SubscribeMagnetic field sensors are generally based on the magneto-resistive or Hall-principle. Compared to optical speed sensors, resolvers and precision potentiometers magnetic field sensors do not contain movable parts, such as shafts, bearings, code disks or slip rings. A fact allowing that they are sealed completely with a sealing compound and thus a completely sealed unit. Therefore they are tolerant to contamination of any type. Strong vibrations, which sometimes occur in rough applications, also do not affect magnetic field sensors.
The variety of magnetic sensors is as extensive, as their versatility of practical applications: the gear wheel sensor with integrated preloaded magnet, the contact-less potentiometer, which generates its signals with an external bar magnet and the incremental speed sensors respectively distance measuring sensors which material measure consists of a magnetic ring or magnetic tape. The resolution thereby ranges from almost “rudimentary” at the gear-wheel sensor up to high resolution with the 12 bit single turn speed sensor with bus connection.
Scanning frequency up to 15 kHz
As a rule the core of a gear wheel sensor consists of a magnetically influenceable Hall-element. It is especially suited to record rotating gear wheels or linear moving racks of ferromagnetic material. Due to the fact that in most standard applications hardly a magnet is ever available – meaning that generally the gear-wheels to be detected are not magnetized and thereby do not have an impact on the sensor – the Hall element is preloaded in the sensor with a permanent magnet. It normally is penetrated by a field of constant strength, which is reflected in a constant output signal.
MHRM gear wheel sensor scans the teeth of a pinion with module 2
If one tooth of the gear-wheel or a rack now moves past the sensor element the sensor field becomes distracted, which interferes with the magnetic penetration. Each individual tooth thus generates a square impulse on the output of the sensor. Starting with module 1, ferromagnetic gear wheels can be scanned over an air gap of approximately 0.7 mm.
With larger modules the air gap increases correspondingly (Picture 1). While the assembly expenditure for a single channel gear wheel sensor is practically negligible, sensors with two outputs have to be adjusted to the gear wheel to be scanned. Only this adjustment warrantees a phase offset of 90 0 between both output signals, which is a prerequisite for forward- and return motion recognition. Due to their scanning frequencies up to 15 kHz gear wheel sensors are well-suited speed monitors.
Linear above 160 0
The magnetic potentiometer of the series MDRM & MDFM function completely contact-less. Due to their magnetic operating mode they are not subject to mechanical wear and tear, which has proven to be an invaluable asset especially during long term operation. Within a mechanical angle of rotation of + 80 0 the potentiometer generates an absolute, linear output signal of 0…10V or 4…20 V (Picture 2). This interesting component is utilized mainly as replacement of highly stressed, often malfunctioning rheostats on compensating systems, tensions disks and similar fixtures. Magnet and sensor can also be separated by nonmagnetic sheet metal or a plastic wall without impairing the function.
Comparison of basic data of magnetic sensors
Use of magnetic rings and magnetic tape
Despite all of the above listed advantages at one point in time both of these technologies reach their performance limits. Either the measurement angle is insufficient, or the resolution does not satisfy the application requirements. In these instances, magnetic incremental speed sensors step into the breach.
A different type of Speed Sensor
The magnetic speed sensor, consisting of magnetic wheel and sensor, can be utilized the same as conventional speed sensors. Just like it’s optical counterparts, it provides incremental square signals, as well as a reference impulse after completion of each rotation. Compared to conventional speed sensors it has a several additional advantages.
The magnetic sensor manages without torque increasing and speed reducing bearings. Most of all the expensive flexible couplings between the speed sensor and shaft, which require a lot of space, are no longer required. A lot of space is gained thereby, which can be used otherwise: As a rule less than 10 mm are required for installation (Illustration 4).
Illustration 3 shows a magnetic incremental speed sensor of the MDFK series in combination with a 64 pin ring-magnet mounted on the shaft of a textile machine to monitor the speed. The magnetic wheel is adapted to the shaft diameter with a reducing flange. Due to 16 fold interpolation the sensor generates 1024 digital impulses per shaft rotation and circuit.
With external flange evaluation the impulse number can actually be increased to 4096. RS-422 drivers provide fail-safe transmission of the speed sensor signal even over long supply lines. Contamination from dust, fibers and oil mist, which are detrimental to optical sensors, do not harm this magnetic speed sensor: even at a large degree of contamination the ring magnet is scanned absolutely precisely.
10 micrometer Resolution
The same technology can also be applied for linear path measurement. Adhesive magnetic strips with 1 mm long magnetic poles are used as material measure. These are polled completely contact- less over an air gap of 0.3 mm. With proper signal processing this measuring method achieves resolutions of 10 micrometer. The sensor is particularly of interest for sections of measurement in the cm- and dm-range.
Incremental MDFK in combination with a 64-pin magnetic wheel on a textile machine
Measurement lengths of several meters can also be obtained. Due to integrated signal interpolation 25 impulses/mm traverse path are available on two outputs. The signals are offset by 90 0 and can thereby also be utilized to determine the traverse path. In addition the high input frequency of the sensor permits traversing speeds up to 40 m/sec.
Application of magnetic field sensors In perfect interaction with magnetic strips, bar- or ring magnets a multitude of applications, such as monitoring and control of rotary- and linear motions, can be made accessible with magnetic field sensors. They are insensitive to dust, humidity and vibrations (IP 67), can be installed quickly and even at high speeds and are not subject to wear. Due to their tolerance and sturdiness they are well suited for speed sensor applications in contaminated environment, which includes outside applications.
MLFK-linear sensor with 10 micrometer