HST Relief Pressure Control for Stable Work Vehicle Torque
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Solution Overview
Problem
The pressure of a continuously variable transmission (HST) in work vehicles like wheel loaders can increase rapidly due to load fluctuations during operations such as excavation and braking, leading to unnecessary increases in output torque and deteriorating riding comfort.
Innovation Solution
A control device that adjusts the relief pressures of the HST using a control device with a processor to manage the HST pressure based on operational inputs and vehicle states, preventing rapid torque changes by setting optimal relief pressures through a control system that includes a processor, memory, and interface, which acquires operational and measurement data to calculate and adjust HST pressures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If the HST circuit pressure increases rapidly due to load fluctuation, then the output torque increases, but the riding comfort deteriorates
Solution Approach 1:
The control device applies preliminary anti-action by detecting load fluctuations before they cause excessive pressure increases, and preemptively adjusting the HST circuit pressure or output torque to counteract the harmful effect. The control device calculates the difference between actual and target HST circuit pressures, and when this difference exceeds a threshold, it restricts the increase in output torque to prevent riding comfort deterioration.
Solution Approach 2:
The control device implements feedback control by continuously monitoring the HST circuit pressure, comparing it with the target pressure, and adjusting the output torque based on the pressure difference. When the actual pressure exceeds the target pressure by a predetermined threshold, the control device restricts further torque increase, creating a closed-loop control system that maintains pressure within acceptable limits.
2Power
If the HST circuit pressure is increased to handle load fluctuation, then the power transmission capability is improved, but the pressure increases more than necessary
Solution Approach 1:
The control device applies dynamics by making the HST circuit pressure and output torque adjustable rather than fixed. It dynamically modifies the pressure-torque relationship based on real-time operating conditions, allowing the system to maintain adequate power transmission capability while preventing excessive pressure increases that would occur with a fixed, conservative pressure setting.
Solution Approach 2:
The control device changes the parameter of output torque restriction based on the HST circuit pressure difference. When the pressure difference exceeds a threshold, the control device activates torque restriction, effectively changing the torque parameter from unrestricted to restricted mode, thereby optimizing the balance between power transmission and pressure control.
3Productivity
If the output torque is allowed to increase rapidly with load fluctuation, then the work capability is improved, but the riding comfort and operational stability deteriorate
Solution Approach 1:
The control device performs preliminary action by establishing a target HST circuit pressure before load fluctuation occurs and preparing the torque restriction mechanism in advance. When load fluctuation causes the actual pressure to deviate from the target pressure beyond the threshold, the pre-configured torque restriction immediately activates, preventing excessive torque increases that would compromise operational stability.
Data Source
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AI summary
A work vehicle is provided with a power source, a travel device, and a power transmission device for transmitting power of a power source to the travel device. The power transmission device includes a hydrostatic continuously variable transmission having a relief valve capable of setting a relief pressure. A control device of the work vehicle is provided with a relief pressure setting unit that is configured to set the relief pressure of the relief valve in accordance with a target output value of the travel device.