What Is a Shaft Encoder?

The spindle encoder uses a photoelectric pulse generator that is synchronized with the spindle, and is driven 1: 1 synchronously by the gear on the intermediate shaft. There is no direct mechanical connection between the rotation of the spindle of the CNC lathe and the feed motion. In order to process the thread, it is required that the pulse number of the given feed servo motor and the spindle speed should have a corresponding relationship. The link between the rotation of the spindle and the feed motion is reached.

Spindle encoder

Threads are continuous protrusions and grooves with the same cross-sectional shape formed along a spiral line on a cylindrical or conical surface. Threads formed on a cylindrical surface are called cylindrical threads, and threads formed on a conical surface are called conical threads. The threads formed on the outer surface of the rotary body are called external threads, and the threads formed on the inner surface of the rotary body are called internal threads.

There are many methods for processing threads. When adding threads to a lathe, the lathe clamps a cylindrical workpiece through a three-jaw chuck and drives it to rotate at a constant speed. The axis direction makes constant-speed linear motion, and the turning tool tip cuts a triangular groove on the surface of the workpiece to form a triangular thread.

There are trajectory starting point (point A, which is the feed point) and trajectory end point (point B, which is the retract point) during thread processing. In the thread processing trajectory, to ensure the processing of the thread, there are sufficient increments and decelerations to eliminate the pitch error caused by servo lag.

The principle of photoelectric pulse generator. On the leaked disc, two circles of stripes are engraved along the circumference, and the outer circle is a line of equal division of the circumference, for example: 1024, for sending pulses, the inner circle has only one. On the light bar, there are engraved strips A, B, C, and the distance between A and B should ensure that when stripe A coincides with any stripe on the disc, the overlap of the stripe BEN and another stripe on the disc Dislocation 1/4 cycle. A phototransistor is placed behind each stripe of the light bar to form an output channel.

The scattered light emitted by the light bulb is converged by the condenser to become parallel light. When the leaked disc rotates synchronously with the spindle, the stripes on the leaked disc overlap with the stripes on the light bar, causing the photosensitive tube to undergo bright and dark changes. , Causing the current in the photosensitive tube to change, and the changed signal current outputs a rectangular pulse through the rectifier amplifier circuit. Because the stripe A is overlapped with the disc of the disc leakage, the stripe B and the other stripe are out of position by 1/4 period, so the waveform phases output by the two channels A and B are also different by 1/4 period.

The pulse outputted when a engraved line on the inner circle of the leaked disc in the pulse generator coincides with the stripe C on the light bar is a synchronous (starting, also called zero) pulse. With synchronous pulse, the CNC lathe can realize the processing control, and can also be used as the exact stop signal of the spindle exact stop device. When turning a CNC lathe thread, the synchronization pulse is used as the control signal for the turning point and the retracting point of the turning tool to ensure that the turning thread will not be buckled.

1. Requirements of CNC machine tools for detection elements and position detection devices

(1) NC machine tool requirements for testing components

The detection element is an important part of the detection device, and its main function is to detect displacement and speed, and send feedback signals. The minimum displacement that a displacement detection system can measure is called resolution. The resolution depends not only on the detection element itself, but also on the measurement circuit.

The main requirements of CNC machine tools for detecting components are: long life, high reliability, and strong anti-interference ability; to meet the requirements of accuracy and speed; easy to use and maintain, suitable for the machine tool operating environment; low cost;

Different types of CNC machine tools have different requirements for the accuracy and speed of the inspection system. Usually large CNC machine tools mainly meet the speed requirements, while medium, small and high precision CNC machine tools mainly meet the accuracy requirements. When selecting the resolution and pulse equivalent of the measurement system, it is generally required to be an order of magnitude higher than the machining accuracy.

(2) Requirements of CNC machine tools for position detection device

The position detection device is an important part of the CNC machine tool servo system. Its role is to detect displacement and speed, send feedback signals, and form closed-loop or semi-closed-loop control. The machining accuracy of CNC machine tools is mainly determined by the accuracy of the detection system. Different types of CNC machine tools have different requirements for the accuracy of position detection elements, detection systems, and the maximum moving speed of the tested components. Now the highest level of detection components and systems is: when the highest moving speed of the tested component is as high as 240m / min, the resolution of its detection displacement (the minimum displacement that can be detected) can reach 1m, such as 0.1 at 24m / min m. The highest resolution can reach 0.01 m.

CNC machine tools have the following requirements for position detection devices:

It is less affected by temperature and humidity, reliable in work, can maintain accuracy for a long time, and has strong anti-interference ability.

It can meet the requirements of accuracy and speed within the moving range of the machine's executive parts.

Easy to use and maintain, adapt to the working environment of the machine tool.

Low cost. Introduction to Spindle Encoder

2. Classification of position detection devices

For different types of CNC machine tools, due to different working conditions and testing requirements, the following different testing methods can be used.

