Page 498 - Mechatronics with Experiments

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which may have different loads, it is necessary to sense all loads and use the highest load
signal in control of the pump. Therefore, the pump displacement is determined by the
maximum load pressure signal among the steering circuit (not shown in the figure), boom
circuit, bucket, and auxiliary circuits. The selection of the maximum load pressure is made
by a series of shuttle valves.
The valve–cylinder pair for each function is connected in parallel configuration to
the pump and tank lines. Furthermore, each directional flow control valve (proportional
type) is accompanied by a post-compensator valve. As discussed in Figure 7.57, the post-
compensator valve works to maintain a constant pressure drop across the main valve so that
the function speed is proportional to the lever command regardless of the load as long as
the load is not so large as to saturate the pump. The load pressure feedback is the maximum
load pressure among all circuit load pressures. This selection is made by a series of shuttle
valves. Notice that due to the load sensing signal needed for the compensator, the main
valve has three additional lines (in addition to the P,T, A, B main lines): two of them are
for the input and output of the compensator valve connection and one of them is the load
pressure sensing line for feedback control of the compensator. In addition, there are two
pilot pressure ports, one for each side of the spool, which come from the pilot valves. The
check valve and fixed orifice in parallel with it between the pilot control valve signal to the
main valve spool on both sides have the effect of dampening the pressure oscillations on
the pilot control line and result in smoother operation of the main valve.
The pilot pressure supply for the main valve control of the implement hydraulic
is derived from the output of the main pump pressure using a pressure reducing valve.
The output of the pressure reducing valve is a constant pilot supply pressure. The pilot
valves allow the operator to control the pilot output pressure to the main valves. For each
function (lift, tilt, auxilary) there are two pilot valves. Each valve has two inports (pilot
pressure supply port and tank pressure port) and one output port (output pilot pressure
used to control the main valve). The output pilot pressure is approximately proportional to
the mechanical movement of the lever controlled by the operator. As the operator moves
the lever, that is to lift the bucket, the pilot valve connected to the lift lever sends pilot
pressure (proportional to the lever displacement) to the main valve spool’s control port.
The main valve spool is spring centered and its displacement is proportional to the pilot
pressure differential between the two sides (one side is at the tank pressure, the other side is
modulated by the pilot valve output). Hence, if we assume a constant pressure drop across
the main valve, the cylinder speed will be proportional to the main spool displacement
which is in turn proportional to the lever displacement. In approximate terms, the cylinder
speed is proportional to the lever displacement.

7.5.5 Directional, Proportional, and Servo Valves
Directional flow control valves are categorized based on the following design characteristics
(ISO 6404 standard):
1. Number of external ports: two-port, three-port, four-port. Number of ports refer to
to the plumbing connections to the valve which can be 2, 3, 4 or more. A four port
valve connects the pump and tank (P, T) ports to two load ports (A, B) (Figure 7.44).
2. Number of discrete or continuously adjustable spool positions: ON/OFF two-position,
ON-OFF three position, proportional valves.
3. Neutral spool position flow characteristics: open center where one or more ports are
connected to the tank (i.e., P to T, A to T, B to T, A and B to T, P and A to T, P and B
to T) or closed center where all ports are blocked. Closed center valves are generally