2017年10月20日星期五

CNC milling machine grating (Part 1)

Grating is a high-precision position sensor, widely used in CNC milling machine, which mainly involves long grating and round grating two types. Long grating for the milling machine table and other moving parts of the linear displacement measurement, also known as linear grating. Round gratings are commonly used for angular displacement measurements such as rotary tables. The picture shows the grating installation diagram.
The grating body is fixed on the milling machine bed, the scanning head is fixed on the work table, the scanning head moves together with the work table, and the straight line displacement amount of the work table is converted into electric signal by cable output.
The picture shows the appearance and the internal structure of the grating. Ruler grating (also known as the main ruler) fixed in the body fixed. The ruler grating is made of optical glass or stainless steel strip. The ruler grating is engraved with equidistant stripe, which is called the pitch ω. 
In addition, the ruler grating is also engraved with a set of zero mark fringes, and according to the law of equidistant arrangement, its purpose is to return to the reference point in the process, to the nearest zero mark, save back to the reference point of time. The indicator grating (also known as the secondary ruler) corresponding to the ruler grating is also engraved with equally spaced stripes. Indicator grating is fixed in the scanning head and moved together with the scanning head. According to the projection of the light, the grating has two kinds of transmission grating and reflection grating.
The above is on the CNC milling machine grating of the basic composition, classification and the role of a brief introduction, you are welcome to continue to pay attention.

2017年10月19日星期四

Milling Machine Absolute Photoelectric Encoder

Milling machine absolute photoelectric encoder structure and incremental encoder is similar to the code, LED, photosensitive components and signal detection circuit and so on. The detection signal is binary coded, and each code corresponds to an angular position. The picture shows the absolute measurement and binary coding.
The milling machine has a code track (also known as track). As shown in the code on the code from the inside to the outside there are 8 yards, the code on the road by the binary law engraved with light or opaque stripes. The side of the disc is the light source and the other side is a group of photosensitive elements. When "0" to a certain angle, and the corresponding channel of the photosensitive element by the light output is "1" level, from the light output is "0" level, which constitutes a set of binary code. The number of bits in the binary code is the number of code channels, and each set of binary codes corresponds to a single angle, as shown.
Binary code has two types of natural binary code and Gray code. Natural binary code, the measured angle changes every time, binary coding may occur in a number of "0" and "1" changes, prone to error code, low reliability. Gray code, the measured angle changes every time, only one binary code "0" and "1" changes, Gray code is conducive to reducing the wrong code, improve reliability.
Because the binary code is distributed within 360°, when the code from 0°to 360°, the code back to the initial value, can not tell the number of turns of the code wheel, so when the angular displacement measurement, to using multi-turn absolute encoder.
The position data of the absolute encoder is stored in the memory, held by an external lithium battery, and the data is not lost after power failure. So the configuration of the absolute encoder CNC milling machine once the reference point after the completion of the set, after each boot power after the need to return to the reference point of the operation.
The above is a simple introduction to the milling machine absolute photoelectric encoder, hoping to help you.

2017年10月18日星期三

Milling Machine Incremental Photoelectric Encoder

Incremental photoelectric encoder in milling machine is mainly composed of light source, code plate, light rail, photosensitive components and conversion circuits and other components, as shown.
The incremental photoelectric encoder is engraved with radially light-colored stripes with equal pitch, and the adjacent two stripe represent a resolution angle α. There is a fixed encoder in the encoder, light rail engraved with A, B two groups of light transmission stripes. When the disc is rotated with the rotating shaft, the light is transmitted to the photosensitive element through the light transmission strip on the disc and the light rail, and the photosensitive element outputs the electrical signals of A and B and the phase difference 90°electrical angle, as shown. A, B two signals used to determine the direction of rotation, if A ahead B 90°, then the encoder forward. If B is advanced by A 90°, then the encoder is reversed.
There is also a light transmission stripes on the inside of the code wheel. When the code wheel turns a turn, the corresponding photosensitive element of the stripe produces an electrical signal Z, called a zero mark pulse (also called a turn pulse).
Milling machine Incremental photoelectric encoder is characterized by the generation of a pulse signal corresponding to an angular displacement increment, the increment is the resolution angle. Counting the pulse is the accumulation of diagonal displacement increments, the cumulative result is the measured angular displacement. In addition, since the count pulse frequency is proportional to the encoder speed, the speed of the milling machine encoder can be obtained by measuring the counting pulse frequency.
When the incremental encoder is de-energized, the current position information is lost. Therefore, the CNC milling machine with the incremental encoder is operated back to the reference point after power-on. The reference point is determined by finding the zero sign signal to establish the measurement reference.
The above is a brief introduction to the CNC milling machine incremental encoder, and welcome to continue your attention.

