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Jitter/glitching on HS-7940TH 2S lipo direct

Old 10-14-2010, 09:49 PM
  #1  
eajohnson
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Default Jitter/glitching on HS-7940TH 2S lipo direct

I'm having problems with some HS-7940TH servos that I'm trying to use on two halves of an elevator on a 35% plane (set up for IMAC not 3D so max surface deflection is very low).

One servo sometimes does not seem to 'boot up' when power is applied but only when batteries are fully charged.

The other jitters and glitches (meaning uncommanded deflections that occur then swiftly revert back to the proper position).

As the rx packs go lower in voltage, the glitching seems to improve. At full charge it is extremely bad. I've tried adding a magnetic rf choke on the extension of the servo that glitches, this had zero effect. Also I've tried directly plugging in to the smart fly unit to eliminate the extensions; this helps but does not always eliminate the problem.

I have not flown the plane yet because of this. Lots of people in my area are looking to this plane as it is a 35% electric which is still something of a novelty here and now it's the glitching servos that are holding up a maiden!

My setup:

Airtronic 92104 10ch rx
Smart Fly power expander 14Mz
2x 1600mAh 2S 25C Hyperion G3 lipo (unregulated in to the smart fly unit)
22ga servo extensions (~2ft)

Measured voltage and current during test (measured at battery, two servos connected): 8.3V 0.9A peak (batts partially depleted, occasional glitching). At 8.4V full charged rx batts, the glitching on the one servo is severe.

Are these servos possibly not capable of handing a 2S lipo direct? (is 8.4V OK?)

- as an afterthought I just tried putting a smart fly equalizer 3d device just before the glitching servo and so far this seems to eliminate the problem. However the equalizer 3d I don't think is rated for 2S lipo voltages so this probably does not present a useful workaround. I also have a 7950 on rudder that seems to be completely fine.

- the glitching appears to always be in the same direction (happens to be down on the right elevator half). Also saw my left elevator glitch once.

- adding a load to the affected servo by applying pressure to the control surface causes it to glitch much worse - but ading the equalizer 3d in line before the servo still completely eliminates the glitching no matter how much pressure I apply to the control surface

- swapping extensions for a different extension makes no difference, swapping channels makes no difference, putting two extensions end to end to make a super long extension does not seem to make the problem worse (or better)

I have an oscilloscope so I should probably also take a look at what the signal and voltage lines look like relative to the ground line (e.g. with an without the smart fly device in place).

One last thought before I go to bed - the rx voltage is regulated at 5V. So, relative to ground the highest V the signal can be is 5V with no extension involved, and the highest the +ive lead will be is 8.4V. Do you think possibly the servo is having a problem with a <5V signal relative to a +8.4V +ive line? Possibly the equalizer 3D might be sending new pulses at a higher voltage thus eliminating the problem?

9:11am - adding to the info, when I looked on the scope this morning I can see some of my assumptions were wrong, the equalizer 3d appears to reduce both voltage and frame rate according to what I see on the scope. Noise visible on the scope that is present without the equalizer in place next to the servo but absent with it there (when signal is measured relative to the ground wire, just before the servo) seems to correspond with the glitching and occurs only when the servo is moved. So it appears that the noise is somehow related to operation of the servo - motor perhaps? This might also explain why glitching occurs when I press against the control surface even though no change has been made to the input signal - if it is the servo itself that is somehow generating the noise and if this noise is exacerbated by the extensions. I wonder if a capacitor would help and whether the noise is on the signal line or is actually on the +/- lines.
Old 10-15-2010, 10:50 AM
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Brendan Lugo
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Default RE: Jitter/glitching on HS-7940TH 2S lipo direct

As a test, plug the servo directly into the receiver bypassing the smartfly and see how it operates. According to the specs for that receiver it should be able to handle the 2s lipo unregulated.
Old 10-15-2010, 11:48 AM
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eajohnson
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Default RE: Jitter/glitching on HS-7940TH 2S lipo direct

Hi, I'll do that this evening once I rig up a battery connection for the direct connect to the receiver.

