Commercial Vehicle Solutions
City Bus CMS: Brake smarter, not harder!

City Bus CMS:
Brake smarter, not harder!

Safety through sensors

The best way to greater traffic safety is avoiding crashes altogether. That’s why one aspect of the City Bus Collision Mitigation (short: City Bus CMS) concept is a forward collision warning feature (FCW). ZF’s state-of-the-art camera and radar continuously monitor traffic. If they detect an obstacle in the path of the bus, City Bus CMS issues a warning – both as an acoustic signal and a visual display on the driver’s dashboard.

At the same time, the active braking assistant – another crucial component of ZF’s City Bus CMS – initiates in order to reduce speed as much as possible. However, calibrating the braking force for a bus is a lot trickier than a conventional car.

ZF’s City Bus CMS relies on inhouse developed state-of-the-art sensor technology

Stopping is easy, braking is hard

Everybody who ever had to suddenly hit the brakes while driving will remember the forces acting upon the body, accelarating both themselves and unsecured bags towards the dashboard. While passengers in a car can rely on seatbelts to break this motion, the situation looks different in a bus packed full of people: no seatbelts, many passengers standing, unaware of the impending collision ahead. “Coming to a sudden full stop might avoid a crash, but passengers can still get injured from falling over and bumping into each other,” explains Helmich. Therefore, balancing these safety requirements was crucial to the development of City Bus CMS. The key question: how much braking force is necessary to maximize the chance of avoiding a crash while still keeping occupants inside the bus safe?

ZF’s City Bus CMS provides active braking support tailored to city bus applications in complex urban environments
Helmet and harness: designing the braking profile

To figure this out, Advanced Driver Assistance Systems (ADAS) experts from ZF’s Commercial Vehicle Solutions (CVS) division went above and beyond the usual testing protocol. At the company’s test track in Jeversen, Germany, the team strapped on safety harnesses and helmets, then picked up a questionnaire and boarded a city bus.

Testing, documenting and evaluating the experience at different speeds and braking forces from a passenger perspective helped to determine how much deceleration intensity is acceptable at which speed. “After that, we had to transfer these findings into algorithms for the system’s control software,” says Jakob Schmidt, Customer Requirements Engineer at ZF. “What we did was integrate the data with ZF’s advanced braking capabilities that are precisely calibrated to the vehicle’s speed and weight. That way, we achieve a seamless interaction between the ADAS and braking system. Braking pressure is carefully applied across the braking profile, making passengers far less likely to be jolted and put off-balance,” Schmidt explains. “Designing the breaking profile like that minimizes the adverse effect on the passengers while still providing the best possible support for the bus driver in these challenging situations.”

Smart braking is the way forward

City Bus CMS is independent of both manufacturer and platform. This means that it can be applied to buses from any OEM, both with conventional engines and electric drives. “As more and more public transit authorities and OEMs switch to sustainable drivetrains, we planned for a system that can be easily integrated into a broad range of vehicle models. Mastering the braking maneuver is also important for autonomous driving, where city bus applications are a remarkably interesting use case,” says Philipp Helmich. “So, with this project we are not only entering a new market segment for our Advanced Driver Assistance Systems technology. We are also developing a critical building block for higher automation levels – in line with our company’s goal of mobilizing commercial vehicle intelligence.”