Structure and Principle of Linear Grid of Domestic Linear Motor
time：2017/5/11 click：14806
The linear motor is a high-precision special motor. The position accuracy of the Tongmao linear motor is 0.01 μm ~ 10 μm. So how does the linear motor achieve the accurate feedback of the linear displacement, that is, the linear grid ruler, the following let the Tongmao linear motor small to briefly introduce the basic structure and principle of the grid ruler.
The raster ruler, also known as the raster displacement sensor(raster sensor), is a measurement feedback device that uses the optical principle of the grating. The raster is often used in the closed-loop servo system of CNC machine tools and can be used as a detection of linear displacement or angular displacement. The signal of the measurement output is digital pulse, which has the characteristics of large detection range, high detection accuracy and fast response speed. For example, in CNC machine tools are often used to detect the coordinates of the tool and workpiece to observe and track the error of the knife, so as to play a role in compensating the tool's motion error.
The grating ruler is divided into transmission grating and reflection grating according to the manufacturing method and optical principle.
The basic structure is as follows:
The raster is composed of two parts: the raster and the raster reading head. The ruler grating is usually fixed on the fixed part of the machine tool. The grating reading head is mounted on the linear motor, and the grating is indicated to be mounted on the grating reading head.
Tongmao linear motor grating
The key part of the grating detection device is the grating reading head, which consists of a light source, a polymer lens, an indicator grating, an optoelectronic element, and an adjustment mechanism. There are many forms of grating reading head structure, which are divided into direct receiving reading heads(or Silicon photocell reading heads, mirror reading heads, spectroscopic reading heads, and metal grating reflective reading heads) according to the characteristics of reading head structure and use occasions.
Basic working principles:
1, Moore Stripe
Take the transmission grating as an example. When a small angle θ is formed between the line pattern on the indicator grating and the line pattern on the ruler grating, and the two grating rulers are placed relatively parallel, under the irradiation of the light source, they are located on the almost vertical grid. On, Form bright and dark stripes. This Stripe is called the "Moore Stripe." Strictly speaking, the direction of Moore's stripes is perpendicular to the bisector of the angle of the two grating lines. The distance between two bright lines or two dark lines in the Moore Stripe is called the width of the Moore Stripe and is represented by W. W = Ω / 2 * sin(θ / 2) = Ω / θ
2, Moore stripes have the following characteristics:
(1) The Law of Moire Stripes
The two gratings move opposite one grid distance and the Moore Stripe moves one Stripe distance. Due to the diffraction and interference of light, the change law of Moire Stripe approximates the positive(residual) string function, and the number of periodic changes is synchronized with the number of grid distances of the relative displacement of the grating.
(2) Amplification
In the case of a small angle of the two-beam grid line, the following relationship exists between the Moire Stripe width W and the grid pitch Ω and the grid line angle θ. In the formula, the unit of θ is rad and the unit of W is mm. Due to the small inclination, sinθ is small, then W = Ω / θ; If Ω = 0.01 mm, θ = 0.01 Rad, the above equation yields W = 1, IE the grating is magnified 100 times.
(3) homogenization error
The Moire Stripe is formed by the common formation of several grating stripes. For example, the grating per 100 mm line, the Moor Stripe with a width of 10mm has 1,000 lines, so that the adjacent error between the grids is averaged. Errors caused by uneven grid spacing, fracture, etc. are eliminated.
III. Testing and data processing
1, electronic subdivision and direction method
The essence of grating displacement measurement is to measure the bit weighing with grid distance as a standard ruler. High-resolution raster rulers are generally more expensive and difficult to manufacture. In order to improve the resolution of the system, it is necessary to subdivide the Moire Stripe. At present, the grid sensor system uses electronic subdivision method. When the two gratings overlap at a small inclination, Moore stripes are generated in a direction roughly perpendicular to the grating engraving line. As the grating moves, the Moore stripes move up and down. In this way, the measurement of grating pitch is converted into the measurement of the number of Moire stripes.
Within the width of a Moore Stripe, four optoelectronic devices can be placed at a fixed interval to achieve electron segmentation and targeting. For example, the grid line is a 50-wire pair / mm grid ruler with a grid distance of 0.02 mm. If four subdivisions are used, a counting pulse with a resolution of 5 μm can be obtained, which has been achieved in industrial general measurement and control. Very high precision. Since the displacement is a vector, that is, to detect its size and to detect its direction, at least two photoelectric signals with different phases are required. In order to eliminate common mode interference, DC components, and even harmonics, a differential amplifier consisting of low drift flux is usually used. The 4-way photoelectric signal obtained by the 4 photosensitive devices is sent to the input end of the 2-division amplifier, respectively. The phase difference of the two-way signal output from the differential amplifier is π / 2. In order to obtain the directional and counting pulses, these two signals need to be obtained. Plastic work, First, they are shaped as square waves with a ratio of 1:1. Then, by comparing the phase of the square wave, we can wait until the direction of the grid. Counting by the other wave pulse can wait until the displacement and speed of the grid ruler.
The above is the basic structure and working principle of the dedicated raster for linear Motors. For more information on the selection and application of linear motors and coil Motors, please log on to www.tomemotion.com(Kunshan Tongmao Electronics Co., Ltd.-Focus on the development and production of linear motors and coil motors). Consultation hotline: 400-007-7724.
Linear motor