Automobile Hands-Off Detection, Part 4: Capacitive solutions

Cars with lane-keeping assistance (LKA) also are mandated to have a hands-off detection (HOD) function; there are several ways to meet this challenging requirement.

Despite the BOM advantages of torque-based HOD, it has performance issues. Capacitive-based sensing is a viable alternative approach.

Capacitive sensing for HOD 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 (Figure 1).

Fig 1: Capacitive sensing using long-established basic principles of physics and electronics, supported by advanced support circuitry. (Image: LinkedIn)

Capacitive-based sensing is a well-understood technique, as it has been applied for many years in products such as touchscreens and touch-sensing buttons. One vendor’s physical implementation (IEE) uses a highly flexible multi-layer sensor mat integrated into the steering wheel, with a miniaturized electronic and the connecting cabling installed in the steering wheel’s center hub (Figure 2). The system measures the current flowing from the sensing electrode towards vehicle ground, proportional to the capacitance. If a driver touches the steering wheel, the capacitance increases, resulting in a change current which can be detected.

Fig 2: The basic “hardware” for capacitance sensing uses a “belt,” which is a highly flexible multi-layer sensor mat integrated into the steering wheel, plus interface circuity and connector (Image: NHTSA/IEE)

However, simple capacitive-based sensing has weaknesses as well. Hands-off/on detection can be inconsistent or fail when the driver wears gloves. Further, false detection signals generated by moisture or humidity undermine the safety performance of this method of hands-on detection. It 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. Thus, implementing this charge-discharge method of capacitive sensing potentially compromises safety.

One solution is to enhance the simple sensor (transducer). More advanced classification is enabled by using a multi-zone HOD system which determines, for example, between a left and/or right-hand touch and the approach used by Alps Alpine (Figure 3).

Fig 3: The Alp Alpine system uses multiple capacitance sensors to improve the localization of the hands-on position and hands-off situation and enhance sensing integrity. (Image: eeNews European Automotive)

By dividing the steering wheel and grip patterns into smaller sections, more-efficient system switching and enhanced touch assessment reliability is possible. The sensor electrode is divided into four sections to identify various grip states, which allows for a more detailed assessment of driving status than a basic touching/not touching determination reached using an existing single-electrode sensor.

Even this approach still has limits and false indications. Some vendors use advanced modulation for the sensor with in-phase/quadrature (I/Q) modulation and synchronous demodulation to compensate for this shortcoming. Like the capacitive charge-discharge method, I/Q modulation and demodulation are proven and well-known techniques for capacitive sensing. Unlike that simpler method, however, the use of I/Q signals measures the resistive and the capacitive element of a system’s impedance, which enhances performance and is conceptually analogous to digital bit error rate.

As a result, its proponents, such as AMS-Osram with their AS8579 capacitive sensor IC (Figure 4), maintain that it works more reliably than basic capacitive sensing even under challenging conditions such as in the presence of moisture or when the driver is wearing gloves. The IC captures the current flow of a metal object and applies algorithms to determine the capacitive and resistive impedance through the steering wheel.

Fig 4: The AMS-Osram AS8579 capacitive-sensor IC uses advanced quadrature modulation and demodulation techniques for enhanced sensitivity in challenging situations. (Image: AMS-Osram)

The IC can be read via an SPI interface used for IC setup and configuration. They also claim an added benefit of the AS8579-based solution: 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.

In summary, automobile engineers have multiple ways of meeting the required HOD mandate imposed due to the presence of lane-keeping assistance. They can use torque-based algorithms or one of several capacitance-based approaches.

But those are not the only possibilities. Researchers are developing and evaluating very different approaches contact-free approaches. One such leading-edge method is based on an infrared camera located over the driver’s shoulder, which is used to image and interpret the position of the backs of the driver’s hands. As of now, these are only investigative projects in the lab, but as the old cliché says, “you never know what the future will bring.”

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