The needle tracing method is also called the stylus method. When the stylus is directly across the surface of the workpiece to be measured, due to the peak and valley of the measured surface contour, the stylus will move up and down in the direction perpendicular to the surface of the measured contour, and this movement is sent to the signal through the electronic device. Zoom in, and then output data or graphics related to roughness through a zero meter or other output devices.
The surface roughness measuring instrument adopting the principle of needle drawing method is composed of sensor, driver, zero meter, recorder and inductance sensor. It is one of the main components of the profiler. A diamond stylus is installed at one end of the sensor measuring rod, and the radius of curvature of the stylus tip is r is very small, when measuring, place the stylus on the workpiece, make contact with the measured surface vertically, and use the driver to drag the sensor at a certain speed. Due to the ups and downs of the profile of the measured surface, the stylus will move up and down when sliding on the measured surface. This movement makes the magnetic core move up and down synchronously through the fulcrum, so that the inductance of the two differential inductance coils surrounded by the magnetic core changes. The coil of the sensor and the measuring line are directly connected to the balance bridge. The change of the inductance of the coil makes the bridge unbalanced, so it outputs a signal proportional to the displacement of the stylus up and down, and the weak power is transferred by the electronic device. After the change amplification and phase-sensitive detection, a signal that can represent the magnitude and direction of the displacement of the stylus is obtained. After that, the signal is divided into three paths: one path is added to the zero indicator to indicate the position of the stylus, the other path is sent to the DC power amplifier, and after being amplified, the recorder is driven to record; the other path is filtered and amplified by the average meter amplifier, and then enters Integral calculator, for integral calculation, the surface roughness Ra value can be read directly from the indicator.
When the iron core is in the middle of the differential inductance coil, the pointer of the zero indicator indicates the zero position, that is, it is guaranteed to be within the linear range of the inductance change. Therefore, before measuring, the zero indicator must be adjusted to make it at zero. After noise filtering and waviness filtering, what is left is a signal proportional to the roughness of the measured surface, and after being amplified by the averaging meter, the absolute ratio between the output current I and the height y of each point of the measured surface profile from the center line The value is directly proportional, and then calculated by the product completed by the integrator to obtain the Ra value, which is displayed by the zero meter. This instrument is suitable for measuring the Ra value of 0.02-10μm, and a few types of instruments can also measure smaller parameter values. The instrument is equipped with various accessories to adapt to planes, inner and outer cylindrical surfaces, conical surfaces, spherical surfaces, curved surfaces, And workpiece surface measurement of small holes, grooves and other shapes. The measurement is quick and convenient, and the measurement accuracy is high.
Traditional surface roughness measuring instruments have the following deficiencies:
(1) There are few measurement parameters, and generally only a small number of parameters such as Ra, Rz, and Ry can be measured;
(2) The measurement accuracy is low, the measurement range is small, and the range of Ra value is generally about 0.02-10μm;
(3) The measurement method is not flexible, such as: selection of evaluation length, selection of filter, etc.;
(4) The output of measurement results is not intuitive. The main reasons for the shortcomings in the above aspects are: the reliability of the system is not high, the error of the analog signal is large and it is not easy to handle.