What is "linear scale"

Grating ruler, also known as raster ruler displacement sensor (sensor grating ruler), is using the optical principle of work of grating measuring feedback device. Grating ruler is often used in closed-loop servo system of nc machine tools, can be used for detection of linear displacement or angular displacement. The measurement of the output signal of digital pulse, with wide detection range, high precision, fast response characteristics. In the numerical control machine tool, for example, often used for cutting tool and workpiece coordinate, to observe and track feeding error, to play the role of a motion error compensation of cutting tool.

Grating ruler according to the methods and different optical principle, divided into transmission grating and reflection grating.

Grating ruler is made up of rod grating and the grating reading of the first two parts. General scale grating fixed on the machine fixed parts, grating reading head installed on the machine moving parts, indicating grating in a grating reading head. The picture on the right is shown in the structure of the grating ruler.

Raster detection device is a key part of the grating reading head, it consists of light source, convergent lens, grating, optoelectronic components and the adjusting mechanism, etc. Grating structure of reading head form a lot, according to the reading head structure characteristic and using occasion are divided into direct receive type reading head (or silicon photocell reading head, image reading, reading head, metal grating spectroscope type reflection type reading head).

The working principle of

Moire fringe

Transmission grating, for example, when the indicator on the grating lines and scale on the grating lines formed between a small Angle theta, and two relative of parallel grating ruler facets, in under the irradiation of light source, is located in a nearly vertical grid lines, the formation of light and dark stripes. The stripes, called "moire fringe" (right). Strictly speaking, the direction of moire fringe arrangement is with two pieces of grating lines perpendicular bisector phase Angle. Moire fringe in the two bright lines or the distance between two dark grain as the width of the moire fringe, expressed as a W.

W = omega / 2 * sin (theta / 2) = / omega theta.

Moire pattern has the following features:

(1) the change of moire fringe patterns

Two pieces of grating relative move through a lattice spacing, moire fringe moving over a stripe distance. By light diffraction and the interference effect, the changing rule of the moire fringe approximation (more than) string function, change cycles and the relative displacement of the grating lattice spacing for synchronization.

(2) the amplification effect

In the case of two raster grid line Angle smaller, moire fringe width W and raster grid, the grid line omega has the following relationship between Angle of theta. Type, theta unit for rad, W unit is mm. Due to the Angle is small, the sine theta is small, then

W = omega/theta.

If omega = 0.01 mm, theta = 0.01 rad, is the type available W = 1, the grating magnified 100 times.

(3) the homogenizing error function

Moire fringe is formed by several grating stripe share, 100 lines per mm grating, for example, 10 mm width of moire fringe has 1000 lines, and such lattice spacing between adjacent error is averageness, eliminated due to uneven blade spacing error caused by, fracture, etc.

Detection and data processing

Electronic subdivision and sentenced to the law

The essence of the grating measuring displacement is a standard ruler grating lattice spacing para weighing measurement. High resolution raster ruler general cost is expensive, and makes them. In order to improve the system resolution, need of moire fringe subdivision, the grating ruler (2006) sensor system using electronic subdivision method. When two pieces of grating with small Angle overlap, in a direction perpendicular to the grating groove is roughly will generate moire fringe, with the moving of the grating, moire fringe is moved up and down. Thus the lattice spacing of grating measurement is converted into a number of moire fringe measurement.

Within a moire fringe width, according to a certain interval placed four photoelectric device can realize electronic division and sentenced to function. For example, the grid line of 50 lines/mm grating ruler, the grating lattice spacing of 0.02 mm, if adopted four subdivision resolution of 5 microns can be got after count pulse, it is common in industry measurement and control has reached a very high precision. Due to the displacement is a vector, that is, to test its size, and to test its direction, so the photoelectric signal of at least two different road phase. In order to eliminate common-mode interference, dc component and harmonics, usually adopt composed of low drift op-amp differential amplifier. Obtained by four photosensor 4-way photoelectric signal to two differential amplifier input respectively, two road from differential amplifier output signal phase difference of PI / 2, to get to and count pulse, for the two way signal is plastic, plastic them, first of all, for the square wave duty ratio of 1:1. Then, by comparing the phase of the wave for the discriminant, can wait until the moving direction of the grating ruler. By the other party wave pulse count, can wait until the displacement and speed of the grating ruler.