In the meantime, I've found that just experimentally adding a 330uF electrolytic cap (just happened to have one sitting around) between - and + lines of the servo extension just before the servo cures the glitching. However, putting the same cap at the smart fly end does not seem to solve anything. The servo extensions are 22GA twisted, perhaps the noise on the power leads is leaking a bit into the signal line?
Old 10-15-2010, 11:56 AM
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Default RE: Jitter/glitching on HS-7940TH 2S lipo direct

Here's a thought - every time your servo moves, there is increased current in the leads. Losses in the wire mean the voltage in the servo ends of the wires will be lower than at the power supply feed ends. WIth respect to the battery, the positive voltage will be somewhat less, and the "ground" wire will actually have a somewhat higher voltage at the end when compared to the battery ground lead itself. Perhaps it's the noise on the ground reference that's causing jitter. Adding the cap to the servo end would smooth out noise transitions due to current in the servo leads. Adding it to the other end of the wires won't have the same effect.

In the high power pulsed amp world (2.5KW or so) we have to use differential voltage control signals rather than ground referenced. There is no such thing as ground...

Old 10-15-2010, 06:47 PM
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Default RE: Jitter/glitching on HS-7940TH 2S lipo direct


ORIGINAL: Brendan Lugo

As a test, plug the servo directly into the receiver bypassing the smartfly and see how it operates. According to the specs for that receiver it should be able to handle the 2s lipo unregulated.
The servo still glitches when I connect a 2S lipo directly to the RX and connect the servo extension directly to the rx, bypassing the smart fly unit (actually I removed the rx entirely from the smart fly unit when doing the test). When I power the rx with a lower voltage (~5V) pack, the servo does not glitch.

Old 10-15-2010, 06:53 PM
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Default RE: Jitter/glitching on HS-7940TH 2S lipo direct

(edited out - sorry this was a duplicate posting of the previous post, caused by a browser hang)
Old 10-15-2010, 06:58 PM
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Default RE: Jitter/glitching on HS-7940TH 2S lipo direct

ORIGINAL: fizzwater2

Here's a thought - every time your servo moves, there is increased current in the leads. Losses in the wire mean the voltage in the servo ends of the wires will be lower than at the power supply feed ends. WIth respect to the battery, the positive voltage will be somewhat less, and the ''ground'' wire will actually have a somewhat higher voltage at the end when compared to the battery ground lead itself. Perhaps it's the noise on the ground reference that's causing jitter. Adding the cap to the servo end would smooth out noise transitions due to current in the servo leads. Adding it to the other end of the wires won't have the same effect.

In the high power pulsed amp world (2.5KW or so) we have to use differential voltage control signals rather than ground referenced. There is no such thing as ground...
It's an interesting idea. Of course, it's not the level of the pulse in terms of relative voltage that's significant, it's the duration of the pulse, so there's more to it than just whether the ground reference floats a bit. It's got to be jumping around enough that the servo recognizes a pulse or to interefere with its recognition of the beginning or end of one of the pulses coming from the rx.

On same topic, I just tried running a parallel 14GA ground wire to the servo so there would be negligible voltage change, and this made no difference.

Incidentally, neither did adding a small ~500 pf cap between ground and signal.

Old 10-15-2010, 07:47 PM
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Default RE: Jitter/glitching on HS-7940TH 2S lipo direct

So to summarize, this HS-7940TH glitches consistently in one direction when run at high voltage (>8.3V) and separate from the rx by a long-ish 22GA twisted wire extension. The glitching occurs only when power drawn e.g. if I put pressure on the control surface or command servo movement from the transmitter.

