping for interaction with humans is a key application for the technology, particularly as the eldercare and service robotics markets grow.

Business Strategy
PPS has already positioned itself as an OEM supplier to Korean mobile phone manufacturer Mobile Input Device Technology Company Ltd., with which PPS has partnered for its ASIC manufacturing capability. The company has also partnered with Artann Laboratories, a research company developing medical diagnostic devices.

These alliances lay the groundwork for similar partnerships with robot manufacturers. The challenge for PPS is to locate and partner with a robotics company that requires tactile sensing in its products and is producing systems on a large scale. Current partners Barrett Technology and Willow Garage both offer standalone hands and grippers with integrated PPS sensors, but to date both companies’ primary customers are researchers, a low-volume market.

The RoboTouch sensors themselves are offered for sale direct from PPS to researchers interested in tactile sensing capabilities. An off-the-shelf evaluation kit is also available. However, though the sensors’ electrical interface is standard, actual integration requires work in partnership with PPS, which customizes the physical configuration and software for each customer.

In high volume-hundreds to thousands of units-the sensors may cost between $100 and $500 each, or up to $2,000 for a single unit, while the end effectors with which these sensors are integrated are individually priced in the tens of thousands of dollars. These prices are at odds for the majority of products, robotic or otherwise, that are produced and sold in high volume. Medical devices and high-end consumer products, both of which would not be described as robotic, are the exceptions.

Future Development
New development work at Pressure Profile Systems is focused less on further innovation of the underlying sensor technology, then on enhancing sensor integration. For example, PPS is developing a version of the sensor that communicates wirelessly with a computer, rather than requiring cabling. Another version of the sensor will be built as a strip that can be trimmed down to any length depending on the application. According to PPS CTO Ables, the RoboTouch sensor is unlikely to benefit from these developments as much as the other products.

In a manner similar to capacitive touch sensing and mobile phones, PPS’ RoboTouch technology requires a high-volume application that will further drive down the manufacturing cost, define a standard interface, and reduce the need for customization. With academic research as a primary market for the RoboTouch technology, this is not likely to occur soon. Though the sensing technology itself may be ready, as of yet there is no “killer app” for the RoboTouch technology, and the robotic end effector market has not matured to the point that it demands commercialized tactile sensing capability.

The Bottom Line
Pressure Profile Systems’ has leveraged capacitive touch sensing research to develop flexible, versatile sensor products for a variety of platforms. The capacitive touch sensing technology, the development of which originated in the robotics arena, has been commercialized in other markets, bringing the costs down and capabilities up to a point that makes integration into robotic end effectors possible. Though PPS faces commercializing challenges for its RoboTouch system because of the limited end effector market, the underlying technology—already mature and ubiquitous on mobile devices—enables safe, reliable grasping in personal and service robot hands and grippers.