Load Sensing Control Systems
Advanced hydraulic control technology for efficient power management in industrial applications, including the hydraulic pto drive motor systems.
Load sensing control systems represent a significant advancement in hydraulic technology, optimizing power usage by adjusting fluid flow and pressure according to system demands. These systems are particularly valuable in applications where energy efficiency and precise control are paramount, such as in the hydraulic pto drive motor configurations commonly found in agricultural and construction equipment.
By continuously monitoring and responding to load requirements, these systems minimize energy waste while maintaining optimal performance. This capability is especially critical in modern machinery where the hydraulic pto drive motor must operate efficiently under varying load conditions.
Classification by Hydraulic Source Type
Open Center System (OLSS)
The Opened Center Load Sensing System (OLSS) utilizes a constant output flow oil source, typically incorporating a fixed-displacement pump combined with a pressure relief valve as the load sensing control component. This configuration ensures consistent flow delivery while protecting system components from overpressure conditions.
In OLSS configurations, the hydraulic pto drive motor receives a constant flow supply, with excess flow diverted through the relief valve when not needed. This approach, while simpler in design, may result in energy losses during periods of low demand, making it less efficient than closed center systems in applications where the hydraulic pto drive motor operates under highly variable load conditions.
Closed Center System (CLSS)
The Closed Center Load Sensing System (CLSS) typically employs an oil source whose output flow varies according to load requirements, utilizing a pressure-compensated pump directly as the load sensing control element. This dynamic adjustment capability makes CLSS particularly suitable for applications featuring the hydraulic pto drive motor.
In CLSS implementations, the hydraulic pto drive motor benefits from precise flow control that matches operational demands. The variable displacement pump adjusts its output based on pressure feedback from the system, ensuring that only the necessary hydraulic power is delivered. This results in significant energy savings compared to open center systems, especially when the hydraulic pto drive motor operates under fluctuating load conditions.
Hydraulic Pump as Load Sensing Control Element
Load sensing control hydraulic systems that use a hydraulic pump directly as the load sensing control element can automatically transmit signals of pressure or flow changes required by the load to the sensing chamber of the load sensing pump's variable control mechanism. This capability is particularly beneficial in systems incorporating a hydraulic pto drive motor, where precise power matching is essential.
These pressure parameters change, thereby adjusting the operating state of the oil supply unit (variable pump) in the system so that it provides almost only the hydraulic power required by the load. This optimization is crucial for the efficient operation of the hydraulic pto drive motor, as it ensures that energy consumption is minimized during both high and low demand scenarios.
This configuration minimizes pressure and flow-related losses to the greatest extent, enabling flow regulation and ensuring that the flow to the actuators is independent of their load, but only related to the size of the control valve spool opening. For the hydraulic pto drive motor, this means consistent performance regardless of external load variations, enhancing both control precision and energy efficiency.
Figure 1-12: Load Sensing Control System (LS System)
Schematic representation showing the integration of a variable displacement pump with load sensing capabilities, including an imported pressure compensator for enhanced control precision, suitable for applications with hydraulic pto drive motor.
Key Advantage for Hydraulic PTO Drive Motor
The ability to match pump output precisely to load requirements is particularly advantageous for the hydraulic pto drive motor, which often operates under varying conditions. By maintaining optimal pressure and flow levels, the system ensures that the hydraulic pto drive motor delivers consistent performance while minimizing energy consumption and reducing wear on components.
Application in Hydraulic Construction Machinery
When applying load sensing control to hydraulic construction machinery, including equipment utilizing a hydraulic pto drive motor, the pressure delivered by the pump must only match the highest load pressure to ensure proper operation. This means load sensing control only acts on the highest load circuit, while pressure compensation is applied to other circuits with lower load pressures to maintain a constant valve port pressure difference.
Flow Distribution Challenges
When the valve port is fully open and the flow required by the working system exceeds the pump's oil supply capacity, the speed of the actuators in the highest load circuit will decrease rapidly until they stop. This phenomenon can cause other actuators to lose the coordination capability for compound actions, which is particularly problematic in machinery where the hydraulic pto drive motor must work in harmony with other system components.
This issue arises because it's impossible to maintain the set pressure difference across all throttle ports under flow saturation conditions. In high-load branches, where the pressure difference across the throttle port is below the set value, the pressure compensator for that branch remains fully open. Consequently, the pump's output pressure cannot rise to the level required to drive the high-load circuit, resulting in reduced flow to the heavy-load branch. This can significantly impact the performance of the hydraulic pto drive motor when it's part of the high-load circuit.
To overcome this limitation, Rexroth developed the LUDV system, which provides superior flow distribution capabilities even when the hydraulic pto drive motor and other actuators are operating under varying load conditions and flow limitations.
LUDV System: Load Independent Flow Distribution
The LUDV system, or Load Independent Flow Distribution system, controls the pump and pressure compensation based on the highest load pressure of the actuators. When the flow required by the actuators exceeds the pump's capacity, the system proportionally distributes the flow to each actuator rather than prioritizing those with lighter loads. This feature is particularly beneficial for equipment incorporating a hydraulic pto drive motor, ensuring consistent operation even under partial flow conditions.
In LUDV systems, the hydraulic pto drive motor receives a proportional share of the available flow based on its control valve settings, rather than being starved of flow by lighter loads. This maintains operational harmony between different system components, enhancing overall machinery performance and operator control.
Compensation Principle
The compensation principle of the LUDV system involves positioning the pressure compensator after the measuring orifice. The highest pressure P (where P₁ > P₂) of the actuators is transmitted to all pressure compensators and the hydraulic pump. A regulated pressure difference of approximately 2MPa, provided by the pressure-flow regulator, acts on the system as the regulating pressure difference Δp across the orifice.
