Proportional hydraulic valves are key components in mobile agricultural and construction machines, as they control motion for a wide variety of functions via cylinders and hydraulic motors.
Electrohydraulic actuation is often used to control valve main-spool movement. For lower flows two solenoids can directly actuate the spool, but such arrangements result in high hysteresis and require standalone sensors for better precision. Higher flows need pilot-actuated valves, where solenoids control pressure in the pilot stage and let the valve actuator move the spool. Valve control is subject to drawbacks like pilot-oil pressure variations, temperature swings and contamination, and is not precise unless an integrated position sensor permits closed-loop control.
Alternatively, spool valves can be controlled with electromechanical direct actuation. This eliminates the need for pilot oil, which improves controllability, reliability and even reduces complexity. Integrated electronics controls the actuator and processes sensor signals.
In a presentation at this year’s 11th IFK fluid power conference in Aachen, Germany, engineers Jan Lux and Christophe Habegger from Sonceboz SA in Switzerland discussed a new generation of electromechanical valve actuators. The Sonceboz S40, they explained, offers increased power density, an enhanced control architecture and on-board diagnostic features, compared with current versions, and is built to address future challenges such as predictive maintenance in mobile applications.
Electromechanical valve actuation
Sonceboz’s current-generation electromechanical valve actuator includes an electric stepper motor with integrated electronics controlled via a CAN bus. The “intelligent” actuator provides enough power to directly actuate the main spool of a hydraulic valve. Electrical control ensures reliability and precision independent of the oil circuit. Hydraulic pressure is unnecessary if the working hydraulic system is not in operation, which increases efficiency.
The design uses a robust electric motor which resists severe vibrations. Its two-speed gearbox transforms rotary movement of the motor to a linear movement of the valve spool. Motor and electronics are enclosed in an IP69K-rated housing to withstand tough ambient conditions.
A high-performance microcontroller directs the stepper motor and signal processing and ensures precise positioning, high torque and holding torque, and moderate residual force. The system approaches opening points with a linear resolution of 7 µm, without overshoot. The 12-V unit attains spool-adjustment speeds up to 80 mm/sec, and up to 100 mm/sec with the 24-V version. For example, together with the acceleration ramps, step-response times of from 0 to 7 mm in 110 msec or in 100 msec are possible.
Configurable software gives engineers the flexibility to optimize valve-performance curves, detect dead zones and store valve-specific spool overlap. Users can set desired valve characteristics without mechanically adapting the shape of the spool. This results in a high-precision hydraulic valve that compensates for mechanical play, tolerances and wear-related inaccuracies throughout the entire service life of the valve and machine. Plus, the compact actuator installs in the same space as do electrohydraulic actuators. A CAN bus J1939 protocol is used as standard, and other protocols and analog interfaces are available.
An intelligent evaluation of motor data without sensors ensures reliable and precise valve control. Any disruptions caused by external influences, such as a mechanically blocked valve spool, are reported to the ECU by the actuator. Additional temperature sensors enable online diagnostics. Actuator characteristics adapted to specific temperatures can also be saved, so that the actuator can respond to valve changes caused by high fluid viscosity at low temperatures. This makes it possible to control the valve reliably and precisely at ambient temperatures as low as -40° C.
To meet the challenges of tomorrow, the new S40 electromechanical valve actuator features an updated design that takes advantage of new control concepts and connectivity features. Heart of the revamped actuator is an innovative three-phase brushless direct current (BLDC) motor designed for lean, high-volume automotive manufacturing. It improves power density by more than 50% compared with current versions.
Actuator width has been reduced by 2 mm to less than 40 mm which, according to the authors, is key to providing a flat device compatible with many different valve modules. This is due to a special motor bobbin arrangement where all three motor phases are located within a 120° surface and gears mount extremely close to the motor. Likewise, a completely re-engineered electronic architecture permits a more-compact printed circuit board assembly with higher control performance.
The controls use position sensors to detect motor rotor position for reliable, precise positioning. Motor closed-loop control is possible, in short, by continuously monitoring rotor position and adjusting the phase current according to the offset between the target and measured position. This results in less thermal stress for the actuator and the possibility of a power boost for a limited time, important to overcome peak hydraulic loads. Further, an absolute position sensor installed on the output gear measures valve-spool position.
Thanks to the new control architecture, any detected errors have a direct impact on control parameters of the PID position controller. Measuring the offset of current and target position allows a dynamic parameter setting of the PID controllers and improves control characteristics and dynamic behavior of the actuator. In terms of performance, the step response of the S40 (12 V) at 300 N load to move the valve spool 7 mm is 11% faster than in the previous generation and maximum force increases by 66%.
The new control structure simplifies on-board diagnostics (OBD). The current-generation actuator works with an algorithm based on stepper-motor phase current and voltage information and detects stall conditions in the spool valve, but additional diagnostics are not possible because spool position in not known. Now, the absolute position sensor enables additional diagnostic functions, like predictive maintenance and implementation of functional safety.
OBD can directly detect valve-spool related failures, such as dirt particles that can clamp the spool. And the S40 can share its sensor data with a central diagnostic unit and address issues like abrasion at the control edges that increases leakage and affects valve control behavior; friction behavior that changes over time; and spring characteristics that change due to fatigue.
Special diagnostic algorithms monitor the valve-actuator status during operation and perform regular tests. Based on a J1939 diagnostic layer, it is possible to connect the Sonceboz diagnostic technology to the main diagnostic tools already used on mobile machines.
The S40’s on-board electronics allows a decentralized system architecture. That means only data that affects an upper level is transmitted to the ECU. Internal data like motor control, condition monitoring and predictive maintenance functions are processed locally.
Other functions are possible, too. For example, an anti-stick-slip function (dithering) can be implemented in the actuator. And additional sensors in the hydraulic circuit makes it is possible to generate spool-position flow curves. The actuator sends flow-rate signals directly to the ECU, so valve behavior can be adapted to an individual user profile or operation mode. The control can also compensate for environmental impacts like a changing oil viscosity.
For autonomous machines, it is important for connected devices to communicate properly. Sonceboz actuator technology permits pre-set autonomous cycles on actuator electronics. This reduces data transfer and simplifies the system design. It is much easier for the ECU to select the duty cycle and the actuator, in turn, performs the action itself. Otherwise, all feedback data must be transferred to the ECU by the CAN bus, which can quickly overload bus communications.
The connectivity features lead to a better integration of hydraulic components to modern networks on the latest mobile machines. Moreover, the S40’s decentralized intelligence reduces system complexity and helps valve suppliers provide innovative electrohydraulic products for OEMs. All of these features improve the efficiency and productivity of mobile machines. Further, it makes autonomous duty cycles possible. The Sonceboz development supports hydraulic component suppliers in integrating their products successfully into the modern OEM networks of tomorrow.