The measuring machine element
Shaft coupling with integrated torque measurement
The torque-measuring shaft coupling ROBA®-DSM is based on the tried-and-tested backlash-free shaft coupling ROBA®-DS. This torque measurement coupling can be used in many different applications, from test stand construction to serial machines, right up to condition monitoring. The system allows easy condition monitoring of machines and systems. Using the data collected by the coupling, machines can be optimally used to capacity.
The measurement of rotating parts requires the transmission of energy onto the rotating part and the transmission of data from the rotating part. Many different systems are available for this purpose. In the past, energy and data were mainly transmitted inductively. However, this principle has several disadvantages in comparison to the data transmission of the new ROBA®-DSM.
A big disadvantage of the inductive systems is that data can only be transmitted over short distances. Therefore, they work mainly using “enclosing” stators, which are also partly bearing-supported. The enclosure makes both installation and replacement far more complex. Furthermore, the installation must be carried out very precisely, as the distances are very short. Another disadvantage is the low tolerances in case of vibrations and shaft run-out. Bearing-supported systems also make a torque support mandatory. Here, it is also important to observe the installation position, so that the bearing friction torques are not included in the measurements. Systems, which are not bearing-supported, require an installation aid in order to centre the rotor and the stator.
The ROBA®-DSM does not require an enclosure and therefore only needs a small installation space. The stator can easily be mounted in any position on the circumference. When adjusting the stator, large tolerances are permitted in all directions. Potential distances of up to 5 mm are substantially larger than on enclosing systems.
Another major disadvantage of the inductive systems is the usually low transmission bandwidth on analogue versions or the low data rate on digital variants. In most cases, such systems are specified up to 1 kHz. Due to the transmission of energy and measurement data via a carrier, these systems require a complex separation of signals.
The new torque-measuring ROBA®-DSM makes use of two completely separate paths for the transmission of energy to the rotor and the transmission of data to the recipient. Due to the high bandwidth of up to 3.5kHz, it can even record fast, dynamic processes reliably. The preparation of the data on the rotor permits optimum amplification and offset compensation. On the rotor there is a programmable amplifier, which is programmed via the radio interface. In this way, the customer can undertake offset compensation on the rotor even after installation. In addition, the user can adjust the address encoding and radio channel with the aid of software.
Another advantage is the use of an encoded radio system in the 2.4GHz ISM band. No authorisation is required for operation within the EU due to a general approval. The system can also be deployed in other markets, but additional approvals are required for the purpose. The system uses up to 80 channels. This means that the operation of several couplings in the same environment is no problem. The data transmission is encoded with an address so that only the appropriate recipient is able to detect and evaluate the signal.
The radio connection function can be checked on the receiver via a simple display. Because the data transmission is bi-directional, and each package is acknowledged, the quality of the radio connection can be monitored. The receiver reports any malfunctions in the radio connection or missing data from the transmitter. Due to the high sampling rate and the fast radio connection, the ROBA®-DSM achieves very good values for Jitter (max. +/-68µs) and Delay (typ. 2ms), which usually are substantially higher on digital systems.
Like industrial standard systems, the ROBA®-DSM supplies an output signal of +/-10V for the right/left torque, which can be used as the input signal for a PLC. However, because more and more control systems are PC-based, the direct collection of digital data is an advantage. The new system ROBA®-DSM provides a USB interface, via which the digital measurement data can be read in on any PC or laptop using a standard USB interface. No additional hardware is required for the recording of measurement data. Therefore the operator can quickly obtain an overview over the current performance data. In addition, records can easily be made, even over extended periods of time. The evaluation of the data can then take place offline using appropriate programs such as DIADEM or also EXCEL, and other tabular calculation programs. In addition, online evaluation is possible, whereby the data are read in directly, for example using LABVIEW, and processed in real time.
Fig.1: The torque-measuring shaft coupling ROBA®-DSM is based on the backlash-free and robust shaft coupling ROBA®-DS.
