How does the grating encoder affect the accuracy of the voice coil motor

How does the grating encoder affect the accuracy of the voice coil motor?

We know that because of its simple structure and light coil mover, the voice coil motor has been successfully used in many linear motion and swing motion applications that require high-precision positioning; the voice coil motor platform module adopts appropriate positioning feedback and induction devices, and its positioning accuracy It can easily reach 10um, the acceleration can reach 300G, and the configuration of appropriate structure technology and position feedback can achieve the nano-positioning accuracy requirements;

A very important device for linear position feedback is a linear grating ruler, which is used to measure the moving position of a linear axis. Due to the direct measurement of the mechanical position, the actual position of the voice coil motor platform module can be reflected most accurately.

For machine tool control, the grating ruler can:

Eliminate the backlash of the ball screw;

Eliminate the position error of the screw and guide rail due to temperature changes; (large)

Eliminate the position error caused by the pitch error of the ball screw;

For machine tool control, grating rulers are often used in the following situations:

1. High-precision machine tools: improve processing accuracy and improve product quality;

2. Large machine tools, such as: large gantry, horizontal machining, boring and milling machines, etc.;

The use of a grating ruler can reduce the transmission error caused by the excessively long transmission chain, while reducing

Deformation errors caused by small temperature changes;

3. New motors with direct drive technology: iron-core linear motors, iron-core linear motors, cylindrical voice coil motors, curved voice coil motors, etc.

For frequently used grating rulers, we usually pay attention to the following technical specifications:

1. The structure of the grating ruler: steel belt, glass;

2. The signal type of grating ruler: serial signal, square wave signal, sine wave signal (1-Vpp);

3. The resolution of the grating ruler;

4. The signal period and frequency multiplication of the grating ruler;

The grating ruler is classified according to the structure:

1. Steel belt structure.

The steel belt structure is suitable for long-distance movement,

2. Glass structure.

The glass structure is suitable for high-precision grating rulers due to its good thermal stability; but the measuring distance

It will not be too long.

In addition, ultra-high-precision measurements may also use ceramic-based grating rulers.

The grating ruler is classified according to the measurement method:

①Incremental grating ruler, ②Absolute grating ruler, ③Distance code grating ruler.

among them:

· Incremental grating ruler measurement position information is obtained by incremental calculation with the current position.

· The position information of an absolute grating ruler is recorded on an absolute position code line on the grating ruler.

· The distance code grating ruler does not require an external power source, and the reference point determined by a fixed algorithm is detected

To determine the zero point coordinates.

The grating ruler signal mainly includes three kinds: serial signal, sine wave signal and square wave signal;

1. Serial signal: Serial signal refers to a signal that conforms to the FANUC transmission protocol.

The grating ruler using this signal transmits the signal as serial data. Therefore, the reliability and stability are relatively high.

2. Sine wave signal: also called 1-Vpp signal. A06B-6061-C201 expandable

The signal is usually a two-phase signal with a 90° electronic angle.

2.1. Grating pitch.

As shown in the figure below, the grating ruler outputs electrical signals, and the grating pitch refers to the actual physical scribe lines on the grating ruler. Each time a grating pitch passes, the electrical signal output by the grating ruler changes by one cycle.

For example: when the grating pitch is 20um, when the moving distance is 20um, the grating ruler will output two sine wave waveforms with a phase difference of 90° and a 360° change.

2.2. Signal period.

With the development of measurement technology, it is now possible to multiply the frequency of the sine wave generated by each grid signal on the grating scale reading head.

Accordingly, the signal output period of the grating ruler can be refined. The signal after frequency multiplication by the reading head will be densified many times than the original grid signal. The length of the signal after densification is called the signal period.

2.5 Example: Common grating ruler specification table

3. Square wave signal: also known as TTL interface output signal. A02B-0303-C205 expandable

The square wave signal received by the system is 2 square wave signals A and B, and its inverted signals A and B.

Among them, the phase difference between the two signals is 90°.

the difference:

1. Normally, through the sinusoidal signal output by the grating sensor (a period is a grating period)

After interpolation and digital processing, a square wave with a phase difference of 90° is obtained.

2. Square wave signal processing, according to its signal characteristics, it can be processed up to 4 times the frequency.

The sine wave can theoretically perform infinitely without signal attenuation and interference.

Multiplication processing of multiples.

3. Sine wave signals are more susceptible to interference than square wave signals.

Choice of grating ruler for voice coil motor platform module:

When selecting a grating ruler, the user needs to consider the following principles:

1. The minimum command unit (system) and the measuring step of the grating ruler should be consistent;

2. In 1 point, if you need to ensure that the positioning accuracy of the actual machine tool reaches the minimum command unit of the system, it is recommended to use a model with a measuring step of the grating ruler that is 1/10 of the minimum command unit;

3. In ultra-slow movement, (especially in the use of grinders) such as F0.1mm/min, in order to maintain the stability of the speed, the choice of grating ruler is more critical. The measurement step should usually be about 0.01um.

4. The model type accepted by the driver; for example, the American copley driver can accept incremental A/B differential digital signals. Choosing the FAOR TA series is a very good guarantee for the stability and accuracy of the voice coil motor platform module. ；