Mutual capacitive touch sensors improve vehicle user interfaces
There is huge potential for touch control technology in the automotive sector but also major challenges to overcome. Before embarking on new designs, engineers need to take these into account. They need to specify semiconductor technology with the signal integrity to boost touch sensor performance and the configurability to optimize the system accordingly.
Figure 1. Functional block diagram of the LC717A30UJ
By Tetsuya Tokunaga, ON Semiconductor Download PDF version of this article
Consumers have developed greater expectations with regard to the performance and intuitiveness of touch sensor interfaces. This is quite clearly a consequence of the experiences they have had with items of portable electronics such as smartphones and tablets. The vast majority of people will therefore not accept anything less effective when it comes to other application areas, as touch control starts to see more widespread proliferation in a wide and varied selection of end markets. They want to derive the same sort of seamless smooth, glitch-free operation that they are used to elsewhere. Here we will look at the implications this has now that touch technology is starting to become popular in automotive designs.
The opportunities for touch-based control within the automotive sector are just starting to be appreciated by manufacturers and their tier one technology partners, providing them with a way to offer product differentiation in what has become an extremely competitive market. There are many places within modern vehicles that have been identified where this can be applied. Among the most prominent of these are HVAC controls, smart key entry systems (based on proximity sensing) and body electronics functions such as power window lifters. Of course, the decision-making process that relates to specifying touch sensing solutions for such functions is very different from what would be applicable for portable electronics, home entertainment systems, white goods and suchlike. There are, as we will see, a number of important and highly distinctive aspects that define automotive deployment, which simply do not appear in other markets. These need to be given adequate consideration, or the touch system will fail to meet the application performance, reliability and longevity requirements.
The automotive environment is typically harsh and uncompromising. This is why electronic components that are used on vehicle systems need to have attributes that address this. When looking to utilize a touch sensor within such a setting, there are key criteria that need to be considered. Firstly, the touch sensor is going to be exposed to elevated levels of electromagnetic noise - with the various electric motors present in the vehicle, as well as the cable harnessing, the alternator coil and a multitude of other sources all contributing. If not properly addressed, this noise could impair the reliable performance of the touch system. Secondly, the nature of where these touch sensors are going to be situated means that they may have to contend with various physical stresses, such as mechanical shock, vibration, and elevated temperatures. Rugged construction thus becomes mandatory. Capacitive touch sensors are, for these reasons, deemed to be the most suitable.
Next, as there can be significant variations in electrical system level parameters from one vehicle to another, there needs to be scope for fine tuning before the car comes off the production line. Finally, given that there is such a plethora of different applications within the cabin and on the car exterior which could benefit from touch functionality, the sensor should include various features to match specific application needs; the ability to support different design arrangements, while keeping the number of components involved as low as possible, is important. As an example, some designs may result in the presence of an air gap between the sensor/PCB and the protective cover. This will normally mean that a light guide has to be incorporated into the set up (with repercussions in terms of both the bill of materials cost and the associated engineering effort). Employing technology that can alleviate problems of this kind will prove beneficial.
Figure 2. Use of mutual capacitance sensing in a smart key application
Capacitive touch sensors can be based on either of two different sensing technologies - these are self-capacitance and mutual capacitance. With self-capacitance, an increase in capacitance level is detected when the finger of the user approaches the sensor electrode. Though widely used, self-capacitance touch sensors can be susceptible to parasitic capacitance effects. They also have recognized limitations in terms of the range they can achieve. This means that they are not suitable for application scenarios where an air gap will be present. In contrast, a mutual capacitance arrangement has two separate electrodes and detection is based on a decrease in electrode force lines between them as the user finger comes closer. This methodology, provided that it is accompanied by good signal conditioning, supports longer range operation and is not impacted by parasitic capacitance to the same extent as self-capacitance. As a result, it is better able to deal with air gaps and inherently noisy application environments.
Using a mutual capacitance approach allows automotive engineers to deploy touch sensors that can serve a number of different purposes. The heightened sensitivity means that they can be utilized not just for touch but in proximity sensing, for instance. Figure 2 describes its use in a smart key vehicle entry application, where the driver is able to unlock the car door without having to come into contact with the door handle, through a change in capacitance being determined from some distance away. Figure 3 shows how this type of sensing system may also be applied equally well to a power window lifter control panel application. The nature of the cabin design means the protective cover that the vehicle occupant touches is not likely to be in direct contact with the PCB on which the sensor is mounted therefore leaving a sizeable air gap. Via a mutual capacitance sensor arrangement, the need to fill this gap with a light guide (along with complex and expensive to implement optical bonding) can be avoided. The result can be a more streamlined and cost-effective system solution.
Figure 3. Mutual capacitance sensing applied to a power window lifter control where an air gap is present.
Advanced capacitance-to-digital converter LSI technology with industry-leading dynamic range from companies like ON Semiconductor can support significant performance improvements in capacitive touch sensors and thereby enhance their operation in automotive applications. Designed for use with mutual capacitance touch sensors, ON Semiconductor LC717A30UJ sets new standards in relation to touch sensitivity thanks to its parasitic capacitance cancellation mechanism. It also has built-in noise rejection capabilities that help to mitigate the effects of electromagnetic interference often present in an automotive surrounding. This LSI can support the detection of capacitance changes down to femto-Farad (fF) levels, with a range of up to 150 mm being attained. As a result of this, it is not just equipped to deal with conventional touch functionality, but also proximity sensing and sophisticated gesture recognition. In addition, it means that this device is able to operate when there is an airgap between the sensor/PCB and the protective cover. The need for inclusion of a light guide in the user interface design is thereby eliminated, allowing marked reductions in the bill of materials costs to be realized.
The LC717A30UJ has eight capacitance sensing input channels, making it highly suited to inclusion within systems that require an array of switches. Among its other features are an analog-to-digital converter for data translation, a dual-stage amplifier for determining capacitance changes on the analog outputs and an integrated multiplexer for input channel selection. I2C and SPI serial interfaces can be selected from, depending on the specific application requirements of the system. Furthermore, this highly robust AEC-Q100 compliant IC has a built-in automatic calibration function. This optimizes and self-calibrates the touch sensor system with regard to the characteristics of the electrode, as well as line capacitance and the surrounding environment. It simplifies the installation process and maximizes the effectiveness of the user interface in-situ, presenting the industry with a single chip touch/proximity sensing solution that is both accurate and reliable.
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