# Instrumentation & Measurement Magazine 24-2 - 110

```Fig. 1. (a) Typical excavator machine. (b) Experimental setup.

acceleration. Notice that the rotational angle θ can be translated as the piston displacement xp since they are geometrically
related.
Fig. 2 shows the novel hydraulic circuit, which is running
the excavator arm.
Elements of the EHA circuit in Fig. 2 are described in
Table 1.
The bidirectional pump is driven by a servomotor. The
auxiliary pump is connected to a relief valve and the one-directional flow control valve. Using the three-position four-way
directional valve, the auxiliary circuit is used to compensate
for the uneven cylinder flows to the single-rod cylinder. Two
check valves have been used in the circuit to ensure the minimum flow and two relief valves have been used to set the
maximum operating pressures in the actuator chambers. The
outlet pressure of auxiliary pump is 80 psi which is adjusted by
a relief valve. The working principles are a modified version

of what is described in [8]. Based on the pressure pL the directional valve has two states: when pL > 0, the solenoid y of the
directional is activated and the solenoid z is deactivated; and
when pL < 0, the solenoid z is activated and the solenoid y is
deactivated.
The flow of the bidirectional pump can be described as:
Q
Q
m Vd  	(2)
1
2

where Q1 is the flow into the pump, Q2 is the flow out of the
pump, ωm is the motor speed and Vd is the pump displacement.
The motor speed equation is described by:

 m   m  m  Kmu 	(3)

where Km is the servomotor gain, τm is the servomotor time constant, and u represents the input voltage to the servomotor and
consequently the excavator
system.
The area ratio of the actuator cap and the rod sides is:

  Ab / Aa	(4)

where A a is cap side area,
and Ab is the rod side area.
The load pressure is defined as:

where p a and p b are the
pressures in the cap side
chamber a and the rod side
chamber b, respectively.
By applying the conservation of mass to sides a
and b of the actuator, we can
write:

Fig. 2. Schematic of the EHA circuit.
110

p
pa   pb	(5)
L

IEEE Instrumentation & Measurement Magazine

April 2021

```

# Instrumentation & Measurement Magazine 24-2

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