NUM introduces accelerometer-based active vibration control system for CNC machine tools

August 23, 2019 — NUM has launched an accelerometer-based active vibration control system for CNC machine tools. Primarily designed to improve machining by eliminating tool head vibration, the system further helps maximize material removal rate (MRR) and can also provide real-time feedback for predictive maintenance purposes.

The system is available as a retrofittable option for any machine tool that uses NUM’s MDLUX digital servo drives, which form part of the company’s Flexium+ CNC platform.

NUM’s active vibration control system is a solution that provides dynamic damping capabilities. Typically, acceleration data can only be retrieved once every 20 milliseconds, which is inadequate for vibration damping purposes on machine tools. The accelerometer in NUM’s system is sampled every 100 microseconds, which means that it can be used for closed loop control at bandwidths approaching several hundreds of Hertz.

The small flange-mounting accelerometer transducer can be attached to the machine’s tool head and has a sensitivity of 0.02 g (1.96 m/sec/sec), with a measurement range of plus/minus 2 g (plus/minus 19.6 m/sec/sec). The transducer connects directly to the digital servo drive, obviating the need for any additional signal conditioning circuitry.

The active vibration control system utilizes 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 behavior of the speed servo control loop. All MDLUX servo drive modules use advanced DSP control techniques to maximize the CNC kernel-to-drive servo bus speed and feature high loop bandwidths and special acceleration algorithms.

One application of NUM’s 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 an encoder (integrated in the servo motor) it has been impossible to damp the TCP vibration, because it is not measured.

However, by using NUM’s 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. Using this approach, tool head vibration can effectively be eliminated, reducing tool wear and maximizing the MRR 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 useful for preventive maintenance applications, where it can provide early indication of potential machine problems. 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.