Editor's note: In this Product How-To design article, Nicky de Jager and Gerrit Avenant of Azoteq investigate the possibilities of capacitive touch technology in the design of alternatives to electromechanical switches. They explain the challenges of such designs and detail how the company’s patented SwipeSwitch technology deals with them.
The limitations of conventional tactile or electromechanical switches have made electronic counterparts not only desirable, but necessary. The susceptibility to mechanical failure and the relatively high cost of tactile switches are only two of the factors fuelling the trend to replace them with electronic switches. With modern processing speeds that allow sampling periods in the nanosecond range, the switching transients or “bounce” of conventional switches have become unacceptable.
For these reasons, electronic switches such as capacitive touch sensors are becoming the technology of choice for modern applications. The benefits of capacitive sensors include reliable, invariable functioning, better sensitivity, higher levels of safety, improved immunity against aqueous substances, and ultra low power consumption, especially for portable and battery operated devices.
Thus capacitive touch sensors are an ideal approach to designing the electronic switch, but the technology does have minor limitations.
All materials or objects possess a certain electrical permittivity or dielectric constant, thus the detection of foreign objects in the sense environment is inevitable and the unintended activation of a device may be produced by metallic objects or electrically conductive or ionic solutions. This may raise safety concerns for use in products such as hot appliances (e.g. stove tops, hairdryers etc.)
Further limitations of capacitive sensors include their uninterrupted power dissipation and the fact that portable devices are introduced to continuously changing environments, which may influence the sensor’s sensitivity due to a varying reference potential.
However, by implementing refined semiconductor technology together with innovative capacitive sense electrode designs and advanced processing algorithms, an intelligent capacitive “swipe-switch” may be implemented to circumvent these limitations.
Design and Implementation
A capacitive swipe-switch is based on a 2- or 3-channel capacitive sense electrode. The signals must satisfy specified control algorithms to register as valid swipe or gesture actions. This concept is illustrated in Figure 1, which depicts a simple 3-channel self-capacitance sense electrode that can be implemented to perform the swipe detection.
User input is identified by sequences of a combination of input states, where a number (e.g. 1, 2, or 3) indicates a touch condition/state on the corresponding channel and a z-character indicates a zero condition/state.
For a 2-channel electrode, a simple swipe or gesture can be seen as a touch on the first electrode (1z), followed by a touch (12) on both electrodes, and lastly followed by a touch on the second electrode (z2). The required sequence of state combinations can be summarized as: 1z, 12, z2. If a swipe event is to be recognized in the opposite/reverse direction, the required swipe sequence will be: z2, 12, 1z. Any combination of states not seen in these orders will clear the current state machine, and the switch will wait for the next valid start condition.
Depending on the design of the sensing electrodes, several small changes can be made to the state machine to improve the quality of the swipe sequence:
A minimum and maximum number of valid samples can be required per state combination. This ensures that a finger should remain a certain amount of time on an electrode, reducing the probability of unintended swipe events. However, this may limit the speed at which the swipe or gesture can be performed.
- An overall sequence time can be imposed.
- Zero state combinations can be allowed. Zero states or “no touch” conditions may be allowed for distant spaced electrodes.
- A zero state combination can be appended to the required sequence. This requires that the touch condition on the last electrode be cleared before a valid swipe event is recorded. This reduces false activation by semi-stationary and foreign objects.
Implementing a 3-channel sense electrode will result in a more robust swipe sensor, due to the requirement of a strict sequence of state combinations. Examples of possible sequences of state combinations include:
- 1zz, z2z, zz3
- 1zz, 12z, z2z, z23, zz3
- 123, z23, zz3
With modern day capacitive sensing solutions, the designer has the choice of implementing either a self- or projected capacitive sense electrode. Figure 2 illustrates examples of simple 2- and 3-channel sense electrodes in configurations that can be utilized for the detection of swipe or gesture events.
