Capacitive sensing finds use in all kinds of industrial, automotive, medical, and consumer applications. The popularity of this technology, especially in human interface devices (HID), has grown rapidly due to its ability to reduce manufacturing cost, increase product lifespan by eliminating mechanical components, and enhance product look-and-feel. Track-pads and touch-screens constitute the major applications, but we are also seeing a growing use of this technology in implementing touch-buttons and sliders. Be it mobile phones, TV controls, automotive dashboards, remote controls, or industrial controls, capacitive sensing-based touch buttons and sliders are proving to be much more appealing than mechanical switches and rotary encoders, both in terms of looks and reliability. Not only do these sensors allow a striking user interface but also offer a highly durable and reliable solution, provided they are properly designed and calibrated. Herein lies the concern of the engineers and designers trying to implement solutions based on this technology.
Designing low-cost, responsive, and low-power capacitive sensors for reliable operation in noisy environments is considered to be a tough job by many engineers. What refutes this myth is that application development with such sensors can indeed be a matter of few days, including prototyping through validation for production requirements. This article discusses various methodologies, guidelines and know-how related to all development phases, from design to production of capacitive sensing-based keypads, all of which contribute to reducing time-to-market by a significant amount.
Capacitive Sensing--An overview
A capacitive touch sensor is based on one of the following capacitance types:
1. Surface capacitance
2. Projected Capacitance: These are subdivided again into two types:
a. Self capacitance
b. Mutual capacitance
In this technology, glass is uniformly coated with a conductive layer. During operation, a voltage signal is applied to all four corners of the panel, resulting in a uniform electrostatic field. When a human finger touches the panel, it forms a capacitance where one plate is the conductive layer and the other being the human finger. Depending upon the location of the finger touch, current drawn from the four corners will be different and thus the capacitance seen by those corners will also be different. This difference can be used to determine the exact location of the touch (See Figure 1).
Figure 1: Surface Capacitance
This technique brings in all the advantages of capacitive touch technology as discussed above but is prone to false detection and requires special calibration during manufacturing.
In the case of self-capacitance, each capacitive sensor is a conductive pad laid on a PCB, surrounded by a ground pattern (See Figure 2).
Figure 2: Sensor pad on a PCB
This sensor forms a parasitic capacitance Cp with the surrounding ground pattern, and the electric field lines can be seen in the area above the sensor. When a conductor like a finger enters the area above this sensor, it alters the electric field lines and effectively adds a finger capacitance Cf to the sensor (See Figure 3).
This results in an increase in capacitance of the sensor from Cp to Cp + Cf.
By continuously measuring the capacitance of the sensor(s) in the system and looking for a sudden change in capacitance, a microcontroller can determine when the finger was placed on the sensor. Here, the absolute value, or the parasitic capacitance of the sensor does not matter. The microcontroller just looks for a sudden change in capacitance and if this change is above a particular threshold, a finger presence is reported.
This is the latest addition to the catalogue of capacitive sensing techniques. Such capacitive panels have two conductive layers stacked together with a very thin separation (See Figure 4).
In this technology, when a finger touches the panel, the mutual capacitance between the row and column is reduced. This reduction in capacitance is used to identify the presence of a finger.
Figure 5: Mutual capacitance between Rx-Tx
As every intersection has its own mutual capacitance and can be independently tracked (See Figure 5), this method provides a distinct advantage for detecting multiple fingers.
Which method is right for your application?
All the technologies mentioned in Table 1 have their merits and demerits for touchscreen-based applications. However, when it comes to implementing buttons or slider for a front panel where two buttons are separated by a significant distance, then the only economical solution is projected capacitance. This is because it works with PCB traces; all other technologies require that a special panel has to be designed. For this reason, this article concentrates on the use of self capacitance.
Measuring capacitance change
Measuring a capacitance is straightforward. Traditional methods are based on measuring the charging time of the capacitance or the resonating frequency of an RC circuit (as is done in LCR meters). However, these methods cannot be used directly for touch sensing because the change in capacitance of such a sensor would generally be in the range of a few tenths of a pF. If we try to measure the charging time/change in the resonating frequency of such a capacitor using a constant current, there would be three concerns:
1. Measuring such a small capacitance would require a high frequency clock and/or an accurate low current value for an optimum measurement.
2. While measuring charging time, a greater part of the time would be spent in measuring the self (parasitic) capacitance of the sensor (which is generally in the order of tens of pF) and the required measurement of the change would constitute only a fraction of the measurement time, thus leading to excessive use of controller time and power.
3. The effect of noise on such a system would be high.
So, how do we ensure less noise with a lower measurement time? The answer is to incorporate an integrating effect. This is similar to averaging the effect of noise on an ADC input (See Figure 6).