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Traction management on the ZF Innovation Tractor enables vehicles to start on uphill gradients of up to 30%, thanks to electrically powered wheel heads installed on the trailer that provide additional drive torque.

Automation, electrification focus of ZF’s Innovation Tractor prototype

The passenger-car industry may be driving development of advanced driver-assistance systems (ADAS) and electrification, but the off-highway segment may be best-positioned to utilize them more widely—and sooner.

“The main movement for [driver-assistance systems] is in passenger cars, although it’s much more difficult to get this on cars because of all the regulations,” said Dr. Manuel Götz, responsible for Advanced Engineering, Industrial Technology at ZF. “So I believe in the future, we’ll be seeing autonomous vehicles in off-highway more regularly and even earlier than on the highway. Due to the fact that we’re in an enclosed environment and don’t have the regulations of on-highway, this could make things easier to implement.”

“A higher degree of automation also addresses the trend of operators being less trained in the future and not being able to find highly trained people to operate more and more complex machinery. So this [Innovation Tractor] is a step toward autonomous operation,” he added.

ZF is making a concerted effort to transfer the intelligent systems it’s developed initially for passenger cars into heavy trucks and buses, as well as off-highway equipment. To effectively illustrate this point, engineers in the company’s Advanced Engineering department have spent the past year designing the Innovation Tractor, an advanced-technology demonstrator loaded with cameras and electronics to allow automated maneuvering and hitching, not to mention pedestrian detection. Electric single-wheel drive for trailers and implements helps to dramatically improve traction management, as demonstrated in Aachen, Germany, on a 30%-grade paved road that was thoroughly watered down.

“We have full control not only of the driveline and the engine on this vehicle but also the steering and the brakes,” he said. “This is necessary to have full control over the vehicle and to do things like trajectory control and steering the vehicle.”

Agricultural was selected as the first application for these automation technologies, but they are viable for construction and other off-highway segments, said Götz.

“The Innovation Tractor brings together in a test prototype all the new functions we believe are practical for agricultural and construction applications,” said Dr. Harald Naunheimer, Head of R&D at ZF Friedrichshafen AG. “The focus was on demonstrating what is already possible and technically feasible today, but sets a benchmark on future innovation.”

Automated operation and hitch detection

The Innovation Tractor is equipped with 10 environmental cameras to monitor its surroundings; the data from these cameras enables the vehicle to maneuver semi-autonomously or via mobile devices operated outside the driver's cab, making it easier to hitch implements, for example.

Six cameras are mounted on the driver’s cab and the hood. A computer analyzes the images and generates a 360° surround-view image of the tractor's environment. The operator can view this image on a tablet from various perspectives, including a bird’s-eye view.

These cameras could be linked to provide warnings for obstacle detection and to brake, if necessary, as is becoming more common in passenger cars, Götz said.

Two cameras located at the rear of the tractor are used for the Hitch Detection function. Two additional rear cameras, monitoring a 180° swath, have pedestrian-detection capabilities. These four rear-mounted cameras have a separate data-processing unit.

In future iterations, the tractor ideally will utilize just four cameras, all mounted on the roof, to handle all of these functions, said Götz.

The Innovation Tractor's SafeRange function allows the driver to leave the vehicle and remotely control the tractor/trailer combination from a safe distance. The Innovation Tractor and trailer are outlined as a bird's-eye view on a tablet display from which all the relevant driving and steering commands are managed. Dragging the tractor or trailer image with a finger to the right or left on the screen causes the actual tractor/trailer combination to maneuver in the chosen direction. For reversing in complicated situations, the user can specify the desired direction for the trailer and the system calculates and executes all the necessary steering movements.

Speed is set by swiping the screen from the center to the edge, across the tractor model or the trailer. The maximum forward speed is 4 km/h (2.5 mph), with the top reversing speed limited to 2 km/h (1.2 mph). When contact with the screen is removed, or if radio contact is lost between the tablet and the tractor, the vehicle stops automatically. SafeRange also works when maneuvering the tractor without a trailer.

