The working principle of grating ruler

Working principle

Working principle of common grating are based on physical moire fringe formation principle of work. When the line and scale grating grating on the line at an angle to place the two grating ruler, will result in line two grating on the cross. Under the irradiation of the light source, near the intersection area as the black line overlap, so the shading area of the smallest, the blocking effect, cumulative effect of light makes this area appear bright band. On the contrary, from the point of intersection area far, because the overlap of two grating opaque black lines are becoming less and less, opaque area becomes larger, i.e. the shading area becomes larger, the shading effect becomes stronger, less light gets through this region through the grating, make this area appear dark band, thus they formed the moire fringes we see.

Moire fringe

The transmission grating as an example, when a small angle theta is formed between the index grating on the line and scale grating on the line, and the two grating facet is placed in parallel, under the irradiation of the light source, is located in the grid lines on nearly vertical, the formation of bright and dark stripes. This is called " fringe moire fringe " ( pictured to the right ). Strictly speaking, moire fringe orientation is the bisector and two pieces of grating line angle perpendicular. Moire fringe in between two bright lines or two dark lines distance is called the moire fringe width, expressed in W. Moire fringe

W=ω /2* sin (θ /2 ) = w / theta. Moire fringe has the following characteristics: ( 1 ) the rule of change of Moire fringe two pieces of grating relative moves over a pitch, moire fringe shift a fringe distance. Because of the light diffraction and interference effects, the rule of change of Moire fringe approximation are (more than) the string function, the number of pitch variation of relative displacement cycles and grating synchronization. ( 2 ) amplification angle in two grating is smaller, the relationship between the moire fringe width W and grating pitch angleω, grid line. In the formula, theta units for rad, W for mm units. Because the angle is very small, sin theta is very small, then W=ω/ theta ifω =0.01mm,θ =0.01rad, then type W=1, i.e. grating magnified 100 times. Moire fringe computation

( 3 ) all of the error of Moire fringe grating formation is composed of several common, such as 100 lines per mm of grating moire fringe width of 10mm, there are 1000 lines of adjacent error, pitch between such is average, and eliminates the error caused by the uneven spacing, fracture etc..

Detection and data processing

The essence of grating displacement measurement is based on the grating pitch as a standard ruler on the weighing measurement. The high resolution grating ruler is generally more expensive, and difficult to manufacture. In order to improve the system resolution, the need for subdivision of Moire fringe, at present ( 2006) grating displacement sensor system using electronic subdivision method. When the two gratings with small angle overlap, and the grating grooves generally perpendicular direction will produce moire fringe, along with the moving grating, moire fringe also moves. So the measurement of grating pitch conversion for measurement of Moire fringe number. In a moire fringe width, according to certain intervals and placed 4 photoelectric device can achieve electronic subdivision and direction judgment function. For example, a gate line is 50 lines of /mm grating ruler, the grating pitch is 0.02mm, if you use the four segmentation can be obtained after resolution of count of 5μ m pulse, which has reached a very high accuracy in the industrial common measurement and control. As the displacement is a vector, that is to measure its size, but also detect the direction, therefore needs at least two different phases of photoelectric signal. In order to eliminate the common-mode interference, DC and even harmonic, usually composed of low drift operational differential amplifier. The 4 optical signal is obtained by the 4 photosensitive devices were sent to the 2 differential amplifier input, from the two-way differential signal amplifier output of the phase difference of PI /2, in order to get the judgment and counting pulse, need shaping of the two signals, first put them plastic for duty ratio for 1: 1 square wave. Then, by phase square wave discrimination comparison, can get the moving direction of grating ruler. Through the square wave pulses are counted, can obtain the displacement and speed of grating ruler.