July 2013 Articles & Features
By Mike Dale | July 2013
Electronic stability control has been working its way into the passenger car fleet, and the safety gains have been great. The next vehicle segment to receive this technology is heavy-duty trucking.
Electronic stability control (ESC) has been working its way into the worldwide automotive passenger car fleet over the last several years. Under federal law, all passenger vehicles from the 2012 model year forward must have it. SUVs and high-end vehicles have had it across the board since the 2008 model year. Early versions of the system were used on BMW and Mercedes vehicles as long ago as the early ’90s. Still, it took federal mandates to make it an across-the-board feature. The benefits in terms of lives saved and accidents avoided have been terrific. This savings will only get better as older vehicles without ESC are phased out.
But there’s still a long way to go. The National Highway Traffic Safety Administration (NHTSA) says that two-thirds of all the heavy truck occupant fatalities occur in rollover accidents. Whether it’s a skid or a rollover, the result of a driver losing control of a big rig can be very serious. There’s tremendous energy in 80,000 lbs. of truck and cargo moving at 70 mph. Totaled up, the Dept. of Transportation (DOT) says that more than 300 truckers per year are killed and thousands more injured in truck rollover and loss-of-control crashes.
NHTSA has been working with the makers of heavy-duty trucks and truck braking systems to bring the advantages of passenger car electronic stability control to heavy trucks. Already more than 200,000 systems have been installed, and it appears likely that the use of ESC will become mandatory on heavy-duty trucks at some point.
While the names for electronic stability control and roll stability might sound the same as for passenger cars, the problems and solutions for big rigs are really quite different. Tractor trailer rigs are very much larger, heavier and longer. There’s also a very large difference between the loaded and unloaded weight that doesn’t happen in cars. Perhaps the most significant factor is that the center of gravity (Cg) for trucks is not only much higher than for passenger cars, but varies depending on the load.
Light vehicles are more likely to lose directional control (left or right) than to roll over. For heavy-duty vehicles, the reverse is true. The Cg is higher and the truck is likely to reach its roll limit first, so any stability control strategy has to be different for trucks than it is for cars.
In heavy-duty vehicles, the centers of gravity are different for the tractor and for the trailer. Some trailers like flatbeds have a low Cg, while others like box trailers may have a higher Cg, depending partly on how it’s loaded and what it’s loaded with. Both parts of the semi have different limits in terms of what sort of lateral forces it takes to make them slide.
Rollovers of semis often start with the trailer, when the trailer’s rear wheels on one side lift off the pavement during sudden changes in the intended path of travel. The roll torque of the trailer is then transmitted to the tractor by way of the articulation point. It’s the trailer that wants to flip the tractor. Rollover of a tractor trailer depends on a variety of factors, including the maneuver, the load and the condition and shape of the roadway surface (icy, dry, wet, crowned, flat, etc.).
Vehicle rollovers are generally divided into two categories—tripped and untripped. Tripped rollovers are caused by forces from an external object, such as a curb or a collision with another vehicle. Untripped rollovers occur when cornering forces destabilize the vehicle. When the tire and the inertial forces are enough to overcome gravity, the vehicle will start to turn over. This often happens even on a dry surface with good contact between the tires and the road surface.
The basic building block for stability systems in general is the antilock braking system (ABS). Beyond its ability to shorten stopping distances, ABS offers three features fundamental to stability control—the hardware that senses the rotational speed of each wheel, a central processing unit (CPU) to analyze that wheel speed information and actuating valves that can individually control the braking on each wheel or wheel pair.
The real star of ABS and all of the stability systems is the CPU. That computer and its software decide when the system needs to respond and how best it should respond.
Using ABS as the basic platform, there are two types of systems now being offered to heavy-duty trucks to help prevent big truck loss-of-control accidents—roll stability control and electronic stability control. Roll stability control (RSC) is the first-level add-on to the basic ABS system. RSC systems add lateral acceleration sensors that detect forces trying to push the vehicle into a rollover situation.
RSC is designed to react by slowing the vehicle using either the trailer or tractor brakes. Engine braking and electronic engine control can also be directed by the RSC system to aid in slowing the vehicle. It does this in response to inputs from lateral acceleration sensors that detect forces on the vehicle that are trying to cause it to roll over.
The sensors that detect these lateral forces are built using MEMS (microelectromechanical systems) technology. They’re fabricated from the same silicon used in semiconductor devices and are usually built with on-chip electronic signal processing. The key element to MEMS sensors is that a tiny beam with a mass at one end is etched out of the silicon. When external acceleration is applied, the mass on the end of that beam deflects from its original position. It can be done in different ways, but one method is to measure the capacitance between that mass and the two rails located on either side of it. As the beam deflects, the separation between the mass and the two rails changes, and so, too, does the capacitance. When the beam is centered, the capacitance to either rail is the same. When the beam deflects due to lateral acceleration, the capacitance rises on one side and falls on the other.
