Discretized valve state control for multi-level hydraulic systems

1. A method comprising:

receiving, by a control system of a robotic device, data indicating a magnitude of a load on a hydraulic actuator;

based on the magnitude of the load on the hydraulic actuator, determining, by the control system, an actuation pressure to actuate the load;

causing, by the control system, one or more valves to select one of a first pressure rail at a first pressure or a second pressure rail at a second pressure, wherein the second pressure is higher than the first pressure, and wherein the first pressure rail is selected when the determined actuation pressure is less than the first pressure and the second pressure rail is selected when the determined actuation pressure exceeds the first pressure;

determining, by the control system, that a pressure difference between the pressure of the selected pressure rail and the determined actuation pressure exceeds a tolerated pressure difference; and

responsive to the determination that the pressure difference exceeds the tolerated pressure difference, causing a metering valve to throttle a flow of hydraulic fluid from the selected pressure rail to the hydraulic actuator such that the hydraulic fluid is at a throttled pressure that is within the tolerated pressure difference from the determined actuation pressure.

2. The method of claim 1 , further comprising:

determining, by the control system, that the pressure difference between the pressure of the selected pressure rail and the determined actuation pressure does not exceed the tolerated pressure difference; and

responsive to the determination that the pressure difference does not exceed the tolerated pressure difference, causing the metering valve to open such that hydraulic fluid flowing from the selected pressure rail to the actuator is unthrottled.

3. The method of claim 1 , further comprising:

receiving, by the control system, data indicating a second tolerated pressure difference between the determined actuation pressure and the pressure of the selected pressure rail;

determining, by the control system, that a pressure difference between the pressure of the selected pressure rail and the determined actuation pressure is less than the second tolerated pressure difference; and

responsive to the determination that the pressure difference is less than the tolerated pressure difference, causing the metering valve to open such that hydraulic fluid flowing from the selected pressure rail to the actuator is unthrottled.

4. The method of claim 1 , further comprising:

supplying, by a hydraulic drive system, the first pressure to the first pressure rail and the second pressure to the second pressure rail.

5. The method of claim 1 , wherein causing the metering valve to throttle the flow of hydraulic fluid from the selected pressure rail to the hydraulic actuator comprises:

causing the metering valve to throttle the flow of hydraulic fluid from the selected pressure rail to the hydraulic actuator such that the hydraulic fluid pressure is reduced to approximately the determined actuation pressure.

6. The method of claim 1 , wherein the robotic device comprises at least one leg rotatably coupled to a frame, the method further comprising:

causing the at least one leg to raise from a support surface and rotate forward relative to the frame;

after causing the at least one leg to raise from the support surface and rotate forward relative to the frame, adjusting the tolerated pressure difference to a smaller tolerated pressure difference; and

after adjusting the tolerated pressure difference to a smaller tolerated pressure difference, causing the at least one leg to lower to the support surface and rotate backward relative to the frame.

7. The method of claim 1 , further comprising:

receiving, from one or more sensors on the robotic device, data indicative of one or more physical features of an environment in which the robotic device is operating; and

determining the tolerated pressure difference based on the data indicative of the one or more physical features of the environment, wherein the tolerated pressure difference is set to a first tolerated pressure difference when the data indicative of the one or more physical features of the environment indicates that the robotic device is traversing even terrain, wherein the tolerated pressure difference is set to a second tolerated pressure difference when the data indicative of the one or more physical features of the environment indicates that the robotic device is traversing uneven terrain, and wherein the first tolerated pressure difference is greater than the second tolerated pressure difference.

8. The method of claim 1 , further comprising:

determining an energy reserve of the robotic device; and

determining the tolerated pressure difference based on the determined energy reserve, wherein the tolerated pressure difference is set to a first tolerated pressure difference when the energy reserve is below a threshold level, wherein the tolerated pressure difference is set to a second tolerated pressure difference when the energy reserve is above the threshold level, and wherein the first tolerated pressure difference is greater than the second tolerated pressure difference.

9. The method of claim 1 , wherein the robotic device comprises at least one leg, the method further comprising:

receiving, from one or more sensors on the robotic device, sensor data indicating an extent of slipping of the at least one leg; and

determining the tolerated pressure difference based on the sensor data indicating the extent of slipping of the at least one leg, wherein the tolerated pressure difference is set to a first tolerated pressure difference when the extent of slipping is below a threshold, wherein the tolerated pressure difference is set to a second tolerated pressure difference when the extent of slipping is above the threshold, and wherein the first tolerated pressure difference is greater than the second tolerated pressure difference.

10. The method of claim 1 , further comprising:

determining a gait for the robotic device to use in moving through an environment; and

determining the tolerated pressure difference based on the determined gait.