For the automatic hitching, currently there are three target shields mounted on the trailer to help guide the system. These shields will eventually go away, said Götz, but additional technology is required for that to happen.

“You have the opportunity to train your algorithm better on different implements, but this is a very lengthy [process]. I believe that in the future we might have a system that combines an optical camera and a laser system. With laser you have additional possibilities of detecting much more accurately, not only distances but also how the environment is changing. So I think we’ll see this sensor fusion for autonomous operation,” Götz explained.

The Hitch Detection system uses cameras to detect the exact position and angle of the trailer in relation to the tractor by using the targets. It works up to a distance of 7 m (23 ft). The position is continuously measured during the hitching process and the angle of the steered wheels corrected. The Innovation Tractor maneuvers automatically until it reaches the optimum position for hitching, which then is completed by hand. The system has a tolerance of 1.5 cm (0.6 in).

While using the tablet for maneuvering and hitching, cameras can detect pedestrians located between the vehicle and trailer up to 7 m away, with this information also displayed on the tablet. If the person controlling the tractor fails to respond, the system stops the vehicle. The interrupted hitching process can be restarted only once the pedestrian has moved out of the way.

“Many accidents happen on the farm with people getting run over, so this is a huge step,” said Götz. “Right now we don’t have the regulations demanding such a solution, but once the industry shows such technology I believe there will be regulations to have it implemented on the vehicles.”

Driveline electrification

Drivetrain electrification for the Innovation Tractor comes via ZF's TERRAMATIC transmission with the TERRA+ generator module, an electric single-wheel drive for trailers and implements, coupled with specially developed traction management. An electric steering system, required for automatic-driving functions, has been built into the control network.

This system generation can provide 50 kW of continuous electrical power and serves as the power source for the electrical consumers in the trailer. Two liquid-cooled, 3-phase asynchronous motors are integrated into the electrical wheel heads, saving space on the trailer axle; for the Innovation Tractor’s trailer, the system was installed on the middle axle. The nominal voltage is 400 V, and the system also can be fitted with a wheel brake.

The motors provide 6000 N·m (4425 lb·ft) maximum torque to each wheel.

“We have not yet implemented [the braking] but it is possible; it will be our next step,” said Götz. This function can help with deceleration when going downhill and making a turn, helping to prevent jackknifing and rollover. “We have to implement it in the control so we can use the electric axle to provide a braking torque. It is just a matter of applying the parameters.”

Torque vectoring—which is employed in automotive differentials and provides the ability to vary the power to each wheel—is another technology currently not available on the tractor but would be possible to implement, he said.

“From a space perspective we can go up to 70, even 100 kW continuous power,” said Götz. “In the vehicle at the moment we have installed a 50-kW generator system which is linked to our power electronics, from which we transfer the electric current to the wheels of the trailer. We’re not only powering them, we’re also controlling them; this way we are able to realize optimized traction control between the tractor and the trailer.”

The all-wheel-drive function of the ZF tractor and the electrical-boost function from the single-wheel drive on the trailer combine to deliver optimum traction management. With electrical assistance from the trailer, the tractor can climb uphill gradients of up to 30%, terrain normally off-limits for a conventional tractor/trailer setup, Götz noted. It also helps in difficult conditions in the open field.

A combination of two inputs determine traction management: the coupling force to determine if the trailer is pushing or pulling, and detection of load in the trailer.

The additional power from the trailer also allows a higher payload to be transported with a downsized tractor, ideal for users who only occasionally tow a fully-laden trailer.

ZF admits that its current CVTs are about as efficient as the full electric drive and less costly, so that hinders the move toward electrification. Other factors, however, such as improved controllability and the possibility for optimized traction control can help spur electric-drive adoption. As, of course, could CO2-emissions regulations, which “might not be that far off” for the off-highway sector, somewhere between five and 20 years down the road, Götz predicted.

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