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.
Project Overview Sheet
TB1 Excavator (PDF)
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 Lab, Purdue University
Statement of Test Bed Goals
The compact excavator test bed was 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 various 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.
Over the past few years, efforts have been focused toward transition of the test bed becoming a demonstrator of a novel hydraulic hybrid configuration with pump switching. The series-hybrid architecture introduces 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. The pump switching architecture introduces a distributing manifold that acts as a logic element to minimize the installed pump power while maximizing the number of actuators available to the operator. This architecture leverages fuel savings above those demonstrated with the non-hybrid DC excavator prototype and the reduction of production costs and improved reliability.
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 and through advanced power management strategies. Through project 1A.2 work was developed to evaluate and ultimately implement 1) throttle-less DC actuation, 2) a novel highly efficient hydraulic hybrid swing drive and 3) pump switching, a reliable and cost effective solution for the reduction of the installed pump power. In addition, the developments of project 1A.2 led to the theoretical development, simulation and formulation of appropriate control concepts for each of the aforementioned proposed technologies. The test bed has also been used for the demonstration of a novel human-machine interface as part of project 3A.1 at Georgia Tech. The test bed is well positioned for testing of energy-efficient fluids researched at MSOE (Project 1G.1), and for evaluation of high efficiency, virtually variable displacement pump/motors that utilize high-speed on-off valves (Projects 1E.3 and 1E.6), at Purdue University. With the transmission 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).
Description and Explanation of Research Approach
The current state-of-the-art in hydraulic drive and actuation technology involves the use of different forms of resistance control through the utilization of valves. Most mobile applications use load-sensing (LS), negative flow control (NFC), positive flow control (PFC) architectures or variations of these architectures. In those systems one or two hydraulically controlled variable displacement pumps provide the required 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 other than 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 wasted or recovered very inefficiently, through these architectures.
Displacement controlled (DC) actuation is a highly efficient throttle-less actuation with simultaneous utilization of energy recovery without energy storage. The basic circuit for linear single rod cylinders was introduced by Rahmfeld & Ivantysynova (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 initial challenge was to demonstrate that pump control could compete with the performance of valve controlled systems with respect to bandwidth and accuracy. Another challenge was to define the maximum required pumps in multi-actuator machines by introducing pump switching architectures and new control concepts. This complete new hydraulic actuation technology has been demonstrated in the past 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 were 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 affiliated project, energy efficient active vibration damping of the boom and machine cabin has been 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, which greatly complicates the actuator control. The INNAS Hydraulic Transformer (IHT) concept is not yet a proven technology that has been demonstrated on mobile multi-actuator machines.
The DC hydraulic hybrid prototype captures the swing drive braking energy in a hydraulic accumulator. Through the use of a secondary-controlled variable displacement motor for the swing drive, both the energy recovery concept and the manipulation of the excavator cabin motion are possible. 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. 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. In such scenario, 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 has released a hydraulic hybrid version of a 37-t excavator (336E H) and has announced the release of a hydraulic hybrid front shovel. The 336E H excavator 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, 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 series-parallel hybrid DC architecture. Also, due to the fact that all remaining functions are still valve-controlled, it is not possible to recover energy from other working functions like boom, arm or bucket in the current 336E-H Caterpillar machine.
Figure 1: Series-Parallel Hybrid DC Excavator System (L) and Detailed System Model (R) used in Simulation