Basic knowledge - Rotational speed sensors

Speed sensors are used in areas where a solution for the contactless measurement of speed and rotation speed is required within inaccessible locations or under special ambient conditions at an affordable price.

RHEINTACHO specialises in developing and producing such sensors to cope with the most demanding environments. The exceptional operability and service life of the sensors reflects their ability to withstand high pressures, temperature fluctuations, electromagnetic disturbances and aggressive media as well as the impact of jets of water or steam.

We constantly leverage the latest findings and technologies when developing, building and producing our speed sensors. Sensors with built-in signal processing are constructed using the very latest technology. Carefully selected components, automatic testing during individual production steps and final testing of each individual sensor in accordance with predefined criteria are key to ensuring and maintaining the top quality of our speed sensors.

RHEINTACHO aims to keep pace with all current and future requirements by developing innovative products. Our remit also includes advising you as a partner on all speed measurement issues and supporting you to plan and implement your projects.


General functionality

The vast majority of speed sensors detect the motion of ferromagnetic structures, such as gearwheels or shafts, based on the variation in magnetic flow. When the sensor traverses a tooth or gap, the impact on the magnetic field varies accordingly. These changes to the magnetic field can be converted into electrical variables and subsequently conditioned. In other words, the variation in the magnetic field reflects the electrical output signal of the sensor.

 

Measuring principles

A whole host of competing technologies are used for this process of contactless scanning via magnetic fields, which can be mainly classified as follows:

  • Hall-effect sensors
  • Inductive sensors
  • Oscillatory sensors
  • Magneto-resistive sensors
  • Eddy current sensors or proximity switches

RHEINTACHO has specialised in developing and producing hall-effect sensors.

These sensors are described in more details as follows:

 

RHEINTACHO's speed sensors in action

  • Industrial trucks
  • Construction machines
  • Mobile working machines
  • Agricultural machines
  • Dumper trucks
  • Rail vehicles
  • Wind power plants
  • Large diesel engines
  • Hydro motors
  • Electric motors
  • Gears

Hall-effect sensors in general

Hall-effect sensors are suitable for proximity rotational speed recording on gears with a small module and high resolution. The various models can handle a wide range of applications as well as the harshest of conditions. Hall-effect sensors are used in wide-ranging mechanical engineering, vehicle and mobile working machine applications, as well as in hydraulic drives.

The sensor detects the movement of ferromagnetic structures such as gears via the change in the magnetic flux and is biased using a permanent magnet. Meanwhile, the teeth and gaps which traverse the sensor have varying impacts on the magnetic field, which changes the Hall voltage on a hall-effect sensor. These changes to the magnetic field can then be converted to electrical values, filtered and processed. The sensor output signal is a square-wave voltage which reflects the change in the magnetic field.

Hall principle of speed sensors

A Hall sensor element is a semiconductor dependent on magnetic fields, the function of which is based on an effect discovered by Edwin Hall. The so-called Hall effect is caused by the Lorentz force, which acts on moving charge carriers in the magnetic field. The semiconductor board has four connections and a constant current (I) flows through two opposing connections. While in a neutral state, there is no difference in voltage between the other two connections. However, if the Hall plate is exposed to a magnetic field with induction B, the current (I) is deflected by the Lorentz force. The charge carriers are then forced to the side to generate an electrical field between the edges to counteract the Lorentz force. This triggers a difference in voltage at two opposing sides, which is known as Hall voltage. This voltage is proportional to the current (I) and the magnetic induction B acting perpendicularly to the plate.

Differential-Hall principle of speed sensors

This measurement method allows the magnetic difference between two Hall elements mounted on a substrate to be assessed. With a gearwheel, this could mean that the maximum difference emerges when a pair of Hall plates are located over a tooth and gap respectively, whereupon the impact of a change in distance relative to the overall object declines in importance.

Differentail-Hall sensors are often used to detect speed at points where harsh environmental conditions could affect the operation of Hall sensors. Differentail-Hall sensors are ideal for use on gearwheels with a slightly out-of-round trajectory, although the measurement principle used means direction must be considered when installing these sensors. Applications for speed sensors of this type include mobile working machines.

 

A short insight into the other measuring principles can be found as follows:

Oscillatory sensors

Oscillatory inductive sensors are particularly well suited to simple rotation speed and standstill detection for indicators on machinery and equipment. These can also be used to measure grooves, bolts, screw heads or likewise for items fixed to shafts. These sensors should preferably be deployed where only one (or a limited set of) impulses per rotation need to be measured within a normal industrial environment.

Magneto-resistive sensors

Magneto-resistive sensors are particularly suitable for measuring rotation speed and standstill on machinery and equipment. They can also measure high frequencies and finer structures (from module 1). In comparison to hall-effect sensors, the advantage of these sensors is that no lower frequency limit applies.

 

Eddy current sensors or proximity switches

Proximity switches are sensors which respond in a contactless manner to being approached, namely without direct contact. For example, proximity switches are used during technical processes to detect the position of parts and tools as well as for triggering safety measures (Source: Wikipedia).

 

Inductive sensors

Magnetic-inductive sensors are ideal for recording high rotation speeds. The sensor tip can scan ferromagnetic gearwheels e.g. within hydraulic motors or oil-filled gearboxes. This means the sensor is also well-equipped for use with working machines and commercial vehicles.