NHTSA says that trailer-based RSC systems have an ECU mounted on the trailer that can monitor wheel speeds and use the position of the trailer suspension as a means of estimating the trailer weight. When high lateral acceleration that could result in a rollover is detected, the trailer ECU can apply full pressure to the outside wheels. The ECU can modulate the braking pressure for maximum braking without locking the wheels. Trailer-based RSC systems can control only the trailer wheels, while a tractor-mounted RSC system can apply the brakes as needed for both the tractor and trailer.
The bottom line to RSC is that it does work to help prevent a rollover event. What it cannot do is to prevent loss-of-control events such as spin-outs and jackknifes.
The newest technology for heavy-duty trucks is electronic stability control. Even though the name is the same as the passenger vehicle system, the function for trucks is more complicated. ESC technology includes all of the features of RSC, plus it can attempt to help the driver deal with loss of directional control.
ESC systems have additional types of sensors, compared to RSC. One is a steering wheel angle sensor that monitors the driver’s intended steering angle by detecting the rotary position of the steering wheel. Another is a yaw rate sensor, which measures the actual turning movement of the vehicle around its vertical axis. The vertical axis of the truck could be pictured as a line in the middle of the vehicle going from the ground to the tallest part of the truck. Rotation around this axis would look like the truck trying to spin like a top. This is called yaw.
In a loss-of-control situation, the tractor starts to slide, which generates a lateral force at the vehicle’s center of gravity. The loss of control happens when the opposing force of the tires is insufficient to overcome the lateral forces. Instead of the vehicle rolling over, it slides. Often, it’s the force of the trailer that pushes the tractor into a jackknife. Typically, this occurs on wet asphalt, ice and/or snow.
The solution is a sensor package that detects the onset of a possible loss of control. When the ECU sees that there’s a difference between the intended vehicle direction (wheel angle sensor) and the actual movement of the truck (yaw rate sensor), it knows that the vehicle is in either an understeer condition (plowing out) or an oversteer condition (spinning out). The system then acts to slow the vehicle and apply the specific pattern of braking that will stop the event from progressing.
There are two basic types of yaw rate sensors—piezoelectric and MEMS. Basically, the yaw rate sensor is a gyroscopelike device that measures the angular velocity of the vehicle around its vertical axis.
The steering wheel angle position sensor is more complicated than you might think. Most steering wheels can be rotated through multiple turns from the extreme right or left position. The sensor needs to know not only where the steering wheel is, but also what rotation the wheel is on. Most wheel position sensors are noncontact optical devices that count pulses of light as the wheel is turned. Shaping the ports through which the light passes creates distinctive information that tells the ECU not only the position of the wheel but what rotation from wheel center (either to the right or left) position the steering wheel is on.
ESC also can include sensors that measure the weight of the vehicle and the pressure in the braking system. In combination with yaw sensing, lateral sensing and steering wheel sensing, ESC can see in advance that a loss-of-control situation may be developing.
Mark Melletat, director of the Meritor-WABCO Corp., says that it’s also possible to measure and take into account vehicle loading without needing sensor inputs from the trailer.
“The system employs advanced algorithms to estimate vehicle information such as engine torque, current gear and measured wheel speed,” he says. “In ESC, we are looking for sensor-reported data that shows a difference between the driver-intended steering path and what the yaw sensor is reporting. When we see understeer or oversteer, we don’t just apply the brakes, we selectively apply them, depending on whether the vehicle is understeering or oversteering. In oversteer, it will be the right front tire; in understeer, it’s the left rear tire that needs braking.”
ESC can intervene in a number of ways, depending on the severity of the problem. Engine power may be decreased, engine braking engaged or the brakes selectively applied. It’s also possible to generate a counter yaw moment. The combination of actions is intended to return the vehicle to its original intended path. The ESC system has a further advantage over the underlying RSC and ABS systems. RSC is typically configured to brake the rear wheels only. ESC brakes front and rear wheels, providing more braking capability. Only the ESC system can activate the tractor’s steering wheel brakes.
At a recent meeting of the American Trucking Association, the brake maker Bendix Corp. made a presentation about its electronic stability control system. According to Bendix, stability control systems must determine that a vehicle is approaching a loss-of-control or rollover situation and then quickly act to prevent it from starting. If the system waits until the vehicle starts to roll over or skid, it will be too late.
There are a couple of service-related cautions. For ESC-equipped vehicles, after mechanical repairs to the front suspension, the system should be recalibrated to confirm the settings in the system for the steering geometry. Steering geometry is an important element due to its effect on the vehicle’s dynamic behavior. Some systems have built-in “learning capability” that allows the system to adapt as the steering system wears and ages.
Both Bendix and Meritor-WABCO made points in their presentations to the effect of “We still need drivers to drive safely.” These systems work in the background. When the driver makes a decision, he or she is in control. But the beauty of ESC or RSC is that they can help before the driver knows that trouble is at hand.