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Old Jan 31, 2011, 12:21 PM
Detail Freak
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Harbor City, CA
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Twisted servos wire: Fact or Fiction?

Here's a question, and I'm looking for facts, not opinion!

Does it really matter if the servo wire is twisted or flat for the wings in our F3X planes, and the fuselages leading to the receiver?

Is a 72Mhz system more in need of this than a 2.4G system due to the longer Rx aerial??

It is much easier for me to deal with flat servo wires, especially in the fuse, than twisted. I've done it both ways, and haven't noticed any difference. Twisted is heavier and uses slightly more wire length, so is heavier.

Thanks for facts inputted here.

R,
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Old Jan 31, 2011, 12:47 PM
Play loud, Fly high
Gil Gauger's Avatar
Vincennes,IN USA
Joined Apr 2007
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Twisting is one method of shielding used in signal wires. Coax, twinax, etc. are others. Though guys with a better understanding of antenna theory than myself could provide better explanations, I do believe for 72 MHz systems servo lead lengths at or near antenna length can lead to multipath interference. Wing servos in unlimited class sailplanes and other aircraft of similar size often exhibit signs of this most commonly when swamped by RF with the transmitter close by. Flaps chattering while hooking up for launch ring a bell? The shielding properties are frequency dependant and the methods used depend on the particular situation. I am an audio guy, not RF, so my experience is much more in line with servo pulses instead of modulation and carrier freq issues. All this same stuff applies to 2.4 transmission but the networking protocols tend to make up for and mask issues that we may have noticed in the analog environment.
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Old Jan 31, 2011, 02:05 PM
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Target,

Thanks for posting this. I was asking the same question this week. Got one response that said not necessary with 2.4. I would like to hear what others think as well.

Shawn
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Old Jan 31, 2011, 02:23 PM
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vespa's Avatar
Thousand Oaks, CA
Joined Mar 2004
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Twisting the wires does nothing for the intended signals but it causes any signals that are common in both (or 3) wires to cancel eachother out. For example if you held a 72MHz TX close to a long wire run it would produce that signal in all 3 wires, adding noise to your power and ground while distorting the signal. Twisting completely eliminates that.

If you use a video TX, telemetry, voltage regulator, or anything else that produces RF noise twisting will help, but modern servos and receivers are pretty good at filtering that out anyway.

I twist my wires because it is lighter! I can save weight by properly sizing the power wires, using small signal wires, and choosing quality wire with tefzel or similar ultra-thin insulation. Flat wire is only available with yucky PVC insulation.
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Old Jan 31, 2011, 03:02 PM
Detail Freak
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Harbor City, CA
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Hi, Vespa-

I understand what you mean, but really twisted wires of the same construction always weigh more since they are longer than flat; but I hear you... I've not gone to the lengths to save weight to use lighter gauge signal wire... I'm mostly just interested in the question about the twisted wire and the signal, and especially the difference between 72 megs and 2.4, and if it even is needed with 2.4Ghz....

Thanks,
Target
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Old Jan 31, 2011, 03:04 PM
Detail Freak
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Gil-

I feel that you are probably correct, but we need a mad scientist to back this up. Know anyone that can prove this to be true?
Thanks for the post.

R,
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Old Jan 31, 2011, 03:49 PM
Don't take your guns to town
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There are 2 issues with FM that no longer affect us with 2.4. First, our long servo wires make great antennas for the MHz range and thus RF hitting your servo wires can efficiently come back to the RX and distort the ground, causing a slight distortion of the FM signal. Second, an analog protocol is inherently sensitive to such slight distortions since analog signals are nothing more than slight distortions themselves.

To upset a 2.4RX you would need servo wires about 1" long and RF noise so strong that it completely masks the clear digital signal. Essentially impossible. Plus, the modern RF units are far better isolated from ground noise anyway.

To upset a servo however, is much easier. The PWM signal is essentially analog in that it is very sensitive to slight distortions. Hold a servo wire near the antenna of a powerful transmitter and you will likely see the servo deflect or jitter. I have done this testing on dozens of servo makes/models and found that most are affected, digital or not.
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Old Jan 31, 2011, 04:34 PM
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Does a radar physicist qualify as a "mad scientist" ?

What you are looking at is an area of physics called "Magnetostatics" it's an extension of Gauss's Law, more specifically labeled the Bio-Savart Law. What it basically says (in lamen's terms) is that when you have a steady state current flowing through a wire you emit both an electric field and a magnetic field (your question is relative to the strength of the magnetic field). The magnetic field's direction goes as the "curl" of the current flow. Take your right hand wrap it as if you are holding a broom stick. Your thumb is the current flow and your fingers curl around the broom stick, that's the magnetic field. In this case the current flow is down the length of the wire. So the magnetic field is perpendicular to the current flow at all points. Maybe it's easier to think of a Peacock's feathers spanning out in all directions as you're looking at its butt. Now I am venturing into the realm of mad scientist. Anyways, the strength of the electric field is proportional to the power (current and voltage) pushed through the wire, and the electric field strength decreases as a 1/(r^2)the distance an object is placed from the wire. The magnetic field strength follows the electric field strength. Yes, the frequency (2.4GHz or 72MHz) of the system also comes into play, but I'll state my opinion of that below. What the heck does all this mean???

