Created on:2021-08-28 09:59

Fault diagnosis and maintenance of marine hydraulic equipment_ Zero bias and zero drift of electro-hydraulic servo system

Fault diagnosis and maintenance of marine hydraulic equipment_ Zero bias and zero drift of electro-hydraulic servo system

(1) Overview

It is a key component in modern electro-hydraulic control system. It can be used in various aspects such as position control, speed control, acceleration control and force control. Its outstanding characteristics are small volume, compact structure, large power amplifier coefficient, good linearity, good dynamic response, small dead zone and high precision, which meet the requirements of high-precision servo control system. Therefore, it has been more and more applied in the automatic control system of steering gear.

Users generally hope that the zero drift and zero bias of the servo valve are small, the insensitive area is small and the linearity is good. However, it is very difficult to reduce the zero drift of servo valve, because the zero drift of servo valve is a comprehensive reflection of the manufacturing accuracy and use environment of servo valve components. In the process of servo valve production and commissioning, it often happens that the zero position of the adjusted servo valve changes again when the oil pressure and oil temperature do not change. Therefore, the zero position is difficult to adjust.

(2) Working principle of electro-hydraulic servo valve

The electro-hydraulic servo valve is composed of electromagnetic and hydraulic parts. The electromagnetic part converts the input electrical signal into the movement of the valve. The hydraulic part controls the flow and pressure of the oil output by the movement of the valve, so as to control the position, speed and force of the system. The torque motor of the electro-hydraulic servo valve is connected with the valve core by a feedback mechanism, which can smoothly convert the electric signal of the servo valve and the motion state of the valve core, so as to achieve the self-control of the system. The purpose of dynamic precise control.

(3) Zero drift and zero bias of electro-hydraulic servo valve

Zero bias is an important performance index of electro-hydraulic servo valve. The zero offset of electro-hydraulic servo valve generally refers to the offset of the actual zero point relative to the coordinate origin, which is expressed as the percentage of the input current value required to make the valve at the zero point relative to the rated current. The zero drift of electro-hydraulic servo valve refers to the change of zero bias caused by working conditions or environmental changes, which is also expressed as a percentage of rated current. The asymmetry of component parameters of electro-hydraulic servo valve in production and manufacturing is easy to cause zero bias and zero drift of electro-hydraulic servo valve. When the oil supply pressure or oil temperature changes, it will also cause the change of servo valve zero point, which is called pressure zero drift or temperature zero drift, and is also expressed as a percentage of rated current.

Generally speaking, the zero drift refers to the change of the zero point (the position where the output flow is zero) of the electro-hydraulic servo valve when the oil supply pressure and oil temperature are constant. In fact, it is the change of the dead zone of the electro-hydraulic servo valve, which is expressed by the percentage of the current value to be controlled relative to the rated current.

(4) Opening form of spool valve of electro-hydraulic servo valve

When the electro-hydraulic servo valve is at zero position, there are three opening forms (Fig. E): negative opening (positive overlap), zero opening (zero overlap) and positive opening (negative overlap). The negative opening is that the shoulder width L1 of the valve core is greater than the width L2 of the hole above the valve sleeve, and there is an overlap, that is, the covering amount of the valve core to the square hole of the valve sleeve is positive; Zero opening is that the shoulder width L1 of the valve core is equal to the width L2 of the hole above the valve sleeve, and the covering amount of the valve core to the square hole of the valve sleeve is zero; The positive opening is that the shoulder width L1 of the valve core is less than the width L2 of the hole above the valve sleeve, there is a pre opening amount, and the covering amount of the valve core to the square hole of the valve sleeve is negative. The opening form of the valve has a great influence on its characteristics, especially near the zero position. It is generally believed that the zero opening characteristic is good and the flow gain is linear, but it is very difficult to achieve the real zero opening. Usually, the opening is close to zero, and the valve core covers the square hole of the valve sleeve slightly.

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