TRW's ARC Toward Better Dynamic Control

TRW's new Active Roll Control (ARC) system does not employ some hot new technology to control body roll. In fact, patents on many of the devices it uses have been around since the 1960s. But what it does do is slash system costs in half. And with the current cost cutting atmosphere in the automotive industry, that fact could clear the way to bringing better handling vehicles to the masses.

How It Works
"The Active Roll Control system we have developed is quite a simple design, compared with an active body control system which uses pumps and control valves and its own ECU and is very expensive," says Phil Cunningham,TRW's product director, Chassis Control (Livonia, MI). At the heart of the ARC system are two actuators that replace the drop links on the front and rear stabilizer bars and are piped to a control valve which is either driven by the engine's hydraulic steering pump or a standalone pump. A lateral accelerometer and a steering angle sensor are added to the usual array of sensors to provide data that determine how the actuators react to a given driving maneuver.

During hard cornering or severe avoidance maneuvers, the accelerometer and sensors detect the roll force created and the actuators are directed to apply a counterbalancing force to the stabilizer bars, which greatly reduces roll angle. In straight-ahead driving, the stabilizer bars are essentially disconnected and can move in concert with the wheels, up to the stroke of the actuators. TRW claims that this setup markedly improves the ride comfort of the vehicle.

According to Cunningham, "What makes our system different than others is that we have integrated the electronics into the hydraulic control unit in a similar way to an ABS unit. We have been able to utilize braking technologies from our vehicle stability control and apply it to ARC, which gives us a considerable cost advantage—up to about 50%—over our competitors."

ARC was originally developed by TRW's Steering and Suspension group about five years ago, but the research was low-key until Cunningham recently picked up the project and began looking at how it could be brought to market cost effectively. One key to making the system work was the braking expertise that came with TRW's acquisition of LucasVarity in May, 1998, which brought, among other things, Cunningham himself.

The Business Case
The original research for the ARC system was conducted with performance cars in mind, since it can help to soften the notoriously harsh rides of performance vehicles while maintaining stiffer suspension characteristics during cornering. But with the attention that vehicle rollover accidents have received in the last year, and the subsequent passage of the TREAD Act by Congress, which requires NHTSA to develop a dynamic rollover test, TRW is now looking to SUVs for the lion's share of the system's sales. Speaking of SUVs, Cunningham says, "We see this system as part of the solution to help the driver maintain control of the vehicle. The data we've gathered show that the vehicles are not rolling on the road, but because they are going off the road. This system should help the driver maintain control and keep that vehicle on the road."

The Big Picture
When the executives at TRW were briefed on the ARC system, they asked if it could replace the company's existing Vehicle Stability Control (VSC) system, no doubt thinking the cheaper ARC could further reduce costs and add market share. But they were quickly told by Cunningham and his team that it could not. Instead of replacing VSC, ARC has been designed to complement it. Though ARC's primary purpose is to improve ride handling, the reduction in body roll it produces does in effect act to stabilize a vehicle at thresholds below those where a VSC system would kick in, but it doesn't correct for understeer and oversteer by applying brake torque the way VSC does.

TRW's future strategy calls for ARC to be one component of its Integrated Vehicle Control System, which will include VSC and electric power steering. Cunningham says that the company is currently testing vehicles with integrated VSC and electric power steering systems that apply steering countertorque to assist the driver in managing severe maneuvers. The company is talking with several potential customers about the integrated system at the moment, but has yet to receive an order. However, TRW has signed nine contracts in the last 18 months for its standalone VSC system, including one for the Cadillac Escalade, which is the first North American-designed SUV to be so equipped. In fact, Cunningham predicts, "In the not-too-distant future, we expect [VSC] fitment rates to be at the same level as ABS today."

TRW is betting that the cost savings it can achieve by integrating all of its systems into one that would share electronics, sensors and software will be great enough to convince automakers to expand the use of these technologies. For example, the company estimates that it would cost 25% less to equip a vehicle with ARC if it already has VSC. With cost reductions of that magnitude, these systems could soon bring greater safety and better handling to a wider range of customers.



TRW's ARC system
In straight-ahead driving the ARC system allows the stabilizer bars to move freely which results in smoother ride characteristics.
TRW's ARC system
TRW's ARC system greatly reduces body roll during extreme cornering or avoidance maneuvers through actuators that apply force to the stabilizer bars.