Bigtrucks, tight turns

DN Staff

October 7, 2002

7 Min Read
Bigtrucks, tight turns

Denali Quadrasteer trucks featuer a four-wheel steering systems that improves low-speed maneuverability, including drastic reduction in turning radius.

Ever watch someone try to squeeze a full-size pickup truck into a parking space down at the local mini-mall? "The more sizeable trucks often take two or three tries to hit the spot," says Paul Grougan, a lead design engineer for steering systems at GM Truck (Pontiac, MI). And on the streets, the biggest pickups can suffer from other maneuverability mishaps, such as clipping curbs when making a tight turn. But graceful handling at low speeds, and size, don't have to be mutually exclusive as GM engineers discovered when they came up with a new four-wheel-steer system for the company's extended-cab Denali models.

Called Quadrasteer, the system features steerable rear wheels that can reduce the turning radius of the company's full-size trucks by as much as a quarter. On a Denali Sierra with a 143-inch wheel base, for example, the Quadrasteer brings the turning radius from 45.3 ft. down to 37.4 ft. "That's very close to a midsize car like a Saturn," Grougan notes. And with the steerable rear wheels now able to follow a tighter turn circle, Quadrasteer trucks can also steer clear of curbs. The improvements to low-speed handling also appear to extend to mid-range and highway speeds. "It doesn't feel like you're driving a truck," says Grougan, who again likens the handling to that of a midsize passenger car.

Four-wheel steering is hardly a new idea, but a couple of things set GM's new system apart from efforts that briefly appeared on passenger cars of years past. For one, GM applied Quadrasteer to a class of vehicle where it can actually do some good. "All previous systems were found on vehicles with much shorter wheel bases," Grougan says. But small vehicles, which already maneuver just fine at low speeds, stand to gain the least from a steering system that primarily enhances low-speed maneuverability. "People thought four-wheel steer for cars was a cool option, but they weren't willing to sign up for it because the benefit of low-speed maneuverability didn't warrant the cost," Grougan says. For another, the Quadrasteer marks GM's first commercial steer-by-wire system, and it takes the by-wire concept all the way: "The system has no mechanical redundancies to generate a steering response in the rear wheels," Grougan points out.

Without any mechanical link to the electronically controlled rear wheels, Quadrasteer had to be engineered first and foremost with safety in mind. So GM and its steering system supplier, Delphi Corp. (Saginaw, MI) built a system that features a variety of mechanical failsafes and control redundancies. "We wanted to make sure nothing could ever happen to the system that would cause the driver to lose control of the vehicle," says Steve Klein, a Delphi systems engineer who helped design Quadrasteer. "We took a belt and suspenders approach," agrees Grougan. "Every aspect of the system has a failsafe of some sort."

At low-speeds, the rear wheels point in a different direction and the front wheels for maximum maneuverability. As speeds pick up, however, the front and rear wheels gradually start to work in phase with the front wheels.

Steering signals. As a true steer-by-wire system, Quadrasteer relies entirely on its electronic control system to steer the rear wheels. That system collects steering input from a position sensor on the hand wheel. A second position sensor on the rear wheel, meanwhile, measures actual wheel position. And the system also collects speed data from engine and braking controllers. The system processes all this speed and position data on a 16-bit custom microcontroller and ultimately actuates the rear wheels with a brushless dc motor that directly drives the rear steering rack. A GM Class II network ties the pieces together.

Running the show is a steering algorithm that sets rear-wheel angle in relation to the front wheels as a ratio of rear-wheel angle to front-wheel angle. At low speeds, the ratio is negative and wheels work "out of phase," Grougan says. The rear wheels thus point in the opposite direction than those on the front, minimizing the turning radius and generally improving maneuverability. As speeds pick up, that negative ratio moves toward zero, hitting it at a predetermined crossover point. "At this crossover point, the rear wheels have no steering authority at all," says Grougan. Speeding up further, the ratio becomes positive and the wheels steer in phase with the ratio of back to front wheel angle up to about 50%. Klein points out that the actual crossover point varies with application. For normal driving it takes place a little over 40 mph, while in a towing mode it would take place around 25 mph. Another aspect of the control, its "swing-out algorithm," initially limits the amount the rear wheels can turn with the vehicle at a standstill. "Its purpose is to keep the rear bumper from swinging into objects beside the vehicle," says Klein. As speeds increase from a dead stop, the allowable angle increases from five degrees to as much as 12 degrees, which is the maximum steering angle for the rear system under any condition.

At first glance, steering doesn't represent the most difficult control task-something Klein readily acknowledges. But putting together failsafe controls suitable for what Klein calls a "safety-critical mission" upped the ante on system complexity. "All the complexity comes from guaranteeing the system always does the right thing under all conditions," he says. To that end, GM and Delphi engineers pursued a design approach that could be described as "do everything twice." Every bit of position data, for example, comes from two different sources. The hand-wheel position sensor package contains both analog and digital encoders for two distinct signals. The rear-wheel position sensor likewise has redundancy. It takes wheel-position data from an analog encoder and from a Hall effect sensor on the dc motor which does double duty for motor commutation. The speed data also comes from two sources, the ECU and ABS controllers.

All these redundant signals become important when they reach the system's microcontroller. According to Terry Smith, a GM software engineer, the Quadrasteer's microcontroller features a dual-core architecture, allowing it to simultaneously perform independent speed and position calculations based on both the analog and digital data. "We then cross check every answer," explains Smith. As a result, every calculation is "verified and validated" prior to the rear wheels moving an inch.

Few mechanical twists. For all its extra steering capabilities, Quadrasteer did not require substantial mechanical innovations or new technology-which just goes to show why the by-wire approach is so attractive. Quadrasteer has the same front steering system, for example, as the company's conventional trucks. And even the rear steering system closely resembles that of a conventional truck. "Quadrasteer has a typical rack-and-pinion design with a yoke and knuckle assembly that allow the wheels to turn in relation to the axle," Grougan says. A typical shaft and CV joint complete the picture, he adds.

The only significant mechanical twists relate to the addition of the motor to drive the steering rack, and the addition of an important mechanical failsafe: The rear- wheel steering system sports a spring (15 N/mm) that returns the steering rack to center in the event of any controller failure. Yet because this spring alone would rein in the wheels too rapidly for some drivers to keep control of the vehicle, the spring works in conjunction with the motor to form a last line of defense against loss of vehicle control. In the event of a blown sensor or any other kind of control failure, Quadrasteer takes the safe route and intentionally shorts out the motor. That shutdown not only prevents it from incorrectly positioning the wheels but also enables it to generate a controlled damping response that limits the speed of the spring return. It's a strategy that Grougan sums up as "gradually fail to center."

And in the event that the system does fail, Quadresteer trucks simply act like conventional two-wheel-steer vehicles. "The front steering system is the ultimate back-up," Klein says. "We can shut down rear-wheel steering and still have a fully functional vehicle."

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