(1) Incremental and absolute measurement

The incremental detection method only measures the displacement increment, and uses the number of digital pulses to indicate the number of unit displacements (ie, the minimum set unit). A measurement signal is sent every time a measurement unit is moved. The advantage is that the detection device is relatively simple, and any center point can be used as the starting point for measurement. However, in this system, the shift distance is read after the measurement signals are accumulated. Once the accumulation is wrong, the subsequent measurement results will be completely wrong. In addition, when a fault occurs (such as power failure), the correct position before the accident can no longer be found. After the accident is eliminated, the workbench must be moved to the starting point and counted again to find the correct position before the accident. Pulse encoders, resolvers, induction synchronizers, gratings, magnetic grids, laser interferometers, etc. are all incremental detection devices.

The absolute measurement method measures the absolute coordinate position value of the component under test in an absolute coordinate system, and is expressed by a binary or decimal digital signal. Generally, it must be converted into a pulse digital signal before it can be sent for comparison. And display. In this way, the higher the resolution requirements, the more complicated the structure. Such measuring devices include absolute pulse encoder disks, three-speed absolute encoder disks (or multi-turn absolute encoder disks), and the like.

(2) Digital and analog measurement

Digital detection is to quantify the unit to be measured and express it in digital form. The measurement signal is generally an electric pulse, which can be directly sent to the CNC system for comparison and processing. Such a detection device includes a pulse encoder and a grating. Digital detection has the following three characteristics.

The measured number is converted into the number of pulses, which is convenient for display and processing;

The measurement accuracy depends on the measurement unit and has nothing to do with the range; however, there is a cumulative error difference;

The detection device is relatively simple, and the anti-interference ability of the pulse signal is strong.

Analog detection is to use measured continuous variables to represent the measurement, such as voltage amplitude changes, phase changes, and so on. When accurate analog testing is performed within a large range, there are higher requirements for technology. Analog testing in CNC machine tools is mainly used for small-range measurement. Analog detection devices include speed generators, resolvers, induction synchronizers and magnetic rulers. The main characteristics of analog detection are as follows.

The measurement is directly performed without quantification.

High precision measurement can be achieved in a small range.

Can perform direct and indirect detection.

The position detection device is installed on the actuator (ie, the end piece) to directly measure the linear or angular displacement of the end piece of the actuator, which can be called direct measurement. It can form a closed-loop feed servo system. The measurement method includes linear grating and linear induction synchronization. Devices, magnetic grids, laser interferometers, etc. measure the linear displacement of the executive components; because this detection method uses a linear detection device to measure the linear displacement of the machine tool. Its advantage is to directly reflect the linear displacement of the table. The disadvantage is that the detection device is required to have the same length as the stroke, which is a great limitation for large machine tools.

The position detection device is installed on the transmission element or the drive motor shaft in front of the actuator, and measures its angular displacement. The linear displacement of the actuator can be obtained after the transmission ratio is changed. This is called indirect measurement and can form a semi-closed-loop servo feed system. . For example, install the pulse encoder on the motor shaft. Indirect measurement is reliable and convenient to use, and has no length limitation; its disadvantage is that a transmission chain error that converts a straight line into a rotary motion is added to the detection signal, which affects the measurement accuracy. Generally, compensation for the transmission error of the machine tool is needed to improve the positioning accuracy.

In addition to the above position detection devices, the servo system often includes a speed detection component to detect and adjust the engine speed. A commonly used speed measuring element is a speed measuring engine.

One. The general shape of absolute encoder: 38MM, 58MM, 66MM, 80MM.100MM.
Two. Absolute encoders are divided into single-turn and multi-turn.
three. Bus encoders are divided into sub-absolute encoders and optical absolute encoders according to their principles. The bus encoder outlet method is divided into: side outlet, rear outlet 5. The bus encoder shaft is divided into: 6MM, 8MM, 10MM, 12MM, 14MM, 25MM.
six. The bus encoder is divided into: shaft, blind hole, through hole.
Seven. Bus encoder protection is divided into: IP54-68.
Eight. The installation method of the bus encoder is divided into: clamping flange, synchronous flange, clamping belt with synchronous flange, blind hole (spring leaf, hold tight), communication

Hole (spring leaf, key pin)
nine. The absolute encoder accuracy is divided into: single-turn accuracy and multi-turn accuracy, which add up to the total accuracy, which is the usual number of bits (conventional 24

It is 25 bits, 30 bits, and 32 bits. . . . ).
ten. The baud rate of the absolute encoder communication protocol: 4800 ~ 115200 bit / s. Refresh period is about 1.5ms
XI. Encoder output optional: SSI, 4-20MA, profibus-dp, DEVicenet, parallel, binary code, BESM58, BiSS, ISI,

CANopen, Endat, Hiperface, etc.