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  1. Sim597

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    Ok, I know all of us here read every bit we can on our devices, Phantoms, and all the goodies that go into the technology, but, BUT I didn't know this, directly copied off of DJIs website and they go on to talk about the P4 specifically.
    Here's what's on the site:
    Active Braking
    DJI ESCs support Active Braking, a feature that increases agility and conserves energy. Put plainly, what Active Braking does is to increase the motor's voltage when braking is initiated so that the time and distance before the aircraft slows to a hover is minimized. While the braking is happening, the motor is automatically put into generator mode so that instead of its mechanical energy being turned into heat energy and going to waste, it is turned into electric energy that is sent back to the battery, charging it up.

    The Active Braking feature increases flight performance and efficiency. To further increase the agility and responsiveness of the aircraft, DJI ESCs run at high frequencies that more accurately translates the pilot's control signals to instructions sent to the motors so that flight is smooth and response immediate.


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  2. alokbhargava

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    Even P3 has active braking. This is the second generation for active braking in Phantoms.
     
  3. Sim597

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    So, your telling me, that you think your P3 actually charges your batteries when u brake?


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  4. LuvMyTJ

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    I highly doubt any appreciatable regeneration is happening. Even my Electric GEM car regeneration takes a lot to make it matter.
     
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  5. Sim597

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    I doubt this terribly, I'll say it, NO WAY does the P4 have the hardware to do this


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  6. Sim597

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    Active braking in the P3 and 4 is more, when you let go of FWD stick, it "actively" kick in opposite force, slowing unit


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  7. sar104

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    From a purely physics perspective, as described this sounds like nonsense. The goal of stopping the aircraft as quickly as possible, which is certainly achieved by reversing the pitch of the aircraft and increasing the motor voltage, thus directing an increased component of the thrust from the props opposite to the direction of motion, is not consistent with allowing the incident horizontal airspeed to drive the propellers in a pseudo-generator mode, which would involve reducing the driving voltage and then harvesting the induced current.

    As described, the process is roughly equivalent to throwing your electric vehicle transmission into reverse and then spinning the wheels backwards to stop quicker. While that wouldn't actually have the desired effect (since the dynamic coefficient of friction is approximately constant and lower than the static coefficient), it is mechanically equivalent and would not recover any kinetic energy.

    Either this is a bit of marketing science fiction or the explanation is completely wrong, possibly convolving two different effects under different conditions.
     
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  8. Ramphex

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    Wonder how many people understand any of what you just said :)
     
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  9. alokbhargava

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    Let's talk about physics :)

    At any point of time, aircraft will will be associated with two energies: potential energy at an altitude and kinetic energy during its motion. If you want to stop the aircraft motion but maintain the height, you need to dissipate the kinetic energy only. The best way to dissipate is to extract it and push back to batteries. But all of the kinetic energy can not be absorbed quickly by the battery as it is not an infinite sink. That means some of the energy needs to be wasted by working against the resisting force which is air.

    I have no data as how much energy is pushed back to battery during braking process.

    I have designed the braking system for steel rolling mills, CNC systems etc and I know how efficient and effective it is.
     
  10. With The Birds

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    I had wondered how the spindles on my cnc routers decelerate so quicly. I had assumed it was by being commanded to a lower RPM by a reduction in drive frequency feed from the controller. I hadnt considered a resistive load being applied and the motor running as a generator (there is no apparent passive load bank available in or connected to the drive unit). I do see the line frequency rapidly step down on the drive unit as the spindle rapidly steps to lower RPM. Had assumed the phantom motors were driven in a simillar fashion albeit in a smaller scale.

    This is interesting.
     
  11. sar104

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    Yes - that works fine for linear or angular kinetic energy in vehicles or tools, where the existing angular momentum of the system drives the electric motor, which then behaves as a generator and induces current that can be used to do other work, or charge batteries. However, the equivalent simple energy balance that you describe for the aircraft is incorrect, because you are forgetting the changes in kinetic energy of the surrounding air - see below.

    In the case of propellers operating in a working fluid (air in this case), that could also be achieved, but only in very limited situations for an aircraft with no other source of lift. For example, if one stopped actively driving the propellers and allowed the aircraft to drop, then the airflow through the propellers would cause them to turn, rather than just stopping (equivalent to a wind turbine), and the work done by the airflow turning the props could be used to generate electrical energy in the motors. In this case one is converting aircraft potential energy (rather than kinetic energy) into electrical energy, but at least the principle is similar.

