Pump-Valve Coordination Control
5.7 Case Study
5.7.3 Multi-Actuator Motion
5.7 Case Study 95
Fig. 5.21 Comparison of energy consumption
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
case B case A
Enegy consuming (J)
PF_MO PP_MI 2.4%
21.3%
rE = E2E−2 E1 × 100% (5.12)
As shown in Fig. 5.9, the proposed PF_MO controller reduces energy consumption by decreasing pressure loss in the meter-in valve pm. Consequently, the ratio rE is determined by the operating conditions, such as the pre-set pressure margin pm,ref, pump pressure ps, and the reference flow Qref. In case B, the rE ratio reaches 21.3%, which has been verified that the PF_MO controller involves a higher energy efficiency than PP_MI. The ratio rE is relatively high in case B and this can be attributed to two reasons: on the one hand, the lower Qref of the arm leads to greater reduction of pm by the PF_MO controller, as described in Figs. 5.16 and 5.19; on the other hand, since the pump pressure ps of working case B is lower, it has a significant energy-saving effect by reducing pm. Compared to case B, the ratio rE of case A is only 2.4%, which is not particularly significant. This is due to the slow reaction of the pump flow by the PP_MI control (shown in Fig. 5.14), which weakens the energysaving performance by decreasing the pressure margin pm. Because the task cycle of case A is longer, the energy-saving performance is also more significant. When the boom rises, the pump pressure ps is reduced by about 0.4MPa compared to Figs. 5.16 and 5.17.
Fig. 5.22 Studied the duty cycle of the measurement machine
of a continuous duty cycle comprising all three actuators. Figure 5.23 depicts the set trajectory of each actuator. The cycle lasts around 15 s and simulates the manipulator raising three actuators, lowering the boom, retracting the bucket to scoop up material, moving out from the pile, forwarding to a dump truck, and unloading the material from an unloading position.
This duty cycle is applied to the evaluation of three distinct systems. The current hydraulic control systems with PP_MI and PF_MO control systems are measured in comparison to the conventional load sensing (CLS) system. To simulate the conven- tional hydro-mechanical LS mechanism, the displacement of the pump in the CLS system is regulated in a pressure-control manner. The CLS system maintains a constant pressure margin between supply and load pressures of 1.2 MPa.
In Fig. 5.24, different fill patterns are used to identify the cylinder modes for different actuators. The arm and bucket both retract under overrunning loads from 8.5 to 11.5 s. However, only the capacity of the bucket’s load is difficult to drive the movements. Consequently, the mode of the arm swiches to LPR while the mode of the bucket remains NO. Due to the heavy gravity load, the NO modes in the current CLS system are swiched to LPR modes when the boom lowers down. Figure 5.25 illustrates the velocity tracking errors referring to the desired one in Fig. 5.23.
Referring to Figs. 5.24 and 5.25, the excellent motion tracking of all three hydraulic control systems are demonstrated. The velocity dynamics of PP_MI is roughly equal to those of CLS. Compared to the PP_MI and CLS systems, the PF_
MO under NO modes exhibits a faster velocity response and higher overshoot. The abrupt maximum opening of the meter-in valve causes a larger overshoot, rather than
5.7 Case Study 97
Fig. 5.23 Reference velocity of three actuators
Fig. 5.24 Comparision of motion tracking performance
Fig. 5.25 Comparision of velocity tracking errors
a low stability margin. Due to the open-loop control manner, the stability of PF_MO is superior to that of the other two pump-valve coordinate control systems. It can be confirmed that both the velocity and pressure of PF_MO converge on a steady value
in a short period. Static errors of velocity trackings are consistent. In conclusion, the three-level multi-mode switching introduced by the pump-valve coordinate control does not decrease the performance of motion tracking.
