Machines such as wheeled compactors, loaders, trucks, and other mobile equipment are used to perform many tasks. To effectively perform these tasks, the machines require an engine that provides significant torque through a drive system to one or more ground engaging traction devices (e.g., wheels, tracks, etc.). Drive systems for such machines usually include either a mechanical power-shift transmission that is connected to the engine by way of a mechanical or hydraulic torque converter, or a hydrostatic transmission (hystat) that pressurizes fluid with an engine-driven pump for powering a motor connected to the traction device. Recently, more attention has been given to the use of hystats because hystats are known to achieve higher fuel efficiency and extended functionality when compared with a power-shift transmission. A hystat provides an infinitely variable torque-to-speed output ratio within its overall range through the pairing of a variable displacement pump and a fixed- or variable-displacement motor.
Although higher fuel efficiency and extended functionality can be achieved with hystats, many operators still do not prefer machines equipped with hystats because hystat systems respond to operator commands (e.g., throttle commands, directional shits, shifts to neutral, work tool commands etc.) with different characteristics (e.g., acceleration rates, deceleration rates, rimpull torque, etc.) than the familiar and comfortable characteristics of power-shift systems. For instance, when an operator releases the acceleration pedal or throttle of a machine equipped with a hystat, displacements of the pump and/or motor are immediately neutralized and the machine either continues traveling at about the same speed for an extended period of time (when both the pump and motor are neutralized) or quickly stops (when only one of the pump and motor are neutralized). Therefore, operators must modulate the acceleration lever or pedal to move through intermediate displacement positions in a gradual manner in order to travel at an increasingly slower speed. This can prove very difficult in rough terrain common to many worksites, especially for unskilled operators, and can cause undue operator fatigue.
One attempt to vary the response characteristics of a hystat drive system is described in U.S. Patent Publication No. 2013/0104532 (the '532 publication) by Ries et al. that published on May 2, 2013. The '532 publication describes a hystat drive system having a transmission input device that allows an operator to select a transmission setting for operating the machine with a desired virtual gear or range of speed-to-torque ratios. Based on the operator's selection, a controller selects a relationship from an acceleration map that corresponds with the transmission setting. The controller then uses the relationship to adjust the displacement of a pump and/or a motor in the hystat system during an acceleration process. When the operator initiates a coasting or braking operation, the controller selects a coasting deceleration map or a braking deceleration map, respectively, and adjusts the pump and/or motor displacements to slow the vehicle by a desired amount over a desired period of time after initiation of the coasting or braking operation. The desired period of time is a function of the machine's travel speed when the coasting or braking operation was initiated.
Although the system of the '532 publication may be somewhat effective at changing the acceleration rate and deceleration rate of a machine equipped with a hystat, it may not be optimal. In particular, because the speed change characteristics of the '532 publication are based on a gear selection, acceleration and deceleration characteristics of the machine may not be optimal under varying conditions. Further, the single control mode of the '532 publication may not allow the machine to be effectively controlled by operators of varying skill and/or familiarity levels.
The system of the present disclosure solves one or more of the problems set forth above and/or other problems of the prior art.