Introduction to absolute encoder

Introduction to Absolute Encoder (Absolute Encoder)
It is relative to the increment. As the name implies, the so-called absolute is that the output code value corresponding to each position and angle of the encoder is uniquely corresponding when the output signal of the encoder is running in one or more cycles. It also has the absolute function of power-down memory.

The absolute encoder is designed based on the bit code in the computer principle, such as: 8-bit code (0000 0011), 16-bit code, 32-bit code, etc. Reflecting these bit code information on the code disc of the encoder is a multi-channel optical channel engraved line, and each engraved line is in sequence of 2 lines, 4 lines, 8 lines and 16 lines. . . . . . Choreography. The result of such an arrangement, for example, for a single-turn absolute type, it is to divide 360° of a circle into the 4th power of 2, the 8th power of 2 and the 16th power of 2, the higher the number of digits, The higher the accuracy, the larger the range.

In this way, at each position of the encoder, by reading the open and dark of each engraved line, a set of unique binary codes (Gray codes) from the zero power of 2 to the n-1 power of 2 are obtained. This is called an n-bit absolute encoder. Such an encoder is determined by the mechanical position of the photoelectric code disc, and it is not affected by power failure or interference.
　　 Each position determined by the mechanical position of the absolute encoder is unique. It does not need to be memorized, no need to find a reference point, and it does not need to be counted all the time. When it needs 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.

Single-turn absolute encoder to multi-turn absolute encoder
　　
Rotate a single-turn absolute encoder to measure each engraved line of the photoelectric code disk during rotation to obtain a unique code. When the rotation exceeds 360 degrees, the code returns to the original point, which does not comply with the absolute code unique principle. The code can only be used for measurement within 360 degrees of rotation, which is called a single-turn absolute encoder.

　　 If you want to measure the rotation range beyond 360 degrees, you must use a multi-turn absolute encoder.
The encoder manufacturer uses the principle of clock gear mechanism. When the center code wheel rotates, another set of code discs (or multiple sets of gears, multiple sets of code discs) are driven by gears, and the number of turns is added to the single-turn encoding. Encoding, in order 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 code, each position code is unique and does not repeat, without the need to remember.
　　 Another advantage of multi-turn encoders is that due to the large measuring range, the actual use is often richer, so there is no need to laboriously find the 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.

The signal output of the absolute encoder (Signal Output)

Absolute encoder signal output has parallel output, serial output, bus type output, and transmission integrated output

1. Parallel output (Parallel):
The output of the absolute encoder is multi-digit digital (Gray code or pure binary code). Parallel output means that all signals occupy one signal line and output at the same time to represent the digital 1 or 0. For absolute encoders with low digits, Generally, the digital output is directly in this form, which can directly enter the I/O interface of the PLC or the upper computer, the output is instant, and the connection is simple. But parallel output has the following problems:
  1. Gray code is best, because if it is a pure binary code, multiple bits may change at the same time when the data is refreshed, and the reading will cause code errors in a short time. However, only one bit of the Gray code changes each time, reducing the possibility of code errors.
  2. All interfaces must be connected well, because if there is a bad connection point, the potential of that point is always 0, which causes an error code and cannot be judged.
　　3. The transmission distance can not be far, generally about one or two meters, for complex environments, it is best to have isolation.
　　4. For a large number of digits, many core cables are required and a good connection must be ensured, which brings engineering difficulties. Similarly, for the encoder, there must be many node outputs at the same time, which increases the failure rate of the encoder.

Serial SSI output (Serial Synchronous Interface):

　　Serial output is through a certain protocol, with sequential data output in time. This agreement is called a communication protocol, and the physical forms of connection include RS232, RS422 (TTL), RS485, etc. Since the good manufacturers of absolute encoders are all in Germany, most of the serial output is matched with Siemens in Germany, such as SSI synchronous serial output. The serial output connection line is few, the transmission distance is long, the protection and reliability of the encoder are greatly improved. Generally, high-bit absolute encoders use serial output.

SSI interface (RS422 mode), connected by two data lines and two clock lines, the receiving device sends interrupted clock pulses to the encoder, and the absolute position value is synchronously output to the receiving device by the encoder and the clock pulse.

Triggered by the clock signal sent by the receiving device, the encoder starts outputting a serial signal synchronized with the clock signal from the high bit (MSB). When the signal of the SSI standard is not transmitted, the clock and data bits are both high bits. After a falling edge, the current value of the encoder begins to store. From the rising edge of the clock signal, the encoder data signal starts to be transmitted after a delay time of T2. t3 is the recovery signal, waiting for the next transmission.

