What is the difference between a rotary incremental encoder and an absolute encoder

Incremental value rotary encoder, also called circular grating, pulse code disc, from these names, it is a circular grating engraved code disc. After rotating, it generates pulses through the change of luminous flux and counts pulses by external equipment. , To incrementally add (or subtract) the number of pulses to measure the angle of rotation. For example, a circular grating engraves 360 reticles every week, and each reticle generates a pulse equivalent to 1 degree. The cumulative increase of the measured pulse is 30, which is a positive rotation of 30 degrees.

Schematic diagram of incremental encoder code disc

In fact, there are two (or four) optical eyes for reading these reticles. Each of the two optical eyes outputs phase A to phase B to determine which direction the scribed line comes from. A is ahead of B. Or B is ahead of A, just like the left and right eyes of a person, thus knowing the rotation direction of the encoder, so that it can judge whether the pulse count is increasing or decreasing, so as to obtain the true rotation angle.

In actual use, the position of phase A and phase B are different by 1/4 pulse period, so that from the positive direction is 1/4 period difference, and from the reverse direction is 3/4, which can be used to judge the rotation direction. If one pulse period is a 360-degree ""phase"" angle, such 1/4 is a 90-degree phase difference, and 3/4 is a 270-degree phase difference. In addition, each revolution of the rotary encoder has a separate engraved line, which is equivalent to the zero position (Zero), also known as the Z phase, which is used to read the starting point of each week.

These circular grating code discs were first obtained by etching round metal plates, and the metal etching accuracy was limited. Instead, they were etched with glass coating. The glass code discs had the highest accuracy, but they were fragile. For some economical encoders, they are also made of plastic film. Recently, there are new technologies using resin materials. The processing technology is the same as that of glass code discs, which can be compared with glass code discs with higher accuracy and stability. It is not easy to damage, which may be the trend of large-scale industrial mass production.

Rotary incremental encoder outputs pulses when rotating, and knows its position through counting equipment. When the encoder does not move or the power fails, the internal memory of the counting equipment is used to remember the position. In this way, when the power is off, the encoder cannot move. When the power is on, the encoder cannot lose the pulse due to interference during the pulse output. Otherwise, the zero point of the counting device will shift, and this deviation There is no way to know the amount of shift, only after the wrong production results appear. In fact, industrial control uses more and more equipment, and more and more interference signals are becoming more and more complicated. For incremental signals, interference signals are more of interference signals. It is impossible to judge whether the pulse is counted or missed, resulting in cumulative errors. .

The solution is to increase the external reference point. Every time the encoder passes the reference point, the reference position is corrected into the memory position of the counting device. Before the reference point, the accuracy of the position cannot be guaranteed. For this reason, in industrial control, there are methods such as first finding the reference point for each operation, turning on the machine and making change.

This method is cumbersome for some industrial control projects, and even does not allow the startup to find change (the exact position must be known after startup), and some work continuously and do not allow frequent change, so the absolute value encoder appears.

There are many engraved code tracks from the inside to the outside on the optical code disc of absolute encoder, and each engraved line is divided into 2 lines, 4 lines, 8 lines and 16 lines in sequence. . . . . . Arrangement, in this way, at each position of the encoder, through n optical eyes to read the open and dark of each engraved line, obtain a set of unique 2 from the zero power of 2 to the n-1 power of 2 Base code (Gray code), this is called an n-bit absolute encoder. Such an encoder is determined by the mechanical position of the code disc, and the encoding of each position is unique and absolute, so it is called an absolute encoder. It is not affected by power outages and interference.

The uniqueness of each position of the absolute encoder determined by the mechanical position, it does not need to be memorized, does not need to find a reference point, and does not need to be counted all the time. When you need to know the position, you can 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 encoder to multi-turn absolute encoder

Rotate a single-turn absolute encoder to measure the engraved lines of each code track on the optical code disc during rotation to obtain a unique set of codes. When the rotation exceeds 360 degrees, the code returns to the original point, which does not comply with the absolute code uniqueness In principle, such encoders can only be used for measurements within 360 degrees of rotation, and are called single-turn absolute encoders.

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

Earlier multi-turn calculations were to add a lap count to the counter for every revolution over 360 degrees (the method of counting laps is similar to an incremental encoder), but this method is used when the encoder stops near 360 degrees when the power is cut off or received Interference is very dangerous. It is possible that the lap may be missed and the code is different by one lap. The built-in battery of the encoder can also be used to count the laps, but the battery life, vibration contact, low temperature failure and other issues are still dangerous. Some batteries use intermittent work to extend their lifespan, but intermittent work limits the speed of encoder rotation. These methods are very risky for multi-circle absolute use.

Real multi-turn absolute encoder: The encoder manufacturer uses the principle of clock gear machinery to add a set of mechanical gear set code discs. When the center code disc rotates, another set of gear code discs (or multiple sets of gears) are driven by gears. , Multiple sets of code discs), on the basis of the single-turn encoding, the number of turns is added to expand the measuring range of the encoder. Such absolute encoders are called true multi-turn absolute encoders. For multi-turn values , The code is also determined by the mechanical position, and each position code is unique and does not repeat, without memory.

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

In the past, true multi-turn encoders for absolute gear sets were mainly produced by European and American encoder manufacturers. Now, Shanghai Jingpu Electromechanical has also been able to produce true multi-turn encoders for absolute gear sets with independent intellectual property rights. Our goal is To be the best encoder in China.

The real multi-turn absolute encoder has obvious advantages in length positioning, especially irreplaceable in reliability, and has been increasingly used in industrial control positioning.