Theory of galvo control

An optimal control of mechanical deflecting units, so-called “galvo scanners”, is a highly complex, physical topic, which one could write his doctoral thesis about without ever completely working it off.

In principle a galvo scanner is a very fast servo with a drive with the lowest friction possible and a positioning sensor as exact as possible. Those both electrical components will be evaluated and powered by a galvo driver like i.e. the MicroAmp. Because of its mechanical components the deflecting unit has got a relatively high mass inertia, which results of high voltage- and power peaks that needed to be accelerated and slowed down very quickly by the driver. The faster and the more precisely this works, the faster the mirror can move and the less flickering will be visible at the actual deflection.

However, even the fastest acceleration phase takes a little time, as well as the following brake phase. Let’s just imagine a line. At the beginning of this line the mirror axle will be accelerated as fast as possible. This is done with high voltage in order to load the coil of the galvo quickly and to feed it the highest power possible. Therefor voltages up to 30V will be fed to the galvo coil. Temporary peak currents up to 10A are possible. Once the axle is in motion, only little power is needed which at the end unfortunately is changed rapidly again. Here the so-called “break” takes place. The voltage at the coil will be inverted and again opened up highly in order to slow down the axle as fast as possible.

During these start and stop phases deformations at the mechanic of the galvo occur, which will become noticeable as an overshooting (first top shaft) or an undershooting (second top shaft). Because of the mass inertia, the mirror axle “overshoots the mark” (first top shaft) just to swing back again (second top shaft). The task of a galvo driver now is to level those over- or undershooting as effectively as possible and to suppress it. This is called “damping”, while the energization at the driver in acceleration- and brake phase is called “gain”. The higher the gain, the higher the powers at a position change.

The time needed by the galvo system to initiate and complete acceleration and deceleration needs to be considered at the control of the software. Now you can imagine that these times do vary at all times, depending on how much deflection is demanded by the galvo. At a biaxial XY system a sort of “integration” therefor takes place by not exactly adjusted wait times. Corners will be rounded because the control is faster than the galvo can do. The task of the software now is to depict the electromagnetic system of the galvo scanner now as precisely as possible and to adjust itself to the inertia moments and driving speed. This is extremely difficult and varies from galvo to galvo and from driver to driver.