ELECTRONIC TORQUE AND PRESSURE CONTROL FOR LOAD SENSING PUMPS

Abstract

An electric torque and pressure control for load sensing pumps includes a variable open circuit pump with a swash plate angle sensor. The pump is connected in line with a pressure compensated load sensing control having an electrically variable pressure relief valve and orifice. Connected to the circuit is an engine speed sensor, a user input device, and a micro-controller. The micro-controller has software that controls a pressure relief setting of the electrically variable pressure relief valve in the pressure sensing control based upon signals from the swash plate sensor and the engine speed sensor and inputs from the user input device.

Claims

1. A control system for a load sensing circuit, comprising: a variable open circuit pump with a swash plate angle sensor; a pressure compensated load sensing control having an electrically variable pressure relief valve and orifice; an engine having a maximum torque output capability delivered to the variable open circuit pump; an engine speed sensor; a user input device; a micro-controller having software that controls a pressure relief setting of the electrically variable pressure relief valve in the pressure compensated load sensing control; and wherein the software continuously calculates a maximum pressure that would result in a torque level delivered by the engine to the variable open circuit pump at a sensed displacement of a swashplate and sends a current to the electrically variable pressure relief valve to produce the calculated maximum pressure.

2. The system of claim 1 wherein the software calculates the maximum pressure required that would result in an operative torque level produced at variable swash plate displacements.

3. The system of claim 1 further comprising a pressured transducer that monitors a pressure required for a lift function.

4. The system of claim 1 further comprising a dump valve to reduce a set point of a pump engine cranking.

5. The system of claim 1 wherein the software adjusts a first function output by a first command when a second command is received to output a second function and a torque set point of the pump does not allow a load to be lifted until pump displacement is decreased to a point that will have a high enough pressure to lift the load on the second function.

6. The system of claim 2 wherein the operative torque level is where and when an input torque to the pump does not exceed a torque output capability of the engine and does not result in a stalled engine.

7. The system of claim 1 wherein a control for the load sensing pump changes a maximum pressure point automatically without manual intervention.

8. A control system for a load sensing circuit, comprising: a variable open circuit pump with a swash plate angle sensor; a pressure compensated load sensing control having an electrically variable pressure relief valve and orifice; an engine having a maximum torque capability delivered to the variable open circuit pump; an engine speed sensor; a user input device; a micro-controller having software that controls a pressure relief setting of the electrically variable pressure relief valve in the pressure compensated load sensing control; and the software is configured to calculate a maximum pressure based on signals received from the swash plate angle sensor, wherein the maximum pressure is equal to a maximum torque level the engine can produce without stalling at a pressure; the software is configured to send a current to the electrically variable pressure relief valve to produce the maximum pressure while maintaining a torque level required by an operator's command that is no higher than the maximum torque level.

9. The system of claim 8 wherein the software is configured to send the current to the electrically variable pressure relief valve while the operator's command is maintained.

10. The system of claim 8 wherein the software is configured to send a current to the electrically variable pressure relief valve to achieve the calculated maximum pressure.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] FIG. 1 is a schematic view of a prior art load sensing system;

[0016] FIG. 2 is a schematic view of an electronic torque/pressure control circuit;

[0017] FIG. 3 is a chart comparing pump displacement with maximum torque pressure;

[0018] FIG. 4 is a chart comparing pump displacement with current to valve;

[0019] FIG. 5 is a chart comparing pump displacement with pressure;

[0020] FIG. 6 is a chart comparing pump displacement with system displacement;

[0021] FIG. 7 is a schematic view of an electronic torque/pressure control circuit;

[0022] FIG. 8 is a schematic view of a torque control circuit with load holding valves;

[0023] FIG. 9 is a schematic view of a torque control circuit with a pressure compensated pump;

[0024] FIG. 10 is a chart showing a margin allocation in torque control by comparing displacement with pressure; and

[0025] FIG. 11 is a chart showing a margin allocation in torque control by comparing displacement with pressure.

DETAILED DESCRIPTION OF THE INVENTION

[0026] The system 10 is comprised of a variable open circuit pump 12 with a swash plate angle sensor 14. The pump 12 has a pressure compensated, load sensing control 16 with an electrically variable pressure relief valve 18 and orifice 19 built into the input side of the control 16. An external micro-controller 20 and software 22 utilize the signal from the swash plate angle sensor 14, as well as engine speed and user programmable inputs to control the pressure relief setting of the valve 18 in the control 16.

[0027] The Electronic Torque/Pressure Control Circuit 24 (ETL) is created by the addition of the items shown in FIG. 2 to a conventional load sensing circuit. The additional items include:

[0028] Micro-controller 20 and software 22

[0029] Electronically proportional pressure relief valve 18 default to max.