Figure 2: Simple 2 and 3-channel sense electrodes in both self- and projected configurations
Typical user interfaces (UIs) that can be designed to complement a swipe-switch configuration include the following:
- Single direction – only one direction is allowed
- Bi-directional – swipes in both directions toggle the same response. This is useful for applications where the first swipe can be in any direction.
- Directional – for specific applications where responses are dependent on the direction such as volume control or a light switch.
- Dual swipe – A swipe in one direction followed by a swipe in the opposite direction toggles the output. This can be used for applications where more switching reliability is required.
- Combined swipe and button actions provide more UI options.
To mitigate the limiting criteria of capacitive sensors, we’ve developed the IQS213 ProxSense SwipeSwitch with proprietary techniques that make it possible to provide an effective swipe or “gesture” activation of an electronic product with a high degree of sensitivity and signal-to-noise ratios (SNR).
Compared to ordinary touch sensors, this new technology not prevents accidental activation, but it also features as a “Zero-Power” electronic switch. With current consumptions in the sub 2μA range with selected low power modes, the IQS213 offers extended battery life without sacrificing performance, while the average device current consumption is negligible compared to almost any realistic load.
Through patented Automatic Antenna Tuning Implementation (ATI) algorithms , the SwipeSwitch can be installed easily in a wide range of applications and designs with various non-conducting overlay materials, including wood, plastic and glass. Furthermore, automatic drift compensation and advanced parasitic capacitance cancellation abilities make the IQS213 a good fit for portable and battery-operated devices. These features, together with smart processing algorithms also results in better immunity against aqueous substances, without the implementation of sensor shield- and/or guard-electrodes (certain layout guidelines apply). In addition, the high sensitivity of the ProxSense devices allows the activation of a product or device by detecting successful swipe gestures through dry or even damp gloves and other protective gear.
The various programmable user-input configurations allow reliable swipe detection over distances as short as 8mm and as long as 100mm, in straight or curved electrode arrangements, and at swipe speeds varying from approx. 35-500 mm/s.
Additional features of IQS213 device include internal voltage regulation, an internal reference capacitor (Cs), and the use of a small outline package (MSSOP10). These features make possible the design of applications with a minimal component count that can be easily integrated into compact devices.
The advantages of a capacitive switch as opposed to conventional tactile switches include:
- Simplified manufacturing
- Waterproofing is easier since no holes have to be made for the switch
- No mechanical wear and tear (no moving parts)
- More intelligence – better user interfaces can be designed, with more features and better security
Ideally these electronic switches should also have the following properties to be viable replacements for normal switches:
- Low power – with sub 5µA current consumption considered as “no current”
- Response rate – low power characteristics not influencing the response time of the switch
- Robustness – false switching should not occur with accidental bumps and touches
- Reliability – a swipe or gesture should be easily recognizable
- Water immunity – since these switches can be implemented in a variety of products and environments, immunity against aqueous substances is imperative
In general, capacitive swipe switches can be constructed using a microcontroller and a capacitive touch solution, which features at least two sense electrodes. However, such configurations are not always practical due to cost and space requirements, and full control of the sampling rate of the required state combinations has not been possible. The IQS213 has been designed to provide this full control, plus the properties listed above, without the lengthy development time and cost that would otherwise be necessary.
Nicky de Jager has a B.Eng. degree in electrical and electronic engineering (2009) and a M.Sc.Eng. degree (2011), focused on the application of carbon nano-materials in gas sensors, at Stellenbosch University. He joined Azoteq in September 2010 as an application engineer, focusing on product development for ProxSense applications.
Gerrit Avenant has a B.Eng. degree in electrical and electronic engineering with computer science (2007) and a M.Sc.Eng. degree in the autonomous flight control of an airship (2010) at Stellenbosch University. He has been employed at Azoteq since 2011 as an application engineer with focus on firmware development for ProxSense applications.
1. J. Viljoen, Azoteq: “Design capacitive touch systems for robustness and manufacturability” . Embedded.com, 2011.
2. Azoteq Datasheets and Application Notes