Difference between incremental encoder and absolute encoder

1、 Classification of encoders

According to the detection principle, encoders can be divided into optical type, magnetic type, inductive type and capacitive type. According to their calibration methods and signal output forms, they can be divided into three types: incremental, absolute and hybrid.

1. Incremental encoder the incremental encoder outputs three sets of square wave pulse a, B and Z by photoelectric conversion principle; the phase difference between a and B is 90. The Z phase is a pulse per revolution, which is used to locate the reference point. Its advantages are simple principle and structure, the average mechanical life can be more than tens of thousands of hours, strong anti-interference ability, high reliability, suitable for long-distance transmission. Its disadvantage is that it can not output the absolute position information of shaft rotation.

2. Absolute encoder The absolute encoder is a sensor that outputs digital directly. On its circular code disk, there are a number of concentric code disks along the radial direction, and each channel is composed of transparent and opaque sector areas alternately. The sector trees of adjacent code channels have a double relationship. The number of code channels on the code disk is the number of bits of its binary number. On one side of the disk is the light source, and on the other side there is a photosensitive element corresponding to each code channel When the plate is in different positions, each photosensitive element converts the corresponding level signal according to whether it is illuminated or not, forming a binary number. The characteristic of this kind of encoder is that it does not need a counter and can read a fixed digital code corresponding to the position at any position of the rotating shaft. Obviously, there must be n Modao. At present, there are 16 bit absolute encoder products in China.

3. Hybrid absolute encoder hybrid absolute encoder outputs two groups of information. One group of information is used to detect the position of magnetic pole with the function of absolute information; the other group is exactly the same as that of incremental encoder.

2、 Application of photoelectric encoder

1. Angle measurement

The photoelectric encoder is selected as the sensor to measure the rotation angle of the steering wheel in the driving simulator. The photoelectric encoder is used in the gravimeter to connect its rotating shaft with the compensation knob shaft in the gravimeter. The torsion angle meter uses the encoder to measure the change of the torsion angle, such as torsion experimental machine, fishing rod torsion fishing ability test, etc. Pendulum impact testing machine, using the encoder to calculate the impact is the swing angle change.

2. Length measurement

The meter counter uses the circumference of the roller to measure the length and distance of the object.

The cable displacement sensor is used to measure the length distance of the object by the circumference of the winding wheel.

It is connected with the main shaft of the power device driving the linear displacement, and is measured by the output pulse number.

Medium detection is used to transmit linear displacement information in straight rack, chain wheel of rotating chain and synchronous belt pulley.

3. Speed measurement

Linear speed, through the instrument connection, measure the line speed of the production line

Angular speed, through the encoder to measure the speed of motor, shaft, etc

4. Position measurement

In terms of machine tool, memorize the coordinate position of each coordinate point of machine tool, such as drilling machine

In the aspect of automatic control, it controls the designated action in the pastoral position. Such as elevator, elevator, etc

5. Synchronous control

Through angular speed or linear speed, the transmission link is controlled synchronously to achieve tension control

3、 Incremental encoder (rotary)

1. Working principle:

Four groups of sine wave signals are combined into a, B, C, d by a photoelectric code disk with an axis in the center, on which there are ring-shaped through and dark lines, which are read by photoelectric transmitting and receiving devices, Each sine wave has a phase difference of 90 degrees (360 degrees relative to a cycle), and the C and D signals are reversed and superimposed on a and B phases to enhance the stable signal; in addition, a z-phase pulse is output per revolution to represent the zero reference position.

Because the phase difference between a and B is 90 degrees, the forward and reverse rotation of the encoder can be judged by comparing whether phase A is in front or phase B is in front. The zero reference bit of the encoder can be obtained by zero pulse.

The encoder code disc is made of glass, metal and plastic. The glass code plate is a thin line deposited on the glass, which has good thermal stability and high precision. The metal code plate can be cut directly through or without lines, and is not fragile. However, due to the thickness of the metal, the accuracy is limited, and its thermal stability is one order of magnitude worse than that of glass. The plastic code plate is economical and its cost is low, However, the accuracy, thermal stability and service life are poor.

Resolution - the resolution is the number of through or dark lines provided by the encoder per 360 degrees of rotation, which is also known as the resolution, also known as the analytic division or the number of lines directly weighed, which is generally 5-10000 lines per revolution.

2. Signal output:

Signal output includes sine wave (current or voltage), square wave (TTL, HTL), collector open circuit (PNP, NPN), push-pull type. Among them, TTL is long line differential drive (symmetrical a, a -; B, B -; Z, Z -), HTL is also called push-pull or push-pull output. The interface of signal receiving equipment of encoder should correspond to encoder.

Signal connection - the pulse signal of encoder is generally connected with counter, PLC and computer. The modules connected by PLC and computer are divided into low-speed module and high-speed module, with low-speed and high-speed switching frequency.

Such as single-phase connection, it is used for counting in one direction and measuring speed in one direction.

A. B two phase connection is used for counting, judging and measuring speed.

A. B, Z three-phase connection, used for position measurement with reference position correction.

A. A -, B -, B -, Z, Z - connection, due to the connection with symmetrical negative signal, the current contribution to the cable electromagnetic field is 0, the attenuation is the minimum, the anti-interference is the best, and it can transmit a long distance.

For TTL encoder with symmetrical negative signal output, the signal transmission distance can reach 150 meters.

For HTL encoder with symmetrical negative signal output, the signal transmission distance can reach 300 meters.

3. Problems of incremental encoder:

The incremental encoder has some problems, such as zero accumulated error, poor anti-interference, power-off memory when the receiving equipment is shut down, and zero or reference bit should be found when starting up. These problems can be solved by using absolute encoder.

General application of incremental encoder:

Speed measurement, rotation direction measurement, movement angle, distance (relative).

4、 Absolute encoder (rotary type)

There are many optical channel lines on the optical code disk of absolute encoder. Each line is divided into 2 lines, 4 lines, 8 lines, 16 lines...... In this way, a set of unique binary codes (gray codes) from the zero power of 2 to the N-1 power of 2 can be obtained at each position of the encoder by reading the pass and dark of each reticle, which is called n-bit absolute encoder. Such encoder is determined by the mechanical position of the photoelectric encoder, and it is not affected by power failure and interference.

Absolute encoder is determined by the mechanical position of each position is unique, it does not need to remember, do not need to find a reference point, and do not have to count all the time, when need to know the position, when to read its position. In this way, the anti-interference characteristics of the encoder and the reliability of the data are greatly improved.

From single turn absolute value encoder to multi turn absolute value encoder

Rotating a single turn absolute encoder is used to measure the lines of each photoelectric code disc during rotation to obtain the unique code. When the rotation exceeds 360 degrees, the code returns to the origin again, which does not conform to the principle of unique absolute coding. Such coding can only be used for measurement within 360 degrees of rotation, which is called single turn absolute encoder.

If you want to measure the rotation beyond 360 degrees, you need to use a multi turn absolute encoder.

The encoder manufacturer uses the principle of clock gear mechanism. When the central code plate rotates, another group of code plates (or multiple sets of gears, multiple groups of code disks) are driven through the gear, and the number of turns is added on the basis of single cycle coding to expand the measurement range of the encoder. Such an absolute encoder is called a multi turn absolute encoder, which is also determined by the mechanical position, Each location code is unique and does not need to be memorized.

Another advantage of the multi turn encoder is that due to the large measurement range, the actual use is often more abundant, so it is not necessary to find the zero point during the installation, and a middle position can be used as the starting point, which greatly simplifies the difficulty of installation and debugging.