encoder
An encoder is a device that encodes signals (such as bitstreams) or data, converting them into a signal form that can be used for communication, transmission, and storage. Encoders convert angular or linear displacement into electrical signals; the former is called a code disk, and the latter a code scale. Based on the readout method, encoders can be divided into contact and non-contact types; based on their working principle, they can be divided into incremental and absolute types. 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 assign a unique digital code to each position; therefore, their reading depends only on the starting and ending positions of the measurement, and is independent of the intermediate steps.
Encoder working principle:
The device uses photoelectric transmitting and receiving devices to read and obtain four sets of sine wave signals, which are 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, or 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 prone to breakage. 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.
How to determine the quality of an encoder
The simplest way to determine if an encoder is damaged is to connect it correctly. If there is no pulse output from AB, it is faulty. As for its functionality, the main factor is its resolution, which is the number of internal etched lines. The higher the resolution, the better, but the price will be relatively higher. Another factor is quality. It is recommended not to buy cheap counterfeit products. Japanese products are widely used in the Chinese market now, and some Korean products are also good. Several well-known PLC, inverter, and servo motor manufacturers all have encoder products. To choose a model, once you have your eye on a brand, you can download samples and user manuals from their official websites, or you can directly request them from the distributor.
① Connect to the PLC to check if the number of pulses or the code value is correct;
② Connect an oscilloscope to view the waveform;
③ Use a multimeter in voltage mode to test whether the output is normal.
When the encoder outputs NPN: Measure the positive power supply and signal output line. When the transistor is ON, the output voltage is close to the supply voltage; when the transistor is OFF, the output voltage is close to 0V.
When the encoder outputs PNP: Measure the negative power supply and signal output line. When the transistor is ON, the output voltage is close to the supply voltage; when the transistor is OFF, the output voltage is close to 0V.
What is rotary encoder resolution?
Resolution, also known as bit count, pulse count, or wire count (this terminology is used in absolute encoders), refers to the number of pulses output by the encoder per revolution of the shaft for incremental encoders. For absolute encoders, it's equivalent to dividing a 360° rotation into 256 equal parts. For example, a resolution of 256 P/R means dividing a 360° rotation into 256 equal parts, with each rotation being 1.4°.
Output one bit value on each side. The unit of resolution is P/R.
How to determine whether a speed encoder is good or bad
1. When the encoder is stationary, the voltage of phases A and B can be measured to be around 15V or 0V.
When the encoder is gently rotated, the two voltages mentioned above should be obtained alternately. A- and B- should respectively provide 0V or -15V.
2. When the encoder rotates continuously, the output is the average value of the effective voltage, which may only be a stable voltage value of about 3 to 5V.
3. A multimeter can only perform a rough check. If the measurement results differ greatly from the above description, it can be preliminarily concluded that the encoder is faulty.
4. However, it is not possible to accurately check whether the encoder is completely normal using only a multimeter.
Because the encoder normally outputs a high-frequency pulse signal, it is recommended that you use an oscilloscope for measurement.
5. The method is as follows: Connect the encoder's output A-phase or B-phase signal to an oscilloscope, and then rotate the encoder shaft. If a high-frequency 15V square wave pulse signal is observed on the oscilloscope at this time, it indicates that the encoder is good.
Additionally, use a multimeter in voltage mode to test whether the output is normal.
When the encoder is NPN output: measure the positive power supply terminal and the signal output line.
When the transistor is ON, the output voltage is close to the supply voltage; when the transistor is OFF, the output voltage is close to 0V. When the encoder is PNP output: measure the negative power supply terminal and the signal output line.
When the transistor is ON, the output voltage is close to the supply voltage; when the transistor is OFF, the output voltage is close to 0V.
Remove the encoder and, with the power on, rotate it by hand while observing the data displayed on the screen. If the data doesn't change, the encoder is faulty; if it does change, the encoder is good.
Encoders generally require live monitoring. Ideally, the encoder should be removed. After powering on, manually rotate the encoder. If the servo motor moves in response to changes in the encoder reading, it's working properly. If, after powering on, the reading doesn't change or changes irregularly, it's faulty. However, be cautious to prevent the motor from running away with the power.
What are the key factors to consider when selecting a rotary encoder?
① Incremental or absolute type
② Appearance size: φ20mm, φ25mm, φ40mm, φ50mm, φ55mm, φ60mm. This needs to be considered when installing the encoder (as indicated in the naming rules).
③ Shaft diameter: φ2mm, φ4mm, φ6mm, φ8mm, φ10mm, used for coupling with other equipment.
④ Resolution: Also known as bit depth, pulse count, or wire count (this terminology is used in absolute encoders). Currently, the maximum resolution is 6000 P/R for incremental encoders and 1024 P/R for absolute encoders. ⑤ Shaft type: Extended shaft, requiring a coupler for use; Hollow shaft, no coupler needed, can be directly fitted onto the other device.
⑥ Operating voltage: DC 5-24V (as indicated in the naming convention) ⑦ Output type: NPN open collector output, PNP open collector output, complementary output, voltage output, linear drive output. The encoder's output type is determined by the signals that the subsequent equipment can receive (as indicated in the naming convention).
⑧ Output code: binary, BCD, Gray binary (as indicated in the naming convention)
⑨ Output phases: Phase A, Phase A and Phase B, Phase A, Phase B and Phase Z. The specific number of output phases required depends on the function the customer wants to achieve (this can be reflected in the naming rules).
⑩ Maximum Allowable Rotation Speed: The maximum allowable rotation speed of the selected encoder must be higher than that of the other device. Currently, the maximum for incremental encoders is 12000 r/min, and the maximum for absolute encoders is 6000 r/min.
⑪ Maximum Response Frequency: The selected maximum response frequency must be greater than the operating frequency of the other device. Currently, the maximum for incremental response is 200kHz, and the maximum for absolute response is 20kHz.
⑫ Protection level: Whether there is water, oil, dust, etc. on site. Currently, the highest protection level is IP65.
⑬ Accessories: Couplers, flanges, mounting accessories for servo devices
