New X-by-wire systems under study for commercial and heavy-duty vehicles such as agricultural tractors are increasingly real autonomous systems, capable of autonomously controlling a vehicle's functionality, actuating the operator's commands, or managing in a complete autonomy a machine function. These applications need a higher performance level from the functional safety point of view, due to the risk of a malfunction consequence.
Researchers from Imamoter have developed a new concept hydraulic spool valve that allows the design of new safer and more compact hydraulic circuit architectures, ensuring higher safety performance levels. The architecture presents advantages both from performance (precision, fastness) and operational points of view.
In hydraulic controls, proportional valves are commonly used in many applications because of their good reliability and low cost. But, in critical applications and if high performance is required, servo-valves are the common choice.
Many studies in recent years were aimed at going beyond traditional architectures of proportional valves, some introducing a completely new concept valve, essentially immune from flow force in which operation is based on a rotary movement. Some works considered new concepts drives such as a piezoelectric actuation servo-valve, exploring the potentials of linear voice coil actuators, and proposing a PWM control strategy, based on an extremely high-frequency switching electronically controlled actuator. On the topic of safety, some work has been done in proposing a safe tolerant digital valve system.
Moreover, recently high performance single-stage servo-proportional valves were introduced to replace the more complicated and expensive pilot operated servo-valves (flapper or jet-pipe type). All these experiences and this innovation trend encourage the development of innovative solutions, with the purpose of enhancing the performance in terms of speed.
Inamoter focused on the features of precision and reliability of a novel hydraulic roto-translating spool valve. The architecture has great advantages with respect to traditional hydraulic valves and is suitable to be used in safety critical applications. The peculiarity of the architecture is that the enhancement of precision and reliability doesn't rely on a feedback signal, but on its intrinsic structure.
Proportional valves, generally speaking, present notches on the spool, and those notches are tailored for a specific operative cycle, to obtain a specific function of area dependent from the spool position. That means each position of spool determines a particular flow gain and particular control accuracy. The design of different spools for different applications is a money and time consuming task. Also, if the real operating condition or work cycle deviates from that specific nominal operative cycle, the control will be degraded in terms of performance and accuracy.
The Imamtoer valve takes advantage of two actuators controlling the metering area, enhancing the performance in terms of precision and flexibility. This component architecture also offers fail safe features.
In safety critical systems like flight controls and brake- and steer-by-wire systems, where the main functionality can't be lost even in case of fault, the most common method to assure functionality in case of a failure is to double the components, increasing the size, resulting in a significant increase of systems weight and complexity, affecting both fixed and running costs.
Because of the diversity of the actuators in the Imamoter valve, it is fault tolerant and can be employed in safety critical systems, offering a different perspective for redundancy concepts in flow regulation techniques and applications.
Having a double actuator opens the possibility of adjusting linear and rotary overlaps and linear and rotary gains to obtain a wide range of control methods. In its initial version the valve is made up of a spool connected to a linear actuator, a rotating sleeve connected to a rotary actuator, and a valve body; an alternative realization has been patented also, having the opposite movement but the same functionality.
From the physical point of view, the sleeve characteristic suggests a native electro-magnetic rotating actuator to act on the hydraulic actuator in a direct way. A direct actuation reduces the mechanical backlashes and speeds up the actuation, and reduces the safety analysis complexity, due to the complex forces to be analyzed if mechanical links connect the hydraulic actuator with the electro-magnetic piloting actuator.
The spool (2) is driven by a linear actuator sliding into the sleeve and has some rectangle notches (20) on one shoulder. A rotational lock of spool is provided by apin (8). The sleeve (3) is actuated by a rotary motor and have holes (31) connecting to the body cavity (A) and rectangular windows (30) that match with the spool's notches. The valve body provides the connection with the rest of circuit through the two cavities (A) and (B), the fastening of the actuators and the anti-rotation pin.
The control of the valve is realized by the mutual movement of the spool with respect to the sleeve. The mutual position of the spool notches with respect to the sleeve windows determines the metering area, controlling the flow between ports (A) and (B).
The double actuator architecture gives the opportunity to implement safety functions, moreover the diversity of actuators guarantee to exclude common mode failures, making the valve fault tolerant. The diversity relies on both actuators’ working principle and physical structure and on the electrical power actuator configuration.
It is possible to obtain a minimum emergency functionality with opportune minimal changes in above described architecture.
Overall, the work resulted in a novel valve type acting as a flow regulation valve for oil flow metering, but being a complex mechatronic system, conceived together with an embedded control system, to provide more complex functionalities and performance.
The component architecture offers advantages both from performance and operational point of view. The valve has plenty of potential in the field of safety and control.
The functional safety features, flexibility, and high precision characteristics of the new roto-translating valve are a promising scenario for a new class of component design and study for innovative electrohydraulic applications. The roto-translating valve concept offers a new valve design perspective. The new architecture, coupled with new automatic control technologies, offers solutions and possibilities that can improve many aspects of the flow control in the high precision, high dynamics or high safety performance level required applications.
While just the main characteristics of the invention were highlighted here, all related to the concept phase and system design, some of those characteristics are very evident and undeniable, thus allowing to further the valve concept development and new control architectures design for safety critical systems.
The application of the system to steer-by-wire systems, brake-by-wire systems, and some other primary control in vehicles, could be a very good test bench for the valve, that could lead at a step ahead in steer- and brake-by-wire designs, as well as in the clutch control in the gear of heavy duty and agricultural machines, proving new solution for safety standard requirement matching.
This article is based on SAE International technical paper 2014-01-2403 by Massimiliano Ruggeri and Pietro Marani, Imamoter.