T=0.9—11us each pulse period n is the total number of bits of the encoder

t1>0.45us each half cycle of pulse

t2≤0.4us data output delay time

t3=12—35us data recovery (off) time

2. Fieldbus type output (BUS)

Fieldbus type encoders are multiple encoders connected together by a pair of signal wires. By setting the address, the signal is transmitted by communication. The signal receiving device only needs one interface to read multiple encoder signals. The bus-type encoder signal follows the physical format of RS485, and its signal arrangement is called the communication protocol. At present, there are multiple communication protocols in the world, each with its advantages, but not yet unified. The commonly used communication protocols for encoders are as follows:

PROFIBUS-DP; CAN; DeviceNet; Interbus, etc.

The bus-type encoder can save connection cables and receiving device interfaces, and has a long transmission distance. In the case of centralized control of multiple encoders, it can also greatly save costs.

4. Transmission integrated output

Transmitting means that the signal is directly transmitted and output after conversion in the encoder. It has analog 4-20mA output, RS485 digital output, 14-bit parallel output, etc.

Common parameters for absolute encoder selection

1. Singleturn absolute type (Singleturn)-Need to know the number of bits required by the customer, also called Resolution, such as 10 bits, also known as 1024 positions, 12bits = 4096 positions, etc.

2. Multiturn absolute type (Multiturn)-At this time, in addition to asking him the resolution of a single turn, there is also the number of revolutions he needs (revolution), so the number of bits of a multiturn encoder is the number of single turns and The total number of laps. For example: the single-turn resolution of an encoder is 4096/12bits and the number of turns is 13bits, then the total output bits of this encoder is 12+13=25bits.

3. Signal output and interface form (Signal and Output)-first there are digital output and analog output, but generally it is mainly digital. Coding output includes: parallel output, serial output, bus interface, etc.

4. Encoder power supply voltage (Power)-ask clearly the working power supply voltage of the encoder he chooses.

5. Output code system (Code)-The code output by the absolute encoder is the same as the code system used in the computer, and there are also natural binary, BCD, Gray code, Cogray code, etc. Among them, the commonly used ones are natural binary code and gray code. Because Gray code is better than natural binary, so Gray code is generally used.

6. Encoder temperature range (Temparature Range)-this corresponds to the specific requirements of the customer and help the other party to choose it. This is divided into use temperature and storage temperature.

7. Encoder speed range (Speed)-This phase also needs to meet the specific requirements of customers. Generally, the mechanical speed of a good encoder can reach 5000~6000rpm.

8. IP protection level (Protection)-this protection level is divided into the outer cover protection level and the protection level at the shaft diameter. Its IP67 is the highest level.

Finally, basically, if we can provide so much data, we can already help customers choose the right encoder. Other specific issues involved will be resolved according to the specific situation.

Electrical secondary equipment connected to absolute encoder:

The equipment connected to the absolute encoder can be a programmable controller PLC, a host computer, or a dedicated display signal conversion instrument, which then outputs signals to the PLC or host computer (data processing and display system).

1. Direct access to PLC or host computer:

If the encoder is output in parallel, it can be directly connected to the input/output contact I/O of the PLC or the upper computer, and the signal math format should be Gray code. The number of bits of the encoder will occupy the number of contact points of the PLC. If it is a 24V push-pull output, the high level is valid as 1, and the low level is 0; if it is an open collector NPN output, the connected contact must also It is NPN type, its low level is effective, and the low level is 1.

2. If the encoder is serial output, due to the limitation of the communication protocol, the subsequent electrical equipment must have a corresponding interface.

For example, SSI serial can be connected to Siemens S7-300 series PLC. There are special modules such as SM338, or S7-400 FM451 and other modules. For other brands of PLC, there are often no special modules or modules are very expensive.

3. If the encoder is a bus-type output, the receiving device needs to be equipped with a dedicated bus module, such as PROFIBUS-DP.

However, if you choose a bus-type output encoder, you cannot add other display instruments between the encoder and the receiving device PLC. If you need on-site display, you must transfer the signal from the PLC to the display instrument that matches the signal. Some protocol-defined RS485 output signals enter the PLC's RS485 interface, which requires the PLC to have an intelligent programming function.

Complex Encoder (Complex Encoder)

As encoders are used in more and more occasions and uses, their products have also appeared in many categories. In some industrial scenes, it is necessary for the incremental signal to appear and measure the absolute value at the same time. In order to solve this problem, a composite encoder came into being.

The composite encoder is on the same encoder disc, with incremental signal tracks and absolute signal tracks engraved at the same time. After the circuit is processed, the incremental pulse signal and absolute value of the signal can be obtained at the output. Encoded signal.

These products are produced by various manufacturers, such as: SSI 674/675/684/685 from Leine linde