Motor encoder signal interference can cause abnormal position control, speed fluctuations and other problems, seriously affecting the accuracy of the equipment. This article will systematically analyze the source of interference and provide a variety of practical anti-interference solutions to help you completely solve the problem of signal transmission quality.
Interference source diagnosis method
- Using an oscilloscope to observe the encoder signal waveform is the most direct diagnostic method. The normal square wave should have steep edges and no burrs. If ringing or deformation occurs, it indicates interference. It is recommended to measure the motor in running and stationary states respectively to compare the signal quality differences.
- The spectrum analyzer can accurately identify the interference frequency band. Connect the probe to the encoder cable and scan the 50Hz-1MHz range. Common interference peaks appear near the switching power supply frequency (20-100kHz) or the inverter carrier frequency (2-20kHz).
- The step-by-step elimination method can locate the interference path, disconnect the power supply of peripheral equipment such as the driver and relay in turn, and observe the signal improvement. Pay special attention to the parallel routing distance between the high current line and the signal line. If it is less than 10cm, it is very easy to introduce inductive interference.
Hardware anti-interference measures
- It is very important to use twisted-pair shielded cables to transmit signals. The shielding layer should adopt an aluminum foil + braided mesh composite structure, and both ends should be grounded through metal connectors. The cable impedance is recommended to match the encoder output characteristics (usually 120Ω), and the length should not exceed 50 meters.
- The signal conditioning circuit can significantly improve the anti-interference ability. Installing an RC filter (typical value 100Ω+100nF) at the receiving end can filter out high-frequency noise. The differential line needs to be equipped with a common-mode choke, and the suppression frequency is usually selected 2-3 times that of the interference source.
- Grounding system optimization is the fundamental solution. The encoder housing, cable shielding layer, and controller ground wire should be connected to the same grounding pile at a single point. The cross-sectional area of the grounding wire is not less than 2.5mm², and the grounding resistance is required to be less than 4Ω to avoid the formation of a ground loop.
Software filtering technology
- The digital filter is simple and effective to implement. The moving average window (recommended 5-11 points) is configured in the controller to smooth the reading jump caused by sudden interference. However, it should be noted that the filtering delay will affect the dynamic response, and it should be used with caution in high-speed occasions.
- The signal verification algorithm can identify abnormal data and automatically eliminate obvious erroneous readings by comparing the rationality of continuous sampling values (such as acceleration limit). This solution does not increase latency, but requires a reasonable threshold setting to avoid misjudgment.
- The redundant verification mechanism improves reliability. The majority voting logic is used for the A/B phase of the orthogonal encoder. The position value is updated only when three consecutive samples are consistent. This method can effectively resist instantaneous pulse interference, and the hardware cost is almost zero.
Installation and wiring specifications
- The cable routing must be isolated from the power line, and the minimum parallel spacing must be maintained at more than 30cm, and the crossing is at a 90-degree right angle. The cable tray should be divided into strong and weak power areas, or physically separated by metal partitions to avoid cross interference.
- The terminal processing should be standardized, and the shielding layer should be completely covered 360 degrees at the connector, and no “pigtail” grounding should appear. Excess cables should not be coiled into a circle. It is recommended to cut them to a suitable length or loosely fix them in a large circle with a diameter of more than 20cm.
- The motor end processing is also critical. The encoder outlet needs to be wrapped with an EMI magnetic ring for 3-5 turns, and the installation position should be as close to the motor housing as possible. Heat-resistant magnetic rings should be used in high-temperature environments to avoid magnetic material failure.
System-level protection design
- Power isolation can block conducted interference. A DC/DC isolation module is configured separately for the encoder to suppress ground noise. The isolation voltage should be no less than 1000V, and a π-type filter circuit (10μF+10Ω+10μF) is installed at the output end.
- The signal isolation module provides double protection. Optical coupler or magnetic coupler isolator is selected. The transmission delay needs to be less than 1μs to ensure real-time performance. Note that the isolator bandwidth must match the highest output frequency of the encoder to avoid signal distortion.
- The cabinet layout is optimized to reduce radiation interference. The inverter and controller are at least 30cm apart, and the encoder interface board is away from the high-current contactor. Sensitive equipment can be installed in a metal shielding box with good grounding.
Motor encoder signal interference requires a systematic solution, from source suppression, transmission isolation to terminal processing. Following these engineering practices, your equipment will obtain stable and reliable signal quality, ensuring optimal motion control accuracy.