Mobile Heavy Equipment - High Efficiency Excavator (Test Bed 1)
See video of the excavator test bed being put through its paces at Purdue University's Maha Fluid Power Center.
Prof. Monika Ivantysynova, Purdue University
One of largest sectors using fluid power is the mobile equipment sector. This sector includes heavy machines such as excavators and wheel loaders that are commonly used in industries such as construction, agriculture, mining, and forestry. Fluid power is essential to this type of its inherent high power density. The power requirements in this mobile equipment are large while the equipment size must be as compact as possible for mobility and packaging. The design of these fluid power systems has generally focused on power and productivity giving little thought to the efficiency of the system. In recent years, however, new and stricter emissions regulations and increasing fuel costs have caused the industry to look for more efficient system designs. With this motivation the CCEFP has designated an excavator, one of the most common multi-actuator mobile machines in use, as a test bed for research. The test bed will be used as a platform to demonstrate the significant improvement in the hydraulic system efficiency of heavy mobile machines that could be achieved by integrating advanced system and component designs being studied by researchers throughout the CCEFP.
CCEFP Compact Excavator at Maha Labs, Purdue University
Statement of Test Bed Goals
The compact excavator test bed has been a demonstrator of throttle-less hydraulic actuation technology since the inception of the Center through spring 2012. This technology, called displacement control (DC) or pump-controlled actuation (PCA), promises fuel savings for multi-actuator machines used widely in the construction, agriculture and forestry industries. Following predictions based on system simulations, significant fuel savings have been demonstrated on the test bed over the standard excavator system.
Beginning February 2012, the test bed has been transitioning to a demonstrator of a novel hydraulic hybrid configuration, called series-parallel hybrid DC system, for which a patent was applied in 2011. The series-hybrid architecture will introduce secondary controlled actuation for the swing drive in combination with the implementation of an energy storage system in parallel to the other DC actuators for the remaining working functions. Such architecture enables energy recovery from all actuators, capture of swing braking energy and 50% engine downsizing. It promises fuel savings beyond those achieved with the prototype non-hybrid DC excavator, as was previously shown through simulation studies in project 1A2 (‘Multi-Actuator Hydraulic Hybrid Machine Systems’), which concluded in June, 2012.
The goals for the project are 50% fuel savings over current state-of-the-art excavator systems, meeting current exhaust emission standards and no degradation in machine performance.
Test Bed’s Role in Support of Strategic Plan
The compact excavator test bed primarily addresses the efficiency thrust of the Center. The prime role of the test bed is to be a demonstrator of energy savings that are possible in multi-actuator machines, through efficient system architectures (utilizing throttle-less actuation, enabling energy recovery and storage) and through advanced power management strategies.
These concepts were investigated in project 1A.2 from 2006-2012 and the test bed draws upon theoretical results achieved to meet these goals. The test bed has also been used for demonstration of a novel human-machine interface as part of project 3A.1. It is well positioned for testing of energy-efficient fluids researched from Project 1G.1 and for evaluation of high efficiency, virtually variable displacement pump/motors from projects 1E2 and 1E3. With the transition of the test bed to a series parallel hybrid DC system, it will also open the door for testing new accumulator technologies researched within the Center (e.g. the advanced strain accumulator, Project 2C.2).
The successful demonstration of DC will open new applications in both large scale mobile machinery and robots, and in human-scale applications like surgery robots or other portable devices where efficient and compact actuator technology is necessary.
Description and Explanation of Research Approach
The state-of-the-art in hydraulic drive and actuation technology involves the use of different forms of resistance control through the use of valves. Most mobile applications use load-sensing (LS), negative flow control (NFC), positive flow control (PFC) or similar architectures. In those systems one or two hydraulically controlled variable displacement pumps provide flow to all actuators by adjusting the system pressure to the highest required pressure of all actuators. Control valves throttle flow from the operating pressure to the desired actuator pressure and meter flow in accordance with respective operator inputs. This leads to large throttling losses across the control valves supplying all actuators except the actuator operating at maximum pressure (in a typical cycle, only one or two actuators operate at high pressures, with the others at low or medium pressures). Further, energy from braking or lowering of actuators is either recovered very inefficiently or not at all, through these architectures.
Displacement controlled (DC) actuation is a very efficient throttle-less actuation with simultaneous utilization of energy recovery without energy storage. The basic circuit for linear single rod cylinders was introduced in 1998 . One variable displacement pump/motor is used per working actuator in a closed-circuit and throttling valves are entirely eliminated. The only control element is the pump displacement and the unit automatically moves over-center to allow energy recovery. The challenge is to demonstrate that pump control can compete with the performance of valve controlled systems with respect to bandwidth and accuracy. Another challenge is to define the maximum number of pumps required in multi-actuator machines by introducing pump switching architectures and new control concepts. This complete new hydraulic actuation technology has been demonstrated on a wheel loader where measurements showed 20% higher fuel efficiency . As a first result of the CCEFP research, a four pump DC system with multiple switching valves was implemented for the eight actuator mini-excavator test bed. 40% fuel savings have been demonstrated through independent, side-by-side testing at a Caterpillar facility over the standard machine in August 2010. The technology offers several new energy efficient features to be introduced to mobile machines. In an associated project, energy efficient active vibration damping of the boom and machine cabin was demonstrated on a skid-steer loader . Competing throttle-less actuation technologies are open-circuit DC actuation  and hydraulic transformers . Open-circuit DC actuation is a feasible alternative. However, it involves the use of several logic valves per actuator and accompanying control laws. The INNAS Hydraulic Transformer (IHT) concept is not yet a proven technology that has been demonstrated on mobile multi-actuator machines.
In the hybrid DC version (Fig. 1) of the test bed (Feb 2012 onward), braking energy of the swing is captured in a hydraulic accumulator, by using a secondary-controlled, over-center, variable displacement motor for the swing drive, as opposed to a fixed displacement motor that was previously in use. The energy stored in the accumulator may be re-used either for reducing the load on the engine or for powering the swing at a later stage. The proposed system architecture does not require any additional units compared to the DC non-hybrid prototype, and energy from the boom, stick and bucket can be recovered through the DC circuits.
Figure 1: Series-Parallel Hybrid DC Excavator
Figure 2: Excavator Truck-Loading Cycle Power Requirement
The typical cyclical operation of these machines, together with added energy storage capability, leads to the idea that engine downsizing is possible with appropriate power management. Peak power requirements would be met by assistance from the accumulator. On the test bed, the engine will not be downsized, however through the use of appropriate power management, engine load will be limited to 50% of peak power in order to demonstrate the feasibility of the concept in a functioning machine.
Caterpillar will soon be releasing (April 2013) the hydraulic hybrid version of a 37 ton excavator (336E H) and next year will announce the release of a hydraulic hybrid loader. The 336E H uses a parallel hybrid architecture, wherein an extra pump/motor is added to the engine shaft, in parallel to the pumps supplying the working actuators. The additional pump is responsible for charging and discharging the accumulator. Caterpillar has claimed 25% fuel savings over the 336E, and although details are not yet available, it is claimed that swing braking energy is captured. However, the addition of another pump in the Caterpillar system will introduce additional power losses to the system. This is not the case in the in CCEFP proposed new series-parallel hybrid DC architecture. Also, due to the fact that all remaining functions are still valve-controlled, it is not possible or very unlikely that energy can be recovered from other working functions like boom, arm or bucket in the 336E-H Caterpillar machine.
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