Mobile hydraulic applications distinguish themselves from other hydraulic applications, such as industrial hydraulics, because the pressure and flow demand varies greatly over time and between different functions. Unlike other hydraulic applications, several functions are often supplied by one single pump. This means that the total installed power on the actuator side is generally considerably higher than the installed pump power. This is possible because the actuators almost never require their maximum power at the same time.

Given these conditions, the development of working hydraulic systems has moved towards load sensing systems. These systems have high energy efficiency since the pump pressure continuously adapts to the highest load. However, load sensing systems suffer from poor dynamic characteristics due to their closed loop pressure control. In specific points of operation, load sensing systems can thereby display oscillatory behaviour.

Another pump control strategy is to remove the load sensing hose and control the pump according to the operator’s command signals. The pump displacement setting is then controlled according to the sum of all requested load flows. This is a change from a closed loop control mode to an open control mode with no feedbacks present. It makes the system design process simpler since the pump can be designed to meet the response requirements without considering system stability.

There are several challenges with an open pump control strategy. One of them is to match the pump flow and the expected load flows, often referred to as flow matching. This problem occurs if the valves are equipped with traditional pressure compensators, controlling the absolute flow through the directional valve. If the flow sent by the pump is higher than the sum of all load flows, the pump pressure will increase until the system relief valve opens and the system will emerge as a constant pressure system. There exist several workarounds to this problem. Feedback signals could be introduced by using sensors or the system could be complemented with a bleed-off valve. Another solution is to use flow-sharing pressure compensators, distributing the entire pump flow relative to the individual valve openings. A drawback with flow sharing compensators is that the highest load dynamically will disturb all lighter loads.

This paper presents another solution to the flow matching problem. Instead of being flow or pressure controlled, the pump could be controlled partly by pressure and partly by flow. Load sensing systems and flow on demand systems are extreme points on this continuous scale. From a control theory point of view, pressure control would be the feedback gain and flow control would be the feed forward gain. By using this hybrid pump control approach, it is possible to take advantage of the benefits with load sensing and flow on demand and at the same time avoid their drawbacks.

The benefits and drawbacks with the respective system design are shown and demonstrated in this paper. One way of finding the optimal control parameters for pressure and flow control are proposed. A higher feed forward gain will increase the system response but exacerbate the problem of flow matching. A higher feedback gain will solve the flow matching problem but the system will get more oscillative. The user benefits of optimal control parameters are fast response, high stability and high robustness.

In this paper, both theoretical studies and practical implementations demonstrate the capability of a hybrid pump control approach. Experiments show that it is possible to improve the system characteristics by finding the optimal compromise between pressure and flow control.

Hydraulic control system is the most important transmission of construction machinery due to its high power density. Many hydraulic circuits, such as open center systems, load sensing systems, flow sharing systems, have been developed in the past few decades. However, these mobile hydraulics should be more efficiency to reduce combustion emission imposed by government legislations. The electronic flow matching control (EFMC after)system, which works in a synchronous mode of electrohyraulic valves and electrohyraulic pumps, has been a research focus in recent years because of its potential to improve the control performance and energy efficiency in comparison of the current mobile hydraulics.
In this paper, to improve the energy efficiency of mobile hydraulics, a flow sharing control system with electronic flow matching is developed, also a novel flow dividing algorithm is proposed to reduce the pressure loss of the control valve. The flow sharing system with electronic flow matching consists of an electrically controlled pump, a relief valve, proportional directional valves, post compensated valves and actuators. The working conditions of flow saturation and typical digging cycles were experimentally investigated on a mini excavator test bench. The results indicated that compared with the conventional flow sharing system in typical digging tests, the energy consumption of the proposed system was reduced by 10%.
In the traditional way, the control signals from the input devices, like joysticks, are directly delivered to the valves or pumps. In this study, a new flow dividing algorithm is proposed for efficiency improvement. Firstly, the controller selects the actuator with highest flow demand, and then the maximum signal is given to the corresponding control valve to fully open it. The control signals of the other valves are modified proportionally according to the control inputs. Based on the anti-saturation ability of flow sharing valves, the operation performance by the proposed method is in accordance with that by the traditional method. Meanwhile, the pressure loss of the directional valve by the proposed method is reduced due to the larger cross-sectional area, therefore the energy efficiency is improved. The experimental results indicated that the energy efficiency was improved by 13% with the proposed method.
The rest of the paper is structured as follows: Firstly the system configuration of the flow sharing system with electronic flow matching is introduced , and then the simulation model is built up in section 2. Section 3 describes the simulation and experimental results to compare the conventional flow sharing system with the proposed system. Section 4 gives the introduction of the proposed flow dividing algorithm and the experimental tests in comparison of the traditional control method. Section 5 describes the conclusions and future works.