By twisting the servo wires you will disrupt the the nice pretty Peacock pattern of the magnetic field. The total magnetic field will no longer be perpendicular to the current. Think of the Peacock's feathers going every which-way. The magnetic field will span all three dimension in a highly non-uniform manner.

The higher the current and voltage in the system, the stronger the magnetic field.

The further away your receiver is from wires the weaker the magnetic field is.

A receiver with a good filter, or set of filters, the less likely you will encounter any glitches due to the magnetic field generated and if the filters are tuned correctly, the same applies to frequency concerns. You wouldn't use a filter tuned at 75MHz on a 72MHz system, and the same is true for 2.4GHz. However in that highly non-linear-magnetic-near-field that you just created, you could have secondary effects called "harmonics" and these could fall outside of the frequency regime that your receiver is filtering. The field strengths of harmonic waveforms typically have a very quick fall-off of around 13dB - that's a huge fall-off in power space, and that's a whole-nother discussion.

Having said all that, will your receiver with your battery and your servos be glitched or not be glitched by twisting or not twisting the wires? The answer to that question is left to an engineer to compute (I'm a theoretical physicist that works in the world of radars). But my gut felling is, the currents involved with our systems, the distances of where things are placed, and the receivers (I never buy cheap receivers) we put in our toy airplanes have sufficient filters to not worry about these things. I don't.

Todd
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Old Jan 31, 2011, 05:00 PM
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Hawaii
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Ahhhhh yes, the righthand rule.
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Old Jan 31, 2011, 05:03 PM
RIP MC
fnnwizard's Avatar
United States, CA, Midway City
Joined Dec 2003
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T,
This is what I got when going through the same thing a couple years ago.
The consensus is that it is a "best practice" to twist.

With our sailplanes I was twisting when using 72Mhz but only because it was considered "best practice" as I never did experience any issues when testing untwisted vs twisted in the sailplane environment. I've been running straight wires since going to 2.4 and never have had any issues.

On 72Mhz... it's probably still a good ideal to twist if you have speed controllers, motors, engines etc. in a crowded fuse or heli body.

In heli environment, using 72Mhz, sometimes we get glitches from various noise sources including static from the tail rotor belt. I don't know if twisting would have helped, but we ended up using ferrite rings on the ESC to help suppress noise.

When I switched to 2.4g, I never got a single glitch in the heli environment (I would say that's probably the toughest place for our RF electronics) and don't use ferrite rings anymore.

It's safe to say, if you are getting some kind of servo chattering while using a 2.4g system, twisting wires or not is not going to be the answer.
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Old Jan 31, 2011, 05:24 PM
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Gil Gauger's Avatar
Vincennes,IN USA
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Target, at my current position (vocational school IT guy and electrical instructor) I have equipment to bench test up to 20 MHz. Probably far too low to provide any meaningful data, but a couple bright students and I will conduct a few experiments to see if we can spot any trends. I do know from my network cabling days that the 'twist' in twisted pair cabling is critical for both controling susceptibility to external noise, and preventing those computer cables from becoming unintended transmitter antennae. Sorry, but my current situation doesn't give me access to the big ticket test equipment I used for compliance testing then.
Of course while I am typing, others have posted. Let me add, like the 'wizard' stated: There is no harmful effect that I can perceive, and a few possible benefits. As a caveat I will say that I have used flat ribbon cable in the past with analog servos with no problems, until one plane that loved to waggle the flaps. On a hunch, I lengthened the wires from the receiver to the wing plugs and tamed the problem. Hence my original post. Was it the added length, or just rearranging the routing? Can't say for sure.
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Old Jan 31, 2011, 05:32 PM
Detail Freak
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Harbor City, CA
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Thanks Gil-

I wouldn't spend too much time on my behalf, but if you want to satisfy your own curiosity, by all means please share your findings.
I'm sure that lots of folks, myself included, would find it helpful.

R,
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Old Jan 31, 2011, 06:31 PM
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Silent-AV8R's Avatar
Orange County, CA
Joined Aug 2004
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I know many people who feel very strongly that twisted wires are a must to avoid errant RF from creeping into your system. Personally I have never noticed an issue using flat ribbon in anything I've flown. (gliders, scale, giant scale power, etc.).

I do suggest to people that want to use twisted wires to make sure to balance the system by using left hand twist on one wing and right hand twist in the other wing. Failure to do this can lead to the coriolis effect causing an induced electromagneto field relationship that oscillates in a semi-harmonic fashion. As we all know, this is to be avoided whenever possible. Runs to the tail are center balanced for matched capacitance/reluctance variance and are therefore not an issue for most non-symbionic installations.

This video explains it far better than I can:

Engineering Gibberish (1 min 49 sec)
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Old Jan 31, 2011, 06:45 PM
Detail Freak
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Harbor City, CA
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All this time, I have been twisting the wires in the same direction; I knew there was room for improvement.
Thanks, Bill!

R,
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Old Jan 31, 2011, 07:06 PM
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Thousand Oaks, CA
Joined Mar 2004
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I'm confused. What if we fly in the southern hemisphere?
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