Proximity measurement systems as speed sensors

When it comes to the design and specification of an overspeed detection system (ODS), the selection of the correct speed sensors for each specific application is essential in order to achieve an accurate and reliable measurement of the rotational speed and/or acceleration.

In every ODS application, particular attention has to be given both to the input signals (sensors/measurement chains) and to the overspeed detection and protection system (monitoring module hardware). They must be considered together as a system to fulfill the safety requirements.

Three main technologies used for speed sensors:

  • Proximity sensors/probes using the eddy-current principle.
  • Hall-effect sensors using the Hall-effect principle, also known as “electronic” or “active sensors”.
  • Magnetic sensors using the variable-reluctance principle, also known as “electromagnetic sensors”, “speed pickups” or “passive sensors”.

The different types of speed sensor chain have various advantages and disadvantages to be considered, depending on the specifications and constraints of the machine, its speed wheel and the application.

Regarding the machine and speed wheel, you must consider:

  • Location: where to install/support the sensor
  • Geometry, dimensions and shape of the speed sensing surface (pole wheel / pole band / integrated slots) to be detected by the sensor, including the number of poles per rotation
  • Gap: distance to target
  • Shaft/pole wheel eccentricity
  • Speed range to be measured – minimum and maximum
  • Frequency range of the resulting speed signal
  • Ferromagnetic: target material
  • Thermal expansion
  • Signal transmission distance: distance to the ODS module.

Regarding environmental constraints, you must consider:

  • Temperature ranges
  • Hazardous area requirements such as potentially explosive atmospheres
  • Safety standards and requirements such as IEC 61508 SIL levels
  • Industry specific standards such as API 670
  • Electromagnetic concerns.

TQ-based proximity measurement systems from the vibro-meter® using eddy-current principle:

  • TQ4xx, EA40x and IQS45x
  • TQ7xx, EA70x and IQS7xx (nuclear)
  • TQ9xx, EA90x and IQS900 (SIL safety)

vibro-meter® TQ7xx/xx sensor and IQS7xx/9xx conditioner vibro-meter TQ7xx/xx sensor and IQS7xx/9xx conditioner


Accordingly, all of these proximity measurement systems are suitable for use with overspeed detection and protection systems, such as the SpeedSys300 ODS301 module. These sensors are widely used in industrial applications and have demonstrated excellent reliability and customer satisfaction over time.

TQ proximity sensors are usually either mounted on brackets or installed directly on the casing of the machine. They are designed for harsh industrial conditions in terms of temperature (−40 to +180°C / −40 to +356°F), environment (IP68, ATEX/IECEx, nuclear for the TQ7xx) and safety (SIL 2 for the TQ9xx). Many configurable options exist including sensitivity range; probe body length, threads and mounting; total system length; and cable protection.

The IQS signal conditioners contain the electronics and can be installed in an industrial housing or enclosure near the sensor for additional environmental protection. When configured with a current output, an IQS can transmit the measurement signal over distances up to 1 km – longer than any other solution. In addition, the bounded range of the output signal provides a simple way to monitor the health of the sensor/measurement chain.

As a result, vibro-meter TQ-based proximity measurement systems are a perfect solution for measuring speed with signal frequencies up to 20 kHz in overspeed applications.

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