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O+P Fluidtechnik 6/2016

O+P Fluidtechnik 6/2016


SIMULATION FORSCHUNG UND ENTWICKLUNG PEER REVIEWED 03–06 Heavy power cycle The optimal swash plate angle (Figure 3–4) and the optimal rotational speed (Figure 3–5) for the BERS are depicted for boom lowering (negative cylinder velocity) and boom rising operation. The maximum power of the hydraulic-electric-combination is limited by the maximal power of the electric machine P el,lim . This limitation is visualised by the dot-and-dashed line. The following performance analysis has been conducted with a measured cycle, called heavy power cycle (cf. Figure 3–6). During the cycle only the boom cylinders are actuated, the stick cylinder is completely retracted, and the bucket cylinder is fully extracted. The overall BERS efficiency is depicted in Figure 3–7. As can be seen, the BERS, consisting of an electric machine and a hydraulic unit reaches a total efficiency of 84 % and above for a wide range of operating points. Low pressure and low cylinder velocity leads to a lower efficiency (approx. 64 %). The parameter combination of pressure (Δp) and cylinder velocity (ν cyl ) during the heavy power cycle is visualised via the thick black line. 4 PERFORMANCE ANALYSIS The advanced control algorithm is analysed concerning its performance. The algorithm is compared to the former system using the heavy power cycle (Figure 4–1). During the heavy power cycle the arm cylinder is fully retracted and the bucket cylinder is fully extended. The arm cylinder and the bucket cylinder do not move during the heavy power cycle. This leads to a high inertia of the boom and a high load on the boom cylinders. System efficiency Main target of the advanced control algorithm is an increase in efficiency of the BERS. In Figure 4–2 the total efficiency of the conventional control (η tot,con ) and the ACA (η tot,ACA ) are compared for boom lowering (left) and rising (right). 03–07 Total efficiency using the advanced control algorithm With the ACA higher system efficiency is achieved. The gain of efficiency depends on the current operating point. The cycle can be divided into three phases (c.f. Figure 4–2): 1. Stop, no power is transferred 2. Movement at partial load 3. Movement at full load The efficiency difference during the stop phase comes from a numerical error, which occurs while calculating values that are nearly zero, for example when the transferred power is almost zero. During the partial load phase swash plate angle and rotational speed can be varied almost freely within the given limits. In these operating points the ACA-controlled system reaches a higher efficiency than the conventionally-controlled system. The highest total efficiency is reached during the full load phase. The boundary conditions (Δp and in particular ν cyl ) require full swash plate angle and full rotational speed. The efficiency difference between both control systems is zero here, since both algorithms demand the full BERS power. In total an efficiency improvement of up to 10 % for boom lowering and up to 15 % for boom rising could be achieved using the ACA. 4.2 SYSTEM PERFORMANCE The system performance of the ACA is compared to the conventional system on the basis of the heavy power mode. For the machine operator the system reaction to the control command is very important. This can be evaluated by means of the boom cylinder velocity and position. A comparison of both control systems is depicted in Figure 4–3. During boom lowering (depicted on the left side) the difference between desired value (x des and ν des ) and actual value (x cyl and ν cyl ) is marginal except in the end of the cycle when the boom is decelerated. Control deviation can also be seen when the boom is rising. The actual value follows the desired value with an almost constant offset while boom rising at partial load. When the BERS is supported by the main hydraulic system under full load the boom starts to oscillate (phase 3). Similar characteristics can be seen in Figure 4–4. The measured pressure at the bottom end of the boom cylinder (Δp cyl,meas ) shows aperiodic pressure pulsation, likewise the simulated pressure difference at the pump respectively the motor (Δp sim ) shows this behaviour. These considerable pressure pulsations occur with both control systems and worsen during full load. 78 O+P – Ölhydraulik und Pneumatik 6/2016

SIMULATION 04–01 Boom velocity and cylinder pressure in heavy power cycle 04–02 Total efficiency in heavy power mode, boom lowering (left), boom rising (right) 04–03 Desired value und actual value of cylinder position and velocity O+P – Ölhydraulik und Pneumatik 6/2016 79


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