Working principle and application of photoelectric encoder

Encoder manufacturers use the principle of clock gear machinery. When the central code disk rotates, another set of code disks (or multiple sets of gears, multiple sets of code disks) are transmitted through gears. The number of turns is increased on the basis of single-turn coding to expand the measuring range of the encoder. Such an absolute encoder is called a multi-turn absolute encoder. It is also determined by the mechanical position. The code for each position is unique and non-repetitive, and no memory is required.

Another advantage of multi-turn encoders is that due to the large measuring range, they are often more practical to use. This way, during installation, there is no need to find a zero point, and a certain middle position can be used as the starting point, which greatly simplifies the difficulty of installation and debugging.

Multi-turn absolute encoders have obvious advantages in length positioning and have been increasingly used in industrial control positioning.

15. Specific information on serial and parallel outputs of absolute encoders

Parallel output:

Absolute encoders output multi-digit numbers (Gray code or pure binary code). Parallel output means multiple high and low level outputs on the interface to represent digital 1 or 0. For absolute encoders with low digits, digital output is usually directly in this form, which can directly enter the I/O interface of PLC or host computer. The output is immediate and the connection is simple. However, parallel output has the following problems:

1. It must be Gray code. If it is pure binary code, multiple bits may change when the data is refreshed, and the reading will cause error codes in a short time.

2. All interfaces must be ensured to be well connected. If there is a poor connection point, the potential of that point will always be 0, resulting in error code and inability to judge.

3. The transmission interval cannot be far, generally one or two meters. For complex environments, isolation is best.

4. For a large number of bits, a multi-core cable is required and the connection must be good, which brings engineering difficulty. Similarly, for the encoder, there must be many node outputs at the same time, which increases the failure and damage rate of the encoder.

Parallel: In terms of time, data are sent simultaneously; in terms of space, each bit of data occupies one cable.

The output of the incremental encoder is usually parallel output.

Serial Output:

Serial output is the output of data in a time sequence through agreement. This agreement is called a communication protocol. The physical forms of its connection include RS232, RS422 (TTL), RS485, etc.

Serial output has fewer connection lines and longer transmission distances, which greatly improves the protection and reliability of the encoder. Generally, high-bit absolute encoders use serial output.

Since some well-known manufacturers of absolute encoders are in Germany, most of the serial outputs are matched with Siemens in Germany, such as SSI synchronous serial output, and the bus type is PROFIBUS-DP output.

It is relatively easy to connect the serial output encoder to the German Siemens equipment, but the interface becomes a problem when connecting to non-German equipment. Our company provides instruments with various interface outputs to solve such problems.

Serial: In terms of time, data is arranged in a certain order according to the agreement; in terms of space, all bits of data are sent out on a set of cables (one after the other).

16. Should all serial encoders be absolute?

Serial means that the digital code signal is output serially according to the time agreement, which is basically absolute, but there are also some incremental encoders that use the built-in battery to remember the origin, and they can also output the position value through serial. If the battery line is not connected, it is still an incremental encoder, which is also called a pseudo-absolute value encoder, which is more common in some Japanese servo systems. Its essence is still an incremental encoder.

17. Q: Why is it called “absolute encoder”?

"Absolute encoder" is relative to "incremental encoder".

"Absolute encoders" use a certain method to indicate and memorize the absolute position, angle and number of turns of an object. That is, once the position, angle and number of turns are fixed, the encoder's indication is always unique and fixed, including power-on after a power outage. "Incremental encoders" cannot do this. Generally, "incremental encoders" output two A and B pulse signals and a Z (L) zero position signal, and the A and B pulses differ by 90 degrees in phase angle. The position, angle and number of turns increment can be known by counting pulses, and the direction can be known by the advance or lag of A and B pulse signals. After a power outage, the counting must be restarted from the agreed reference. "Incremental encoders" indicate that the position, angle and number of turns need to be post-processed, and a "reset to zero" operation must be performed when power is re-on. Therefore, "incremental encoders" are much cheaper than "absolute encoders" in price.