2017年10月17日星期二

CNC milling machine encoder

In the process of processing, milling machine CNC system to real-time detection of the current table and other moving parts of the actual location and speed, and with the command position and speed to compare, to achieve position control and speed control to meet the accuracy of position and speed stability control requirement.
 In the same way, CNC system also carries on the speed detection and the position detection to the spindle, in order to carry on the speed and the position control to the spindle. Modern CNC milling machine commonly used position and speed detection device, such as encoder and grating.
The encoder is used for angular displacement measurement and speed measurement, either in the servo motor and spindle motor, or independently. The picture shows the application of the encoder in the numerical control machine tool.
The servo motor is connected to the ball screw through the coupling. Servo motor rotation, the servo motor built-in encoder real-time detection of the servo motor rotor angular displacement and speed, indirect measurement of linear displacement. If the screw pitch is t, the CNC system obtains the linear displacement x of the milling machine table according to the angular displacement θ detected by the encoder.

The spindle encoder and spindle are connected with a 1: 1 timing belt for real-time detection of spindle speed and angular displacement. In addition, the encoder installed in the milling machine spindle motor can directly measure the motor speed and rotor angle.

2017年10月16日星期一

Milling machine linear motor

Feed drive being composed of servo motor, ball screw and milling machine worktable, there are mechanical transmission error (such as backlash, screw pitch reducer and screw nut cumulative error) and the transmission stiffness is low, these factors will affect the accuracy of position. In addition, by the servo motor and transmission mechanism, the maximum speed and acceleration are also limited.
Feeding drive using milling machine linear motor, the elimination of the motor to the workbench between all the intermediate links, reducing the mechanical transmission error, transmission stiffness and positioning accuracy, fast response. At present, the linear motor is more and more used in high-speed, high-precision CNC milling machine. The picture shows the appearance and composition of the linear motor.
Linear motors are essentially AC servo motors, consisting of primary and secondary sides. On the secondary side of the linear motor, the permanent magnet is mounted alternately on the base plate. A three-phase winding is mounted on the primary side slider, which is connected to the milling machine table. The table is supported and guided by a linear rolling guide and maintains a certain air gap between the slider and the permanent magnet.
When the primary side of the three-phase AC power, the resulting air gap magnetic field along the line, the magnetic field known as the traveling wave magnetic field. The magnetic field of the secondary side permanent magnet interacts with the traveling wave magnetic field to generate electromagnetic thrust, and the primary side slider moves linearly under the action of electromagnetic force. The linear motor can be driven by a servo motor drive. The primary and secondary sides of the linear motor can be fitted with cooling components to cool and cool the linear motor. The picture shows the application of linear motor in CNC milling machine feed.

2017年10月15日星期日

Milling machine step driver

The function of the milling machine step driver is to process and enlarge the input control pulse signal, and output the pulse power to each phase winding of the stepping motor, as shown. Step driver involves pulse distribution and power amplification.
1.Pulse distribution
Pulse distribution is used to control the running mode of the stepper motor, and its function is to send a series of command pulse signals issued by the numerical control system according to the power phase sequence. The assigned pulse signal is used as the control signal of each phase winding. Taking the milling machine three-phase stepper motor as an example, the number of pulse signals CP controls the angle of the stepping motor rotation, and the stepping motor turns a step angle every time a CP pulse is input. The frequency of the pulse signal CP controls the rotational speed of the stepping motor. The direction of the signal is TTL level, when the TTL is high, the stepper motor is running, and vice versa. The pulse distributor (ring distribution integrated circuit) assigns the CP pulse signals to the stepping motors A, B, and C windings in sequence according to the direction signals.
2.Power amplification
Each phase winding of the milling machine stepper motor has a power switch, the distribution of the pulse power amplification, in order to obtain the drive current of each phase winding. Single-supply power amplifier at high frequencies due to the winding current to reach the specified value, resulting in stepping motor output torque is insufficient. Low-frequency winding current easily intermittent, resulting in stepper motor speed instability. In order to overcome the shortcomings of single-supply power amplification, stepper drive there are high and low voltage power supply drive, constant current chopper drive and subdivision drive, these drive devices can improve the smooth operation of the milling machine stepper motor to eliminate oscillation.

2017年10月12日星期四

Milling machine stepper motor(Part 2)

In the previous article, we talked about the composition of the milling machine stepper motor and its control ways. Today we look at what features it has.
1. Hold torque
The stepping motor remains energized, but the rotor does not move and has torque output. At this time, the torque is called the holding torque, also known as the maximum static torque. Holding torque is an important measure of the ability of the milling machine stepper motor to withstand external load disturbance.
2. Torque-frequency characteristics and acceleration / deceleration
The output torque of the stepper motor varies with the energization frequency, which is called the torque-frequency characteristic. As the picture shows.
As shown in the figure, fs is the starting frequency, also known as the sudden jump frequency, the corresponding torque for the starting torque Ts. At the time of starting, the starting torque is greater than the torque Tr at the time of continuous operation due to the presence of the acceleration torque. The starting frequency is much lower than the continuous operating frequency, the starting frequency is generally only a few hertz.
If the starting frequency is too large, the milling machine stepping motor output torque is not enough to overcome the starting torque, it will lead to stepper motor can not start, out of step or stall phenomenon. After the start of the stepping motor, the highest frequency with the continuous rise of the frequency of the energization frequency is called the running frequency fr, and its value is much larger than the starting frequency. If the stepper motor running frequency is too high, will lead to the output torque is not enough to overcome the load torque, it will be out of step or stall phenomenon, accompanied by howling.
Understand the milling machine stepper motor these characteristics, I hope to help you some.