This paper presents an innovative experimental auto-tuning method of a controller used to operate an electro-hydraulic crane with the aim of reducing system oscillations. Commonly, the control tuning is conducted using two different strategies: analytical derivation from system model, or trial-and-error based methodologies, in many cases applied directly on an experimental set up. Both mentioned approaches are in most of the cases time consuming and sometimes inaccurate. For the first approach, the analytical derivation may lead to poor control performance in case of inaccurate model parameters estimate, while, for the second case, there is often not guarantee the controller parameters are optimal.
The proposed auto-tuning method is applied to a particular control strategy developed by the authors to reduce oscillations in mobile hydraulic machines. The pressure feedback control method, presented in [1] solely requires pressure sensors mounted on the workports of the flow control valves of the hydraulic system. Despite the generality of the proposed technique, this paper presents the application of the controller, and of the novel auto-tuning method, on the particular case of the mechanical arms of an hydraulic crane installed at Maha Fluid Power Research Center of Purdue University.
The experimental-auto-tuning method proposed in this research in based on an iterative loop in which the optimal control parameters are found through an optimization process. The process iterations are represented by real experiments. At each iteration, the effectiveness of the controller is evaluated through a proper objective function used to quantify the system oscillations. The control performance is measured as deviation of the actual dynamics from the ideal one, in terms of system acceleration. Also, the procedure always verifies that the considered control parameters always meet the requirements in terms of machine productivity. Particular effort is dedicated to the optimization procedure used to minimize the total number of experiments to be performed in a fully automated tuning procedure.
The paper describes the control performances in terms of measured acceleration of the mechanical arms for different drive cycles of the machine taken as reference. The results show also the capability of the presented tuning method to produce better control performance with respect to a traditional tuning method previously utilized to control the reference machine.
The proposed experimental-auto-tuning method has potential to become an industry practice since it offers an automatic and time-effective way to tune the control parameters of any electro-hydraulic machine, without requiring significant expertise in the controller design.


References:
[1] Cristofori D., Vacca A., 2012, A Novel Pressure-Feedback Based Adaptive Control Method to Damp Instabilities in Hydraulic Machines, SAE Int. J. Commer. Veh. 5(2):2012.

Steering systems of mobile machinery have not seen adequate innovation and breakthroughs in the past few decades, except for a few incremental developments. Hydrostatic steering dominates the state-of-the-art, and is ubiquitous on most agricultural and construction machinery. This paper introduces an innovative steering concept for articulated frame machines, namely wheel loaders. The new system is based on pump displacement control (DC), a well-established energy efficient alternative to valve control. DC technology has been successfully researched and applied to the implement functions (working hydraulics) of mobile machinery resulting in significant fuel savings. However, DC has never been investigated for the steering function of such machines, which is the focus of the research conveyed in this paper.
In addition to its potential fuel savings, a DC steering system also promises multiple other advantages in regards to machine safety, productivity, packaging, and comfort. DC steering can be classified as an electrohydraulic steer-by-wire system, which offers the numerous benefits, and challenges, typically associated with by-wire systems. On top of improved fuel economy, a DC steered machine can be made safer via active steering intervention to stabilize lateral instabilities and reject external disturbances. Also, DC steering increases the productivity and safety of the machine by employing variable-rate / variable-effort steering algorithms, which adjust the sensitivity and gain levels based on the machine’s operating conditions. At low vehicle speeds the number of steering wheel turns and the amount of torque feedback are reduced, resulting in reduced operating time per unit work as well as reduced operator fatigue. On the other hand, the number of steering wheel turns and the amount of torque feedback are increased to prevent sudden steering perturbations from destabilizing the machine while traveling at high speeds.
To permit for generating appropriate control algorithms that offer suitable performance, a high fidelity dynamic model of the ‘plant’ had to be developed. The plant model constitutes of two main subsystems: an electrohydraulic component and a mechanics one. The electrohydraulic module includes models of the variable axial piston pump, pump adjustment system, low pressure system, steering actuator, and others; whereas the mechanics module includes an advanced multi-DOF vehicle dynamics model based on the Lagrangian principle. Complex nonlinear models were first generated, after which linearized models were developed and validated, paving the way in front of controller synthesis and design.
Modern control theory techniques were utilized for controlling the linear time invariant (LTI) system. State space formulation was facilitated by the system of linear first order differential equations derived for the two subsystems. Controller design focused on the following approaches: full state feedback, reference tracking, output feedback, and state observer/estimator design to reduce the number of required sensors by employing virtual sensing instead. Attention was then turned to the controllable part of the system where the input (pump displacement) results in a direct adjustment of the desired output (articulation angle).
A designated test vehicle, a wheel loader, has been baseline tested with its stock hydrostatic steering system and is currently undergoing the necessary hardware and software modifications for implementing the new DC steer-by-wire system. Upon completion, the machine will be retested at which point the generated models and the designed controller will be validated and fine-tuned. The measurement and simulation results will be included in this paper.