Sensors for speed measurements

Choosing the right speed sensor for turbomachinery is of crucial importance for an accurate and reliable measurement. After all, the signal of the sensor is the input for an overspeed protection system. A faulty sensor leads to an unreliable input signal, which has a negative influence on the accuracy and reliability of the protection system.

There are several considerations that have to be made when choosing the right sensor, which can be categorized in environmental and machine considerations.

Machine considerations:

1. What is the expected min and max speed?
2. What is the target that is measured and what are its specifications?
3. Are there any limitations on the weight and size at the mounting location?
4. What is the necessary cable length?
   

Environmental considerations:

1. What is the expected ambient temperature?
2. Does the measurement take place in explosion hazardous areas (ATEX)?
3. Are strong electromagnetic fields present?
4. Does the measurement take place in a corrosive environment?

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

• Variable reluctance (VR) sensors – also known as: passive sensors, electromagnetic sensors or magnetic pickup sensors (MPU).
• Eddy current sensors – also known as: proximity sensors or displacement sensors.
• Hall-effect sensors – also known as: active sensors.
  

Variable reluctance (VR) sensors

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.

Advantages

An advantage of VR sensors is their applicability to high-temperature applications. There are specific types of sensors that are suitable to function with temperatures of more than 300°C. Moreover, VR sensors are easy-to-use and highly reliable. Another advantage is that the sensor has a two-wire connection and therefore often fits within legacy infrastructures.

Disadvantages

A major disadvantage of VR sensors is that 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 big, 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 (80VRMS). 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 not suitable for low or zero-speed detection.

Eddy current sensors (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 determined based on the time between these events.

Advantages

A major 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.

Eddy current sensors are also 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 impedance as compared to Hall-effect sensors, eddy current sensors based on voltage signals, and VR sensors.

Disadvantages

The use of eddy current sensors for speed measurements has a disadvantage. At a high speed, saturation may occur, causing the signal shape to flatten increasingly. 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 conditioners, 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 stationary parts and high-speed parts.

Advantages

An advantage of a Hall-effect sensor is that the sensor directly provides a digital output which is easy to transmit and process. Another advantage is that Hall-effect sensors usually feature internal signal processing. The signal is digitalized and amplified, making it less susceptible to electromagnetic interferences (EMI).

Disadvantages

Due to built-in electronics, Hall-effect sensors are limited to applications that operate in temperatures ranging from -40 °C to +150 °C. Moreover, Hall-effect sensors require a 3-wire connection. Also, the trigger level is defined in the Hall-effect sensor and cannot be changed.