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Introduction

The VariStroke-II is a linear electro-hydraulic actuator that utilizes a double-acting power cylinder with

integrated electronic driver module, servo valve, and redundant LVDT (Linear Variable Differential

Transformer) position feedback sensors to precisely control steam turbine valves. The actuator’s driver

module accepts one 4–20 mA position demand and compares it to the sensed actuator shaft position to

accurately control output shaft position.

The actuator’s output shaft position is controlled by a digital controller combined with an integrated

rotary

servo valve that ports supply oil to and from its power cylinder. The actuator digital controller architecture

allows it to perform stable position control during normal conditions, and also respond quickly to desired

valve step changes during system or plant transients. As a means of protecting the turbine, an internal

servo valve return spring forces the actuator to a failsafe position upon any internal unit failure (electrical

input power failure, position sensor failure, processor failure, etc.).

Hydraulic Power Cylinder

The VariStroke Remote Servo can be connected to any hydraulic cylinder,

however; proper operation requires that the VariStroke Stability Equation be

satisfied (see Chapter 2. Stability Specifications). In order to control cylinder

position, the Cylinder must be equipped with a position feedback sensor. The

position sensor must meet the following specifications:

 Output Signal: 4–20 mA

 Input voltage (provided by the VariStroke Circuit Board): 15 Vdc

 Update Rate: ≤1 ms

 Linearity: ±0.04% Full Stroke

 Current Drain: < 100 mA

 Sensor Length must not exceed 2 times the Cylinder Stroke Length

Cylinder Position Control

The cylinder position controller adjusts the hydraulic power cylinder position to

match the feedback signal to the demand.

Both the servo position controller and cylinder position controller are monitored to

ensure accurate tracking.

The position controller regulates a pulse width modulated (PWM) drive signal to

the actuator. The drive current to the actuator is regulated, transiently allowing up

to 10 A to be provided to move the actuator at its maximum speed and torque. A

steady-state current limit becomes active after a period of a few seconds to

protect the actuator and electronics.

The shield connections for the Analog Output (terminal #20), CAN1 (terminal

#23), and CAN2 (terminal #29) are through capacitors only as indicated in the

wiring section of this manual.

The power supply section performs the EMI filtering on the (18 to 32) V (dc) input

voltage and generates controlled voltages for several electronics sub-systems.

The power supply system is monitored for proper operation. If input voltage or

internal power systems are detected outside of allowable operating ranges, a

diagnostic alarm will be enunciated.

Calibration and configuration of alarms and shut down and redundancy operation

are configurable via the PC Service Tool.

The primary demand and redundant demand / feedback input signals are

designed for a (4 to 20) mA control signal. Each input signal is EMC protected.

Discrete outputs are provided for alarm and shut down enunciation. An internal

LED also is illuminated when a fault condition is detected. Cover needs to be

removed to see this LED. The configurable discrete output can be customized to

output a variety of enunciations using the PC Service Tool. All of the discrete

outputs are configurable for normally-open or normally-closed action using the

PC Service Tool.

Electronic Driver Module (PCB)

The printed circuit board (PCB) is mounted on top of the housing (see Figure

1-6). The PCB performs the following tasks:

 Power Supply

 Isolated Input and Outputs

 Dual Redundant Demand Inputs

 Dual Redundant Inputs for Final Cylinder Feedback

 Microprocessor-based Control

 Actuator H-Bridge Drive

 Current Limiting for Thermal Protection

 Advanced Diagnostics

 Discrete Outputs for Fault, Alarm, and Shutdown Enunciation

The shield connections for the Analog Output (terminal #20), CAN1 (terminal

#23), and CAN2 (terminal #29) are through capacitors only as indicated in the

wiring section of this manual.

The power supply section performs the EMI filtering on the (18 to 32) V (dc) input

voltage and generates controlled voltages for several electronics sub-systems.

The power supply system is monitored for proper operation. If input voltage or

internal power systems are detected outside of allowable operating ranges, a

diagnostic alarm will be enunciated.

Calibration and configuration of alarms and shut down and redundancy operation

are configurable via the PC Service Tool.

Rotary Servo Valve

The hydraulic servo valve has five ports: Supply, two Control Ports, Over Board

Drain (OVBD), and Drain/Tank. With the hydraulic valve in its middle position,

both control ports are blocked. As the valve rotates, supply pressure is

connected to a control port while simultaneously connecting the drain to the other

control port. The combined action of the servo position controller and cylinder

position controller modulate the power cylinder position as necessary to match

the input demand. OVBD is permanently connected to drain and can (optionally)

be connected to the OVBD connection on Hydraulic Power Cylinder to drain any

leakage pass the primary rod seal.

A unique function of the software is a periodic, symmetrically opposed impulse

(called “Silt Buster”) which flushes silt and debris from the servo valve without

causing undue wear. At the interval and amplitude selected by the user, this

function provides a very rapid motion of the hydraulic valve, allowing any silt to

be flushed to the drain passage. This motion is followed immediately by a step of

equal amplitude in the opposite direction. The opposing symmetry of the impulse

results in no net change in fluid volume to the controlled servo valve, and thus

does not interrupt the control of the turbine. This unique function provides a

higher degree of stability, reliability, and silt resistance.

