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Knowledge base ds760 Servo

Question #85
Does the new 560Hz frame rate shorten the lifetime of the servos?

Before implementing the 560Hz mode Spartan RC has studied the datasheets of the servo controller chips used in all popular 760uSec servos and verified compatibility. Subsequently the manufactures of 760uSec servos like the S9256, BLS251 and MKS DS8910 have rated their servos as suitable for 560Hz.

The theory

With analogue servos the frequency of the control pulse (aka frame rate) affects the operation of the internal electronics which in turn affects the voltage that is applied to the motor terminals. Increasing the frequency of the control pulse often affects the frequency and thus average current at the motor windings which may be considered as making the servo working harder than intended. Other undesirable behaviour may also manifest at these higher frequencies depending on the electronic design.

Unlike analogue servos, for most digital servos the frequency of the control pulse has no negative effects to the servo. The control pulse is simply a position command which the servo controller reads. The process that drives the motor runs independently and thus the frequency and current at the motor windings remain unaffected.

Where is the benefit of the higher frame rate then? As an analogy consider a train that departs the station every hour for a 3 hour long journey. The hourly departure time is the equivalent of the servo control pulse frequency and the journey duration equivalent to the transit time of the servo arm. Assuming a passenger arrives at the station at a random time he may have to wait up to one hour to catch the next train. Thus the total time to get to his destination would vary between 3 and 4 hours. If we now double the departure frequency of the trains so a train leaves very 30 minutes the total time for the passenger to get to his destination would be between 3 and 3.5 hours. The train still travels at the same speed and it does not have to work any harder to get to the destination. By doubling the frequency things just get in motion sooner.

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