Page 423 - Mechatronics with Experiments

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JWST499-c07
JWST499-Cetinkunt
ELECTROHYDRAULIC MOTION CONTROL SYSTEMS 409
Controller
DC P.S.
DSP
Amp
Cylinder
Valve
Pump
A
P
M/E
T B
Motor/Engine

Reservoir
FIGURE 7.3: Symbolic representation of the components of an electrohydraulic motion
system for one axis motion: hydraulic power unit (pump and reservoir), valve, actuator,
controller. Since there are no sensors used for the control decision, this is an open loop control
system. It is also called an operator in the loop control system.

the pump more difficult. There are other types of hydraulic power units, such as L-shaped,
vertically mounted motor on reservoir, and T-shaped reservoir.
The valve is the key control element. A valve is used to control (also called modulate or
throttle or meter) the fluid flow. The valve is the metering device. The flow can be regulated
in terms of its rate, direction, and pressure. The type of the valve used and the “control
objective” that drives the valve vary from application to application. The most common
form is a proportional flow control valve. The valve is controlled by a solenoid or torque
motor where the current in the windings generates a proportional spool displacement. The
spool displacement opens the metering ports, and hence controls the flow rate. The valve
amplifier (considered as part of the controller block in Figure 7.3) provides the amplified
version of the low power command signal from the controller. A proportional valve (or
a proportional hydraulic control component) is a device which provides an output that is
proportional to the input. For instance, an electrohydraulic proportional flow control valve

Electrohydraulic servo system
Operator commands Machine
Motor Pump
Power supply &
amplifier Tank
Valve
Hydraulic
actuator

Valve sensor

Actuator sensor

Process sensor

FIGURE 7.4: Basic components of a closed loop controlled electrohydraulic motion system for
one axis motion (pump, valve, cylinder, controller, sensors).