Safety options expand

Thanks to radar, sensor technology, and on-board computer processing power, safety is spreading in cars. Two new developments: radar-based adaptive cruise control and rollover injury protection.

Radar love. Automotive radar is one of those technologies that, from the 1950s on, was always to be in the cars of the future. Now it's here.

Radar systems have been proposed for collision avoidance and intelligent cruise control. Mercedes'introduction of radar-based Distronic (distance electronic) Adaptive Cruise Control, optional on its S- and CL-Class cars, goes a long way to such complete systems while providing a driver aid that is more than just technology for technology's sake. DaimlerChrysler and Automotive Distance Control Systems (ADC; Lindau, Germany) engineers developed the system. ADC is a joint venture of Temic, Continental Teves, and Leica, and supplies the Distronic cruise control to Mercedes.

The Mercedes Distronic Adaptive Cruise Control 'following distance' display within the speedometer gives a driver radar-determined distance to the vehicle ahead, out to 150m (490 ft). The three radar antennae at the front of the S- or CL-Class cars project a 10 degree-wide beam to capture vehicles ahead.

Many drivers feel that cruise control is often an annoyance rather than a convenience, because "cruising" is becoming less and less possible. On today's heavily-travelled roads, drivers often encounter a cluster of traffic that causes them to disengage the system or else start maneuvering so they won't have to. Now Distronic control looks to take away much of that hassle and add a safety benefit.

The system is designed to interface with the driver as much like conventional cruise control as possible. The driver sets the speed as usual, but uses a thumbwheel on the center console to select a desired radar-aided following distance within a safe range, as indicated on a display within the speedometer. Normally the system initializes on a separation distance corresponding to 1.5 seconds of vehicle travel. This interval (and corresponding distance) can be varied between one and two seconds. The system also alerts the driver with an audible chime and flashing red triangle on the display if the car ahead slows rapidly.

While conventional cruise control governs only the throttle to maintain a constant, open-loop speed, Distronics adds braking to maintain separation distance. Three front-looking microwave radar transmitters and receivers, positioned in the grill, generate microwave (77 GHz) radar pulses. These are used two ways: radar ranging to determine distance to the car ahead, and Doppler-shift of the returning pulses, to provide relative velocity between the two cars. To keep following distance control sufficiently updated, 40 MHz digital signal processors are necessary.

The Distronic system uses an additional solenoid valve to activate no more than 20% of the braking power, thus leaving control of hard braking in critical situations with the driver. The system produces a maximum 2-ft/sec ² deceleration, and often braking inputs are so slight as not to be noticeable, according to ADC. Algorithms allow for speeding up, with an accompanying increase in safe separation. For instance, if programmed for 30 mph and 150 ft following, and the driver accelerates to 60 mph, the distance before warnings and braking increases proportionally to 300 ft. When not in the Distronic mode, the system can still provide the separation alerts via the display and chime or display alone.

ADC says that in testing, stress on drivers using Distronic, based on monitored heart rates, "clearly decreased."

Rollover protection.

"SUVs are as safe or safer in protecting their occupants from serious injuries as like-size passenger cars," says Helen Petrauskas, Ford vice president for environmental and safety engineering. "But they tend to be involved in different types of accidents," she adds. By simple physics, because of their high center of gravity relative to their width (wheel track), SUVs are more prone to rollovers than cars. Occupants' upper bodies may be repeatedly slammed against the sides of the vehicle interior during rollovers caused by impacts or road conditions and maneuvering.

About half of all SUV fatalities involve rollover, the company says. Because they were not wearing seatbelts, many of these victims were ejected from the vehicle—and were ten times as likely to be killed. "Occupants are best protected in SUV rollovers when they buckle up," notes Petrauskas. She adds that inflatable side-curtain-style airbags, or safety canopies, along with the rolling sensor and control software Ford is introducing as part of its SUV rollover protection system will help in protecting occupants, even if they don't use a belt. While side curtains have been around for a few years, this is the first time they are being used for rollover protection—with the key being sustained inflation for the duration of the accident.

Originally slated to be introduced on 2001 Ford SUVs, the side curtains will come out as an option in the all-new 2002 Ford Explorer/Mercury Mountaineer, which goes on sale in January 2001. The rollover function for the side-curtain system will debut as a follow-on option later next year.

In operation, a vibrating-mass roll-rate sensor, in a Visteon-supplied Restraints Control Module (mounted near the vehicle longitudinal axis between the front seats), measures the rolling of the vehicle. The module then determines if the vehicle is rolling over, triggering the canopy as fast as 130 msec. The bags extend from the headliner trim along the sides, and a single long canopy protects both front and rear seat occupants. As opposed to conventional airbags that deflate within 100 msec, the rollover-protection curtain bags stay inflated for six seconds, good for several tumbles of the vehicle. In addition, attachments at the front and tethers at the rear of the curtains hold it in place, helping to prevent occupants from being thrown from the SUV.

Ford's SUV rollover protection system uses a roll-rate sensor to activate side-curtain airbags which remain inflated during rolling for up to six seconds.

Jerry Chao, occupant safety supervisor for Ford, says other keys to the system include "sealed-bag technology with cool-gas inflators." The sealed bag features a weave coating that limits gas permeability, keeping the bag resilient for the required six seconds. The bag is also strengthened in its joint areas. He notes that the cold-gas inflator means heat transfer from the gas is limited, which also tends to maintain canopy pressure. If a more conventional, hotter gas were used for inflation, the higher rate of heat loss from a hot gas in a limited volume would reduce bag pressure too rapidly; thus turgidity would drop before the needed time interval. After six seconds, the curtain deflates via the gas permeating out through the fabric.

Chao adds that the rollover technology won't be retrofit into vehicles with only conventional side curtains because "there is a different wiring harness, controller, inflator, and bag."