Capacitive tactile sensors are putting pioneering products on the map that run the gamut from enabling early detection of cancer to enhancing headset comfort. Yet despite capacitive tactile sensing’s contribution to the development of cutting-edge products, it is—like any technology—not without its drawbacks. Below we touch upon three distinct drawbacks of capacitive sensors to provide a clearer understanding of the technology and its limitations.
Capacitive Tactile Sensors Aren’t Scales
There are long-term loading issues associated with both resistive and capacitive tactile sensors. Capacitive sensors, however, are typically engineered using a silicone elastic-type material. So, when weight is applied to the pressure sensor, the material between the two electrodes can creep. This creep, in turn, can cause sensor output to change very slightly over a period of time—even though the load itself doesn’t change.
The fact that capacitive sensors aren’t really intended for weight-measurement applications is typically not a problem. But some applications do require the stability of a load cell. In those cases, PPS typically employs metal flexors to act as the compressible layer in order to improve capacitive tactile sensor stability over long-term use.
Capacitive Tactile Sensors Aren’t the Fastest Technology
Owing to their complex array construction, capacitive tactile sensors can be a bit slower to scan pressure measurement than competing technologies. Electrodes in the sensor array, after all, are arranged as orthogonal, overlapping strips. Each point of intersection in this lattice design forms a distinct capacitor, thereby enabling the measurement of capacitance—and, thus, local pressure—at a single point by selectively scanning an individual row and column in the array.
And while proprietary drive and conditioning electronics can scan through a tactile sensor array at a relatively high speed of 10,000 elements/second, these particular tactile pressure measurement technologies may not yield readings as quickly as other sensor types. We’re talking about a delay of fractions of seconds that wouldn’t have any significant impact on most applications. However, capacitive tactile sensors would not be the optimal choice for an application in which speed is a critical criterion, such as airbag impact testing, for instance. Nor would they be suited for use in applications where artillery was impacting the sensor. PPS does offer single-element sensors that can be used for such applications, but not an array at this point.
The speed of the capacitive sensor can actually be increased by adding more electronics, but cost would also increase significantly as a result.
Capacitive Tactile Sensors Aren’t the Cheapest Option
On that note, the expression “you get what you pay for” is particularly apt when it comes to capacitive tactile sensors. Because they are difficult to surpass in terms of accuracy and reliability, these cutting-edge tactile sensor solutions can get quite expensive, depending on the degree of customization required.
While the cost of capacitive tactile sensors over the past few years has dropped dramatically, resistive sensors remain significantly cheaper and are therefore still the most commonly used tactile sensor technology on the market today. While typically not as elegant looking or as accurate, resistive tactile sensors are cheaper and easier to replace owing to their more off-the-shelf nature.
Custom capacitive tactile sensors take time to develop, and, as we all know: Time is money. At PPS, for example, we enjoy solving problems for clients and pushing boundaries with capacitive sensors. But more often than not, addressing these complex needs requires custom tactile sensor solutions that are carefully engineered using extensive hand wiring and expensive flexible cable wires. On the upside, however, the price of capacitive tactile sensors can drop dramatically with larger-volume applications.