Page 96 - Biosystems Engineering

Biosystems Analysis and Optimization 77

Front axle with Mechanical
differential gearbox

Engine Steering
speed current

diesel Hydrostatic Hydrostatic
engine pump engine

FIGURE 2.24 A scheme of the propulsion system of a combine harvester.

2.6.2 System Description
The propulsion system of a combine harvester consists of a diesel
engine, which drives a variable hydrostatic pump (Fig. 2.24). The
hydrostatic flow thus depends on the speed of the diesel engine and
on the pump setting. Hydrostatic flow, as well as engine speed, can
be set independently by the operator. On our test machine (a New
Holland CR combine harvester), engine speed can vary between a
minimum (1300 rpm) and a maximum level (2100 rpm). Note how-
ever, that different combinations of engine speeds and pump settings
result in the same machine speed.
During road transport, the operator can vary the pump setting as
well as the speed of the diesel engine. This is not possible in field
conditions. Because some components such as the cleaning system
are mechanically connected to the diesel engine, the engine has to run
at maximal speed while harvesting. Under road conditions, the speed
of the machine can thus be controlled by two independent inputs: the
pump setting and the engine speed of the diesel. It is clear that an
operator does not have the time to use this extra degree of freedom
and will always run the diesel engine at maximal engine speed while
controlling the machine speed with the hydrostatic flow. However,
the most straightforward way to reduce exhaust fumes and noise is to
run the engine speed as low as possible.

2.6.3 Control Objective
The purpose of this controller is to run the combine harvester at the
lowest possible engine speed without decreasing acceleration perfor-
mance. In other words, the engine speed is allowed to increase dur-
ing acceleration, but after the acceleration to the set point, the engine
speed has to return to the lowest possible value without causing a
set-point error. To reach this goal, the objective function is defined as
follows: