In the steam turbine digital electro-hydraulic control system (DEH), the electro-hydraulic servo valve G771K208 is the core component for achieving precise control. It provides valve position feedback through LVDT to ensure the accuracy and stability of the turbine valve adjustment action. This article will analyze how the servo valve G771K208 can achieve high-precision valve position feedback from the perspectives of structural principle, signal transmission, closed-loop control, etc.

1. Structure and working principle of servo valve G771K208

The servo valve G771K208 adopts a torque motor + two-stage hydraulic amplification design. The core components include electromagnetic coil, armature, baffle, nozzle and slide valve. When the DEH controller sends a valve position command, the electromagnetic coil generates a magnetic field, drives the armature to deflect and drives the baffle to move. The change in the gap between the baffle and the nozzles on both sides will form a pressure difference, push the slide valve displacement, and thus control the flow of high-pressure fire-resistant oil into the oil motor.

Key features:

1. Torque motor sensitivity: The armature deflection angle is linearly related to the input current, and the resolution can reach 0.1%, ensuring fine-tuning capability.

2. Hydraulic amplification efficiency: The first-stage nozzle baffle mechanism amplifies the electrical signal into hydraulic energy, and the second-stage slide valve further amplifies the flow rate, with a total gain of up to 10^4 times.

2. Implementation process of LVDT valve position feedback

1. Physical installation and signal generation of LVDT

The oil motor piston of the servo valve G771K208 is connected to the regulating valve through a mechanical connecting rod. The LVDT is directly fixed on the oil motor housing, and its iron core is rigidly connected to the piston rod. When the piston moves, the position of the LVDT iron core changes, resulting in a change in the magnetic coupling between the primary coil and the two secondary coils, and the output differential voltage signal Vout=k⋅x (k is the sensitivity coefficient, x is the displacement).

Installation points:

- The LVDT zero position needs to be aligned with the fully closed position of the oil motor, and the deviation needs to be controlled within ±0.1mm.

- The bracket stiffness needs to meet the vibration frequency>100Hz to avoid mechanical resonance interference signal.

2. Signal conditioning and closed-loop control

The AC differential signal output by the LVDT needs to be demodulated by the servo card:

1. Modulation and demodulation: The servo card has a built-in carrier generator (usually a 3-10kHz sine wave) to power the LVDT primary coil, and the secondary signal is converted into a DC voltage through phase-sensitive rectification.

2. Linearization correction: The nonlinear error of the LVDT is compensated by the software algorithm to ensure that the 0-100% valve position corresponds to the 0-5V output, and the linearity error is <0.5%.

3. Closed-loop comparison: The DEH system compares the valve position command signal with the LVDT feedback signal, and the difference is driven by the PID operation to form a closed-loop regulation.

Typical parameters:

- Feedback signal update frequency: 1kHz, response delay <1ms.

- Resolution: When the full stroke is 100mm, the position detection accuracy reaches 0.01mm.

3. Anti-interference and reliability design

1. Electromagnetic compatibility optimization

Servo valve G771K208 uses double-shielded cable to transmit LVDT signals:

- The inner shielding layer is grounded to the servo card to suppress common-mode interference;

- The outer shielding layer is connected to the cabinet ground to isolate the external electromagnetic field.

Experiments show that this design can improve the signal-to-noise ratio by 20dB and ensure the stability of the signal in a strong electromagnetic environment.

2. Redundancy and fault diagnosis

- Dual LVDT redundancy: Some units are equipped with two sets of LVDTs, and the signal takes the median or optimal value, and automatically switches when a single-channel fault occurs.

- Online self-test: The servo card periodically injects test signals to detect the impedance of the LVDT coil (normal value 50-200Ω), and triggers an alarm when abnormal.

4. Typical fault analysis and maintenance

1. Common fault modes

- Signal drift: Wear of the LVDT core or oil adhesion leads to increased nonlinear error, which needs to be cleaned or replaced.

- Zero offset: Mechanical vibration loosens the mounting bracket, which needs to be recalibrated and reinforced.

- Electrical break: Cable connector oxidation or coil break, manifested as a sudden drop in feedback signal to zero, need to check line continuity.

2. Maintenance strategy

- Regular calibration: Perform full stroke calibration every 6 months, adjust servo card zero position and full scale parameters.

- Oil quality management: Keep EH oil clean at NAS 1638 level 5 to prevent particles from blocking the slide valve or wearing the LVDT core.

Servo valve G771K208 uses LVDT to achieve valve position closed-loop feedback. Its core advantages are high-precision signal conversion, strong anti-interference ability and redundant design. Reasonable maintenance and calibration can ensure the long-term stability of the feedback system and provide reliable protection for turbine regulation.

When looking for high-quality, reliable DEH servo valves, YOYIK is undoubtedly a choice worth considering. The company specializes in providing a variety of power equipment including steam turbine accessories, and has won wide acclaim for its high-quality products and services. For more information or inquiries, please contact the customer service below:

E-mail: sales@yoyik.com
Tel: +86-838-2226655
Whatsapp: +86-13618105229

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