A young engineer recently recounted that his Senior Design professor would frequently repeat this mantra: “A good engineer designs to spec, to budget, and to deadline.”
Therefore, it came as little surprise when a recent survey of medical device design engineers, conducted in April 2011 by medical device media company UBM Canon, revealed accuracy and cost as the two key attributes they seek in a force sensor when designing a new product. A follow-up question asked participants about the greatest challenges they face in sourcing sensor technologies. Cost and ease of integration topped the list.
What does this mean for engineers seeking an accurate, cost-effective, customizable force sensor? Perhaps the most well known transducers for measuring force are load cells. They can use a variety of technologies to sense loads. Strain gauges, piezoelectric elements, and variable capacitance are among the methods in wide use.
Load cells, while known for high precision, have some drawbacks, particularly in applications where weight, size, and/or power are at a premium. Load cells can be bulky. In situations where the application of interest involves relatively lightweight elements, a load cell may actually weigh more than the component being tested. Load cells can also be expensive, in terms of piece-price as well as integration. Another disadvantage of using a load cell is that, rather than providing direct force measurements, they measure the correlating strain of an assembly based on the applied load. This adds complexity to the setup.
In recent years, tactile force sensors have become commercially available. The term comes from their frequent use as a means of sensing a nuanced or delicate touch, as with a robotic hand from a humanoid robot. However, tactile sensors have applications extending far beyond anthropomorphic robots. They typically are built on flexible circuit material so they can be thin, light, and flexible.
One such tactile sensor is designed for applications requiring unobtrusive, cost-effective force measurement. The FlexiForce® sensor, manufactured by Tekscan, Inc. (Boston, MA), consists of piezoresistive material sandwiched between two pieces of flexible polyester with printed-silver conductors on each inner half. The conductive traces form electrical connections to external circuits (Fig. 1). The resulting sensor is only about 8 mils thick. The sensor is also very light; an 8-inch length sensor weighs 0.02 oz.
FlexiForce sensors are resistors that vary linearly in terms of conductance vs. force under an applied load. With no force applied, their resistance is on the order of megaohms; it is essentially an open circuit. As applied force rises, output resistance drops, eventually reaching about 10kΩ or lower, depending on the application. The output, expressed in conductance vs. force, is quite linear (linearity error <±3%). External circuitry to convert the output into a linear analog voltage can be relatively simple, which is an important consideration in keeping costs down and keeping the product design simple. Adding to the simplicity of the design is the fact that FlexiForce sensors measure forces directly, which means there is no threat to the integrity of the structure (Fig. 2).
FlexiForce sensors come in two standard shapes for off-the-shelf for testing/proof of concept and are available in several lengths. These sensors can also be fabricated in custom form factors and force ranges for OEM applications.
Also available are 'matrix' sensors, which come in a variety of standard shapes and sizes. These sensors can be built with sensing locations as small as 0.0009 sq-in. or covering areas up to 256 sq-in. These packages are considered pressure mapping systems as they are designed to be used with Tekscan's patented electronics and software to display areas of high and low pressure across the surface of the sensor. They typically find use in R&D, test & measurement, and quality assurance applications.
However, they can also be found in OEM applications. For example, Dr. Scholl's® (Schering-Plough Health Care Products) wanted to implement a technology that would help drugstore customers select the appropriate Custom Fit™ Orthotic based on their unique foot pattern, support, and cushioning needs. This system uses the patented science of Tekscan's pressure sensing technology along with custom developed software and sophisticated algorithms to recommend the right fit. The orthotic, designed with special cushioning and support layers, offloads high pressure areas with support and comfort in mind.
Case Study 1: Measuring Palpation Proficiency
A National Cancer Institute-sponsored project in the late 1970s began to use technology to standardize breast examination. The problem: Most breast lesions are palpable and discovered by hand, not X-ray, which works on bone and not on soft tissue. At the time, it was routine for tumors the size of golf balls to go unnoticed, not a good situation when the larger the tumor, the more chance of malignancy. Over time, the group developed MammaCare®, now recognized as the medical and scientific standard for measuring clinical breast examination efficiency. Most women survivors of breast cancer report a detection method other than mammographic examination. Therefore, the goal is to reduce the incidence of missed palpable breast cancers, which are frequently not seen on imaging. More efficient, effective methods of palpation are the way to achieve this.
Over the years, the group developed many design iterations to come up with what today is called the Palpation Proficiency and Assessment Device (PAD), which comprises tactually accurate breast models instrumented with custom FlexiForce sensors. The sensors locate and relay over 1,000 levels of examination pressure within each square centimeter by way of a digital signal processor. The device lets MammaCare confidently certify clinicians who have demonstrated effective examination skill.
Case Study 2: Monitoring the Effectiveness of Drug Delivery Systems
Automated infusion pump systems continuously deliver vital drugs to the subject. A crucial design consideration for such devices is a sensor that will detect potentially life-threatening blockages in the system and alert the user or caretaker in time to take appropriate steps to correct the problem or minimize any negative effects. The sensor must be small and thin, in keeping with the size and weight requirements of the device.
Some drug delivery systems are designed so that when a blockage occurs, the tubing expands. A FlexiForce sensor incorporated where the tubing meets the housing detects this expansion by monitoring the force applied to the sensor by a section of the tubing. The sensor can trigger an alarm to alert the user if such expansion is detected. The same principles of operation can apply in infusion pumps and devices designed for operation by medical professionals in settings such as hospitals, hospices, and home care. The construction of the FlexiForce sensor means that it does not add extra weight to the device, so it encourages portability. Since the sensor is a passive device which is only powered when a force is applied, it does not consume as much power as other measurement methods.
FlexiForce sensors, with their very thin and flexible forms, are easy to integrate into any number of designs and applications. OEMs find that these low-cost, off-the-shelf sensors provide them an easy and quick way to prototype products. Further, the user-friendly, single-cell sensors do not require complex electronics to achieve an output.
Given these characteristics, OEMs can incorporate this technology into a range of applications where efficiency and effectiveness are crucial. In addition, they will find that these sensors play a key role in the development of sleeker, smaller designs.
This article was written by Elizabeth Hood, Marketing Specialist for Tekscan, Inc., Boston, MA. For more information, Click Here
- MammaCare® is the registered trademark of the Mammatech Corporation. All rights reserved.