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)
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.
Statement of Test Bed Goals
The project focus is to develop a multi-actuator mobile machine (an excavator in this case) with dramatically improved fuel economy and a significant reduction in engine size that uses displacement controlled hydraulics and hybrid technologies. The machine should be capable of equaling or exceeding the performance capability of the standard version of the machine.
The primary question to be answered is what are the technological barriers, solutions and potential for displacement controlled actuation and hydraulic hybrid technologies to be successful in drastically improving fuel consumption in multi-actuator mobile machines?
Task definition and functional requirements:
- Reduce fuel consumption to 50% of standard excavator
- Reduce engine size by 50% of standard excavator
- Emissions: Meet current TIER emission standards
- Maintain standard machine performance
Test Bed’s Role in Support of Strategic Plan
This test bed supports the center first goal to achieve a drastic improvement in efficiency of existing fluid power applications and to reduce petroleum consumption and pollution. The test bed will be used to demonstrate fuel savings by more efficient fluid power actuator technology and effective machine power management, especially for large and high power equipment. The demonstrated new actuator technology will open new applications in both large scale heavy duty machinery and robots and in human scaled applications like surgery robots or other portable devices where efficient and compact actuator technology is necessary.
Description and Explanation of Research Approach
Test Bed 1, the excavator, was selected primarily to demonstrate potential energy savings which could be achieved for multi-actuator mobile machines through innovative system designs and advanced control strategies. However, the system is also very suitable for demonstrating the capabilities and performances of individual components developed by projects throughout the CCEFP, thus while the focus of the test bed research is to improve the energy efficiency and performance of multi-actuator mobile hydraulic machines, the scope of the test bed also includes demonstrations of individual components and evaluations of their effect on system performance.
The core of the test bed will be based upon the theoretical results from project 1A.2 although technologies developed within the scope of several projects throughout the CCEFP will be integrated onto the test bed for demonstration. The contributions are as follows:
Project 1A.2 (Dr. Ivantysynova, Purdue):
o Controls for optimal power management of multi-actuator DC hydraulic system
o Controls for energy based trajectory optimization
o Design and installation of hybrid hydraulic system and downsizing of excavator engine
o Reduction of hydraulic cooling power due to improved system efficiency
o Design and installation of smart pump with integrated electronic pump controls
Project 1B.1 (Dr. Ivantysynova, Purdue):
o Development of next generation of highly efficient and smart variable displacement pumps
Project 1E.2 (Dr. Lumkes, Purdue):
o Development of virtual variable displacement pump for the excavator low pressure hydraulic system using high
speed on-off valves
Project 1E.3 (Dr. Lumkes, Purdue):
o Development of a high efficiency, high bandwidth, actively controlled variable displacement pump/motor
Project 1G.1 (Dr. Michael, Milwaukee School of Engineering)
o Testing of energy efficient hydraulic fluids
Project 3A.1 (Dr. Book, Georgia Tech)
o Tele-operation of the test bed using haptics controls and the Phantom controller
Project 3D.3 (Dr. Klamecki, University of Minnesota)
o Improved seal design based on adaptive materials
- Four variable displacement pumps were installed on TB1 (compact excavator) along with associated sensors and electronic control hardware. All 8 functions (swing, boom, stick, bucket, track drives, boom offset, and blade) are now displacement controlled.
- Control laws for pump displacement, actuator position and actuator velocity were designed and implemented on TB1.
- The DC hydraulic system is operational and was demonstrated by video at the CCEFP annual meeting on October 7, 2009 and in person to a delegation from Caterpillar on November 4, 2009.
- Performance measurements made on the test bed indicated 50% energy savings compared to original LS valve-controlled hydraulic system for soil digging duty cycle.
Productivity and Fuel Test
The productivity and fuel test for Test bed 1 with DC hydraulics was conducted in cooperation with Caterpillar, Inc. who is a member company of the CCEFP. Two mini excavators were tested: Tested 1 with DC actuators and a standard excavator of the same model. The test site is shown in Figure 1. Measured quantities included the mass of soil loaded, fuel mass consumed, and cycle times. The excavator loaded soil into a 6-ton dump truck, after which the truck was weighed to determine the soil mass. Fuel measurements were obtained by weighing an external fuel tank with a precision scale (5 g resolution). Data was acquired on the DC excavator from all onboard sensors. The standard excavator was not instrumented. All testing was conducted at the same location with the same professional operator on the same day. Identical fuel was used for all tests.
Tables 1 and 2 summarize the results of the test and it can be seen that Test Bed 1 consumed 40% less fuel on average than the standard machine while moving the same amount of dirt. This shows that the goal of reducing the energy consumption of the system by 40% was achieved. The results not only show that the fuel consumption was reduced, but the productivity of the machine was increased because on average the Test bed with DC actuators was able to move 16.6% more tons of dirt per hour.
Figure 1: Productivity test site
Machine Power Management
A fuel efficiency test was conducted to evaluate the proposed optimal power management algorithm from Project 1A.2. The duty cycle consisted of moving a 250 kg mass suspended from the bucket on a chain. Targets were placed on either side of the excavator. While rotating the cabin 180°, the weight was raised from one target and then lowered onto the other. Each trial consisted of 20 repetitions, after which an external fuel tank was weighed to determine the fuel mass consumed. Five trials each were conducted with and without power management. In the latter case, the engine speed was set to high idle (~2700 rev/min).
Figure 2: Power managemetn fuel test setup
Results are tabulated in Table 3. Mean values are listed along with 95% confidence intervals based on a two sided t-distribution.
Using power management, the engine operates at a lower speed and the pumps operate at higher displacement. In this way, the same actuator motion is attained more efficiently. The measured duty cycle was intentionally selected because it requires slow, careful motions to prevent the weight from swinging. The cycle is comparable to pipe laying or other realistic tasks for an excavator. In a more demanding cycle, there would be less opportunity for reducing engine speed and fuel consumption.
DC Hydraulic Hybrid Feasibility Study
Through project 1A2 a feasibility study was done for a DC hydraulic hybrid system on TB1. The simulation model previously created for the DC excavator test bed was modified to include an additional pump/motor (18 cc/rev) and accumulator (5 L) to create a parallel hybrid system. Measurements from the productivity study where an expert operator was performing a truck loading cycle as fast as possible were used to generate actuator trajectories and loads for the cycle. This cycle was selected because it is very aggressive representing the extremes of the power requirements for the DC actuators. As previously stated one of the project goals is to be able to reduce the required engine power of the machine by 50%. To check the feasibility of this goal the simulation was controlled to limit the engine power output to be 50% of the current test bed engine power where power requirements of the cycle above that level would be met by the hydraulic accumulator and the additional pump/motor.
Figure 3 shows the simulated engine power for the non-hybrid and the hybrid DC hydraulic systems during the digging cycle. From the figure it can be seen the hybrid system power was able to be limited to be half of the maximum engine power suggesting that the engine size could be reduced without sacrificing the productivity of the machine for the truck loading cycle.
Figure 3: Simulated engine power for non-hybrid DC hydraulic systems