The electronic gear ratio of a servo motor amplifies or reduces the frequency of pulses received from the host computer. One parameter is the numerator, and the other is the denominator. If the numerator is greater than the denominator, it's amplification; if the numerator is less than the denominator, it's reduction. For example, if the host computer inputs a frequency of 100Hz, and the electronic gear ratio numerator is set to 1 and the denominator to 2, then the servo's actual operating speed will be based on 50Hz pulses. If the host computer inputs a frequency of 100Hz, and the electronic gear ratio numerator is set to 2 and the denominator to 1, then the servo's actual operating speed will be based on 200Hz pulses. The electronic gear ratio is relative to the gear meshing and gear pair ratio in mechanical gears. It is also a commonly used form of shaftless transmission.
The role of electronic gear ratio
Taking a motor with a 17-bit encoder as an example, for every revolution of the servo motor , the servo amplifier needs to send 131,072 pulses to the servo motor, and the encoder simultaneously sends 131,072 pulses back to the servo amplifier. If we want the motor to rotate at 20 r/s, without setting an electronic gear ratio, we would need to send 2,621,440 pulses per second to the servo amplifier, which translates to a pulse frequency of 2,621,440 Hz. However, the pulse-sending devices we use, such as PLCs, have limitations on their pulse transmission frequency, typically 200 kHz or 500 kHz. Introducing the concept of an electronic gear ratio allows us to reduce the frequency of pulses sent to the servo amplifier.
Servo Motor Electronic Gear Ratio Calculation Method
Motor encoder resolution
Servo motor encoders typically have 2000 or 2500 lines, meaning they generate 2000 or 2500 pulses per revolution. The servo driver multiplies these pulses by 4, so the motor generates 8000 or 10000 pulses per revolution, resulting in a resolution of 8000 or 10000.
Motor Model
Encoder Line Count
Motor encoder resolution
Sanyo P2 and P5 motors
2000
8000
Dahao Servo
2500
10000
Taking a Sanyo servo motor as an example: when the controller sends a pulse to the driver, the servo motor rotates by the following angle:
After the two-stage transmission, the angle of the frame's movement, when translated to the motor angle, is inversely proportional to the two-stage transmission ratio. For example, if the two-stage transmission ratio is 1/4, then the angle the motor rotates is four times the angle the drive shaft rotates. Frame gear size:
Currently, there are two main types of gears on the market: those requiring a rotation angle of 0.36° and 0.45° to move the embroidery frame by 0.1mm. Most machines use the 0.36° gear.
In summary, the formula for the electronic gear ratio is as follows:
If a lead screw structure is used, the calculation method for the electronic gear ratio is slightly different.
Because typically, the motor and lead screw shaft are connected by a 1:1 belt drive, and the lead screw pitch is M millimeters per revolution, the calculation formula is:
Setting the electronic gear ratio of the servo motor
Setting with the aim of achieving the highest motor speed
When a servo motor rotates and speed performance is more important than precision performance, it's desirable to fully represent the motor's speed capabilities; however, when rotational resolution requirements are lower, the following settings are recommended.
1) Conditions and requirements
Assuming the desired servo motor rotation speed is 3000 R/min, and the encoder pulse count is 8192 pulses/rev,...
2) Calculation instructions
The pulse frequency relative to a speed of 3000 RPM is 8192 × 3000/60 = 409600 Hz = 409.6 kHz.
When the controller pulse output can only be up to 100kHz, first set both the numerator (CMX) and denominator (CDV) of the electronic gear ratio to 1. Then, use the 10kHz pulse sent by the controller JOG as the pulse frequency for 1/10 of the maximum speed. At this time, the servo motor speed is...
(10/409.6)&TImes;3000≈73R/min
If the rotational speed is not calculated, you can directly monitor the drive speed value, which should also be 73 R/min.
3) Setup method
The desired speed for a 10kHz pulse is 3000 r/min, but the actual speed is 73 r/min. To correct the actual speed to 300 r/min, the electronic gear ratio must be modified.
73&TImes;CMZ/CDV=300（R/MIN）
Therefore, the numerator of CMX can be set to 300, and the denominator of CDV can be set to 73.
The rotational speed when the controller's pulse output frequency is 100 kHz is
3000&TImes;［﹙300/73﹚×100000］/409600=3009R/MIN
This example ignores all structural conditions, but in practical applications, the resolution of the transmission part must be considered. If the resolution is ignored, the product will eventually become unusable.
Setting the electronic gear ratio for the purpose of mechanical mechanism resolution
Servo motors are widely used in machining control and operation, where machining accuracy should take precedence over speed feed. Speed should only be considered after accuracy is achieved. Therefore, the electronic gear ratio setting must prioritize ensuring the controller's output pulses are not arbitrarily amplified, as the output pulse frequency will affect resolution. It is recommended to prioritize setting the electronic gear ratio.
1) Conditions and requirements
Assuming the servo motor's rated speed is 3000 RPM, the encoder pulses per revolution are 8192 pulses/rev, the controller's maximum pulse output frequency is 100 kHz, the servo motor is connected to a reduction gear mechanism, and the input pulse:output pulse ratio is M:N = 3; the reduction gear mechanism's output shaft uses a 10mm ball screw lead.
2) Calculation instructions
The resolution of the worktable driven by the servo motor via the reduction mechanism is...
﹙10/8192﹚×﹙1/3﹚≈0.0004069（mm/pulse）=0.4069μm/pulse
The number of pulses per revolution of the lead screw is
10/0.0004069≈24576（pulse/rev）
If the desired output variation rate of the controller is 1 μm/pulse, then the number of pulses per revolution of the leadscrew becomes 10/0.001 = 10000 pulses/rev.
10000×﹙CMX/CMD)=24576pulse/rev
The controller sends out pulses, and the servo receives pulses.
Therefore, the numerator of CMX can be set to 24576, the denominator of CDV can be set to 10000, and so on, the maximum speed of the servo motor can be calculated.
﹙100000×24576×60﹚/﹙8192×10000﹚=1800r/min
1800r/min < 3000r/min. Therefore, the motor rotation speed cannot reach 3000r/min, but the resolution of the servo motor will achieve the expected effect.
