AVAS and the dilemma of ‘too good to be true’ designs

Article By : Bill Schweber

Acoustic vehicle alerting system is a classical example of no good engineering deed goes unpunished meets the law of unintended consequences.

One of the many challenges which engineering faces as a profession and a discipline is that doing good and delivering of technical advances soon leads to additional expectations. I’m not talking about foreseeable situations such adding more memory at low cost to a PC leading to more-powerful applications running on that PC, followed by the demand for even more memory to handle these applications more efficiently. What I mean is where new features trigger a need for unanticipated enhancements. I think of it as “no good deed goes unpunished” meets “the law of unintended consequences.”

For example, automotive designers have worked over the past few decades to make the cars quieter inside and out. Through the use of better streamlining and airflow, acoustic insulation, specialty layered materials, and countless improvement throughout the powertrain, they have cut the dB level to where it’s hard to know if the engine is actually running even in gasoline-powered vehicles. The transition from internal combustion engines (ICE) to all-electric and hybrid vehicles (EV/HEV) has been the final “triumph” in drastically reducing the noise generated by the car.

And that’s a problem. At low speed, the inherent wind and road noise generated by any vehicle is quite low, and the EV/HEV powertrain is also low to inaudible. The result is that pedestrians—whether vision-impaired or the ones not paying attention—and cyclists might not know a vehicle is coming their way.

Consequently, regulatory authorities in the United States, Europe, and elsewhere have mandated a minimum noise level that must be emitted from the vehicle when it is traveling under 30 km/hr or 18.6 mi/hr. In some ways, the message to engineers is “sorry, but you’ve done too good a job at quieting the car.”

The function even has a snappy name along with an easy-to-say acronym: acoustic vehicle alerting system (AVAS), which makes for a nice verbal pairing to the better-known advanced driver assistance system (ADAS). It’s estimated that implementing AVAS will add between $100 and $150 to the cost of the car, although that figure has lots of “maybe’s” embedded in it.

Of course, every such challenge also provides an opportunity for some. There are different ways to meet the AVAS requirements, starting with the infotainment subsystem and its audio amplifier (Figure 1).

Figure 1 The audio system in the car is the basis for whatever enhancements are needed for AVAS. Source: STMicroelectronics

But that’s where things get tricky. While the audio subsystem is nice, it is not safety-enabled; on the other hand, AVAS is. Now, all the elements of the AVAS subsystem need to be compliant with the more-stringent Automotive Safety Integrity Level (ASIL) rating in addition to the standard AEC-Q100 reliability rating.

ASIL is a risk classification scheme defined by the ISO 26262 and is established via risk analysis of potential hazards by looking at the severity, exposure, and controllability of the vehicle operating scenario. There are four ASILs identified by the standard: ASIL A (least stringent), ASIL B, ASIL C, and ASIL D (most stringent), as shown in Figure 2. AVAS is classified as an ASIL B function.

Figure 2 There are four levels of risk classification scheme associated with the ASIL, assigning a criticality rating to each subsystem. Source: Synopsys

Vendors see the opportunity and are introducing components which are both AEC-Q100 and ASIL B qualified, such as the STMicro’s HFDA801A, a digital input class-D automotive audio amplifier with a wide set of advanced diagnostics, attributes you would barely grasp from looking at its block diagram (Figure 3).

Figure 3 The HFDA801A Class D audio amplifier, targeting AVAS applications, must meet ASIL-B standards in addition to its AEC-Q100 reliability rating. Source: STMicroelectronics

Have you ever been involved in a project where “doing too good a job” has led to requests or demands to do more or where the unforeseen consequences of doing that good job bit back at you? Did these add-ons make sense or were they semi- or even non-sensical?

This article was originally published on Planet Analog.

Bill Schweber is an electronics engineer who has written three textbooks on electronic communications systems, as well as hundreds of technical articles, opinion columns, and product features. In past roles, he worked as a technical website manager for multiple EE Times sites and as both Executive Editor and Analog Editor at EDN. At Analog Devices, he was in marketing communications; as a result, he has been on both sides of the technical PR function, presenting company products, stories, and messages to the media and also as the recipient of these. Prior to the marcom role at Analog, Bill was Associate Editor of its respected technical journal, and also worked in its product marketing and applications engineering groups. Before those roles, he was at Instron Corp., doing hands-on analog- and power-circuit design and systems integration for materials-testing machine controls. He has a BSEE from Columbia University and an MSEE from the University of Massachusetts, is a Registered Professional Engineer, and holds an Advanced Class amateur radio license. He has also planned, written, and presented online courses on a variety of engineering topics, including MOSFET basics, ADC selection, and driving LEDs.


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