This example shows the usage of secondary control in hydrostatic transmissions with a variable-displacement motor. It uses a pressure-compensated variable-displacement pump, a variable-displacement motor, and is controlled by a secondary control unit. The secondary control unit is a servo-cylinder controlled by a proportional 4-way valve. The servo-cylinder drives the control member of the variable-displacement motor, which is represented by a mass, spring, and damper. The hydrostatic transmission is used in a closed-loop control system with an angular velocity feedback. It drives a winch which lifts a mass and acts against a viscous damper. A dynamic compensator is included in the secondary control unit to improve system stability.
The plots below show the commanded and measured speed of the motor as well as the displacement of the control member which sets the displacement of the motor.