Capacitance Sensing in Human Body Contact ApplicationsJuly 1, 2010 By: Wayne Palmer, Analog Devices Inc. Sensors
Capacitive sensing isn't just for your smartphone; it also holds promise for products such as medical devices that must come in contact with human skin. Here's how capacitive sensing can be used to determine the quality of the contact between the device's surface and the user's skin.
Capacitive sensing technology continues to gain popularity in traditional human interface applications such as laptop track pads, MP3 players, touch screen monitors, and proximity detectors. In addition to using capacitive sensors to replace mechanical buttons, a little imagination—combined with the basic principles used in human-interface designs—will allow many other applications to take advantage of this technology. Figure 1 shows a few example application concepts that can be enhanced to include human body contact sensing.
Figure 1. Devices using capacitive sensor electrodes
For the devices shown in Figure 1, it is often beneficial to have information about the quality of contact between the device and the skin before the device is activated or a measurement is taken. The range of devices could include a medical probe that needs to rest flush against the skin, a bio-potential electrode sensor, or the housing holding a catheter tube in place. To determine the contact condition, several capacitive sensor electrodes, shown in green, could be embedded directly into the device's plastic housing at the injection-molding stage during manufacturing. The host microcontroller reads a few status registers on the capacitive sensor controller IC representing exactly how close the capacitive sensors are to the skin. A basic detection algorithm running on the host microcontroller then processes the status register information to determine when all of the sensor electrodes are making proper contact with the skin.
In traditional capacitive sensing human-interface applications, a person initiates contact with the sensor electrodes, typically by finger touch. The examples shown in Figure 1, on the other hand, use capacitive sensors in a nontraditional way, where the user places a device containing the capacitive sensing electrodes on the human body. Developing this type of application is straightforward, but following a few key guidelines should ensure a robust and reliable system.
The capacitance-to-digital controller. Developing a high-performance contact sensing application starts with selecting an appropriate capacitance-to-digital controller (CDC). For applications such as those shown in Figure 1, the device's surface-to-skin contact is measured directly from the small change in energy—distributed across an array of capacitive sensor electrodes—that results when contact is made between the device and the skin. The accuracy of this type of measurement depends on the sensitivity of the CDC's analog front end and the number of sensor electrodes. Capacitive sensors manufactured on traditional PCB processes are usually in the range of 50 fF to 20 pF, so a high-precision measurement technique employing a 16-bit CDC would be ideal.
When choosing a CDC, start by identifying some key features, such as a high-resolution analog front-end with a 16-bit ADC, programmable sensor sensitivity settings, programmable sensor offset control, onchip environmental calibration, sufficient capacitive input channels to support the desired number of sensor electrodes, and an integrated design that does not require external RC components for sensor calibration. These features all support a reliable, flexible application and optimal user experience. For example, programmable sensitivity enables the interface designer to preprogram the optimal sensor sensitivity for the application, rather than using a fixed solution that can result in poor sensitivity. Programmable offset control is another important feature for the interface designer, as each production lot of sensor boards can be expected to have slightly different offset values. A quick pre-characterization allows the host firmware settings to be changed before releasing the new sensor board into mass production. Onchip environmental calibration will result in a more reliable solution for applications where the ambient temperature or humidity is expected to vary. Recall that the electrode sensors are constructed using standard copper PCB traces; the properties of the substrate will change with temperature and humidity, thus changing the baseline level of the sensor output. This baseline drift can be dynamically compensated while the product is in use, providing the CDC supports onchip calibration.
Small electrodes require high sensitivity. The goal of the measurement is to determine exactly how flush the device is to the skin; the better the contact between skin and device, the more accurate the device's readings. The accuracy of this measurement will be determined by the number (more electrodes means higher resolution) and size of the electrode sensors distributed across the device's contact surface area. For the applications described in Figure 1, the surface area involved is typically small, requiring the designer to develop the application using small sensor electrodes.
Reliably measuring the small capacitance changes, typically <50 pF, associated with small sensor electrodes requires a highly sensitive analog front-end controller. Keep in mind that the type and thickness of the plastic overlay material further attenuates the small signal emitted through the plastic by the sensor. The controller's analog front-end measurement must be sensitive enough to measure this small signal while maintaining a good signal margin between the measured signal and the threshold level detection setting under all operating conditions (e.g., varying supply voltage, temperature, and humidity and the thickness and type of overlay materials). A low signal margin increases the risk of false detection and erratic sensor behavior. To minimize this risk, try to maintain at least 1000 LSBs of margin between the sensor baseline level (no sensor contact with skin) and the contact threshold level when using a CDC with a 16-bit ADC.
The AD7147 and AD7148 CapTouch programmable controllers for single-electrode capacitance sensors have 16-bit resolution, allowing femtofarad measurement as well as 16 programmable threshold detection level settings across the full scale. They can support sensor electrodes as small as 3 mm × 3 mm beneath a 1 mm plastic overlay material, which has a dielectric constant of 3.0, while still maintaining a full scale signal margin of 1000 ADC LSBs. Full-scale signal margin is the difference between the sensor output when the sensor is not in contact with skin and the level when the sensor is in contact.
Maintain reliable performance. Capacitive sensor electrodes are fabricated using standard copper material on PCB or flex material. The characteristics of this material will vary as the temperature and humidity changes. This variation will create a shift in the baseline level to which all of the sensor threshold levels are referenced. Large baseline shifts increase the risk that the contact threshold levels will be too low or too high, depending on the direction of the baseline shift. This can lead to false contact errors or threshold levels that are either too sensitive or not sensitive enough, leading to erratic contact behavior. To maintain the original sensor signal-to-contact threshold detection level margins (sensitivity), the CDC needs to automatically track the magnitude of the baseline shift error and rescale the threshold settings accordingly. Figure 2 provides an example of how the AD7147 and AD7148 threshold levels automatically track and adjust for any baseline offset changes due to changing environmental conditions.
Figure 2. AD7147/AD7148 onchip environmental calibration
Eliminate measurement errors. Retrofitting a device to include an array of capacitive sensor electrodes may result in space limitations, forcing the designer to locate the CDC far from the capacitive sensors. This can result in long, closely routed parallel sensor traces that are detrimental to capacitive sensing applications because the traces at different DC potentials will establish stray coupling paths as shown in Figure 3A. A ground plane on the PCB will not prevent this because the traces and the ground plane will be at different DC potentials, and stray capacitances will still be formed (Figure 3B).
Figure 3. Paths for stray capacitance showing the outcomes for parallel traces with no flooded plane (A), parallel traces on a grounded flooded plane (B), and parallel traces on a flooded plane where both traces and plane all have the same DC potential (C)
One way to eliminate stray capacitance errors is to surround the adjacent traces with a plane driven at the same DC level as the capacitive sensor electrodes and traces. The AD7147 and AD7148 devices eliminate stray capacitance problems by providing a dedicated ACSHIELD output that is capable of this as shown in Figure 3C.
Consumer healthcare products, including spa skin-treatment products, are transitioning out of the professional office and into homes where the user is no longer a trained technician familiar with the product and its application. As a result, many of these products require a more intelligent user interface to close the loop between how the product should be used and how the product is actually used by an untrained user. Capacitive sensing offers a user interface designer another option to consider when exploring novel ways to meet these new user interface requirements. The capacitive sensor electrode-to-skin contact information can be applied to maintain optimal product performance and safety using today's capacitive-to-digital technologies.
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