Encoder working principle
It consists of a centrally located photoelectric code disk with circular, dark and light-colored engravings.
Photoelectric transmitting and receiving devices read and obtain four sets of sine wave signals combined into A , B , C , and D. Each sine wave is 90 degrees out of phase ( 360 degrees relative to one cycle ) . The C and D signals are reversed and superimposed on the A and B phases to enhance the stability of the signal . In addition, a Z -phase pulse is output every revolution to represent the zero reference position.
Since phases A and B are 90 degrees out of phase, the forward and reverse rotation of the encoder can be determined by comparing whether phase A or phase B comes first. The zero-position reference position of the encoder can be obtained through the zero-position pulse. Encoder code disks are made of glass, metal, and plastic. Glass code disks have very thin lines deposited on glass, resulting in good thermal stability and high precision. Metal code disks have lines directly engraved with both through and non-through surfaces, making them less fragile. However, due to the thickness of the metal, the precision is limited, and its thermal stability is an order of magnitude worse than that of glass. Plastic code disks are economical, with low cost, but their precision, thermal stability, and lifespan are all inferior.
Resolution - The encoder's resolution is defined by the number of through or dark lines it provides per 360 degrees of rotation. It is also called resolution scale or simply the number of lines, and is generally between 5 and 10,000 lines per revolution .
Main function of encoder
It is a rotary sensor that converts rotational displacement into a series of digital pulse signals.
These pulses can be used to control angular displacement, and if the encoder is combined with a rack and pinion or a lead screw, it can also be used to measure linear displacement.
After the encoder generates an electrical signal , it is processed by numerical control (CNC) , programmable logic controller (PLC) , control system, etc. These sensors are mainly used in the following areas : machine tools, material processing, motor feedback systems, and measurement and control equipment. In the ELTRA encoder, the conversion of angular displacement adopts the photoelectric scanning principle. The reading system is based on the rotation of the radial indexing plate, which consists of alternating transparent and opaque windows. This system is entirely illuminated vertically by an infrared light source, so that the light projects the image on the plate onto the surface of the receiver, which is covered with a grating called a collimator, which has the same window as the optical disc. The receiver's function is to sense the light changes generated by the rotation of the optical disc and then convert the light changes into corresponding electrical changes. Generally, the rotary encoder can also obtain a speed signal, which is fed back to the frequency converter to adjust the frequency converter's output data. Fault phenomenon : 1. When the rotary encoder is faulty ( no output ) , the frequency converter cannot work normally, becomes very slow, and after a while, the frequency converter will protect itself, displaying "PG disconnected "... Only the combined action can work. To raise the electrical signal to a higher level and generate a square wave pulse without any interference, electronic circuitry is required. The wiring and parameters of the encoder PG (Power Generator ) must correspond to the encoder PG model. Generally, encoder PG models are divided into three types: differential output, open collector output, and push-pull output. The signal transmission method must take into account the inverter PG card interface; therefore, selecting the appropriate PG card model or setting it correctly is crucial .
Encoders are generally divided into incremental and absolute types, and their biggest difference lies in the type of encoder: In the case of incremental encoders,
The position of an absolute encoder is determined by the number of pulses counted from the zero mark, while the position is determined by the reading of the output code. In one revolution, the output code reading for each position is unique ; therefore, when the power is off, an absolute encoder does not become disconnected from the actual position. If the power is restored, the position reading remains current and valid ; unlike an incremental encoder, it does not require searching for the zero mark.
Nowadays, encoder manufacturers produce a wide range of series, which are generally specialized, such as encoders for elevators, encoders for machine tools, encoders for servo motors, etc. Moreover, the encoders are all intelligent and have various parallel interfaces that can communicate with other devices.
An encoder is a device that converts angular or linear displacement into electrical signals. The former is called a code disk, and the latter a code scale . According to the reading method, encoders can be divided into contact and non-contact types . Contact encoders use brushes for output, with one brush contacting a conductive or insulating area to indicate whether the code state is "1" or "0". Non-contact encoders use photosensitive or magnetic sensitive elements. When using photosensitive elements, the light-transmitting and opaque areas indicate whether the code state is "1" or "0" .
Encoders can be divided into two categories based on their working principle: incremental and absolute.
Incremental encoders convert displacement into periodic electrical signals, then convert these signals into counting pulses, using the number of pulses to represent the magnitude of the displacement. Absolute encoders, on the other hand, assign a unique digital code to each position; therefore, their readings depend only on the starting and ending positions of the measurement, and are independent of the intermediate steps.
Rotary incremental encoders output pulses as they rotate, and their position is determined by a counting device. When the encoder is stationary or there is a power outage, the position is remembered by the internal memory of the counting device. Therefore, the encoder cannot move at all after a power outage, and when power is restored, there must be no interference during the encoder's pulse output process to avoid pulse loss; otherwise, the zero point remembered by the counting device will shift, and the amount of this shift is unknown until an erroneous production result occurs. The solution is to add a reference point. Every time the encoder passes the reference point, it corrects the reference position into the memory position of the counting device. Before the reference point, the position accuracy cannot be guaranteed. Therefore, in industrial control, methods such as finding the reference point before each operation and zeroing upon power-on are used. Such encoders are determined by the mechanical position of the code disk and are unaffected by power outages or interference.
An absolute encoder ensures the uniqueness of each position determined by its mechanical position. It requires no memory, no reference points, and doesn't need to constantly count; it only reads the position when needed. This significantly improves the encoder's anti-interference capabilities and data reliability.
Because absolute encoders are significantly superior to incremental encoders in positioning,
Absolute encoders are increasingly being used in industrial control positioning. Due to their high precision and large number of output bits, if parallel output is still used, each output signal must be well connected, and isolation is also required for more complex working conditions. The large number of cores in the connecting cable brings many inconveniences and reduces reliability. Therefore, absolute encoders with multi-bit output generally use serial output or bus output. The most commonly used serial output for absolute encoders manufactured in Germany is SSI ( Synchronous Serial Output ) .
Multi-turn absolute encoders . Encoder manufacturers utilize the mechanical principle of clock gears. When the central code disk rotates, it drives another set of code disks ( or multiple sets of gears and code disks ) through gears , adding more turns of encoding on top of the single-turn encoding to expand the encoder's measurement range. This type of absolute encoder is called a multi-turn absolute encoder. It also uses mechanical position to determine the encoding, with each position having a unique and non-repeating code, eliminating the need for memorization. Another advantage of multi-turn encoders is their large measurement range, often providing ample margin for error in practical applications. This simplifies installation and debugging by eliminating the need for painstaking zero-point finding; a middle position can be used as the starting point. Multi-turn absolute encoders have significant advantages in length positioning and are increasingly used in industrial control positioning.