If the unit detects any diagnostic shut down condition, or if the detected

diagnostic condition prevents reliable control, or if a loss of power occurs, the

servo valve return spring forces the valve to connect the appropriate control

pressure to drain, causing the cylinder to move to the fail-safe position.

Cylinder Position Control

The cylinder position controller adjusts the hydraulic power cylinder position to

match the feedback signal to the demand.

Both the servo position controller and cylinder position controller are monitored to

ensure accurate tracking.

The position controller regulates a pulse width modulated (PWM) drive signal to

the actuator. The drive current to the actuator is regulated, transiently allowing up

to 10 Amps to be provided to move the actuator at its maximum speed and

torque. A steady state current limit becomes active after a period of a few

seconds to protect the actuator and electronics.

The primary demand and redundant demand / feedback input signals are

designed for a (4 to 20) mA control signal. Each input signal is EMC protected.

Discrete outputs are provided for alarm and shut down enunciation. An internal

LED also is illuminated when a fault condition is detected. Cover needs to be

removed to see this LED. The configurable discrete output can be customized to

output a variety of enunciations using the PC Service Tool. All of the discrete

outputs are configurable for normally-open or normally-closed action using the

PC Service Tool.

Electronic Driver Module (PCB)

The printed circuit board (PCB) is mounted on top of the housing. The PCB

performs the following tasks:

 Power Supply

 Isolated Input and Outputs

 Dual Redundant Demand inputs

 Dual Redundant inputs for Final Cylinder Feedback

  Microprocessor based control

 Actuator H-Bridge Drive

 Current Limiting for Thermal Protection

 Advanced Diagnostics

 Discrete Outputs for Fault, Alarm and Shutdown Enunciation

The shield connections for the Analog Output (terminal #20), CAN1 (terminal

#23) and CAN2 (terminal #29) are through capacitors only as indicated in the

wiring section of this manual.

The power supply section performs the EMI filtering on the (18 to 32) V (dc) input

voltage and generates controlled voltages for several electronics sub-systems.

The power supply system is monitored for proper operation. If input voltage or

internal power systems are detected outside of allowable operating ranges, a

diagnostic alarm will be enunciated.

Servo Valve Actuator

The VS-I uses a rotary limited angle torque (LAT) actuator. The permanent

magnet rotor is directly coupled to the servo valve.

The position of the rotor is measured by a solid state integrated circuit on the

PCB which detects the orientation of the sensing magnet on the shaft. The

H-bridge drive is regulated by the microprocessor to precisely control the servo

valve position and maintain the cylinder stroke position demand.

A unique function of the software is a periodic, symmetrically opposed impulse

which flushes silt and debris from the servo valve without causing undue wear

called “Silt Buster”. At the interval and amplitude selected by the user, this

function provides a very rapid motion of the hydraulic valve allowing any silt to be

flushed to the drain passage. This motion is followed immediately by a step of

equal amplitude in the opposite direction. The opposing symmetry of the impulse

results in no net change in fluid volume to the controlled servo valve, and thus

does not interrupt the control of the turbine. This unique function provides a

higher degree of stability, reliability, and silt resistance.

If the unit detects any diagnostic shut down condition, or if the detected

diagnostic condition prevents reliable control, or if a loss of power occurs, the

servo valve return spring forces the valve to connect the appropriate control

pressure to drain causing the cylinder to move to the fail-safe position.

Rotary Servo Valve

The servo valve has four ports: Supply, two Control Ports, and Drain/Tank. With

the hydraulic valve in its middle position, all ports are blocked. As the valve

rotates, the supply is connected to a control port while simultaneously connecting

the drain to the other control port. The combined action of the servo position

controller and cylinder position controller modulate the power cylinder position as

necessary to match the input demand.

Additionally, the Remote Servo has an OVBD (Over Board Drain) port which is

permanently connected to drain. This port can (optionally) be connected to the

OVBD connection on Hydraulic Power Cylinder to drain any leakage pass the

primary rod seal.

A unique function of the software is a periodic, symmetrically opposed impulse

which flushes silt and debris from the servo valve without causing undue wear

called “Silt Buster”. At the interval and amplitude selected by the user, this

function provides a very rapid motion of the hydraulic valve allowing any silt to be

flushed to the drain passage. This motion is followed immediately by a step of

equal amplitude in the opposite direction. The opposing symmetry of the impulse

results in no net change in fluid volume to the controlled servo valve, and thus

does not interrupt the control of the turbine. This unique function provides a

higher degree of stability, reliability, and silt resistance.

The VariStroke-I Remote Servo Kit contains the same primary

components as Integrated version, This kit allows the Hydraulic Power Cylinder

to be mounted separately from the servo in applications where space is

constrained.

Hydraulic Power Cylinder

The simple and robust design of VS-I hydraulic cylinder (Figure 1-3) is capable of

consistent performance for extended periods in challenging environments.

Hydraulic cylinder is designed to operate in wide range of hydraulic pressures

and with high oil contamination. The actuation stroke range can be adjusted

precisely using PC service tool allowing the same actuator to accommodate a

variety of strokes.

The hydraulic power cylinder is designed to be field replaceable in turbine shut

down condition.