New capacitive sensing technology provides a more reliable and safer hands-on detection

The AS8579 sensor offers the simplest way for car makers to comply with the UN Regulation 79, while giving the best detection performance


For automotive design engineers, it is unusual to find a new technology solution which performs better than existing approaches, and which reduces cost, and which is easier to implement in the application. But that is exactly what a new capacitive sensing chip, ams’ new AS8579, offers when used for hands-on detection (HOD) in cars which provide driver assistance functions. 
It is the result of the application of a familiar and proven measurement principle – I/Q demodulation – to the job of sensing the position of the driver’s hands on the steering wheel. And it is markedly superior to any of the existing technologies in use for HOD in cars. Watch the highlights in our video:

 

 

Essential safety requirement in new car designs

The HOD function is required by the United Nations Regulation 79, and applies to all new cars that have a Lane Keeping Assist System (LKAS) wherever ratified. It has already been adopted by the European Union for new production vehicles from 1 April 2021. The purpose of the HOD system is to continuously monitor the readiness of the driver to assume control of the steering system in an emergency, or in the event of the failure of the LKAS.

Various technologies have been developed to provide this HOD function, but have had limitations: it is possible for drivers who want to avoid holding the steering wheel to fool the current monitoring system, which could compromise safety. And some existing solutions also perform poorly in certain operating conditions. 

One approach to HOD has been the torque sensor: this detects the continual, minute deflections produced when the driver grips the steering wheel. The big drawback of this technology is that it can be easily fooled: the driver may take their hands off the wheel and ‘hold’ it by pressing upwards against it with their leg. 

The problems with torque sensors have led the car industry to adopt a form of capacitive sensing for HOD: it monitors the driver’s grip on the steering wheel by detecting the change in capacitance of the steering wheel when the driver’s hands – which absorb electrical charge – come into contact with it. This technique only requires a single sensor chip connected to a metal sensor element built into the steering wheel. 

Until now, automotive system manufacturers have used the charge-discharge method of capacitive sensing: this is a well understood technique, as it has been applied for many years in products such as touchscreens and touch-sensing buttons. But detection fails when the driver wears gloves, and false detection signals generated by the presence of moisture or humidity undermine the safety performance of hands-on detection based on this method of capacitive sensing. This type of capacitive sensor can even be fooled if the driver wedges a capacitive object, such as a piece of fruit or a plastic water bottle, into the frame of the steering wheel. So again, the implementation of this charge-discharge method of capacitive sensing potentially compromises safety. 

It is true that other technologies are already applied to other driver-monitoring functions. For instance, 2D optical sensing is in use in systems for monitoring the position of the driver’s head. However, these 2D optical-sensing systems are not capable of performing HOD. This means that capacitive sensing is the most viable technology for HOD that is ready for deployment today. And now ams has a new approach to capacitive sensing which will meet all the safety requirements imposed by the automotive industry, and which is simple to implement.
 

Better performance, lower cost

This new solution from ams provides better performance, and with fewer components than the existing charge-discharge technique for capacitive sensing. 

By implementing reliable capacitive sensing based on I/Q demodulation, the AS8579 capacitive sensor performs HOD in a way which cannot be fooled. Like the charge-discharge method, I/Q demodulation is a proven and well-known technique for capacitive sensing. Its advantage is that it measures the resistive as well as the capacitive element of a system’s impedance. The effect of this is that, unlike the charge-discharge method, it works reliably in difficult conditions, such as in the presence of moisture, or when the driver is wearing gloves. And it cannot be fooled, so provides for assured detection of the driver’s grip on the steering wheel. And the added benefit of the AS8579-based solution is that it can operate via a heated steering wheel’s heater element, so it does not require a separate sensor element to be built into the steering wheel. 

This is how the AS8579 eliminates the normal trade-offs in engineering design: 

  • It performs better – it cannot be fooled, and it operates in all conditions
  • It costs less – it is a single-chip solution, and requires no dedicated sensing element in a heated steering wheel
  • It is easy to implement – the chip’s output is an impedance measurement, and the system controller simply applies a threshold value to determine whether hands are on the steering wheel or not. 
     

Ready for use in automotive designs

The AS8579 is fully automotive qualified, and offers multiple on-chip diagnostic functions, ensuring support for the ISO 26262 functional safety standard up to ASIL Grade B. Operating at one of four selectable driver-output frequencies – 45.45kHz, 71.43kHz, 100kHz or 125kHz – the AS8579 offers high immunity to electromagnetic interference.

Automotive designers can start developing with the AS8579 automotive capacitive sensor immediately using its dedicated evaluation kit, the AS8579-TS_EK_DB. 

For more technical information or for sample requests, please go to capacitive sensor AS8579.