The Positioning measurement is the very basic measurement performed with the linear optical components: linear retroreflector RL1 and the linear interferometer IL1. Any change in the distance between IL1 and RL1 is detected by the laser head and shown in the HPI Software.
As it is illustrated in the Figure 5.2 both elements are normally aligned along the laser beam. Although IL1 is usually treated as a reference element with the movement of the RL1 measured but the configuration can also be reverse, i.e. RL1 can be stationary with IL1 being translated.
The distance L measured in the linear configuration depends significantly on the actual wavelength lair of the laser beam with the formula
,where N denotes the number of interference fringes, is the laser wavelength measured in vacuum and n(T,P,H) is the refraction coefficient of the air. The wavelength changes with the fluctuations of the parameters of the air: humidity, pressure and humidity. From the experimental formulas it may be obtained the refraction coefficient dependences on T, P and H in usual conditions (T=293K, P=1000hPa, H=50%):
The changes of the wavelength are compensated automatically by the HPI-3D laser head only if the TH sensor is used properly, i.e. placed near the laser beam path. The air pressure is measured inside the laser head.
One of the important factors limiting the precision of every machine is the temperature. In the figure below there is schematically shown a milling machine. On the machine there is a work table with a workpiece. There is also schematically shown the measurement subsystem of the machine, i.e. the Scale. The position encoder (marked as Scale) is the part that is connected to the CNC control. It can be of different construction – magnetic, glass, laser, etc. Its expansion is corrected with positive sign in order to force the CNC control to leave the table in the same position despite the thermal expansion of the scale.
Positioning measurements require optical elements IL1 and RL1 to be aligned along laser beam. Each of the elements can be moved. During positioning measurements the Abbe, Dead Path and Cosine errors (for details – click here) have to be taken into consideration. The usage of the air temperature sensor and at least one base temperature sensor (T1 or T2 or T3) is absolutely necessary! More than one base temperature sensor should be used on long measurement axes, especially where a temperature gradient is possible.