Page 607 - Mechatronics with Experiments

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ELECTROHYDRAULIC MOTION CONTROL SYSTEMS 593
module (ECM) samples the sensor signal and then sends a current to the pilot valve to move
its spool. The rest of the power hydraulic circuit is the same.
There are two pilot valves per cylinder function, for example lift raise and lift lower
pilot valves for the lift circuit. Each pilot valve is proportionally controlled by a solenoid
in the EH system. Hence, there are two solenoid outputs per function. Each pilot valve
has two inports: pilot pressure supply and tank ports. The outport carries the pilot control
pressure to the main flow control valve. The outport pilot control pressure is proportionally
controlled by the solenoid current between the tank pressure (minimum outport pilot
pressure) and the pilot pressure supply (maximum outport pilot pressure). In short, these
are proportional solenoid controlled pressure reducing valves. As the operator moves a
lever (such as a lift lever), a sensor senses the displacement of the lever and the signal is
sampled by the electronic control module (ECM). The ECM sends current approximately
proportional to the lever displacement signal to the pilot valve solenoids, which in turn
generate a proportional pilot control pressure to the main valves. Note that in wheel loader
applications, the solenoid current sent to the solenoid by the ECM is not exactly proportional
to the lever displacement, but includes a deadband around the neutral position of the lever
and a modulation curve beyond the deadband region that is not necessarily perfectly linear.
The pilot valve control is enabled/disabled by a solenoid controlled two position valve (E1).
The operator select the desired function by a switch in the cab, and the ECM controls the
solenoid (ON/OFF) based on that switch state (ON/OFF).
The displacement of the main flow control valve, and hence the flow rate, is pro-
portional to the pilot control pressure since the force (due to pilot control pressure) acting
on the sides of the valve spool is balanced by a pair of centering springs. It is important
to note that the main flow control valves do not have any flow compensator valves. As
a result, as the load changes, the cylinder speed changes for the same lever command.
The cylinder speeds are load dependent. Furthermore, the lift, tilt, and auxilary function
main flow control valves are hydraulically connected in series and they are of open-center
type. The open-center hydraulic system (fixed displacement pump and open-center valves
combination) are less energy efficient (waste more energy) than the closed-center hydraulic
systems (variable displacement pump and closed-center valves combination). Since the tilt
valve is closer to the pump in the serial connection, the tilt circuit has priority over the lift
circuit. Tilt and auxilary function cylinders have anti cavitation make-up valves connected
to the tank as well as line pressure relief valves. The lift cylinder has a ride control circuit
that is engaged or disengaged by the operator using a selector switch.

7.14 PROBLEMS

3
1. Consider a pump with fixed displacement D = 50 cm ∕rev and the following nominal operating
p
conditions: input shaft speed w shaft = 600 rpm, torque at the input shaft T = 50 Nt ⋅ m, the output
3
pressure p out = 5 MPa, and output flow rate Q out = 450 cm ∕s. Determine the volumetric, mechanical,
and overall efficiency of the pump at this operating condition. Consider another version of this problem
statement where p out = 10 MPa, and all other data is the same. Is there any error in this pump data?
If so, discuss what might be the source of error and suggest ways to correct the data.
3
2. Consider a bidirectional hydraulic motor with fixed displacement of D = 100 cm ∕rev. The
m
load torque it needs to provide at its output shaft is 100 Nt ⋅ m and to maintain a rotational speed of
600 rpm. Determine the necessary flow rate and differential pressure between the two ports (input and
output) of the motor that must be supplied by a hydraulic circuit (i.e., a pump and valve preceeding
the hydraulic motor). What is the power rating of the pump to supply this motor assuming an overall