The use of eddy currents and the magnetic field strength of magnets for distance measurement have long been established procedures. Less well known, however, is the ability to combine both methods in one sensor. In the following article, Micro-Epsilon explains the measuring principle, possible designs, the advantages and the limitations of the magneto-inductive measuring method.
The magneto-inductive sensor evaluates the distance between the target magnet and the sensor. The larger the distance, the lower the magnetic field strength. If one were to use only the magnetic field strength for the evaluation of the distance, the characteristic of the sensor would be strongly non-linear. However, in combination with the eddy current method extended to a magnetically sensitive element, this nonlinearity is compensated. One speaks also of "Selbstlinearisierung". Thus, the characteristic curve of a magneto-inductive sensor is also linear at the limit of the measuring range. In contrast, with Hall sensors, the signal swing at the end of the measuring range weakens significantly, which is why it is reluctantly used for long distances.
Possibilities with the procedure
Since the sensor reacts to the magnetic field strength, the measuring range can be determined by the choice of the target magnet. A stronger magnet increases the measuring range. By default, up to 80 mm are possible. Despite this large measuring range, even the smallest path changes can be detected by the high sensitivity. Combining both is a clear advantage to the Hall process, which can only provide measuring ranges up to 30 mm and the sensitivity to the measuring range end decreases sharply and must be re-linearized. With non-ferromagnetic materials, the sensor measures perfectly, making it suitable for pressure-tight containers and closed systems. In addition, the sensor is characterized by its high dynamics.
For small quantities standard models in stainless steel or plastic are available. At high volumes, the smart circuit design allows a high degree of freedom in the design of the sensor. For example, the sensor elements can be arranged next to one another in order to measure precisely over a linear distance. The magnet then moves parallel to the sensor.
The electronics around the sensor element are customized. So had Micro-Epsilon for the damper of washing machines already an OEM version of the so-called "Main sensor"in which the sensor is clipped on the outside of the damper and there detects the vibration of the drum by means of an integrated magnet.
Influence of the magnetic field strength
The magnetic field strength at the sensor element can be influenced in many different ways. This can result from a change in the linearity, the offset, the resolution and the measuring range up to a failure of the sensor.
Ferromagnetic materials close to the sensor affect the course of the magnetic field lines, in addition, these materials can not be penetrated by them. Especially when installing the sensor and the magnet must be paid attention to this. Numerous tests have shown that when the sensor is installed in ferromagnetic material, the characteristic curve of the sensor is damped. This also applies to a ferromagnetic material behind the target. In addition, external magnetic fields of, for example, targets of adjacent sensors or magnetic fields of electric motors can influence the signal. Therefore, you should attach the sensors so that they are not exposed to additional magnetic fields. In addition, sufficient distance must be maintained between two adjacent MDS sensors in order to minimize the influence.
For measurement setup and mounting, only non-ferromagnetic materials such as aluminum should be used.
The best results are obtained when the magnet is positioned towards the sensor so that it moves frontally and centered in front of the sensor. But also an offset or a lateral measurement are possible. However, this measurement must be considered in detail in individual cases, since changes in the characteristic curve result here.
In use for speed measurement
When starting spinning machines, the machine is slowly driven up to working speed. In this piecing process, it is necessary to detect the rotational speed of the rotor of the rotor spinning machine. Due to the lifetime and freedom from maintenance, the measurement must be carried out without contact. In the rotor spinning machines, the rotor speed is measured indirectly for the piecing process. For this purpose, the magnetic field of two magnets in one of the two support disks of the rotor bearing is detected. The sensor looks at the end face of the support disk so that the magnets rotate in front of the sensor. By approaching and removing the magnets to the sensor, the magnetic field at the sensor and thus the output signal changes constantly. A cylinder brings the sensor to the working position. There is a cover made of plastic between the sensor and the support disk, through which the magnetic field must be detected. Sensor and transmitter are housed in separate housings. By using the main sensor, the start-up time of the spinning machines can be significantly reduced.