The electronic gear ratio of a servo motor is used to either amplify or reduce the pulse frequency sent from the upper control system to the motor. This is typically represented by two parameters: one as the numerator and the other as the denominator. For instance, if the numerator is greater than the denominator, the pulse frequency is increased. Conversely, if the denominator is larger, the frequency is reduced.
For example, if the upper computer sends a 100Hz signal and the electronic gear ratio is set to 1 (numerator) and 2 (denominator), the actual speed of the servo will be based on a 50Hz pulse. On the other hand, if the gear ratio is set to 2 (numerator) and 1 (denominator), the effective pulse frequency becomes 200Hz, thereby increasing the motor speed.
This concept is similar to mechanical gear ratios but is implemented electronically, allowing for more flexible and precise control without the need for physical gears. It's often referred to as a "shaftless transmission" system, where the relationship between input and output pulses is adjusted dynamically.
**The Role of Electronic Gear Ratio**
Consider a motor equipped with a 17-bit encoder. In one full revolution, the servo amplifier receives 131,072 pulses from the encoder. If we want the motor to rotate at 20 revolutions per second (r/s), without any electronic gear ratio, the controller would have to send 2,621,440 pulses per second, resulting in a pulse frequency of 2.62 MHz. However, most controllers, such as PLCs, are limited in their maximum pulse output frequency—often around 200 kHz or 500 kHz. To address this limitation, the electronic gear ratio is introduced to lower the required pulse frequency while maintaining the desired motor speed.
**Servo Motor Electronic Gear Ratio Calculation Method**
The resolution of a servo motor’s encoder is crucial in determining how many pulses it generates per revolution. For example, a 2000-line encoder produces 2000 pulses per revolution, which the servo driver then multiplies by four, resulting in an effective resolution of 8000 pulses per revolution. Similarly, a 2500-line encoder can produce 10,000 pulses after processing.
| Motor Model | Encoder Line Number | Motor Encoder Resolution |
|------------------|---------------------|--------------------------|
| Sanyo P2, P5 | 2000 | 8000 |
| Dahao Servo | 2500 | 10000 |
When the controller sends a pulse to the driver, the motor rotates by a specific angle. After secondary transmission, the movement of the frame is inversely proportional to the gear ratio. For example, if the gear ratio is 1/4, the motor must rotate four times to move the frame by one unit.
Most machines use a gear that moves the frame by 0.1 mm, corresponding to an angular rotation of 0.36° or 0.45°. The formula for calculating the electronic gear ratio depends on the encoder resolution, the desired speed, and the mechanical transmission ratio.
**Setting the Electronic Gear Ratio for Maximum Speed**
When the primary goal is to achieve the highest possible motor speed, the focus should be on maximizing the pulse frequency while ensuring the motor operates within its limits. For example, if the desired speed is 3000 RPM and the encoder has 8192 pulses per revolution, the required pulse frequency would be:
$$ \text{Pulse Frequency} = \frac{8192 \times 3000}{60} = 409,600 \, \text{Hz} $$
If the controller can only output up to 100 kHz, the electronic gear ratio must be adjusted accordingly. By setting the numerator and denominator of the gear ratio, the actual motor speed can be fine-tuned to match the desired performance.
**Setting the Electronic Gear Ratio for Mechanical Resolution**
In applications where precision is more important than speed, the electronic gear ratio should be set to ensure the controller’s pulse output remains within its limits. This helps maintain high resolution and accuracy, even if it means reducing the maximum achievable speed.
For example, if the desired resolution is 1 micrometer per pulse, the number of pulses per revolution must be adjusted to match the lead screw pitch and the mechanical reduction ratio. This ensures that the system achieves the required precision, even if the motor speed is slightly reduced.
By carefully selecting the electronic gear ratio, engineers can balance speed, precision, and system limitations to optimize the performance of servo motor systems in various industrial and automation applications.
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