CONTROL SYSTEM FOR ACTUATING LIFTING FUNCTION

Abstract

A hydraulic control system for linear actuation that provides a bypass flow during startup of a lifting command, provides a split flow between a cylinder and a reservoir once a minimum operating speed of a pump has been reached, and provides complete flow to a cylinder after the minimum operating speed of the pump has been reached. This is achieved through control of a flow control valve and a proportional flow control valve by a processor.

Claims

1. A hydraulic control system for linear actuation, comprising: an electric motor connected to a hydraulic pump; a hydraulic cylinder connected to the pump via a flow line; a pressure transducer, a pressure control valve, and a check valve connected to the flow line between the pump and the cylinder; a tank connected to the pump and to the cylinder via a return line; a control valve connected to the flow line between the check valve 24 and the pump and also connected to the return line; a proportional control valve connected to the flow line between the check valve and the pressure control valve and also connected to the return line; and the proportional control valve connected to a processor having software to control the system state during startup of lifting command and during operation at commanded lifting speeds requiring pump speeds less than the minimum pump operating speed.

2. The system of claim 1 wherein before energizing the proportional flow control valve to close pump speed is increased to a level corresponding to a greater value of a command cylinder extension rate or a minimum allowed pump speed.

3. The system of claim 1 wherein all flow is bypassed to the tank during startup through the proportional flow control valve.

4. The system of claim 1 wherein once an operating speed for the pump has been reached, the proportional control valve is energized to become partially opened to a position that bypasses a fraction of the pump flow away from the cylinder directly to the tank.

5. The system of claim 4 wherein a remaining fraction of the pump flow proceeds to the cylinder.

6. The system of claim 1 wherein at commanded speeds requiring speeds less than a minimum operating speed of the pump flow is applied regardless of whether speed needs to be ramped up or ramped down.

7. The system of claim 1 wherein once a minimum operating speed for the pump has been reached the proportional control valve is energized to become completely closed to direct all pump flow to the cylinder.

8. They system of claim 7 wherein the proportional control valve is ramped closed at a configurable rate to produce a desired acceleration rate of the cylinder.

9. They system of claim 7 wherein as the proportional control valve closes, pump speed increases to a level required based on a lifting speed command.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] FIG. 1 is a perspective view of a scissor assembly;

[0024] FIG. 2 is a perspective view of a hydraulic power pack;

[0025] FIG. 3 is a schematic diagram of a hydraulic control system in a de-energized state;

[0026] FIG. 4 is a schematic diagram of a hydraulic control system during lifting;

[0027] FIG. 5 is a schematic diagram of a hydraulic control system during lowering;

[0028] FIG. 6 is a chart of an operating envelope for a pump comparing pump pressure and speed of a pump;

[0029] FIG. 7 is a schematic diagram of a hydraulic control system during startup of a lifting command; and

[0030] FIG. 8 is a schematic diagram of a hydraulic control system during operation at commanded lifting speeds requiring pump speeds less than the minimum pump operating speed.

DETAILED DESCRIPTION

[0031] With reference to the figures a hydraulic control system for linear actuation 10 includes an electric motor 12 connected to a hydraulic pump 14. The pump 14 is connected to a hydraulic cylinder 16 by flow line 18. Connected between the pump 14 and cylinder 16 on the flow line 18 is a pressure transducer 20, a pressure control valve 22, and a check valve 24. The pump 14 is also connected to a tank 26 via flow line 28.

[0032] The cylinder 16 is connected to the tank 26 by return line 30. Connected between flow line 18 and return line 30 is a control valve 32. Control valve 32 is connected to flow line 18 between check valve 24 and pump 14. Also connected between flow line 18 and return line 30 is a proportional control valve 34. The proportional control valve 34 is connected to the flow line 18 between the check valve 24 and the pressure control valve 22.

[0033] The proportional control valve 34 is connected to a processor 36 having software 38. Also connected to the processor 36 is an operator command 40 such as a joystick command. Also, a speed sensor 42 is attached to the pump 14 and or the prime mover shaft. A common approach to achieve proportional speed control and load independent control is to use a proportional flow control valve together with a hydraulic compensator where the proportional control valve controls the flow rate, or lower speed, and the hydraulic compensator maintains the speed regardless of the load weight.

[0034] In one example of the present system, the pressure transducer 20 is used to estimate the pressure in the cylinder 16. Thus, the hydraulic compensator may be eliminated and the software 38 is used to change the proportional control valve’s 34 opening area to maintain the speed regardless of load weight.

[0035] In operation, for startup from zero pump speed, if the pump 14 is not already running and the platform is commanded to lift, it is impossible to reach any pump speed without first passing through the lower area of the pump speed range which is outside of the operating envelope. However, to minimize damage to the pump 14 to the greatest extent, the following steps are performed to minimize the pump pressure at these low speeds.

[0036] First, before energizing the proportional flow control valve 34 to close, the pump speed is increased to the level corresponding to the command cylinder extension rate or the minimum allowed pump speed, whichever value is greater. As the pump rotates, its generated flow has an open, low-resistance pathway through the flow control valve 34 directly to the reservoir 26 as shown in FIG. 7. By bypassing all flow to the reservoir during startup, the pump 14 is able to ramp up in speed with minimal pressure at its output port. This minimizes the potential for damage to the pump during startup.

[0037] In operation at the commanded lifting speed, the next step depends on the magnitude of the lifting speed command by the operator. For commanded lifting speeds requiring pump speeds less than the minimum operating speed of the pump 14, the following steps are performed.

[0038] Once the minimum operating speed for the pump has been reached, the proportional flow control valve 34 is energized to become partially closed, to the position that bypasses a specific fraction of the pump flow away from the cylinder 16 and directly to the reservoir 26. The fraction of generated pump flow that does proceed to the cap end of the cylinder 16 produces the platform lifting speed that is desired. The flow control valve 22 should be ramped closed at a configurable rate that produces the desired acceleration rate of the cylinder 16.

[0039] The system state in this condition is shown in FIG. 8, where the flow control valve 22 is at a position between fully closed and fully open, and the arrows show that the generated pump flow is split between the cylinder and the reservoir. Since the proportional flow control valve is pressure compensated with the pressure compensated valve 44, it is possible to control the flow independent of the system pressure.

[0040] This method for lifting the platform at commanded speeds requiring pump speeds less than the minimum operating speed of the pump applies whether the pump 14 is being ramped up from zero speed or whether it needs to be ramped down from some greater speed where it is already operating. Although this method intentionally converts some energy into heat in order to achieve low lifting speeds, operation at these speeds is normally brief and intermittent. This could be seen as inefficient, but the relatively small amount of extra energy consumed to achieve lower lifting speeds increases the controllability of the machine.

[0041] For commanded lifting speeds requiring pump speeds greater than or equal to the minimum operating speed of the pump 14 the following steps are performed. Once the minimum operating speed for the pump has been reached, the flow control valve 34 is energized to become completely closed and to direct all pump flow to the cylinder 16 as shown in FIG. 4. The valve 34 should be ramped closed at a configurable rate that produces the desired acceleration rate of the cylinder 16. While the flow control valve closes, the pump speed continues to increase to the level required based on the lifting speed command.

[0042] It will be appreciated further by those skilled in the art that other various modifications could be made to the device without parting from the spirit and scope of this invention. All such modifications and changes fall within the scope of the claims and are intended to be covered thereby. It should be understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in the light thereof will be suggested to persons skilled in the art and are to be included in the spirit and purview of this application.