Thrust 1 - Efficiency
Title
1A.2: Multi-Actuator Hydraulic Hybrid Machine Systems
Project Leader
Prof. Monika Ivantysynova (Purdue)
Statement of Project Goals
The goal of the original project 1A.2 was to develop system architectures and control methods for optimal power management in multi-actuator mobile hydraulic machines using displacement-controlled linear and rotary actuators. These concepts reduce overall machine fuel consumption through use of displacement-controlled actuators by avoiding throttling losses, allowing energy recovery and using control methods to achieve effective machine power management. The goal is to demonstrate that at least 40% reduction of energy consumption for typical working cycles of multi-actuator machines compared to the state of the art of machines is achievable.
The project 1A.2 goals are to reduce energy consumption, reduce production costs, and simplify systems for displacement controlled mobile, multi-actuator machines. In the previous phase of project 1A.2, energy efficiency was doubled by implementing displacement controlled actuator circuits. The renewal project will further reduce energy consumption by developing hydraulic hybrid circuits for energy storage and regeneration which will allow load leveling of the engine for more efficient operation. One goal is to further improve the fuel savings beyond the 40% already seen by the test bed to a total fuel savings of 50% from the standard Bobcat excavator system. Simultaneously, it is a target to reduce the rated engine power of the machine by 50% while maintaining the productivity of the original machine. The new system design will also allow cost savings by downsizing the combustion engine and hydraulic oil cooler. The hydraulic and electrical systems will be simplified by the development of methodologies for variable displacement "smart pumps" with optimized swash plate controls and integrated electronics. System design and simulation will be completed by Q2 2011 and testing on TB1 by Q2 2012.
Project's Role in Support of the Strategic Plan
The project primarily addresses the efficiency barrier by developing new system concepts and control strategies for multi-actuator mobile machines. The project also addresses the compactness barrier since displacement-controlled systems allow higher operating pressures and a reduction of interfaces and components. The project will result in an excavator hydraulic system with hydraulic energy storage and integrated electro-hydraulic control hardware. The project leverages past and current research in multi-actuator systems and on-road hybrid vehicles while confronting the barriers of efficient systems, control and energy management, and compact integrated systems. The emphasis on reducing the cost of both operation and production is significant since cost has previously been a limiting factor in market acceptance of displacement controlled hydraulic systems. Smart pump technology will facilitate advancements in hybrid on-road vehicles as well as displacement controlled linear actuators.
Description and explanation of research approach
Project 1A.2 focuses on improving the overall efficiency of mobile hydraulic machines with multiple linear and rotary actuators. Advances in system efficiency are obtained through:
1. Displacement-controlled (DC) actuator systems that eliminate valve metering losses.
2. Real-time control of power generation and transmission in order to maximize operating efficiency of diesel engine and hydraulic pumps.
3. Energy recovery without additional storage devices by sharing power between actuators.
4. Optimizing swash plate regulation systems to create smart pumps for DC actuator technology.
5. Development of multi-actuator DC hydraulic systems with energy storage (DC hydraulic hybrid systems) to maximize energy recovery and reduce rated engine power requirements.
6. Real-time control of power generation and transmission of DC hydraulic hybrid systems in order to maximize operating efficiency of diesel engine and hydraulic pumps.
Actuators for hydraulic manipulators are traditionally powered by a single hydraulic pump and controlled by directional control valves arranged in series or parallel. Such configurations incur energy losses due to metering flow through the valves and do not allow energy to be recovered from aiding loads. Alternative system designs, such as displacement control, that reduce valve power losses and allow energy recovery promise significant fuel savings.
Another area for improvement is power management where the engine, pumps, and actuators are actively controlled to produce and distribute the required power while each component operates as close as possible to its highest efficiency. In the first phase of the project, power management of the displacement control system with no energy storage capabilities was studied. In the second phase of the project, hydraulic hybrid displacement control systems with energy storage capabilities will be analyzed.
Hybrid vehicles have been studied for many years in the transportation sector and more recently there has been a growing interest in the hybridization of off-highway vehicles such as construction, mining and agricultural machines due to both increasing fuel costs as well as more stringent emission regulations continuing to be placed on the industry. Much of the focus has been on electric hybrid systems where companies such as Case, Kobelco, and Komastsu have released or announced the release of hybrid construction equipment to the market. Little focus however seems to have been placed on purely hydraulic hybrid systems. The completion of the first phase of Project 1A2 and demonstrations on Test Bed 1 within the CCEFP have shown that displacement controlled actuation could prove to be an enabling technology for purely hydraulic hybrid systems because it provides for energy recovery and the ability to implement optimal power management strategies for minimizing fuel consumption.
