SMR Control Servo Motor

Control Servo Motor Type SMR

A Voith control servo motor operates a valve that controls the mass flow needed to drive machinery. Voith servo motors ensure very precise and highly 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.


Voith Turbo GmbH & Co. KG
Electronic Drive Systems (aee)
Voithstr. 1
74564 Crailsheim, Germany

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

Product inquiry

Technical data

Full-stroke closing time < 0.3 s
Maximum pressure Up to 200 bar
Effective forces Up to 500 kN
Supply voltage 24 VDC
Current consumption 1.0 A (3.0 A for t < 1 s)
Input signal 4 – 20 mA
Ambient temperature -20 to +80°C (standard version, not rated for
use in potentially explosive areas)
Explosion proof rating (optional) ATEX (II 2G IIC T4) or
FM/CSA (Class 1, Div. 1, 2, Group B, C, D,
Temp. Code T4)
Protection class IP 65
Safety class SIL 2 (on request)

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 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

Control of the valve that regulates the mass flow for

  • Steam turbines
  • Gas turbines
  • Compressors

Technical information

Control servo motor with 3/3 way valve

for single-acting hydraulic cylinders

Control servo motor with 4/3 way valve

for double-acting hydraulic cylinders

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


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


We would welcome the opportunity to help you design your control system. Working together, we will select the control servo motor that is best suited for your application.

You, too, can benefit from our many years of engineering experience in all matters related to turbine and compressor control.

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  • Original spare parts/spare parts kits
  • Repairs, maintenance
  • Overhaul, maintenance
  • Installation


  • Commissioning
  • Modernization, retrofits
  • Training

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