Because the pressure Pₘₐₓ = P₂ is equal, the pump supplies oil in proportion to the orifice cross-sectional areas A₁ and A₂. This proportional flow distribution ensures that the hydraulic pto drive motor and other actuators maintain their operational relationship regardless of load variations.
Figure 1-13: LUDV System
Schematic of the Load Independent Flow Distribution system, demonstrating how pressure signals from the highest load are distributed to all pressure compensators.
Benefits for Hydraulic PTO Drive Motor
- Proportional flow distribution during partial load conditions
- Maintained operational harmony with other system components
- Consistent performance regardless of load variations
- Enhanced control precision in complex operations
Mathematical Relationships in LUDV Systems
From Figure 1-13, the relationship between the pressure at the outlet of the flow distribution valve and the maximum load pressure p can be derived. These relationships are critical for understanding how the system maintains proportional flow distribution, even when the hydraulic pto drive motor is operating under varying load conditions.
Pₛ - P₂ = Pₖ₂
If the opening pressures of the two compensators are adjusted such that Pₖ₁ = Pₖ₂ = Pₖ, then Pₘ₁ = Pₘ₂. This means that all compensators are controlled by the higher load pressure, so the pressure after the flow distribution valve in each circuit can be kept equal.
The flow through the two valves can be expressed as:
q₂ = C₂A₂√(2Δp/ρ) (1-20)
Where:
C₁, C₂ = flow coefficients
A₁, A₂ = valve opening areas
ρ = oil density
Δp = pressure difference across the valve, Δp = pₛ - Pₘ₁ or Pₘ₂
Equations (1-19) and (1-20) can be written as:
This indicates that the flow obtained by both circuits is proportional only to the opening of the throttle valve. This relationship is fundamental to the operation of the hydraulic pto drive motor in LUDV systems, as it ensures that the motor's speed remains proportional to the control input regardless of load conditions.
Flow Distribution Characteristics
When the oil supply qₛ of the hydraulic pump cannot meet the demand during simultaneous operation of multiple actuators, Δp₁ and Δp₂ will decrease accordingly. Since all pressure balancing valves are acted upon by the maximum pressure signal Pₘₐₓ, the flow continues to be distributed independently of the load pressure.
This achieves proportional flow distribution. As the undersaturation increases, the pump pressure decreases, the pressure difference across the control valve orifice and the flow rate both decrease, and each actuator reduces speed in proportion to the opening of its respective control valve orifice. Even under saturation conditions, the high-load actuator, such as a hydraulic pto drive motor operating under heavy load, will not stop immediately but will reduce speed proportionally with other system components.
Advantages and Disadvantages of Load Sensing Systems
Advantages
- In both load sensing technologies, the pump acts as a hydro-mechanical pressure closed-loop controller to ensure that the supply pressure exceeds the maximum load pressure by a fixed pressure difference Δp. This is particularly beneficial for the hydraulic pto drive motor, as it ensures adequate pressure margin under all operating conditions.
- Since the pump supply pressure continuously adjusts with the maximum load pressure, the system avoids flow losses, thus saving energy compared to "open center systems". This energy efficiency directly benefits operations utilizing a hydraulic pto drive motor, reducing fuel consumption and operating costs.
- Precise control over actuator speeds regardless of load variations, enhancing the performance of the hydraulic pto drive motor in applications requiring consistent operation.
- LUDV systems provide proportional flow distribution during flow saturation, maintaining operational harmony between the hydraulic pto drive motor and other system components.
- Reduced heat generation due to minimized pressure losses, extending the life of hydraulic fluids and components including the hydraulic pto drive motor.
Disadvantages
- Changes in load pressure transmit through the load sensing circuit to the pump, and this pressure control loop is affected by many factors, making control more challenging. This complexity can impact the stability of the hydraulic pto drive motor under rapidly changing load conditions.
- The load sensing circuit connects to both the pressure compensator of the control valve and the pump pressure controller, which may cause interference between the two, increasing the tendency for hydraulic oscillations that can affect the hydraulic pto drive motor's performance.
- There is still room for improvement in system response and energy efficiency, particularly in applications where the hydraulic pto drive motor undergoes rapid load changes.
- Variable pumps can only respond after the joystick generates a pilot pressure command, the spool moves, and the load signal is sent to the pump's control circuit, creating potential response delays for the hydraulic pto drive motor.
- Higher initial cost compared to conventional hydraulic systems, though this is often offset by energy savings and improved performance of the hydraulic pto drive motor over the equipment's lifespan.
Performance Comparison
The following chart compares the energy efficiency of different hydraulic systems under varying load conditions, including configurations featuring a hydraulic pto drive motor. The data demonstrates the advantages of LUDV systems in maintaining efficiency across a wide range of operating scenarios.
Energy efficiency comparison between open center, standard load sensing, and LUDV systems when operating a hydraulic pto drive motor under varying load conditions.
Conclusion
Load sensing control systems, particularly the advanced LUDV configuration, represent a significant advancement in hydraulic technology. By precisely matching pump output to system demands, these systems optimize energy usage while maintaining consistent performance across all operating conditions. This is especially valuable for applications utilizing a hydraulic pto drive motor, where efficiency and control precision are critical.
While challenges remain in terms of system complexity and response times, the benefits of load sensing systems—including reduced energy consumption, improved component lifespan, and enhanced operational control—make them the preferred choice for modern hydraulic machinery. As technology continues to evolve, further improvements in the performance and cost-effectiveness of these systems, including those incorporating a hydraulic pto drive motor, can be expected.