Swiss-based CNC specialist NUM has launched an innovative accelerometer-based active vibration control system for CNC machine tools. The system, primarily designed to improve machining by virtually eliminating tool head vibration, further helps increase productivity by maximising material removal rate and can also provide valuable real-time feedback for predictive maintenance purposes. It can be retrofitted on any machine tool that uses NUM’s MDLUX high-performance digital servo drives, which form part of the company’s Flexium+ CNC platform.
The system is a very cost-effective solution that provides unprecedented dynamic damping capabilities. Most accelerometers on the market are only capable of low frequency-sampling rates and require additional signal conditioning electronics before they can be connected, usually via Fieldbus, to the CNC or PLC system. Typically, acceleration data can only be retrieved once every 20ms, which is inadequate for vibration damping on machine tools. The accelerometer in NUM’s system is sampled every 100μs, which means that it can be used for closed-loop control at bandwidths approaching several hundred Hertz.
The small-flange-mounting accelerometer transducer can easily be attached to the machine’s tool head and has a sensitivity of 0.02g (1.96m/s2), with a measurement range of ±2g (±19.6 m/s2). The transducer connects directly to the digital servo drive, obviating the need for any additional signal conditioning circuitry.
The system utilises the drive-embedded macros (DEM-X) option that is available on NUM’s MDLUX high performance digital servo drives. This option enables real-time macros to be embedded within the drive in order to manipulate its regulation algorithms. The output signal from the accelerometer transducer can thus be used to directly influence the behaviour of the speed servo control loop. All MDLUX servo drive modules use advanced DSP control techniques to maximise the CNC kernel-to-drive servo bus speed, and feature high loop bandwidths and special acceleration algorithms for uncompromised speed and positioning accuracy.
One of the most interesting applications of NUM’s new active vibration control system concerns tool centre point (TCP) vibration due to the various vibration modes of a machine tool's mechanical structure. Until now, even if the machine is equipped with a high-resolution encoder (integrated in the servo motor) it has been impossible to damp the TCP vibration, simply because it is not measured. Although complex system modelling and state-space control could possibly provide some effective results, the complexity is very high and the compensation robustness is quite weak. For example, in the case of a small parameter change, such as inertia, the effect of the compensation would be lost entirely.
However, by using NUM’s new active vibration control system to measure and dynamically alter the TCP acceleration in each of the main x, y and z axis directions, it is now possible to damp the vibration very accurately. Using this approach, tool head vibration can effectively be eliminated, significantly reducing tool wear and maximising the material removal rate of the machining process.
All acceleration data is processed within the drive in the digital domain, which means that it can be cyclically transmitted to the Flexium+ NCK and stored in the NCK buffer. The data can then be uploaded to the Windows operating system for further evaluation, or for use with NUM’s process monitoring software. It is especially useful for preventive maintenance applications, where it can provide early indication of potential machine problems before they become expensive repair tasks. Typically these include detecting spindle vibration caused by bearing problems, detecting the onset of performance deterioration such as increased friction or backlash, and detecting excessive tool wear or breakage.
Australian Industrial Machine Services
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