1. Basic principles
Capacitance principle: Capacitance refers to the ability to store an electric charge between two conductive bodies separated by an insulating material. Capacitive sensing technology is not only suitable for human touch, but can also be used to detect a wide range of materials and objects. According to the principle of parallel plate capacitance, the change in capacitance is proportional to the area and dielectric constant of the electrodes, and inversely proportional to the distance between the electrodes.
How it works: A capacitive sensor detects a touch or proximity event by measuring the change in capacitance between two conductive objects. When fingers or other objects are brought close, the electric field distribution is altered, causing the capacitance to increase or decrease. This change is represented by an equivalent circuit model of the capacitance, involving multiple capacitance types.
Measurement Method: Capacitive sensing typically uses the charge transfer method, where the charge is transferred from the sensing electrode to the sampling capacitor through repeated charging, and these charges are recorded. By observing the change in the number of charges, a small change in capacitance can be detected. For example, imagine a smaller container (representing a variable capacitance) being filled with a liquid (symbolizing an electric charge) and then transferring its contents into a larger container (corresponding to a fixed capacitance). The number of times a larger container needs to be filled reflects the capacity of the smaller container. When this number changes, it means that the capacity of the smaller container has changed, allowing touch or other interaction events to be detected.
The other two methods of capacitance measurement include the relaxation oscillator method and the fixed-frequency AC signal method: the relaxation oscillator converts the change of capacitance into a change in frequency, and the change of capacitance can be measured indirectly by monitoring the change of the oscillation frequency, and the fixed-frequency AC signal method uses a known capacitance and an unknown capacitance, and the change of capacitance will lead to a difference in voltage under the action of a fixed-frequency AC signal. The change in capacitance can be measured by comparing the voltage changes caused by known and unknown capacitances.
2. Classification and application
Self-capacitance vs. mutual capacitance: Self-capacitance sensors primarily detect changes in the capacitance of individual electrodes relative to the ground to achieve simple touch recognition. On the other hand, mutual capacitance sensors are more suitable for complex multi-touch functions by measuring the change in capacitance between two electrodes.
The focus of self-capacitance measurements is to monitor the change in capacitance between the electrode and the ground, which is typically between 1 and 10 picofars. In large applications, the use of a ring design reduces overall capacitance and increases the detection range. For smaller applications, a single solid electrode plate is recommended for inspection needs.
Mutual-capacitance technology relies on two independent electrode structures—a transmitting electrode (Tx) and a receiving electrode (Rx)—to identify touches, which require them to be connected to two pins on the microcontroller. When the user's finger touches the intersection of the Tx and Rx electrodes, it disturbs the electric field between the two electrodes, resulting in a reduction in mutual capacitance, which is equivalent to introducing a grounding effect between the two electrodes. This typically results in a change in mutual capacitance of no more than 1 pF.
3. Design optimization strategies
Design Goal: The key to sensor design is to balance sensing range and system stability to improve sensitivity and accuracy. The goal is to maximize the capacitance increment (CTouch) and minimize parasitic capacitance on touch. This requires a careful design of the shape and size of the electrodes, as well as the selection of the appropriate covering material and thickness.
Electrode design: The shape and area of the electrode are critical to maximizing the signal. The larger the size of the sensor, the wider the contact area with the target object, and the wider its sensing range. Unshielded proximity sensors provide a wide 360-degree sensing range, but interference from the grounding structure can reduce their sensitivity.
The presence of a grounding structure will cause the electric field lines emitted by the electrode to mainly gather between the electrode and the ground, and it is necessary to limit the direction of the electric field and prevent the sensor from being triggered from an undesirable angle. For this purpose, grounding shielding or drive shielding technology can be employed.
Ground shielding: The ground shield prevents the electric field from spreading in this direction by adding a ground layer to the back of the sensor, but this shortens the sensing distance because it introduces additional parasitic capacitance while the ground absorbs the surrounding electric field lines.
Drive shielding: Drive shielding technology adds a conductive layer underneath the sensor that carries the same drive signal as the sensor, and maintains zero potential difference through an op amp configured as a voltage follower. When the phase and potential of the drive signal of the sensor and the shield are the same, no electric field is formed between the two electrodes. In addition, the shielding layer pushes the electric field of the sensor in front of it, providing an effective shielding effect. This not only reduces the interference of parasitic capacitance, but also maintains the sensor's high sensitivity to approaching objects, thus improving the signal-to-noise ratio.
Reduced parasitics and interference: In order to enhance the charge-discharge efficiency, low-resistivity materials are used in the electrode and trace design. However, parasitic capacitance within the system can weaken touch-induced capacitance changes, making capacitance changes difficult to measure accurately. For example, at 20 pF parasitic capacitance, touch can cause a change in capacitance of up to 25 percent, but at 100 pF, the change caused by touch is only 5 percent. To reduce the effects of parasitic capacitance, we need to extend the distance between the traces and the ground plane, and we can use a hollow ground design to further optimize performance.
For larger electrode applications, such as proximity sensing, the maximum length of the traces should be controlled within 120 mm, while the traces should be as fine as possible. When designing a mutual capacitance system, avoid placing the transmit (TX) and receive (RX) traces too close together to prevent the detection distance from being compromised.
Overlay: The overlay design can enhance both appearance and protection, as well as affect sensitivity, so non-metallic materials need to be carefully selected. The thickness and dielectric constant of the overlay directly determine the conduction efficiency of the electric field, and the negative effects of thick overlay on sensitivity can be mitigated by using materials with high dielectric constant. For low dielectric constant situations such as air gaps, it is necessary to use specific bridging materials, which are in descending order of effect: metal springs, conductive foams, carbon fibers, polycarbonates, ABS, silicone.
In self-capacitive sensing applications, higher sensitivity can be achieved by using a thinner overlay. In mutual-capacitance applications, a modest increase in the thickness of the overlay can increase sensitivity to some extent. Comparing the properties of different materials, we find that the temperature drift of FPC is relatively large, while that of LDS technology is smaller.
Power Delivery: The power delivery method significantly affects the sensitivity of proximity detection, especially in battery-powered systems, where the coupling between the device and the earth is reduced. For example, in the capacitive closed loop shown in the figure below, the human body of the touch sensor (C1), the path of the electrode and the circuit module (C3, which indicates the coupling of the module to the earth), and the coupling of the human body to the earth (C2). In this case, the detection effect of C1 will be weakened due to the relatively small size of C3. In order to enhance the detection performance of the battery-powered system, the coupling between the device and the earth can be strengthened by increasing the ground area of the system or using a physical grounding method, so as to increase the value of C3.
Mains power supply systems exhibit higher sensitivity due to the fact that they share ground with the earth. In such a system, the reference system is more stable (only one variable capacitance is involved: C1) because there is no need for capacitive coupling between the module and the earth (C3). As a result, well-grounded systems are more sensitive than battery-powered devices, so subtle changes in C1 can be more easily detected.
4. Technical challenges and future trends
Technical challenges: As devices tend to move towards thinner and lighter designs, maintaining high performance in a confined space is a major challenge for capacitive sensors.
Influence of power supply mode: The sensitivity of the sensor is significantly affected by the power supply method, and the design needs to consider optimizing capacitive coupling to improve detection performance.
Future trends: Capacitive sensing technology is moving towards higher sensitivity, smaller size, and lower power consumption due to low cost, which indicates more innovative applications and devices in the future.