Selecting the best pressure sensor for a given application can be a pressing issue for design engineers. Luckily, when it comes to capturing the sense of touch in a product, there are two primary tactile sensor technology options from which to choose.
Resistive or piezoresistive tactile sensors are by far the more commonly used technology, having been on the market for quite some time. Capacitive tactile sensing technology, in contrast, is the new kid on the block and the subject of a great deal of ongoing cutting-edge research.
Resistance tactile sensors may have their place. But in more-sophisticated applications that require more in-depth data, resistance can be futile.
RESISTIVE TACTILE SENSORS
As the name implies, resistive tactile sensors measure the change in resistance of an elastomer, foam, or conductive ink between two points when pressure is applied. Conductive carbon ink, in particular, has semi-conductive properties. And while carbon ink may not be as conductive as copper or silver, for example, it does serve as an effective conductor. The caveat, however, is that the ink’s conductivity depends on the size of the contact area.
Conductive carbon ink is printed onto a flexible cell, and electrodes are layered onto one another with a gap left between each layer. When pressure is applied, the system changes from a state of no-contact to a state of contact in which resistance can be detected.
CAPACITIVE TACTILE SENSORS
Capacitive tactile sensing, on the other hand, employs highly conductive electrodes that are separated by a very small gap. The electrodes never come into contact with one another, however.
They can be supplied as single elements or multiplexing levels, which form arrays. To build tactile array sensors, the electrodes are arranged as orthogonal, overlapping strips. A distinct capacitor is formed at each point where the electrodes overlap. By selectively scanning a single row and column, the capacitance at that location—and thus the local pressure—can be measured.
TACTILE SENSOR SELECTION
If you just need to see where contact is occurring or obtain a basic pressure map, then resistive sensors do the trick. They can also be significantly cheaper than their capacitive-based counterparts, if you don’t need to use data in an absolute manner. Resistive tactile sensors tend to be marginally thinner as well, measuring approximately 0.2 mm compared with the 0.3- to 3-mm width of capacitive tactile sensors.
But they are not quite as stable or as accurate as capacitive tactile sensors. The stability of the resistive tactile sensor degrades over time with use—even when the sensor is not in use—through aging of the ink. In addition, the sensors often need a sort of “breaking-in” phase in order to calibrate and stabilize initial results.
With capacitive tactile sensors, in contrast, the electrodes are compressed within an elastic region, which ultimately makes them more stable. Furthermore, because capacitive sensors don’t have to make a contact-to-non-contact-state transition, they are inherently more stable and, thus, the better option for highly sensitive applications. They also experience significantly less wear and tear under load and can be made out of a broad set of materials that offer increased design flexibility. Moreover, since the scale of the deflections is so small, capacitive tactile sensors require far less calibration than resistive tactile sensors do.
Resistive tactile sensors are well-suited for certain types of medical applications, such as gait analysis and the study of biomechanics. But capacitive tactile sensors enable a medical device manufacturer to provide data with a high degree of accuracy and repeatability, such as how much pressure a product exerts on the human body in order to obtain FDA clearance. Similarly, in the case of pressure ulcers, capacitive tactile sensing provides a validation method that can yield highly accurate readings over time from a variety of locations.
In addition, resistive tactile sensors require a certain amount of applied pressure to provide valuable data, and, consequently, are not ideal for low-pressure applications. Commercial headsets, for instance, need to exert pressures below 1 psi. Such a low pressure is not easy to measure with resistive sensors, but is clearly important for the comfort of the headset wearer.
The bottom line is that while resistive tactile sensors certainly offer advantages for particular applications, you should get your hands on capacitive tactile sensors if repeatability, consistency, and accuracy are required.
