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Design engineers interested in integrating miniature force sensors into product designs now have an option to evaluate and integrate a new breed of capacitive technology-based that can accurately measure forces as low as 1 gram with high repeatability. These capabilities enable the creation of devices which measure applied force at discreet points, even at very low levels, for next-generation medical devices, robotic systems, consumer electronics, and many other applications.

Although capacitive sensor technology has largely supplanted resistive for touch screens, until recently product designers have been limited to resistive sensors to measure force.  This type of sensor measures the resistance of conductive material, such as an elastomer, foam, or conductive ink, to detect pressure.  In this approach several layers of circuitry touch each other, changing the circuit’s resistance. Because there is a transition from contact to non-contact, however, there is a dead-space that limits the sensitivity of the sensor at lower levels of pressure.

PPS capacitive force sensors. Image courtesy of PPS.

Capacitive sensors, on the other hand, involve two electrodes separated by a compressible dielectric structure.  When pressure is applied, the gap decreases and capacitance rises.  Unlike resistive technology, the two electrodes never touch.  Consequently, capacitive sensors are less susceptible to wear or failure even if subjected to multiple, repeat loads. 

Capacitive sensor technology has several advantages over resistive, including greater stability in terms of repeatability and durability, and can measure low levels of pressure with accuracy. Some of the applications which are being explored include integrating pressure (using arrays that measure thousands of points of force) and force sensors (that measure discreet points) in consumer electronics, robotic, ergonomic, medical and automotive products.

According to Dr. Jae Son, founder of Pressure Profile Systems (PPS), one area that has generated quite a bit of interest is in the testing and development of wearables, which can include clothing, shoes, headphones, wristbands, bras and pressure garments. “By integrating capacitive tactile sensors in the testing and development of wearables, for example, manufacturers could capture and quantify the amount of pressure experienced by customers at specific spots to optimize fit and function,” says Dr. Son.  “Comfort, after all, plays a large role in determining whether someone will like a product or not.” To accomplish this, sensors can be embedded in mannequins or sewn into straps that go between the clothing and a mannequin or human tester during the development process.

There are also many applications in medical equipment design.  According to Dr. Son, PPS sensor technology has contributed to the creation and research of an array of novel medical applications such as advanced catheter system that provides clinicians with more-useful and detailed data to aid in diagnosis of esophageal problems and a Screening Clinical Breast Exam called SureTouch that detects breast lumps and, essentially, quantifies the sense of touch. One manufacturer wanted to use a miniature force sensor to control the amount of force used when injecting insulin using a syringe, but there wasn’t a precise miniature force sensor available.  That is about to change with capacitive miniature force sensors.  

The technology is also well suited for monitoring blood pressure, where extremely low pressure measurements are required, and could be used to create an alternative to pressure cuffs or embedded into future fitness and smart watches that currently only measure heart rate.

Even within the realm of capacitive sensors, there are different methods to detect a person’s input. In the most familiar use, capacitive touch screens used in Smartphones utilize a layer of capacitive material to hold an electrical charge.  Touching the screen changes the amount of charge at a specific point of contact.

PPS sensors, however, differ by utilizing two built-in electrodes separated by a compressible gap, eliminating the need to use the electrical charge delivered by the human finger.  As a result, sensor activation can be performed by a user with a gloved hand, for example medical or laboratory personnel, or any other object that can apply force. In some applications, this can protect against unintended activation of a sensor by a user accidentally brushing past it.  It also facilitates the measurement of proportional force, data which can be utilized to improve the user interface and perform different functions – such as accelerated scrolling – based on the amount of force applied.

If there has been a barrier to adoption of capacitive-based force sensors, it can be attributed to the complexity of design – adding to the costs – versus resistive technologies.  However, with the decreasing cost of capacitive sensor ICs and microcontrollers due to its broad adoption for touch screen applications as well as advancements in capacitive sensors manufacturing techniques, more affordable capacitive force sensors are now available.  PPS, for example, has developed a new breed of miniature capacitive force sensors called SingleTact, which are designed to allow engineers to conduct feasibility studies, create prototypes, and even integrate into next generation products.

The miniature force sensor is available in two sizes, each with three force options, and comes with a small microcontroller that performs the capacitance conversion.  For more sophisticated applications, up to 128 single element sensors can be daisy-chained together with a single interface.  

The evaluation kit for PPS's force sensors includes support for both Windows PCs and the Arduino open-source platform. Image Courtesy of PPS.

To further facilitate the integration of the technology, PPS is providing all the schematics, data acquisition software, and open source sample code for Arduino and Windows PC.  For those engineers that require it, the SingleTact can also operate in Analog mode.

“As capacitive sensors have become more pervasive in the industry, more people want to understand how to measure different levels of force,” says Dr. Son.  “So we felt it was important to provide force sensors that were affordable and available in an open source environment, so that product designers can evaluate and experience the technology.”

For more information, visit www.singletact.com or e-mail info@singletact.com.

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