MIMIC Jitters and Twitches -- found a solution!

Discussion in 'MIMIC' started by Graham Futerfas, Sep 15, 2018.

  1. Graham Futerfas

    Graham Futerfas Well-Known Member

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    My Movi Pro's MIMIC has been jittery and unreliable for a while, but I finally figured out the simple thing that's been causing the problem. It's nothing to do with range\antenna\strength of signal in this case, and I tried taking the antenna off, tried increasing distance to the Movi Pro. I don't use it all that often, since I typically prefer Alpha Wheels, but I had a request for it recently and it wasn't usable because of the jitter. I just figured out why it was jittery, and thought I'd share this simple solution with you all just in case someone else has the same problem.

    My Alpha Wheels use an SBus XR Receiver that typically lives on the back of my Movi Pro. Maybe some of you use a Movi Controller receiver, too, and I haven't been able to test if this is also a culprit. I figured unplugging the Alpha Wheels transmitter would stop the signal going to the Receiver, or putting the knobs to Majestic would also turn them off, but apparently if the Receiver is plugged in to the back of the Movi, it's still getting some noise that is affecting the Mimic and causing jitters. I need to physically unplug the Receiver from the back of the Movi.

    Actually, in my testing, I can just remove the antenna from the Receiver and it instantly cleans up the Mimic, so maybe it's just noise in the frequency (which I thought was digital and not subject to noise?). Or maybe it has to do with the signal-hopping?

    Anyway, I'm an idiot and it took me a long time to sort this out, mostly because I don't use the Mimic for Pan\Tilt\Roll control all that often. Also, it's the same jitter that you get from the signal-swamping from being too close to the Movi with the Mimic, so I had assumed they were related.

    Hopefully this information will help someone in the future.
     
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  2. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Hi Graham: Just to be clear, the problem went away when you unplugged the antenna for the SB XR Receiver?

    Can you clarify how the antenna is connected to the Movi Pro, please?

    I'm puzzled, because there seems to be several possibilities:

    1. RF Cross-talk (when you get antennas close enough to anything splatting out RF, they can pick it up even on different frequencies).
    2. Electrical noise coming in via the wiring (e.g. noise propagating on the alleged "ground" wire.
    3. That SB XR receiver might also be acting as a transmitter -- in the world of radio control, many modern receivers transmit telemetry back to their controlling transmitter (which is also a receiver -- confusing isn't it?)
    4. Perhaps something else...

    Andy
     
  3. Graham Futerfas

    Graham Futerfas Well-Known Member

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    Hi Andy, Here's a photo of my SBUS XR Receiver, for clarification. It has a removable Antenna, and it plugs into COM 2 on the Movi's GCU.

    IMG_4943.JPG


    If I leave it plugged in (with Alpha Wheels Tx OFF), it doesn't cause a problem unless it has the antenna on.

    In Dual Op Settings, I leave it set to Radio Type: Movi Controller when using both Mimic and the SBus Receiver.

    It's supposed to be Digital, as opposed to a Spektrum or Futaba that is Analog, and that is supposed to mean it's all 1's and 0's, so there shouldn't be any noise floor... but this is far from my area of expertise, so I don't really know much about the electronics.

    The practical advice is to unplug your receivers when using the Mimic, if you're experiencing any jitter. Traditionally, we've advised people that it's a "Radio Swamping" issue, which it can be, because the Mimic has a powerful transmitter, but this is a second possible cause of the jitter problem, and I've never seen anyone mention it before, so thought I'd bring it up.
     
  4. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Spektrum and Futaba use a digital signal protocol Graham, but the problem is, as the old adage in the computer industry says, "underneath, everything digital is implemented using an analog signal" (that has an electronic threshold applied to it to determine whether it's a zero or a one.

    Radio swamping is a possibility (see my other post in response to you). If you get a transmitter close enough to a receiver, it doesn't matter what radio frequency you're using, it will jam a signal into the receiver.

    Andy

    Forensic Software & sUAV / Drone Analyst : Photographer : Videographer : Pilot (Portland, Oregon, USA): Trees=2, Ground=1, Props=11. :(
    The Ground Is The Limit™
    ---------- Forensic Drone Analyst : Forensic sUAV Analyst : Forensic Unmanned Aircraft Analyst : Forensic Drone Expert
     
  5. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Sorry...I didn't finish my reply (phone call)....

    Hmm. So if you remove the antenna, all is well but the problem happens when the antenna is mounted. Hmmm (again).

    When you remove the antenna, you're effectively attenuating the RF signal that the SB receiver can pick up -- so it doesn't detect the RF signal any more (it's like removing a person's hearing aids).