In the future this project will focus on multi-actuator hydraulic hybrid machine systems to further reduce fuel consumption beyond that achieved in the first phase of Project 1A.2. The research approach will encompass a range of topics each aimed at further optimizing displacement controlled actuation on multi-actuator hydraulic hybrid machines. These research topics can be grouped into two main categories: first, research pertaining to the design and simulation of displacement controlled actuation hydraulic hybrid systems and second, research aimed at optimal design of pump control systems to improve performance, compactness, and cost of displacement controlled actuators.
State of the art in multi-actuator off-highway machine power management
Significant research has been done on power management for vehicle drivetrains based on hydrostatic transmissions (HST) and power-split drives. Ossyra (2004) presented a control method for HSTs involving two real-time optimization loops: one feedback loop for the engine based on steady-state efficiency characteristics and the other for the HST based on detailed steady-state loss models of the hydraulic pump/motor units. However, there has been little work on engine power management for mobile hydraulic machinery in which the primary energy consumers are working functions rather than the propulsion drive. A Japanese industrial R&D group controlled pump flow rates and engine speed on an excavator to improve overall efficiency by about 10% (Kakuzen et al., 1988). An Asian research group showed 26% fuel savings using similar methods (Chun and Seo, 1993). A group in Canada constructed a displacement-controlled forestry machine, but did not report fuel measurements (Lawrence et al., 1995). Recently, Alleyne et al. have developed control methods for optimizing the powertrains of earthmoving vehicles with respect to energy consumption (Montgomery and Alleyne, 2006). No previous research exists on power management for excavators or similar machines using pump-controlled actuators.
One advantage of displacement control for power management is that each actuator is powered by an independently controllable pump. This arrangement offers more degrees of freedom than valve-controlled systems in which the actuators are arranged in parallel and powered by a single pump, thus allowing more opportunity for optimizing operation.
State of the art in multi-actuator off-highway hybrid vehicles
Research on hybrid highway vehicles has been ongoing for many years. The motivation for this is clear because the primary operating cost of a passenger vehicle is fuel. In off-highway machines, such as construction equipment, time is much more costly than fuel so system design has traditionally been focused on maximizing machine power and productivity and not on fuel consumption. Recent increases in fuel costs have given some motivation to look for more fuel efficient designs although the greatest push for fuel efficient hybrid systems has been increasing emission regulations placed on OEMs by governments (Kagoshima et al., 2007). In the United States a new set of federal emissions regulations referred to as the Tier emission standards was implemented in the 1990s. The Tier standards represent a scheduled reduction in emissions to be allowed in off-road diesel engines which are to be phased in over time. As each new phase rolls in manufacturers are being forced to find new innovative methods to reduce their machines emissions without reducing productivity. This and similar standards being issued by governments around the world are driving advancements in hybrid technology for off-road machines.
Figure 1: Block diagram of multi-actuator control system with power optimization
Current research trends in hybrid off-road machinery seem to be copying the electric hybrid approach taken by hybrid passenger vehicle manufacturers. Several systems have already been announced for sale on the market. Komatsu released the PC200-8 hybrid 22 ton crawler excavator for sale in 2008. The series hybrid systems consists of a generator driven by the diesel engine, an electric motor to drive the swing (rotation of the upper structure) and a capacitor bank to recover and deliver energy rapidly (Evans, 2009). Komatsu is claiming 25-30% fuel savings from the standard model although the machine cost has been reported to be from 25-50% more (Evans, 2009; Heavy Construction Equipment Going Hybrid, 2008; Komatsu Scores First with Hybrid Excavator, 2007; Yuko, 2008). Since Komatsu's release of their hybrid excavator several other companies including Case, Doosan, Hitachi, and Sumitomo have developed electric hybrid excavators of the same general design and size and either announced their release to the market or unveiled prototypes (Wilkins, 2009; Yuko, 2008).
All of the above mentioned hybrids have maintained their original hydraulic systems for controlling the boom, stick, and bucket functions which require linear actuation. Kobelco has introduced a more complex hybrid system (Kagoshima et al., 2007) which allows for energy recovery from the boom cylinder and reduced metering losses. This system is again a series hybrid where the engine drives a generator and there is an electric motor for the swing drive. However, in this system there are also three more electric motors for powering the pumps controlling the remaining actuator functions. Because the boom function has the most potential for recoverable energy it is actuated with electro-hydrostatic actuation. Besides excavators, Volvo announced the release of the market's first electric hybrid wheel loader which offers 10% fuel savings and higher performance than the traditional machine and Deutz and Atlas Weyhausen have teamed up to build a prototype electric hybrid wheel loader.
There seems to have been much less interest in industry in purely hydraulic hybrid systems using accumulators to recover and store energy. CAT developed such a system for a 50 ton excavator (Gaved, 2004) where energy was recovered from boom operations. They claimed an average fuel savings of 25-30% with the machine operating 5% faster than the standard system. While the savings were similar to that promised by newer electric hybrids it has received little attention.
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