Bobcat's A300 loader features a true
steer-by-wire system that toggles between traditional skid steering and
all-wheel steering with the flip of a switch.
West Fargo, ND—In the minds of many engineers, off-highway equipment probably conjures up images of heavy metal and hydraulics. Yet, Bobcat engineers recently enhanced the steering capabilities of the company's skid-steer loaders by focusing first and foremost on embedded systems design. Their new electronic steering control system not only paved the way for all-wheel, steer-by-wire capabilities but also helped preserve proven aspects of the company's undercarriage and drivetrain designs. Thomas Sagaser, Bobcat's engineering manager for loaders, sums up the development strategy as "using electronics to do more with less."
So far, Bobcat has brought out two models with the enhanced steering capabilities, the year-old A220 and the new A300, which have respective capacities of 2,000 and 3,000 lbs. On loaders of this size, which see plenty of use in commercial landscaping jobs, electronic steering control has a couple of important benefits. For one, the machine's four steerable axles take the skid out of steering, so the loader can tread lightly on the ground. "All-wheel steer is friendlier to turf than skid steer, and there's less tire wear," says Sagaser. At the same time, the new electronic steering system still allows users to switch back to skid steering for those times when maneuverability in tight space counts—skid steering can turn the machine about face on a dime.
From where the operator sits, the electronic steering system does more than take it easy on the turf. Compared to hydraulic pilot-style controls, electronic joysticks take less effort to push around, reducing operator fatigue, according to Chuck Krause, the senior design engineer responsible for all-wheel steer. "Several other skid-steer loaders do have pilot-style controls, but this is the only one with electronic joystick controls, much less with four steerable axles," he says.
Most important from a design and manufacturing standpoint, the use of an electronic steering system limited the mechanical complexity associated with squeezing a new steering system for all wheels into an existing machine. "I don't know how we would have done it without all the electronics, unless we had added all sorts of hydraulic components and all the related plumbing," Krause says. Beyond limiting the need for more hydraulics, the electronic control system also did away with the need for a mechanical differential. "We don't have one," Sagaser points out. "We achieve the function of a differential by the way we electronically control speed of the inside and outside wheels."
A different differential. Bobcat's electronic control system governs a turning routine that's more complicated than it might look. As Krause explains, the all-wheel steer loaders turn with the inner wheels at a sharper angle than the outer ones in order to achieve a common turning center, minimizing the "scuffing" that would otherwise damage turf and tires. And because the inner tires describe a smaller circle than the outer ones, they need to move at slower relative speed. "The way we turn requires us to keep all the wheels at both the proper angle and the proper rpm," he says. Bobcat does so with an embedded system. This electronic steering controller starts with speed and position information from a set of six sensors. This data, along with input from the operator's joystick, feed control algorithms that govern the machine's two hydrostatic pumps. And these pumps, in turn, both drive and turn the wheels. The closed-loop control system runs on a Motorola microprocessor and was designed and packaged with help from Vansco Electronics (Winnipeg, Manitoba, Canada).
The new all-wheel steer loaders not only
feature steerable axles but they also rely on Hall effect sensors (shown
in red) to monitor wheel speed and angle as part of a steer-by-wire
To collect the speed and wheel angle information that feed the electronic steering controls, Bobcat relies on Hall effect sensors, whose output signals vary in proportion to strength of a magnetic field from nearby permanent magnets. Non-contact, insensitive to changes in air gap conditions, and adaptable to both speed and position sensing, these sensors are perfect for the dirty environment underneath a loader, notes Krause.
The sensors, which were custom designed by the Torrington Company (Torrington, CT), all work on the same fundamental operating principle, but they do differ somewhat in their location and packaging. The angle sensors mount above each wheel, underneath a kingpin bearing that attaches the swiveling and fixed portions of the axles. According to Krause, these angle units house the sensor and its magnet within a common housing that allows the magnet to move relative to the rest of the sensor as a wheel turns, thereby changing the sensor's output signal. The twin speed sensors, meanwhile, mount in the chain case and consist of two parts: a two-pole magnetic disc that spins with the drive pump's output shaft and a stationary sensing component that picks up rpm as the shaft revolves.
Common components. With the differential function controlled electronically, relatively few extra components had to be added to the machine to impart the all-wheel steering capabilities. Bobcat did upgrade its conventional hydrostatic pumps to electronically controlled "smart" servo pumps, but all-wheel steer capabilities did not increase total number of drive pumps the loaders use. The four driven and steerable axles required changes, including a universal joint that allows them to swivel relative to the body of the loader. Each wheel also needed steering actuators consisting of a 2.5-inch-bore hydraulic cylinder and a linkage to turn the wheel.
Other than these components, though, the all-wheel steer loaders used many existing drive train components. "Common components help keep our costs down and also make maintenance easier for our customers," says Mike Fitzgerald, a Bobcat product representative for loaders. For example, the A220 model and the 863 loader, its skid-steer inspiration, share the same chain case and undercarriage. The same goes for the A300 and the skid-steering S250. On the new models, however, the mechanical components associated with all-wheel-steering resulted in a 700-lb weight gain, or about a 10% increase over the skid-steer original, according to Fitzgerald.
That may sound like a lot of weight, but it's far less than it could have been had the company tried to do with hydraulic and mechanical components what it did electronically. "We took out mechanical components and put in electronics," says Sagaser. "No one else has done it to this degree on a loader of this size."