Abstract
A Dynamic Hydromachine Control concept is presented here with specific application as a
specialized transmission for a heavy vehicle, which would provide such a vehicle with
good mobility and good ride and handling characteristics as well as the provision of a
large torque/speed range which is essential for multi purpose vehicles. Dynamic Hydromachine
Control is a term given to a hydrostatic transmission which is subjected to both
primary unit dynamic pressure control and secondary unit dynamic speed control, so as to
obtain an extremely versatile hydrostatic transmission with unique characteristics. The
Dynamic Hydromachine Control concept is realized by a digital Transmission Control
Computer and analogue controllers, which interpret the operator/driver inputs and generate
control signals for the hydromachines according to a Control Law, which is petrol
engine speed dependent. This control strategy ensures that the petrol engine cannot stall
(nor overspeed for long periods) and it would also provide a vehicle with handling characteristics
similar to those of a conventional vehicle with an automatic transmission. The
Control Law may be operated in three main modes, namely those of normal differential
action (with limited wheel slip control capability), differential lock action or forced
steering action. The Dynamic Hydromachine Control concept has been evaluated on an experimental
power train, the hydrostatic transmission of which is to be directly implemented in a
technology demonstrator vehicle (at a later stage of the research program). The hydromachines
have been dimensioned (in terms of torque and speed) so as to fulfil the functional
requirements of the eventual technology demonstrator vehicle. No—load characterisation
tests as well as vehicle traction simulation tests were performed on the experimental
hydrostatic transmission. It is shown that the system pressure, the high pressure
accumulator, and the inertial loads, all have significant influences on the overall dynamic
response of the system. The high pressure accumulator which is normally included in a
hydraulic circuit where secondary unit speed control is performed, has been found to be
redundant due to the fast and effective dynamic pressure control which is exercised. It is
however this controlled variation of system pressure together with the absorbing effect of
the high pressure accumulator, which cause pressure oscillations and possible system instability,
thus providing further motivation for the omission of the high pressure accumulator
from the final hydraulic circuit which is to be implemented in the technology demonstrator
vehicle. Apart from these findings, the experimental results prove that the
different operational modes of the Control Law, operate successfully under various simulated
traction load conditions, thus validating the use of the Dynamic Hydromachine Control
concept for multi wheeled heavy vehicles.
M. Ing. (Meg)