Deep Dive: Curve Control on the 2011 Ford Explorer [w/video]

Curve Control on the 2011 Ford Explorer – Click above for high-res image gallery

When the new 2011 Explorer goes on sale later this year, Ford plans to highlight the new unibody 'ute's safety and fuel efficiency. The Explorer will be exclusively powered by four- and six-cylinder EcoBoost engines, which are expected to increase efficiency by 20-25 percent, while delivering the power consumers crave. Expect more drivetrain details to be forthcoming, but before then, Ford invited us out to its Dearborn proving grounds to see what it has in store on the safety front.

Ford confirmed last fall that the Explorer would be the first vehicle to get its new inflatable rear seat-belts, but the latest – and arguably more interesting – feature on the Explorer is dubbed "Curve Control." The system is a software enhancement built on top of the existing roll stability control (RSC) and electronic stability control (ESC) systems, and we had a chance to experience it first hand. Make the jump to find out more.

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[Source: Ford]

According to Ford, over 50,000 accidents a year happen in the United States as a result of people trying to drive through curves (think highway interchanges) too quickly. This is especially problematic for SUVs, crossovers and pickups, which have a higher center of gravity and are more prone to trip-over curbs and shoulders when they run wide. The engineer's answer to this problem is Curve Control, which goes above and beyond existing ESC to more actively slow down the vehicle when these conditions are detected.

Before we get into the nitty-gritty of Curve Control, let's do a quick review of ESC. Electronic Stability Control is based on the hydraulic brake control technology originally developed for ABS and later, traction control. The hydraulic control unit has the ability to individually manage the pressure to each brake and, in the case of traction control, build pressure to the brakes on the driven wheels independent of the driver. For most ESC systems, the ability to apply brake pressure is expanded to all four wheels.

In order to manage the vehicle dynamics, ESC also includes a set of inertial sensors, including lateral and longitudinal accelerometers and a yaw rate sensor. Yaw is the rotation around the vertical axis of the vehicle when it's turning or worse, spinning. These sensors help the system know what the vehicle is doing. Throttle, brake and steering angle sensors are used to determine what the driver is asking the vehicle to do.

The electronic controls include models of the vehicle's dynamic behavior which are compared against the actual behavior and driver commands. The sensors are used to estimate factors like side slip at the wheels, front slip angles and overall body slip angle (slip angle is the difference between the direction that the wheels are pointed and the actual trajectory of the vehicle). A set of software control functions determine when the vehicle is not tracking where the driver wants, and the system uses a combination of individual brake applications and management of the engine torque to get the vehicle back on track.

Traditional ESC will typically only apply the brakes at one or two wheels at a time and vehicle speed reductions are kept to a minimum. The intent is to apply only enough force to help the vehicle turn where the driver is pointing the wheel. Think of the vehicle like a teeter-totter rotating about its center of gravity. If the driver is trying to turn right, but the vehicle is going too straight, a bit of brake applied on the right side wheels is like pushing on the right side of the teeter-totter, causing the vehicle rotate.

On its trucks, SUVs, CUVs and vans, Ford has added an extra layer called Roll Stability Control (RSC) which brings with it two extra sensors. A vertical accelerometer and body roll sensor provide extra information about possible impending roll-over conditions which are then alleviated by the brakes and engine management.

Curve control is yet another software function added in to the system to help detect when the vehicle is going through a curve too quickly. The combination of sensors detects whether the driver might be trying to negotiate a larger radius turn (an on or off-ramp) and if the vehicle speed it too high, the new control kicks in. While ESC might only apply 0.1 g of deceleration (equivalent to just barely touching the brake pedal), Curve Control applies all four brakes much more aggressively.

2011 Ford Explorer prototype plowing through cones with curve control disabled

The amount of braking at each wheel is still individually controlled to help the vehicle turn in, but over all deceleration could be as much as 0.5 g which is enough to scrub off up to 10 mph of vehicle speed in just one second. On a curve, where the driver has misjudged their speed, that could be enough to make the difference between staying on the pavement and sliding off the side.

Back in the early days of ESC, the systems were much more aggressive and had harsher control than today. Over time, drivers didn't like the sensation and engineers learned to improve the control so that it was more transparent by ramping brake pressure and engine torque up and down rather than using step inputs. Ford's roll stability sensors allow the engineers to do even more because they can better detect conditions like banked or off-camber curves based on the vertical motion and body roll.

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Ford didn't have to add any new hardware to implement Curve Control, but developing and calibrating the algorithm has taken about 18 months. We were given a demonstration on Ford's vehicle dynamics pad in a prototype Explorer and the course simulated a typical off-ramp with the test driver went approaching the coned-off area at 50 mph with the Curve Control disabled. The driver simply steered without applying the brakes or throttle when he hit the curve and the Explorer drifted wide through the cones. With the system enabled, he repeated the same maneuver and as the vehicle turned, we could feel the brakes being applied, causing the mule to track right through the curve.

If the driver decides that they actually want to do something different than what the control is commanding it can be over-ridden by either the gas or brake pedal. For example if the vehicle is pulling out of a driveway, hitting the accelerator will prevent the system from braking the vehicle. Similarly, if the driver wants to slow down even more, they can press through the brake pedal and only the ABS and ESC will still come into play.

2011 Ford Explorer prototype tracking past the cones with curve control enabled

In spite of the fairly aggressive braking, the control was very smooth and the Explorer never twitched or jerked around. Obviously, this was done under very controlled conditions and we'll have to judge how good it feels when we get a production model in our hands. The other question that remains unanswered is how robust the Curve Control system is to false activation. We would hate to see it slamming on the brakes while traveling around a regular corner at even slightly elevated speeds.

The new Explorer will have a driver selectable terrain management system that adjust various control features when off-roading. If the driver selects either the mud or sand modes, the curve control will be disabled to prevent it from causing problems when the Explorer is pretending to be a Jeep. After the Explorer launch, Ford will be adding Curve Control to 90 percent of its trucks, vans, SUVs and CUVs by 2015 and eventually to cars as well.

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