Torque sensor

Torque sensors ( transducers) that determine the shape change of the measuring body, the so-called spring body, the torque. Most torque sensors use strain gauges. There are also torque transducers that operate on the piezoelectric, the magnetoelastic or the optical principle, and sensors that work with the SAW method.

Embodiments

According to the different application areas, there are also different versions:

• With or without bearing
• Rotating or non-rotating capturing the moment
• With slip rings or with contactless transmission

Transducers without bearings have no losses due to bearing friction and are virtually maintenance free. A non-rotating acquisition occurs, for example for torque wrenches or testing machines for the calibration of torque transducers. Here only relatively small angle of rotation on ( 0.01 to 2 degrees) of the Aufnehmerfederkörpers. Slip rings at the possible speeds are limited both due to wear and due to the increasing interference during transmission. Progresses, duration of use, the slip rings " out of round ". This leads, especially at higher speeds drop outs and increased noise of the signal. Usually worked over about 3000 rpm in continuous use or 6000 rpm for short-term use no more slip rings used, but with contactless transmission. In most cases, one of the following methods is used:

• Transmission over coil (magnetic)
• Transmission via flat contacts like two capacitor plates facing each other ( electrically conductive rings or transfer at the end of the shaft )

The frequencies used for transmission are very different, and are in the range of 10 kHz to 20 MHz. Many transducers do not have, in addition to detection of the torque, the ability to detect the rotational speed. This can then be directly calculated performance.

Spring body

The spring assemblies are made ​​of metallic materials, the specific "favorable" characteristics: a small hysteresis, low or possible linear temperature dependence, etc. For small torques, there is the squirrel-cage rotor, in the central region the classical solid cylinders or, when used only on one side of a warehouse to the more rigid hollow cylinder, in the interior of the electronics can be housed. Today, so-called orifice flanges are used for short build and very stiff transducers often used ( shear beam principle).

Metrological properties

The rated load indicates to what torque the sensor is capable of measurement, that is, where the deviations are within the limits specified in the data sheet. In addition, sometimes a range of use is not specified, within which, however, the measurement errors may be exceeded. For use important are the details of the limit torque, ie the load that the the pickup without taking damage and the permissible stress range of the load ( peak / peak according to DIN 50100 ). Torque may not be dynamically loaded usually at its maximum static torque. The peak / peak values ​​for alternating loads are lower than for static loads. The breaking torque is relevant to safety. If this torque is exceeded, the pickup can be destroyed. Other important aspects are the sensitivity to disturbance forces and in particular electrical noise as well as the torsional stiffness and the mass moment of inertia of the transducer. The torsional rigidity and the inertia effect, the natural frequency of the overall system, and thus the measurable speeds where it may not be natural oscillations and resonance occurs.

Application Notes

The installation of a torque transducer must be done very carefully. There must be no eccentricities occur, that is, the arrival and output shafts must be aligned, there may be no angle errors etc. The transducers may receive only small superimposed bending or tensile forces, with many pickups less than 10 % of rated torque are as respective disturbance force maximum allowed. Save over conventional designs, the torque flanges usually. Hence the appropriate installation parts must be used, so-called couplings ( not to be confused with a clutch like in a car) to intercept disturbing forces. Other common installation aids are drive shafts. However, the maximum error values ​​are limited to a degree even with the use of built-in tools. Therefore, all superimposed loads should be reduced as far as possible so that the sensor does not receive any additional burdens. The permissible values ​​are true for each component specified in the data sheet of the sensor disconnected, the values ​​apply only for pure exposure to this type of error. A combination of error, which is always the case in practice, since the installation can not be 100 % perfect, the values ​​typically reduced to less than 40 % of the stated in the data sheet for only a disturbance. It is recommended that for larger structures to calculate at higher speeds or large masses, the dynamic behavior and check. If the entire system of a drive, and transducers or load brake is operated in the vicinity of the natural frequency of the system, there is the danger that the total system will be overloaded and destroyed it.