PLC sends pulse signals to control servo operation. One of the most challenging aspects is how the pulses sent by the PLC (i.e., the position set in the PLC) correspond to the distance the servo moves the mechanical system.Here, I will explain this in the simplest way possible.1. Before discussing the method, it is essential to clarify a concept regarding servos: the number of pulses required for a servo motor to complete one full rotation and the derived electronic gear ratio. In simple terms, it refers to how many pulses the PLC needs to send for the servo to rotate the motor one full turn. The high-speed output ports of PLCs have a maximum frequency limit,which varies by brand. Currently, it is generally 200 kHz, meaning the PLC can send a maximum of 200,000 pulses per second. However, modern servo encoders often have a high resolution, such as a 24-bit encoder, which requires 16,777,216 pulses for the motor to complete one rotation. If we calculate based on the PLC’s maximum output of 200,000 pulses per second, this particular servo can only rotate approximately 0.012 turns per second (200,000/16,777,216). Clearly, this does not meet normal operational requirements, which is why we have the concept of electronic gear ratio. The following formula illustrates this clearly. Electronic Gear Ratio NumeratorPLC Pulse Count * ———————— = Servo Motor Pulses per Revolution Electronic Gear Ratio DenominatorOnce the electronic gear ratio is established, if the numerator is larger than the denominator, the number of pulses sent by the PLC for the servo motor to complete one rotation can be reduced, alleviating the hardware burden on the PLC.2. The above formula applies when the servo directly drives the load without passing through a gearbox; in this case, the servo motor’s one rotation corresponds to one rotation of the load. However, in practical applications, there is often a gearbox between the servo motor and the load. In this case, if the load rotates once, the servo motor may need to rotate 2.5 times. Assuming the load requires 1,000 pulses for one rotation (this is a set value as long as it meets the servo speed requirements; further explanation will follow), then the servo motor needs the PLC to send 1,000/(5/2) = 400 pulses for one rotation. Based on the above formula, we can calculate the electronic gear ratio with a numerator of 16,777,216 and a denominator of 400, which can be simplified.Thus, the PLC sends 400 pulses for the servo motor to complete one rotation, with the condition that the servo speed meets the required speed.If the load speed requirement is 20 revolutions per second, then the servo must achieve 50 revolutions per second, and the PLC must send 50*400 = 20,000 pulses per second. This PLC sending frequency is <200 kHz, which is within the hardware requirements of the PLC.In summary, using this formula, the number of pulses sent by the PLC for the servo motor to complete one rotation must satisfy the maximum speed setting and be less than or equal to the PLC’s maximum pulse frequency.3. Finally, let’s clarify the number of pulses for one rotation of the load, such as 1,000 pulses. For instance, if the load moves 1 mm for one complete rotation, then 1,000 pulses correspond to 1 mm. Therefore, the PLC sends 1,000/(5/2) = 2,500 pulses for the load to move 1 mm.With this formula, everything can be resolved.