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19 October, 2019

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Servo-based vibration control dampens machine tool shakes

17 September, 2019

The numerical conrols specialist NUM has announced an accelerometer-based vibration control system for CNC machine tools that, it claims, will deliver “unprecedented” dynamic damping. Designed to improve machining by effectively eliminating tool head vibration, the system can help to boost productivity by maximising material removal rates (MRRs) and providing real-time feedback for predictive maintenance purposes. The system can be retrofitted to any machine tool that uses NUM’s MDLUX digital servodrives – part of its Flexium+ CNC platform.

According to NUM, most rival accelerometers are only capable of low sampling rates and need extra signal-conditioning electronics before they can be connected – usually via a fieldbus – to a CNC or PLC. Typically, acceleration data can be retrieved only once every 20ms, which NUM argues is inadequate for vibration damping. In its new system, the accelerometer is sampled every 100µs, which means that it can be used for closed-loop control at bandwidths of several hundred Hertz.

The small, flange-mounting accelerometer, with a sensitivity of 0.02g (1.96m/s2) and a measurement range of ±2g (±19.6 m/s2), is attached to the machine’s tool head. It connects directly to the digital servodrive, avoiding the need for extra signal-conditioning.

The vibration control system uses real-time macros in NUM’s MDLux digital servodrives to manipulate control algorithms. The accelerometer output can thus affect the behaviour of the servo control loop.

One potential application for the new vibration control system involves TCP (tool centre point) vibration, which has its origins in a machine tool’s mechanical structure. Until now, even if a machine was equipped with a high-resolution encoder in its servomotor, it was impossible to damp the TCP vibration, because it was not being measured.

In the top figure, vibration damping is off. Trace 2 shows the X-axis speed, while Trace 1 shows that a measured point on the machine is vibrating at its natural frequency (about 3Hz). In the lower figure, vibration damping is on. Trace 2 again shows the X-axis speed, while Trace 1 shows that the measured point on the machine is moving and that the acceleration is at the programmed rate (and that the natural frequency has been compensated).

Although system modelling and state-space control can provide effective results, this approach is complex and the compensation robustness is quite weak, says NUM. For example, a small change in a parameter such as inertia could lose the compensation effect.

The new vibration control system measures and alters the TCP acceleration dynamically in each of the X, Y and Z axis directions, resulting in accurate vibration-damping. NUM claims that it can effectively eliminate tool head vibration, reducing tool wear and maximising material removal rates.

The acceleration data is processed digitally within the drive and can be transmitted cyclically to NUM’s Flexium+ NCK, and stored in a buffer. The data can then be used with process monitoring software, or uploaded for further evaluation. It is especially useful for preventive maintenance applications, where it can provide an early indication of machine problems before they need expensive repairs. For example, it could be used to detect spindle vibration caused by bearing problems, spot the onset of performance deterioration such as increased friction or backlash, or identify excessive tool wear or breakage.




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