The measurement and processing of encoder signals play a critical role in servo motor control. The position, speed, and acceleration of the motor rotor are derived from these signals. Accurate signal processing is essential for reliable control; otherwise, it can lead to incorrect motor behavior.
Encoder signals typically include A, B, and Z pulses, with some models also featuring U, V, and W pulses.
1. **A and B Signals**: These are position pulses used to determine the motor's angular position. The A and B signals have a 90° phase difference and the same frequency, with a 50% duty cycle. The rising and falling edges of these signals can be converted into four pulses per A pulse period, which helps in determining direction and position.
2. **Z Signal**: This is the zero reference pulse, generated once per revolution at the motor’s zero position. To ensure reliability, the Z signal is usually only considered valid when both A and B signals are high. This helps reduce false triggers due to noise or interference.
3. **U, V, W Signals**: These are used for initial position detection and are often found in more advanced encoder systems. Their exact use depends on the specific application and encoder type.
**Important Considerations for A and B Signal Measurement**:
When measuring the A and B signals using the T-method, it's crucial to avoid using adjacent pulses. Instead, it's recommended to measure the time between the same edge of the same signal (e.g., rising edge of A to rising edge of A). This method ensures higher accuracy, though it increases the measurement delay by a factor of four. Alternatively, you can pre-measure the spacing between each edge and apply real-time corrections. While this reduces delay, it may compromise accuracy if the timing between pulses changes due to motor vibration or speed variations.
**Z Signal Measurement and Interference Handling**:
Misinterpreting the Z signal due to interference can cause serious issues, such as sudden current spikes and loss of synchronization. Therefore, special attention must be given to filtering and processing the Z signal. One effective approach is the "window method," where a small time window is set around the expected Z signal occurrence. Only Z pulses within this window, and under the correct condition that A and B are high, are considered valid. For example, in an encoder with 2,500 lines per revolution, the Z signal might be valid between 9,700 and 10,300 pulses per revolution.
Some may question why we don’t apply the same windowing technique to A and B signals. However, since A and B signals are used to build the overall position information, they are generally more reliable. Interference affecting them would likely be rare, and even if it occurs, the accumulated position data from A and B pulses would still provide a good approximation.
In cases where the Z signal is disturbed within the window, the resulting position error is minimal, especially if the window is close to the zero point. After the next Z signal correction, the system will quickly return to normal operation.
By implementing the window method, the risk of Z signal interference is significantly reduced. Even if an occasional disturbance occurs, its impact on motor performance is limited and temporary. This approach ensures greater stability and reliability in motor control systems.
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