INSTRUMENTATION • ELECTRONICSSECTION TITLE
Picking the correct sensor is critical
SPEED SENSOR SELECTION
C
hoosing the right speed sensor for an application is of crucial importance for an accurate and reliable measurement. After all,
the sensor’s signal 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. Tere are several considerations that must be made to select the right sensor, which can be categorised in environmental and machine-related considerations. Machine considerations include: the expected minimum and maximum speed; the target that is measured and its specifications; and whether there are any limitations on the weight and size at the mounting location. Establishing the necessary cable length is also important. Environmental considerations include: the expected ambient temperature; whether or not the measurement takes place in explosion hazardous areas; whether strong electromagnetic fields are present; and
Michel Faas reveals what engineers need to consider when choosing sensors for speed measurements
whether the measurement takes 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); and Hall-effect sensors (also known as active sensors).
VR SENSORS A VR sensor uses a magnetic field to measure changes in the distance between the sensor
tip and the target object. Te 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. Te moving gear creates a varying flux that induces a voltage in the coil; the frequency of which is related to the rotational speed. Te 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. Tere 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.
Eddy current speed sensor from Istec
DISADVANTAGES A major disadvantage of VR sensors is that the amplitude of the signal depends on a
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