    In the "active braking" case described, however, this isn't going to work, because rather than the motor drive being reduced and allowing the aircraft to coast to a halt (which would not work well anyway because most of the thrust is still needed to maintain altitude) energy from the battery is being used to produce extra opposite thrust to slow down the aircraft. More work is being done on the working fluid (the air), not less, so it isn't even reducing the power consumption of the motors, let alone allowing energy to be recovered.

    The main reason that this is confusing, compared to cars or machine equipment, is that the energy partition is not as simple for an aircraft. In the former cases one can simply consider the linear kinetic energy of the vehicle, or angular kinetic energy of the machine, and if one reduces that by motor braking then one can recover significant energy by using the resulting induced currents in the motor windings. But in the case of a quadcopter, it's not simple motor braking - the props change the momemtum and kinetic energy of the aircraft by pushing air around - i.e. changing the kinetic energy of the working fluid in which it moves. As a result, most of the kinetic energy of the aircraft cannot be recovered by the props/motors and is instead converted (and lost) to increased kinetic energy of the surrounding air.
     
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  12. alokbhargava

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    Feed motors and Spindle motors use heavy regenerative drives to achieve the fast dynamics. Motors also are designed for high efficiency and low inertia. They are usually long motors.
     
  13. Sim597

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    I didn't but, I did get what he was saying, basically what I said, it's BS.
    Where is the hardware to do this? Regeneration or regenerative braking involves heat and thermal management and is no small task, a whole subsystem is put into place on say cars that utilize this great concept.


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  14. Sim597

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    So your saying I am wrong? I mean, wouldn't be the first time, but I would have thought there has to be more hardware then what is involved to do this, or are the P4s capable of doing this with what is inside?
    I am in over my head here so, I'll have to defer to someone over my pay grade, I'll install and even engineer whatever system the "man" wants or needs, just don't ask me to explain the theory behind it, this is with about 30yrs as a Union Fitter with nuclear grade welding experience and ran many many jobs, but this isn't something I am familiar with at all.


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  15. Sim597

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    So, anyway, now I really don't know what to think, I still say it isn't really true, but maybe the guys here will say it's theoretically possible with only the parts inside. But the cases where the guys seem to know and understand this make compelling arguments that it's using as much if not more energy to brake.
    The case made by the "anti-rotation" like in a helicopter, that uses the stored energy (height) to slow a descent makes sense, if DJI had said when descending it harnesses extra power (I'd still want to see the hardware, maybe it's not needed) then ok.
    I always though "active" braking in this sense meant instead of just slowing down when letting off the stick, it actively flies against its own motion to brake.


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  16. sar104

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    I don't think that the problem is with the hardware per se - if you can drive a motor externally then extracting power from the resulting induced current in the motor windings is just a matter of appropriate electronics. The problem is that the basic physics of the situation does not support driving the motor in that manner.
     
  17. Sim597

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    So, it can be done?
    If what they're saying can be done, why wouldn't they incorporate it into descent mode?
     
  18. sar104

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    Even during descent the motors are working to provide some lift and the airflow through the props is downwards. To recover energy the airflow would have to be reversed - flowing upward through the props, but that would lead to an uncontrolled fall.
     
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  19. John Locke

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    DJI can take credit for Lightbridge, which is a technology that's way ahead of others for the time being, however this regenerative sales pitch is total BS. It's certainly not a leap of technology like Lightbridge, far from it. SAR104 is right, there is no way the props can retrieve enough force from air to generate anything of significance at the speeds these birds fly. This is total crazy talk. I think something was lost in translation from Chinese to English. That's likely what's going on here.;) Just like when DJI marketing states 28min flight time, we all know that's total BS. I tend to think this regenerative pitch came from the same marketing genius, or translator. As great as this DJI technology is, and it is great, DJI needs to oust the guys that continue to embarrass the company with false or misleading information.
     
    #19 John Locke, Jul 1, 2016
    Last edited: Jul 1, 2016
  20. Sim597

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    Well, it's not like it was or is prominent, it's kinda deep into the site where you can "learn" about motors, or props, or other stuff, I copied the info word for word. It's worded to well for it to be a translation issue, I'm more with you on its outright bullshyt.


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