So far the only remedies that have worked are

+ add a big electrolytic between power and ground, at the servo end
+ put in an equalizer 3D at the servo end
+ lower the voltage
+ helps a lot but not completely eliminates the problem - direct connect servo to smart fly unit with no extension (obviously this can't work in practice because the servo lives in the tail)

Things that did not make a difference:

- directly connect servo to RX running on a 2S lipo with no Smart-Fly Power Expander device in the circuit
- add a small 500pf cap between ground and signal lines at the servo side
- swapping servo extension for different servo extensions
- adding a 2nd 14GA heavy ground line from the smart fly unit to ground on the servo lead
- add a big electrolytic between power and ground at the rx end
- add a 200 or 300 Ohm resistor to the signal line at the servo end (to help with possible ringing)
- add a ferrite ring at the servo end
Old 10-16-2010, 12:00 PM
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Default RE: Jitter/glitching on HS-7940TH 2S lipo direct

This morning I tried a smaller (22uf) tantalum cap between power (+) and ground (-) leads at the servo end of the extension and this also eliminates the glitching.

What gave me this idea in the first place is that we use caps on the battery to ESC power lines, at the ESC end, with our electric powered planes to control back EMF and I thought possibly the same thing might be happening here and causing the glitching.

My other identical elevator servo glitches only very rarely when plugged into the exact same extension, but I _have_ seen it glitch also so I'm considering putting caps on both of these extensions at the servo ends. There definitely is variability from servo to servo in terms of the degree to which they are susceptible or perhaps even cause the glitching.
Old 10-18-2010, 09:46 AM
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Default RE: Jitter/glitching on HS-7940TH 2S lipo direct


ORIGINAL: eajohnson


ORIGINAL: Brendan Lugo

As a test, plug the servo directly into the receiver bypassing the smartfly and see how it operates. According to the specs for that receiver it should be able to handle the 2s lipo unregulated.
The servo still glitches when I connect a 2S lipo directly to the RX and connect the servo extension directly to the rx, bypassing the smart fly unit (actually I removed the rx entirely from the smart fly unit when doing the test). When I power the rx with a lower voltage (~5V) pack, the servo does not glitch.


Servo directly to the receiver, no extensions hooked between the servo and rx. 2s lipo, does it glitch?
Old 10-18-2010, 10:01 AM
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Default RE: Jitter/glitching on HS-7940TH 2S lipo direct

ORIGINAL: Brendan Lugo


ORIGINAL: eajohnson


ORIGINAL: Brendan Lugo

As a test, plug the servo directly into the receiver bypassing the smartfly and see how it operates. According to the specs for that receiver it should be able to handle the 2s lipo unregulated.
The servo still glitches when I connect a 2S lipo directly to the RX and connect the servo extension directly to the rx, bypassing the smart fly unit (actually I removed the rx entirely from the smart fly unit when doing the test). When I power the rx with a lower voltage (~5V) pack, the servo does not glitch.


Servo directly to the receiver, no extensions hooked between the servo and rx. 2s lipo, does it glitch?
No, it does not glitch if there is no extension. The extension is definitely part of the problem but I'm convinced it's not a fault in the extension - swapping for different extension makes zero difference better or worse, and running big thick power wires back to the servo makes zero difference better or worse. It's just like back EMF on our speed controls where it doesn't matter how thick the wires are, you need to add a cap.

Anyway, I've found a solution that works (adding a capacitor across the power leads at the servo side of the extension) but it would be great if Hitec was able to confirm and provide an engineered work-around rather than one that has been merely experimentally devised. There are lots of people that have apparently had similar problems with similar servos (HS-7950TH etc.) at 8.4V when running extensions.
Old 01-31-2011, 04:49 PM
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Default RE: Jitter/glitching on HS-7940TH 2S lipo direct

i'm just begining to look at the forums for this problem....i have 7950th 's on my elavator that have started the same problem....they twitch at rest and when giving them up or down command....twitch, flutter glitch...all the same...and i have 24 inch hitec heavy duty twisted extensions on them also.....havent flown this plane yet....just fully charged my 2s reciever battery and this problem turned up.....what is hitec gonna do or recommend?????

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