Master's Thesis
Nonlinear Control of a Hydraulic Servosystem
Abstract:
This thesis presents the derivation, simulation and implementation of
a control system for hydraulic servosystems. Hydraulic systems allow
faster motion and more efficient power transition when compared to electrical
motors. However, the highly nonlinear behavior of these systems
makes accurate tracking control difficult.
The nonlinear properties inherent in hydraulic systems include
the relationship between input current and output flow from the
servovalve, fluid compressibility and friction in the hydraulic
cylinder. The proposed controller is derived using a Lyapunov based
analysis of the nonlinear system equations. The controller is
shown to be stable for both force and position tracking.
Matlab's Simulink program is used to simulate the control law.
This simulation allows the user to
examine the effects
of errors in important physical parameters.
These parameters include the valve coefficients, fluid bulk modulus
and system mass. The simulation also
shows the importance of a good friction model for the
position tracking problem.
After achieving good performance in simulation, the controller
is implemented on an existing hydraulic system. While
industrial controllers often use an oversized cylinder to increase
the stiffness of the fluid column, the system examined in this thesis
employs a long stroke, small diameter
piston to allow fast motion with low flow rates.
Estimates for the valve coefficients, fluid bulk modulus and friction
effects are derived
during off line system testing.
Finally, several tests are run on the experimental
system to show the force and position tracking performance of
our controller.