author: MSc. Marek Szypuła
The current development of machines is putting more and more demands on hydraulic drive systems, in terms of smooth regulation as well as and speed of operation. Proportional valves are the future of hydraulic drives, they combine the advantages of a hydraulic drive (high force with small dimensions) with the possibilities offered by automation.
The link between hydraulics and automatics lies within proportional electromagnet. By using proportional valves, we can automatically adjust the pressure (force or torque of the mechanical system) or the flow (speed of the mechanical system).
Moving large masses is associated with inertia forces, the use of proportional directional control valves allows for deceleration and a gentle start of inert masses, thanks to which proportional hydraulics ensures safety, preventing damage to the structure, and also ensures stability in reloading devices.
The usage of proportional hydraulics allows for solutions of machines working remotely, in places dangerous for the operator, or, when the need arises, to more precisely define the movement of the machine.
Directional control valves can operate in two control systems, open-loop and closed-loop.
Below is an example of a USAB6 valve operating in an open-loop system; the 30RE20D card is an amplifier that converts standard input signals (-20mA + 20mA, ± 10V) into the current supplying the electromagnet.
Below is an example of a USEB6 valve operating in a closed-loop system; The 30RE21D card is an amplifier that converts standard input signals (4-20 mA, ± 10V) into the current supplying the electromagnet, it also has a PID controller that reacts to the feedback signal of the spool position. The ECI9D-01 card is a feedback element, it converts the signal from a sensor of directional valve spool position (the spool position sensor is built into the directional valve) into standard 4-20 mA, ± 10V signals.
When comparing the two control systems, closed-loop control is more expensive because the feedback loop has to be resolved. Thanks to this, a closed-loop system has a hysteresis of less than 1%, compared to an open-loop system, where we obtain a hysteresis of 6%; repeatability of set values also below 1% for a closed-loop system, for an open-loop system, we obtain repeatability of 3%.
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