Page 102 - Biosystems Engineering

Biosystems Analysis and Optimization 83

Three of these five system states cannot be measured: the two
model states of Eq. (2.100) and the input state of Eq. (2.105). These
states thus need to be estimated using a state estimator to compensate
for model inaccuracy.
Disturbances
Several experiments have been performed to investigate the sensitiv-
ity of the model to disturbances as road slope and machine mass. The
mass of the combine harvester may vary considerably as a function of
the grain in the bin. The effect of this varying machine mass can be
explained by the physical construction of the propulsion system.
The current to the electric coil of the hydraulic pump is called the
pump setting. This electric coil is coupled to a servovalve through a
flipper, which on its turn controls the swash plate position of the
hydraulic pump. Note that there is mechanical feedback present
between the swash plate and the flipper. A current change causes the
flipper to move to the left or to the right. The servovalve then creates
a pressure difference between both cylinders connected to the swash
plate, causing the swash plate to tilt. The mechanical feedback places
the flipper back in its central position. A current thus creates a force
on the flipper, which is counteracted by the force of the mechanical
feedback. The pump itself is driven by the diesel engine. Because of
its construction, the flow only depends on its configuration (pump
setting). As long as the diesel engine can deliver the necessary power,
machine speed will only vary if the pump setting is changed. Thus,
the mass of the grain bin and the terrain slope theoretically do not
influence the propulsion system. However, because of the limitations
of the diesel engine, the mass in the grain bin and the terrain slope
may affect on the machine speed under field conditions (threshing
mechanism on). In road conditions, however, it is very rare that the
diesel engine cannot provide the required power.

2.6.5 Nonlinear MPC
The operator has two degrees of freedom to control the machine
speed while driving on the road: pump setting and engine speed. The
objective of the cruise control system is to use this extra degree of
freedom to minimize the exhaust fumes and the noise production of
the combine harvester without losing performance. The lower the
engine speed of the diesel engine, the less exhaust fumes and noise.
Model-based Predictive Control
MPC is a control technique that calculates an optimal input sequence
by minimizing an objective function subject to a plant model and
other constraints over a fixed prediction horizon (Maciejowski 2002;
Camacho and Bordons 2004). Most MPC approaches are based on
time-domain techniques, which is beneficial for its interpretability by
a wider public.