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2023

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04

Valve driven hydraulic actuator - Selection considerations 3

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【Summary description】More and more automation controls require precise opening or positioning of valves, resulting in an increasing number of occasions where valve actuators are required to accurately position and control valve speed and position. This involves control when the actuator runs near the target area.

More and more automation controls require precise opening or positioning of valves, resulting in an increasing number of occasions where valve actuators are required to accurately position and control valve speed and position. This involves control when the actuator runs near the target area.

One way is to use PLC programming or specialized controller programming, combined with various sensors, such as pressure sensors, position sensors, angle sensors, etc., to correct the error between the feedback signals provided separately or comprehensively and the demand signals.

Continuously comparing the difference between the set signal and the actual feedback signal is to continuously adjust the adjustment input to adjust the position of the actuator. In fact, it can be understood as adjusting the buffering of hydraulic actuators through electrical signals, etc. This is a buffering implemented from a control perspective.

In practical applications, some applications require low costs, and buffering can be achieved by considering the structure of hydraulic actuators, referring to the buffer design of oil cylinders, and designing the buffer structure.

Under heavy load conditions, the actuator generates significant impact loads on the cylinder head and tail. Introduce cushioning at the end of the stroke in the cylinder. The purpose of this built-in buffer device is to minimize the deceleration force and peak hydraulic pressure caused by sudden changes in acceleration. Ideal cushioning device during deceleration period

Establishing a constant pressure can achieve uniform deceleration of speed until it reaches zero. The common buffer designs currently include:

1. Constant throttling buffer

Cylindrical buffer, the principle shape is shown in the figure below.

 

Its characteristic is to start buffering and generate a high pressure peak, then gradually decrease as the stroke continues, while the oil is throttled through a fixed annular gap. Due to the high peak pressure, high impact values are generated, causing machine vibration, noise, and wear, resulting in poor cushioning effect. The buffering performance curve is shown in the comparison chart below.

2. Variable throttling buffer

Variable throttling flow pattern, the principle structure and shape are shown in the following figure

The characteristic of variable throttling flow is to change the throttling area through structural shape changes during movement, so as to maintain a uniform or regular change in buffering pressure. He has a lower initial impact than cylindrical buffering, but often the pressure builds slowly, sometimes resulting in insufficient buffering.

The impact of various buffer structures on the establishment of buffer pressure is shown in the following figure.

These structures need to be designed in detail based on actual project parameters, which is actually a bit like non-standard products, and the cost advantage needs to be discussed.

A European company has designed a special structure on the actuator cylinder head to achieve buffering.

When the gear and rack movement contacts the right part 1, the spring 2 is compressed through the movement of part 1, and the spring begins to play a braking and buffering role. At the same time, part 4 also has a throttling effect. Similar to the function of part 4 in the constant throttle diagram. For different needs, simply replace or adjust the spring compression or spring specifications, and adjust part 4 to achieve the buffering effect. This design concept includes a stroke limit structure and a stroke limit+buffer structure design.

Because hydraulic actuators are powered by hydraulic oil, the design of hydraulic circuits can be used to achieve buffering. Generally, the manufacturer's standard products are used, and through precise calculation of circuit data, suitable hydraulic components are selected to form a hydraulic circuit for buffering. For example, the following image.