Page 603 - Mechatronics with Experiments

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ELECTROHYDRAULIC MOTION CONTROL SYSTEMS 589
The IMV concept offers the following advantages and disadvantages. The advantages are
as follows.
Valve geometry is defined in software. Therefore, only three or four mechanical

valves would be needed to cover the low, medium, and high power applications. The
application specific spool geometry would be defined in the software.
Emulated valve geometry in software does not have be the equivalent of a single
mechanical valve. It can function as the equivalent of different types of mechanical
valve during different operating conditions to optimize the performance.
Through the added flexibility in valve control, regenerative energy can be used to

operate the EH system more energy efficiently.
The disadvantages are as follows.

The control task is more complicated. While a standard valve has a single controlled

solenoid, IMV has six controlled solenoids.
Because of the increased number of electrical components, the number of possible

failures is higher.
The IMV concept is one of the most significant new EH technologies in recent years. As
embedded digital control of valves increases, the functionality of a valve component is
defined in software. The mechanical design of it becomes simpler, since the functionality
no longer needs to be machined into the valve. However, the software aspect of the valve
becomes more complicated and without software the component would not be functional.

7.13 CASE STUDIES

7.13.1 Case Study: Multi Function Hydraulic Circuit of a
Caterpillar Wheel Loader
Figure 7.117 (also see Figure 7.14) shows the complete hydraulic system schematics for a
wheel loader model 950G by Caterpillar. The steering sub-system is discussed separately
in detail below. The hydraulic system power is supplied by four major pumps: a variable
displacement pump for the steering sub-system, and three fixed displacement pumps for
implementing the hydraulics main pressure lines, for the brake charging and pilot pressure
lines, and for the cooling fan motor. The steering system also has a secondary power
source as a safety backup using a battery, electric motor, and pump (the fifth pump). This
design uses:
1. Mostly fixed displament pumps (except the primary steering pump) and each pump
is dedicated to one circuit (for example, the steering pump is dedicated to support the
steering sub-system, the implement pump is dedicated to the implement sub-system).
2. lift, tilt, and auxilary function valve–cylinder functions are connected in series to
the hydraulic supply lines (P and T) and are necessarily os open center type (see
Figure 7.14). Furthermore, the function closer to the pump has priority over the
functions following it.
3. The main flow control valves do not have pressure compensation (no compensator
valves), hence the function speed will vary with load for a given command.

The valves of the implement hydraulics are of electrohydraulic (EH) type, meaning
that the pilot control valves are controlled proportionally by an electric current sent to a
solenoid for each pilot valve. The hydraulic system is also available with mechanically