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Automation Technologies 4/2015

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Automation Technologies 4/2015


SENSORS AND MEASUREMENT previous page This is a decisive advantage in harsh ambient conditions, as even larger engines or frequency inverters with their electromagnetic fields do not adversely affect measurement quality. The most important metrological properties of torque sensors include: accuracy class, sensitivity tolerance, temperature stability, linearity deviation and hysteresis. However, the user should also take into account the application areas and load limits: Rotational speed limits, permissible oscillation bandwidths, lateral and longitudinal limit forces, maximum temperatures Enhancing the performance of torque measurement Plug-in modules for frequency measurements such as the PX460 work with an accuracy of 0.01 % enable up to four torque sensors to be operated, even in mixed operation. A whole series of internal computing channels has been specially designed for operation and use of torque transducers. They work in real time, exactly like the measurement channels, at a calculation rate of 50 microseconds. High-precision calibration equipment is used to capture the behavior of the sensor under various load cases, for example, operation with dynamic right or left rotation. Besides the 100 % measuring range also an accurate measurement in partial ranges is needed, e.g. to capture the residual breaking torque. Other important functions include parallel, independent processing of raw measurement values, for example filtering. Special filter for testing combustion engines: Due to the work cycle with compression and expansion in the individual cylinders and the corresponding fluctuations in combustion, the torque generated by an engine exhibits highly dynamic behavior. In many measurement systems it appears as “noise”. This can be eliminated by using a Casma filter. The Casma filter achieves excellent stabilization of torque measurements in correlation with the engine speed, which also varies. Fieldbus based measuring technology The spectrum of measurement signals to be acquired, ranges from simple signals, acquired at a low frequency, for example a slowly changing temperature value, to complex measurement data that have to be simultaneously measured at a high measurement frequency, for example torque signals, with angle of rotation signals and rotational speeds that must be acquired synchronously. In both cases, a decisive factor in addition to the robust and precise sensors, is measured value acquisition. Both should lie within the same accuracy class and be at least 0.1 % or rather 0.01 %. The sample rate of the signals is just as important as measurement accuracy. It should be high enough so that fast and small partial changes can still be reliably resolved and displayed. To ensure adequate peak value acquisition, calculation speed and control quality, all measurement and calculation channels must be sampled in parallel with at least 20 kHz, which is equivalent to a measurement and calculation rate of 50 µs. PC based system or embedded system ? Embedded systems are used when high real time capability is required. The volume of resulting data is relatively low here, however, it is very time-critical. Control operations cannot be executed on PC-based systems in real time. The resources here are distributed evenly across all components so that control tasks sometimes need to “wait” before they can be executed. The cycle times here therefore are around AUTOMATION TECHNOLOGIES 4/2015

SENSORS AND MEASUREMENT 50 ms or more, which is totally insufficient for rapid and secure test bench control. Embedded systems can demonstrate their full strengths here as their resources are fully reserved for the control tasks via the internal CPU. Two distinctions have to be made for data storage in the test bench sector. If it is only necessary to log or save test end results, this can be implemented by the embedded system itself. But, if larger data volumes and raw data need to be stored, PC systems have a clear advantage. systems and automation solutions. In addition to controlling the measurement sequences, such modern systems also enable machines to be controlled and modern, future-oriented test benches to be implemented. Photographs: Hbm Conclusions Modern and powerful torque sensors combined with embedded systems with open communication interfaces are equally suitable for use in high-quality measurement and regulation tasks. The general trend that can be seen here is a process of convergence in classic measurement technology About Company name: Hottinger Baldwin Messtechnik GmbH Headquarters: Darmstadt, Germany Employees: 1 700 worldwide Products: Force sensors, torque sensors, displacement sensors, load cells, optical fiber sensors, data acquisition systems and software, signal conditioners AUTOMATION TECHNOLOGIES 4/2015


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