Arguably the most notable trend in automobile design over the past decade has been their increased electrification and intelligence. Yet, as our cars have been growing significantly in intelligence for a while now, the way that we interact with vehicles has only recently begun to change.
Touch screens in cars are a popular development in automobile HMI.
The human-machine interface (HMI) aspect of automobile design has found greater importance in the past couple of years. One technology that has greatly bolstered this development has been capacitive sensing, which has found itself integrated into a plethora of new in-vehicle applications. For this article, we’ll take a closer look at this technology and the different ways it has been designed into automobiles.
Capacitive Sensors: A Brief Overview
At its simplest, a capacitive sensor consists of a pair of adjacent electrodes that exhibit some capacitance between each other.
When a human hand, which is a conductive object, comes in close contact with these electrodes, an additional capacitance is introduced and can indicate the location of the hand relative to the electrodes.
Working principle of capacitive touch sensing. Image used courtesy of Cypress Semiconductor
The real trick to designing these sensors actually lies in the measurement of the change in capacitance: how are you supposed to accurately and reliably detect a change from 10.00pF to 10.05pF? While this topic could probably have its own textbook, it’s at least good to know some of the most popular methods of measurement.
These include charge transfer, successive approximation, sigma-delta modulation, and mutual capacitance measurement.
Capacitive Sensing and Automobiles
In more modern cars, you can find capacitive sensing in dozens of different applications. One of the most ubiquitous applications for capacitive sensing is the touch screen control of multimedia like the radio, navigation, and phone calls. Beyond that, capacitive touch has also started being integrated into door handles, used to detect a user’s hand and automatically unlock the car for them.
Example of a capacitive controller in an HMI center console system. Image used courtesy of Cypress Semiconductor
Now, capacitive touch sensing is also being used to detect the presence of a driver’s hands on the steering wheel. Historically, torque sensors have been used for this purpose, detecting the small deflections produced when the driver grips the steering wheel. The problem with this technique is that it can be easily fooled, causing a risk to drivers and other road users.
Capacitive sensing looks to be the answer to this problem, except it introduces a problem of its own: most sensors fail when a driver decides to wear gloves or if there is other moisture or humidity present on the sensor.
ams’ New Capacitive Sensor Uses I/Q Demodulation
Aiming to address this problem, ams has recently announced their newest product, a capacitive sensor that leverages “novel sensing techniques.” These techniques, ams claims, allows more reliable hands-on detection under all conditions.
AS8579 block diagram. Image used courtesy of ams
Instead of using the popular charge transfer method of capacitive sensing, which tends to fail under the aforementioned conditions, ams’ newest sensor, the AS8579, employs a method based on I/Q demodulation.
ams says that this new technique allows the AS8579 capacitive sensor to accurately and reliably detect a driver’s hands, helping to improve the safety of Advanced Driver Assistance Systems (ADAS), while also reducing the cost of the hands-on detection system.
A Boon for ADAS?
As automotive manufacturers continue to implement increasingly sophisticated forms of autopilot functions, the need for the proper detection of a driver’s hands has become crucial. For safety, these autopilots require the driver to be ready to assume control of the vehicle immediately in case the vehicle system fails, and hands-on detection is a key part of all systems for monitoring driver readiness.
As such, technologies to accurately and reliably detect a driver’s hands, regardless of the conditions, are crucial to the development of autonomous vehicles. This, amongst many other reasons, is why a strong understanding of capacitive sensing is so crucial.
