Engineers at Case IH knew that they could help farmers improve crop yields by tightening up the turning radius on their new STX agricultural tractors. But that seemingly simple change led to a total redesign and expansion of the tractor's steering capabilities.
And some engineering challenges. For example, any change in steering technology was likely to mean the addition of extra valves, fittings, and connections, which would further complicate an already complex hydraulic system. So one goal at the onset: Design the extra hydraulic valves into an already cramped space without adding any parts or plumbing.
The main control spool, seen here from the top looking down, controls the direction of the steering.
Steering provides the prime direction control for a tractor. When the steering is very coarse, the turning radius of the vehicle is large. The finer the steering, the tighter the turning radius and the better the vehicle control, increasing the amount of square footage tilled and crop harvested.
A typical articulated vehicle rotates about a single pivot point located at its midpoint. Hydraulic cylinders, controlled by a hydraulic steering valve, rotate the joint. Some vehicles have only articulated steering, such as a front-end wheel loader. Case IH's new STX agricultural tractor steers by pivoting about two points in order to turn its front wheels.
Positioned horizontally across the top half of this cross-section view is the main control spool for Eaton's electrohydraulic valve, part of Case IH's new patented steering system. The pilot-operated check valve is directly below the main spool valve.
Case IH engineers worked with engineers at Eaton Corp. (Eden Prairie, MN) to develop the new, patent-pending steering system called AccuSteer. Two pivot points bring the tractor's turning radius down from almost 16 ft to slightly more than 12 ft. "The system enables the use of two articulation points, which allows a large tractor like the STX to work 30-inch rows with wider implements and turn sharply at the end of the rows," says David Susag, who is the principle hydraulic engineer at Case IH.
In this steer-by-wire application, data from steering wheel sensors is sent to a controller, which instructs the valve how much flow to send to the front steering cylinders. Under instruction from the controller, a mechanically operated valve performs the articulated portion of the steering.
The steering system uses both front-wheel steer-by-wire steering and mechanical main-articulation steering. The steer-by-wire portion allows for precise steering adjustments and improved tracking down crop rows. When the front articulation joint reaches 10 deg, the controller hydraulically activates the rear articulation point, turning the wheel another 45 deg to make a full turn.
The electronic steering controller switches between the steer-by-wire system and the mechanical steering by looking at the position of the front wheels and the angle of the main articulation joint. The switching is accomplished via sensors located in the steering cylinders that measure their relative extension. The electronic steering controller uses a closed-loop PID control algorithm to take the operator's command at the steering wheel through a steering wheel motion sensor. The sensor provides the corresponding valve command that moves the front wheels.
"The hydraulic flow to the main articulation steering is controlled by a proportional valve that allows us to control the flow and feather the mechanical steering on or off while we slowly transition between the steer-by-wire system," Susag explains.
The new Eaton valve mounts between Case IH's existing steering valve and the hose lines already in place for the steering valve. The Eaton valve is unique in its position and the way it interfaces with the steering system because it integrates into the system without any other extra parts.
The steering system keeps the main rear articulation joint straight while the steer-by-wire portion of the system is in use. Two trim valves located in the valve assembly keep the main joint straight and prevent damaging "tail-wag." "The benefit of this is that we look at the position of the main articulation joint and make adjustments to its position as loads on the vehicle change," he says.
New capability, few parts. The addition of a four-way electrohydraulic proportional valve to an existing manual steering valve would normally require three T connections, seven hoses, and nine fittings. "The plumbing would also have to include an extra shuttle valve for accomplishing the necessary logic required by load-sense systems," says Doug Gilbert, systems engineer with Eaton Corp.
Typical stand-alone valves have threaded ports for hydraulic fluid. Fittings are screwed into these ports. Then, hoses are connected to fittings. "Normally, a stand-alone valve would require its five ports and the manual steering valve would require its five ports," says Gilbert.
The AccuSteer system pivots up to 10 degrees at the front of the STX tractor frame before articulation and has a total steering angle of 52 degrees.
He and other Eaton engineers simplified this plumbing arrangement by changing how the four-way proportional valve interfaces with the existing steering valve.
"All of the necessary T connections are made in the internal core of the electrohydraulic valve casting," explains Gilbert. When bolted to another valve, the holes line up and hydraulic fluid passes through. The oil inputs and outputs share common connections.
Eaton's new electrohydraulic valve includes a new internal shuttle that eliminates the need for a separate, external shuttle valve. A shuttle is necessary for proper operation of the load-sense circuit.
The electrohydraulic valve also achieves smoother transitions between the modes of steering, says Susag. The other major improvement that the valve provides is a tractor ready for row-guidance by Global Positioning System (GPS). In fact, one of the deciding factors for going to a steer-by-wire system is electronic input devices enabled by GPS satellites, row finders, and vision systems, all of which steer the vehicle.
"The advantage of GPS is that it can drive straight and parallel to a heading, something that sounds easy enough, but very few operators can achieve," says Susag.
GPS makes less experienced operators much more efficient by reducing overlap and misses. It also improves the application of chemicals and fertilizers and helps with harvesting and tilling.
|Contact Mark Hyde, Eaton Corp., 400 Lincoln Ave, Searcy, AR 72143; Tel: (501) 279-2324; Fax: (501) 279-2116; E-mail: [email protected] ; or Enter 501.|