Off the highways, steer-by-wire is on the fast lane
In a meeting associated to the Bauma construction machines trade fair in Munich, industry experts discussed the technology and application options for steer-by-wire for their specific markets. It turned out that apparently design engineers for cranes, material handling vehicles and the like are much easier to convince of the x-by-wire benefits than their colleagues in the car industry. There was a broad consensus that in particular steer-by-wire offers superior design options – and it appears that with regard to the implementation of x-by-wire solutions this industry is far more advanced than the automotive industry.
In vehicles where designers seek to replace conventional steering columns for reasons such as space reduction or ergonomics, steer-by-wire is already widely in use, explained Joachim Stieler who runs a technology consulting company focused on off-highway vehicle markets. Examples are forklifts of different kinds where frequently the driver's working position rules out a direct mechanical connection to the directed wheels. Massoud Karimi, senior system engineer for forklift manufacturer Crown presented an electronic “steering column” used in the company's palett trucks. “With this steering unit, we could greatly improve the knee clearance for the driver,” Karimi said. On top of the enhanced design freedom, the solution offers another benefit for the user: Unlike mechanic or hydraulic designs, electric motors used in x-by-wire contexts are almost maintenance-free, Karimi said.
Another rationale to use x-by-wire technology is to replace danger-laden high-pressure hydraulic hoses in the driver’s cabin. The implementation of electronic steer-by-wire systems does away with this hazard but at the same time with noise associated to hydraulic systems, Stieler said.
There is also another strong motivator for mobile working machine vendors to replace conventional mechanic or hydraulic control schemes by their electronic counterparts: X-by-wire helps to significantly reduce the weight of the vehicle and thus the fuel consumption and carbon dioxide emission.
A class of vehicles where steer-by-wire approaches quickly gain ground is tractors. According to Stieler, tractor manufacturer John Deere has implemented a gyroscope-supported system dubbed “ActiveCommand Steering” which replaces the traditional steering column. With ActiveCommand Steering, sensors at the steering wheel along with electronic and (still) hydraulic actuators, the number of steering wheel turns and the manual force required to maneuver the vehicle is automatically adjusted to the tractor speed. In addition, the system is equipped with a dynamic stability control system to reduce over-steer – a typical problem if a tractor hauls a hanger. And since tractors get faster and in some cases reach speeds of more than 60 km/h (which actually makes them highway-fit vehicles), steer-by-wire becomes indispensible for them. “Due to their coarse tire profiles, tractors tend to lose stability at higher speeds,” Stieler explained. “X-by-wire concepts allow automatic stabilization by means of sensors and control loops – much better than a manual driver can do.”
At the same time, off-highway vehicles get electrified or at least “hybridized”, in some cases even faster than in the car industry in general. X-by-wire goes hand in hand with the electrification trend. For instance, multi-axle steering and drive concepts can be implemented more easily if the drives are integrated in the wheels, which of course is possible only with electric motors. “Electrification of tractors will push steer-by-wire”, Stieler notes.
At the fair, suppliers showed innovative components and subsystems facilitating steer-by-wire designs. SKF for instance displayed a steering encoder unit equipped with non-contact sensors. This makes it a maintenance-free solution with particularly high life expectancy, the company explained. Mobil Elektronik GmbH had a redundant steer-by-wire system with approval for public roads on display.
In contrast to the passenger car industry where FlexRay is regarded as a data bus system required for advanced solutions in the x-by-wire realm, the mobile working machine industry still bets predominantly on CAN bus variations. All x-by-wire systems presented were designed to exchange its sensor and actuator data by means of CAN busses, albeit in some cases the designers applied modifications regarding safety or bandwidth.
As a consequence of its benefits, steer-by-wire systems are expected to significantly gain market share. In the time span between 2010 and 2015, the percentage of off-highway vehicles equipped with steer-by-wire systems will rise significantly. According to Stieler, forestry machines will lead the pack – by 2015, 100 percent of them will use electric steering. Similarly, wheel loaders will electrify steering rapidly. Today, 20 percent of all wheel loaders are steered with such a system; by 2015 it will be 80 percent. Material handling systems are expected to move to steer-by-wire systems at a similar pace. And even with tractors, a leading category by production numbers, the percentage of electrically-steered vehicles will rise from less than 5 percent in 2010 to almost 40 percent in 2015. “Things move much faster in this segment than in the overall automotive industry”, Stieler concluded.
Also the market size is impressive – and high enough to justify the development costs. According to Stieler, over the coming years some 10.000 to 15.000 mobile working machine designs have to be updated, which in most cases will mean their steering systems will be electrified. The production figures are significant as well: In 2007, the year before the big crisis, worldwide production amounted to 2.5 million units. Currently the industry appears to be well on track to reach its old heights.
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