In Fig. 5.26a, the supply pressures of the CLS, PP_MI, and PF_MO systems decrease in sequence at any given time. Under NO modes, the supply pressure is reduced, on the one hand, by the decrease in outlet pressure losses. As seen in Figs. 5.27 and 5.28, the backpressures of the boom (0.5~3.5 and 12~14.8 s), arm (3.5~6.5 s), and bucket (8.5~11.5 s) are only 0.3 MPa, compared to approximately 0.9 MPa in the CLS system in Fig. 5.29. On the other hand, the optimal pressure margins regulated by the ECP contribute to the decrease of supply pressures. With the change of the flow, the pressure margins of PP_MI are lowered to 0.6~1.0 MPa. The supply pressure of PF_MO is further reduced compared to PP_MI due to the combination of pump flow control and meter-out valve control, which yields pressure margins of only 0.25 MPa. During periods of potential energy regeneration (6.5~11.5 s), supply pressures are naturally reduced because the pump is no longer required to deliver supply flows.
In Fig. 5.26b, the supply flows of the CLS, PP_MI, and PF_MO systems also decrease in sequence at any given time. This tendency is evident when the potential energy is recovered, since the supply flows of PF_MO and PP_MI come from the tank rather than the pump. Under NO mode, despite there is no flow regen- eration, there are still slight decreases in supply flow because lower supply pressures result in fewer pump volume losses.
Fig. 5.26 Comparison of supply pressure, flow, and power
Fig. 5.27 Cylinder pressure of PF_MO mode
5.7 Case Study 99
Fig. 5.28 Cylinder pressure of PP_MI mode
Fig. 5.29 Cylinder pressure of CLS system
According to the declining trends of supply pressures and flows, Fig. 5.26c depicts the supply powers. To investigate energy efficiency in-depth, the saved energy is further categorized into three parts: reduced inlet losses, reduced outlet losses, and regenerated potential energy.
Figure 5.30a shows the energy consumption for each action. According to Eqs. (5.11) and (5.12), Fig. 5.30b depicts the overall energy consumptions of the three different control methods in the duty cycle, together with the saved energies of the three parts. Compared to the CLS system, PP_MI and PF_MO can achieve energy savings of 25.8% and 35.3%, respectively. The utilization of three-level multi-mode switching yields significant energy savings. The major contributors to energy savings are pressure loss reductions, which account for 81.1% and 86.2% of the total saved energy in PP_MI and PF_MO control modes, respectively. Under the PP_MI mode, the decreased energy outlet losses reach 4380 J, which is much more than decreased energy inlet losses. On the contrary, PF_MO control mode increases outlet losses than PP_MI mode but saves more inlet losses (3301 J), which contributes to better efficiency. It is inferred that the energy losses with PF_MO mode are switched from inlet to outlet. The experiment results correspond closely with the theoretical analysis in Fig. 5.9.
The decreased inlet losses of PP_MI are observed to be negative, indicating that its inlet losses are even greater than those of CLS. The energy-saving properties of the three actuators in Fig. 5.31 can be used to investigate this phenomenon in detail.
As shown in Fig. 5.31a and b, the outlet loss in PP_MI mode is the same as PF_
MO, because the boom and arm are heavy loads during movement, and their meter- out valves are operated in pressure control mode. During the periods (3.5~6.5 s) and
Fig. 5.30 Experimental results of energy consumption
(12~14.8 s) in Fig. 5.31c, the bucket is the light load under NO mode. Its meter-in valve with PP_MI mode is operated in the flow control way. Thus, the difference in loads between the arm and the bucket is dissipated at the meter-in orifice of the bucket. The decreased inlet losses with PP_MI mode are therefore negative. Under PF_MO mode, as opposed to PP_MI mode, the meter-in valve of the bucket is fully open and its meter-out valve is operated in the flow control mode. Consequently, PF_MO captures obvious decreases in the inlet losses, and decreased output losses of PF_MO are fewer than PP_MI mode. In conclusion, for all three actuators, the comprehensive reductions of pressure losses with PF_MO are greater than those with PP_MI.
However, potential energy regeneration in these experiments just accounts for a small amount of saved energy. It can be explained that the measurement device is a mini-excavator, and the potential energy is relatively low. The saved energy by the possible energy regeneration using the proposed multi-mode switching system would be significant for a heavier machine, for instance, a 20 t excavator or crane.
Fig. 5.31 Saving energy of three actuators
References 101