The objective for this Master’s Thesis is to investigate and design controllers for the concept
of separate meter-in separate meter-out control for a hydraulic cylinder. The separate
meter-in separate meter-out concept is investigated under various operation conditions
where the velocity of the cylinder is positive and negative, an overrunning and resisitve
load force is applied to the system and the result of the analysis is that the concept has
some limitations regarding certain operating conditions. The limitations associated with
the concept is that cavitation and excessive pressure build up could occur. Cavitation can
occur when the velocity is positive, an overrunning load is present and meter-in flow rate
control is being used, causing a restriction on the inlet flow, which results in insufficient
supply flow to the piston chamber and eventually causes the hydraulic oil to cavitate. A
similar scenario where cavitation can occur is when the velocity is negative, a resisitve load
is present and meter-in flow rate control is being used, which causes the inlet flow to be
restricted and causes insufficient flow to the rod chamber which can lead to caviation in the
chamber. Another limit associated with the concept is excessive pressure build up which
occur in the scenario where meter-out flow rate control is being used and the velocity of
the cylinder is positive and an overrunning load is present and in the scenario where the
velocity is negative and a resisitve load is present. In these critical operating conditions
the caviation or excessive pressure can result in unwanted behavior for the system and to
overcome these suitable control strategies are investigated.
The possible control strategies are compared and two chosen control strategies are seen
suitable to control the system. The control strategies investigated further are a slave
function control where one input signal is dependent on the other input signal which results
in the system only being able to control one state. The other control strategy is where the
two proportional valves are controlled independently which allowing more flexibility and
better performance for the system. Due to the limits for the slave function control the
chosen control strategy that is further investigated is when the two proportional valves are
controlled independently.
When the proportional valves are controlled independently a coupling analysis is conducted
to observe the input-output parring when choosing the primary control state as the velocity
and the secondary control state as either the piston or the rod pressure. The coupling
analysis includes two analysis, the relative gain array and a singular value decomposition.
The results of the coupling analysis shows the least input-output parring between the
velocity and the rod pressure. The control method where the velocity is controlled by input
signal up and the rod pressure is controlled by the input signal ur is further investigated
with respect to designing controllers for the control method.
To decouple the system a pre-compensator is designed and implemented in order to treat the system as two single-input single-output systems. One single-input single-output
system where the velocity is controlled by the input signal up and is independent on
the input signal ur and another single-input single-out system where the rod pressure is
entirely controlled by the input signal ur. For this control strategy two PI-controllers
are designed where the design procedure is that the pressure controller is a factor of 10
faster than the velocity controller to eliminate eventually controller interference. The
controllers are implemented and firstly tested on the linear system where the performance
for the velocity and rod pressure tracking is acceptable. The controllers are then tested
on the non-linear model where the velocity tracking is seen to not be acceptable when
the velocity reference is negative. The poorly performance for the velocity tracking in
the non-linear model is due to changing model parameters when the velocity changes to
negative. To overcome the varying model parameters and the poorly velocity tracking for a
negative velocity reference, a set of new PI-controllers are designed based on a linear model
conducted for negative velocities, which should be activated when the velocity reference
is negative. This is implemented with a switch depending on the reference velocity which
switches between the controllers designed for positive and negative velocity. Implementing
this control structure improved the overall tracking results of the velocity and thereby it
can be concluded that a control method for a separate meter-in separate meter-out setup
can be designed.