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deviation from the straight path would be due to the control resolution of the track speeds
and slip between the tracks and ground. A further level of path tracking accuracy can be
added by closed loop path tracking using GPS signals.

Remark 2 It is assumed that the flushing circuit and charge circuit components are
properly sized so that a certain percentage of the flow is flushed out of the circuit, then
cooled and filtered, and reintroduced back to the circuit via the charge circuit,

Q = Q (7.761)
charge flush
Typically, the charge circuit pressure is used for the swash plate control circuit as the supply
pressure. Hence, for predictable pump and motor displacement control, it is important to
maintain the charge pressure as constant as possible.

Remark 3 In hydrostatic drives which have both their pump and motor as a variable
displacement type, the control of pump and motor order depends on the application mode.
When the vehicle just starts to move, we first up-stoke the pump to start generating hydraulic
power with the motor at its maximum displacement to provide maximum torque, then
slowly reduce motor displacement to get more speed (and less torque). For a given engine
speed, pump displacement controls how much power the hydrostatic circuit draws from
the engine. Motor displacement control is how that power is converted to speed and torque
combination.

7.12 CURRENT TRENDS IN ELECTROHYDRAULICS

The current trend in future electrohydraulic technology is to increase

the power/weight ratio to reduce physical size of components, hence to reduce cost,

software programmable components.

In order to increase the power/weight ratio, the system pressure must be increased, which
results in smaller size components to deliver the same power. However, increased system
pressure reduces the resonant frequency of a hydraulic system due to oil compressibility,
hence the control loop system bandwidth limit is lower. As the supply pressure gets higher,
it is more important to minimize cavitation and air bubbles in the hydraulic lines. Otherwise,
the system response will be significantly slower or even become unstable in the case of
closed loop control. Furthermore, cavitation leads to damage to the hydraulic components
and increases noise.
Another way of reducing the size of components and cost is to make more effective
use of the components. Consider the hydraulic circuits (implement, steering, brake, cooling
fan, pilot hydraulics) and the pumps used to support them in construction equipment
applications. Traditional designs dedicate one or more pumps per circuit. Since all systems
are not used in maximum flow demand at all times, all of the pumps are not used in
their maximum capability. Furthermore, duplicate pumps are provided for safety backup
reasons for critical systems (i.e., steering). The concept of sharing pumps among multiple
circuits through a controllable power distribution valve has been emerging in recent years
(Figure 7.115). Instead of dedicating a pump to each circuit in hardware, the total hydraulic
power of all pumps is combined at a distribution valve, and under program control, the
hydraulic power is distributed to the different subsystems based on demand. This approach
has the promise of making better use of available component capability, reduced cost, and
improved performance.