SENSORS FOR SPEED MEASUREMENTS

BY DANIËL VISSER

Speed sensors for turbomachinery provide the input for overspeed protection systems. A faulty sensor leads to an unreliable input signal, which has a negative influence on the accuracy and reliability of the system.

There are several considerations that must be made to select the right sensor, which can be categorized in environmental and machine-related considerations. Machine considerations include: expected minimum and maximum speed; the target to be measured and its specifications; any limitations on the weight and size at the mounting location; and the necessary cable length. On the environmental side, factors to consider include: the expected ambient temperature; if the measurement takes place in explosion hazardous areas (ATEX) or in a corrosive environment; and if strong electromag-netic fields are present.

For industrial speed measurements there are three main types of measurement sensor:

• Variable reluctance (VR) sensors – also known as: passive sensors, electromagnetic sensors or magnetic pickup sensors (MPU).

• Eddy current sensors – also known as: proxim-ity sensors or displacement sensors.

• Hall-effect sensors – also known as: active sensors.

VARIABLE RELUCTANCE (VR)

A VR sensor uses a magnetic field to measure changes in the distance between the sensor tip and the target object. The sensor contains a coil that is wrapped around a magnet which causes a change in the magnetic field (flux) and the coil as the teeth of a gear pass the sensor. The moving gear creates a varying flux that induces a voltage in the coil; the frequency of which is related to the rotational speed. The signal is a sinusoidal wave of which the amplitude is dependent on the target size, speed and distance.

VR sensors do well in high-temperature applica-tions. Some can function in temperatures of more than 300°C. Moreover, VR sensors are easy to use and reliable. As this type of sensor has a two-wire connection, it often fits within legacy infrastructures.

However, the amplitude of the signal depends on a factor of the size, speed, and distance of the target. If the speed is too low, the gear tooth too small, or the distance to the target material too much, the signal will be flattened and not usable. On the other hand, if the speed is high, the gear tooth large or the distance is small, the signal will show high pulses. The application and positioning of VR sensors requires special attention and expertise to function properly. As these types of sensors do not function well with low speed, they are unsuitable for low or zero-speed detection.

EDDY CURRENT (PROXIMITY)

An eddy current sensor uses an electromagnetic field to measure changes in the distance to an object. As a pole wheel moves past the sensor, it measures a variation in distance; close (tooth) and far (notch). The rotational speed can be deter-mined based on the time between these events.

One advantage of eddy current sensors is that the measuring principle shows both the pulses and the position with respect to the teeth. This provides insight into the set distance to the teeth of the target object. They are available with a dynamic current output, which allows for long cabling (up to 1000m). Sensors with a dynamic current output are less affected by cable imped-ance as compared to Hall-effect sensors, eddy cur-rent sensors based on voltage signals, and VR sensors.

But it should be understood that at high speed, saturation may occur, causing the signal shape to flatten. When the gear teeth move past the sensor at high speed, an eddy current sensor barely detects a difference in distances. The higher the frequency, the less effective an eddy current sensor will be for speed measurements.

HALL-EFFECT SENSORS

A Hall-effect sensor measures changes in magnet’s magnetic field, caused by the ferromagnetic target material. The sensors have built-in signal condi-tioners, which generate a clear square wave signal. In contrast to VR sensors, Hall-effect sensors are sensitive to the size of the magnetic flux rather than the speed at which it changes. Hall-effect speed sensors have a broad measurement range and can be used to measure both low-speed or sta-tionary parts and high-speed parts.

A Hall-effect sensor directly provides a digital output which is easy to transmit and process. Further, they usually feature internal signal pro-cessing. The signal is digitalized and amplified, making it less susceptible to electromagnetic inter-ferences (EMI). However, Hall-effect sensors are limited to applications that operate in tempera-tures ranging from -40 °C to +150 °C due to built-in electronics. Moreover, Hall-effect sensors require a 3-wire connection. Also, the trigger level is defined in the Hall-effect sensor and cannot be changed.

TYPICAL SENSOR APPLICATIONS AND REPLACEMENT OPTIONS

Any application requires different sensors and sensor features. However, three types of turboma-chinery can be distinguished, with each having a default sensor type provided by the OEM. The recommended sensor replacements after end-of-life are may differ for specific applications.

• Large land-based turbines (power industry) are typically provided with eddy current or Hall-effect sensors by the OEM. Based on the frequency range and signal stability, a Hall-effect sensor is recommended when the default sen-sor has reached end-of-life.

• Industrial turbines (process industry, oil & gas industry) are typically provided with eddy cur-rent or Hall-effect sensors by the OEM. Based on the frequency range and signal stability, a Hall-effect sensor is recommended when the default sensor has reached end-of-life.

• Aeroderivative turbines are typically provided with magnetic pick-up sensors (VR) by the OEM due to their normal operation in harsh environments. When it suits the application, Hall-effect sensors are recommended after the default sensor has reach end-of-life.

AuthorL Daniël Visser works for Istec, a provider of machine protection and monitoring solutions that suit an increasingly demanding environ-ment and takes care of sensors and systems during operational life. An in-depth overview of each sen-sor can be found in the book Speed. It covers topics such as speed measurements, overspeed protection, sensor types, functional safety & SIL, and API 670. It can be requested for free at www.istec.com/book-speed