Page 504 - Mechatronics with Experiments

JWST499-Cetinkunt
JWST499-c07
490 MECHATRONICS Printer: Yet to Come October 9, 2014 8:41 254mm×178mm
Operator lever

Pilot Main
valves valve

P
P
T P PB
P
PA
P
S
T

(a)
Operator lever
P
PA Main
valve

Pilot
Sensor ECU
valves

P
S
P p
T
T
P
PB
(b)
FIGURE 7.66: Two-stage proportional valve with two pressure reducing valves at the pilot
stage. Pilot stage output is a pressure proportional to the input command (mechanical or
electrical). The main spool displacement is proportional to the pilot output pressure (control
pressure) as a result of the centering springs. (a) Mechanically actuated pilot valves (for
instance, this type of valve is used on Komatsu wheel loader model WA450-5L for its bucket
control system), (b) electrically actuated pilot valves.

relations in two stage proportional valves where the first stage is a pair of pressure reducing
valve are as follows,
K pi
p pc = ⋅ i sol (7.236)
( s + 1)
pi
1
x main = A main ⋅ p pc (7.237)
K spring
K A main
pi
= ⋅ i (s) (7.238)
sol
K ( s + 1)
spring pi
where p is the pilot pressure output from the pressure reducing pilot valves to the main
pc
spool end caps, i sol is the solenoid current, x main is the main spool displacement, K spring
is the centering spring constant of the main spool, A main is the cross-sectional area of the
main spool at the end caps where the pilot control pressure acts, is the time constant
pi
of the transient response between. Notice that each pilot control valve (pressure reducing
pilot valve) may be controlled electrically by a solenoid or manually by a mechanical