Often intimidated by the relatively new technology, many engineers tend to err on the side of caution and overspec capacitive tactile sensor solutions. They want it all: the best resolution, the most sensors, the fastest speeds, and the best performance. Unfortunately, that wish list simply isn’t feasible.
In reality, engineering the optimal capacitive tactile sensor solution for a given application is a delicate balancing act that requires a basic understanding of the technical tradeoffs associated with capacitive tactile sensor design. Below are three key technical tradeoffs you need to consider when specing a capacitive tactile sensor.
Number of sensors vs. speed. More isn’t always better. In the case of capacitive tactile sensor solutions, for example, speed is inversely proportional to the number of sensors. So, the more elements you have, the more challenging it is for the electronics to scan through them. Consequently, the sensors scan at a slower rate. This issue is further compounded by one of the few drawbacks of capacitive sensing measurement: Because capacitive sensing measurement is not as easy to perform as resistive sensing, the sample rate is slower. The good news, however, is that engineers often underestimate what you can actually achieve with a limited number of sensors. As few as twelve sensors, for instance, can often meet or exceed the needs of a particular application, depending on the requirements. After all, outfitting a product with 1000 sensors is pointless if the data obtained isn’t useful; capacitive tactile sensing is more about the quality of data, not the quantity of sensors.
Resolution vs. maximum pressure. When specing out a tactile pressure sensor, engineers frequently overspec pressure range, in particular, because they are unsure of the pressures to which the sensors will be exposed. For example, an engineer might spec a 100 psi sensor “to be on the safe side” even though the actual pressure may only be 3 or 5 psi. Using a high-pressure sensor for low-pressure applications, however, can unnecessarily compromise resolution. The sensor in this scenario may ultimately experience noise, which, in turn, can affect its ability to reliably differentiate the smallest possible change in measurement for an application.
Flexibility vs. stability. Flexibility and conformability are among the key advantages of capacitive tactile sensor solutions compared with load cells and strain-gauge technologies. In fact, these characteristics are enabling the development of novel products, notably medical devices that are able to follow the curvature of the human body for cancer detection and other healthcare-related functions. But as sensors become thinner and more-conformable to accommodate increasingly sophisticated product designs, stability and performance are affected. At present, the technology does not exist to achieve the accuracy of a load cell in a form as thin and flexible as that of a capacitive sensor. And while this particular tradeoff between precision and conformability can be frustrating, it doesn’t have to be a deal-breaker for capacitive sensors. Rather, designers need to really assess the degree of repeatability essential for the given application; it may not be as high as you would expect.
Designing a tactile sensor that accommodates the needs of a specific application is not without its challenges. However, understanding the technical requirements of the sensor and the environment in which it will be used will help engineers to spec a tactile sensor that is optimized for its intended application.