[0030] Orifice 19 at LS input of the pump control 16

[0031] Swash plate angle sensor 14

[0032] Engine speed sensor 26

[0033] User input device 28

Basic ETL Circuit Operation

[0034] Oftentimes with load sensing open circuit systems, the torque requested to be supplied by the engine exceeds the engine's capabilities. When this happens, the operator is required to reduce his commands, slowing the machine which can make it difficult to operate efficiently. Alternatively, the engine simply stalls requiring the operator to restart the machine.

[0035] Starting with the engine torque calculation in example 1.

[0036] Assume the operator of that machine were commanding this operation, and then encountered some resistance to the circuit that raised the force on the cylinder, and the resultant pressure in the circuit to 300 bar (320 bar at the pump). With no change in the valve command, the pump will try and maintain the same output flow at the new higher pressure. The resulting new torque requirement to the engine is shown in Example 2.

[0037] If the engine on the machine is only capable of 150 Nm of output torque, this new load and sustained flow command would overwhelm the engine and result in a stalled condition if the operator continued the command. With basic ETL, the system 10 can control the stroke of the pump 12 by regulating the LS pressure in the control 16, in turn maintaining a torque level at or below the maximum torque that the engine can provide and keeping the engine from stalling.

[0038] As shown in FIG. 3, as an example there is a large area in which the pump 12 is capable of operating in, that would result in an engine stall condition. The line 30 marked by triangles shows the maximum torque level that the engine is capable of delivering to the pump 12. The line 32 marked by squares shows the constant maximum pressure limit usually employed with a traditional load sense system.

[0039] During machine operation, the software 22 is continually monitoring the angle of the swash plate in the pump 12. The software 22 uses the swash plate angle to calculate a maximum pressure that would result in a torque level that the engine could produce at the given displacement, and sends the correct current to the proportional pressure relieving valve 18 in the pump control 16 to achieve that maximum pressure. Shown in FIG. 4, as swash plate angle increases, the current to the pressure relief valve 18 increases (decreasing its setting) limiting the amount of torque the pump 12 can absorb.

[0040] Using this control logic, electronic torque limiting is able to clip off the area 34 in FIG. 3 that results in engine stalling, and instead allows the hydraulic system 10 to always deliver maximum possible pressure for a given displacement without engine stalling.

[0041] Revisiting the example once again, this time with ETL active: [0042] 1. The operator commands a flow and displacement equal to our first example: 45 cc's and 200 bar. [0043] 2. The machine encounters a load which raises system pressure to 320 bar. [0044] 3. ETL is constantly active, and the pump 12 quickly destrokes to an angle that will allow the load to be lifted without stalling the engine.
ETL Operation from a Mechanical Standpoint [0045] 1. The operator commands a flow and displacement equal to our first example: 45 cc's and 200 bar. [0046] 2. The machine encounters a load which raises load pressure to 300 bar (320 bar seen at pump). [0047] 3. The operator maintains the same command. 300 bar load pressure is transferred down the LS line to the electronically proportional pressure relief valve 16. 320 bar pressure is transferred through the variable orifice 19 to the pump 12 and to the pump controls 16. [0048] 4. The LS pressure is relieved at a setting calculated by the micro controller 20 based on the angle of the swash plate. This lowers the pressure on the LS side of the pump control 16. [0049] 5. High pressure on the pump side of the pump control 16 shifts the control to port oil to the servo piston, de-stroking the pump. [0050] 6. As the pump 12 de-strokes, the software 22 is reducing current command to the LS variable relief valve 18, allowing LS pressure on the pump control 16 to increase. [0051] 7. The pump 12 will continue to de-stroke and the LS pressure will continue to increase based on swash plate angle until a 20 bar delta between pump output and LS pressure is reached.
Torque Control with Load Holding Valves

[0052] A system comprised of a traditional mechanical torque control with multiple functions and a load holding or load drop check valve can encounter conditions when the pump outlet pressure is limited below a pressure that can lift the checked load, and when that function is enabled, it is unable to move. The use of electronic torque control along with electronically controlled valves, a pressure transducer, and a software solution can alleviate this problem.

[0053] In FIG. 8, for example, the valve 18 for function 1 is opened and demands a pressure of 150 bar to lift the load and a flow that together will exceed the current torque limit setting of the ETL software 22. In this scenario, the ETL will be regulating the displacement of the pump 12. If the valve 18 for function 2 is opened, which requires a pressure of 250 bar to lift the load, the check valve 36 will continue to support the load, and the required pressure will not be communicated back to the pump control 16 to allow ETL to function properly and lift the load. To solve this problem, a pressure transducer 38 is added to monitor the pressure required to lift function 2 when it is commanded by the operator. When a command is issued for function 2, but the current torque set point of the pump 12 does not allow the load to be lifted, the software 22 will pull back the command of function 1 (or multiple other functions) until the pump displacement is decreased to a point that will allow a high enough pressure to lift the load on function 2. In considering this function, one must remember that the ETL software 22 continuously monitors swash plate angle and will increase the pressure limit of the pump 12 as pump displacement decreases so as to maintain an acceptable torque level to the engine.

