I was looking at ESC settings for whoops/toothpicks and have seen how the 48KHz JESC firmware can increase flight time. Why is this? Thanks!
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asked Apr 14, 2020 at 20:11
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$\begingroup$ Welcome! Increased flight time as compared to what other sampling rate? $\endgroup$– RockPaperLz- Mask it or CasketApr 14, 2020 at 20:18
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$\begingroup$ Sorry - as opposed to the default rate of 8KHz (as far as I know). $\endgroup$– Drones and WhatnotApr 14, 2020 at 20:19
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$\begingroup$ The default on BLHeli 32 is 24khz, and it cannot be changed without custom firmware on BLHeli_S. The non-custom firmware on BLHeli_S runs at 24khz as well. $\endgroup$– QuadMcFlyApr 15, 2020 at 1:34
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1 Answer
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To clarify, the 48kHz is NOT running on the gyroscope or the FC firmware. What 48kHz is referring to is the PWM Frequency in the ESC, namely the frequency at which the power MOSFETs on the speed controller switch on and off in order to provide power to the brushless motors and therefore thrust to the craft. The reason in whoops and small/light craft 3" and smaller will see a flight time improvement is that the RPm ranges these crafts fly at, the switching down-time at lower PWM frequencies means the motor/ESC is having very small active braking events in the switching cycle, then overcompensating by driving higher throttle values to compensate (both of which cost energy).
In some cases, 96kHz will also work very well, although this is often limited to smaller whoop class quads, and I would caution against it on specific instances where the ESC design has a higher dead time value because these tend to suffer failures more often at higher PWM frequency when pushed hard.
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$\begingroup$ Thank you for the explanation, that makes sense. $\endgroup$ Apr 14, 2020 at 20:21
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$\begingroup$ It should also be noted that this ONLY works with micro motors and very high RPMs, and may have minimal or opposite impact on anything larger and heavier than micro drones. $\endgroup$ Apr 15, 2020 at 1:33
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$\begingroup$ @QuadMcFly I get why it would have minimal effect on motors with a lower RPM but how would it have a negative impact? $\endgroup$– BrechtApr 15, 2020 at 16:52
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1$\begingroup$ As mentioned by @tehllama, if the deadtime values are less than ideal, faster PWM rates can create heat in the ESC which is essentially lost energy. It can also cause shoot-through if the deadtime is off, which could burn out the ESC. Unfortunately what we've seen is a lot of manufacturers either miscalculate the deadtime, or have a large enough variation on the quality and specifications of the components used that the appropriate deadtime varies as QC varies. $\endgroup$ Apr 15, 2020 at 16:56
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1$\begingroup$ With the fairly low current draw and ESCs calibrated for very fast switching on small motors this is low risk, but on larger motors and ESCs it becomes more of an issue. $\endgroup$ Apr 15, 2020 at 16:58