Torque transducer
Torque transducers are sensors (transducers) that determine the torque by changing the shape of a measuring body, the so-called spring body . Most torque sensors use strain gauges . There are also torque transducers that work according to the piezoelectric , magnetoelastic or optical principle, and transducers that work with the SAW method .
Embodiments
There are also different versions depending on the different areas of application:
- with or without bearings
- rotating or non-rotating acquisition of the moment
- with slip rings or with contactless transmission
Transducers without bearings have no losses through bearing friction and are practically maintenance-free. A non-rotating detection is used, for example, in torque wrenches or testing machines for calibrating torque sensors. Here only relatively small angles of rotation (0.01 to 2 degrees) of the transducer spring body occur. In the case of slip rings, the possible speeds are limited both because of wear and tear and because of the increasing disturbances in transmission. As the period of use increases, the slip rings become "out of round" . This leads to dropouts and increased noise in the signal, especially at higher speeds. Usually no more slip rings are used above approx. 3000 / min with continuous use or 6000 / min with short-term use, but work with contactless transmission. Usually one of the following procedures is used:
- transmission via coils (magnetic)
- the transmission via flat antennas (capacitive), which face each other like two capacitor plates (electrically conductive rings or transmission on the end of the shaft)
The frequencies used for transmission are very different and range from 10 kHz to 20 MHz. In addition to recording the torque, many transducers also have the option of recording the speed. The power can then be calculated directly.
Spring body
The spring bodies are made of metallic materials that have certain "favorable" properties: a small hysteresis , low or linear temperature dependency, etc. For small torques there is the squirrel cage, in the middle area the classic full cylinder or, if a bearing is only used on one side should be, the more rigid hollow cylinder, inside which the electronics can be accommodated. For short and very stiff transducers, so-called measuring flanges are often used today (shear bar principle ).
Metrological properties
The nominal load indicates the torque up to which the transducer can measure, i.e. H. in which the measurement deviations are within the values specified in the data sheet . In addition, a range of use is sometimes specified, within which the specified measurement errors can, however, be exceeded. The information about the limit torque , i.e. the load that the transducer can withstand without being damaged, as well as the permissible amplitude of the load (peak / peak according to DIN 50100) are important for use. As a rule, torque transducers must not also be dynamically loaded with their 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 transducer can be destroyed. Other important aspects are the sensitivity to interference forces and in particular electrical interference signals as well as the torsional stiffness and the mass moment of inertia of the transducer. The torsional stiffness and the moment of inertia influence the natural frequency of the overall system and thus the measurable speeds at which natural vibrations and possibly resonance do not occur.
Instructions for use
A torque transducer must be installed very carefully. There must be no eccentricities, d. In other words, the input and output axes must be aligned, there must be no angular error, etc. The transducers may only receive low superimposed bending or tensile forces, with many transducers less than 10% of the nominal torque is permitted as a maximum disturbance force. Torque measuring flanges are usually cheaper than conventional designs. Therefore, appropriate built-in parts, so-called clutches (not to be confused with a clutch like in a car), must be used to absorb disruptive forces. Other common installation aids are cardan shafts. The maximum permissible errors are, however, limited to values of one degree , even when using installation aids . Therefore, all superimposed loads must be reduced as far as possible so that the transducer does not receive any additional loads. The permissible values are specified separately for each component in the data sheet of the transducer, but the values only apply to pure loads with this type of error. A combination of various errors, which is always present in practice, since the installation cannot be 100% ideal, typically reduces the values to below 40% of those specified in the data sheet for just one disturbance variable. It is advisable to calculate and check the dynamic behavior for larger structures, at higher speeds or large masses. If the entire system consisting of the drive, transducer and consumer or brake is operated near the natural frequency of the system, there is a risk that the entire system will be overloaded and thus destroyed.
For some years now, contactless torque transducers based on the magnetoelastic measuring principle have also been available. The advantages of this technology are that with this measurement method, the sensor is not in the force flow and therefore the application can be implemented much more easily.