Torque Control On Pressure Compensated Pumps

[0054] In backhoe systems it is common to use a pressure compensated pump with torque limiting pump control and a manually operated open center valve stack. All the advantages previously listed in the load sensing circuit still apply to the pressure compensated system. Additionally, as shown in FIG. 9, it is common to have a special dump valve 40 to reduce the set point of the PC pump during engine cranking (primarily in cold conditions). The issue is that when the oil is cold, there is a substantial amount of pressure required to push the oil through the open center valve 42. Without any additional components the torque limiting system can reduce the pressure set point of the PC during cranking to reduce outlet pressure and displacement, thus reducing the load on the engine's starter.

Torque Control and Margin Erosion Across Valves

[0055] In proportional valve groups, especially compensated valves, the design of the valves usually requires a minimum pressure drop across the valve (or margin) for it to operate properly, and properly communicate the load sense pressure back to the pump. As discussed previously, torque control functions by shifting the margin across the valve to an orifice 19 located in the pump control 16. As torque control further reduces torque, the margin across the valve 18 can drop to levels where it may not function correctly. This can be especially noticed during low engine RPM operation where the level of torque reduction is quite high.

FIG. 10 outlines the pump outlet pressure (Ppump), the actual load pressure (PLS) which is the pressure actually working on the load, and the pressure seen at the load sense control of the pump (Pctrl) which is after the relief valve 18 and orifice 19.

[0056] A starting condition shown by the X at the end of the arrow requires a displacement of I47 cc to maintain the margin across the valve 18 and a pressure of 75 bar to lift the load. At this condition, the point is not under influence of the torque control, and the entire margin is satisfied by the drop across the proportional control valve 44. If the command to the valve 44 remains the same, as the load pressure increases, it will first travel upward until the PLS line tums to the left. It is at this point that torque control is starting to become active and relieve pressure at the control. As the pressure continues to increase (following the PLS line), the pump 12 continues to destroke which will reduce the flow through the control valve 44. As previously stated this valve is still receiving the same command, so the reduction in flow lowers the pressure drop across this valve 44. The total pressure drop between the pump outlet (Ppump) and (Pctrl) is still being satisfied by the increasing pressure drop across the orifice 19 in the LS control 16, thereby satisfying the required margin to keep the pump 12 from going into stroke. As the pressure continues to rise, one can see that the pressure drop to satisfy the margin requirement of the pump 12 continues to shift away from the control valve 44 and to the orifice 19 at the LS control 16 on the pump 12. The point at which it reaches the vertical line is the point at which the margin across the control valve 44 has dropped to a point where it may no longer function correctly. It is at this point machine performance may begin to suffer, and further pump angle reduction can cause poorer valve performance.

[0057] To solve this problem, a method of controlling the total valve flow request has been utilized. The employed algorithm seeks to limit the valve opening so that the torque limiter is not impacted by margin erosion while avoiding unnecessarily limiting the valve output when the torque limiter is not actively regulating. By using electronically controlled valves in conjunction with the pump angle sensor and a microcontroller, it is possible to manipulate the shift of the margin from the control valves 44 to the orifice 19 in turn, allowing further destroking the pump to meet load and output torque requirements.

[0058] Looking once again at FIG. 10, we can take a closer look at the vertical line in the graph which represents the minimum margin requirement for proper control valve function (let's assume 7 bar for this example). That means the difference between the middle curve (PLS) and the upper curve (Ppump) is 7 bar at the intersections of the vertical line. If the load pressure were to continue under the steady valve command in this example, the standard torque control would continue to destroke the pump to the left of this line and control valve performance would start to deteriorate. The creation of these performance lines are based on the initial conditions of the valve, load, and pump. If we were to change the opening of the control valve (flow request) it is possible to change the nature of these curves, and allow the pump to further destroke without further margin erosion. Continuing the example, if the request from the pump is lowered from the full I47 cc to 115 cc, the characteristics of the PLS curve are re-shaped, and in turn changes the shift of margin discussed above. The now slightly more restrictive valve opening increases the relative margin across itself, allowing for further pump destroking meeting the increased load demands. As you can see in FIG. 11, reducing the valve request from 147 cc to II5 cc for this example allows full system pressure to be reached before the margin erosion across the valve becomes an issue.