Machines such as wheel loaders, excavators, dozers, motor graders, and other types of heavy equipment use multiple actuators supplied with hydraulic fluid from one or more pumps on the machine to accomplish a variety of tasks. These actuators are typically velocity controlled based on, among other things, an actuation position of an operator interface device. For example, when the operator of a wheel loader pulls a joystick controller backward or pushes the joystick controller forward, one or more lift cylinders mounted on the wheel loader either extend to lift a work tool of the machine away from a ground surface or retract to lower the work tool back toward the ground surface at speeds related to the fore/aft displacement positions of the joystick controller. Similarly, when the operator pushes the same or another joystick controller to the left or right, tilt cylinders mounted on the wheel loader either extend to tilt the work tool downward toward the ground surface or retract to tilt the work tool backward away from the work surface at speeds related to the left/right displacement positions of the joystick controller.
In some machine configurations, when a work tool is lifted away from or lowered toward the ground surface, a tilt angle of the work tool relative to the ground surface naturally changes (e.g., the work tool may tilt backward toward a cab of the machine during lifting, and tilt downward toward the ground surface during lowering) due to mechanical linkage connected to the work tool, even though tilting had not been requested by the operator. In this situation, it may be possible for material within the work tool to spill over an edge of the work tool, in some cases onto the machine and/or operator of the machine. Historically, the operator of the machine was responsible for simultaneously adjusting movement of the tilt cylinder during lifting to ensure that the tilt angle of the work tool remained at a desired angle (i.e., to counteract the naturally occurring tilt of the work tool caused by lifting). This dual-control manual procedure, however, can be difficult to control and error prone.
One attempt to automatically reduce the likelihood of material spilling from a machine's work tool during lifting is disclosed in U.S. Pat. No. 7,530,185 that issued to Trifunovic on May 12, 2009 (the '185 patent). In particular, the '185 patent describes an electronic parallel lift system for a backhoe loader. The electronic parallel lift system includes a controller that causes an angle of the backhoe's tool to be automatically adjusted based on measurement of the tool's angle relative to the backhoe's frame, regardless of any particular mechanical relationship between supporting tool linkage, the backhoe's boom, and the tool. The controller uses at least one sensor to detect the angle of the tool relative to the vehicle frame, and then responsively commands a tool actuator to adjust the tool position as a function of the measured angle during boom movement.
Although the system of the '185 patent may help to reduce the likelihood of material spillage during boom lifting, it may be less than optimal. In particular, because the system of the '185 patent adjusts tool angle in a responsive manner based on only angular measurements, the tool angle must first become an angle other than desired before the controller will respond to correct the angle. This type of operation could result in sluggish and inaccurate work tool movements.
The disclosed hydraulic system is directed to overcoming one or more of the problems set forth above and/or other problems of the prior art.