Variable Hydraulic Pressure Relief Systems and Methods for a Material Handling Vehicle

A method for controlling a hydraulic control system for a material handling vehicle, the hydraulic control system including a pump having a pump outlet in fluid communication with a supply passage, a reservoir in fluid communication with a return passage, one or more hydraulic actuators configured to raise and lower a fork assembly attached to a mast of the material handling vehicle, a variable pressure relief valve configured to selectively provide fluid communication from the supply passage to the reservoir, and a controller in communication with the variable pressure relief valve and a pressure sensor configured to measure a fluid pressure in the supply passage, the method comprising:

• calibrating the hydraulic control system by performing steps of:
• supplying, via the controller, a minimum current magnitude to a solenoid of the variable pressure relief valve;
• commanding the a motor a motor driving the pump to run at a maximum pump motor speed thereby increasing a pressure in the supply passage;
• incrementally increasing a magnitude of a current supplied to the solenoid of the variable pressure relief valve, thereby further increasing the pressure in the supply passage;
• determining, as the magnitude of current supplied to the solenoid is incrementally increased, if a pressure measured by the pressure sensor reaches a target pressure;
• and
• upon determining that the pressure measured by the pressure sensor reaches the target pressure, recording the magnitude of current supplied to the solenoid that corresponds with the target pressure.

A hydraulic control system for a material handling vehicle, the material handling vehicle including a pump having a pump outlet in fluid communication with a supply passage, a reservoir in fluid communication with a return passage, one or more hydraulic actuators configured to raise and lower a fork assembly attached to a mast of the material handling vehicle, the hydraulic control system comprising:

• a variable pressure relief valve configured to selectively provide fluid communication from the supply passage to the reservoir when a pressure upstream of the variable pressure relief valve exceeds a variable pressure threshold;
• a controller in communication with a pressure sensor, a height sensor, and the variable pressure relief valve, wherein the pressure sensor is configured to measure a pressure in the supply passage, and the height sensor is configured to measure a height of the fork assembly, the controller being configured to set the variable pressure threshold based on the height of the fork assembly by supplying a control signal to the variable pressure relief valve, and
• wherein the controller is configured to calibrate the hydraulic control system by performing the steps of:
• commanding the variable pressure relief valve to move to a fully open position;
• controlling the pump to increase a pressure in the supply passage;
• controlling the variable pressure relief valve to incrementally move from the fully open position toward a fully closed position by adjusting the control signal supplied to the variable pressure relief valve;
• monitoring the pressure detected by the pressure sensor;
• and
• recording a parameter of the control signal supplied to the variable pressure relief valve when the pressure detected by the pressure sensor reaches a target pressure.