Hydraulic system for a working machine and a method for controlling a hydraulic system

A hydraulic system comprising:

• a rotating hydraulic machine arranged to provide a torque via an output shaft;
• first valve means for providing a first differential hydraulic pressure level over the hydraulic machine by using two sources of hydraulic fluid having different hydraulic pressure levels,
• second valve means for providing a second differential hydraulic pressure level over the hydraulic machine by using two sources of hydraulic fluid having different hydraulic pressure levels, an absolute value of the second differential hydraulic pressure level being different from an absolute value of the first differential hydraulic pressure level;
• and
• a control unit configured to control the first valve means and the second valve means such that different discrete levels of torque are provided via the output shaft of the hydraulic system.

A method for controlling a hydraulic system to provide discrete levels of output torque, the system comprising:

• a plurality of hydraulic machines arranged to provide a torque via a common output shaft, and valve means for providing a plurality of differential hydraulic pressures to each of the hydraulic machines, via the valve means, from at least three sources of hydraulic fluid;
• the method comprising:
• controlling the valve means in response to a requested output torque level such that discrete output torque levels are provided by the output shaft by:
• if the requested output torque level is equal to or lower than a minimum torque level of the hydraulic system, providing the minimum output torque level by applying the lowest differential hydraulic pressure to a hydraulic machine having the lowest fixed displacement;
• if the requested output torque level is equal to or higher than a maximum torque level of the hydraulic system, providing a maximum output torque level by applying the highest differential hydraulic pressure to all of the plurality of hydraulic machines;
• and
• if the requested output torque level is between the minimum output torque level and the maximum output torque level, applying a differential hydraulic pressure to at least one of the hydraulic machines such that a torque level closest to the requested torque level is provided.

A method for controlling a hydraulic system to provide discrete levels of output torque, the system comprising:

• a hydraulic machine arranged to provide a torque via an output shaft, and valve means for providing at least three differential hydraulic pressures to the hydraulic machine, via the valve means, from at least three sources of hydraulic fluid;
• the method comprising:
• controlling the valve means in response to a requested output torque level such that discrete output torque levels are provided by the output shaft by:
• if the requested output torque level is equal to or lower than a minimum torque level of the hydraulic system, providing the minimum output torque level by applying the lowest differential hydraulic pressure to the hydraulic machine;
• if the requested output torque level is equal to or higher than a maximum torque level of the hydraulic system, providing a maximum output torque level by applying the highest differential hydraulic pressure to the hydraulic machine;
• and
• if the requested output torque level is between the minimum output torque level and the maximum output torque level, applying a differential hydraulic pressure to the hydraulic machine such that a torque level closest to the requested torque level is provided.

A method for controlling a hydraulic system to provide discrete levels of output torque, the system comprising:

• a first hydraulic machine having a fixed displacement, arranged to provide a torque via a common output shaft;
• a second hydraulic machine having a variable displacement, arranged to provide a torque via the common output shaft,
• first valve means for providing a differential hydraulic pressure level over the first hydraulic machine by using a plurality of sources of hydraulic fluid having different hydraulic pressure levels;
• second valve means for providing a differential hydraulic pressure level over the second hydraulic machine by using the plurality of sources of hydraulic fluid having different hydraulic pressure levels,
• wherein the hydraulic system comprises sources of fluid at at least three different pressure levels;
• the method comprising the steps of:
• controlling the valve means and the second hydraulic machine in response to a requested output torque level such that discrete output torque levels are provided by the output shaft by:
• applying a differential hydraulic pressure to the first hydraulic machine such that a torque level closest to the requested torque level is provided;
• and
• controlling the displacement and direction of the second hydraulic machine to add torque to or receive torque from the output shaft such that the requested output torque is provided.