18. Q: What are the advantages and disadvantages of photoelectric encoders, optical electronic scales and static magnetic grating absolute encoders?

Photoelectric encoder:

1. Advantages: small size, precision, high resolution (currently our company can reach 54000 cpr on the encoder with diameter φ66 through segmentation technology), no contact and no wear; the same type can detect both angular displacement and linear displacement with the help of mechanical conversion device; multi-turn photoelectric absolute encoder can detect linear displacement with a long range (such as 25-bit multi-turn). Long life, free installation, rich interface forms, reasonable price. Mature technology, has been widely used at home and abroad many years ago.

2. Disadvantages: Although it is precise, it has high protection requirements for outdoor and harsh environments; the measurement of linear displacement requires conversion by mechanical devices, and the error caused by mechanical clearance needs to be eliminated; it is difficult to overcome slip when detecting objects running on the track.

Optical electronic ruler:

1. Advantages: Precision, high intrinsic resolution (up to 0.005mm); moderate size, direct measurement of linear displacement; no contact, no wear, wide measurement gap; moderate price, rich interface forms, and has been widely used in metal cutting machinery industry at home and abroad (such as wire cutting, electric spark, etc.).

2. Disadvantages: Different types are required to measure straight lines and angles; the measuring range is limited (the measuring range exceeds 4m, and the production is difficult and expensive), which is not suitable for displacement detection in a large range and harsh environment.

Magnetostatic absolute encoder:

1. Advantages: moderate size, direct measurement of linear displacement, absolute digital coding, unlimited theoretical range; non-contact and wear-free, resistant to harsh environments, and can be used 1,000 meters underwater; rich interface forms and various measurement methods; the price is acceptable.

2. Disadvantages: The resolution is not high, 1mm; different types are required to measure straight lines and angles; not suitable for displacement detection in small places (greater than 260mm).

19. Example: A disk is divided into 50 points. To achieve positioning control, the speed is very slow. Do I need to use an absolute encoder? How to find the origin? Are the 50 positions evenly divided into 360 degrees?

The codes of absolute encoders are all powers of 2, and there is no 360 degrees divided into 50 equal parts. To approximate, you need to see how high the accuracy requirement is and how high the number of lines of the encoder should be. If the accuracy requirement is not too high, an 8-bit 256-line encoder can be used. Each position of the encoder has a unique code, and the code of zero can be used as the zero point, or any position can be defined as zero, and other positions can be compared with it for calculation.

If you can use reference points, you can also use incremental mode. Due to the slow speed, you should choose 3000 lines or more, with one zero position per circle.

20. Brief introduction: RS-232, RS-422 and RS-485 standards and applications?

RS-232, RS-422 and RS-485 are all serial data interface standards, originally developed and published by the Electronic Industries Association (EIA).

Currently, RS-232 is the most widely used serial interface in PC and communication industries. RS-232 is defined as a single-ended standard that increases the communication interval in low-speed serial communication. RS-232 adopts an unbalanced transmission method, which is the so-called single-ended communication.

RS-422, RS-485 are different from RS-232. The data signal adopts differential transmission, also known as balanced transmission. It uses a pair of twisted pair wires, one of which is defined as A and the other as B.

Normally, the positive level between the transmission drivers A and B is +2 to +6V, which is one logic state, and the negative level is -2 to 6V, which is another logic state. There is also a signal ground C. In RS-485, there is also an "enable" terminal, which is optional in RS-422. The "enable" terminal is used to control the disconnection and connection of the transmission driver and the transmission line. When the "enable" terminal is in effect, the transmission driver is in a high-impedance state, called the "third state", that is, it is a third state different from the logic "1" and "0".

Since RS-485 is developed from RS-422, many electrical regulations of RS-485 are similar to those of RS-422. For example, both use balanced transmission mode and require terminating resistors on the transmission line. RS-485 can use two-wire and four-wire modes, and the two-wire system can realize true multi-point two-way communication.