    But the other possibility is that the SB receiver is acting as a transmitter and also emitting an RF signal.

    The way to disambiguate between whether the SB receiver is receiving or transmitting a signal this might be to unmount the SB receiver from the Movi so that you can move it further away from the Movi.

    The mere act of doing that might solve the problem.

    But there is another experiment: with the receiver dismounted from the MoVI, interpose some kind of metal sheet (e.g. biscuit tin lid, or "hide" the receiver into an empty coffee bean can with the open end facing away from the MoVI). The idea is to (a) have the antenna on, but (b) isolate the receiver in what is called a Faraday Cage, so that if it is actually transmitting, the signal it's transmitting is greatly attenuated.

    Solving RF interference can be more of an art sometimes. You may need to find a sacrificial chicken or something.... (I'm joking, of course, chickens being sacrificed are not know to solve RF problems).

    EDIT: In hindsight, I'm not sure the above really does disambiguate between the SB receiver receiving a bogus signal or transmitting an interfering signal....but the experiments are worth doing (let the chicken go free).

    Andy

    Forensic Software & sUAV / Drone Analyst : Photographer : Videographer : Pilot (Portland, Oregon, USA): Trees=2, Ground=1, Props=11. :(
    The Ground Is The Limit™
    ---------- Forensic Drone Analyst : Forensic sUAV Analyst : Forensic Unmanned Aircraft Analyst : Forensic Drone Expert
     
  6. Graham Futerfas

    Graham Futerfas Well-Known Member

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    Interesting. My theory is that since both the Mimic and the SBus Rx are "talking" to the Movi, even though the Mimic is supposed to have priority, there may be some chatter from the SBus and the Movi is seeing that too.

    Here's the page from 1A Tools about the different receivers, their pros and cons, etc.

    https://1a.tools/mvi-guide

    I'm not an expert in Radios, but it says that Spektrum and Futaba use PPM Analog input, and convert to Digital SBus. The advantage of SBus XR over Spektrum has been promoted to me as the fact that it's an all-digital signal all the way through the chain, which means there's no A to D conversion, which can cause delay. Thus, there's zero (almost zero) latency in the Wheels, verses a slight delay I experienced in my PLC Wheels which used Spektrum. Also, I understood that there's a noise floor in the Analog signal, which can cause drifting, and because SBus XR is an all digital path, this is not supposed to be the case.

    I can look through your suggestions of determining interference, but I don't have really long cables for the COM ports. I do have an RF Explorer 6G, but I can't seem to find these radios with it. I'm not really used to using it yet, and I think some radios use signal-hopping technology, so I can't seem to pinpoint them.

    Thanks for your help and suggestions!
     
  7. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    I checked out the 1A link and I understand what they're trying to say.

    The following explanation may be classified (at least by Internet geeks) as TL;DR !! :)

    The Spektrum/Futaba radios using PPM (pulse position modulation) are using a chain of variable length pulses, the time interval between the rising edge of a pulse and the one following it determines the magnitude of the servo signal for that channel (sorry, but this is an oversimplification, but I hope you get the drift).

    So the time interval between the rising edge of pulse 1 and the rising edge of pulse 2 determines the magnitude of the signal for channel 1. The time between the rising edge of pulse 2 and pulse 3 determines the signal for channel 2 and so on.

    That's how PPM "multiplexes" (aka mux) multiple channels' signals on a single wire/radio signal.

    On the other hand, S.Bus (Futaba proprietary) transmits groups of pulses, swinging the signal between two voltages (e.g. 0 volts and 5 volts) which are then "thresholded" (measured) to determine which are defined to be zeros and which are ones. The two voltages are actually analog (analog means that they can be continuously variable between 0 and 5 volts, say). The magnitude of the voltage is used to determine whether it's a zero or one (so that's the digital part because there can only be zeros and ones).

    So for S.Bus, you need a servo that's going to decode those analog voltages and convert them back into the zeros and ones. For PPM, the servo just looks for the pulses and figures out the time gaps it doesn't "measure" the voltage beyond looking for the rising edge of the signal.

    But either way, just try dismounting the SB receiver and getting even a little bit more distance between it and the MoVi -- the radio signal is subject to the inverse square law, so if you can double the distance you reduce the signal by a quarter, and so on...it would tell us something. :)

    Andy.

    Forensic Software & sUAV / Drone Analyst : Photographer : Videographer : Pilot (Portland, Oregon, USA): Trees=2, Ground=1, Props=11. :(
    The Ground Is The Limit™
    ---------- Forensic Drone Analyst : Forensic sUAV Analyst : Forensic Unmanned Aircraft Analyst : Forensic Drone Expert
     
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