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An introduction to capacitive sensing

02 Jul 2012  | Pushek Madaan;Priyadeep Kaur

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Capacitive sensing finds use in all kinds of industrial, automotive, medical, and consumer applications. The popularity of this technology, especially in HID (human interface devices), 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 touchscreens 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: self and mutual capacitance

Surface 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

(Figure 1).

surface capacitance

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.

Self capacitance

In the case of self-capacitance, each capacitive sensor is a conductive pad laid on a PCB, surrounded by a ground pattern (Figure 2).

sensor pad

sensor pad on a PCB

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 (Figure 3).

finger capacitance added to a sensor

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.

Mutual capacitance
This is the latest addition to the catalog of capacitive sensing techniques. Such capacitive panels have two conductive layers stacked together with a very thin separation (Figure 4).

dual layer ITO-mutual cap

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.

mutual capacitance

Figure 5: Mutual capacitance between Rx-Tx

As every intersection has its own mutual capacitance and can be independently tracked (Figure 5), this method provides a distinct advantage for detecting multiple fingers.

Which method is right for your application?

sensing technologies comparison

Table 1: Sensing technologies comparison

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.

4. 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).

Sigma delta circuit for touch sensing

Tutorial capacitive sensing
Here, the sensor capacitance is continuously switched between two voltage levels using two non-overlapping clocks driving Sw1 and Sw2 respectively. This emulates a resistance of R=1/fCsensor where f is the switching frequency of clock1 (Figure 7).

sigma delta circuit with sensor capacitance

The output of the flip flop in the above circuit is a density modulated bit stream, the duty cycle of which increases with a decrease in resistance. This bit stream goes to the enable signal of a timer, which measures the amount of time for which the F/F output is high during a given time T (decided by the timer resolution and clock frequency).

Bringing a finger close to the sensor increases Csensor and hence decreases Rs. This leads to an increase in the average duty cycle and hence the timer output value. Note that any noise on the input of the comparator is now integrated over n cycles of clock (here n is the timer resolution) and so the effect of noise is lower in such implementations.

Reliability considerations
It is important to quantify various signal and noise parameters associated with any system in order to evaluate its performance and reliability. While reporting a finger press, it becomes crucial that only a valid finger press should be reported i.e. a noisy environment should not cause spurious detections.

Any unwanted change in the signal, whether from devices operating in its vicinity or environmental changes, can be termed as noise. When measuring a change in capacitance of the order of femto Farads, a small amount of noise can also lead to spurious detections. Before going into the techniques of how to build a system where the effect of noise is minimal, it is important to understand the possible sources of noise.

Different types of noise
A capacitive sensing system is mainly susceptible to noise generated from the following three sources:

1. Radiated noise—Any operating circuitry radiates energy that can potentially create problems with the operation of other circuits in its vicinity. Capacitive sensing buttons constitute only the user interface part of any system, and generally there is much circuitry sitting behind the user interface. This nearby circuitry can also radiate noise if not properly designed. Sources of noise can be the LCD (liquid crystal display), switching power supplies, mobile phone, Wi-Fi radio, etc.

2. Conducted noise—A noisy power supply is the most common source of conducted noise. The increasing demand of low cost implementations forces developers to use less expensive supplies which in turn generate more noise. This can adversely affect the operation of the sensor. A human body touching the sensor can also couple a 50/60Hz common mode noise into the system.

3. Environmental changes—Changes in environmental parameters such as humidity, temperature, and device aging also change the capacitance of the sensor. Such unwanted changes can also be termed as noise.

So how do we ensure that noise will not cause spurious detections? If we define a signal-to-noise ratio for a particular system and ensure that the noise in the system is much less than the required signal, we can ensure spurious free detections. Here is how we define signal and noise in such a system:

Signal: Signal can be defined as the change introduced by the finger in the timer output value.

Noise: When a finger is not present, the timer output will vary by a small amount for each scan because of various noise sources. The consolidated effect of such noise can thus be measured by monitoring the peak-to-peak variance in the timer output value for a number of scans.

Figure 8 illustrates SNR.

signal and noise

It might look a little tricky, but maintaining a good SNR is not very tough. There are multiple techniques for achieving a good SNR, which are as follows:

1. Tuning (Auto or Manual)—Calibrate the device during the design phase to ensure that it exhibits a minimum of 5:1 SNR for fail-safe operation. Using some software overhead, this manual tuning effort can be switched to auto-tuning wherein the device calibrates itself in the field to ensure that it achieves the minimum SNR required. Cypress' SmartSense solution is an example of such an innovative technique.

2. Auto correction—Gradual changes in capacitance because of temperature, humidity, or component agingcan be compensated by monitoring the counts obtained (digital representation of capacitance) in firmware and updating the reference signal with the gradual change observed. Note that in the case of auto-calibrating solutions, it is also possible to recalibrate the system if the gradual change in counts exceeds a particular threshold.

3. Layout—A proper schematic and PCB layout is address all the problems mentioned above.

4. Filters—Software filters used to process the digital counts obtained can also be used to improve the SNR. The use of filters increases response time but improves SNR dramatically. Depending upon system requirements like response time and power consumption, the use of software filters may be feasible.

Authors' biographies
Priyadeep Kaur has completed her BE in Electronics and Electrical Communication Engineering from PEC University of Technology, Chandigarh and is currently working with Cypress Semiconductor India Pvt Ltd as an Application Engineer. Her interests are embedded systems, analog circuits, and DSP. She can be reached at pria@cypress.com.

Pushek Madaan is currently working with Cypress Semiconductor India Pvt Ltd as a Senior Application Engineer. His interests lie in designing Embedded system applications in C and assembly languages, working with analog and digital circuits, developing GUIs in C# and, above all, enjoying adventure sports. Pushek can be reached at pmad@cypress.com.




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