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Control Servo Motor SMR

Control Servo Motor;
Linear Electro-hydraulic Actuator for Steam Turbines, Gas Turbines and Other Applications

A Voith control servo motor is a linear electro-hydraulic actuator. It operates a valve that controls the mass flow needed to drive machinery. Voith servo motors ensure very precise and high dynamic control of the mass flow.

The control servo motor is an intelligent and compact functional unit consisting of a hydraulic cylinder, control magnet with integral position control, hydraulic pilot valve and position sensor. When a servo motor is used, no additional components such as pilot valve and hydraulic cylinder, for instance, are needed to operate the control valves. A standard 4 – 20 mA signal specifies the position of the piston rod.

The control servo motors feature a simple and compact design. This ensures con­sist­ent and reliable operation. The Mean Time Between Failures (MTBF) for Voith control servo motors is over 103 years. Your equipment runs for a very long period of time without unplanned downtime; availability is increased.

The control servo motor is also available in a redundant design (position feedback system and way valve control).


Voith Digital Solutions GmbH
Actuators and Governors
Voithstr. 1
74564 Crailsheim, Germany

Tel. +49 7951 32-470
Fax +49 7951 32-605

Product inquiry

Technical data

Full-stroke closing time (Trip) < 0.3 s
Maximum pressure Up to 200 bar [2,900 psi]
Effective forces Up to 500 kN [112,000 lbf]
MTBF (Mean Time Between Failures) 103 years (calculated in accordance to MIL‑HDBK‑217 and Siemens SN 29500, actual MTBF evaluations from installed products show even higher values)
Supply voltage 24 VDC
Current consumption 1.0 A (3.0 A for t < 1 s)
Input signal 4 – 20 mA
Communication (optional)
  • EtherNet/IP
  • Sercos
  • EtherCAT
  • others on request
Fail-safe operation Internal return spring in way valve and additionally in hydraulic cylinder on single acting designs
Redundancy (optional) Position feedback system and way valve control magnets
Ambient temperature -20 to +80°C [-4 to 176°F] (standard version, not rated for use in potentially explosive areas)
Explosion proof rating (optional)
  • ATEX (II 2G IIC T4, Ta = -20...+60°C)
  • others on request
Protection class IP 65
Safety integrity level SIL 2

Advantages and benefits

  • The Mean Time Between Failures (MTBF) is over 103 years. Your equipment operates very reliably. Productivity increases and the costs associated with lost production remain low.
  • Voith control servo motors are not sensitive to contaminated pilot oil. Turbine lubricating oil is suitable as the operating fluid. One oil supply system for both lubricating and pilot oil suffices. Maintenance is simple, which helps you lower operating costs.
  • During load rejection, for instance, the control servo motor reaches the safe position in less than 300 ms. This ensures that your equipment operates with high reliability and availability. After a malfunction, the equipment is ready to operate again very quickly.
  • The high dynamic behavior (response time in the closing direction: < 200 ms) and almost no hysteresis (< 0.01% of maximum stroke) ensure fast, accurate control. You benefit from having a stable process with high product quality.
  • The integral electronic position control in the Voith servo motor eliminates the need for time-consuming tuning of controller, position sensor and actuator during test runs and commissioning. This reduces the time required for commissioning and saves money.
  • We design our servo motors to be compatible with existing machine designs. This gives you design flexibility. Engineering costs are lowered and the time you need for design is shortened.
  • For use in hazardous areas, we offer explosion-proof control servo motors with the required certificates. The interface to the machine control system is the same for conventional and explosion-proof versions.

Typical fields of use

  • Actuation of the control valve of
    • steam turbines
    • gas turbines
    • compressors
  • Retrofit of mechanic-hydraulic valve controls
  • Retrofit of hydraulic, electro-mechanical and pneumatic linear actuators
  • Upgrade of systems with Voith I/H converters DSG or way valves WSR
  • Upgrade of systems with Woodward actuators:
    • Varistroke Linear Electro-hydraulic Actuator
    • EHPC Electrohydraulic Power Cylinder
    • EHPS Electrohydraulic Power Servo
    • PGPL Hydraulic Powered Electric Actuator
    • EG-3P Hydraulic Amplifier
    • TM‑25LP or TM‑200LP Linear Proportional Actuators
    • CPC or CPC‑II Current to Pressure Converters

Technical information


Control servo motor with 3/3 way valve

and single-acting hydraulic cylinder

Control servo motor with 4/3 way valve

and double-acting hydraulic cylinder

The servo motor is controlled by a standard 4 – 20 mA signal from the control system. The piston rod position is proportional to this standard signal w. In the position control loop, the setpoint w is compared with the actual position x of the piston rod measured by the position sensor and any deviation causes the controller to adjust the magnetic force FMag. This force (FMag) shifts the control piston in the way valve against a spring, causing a change in the direction and magnitude of the volume flow to the actuating chamber of the cylinder. In conjunction with the spring pack, the hydraulic force resulting from the volume flow changes the position s of the piston rod. The piston rod moves at a speed determined by the volume flow into or out of the actuator.

T Tank connection
P Oil supply connection (input)
X0, X1 Parameters for stroke adjustment (potentiometers)
KPU, KPD Control gain settings (potentiometers)
w Position setpoint
x Position actual value
F Force
FMag Magnetic force
FF Control spring force
U Voltage
UHall Hall voltage
UMag Voltage applied to magnetic force
s Stroke
i Current signal from position sensor


Selection table

Low pressure High pressure
Type SMR-A51xxx SMR-D13xxx SMR-A11xxx SMR-A41xxx
Max. piston
spindle diameter
90 – 250 mm 105 – 210 mm 85 – 200 mm 55 – 120 mm
Full-stroke closing time < 0,3 s < 0,4 s –
0,8 s
< 0,3 s < 0,3 s
Opening force for piston
spindle stroke of 0 mm
13.000 – 97.000 N 13.000 – 56.000 N 39.000 – 108.000 N 75.000 – 230.000 N
Opening /(closing) force
for max. piston
spindle stroke
7.000 – 91.000 N 14.000 – 58.000 N 33.000 – 94.000 N 67.000 – 219.000 N
Principle of operation Single-acting Double-acting Single-acting Single-acting