Principle and operation of stylus probe instruments
Direct Instrument Measurements (Metrology):
These methods enable to determine a numerical value of the surface finish of any surface. Nearly all instruments used are stylus probe type of instruments. These operate on electrical principles. Further, these electrical instruments can be of two kinds. In first type, they operate on the carrier-modulating principle. The movements of the stylus exploring the surface are caused to modulate a high frequency carrier current. The second type includes those operating on voltage-generating principle. In these the movements of the stylus are caused to generate a voltage signal. In both these types the output has to be amplified and the amplified output is used to operate a recording or indicating instrument .The carrier modulated frequency type of instruments have the advantage that the signal fed to the recorder depends only upon the position of the stylus. While in the voltage generating type, when the oscillatory movement of the stylus stops, the output falls to zero no matter where the stylus may be.Some pneumatic instruments are also used for measuring surface finish.