11. The method of claim 1 , further comprising:

determining a speed at which the robotic device is moving through an environment; and

determining the tolerated pressure difference based on the determined speed, wherein the tolerated pressure difference is set to a first tolerated pressure difference when the determined speed is above a threshold, wherein the tolerated pressure difference is set to a second tolerated pressure difference when the determined speed is below the threshold, and wherein the first tolerated pressure difference is greater than the second tolerated pressure difference.

12. A system comprising:

a hydraulic actuator of a robotic device;

a first pressure rail at a first pressure;

a second pressure rail at a second pressure higher than the first pressure;

one or more valves;

a metering valve; and

a control system configured to:

receive data indicating a magnitude of a load on the hydraulic actuator;

based on the magnitude of the load on the hydraulic actuator, determine an actuation pressure to actuate the load;

cause the one or more valves to select one of the first pressure rail or the second pressure rail, and wherein the first pressure rail is selected when the determined actuation pressure is less than the first pressure and the second pressure rail is selected when the determined actuation pressure exceeds the first pressure;

determine that a pressure difference between the pressure of the selected pressure rail and the determined actuation pressure exceeds a tolerated pressure difference; and

responsive to the determination that the pressure difference exceeds the tolerated pressure difference, cause the metering valve to throttle a flow of hydraulic fluid from the selected pressure rail to the hydraulic actuator such that the hydraulic fluid is at a throttled pressure that is within the tolerated pressure difference from the determined actuation pressure.

13. The system of claim 12 , wherein the control system is further configured to:

receive data indicating a second tolerated pressure difference between the determined actuation pressure and the pressure of the selected pressure rail;

determine that a pressure difference between the pressure of the selected pressure rail and the determined actuation pressure is less than the second tolerated pressure difference; and

responsive to the determination that the pressure difference is less than the tolerated pressure difference, cause the metering valve to open such that hydraulic fluid flowing from the selected pressure rail to the actuator is unthrottled.

14. The system of claim 12 , further comprising:

a hydraulic drive system configured to supply the first pressure to the first pressure rail and the second pressure to the second pressure rail.

15. The system of claim 12 , wherein the control system is configured to cause the metering valve to throttle the flow of hydraulic fluid from the selected pressure rail to the hydraulic actuator by:

causing the metering valve to throttle the flow of hydraulic fluid from the selected pressure rail to the hydraulic actuator such that the hydraulic fluid pressure is reduced to approximately the determined actuation pressure.

16. The system of claim 12 , wherein the robotic device comprises at least one leg rotatably coupled to a frame, and wherein the control system is further configured to:

cause the at least one leg to raise from a support surface and rotate forward relative to the frame;

after causing the at least one leg to raise from the support surface and rotate forward relative to the frame, adjust the tolerated pressure difference to a smaller tolerated pressure difference; and

after adjusting the tolerated pressure difference to a smaller tolerated pressure difference, cause the at least one leg to lower to the support surface and rotate backward relative to the frame.

17. A non-transitory computer readable medium having stored thereon instructions that, when executed by a computing device, cause the computing device to perform operations comprising:

receiving data indicating a magnitude of a load on a hydraulic actuator;

based on the magnitude of the load on the hydraulic actuator, determining an actuation pressure to actuate the load;

causing one or more valves to select one of a first pressure rail at a first pressure or a second pressure rail at a second pressure, wherein the second pressure is higher than the first pressure, and wherein the first pressure rail is selected when the determined actuation pressure is less than the first pressure and the second pressure rail is selected when the determined actuation pressure exceeds the first pressure;

determining that a pressure difference between the pressure of the selected pressure rail and the determined actuation pressure exceeds a tolerated pressure difference; and

responsive to the determination that the pressure difference exceeds the tolerated pressure difference, providing instructions to cause a metering valve to throttle a flow of hydraulic fluid from the selected pressure rail to the hydraulic actuator such that the hydraulic fluid is at a throttled pressure that is within the tolerated pressure difference from the determined actuation pressure.

18. The non-transitory computer readable medium of claim 17 , wherein the operations further comprise:

receiving data indicating a second tolerated pressure difference between the determined actuation pressure and the pressure of the selected pressure rail;

determining that a pressure difference between the pressure of the selected pressure rail and the determined actuation pressure is less than the second tolerated pressure difference; and

responsive to the determination that the pressure difference is less than the tolerated pressure difference, providing instructions to cause the metering valve to open such that hydraulic fluid flowing from the selected pressure rail to the actuator is unthrottled.

19. The non-transitory computer readable medium of claim 17 , wherein the operations further comprise:

providing instructions to cause a hydraulic drive system to supply the first pressure to the first pressure rail and the second pressure to the second pressure rail.

20. The non-transitory computer readable medium of claim 17 , wherein providing instructions to cause the metering valve to throttle the flow of hydraulic fluid from the selected pressure rail to the hydraulic actuator comprises:

providing instructions to cause the metering valve to throttle the flow of hydraulic fluid from the selected pressure rail to the hydraulic actuator such that the hydraulic fluid pressure is reduced to approximately the determined actuation pressure.