Thread: Discussion Hitec Aurora 9 Transmitter
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Old Dec 31, 2009, 07:36 PM
A.T. is offline
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Servo Wag & Jerky Servo Movements

Originally Posted by The Don View Post
I have a question, when I plug in my battery on my ESC to power up my plane all my servos twitch until it links, why is this happening? I thought at first it was my fail safe positions so I made them so my retracts were up and all flying surfaces neutral. So with the gear up I plug in my battery then all the retracts try to come down then once it links they go back up. Also my ailerons twitch badly to full deflection and so does my rudder channel? I do not recall my other Optima 7 receiver doing this.
Servo Wag Answered
"Here is the answer: Use the SPC plug on the receiver. Let me explain the reason why.

Though I have not been able to get hard numbers on the ESC’s actual BEC amperage ratings, from my tests, this appears to be the issue. The Hitec 2.4GHz receivers use bi-directional communication with the transmitter module. In other words, they are transmitting as well as receiving. Because of this their current draw is higher than standard 72MHz receivers and even some 2.4GHz receivers. When coupled with the current draw of 4 micro servos, this can reach 1A or more. To compound the problem, when the receiver first initializes, it uses more current than it does during normal operation. The excessive servo movement during power on is a result of the BEC circuit being overloaded.

“Boot up” time is another factor. The initial startup time of the receivers can vary depending on the environment and due to complexity of the receivers themselves. This may cause the servos to perform a quick jump. The jump could range from a small hop or as much as a ¼ inch. The problem does not occur in most digital servos because of how their circuits are designed. The servo jump will only occur during the initial boot up, not when the receiver experiences a brown out.

By using the SPC plug, you power the receiver directly from the battery and bypass the BEC circuit. It will also allow you to turn on the receiver first, letting it boot up before applying power to the servos. The SPC circuit eliminates any receiver brown outs due to the drain on the BEC.

The only other option would be to use an ESC with at least a 2A BEC when using 4 micro servos.

Tony Ohm
Hitec Multiplex USA "

. Transmitter On First - Off Last

. Analog Servo Wag when Optima turned on see also post #1560 re start up procedure
being same as 27 - 72 MHz refer manual and

Optima RX - SPC Cable and use

see also comprehensive thread
Spectra Module & Optima 7 Failsafe issue last post is Firmware update V1.06

02 Feb 2012: added
Jerky Servo movements & Range Test

"For future reference, jerky moving servos is caused by lost packets of data. I've not seen this explained or described why anywhere else, so heres a short-ish reason why its happening.

This can be caused by a number of reasons on the majority of 2.4GHz kit, with one in particular on Hitec 2.4Ghz kit.
1. When your at the edge of range or a lot of interference, packets will start to be lost, and only those that come through will move the servos. On the older 72Mhz/35Mhz kit, the servo will only receive pulses when signal is received. Intermittent signal will mean the servos are driven less often (analog servos only move when they are receiving pulse data), and so will cause the slow/sluggish servo movement at the edge of range.

However on most modern 2.4GHz kit (all bar the really cheap Chinese stuff with no failsafe), the servos always continuously receive a pulse, even with no signal. So if there is a valid packet received, the receiver will change its pulse output to reflect the new data. Now if there is a delay of, say, 0.5 seconds before the next valid packet is received (as the other packets have been lost from interference or noise), it will cause a 0.5 second delay in moving the servo to the new position. So in large stick movements, that delay will make it noticeably jerky.

2. You may also notice at the edge of range, that the servos move around, as if they are jittering. This can be caused by the receiver not receiving enough valid packets to determine that it has a valid signal and then switching into Failsafe mode - moving the servos to their preset failsafe position. If it then receives a valid packet after switching in failsafe, it will then switch out of failsafe and move to the commanded position given by the Transmitter. At the edge of range this can happen quick enough, to look like the servos are jittering.

3. Another symptom of jittery servos is the receiver having a brown out event.
On frequency hopping schemes, at start-up, the receiver must lock onto the channel that the transmitter is on. Once locked on, the two will hop together, kept in-sync with their internal timers.
Now a brown out is caused by the the receiver voltage drops too low, which is usually caused by a weak/nearly empty battery (or overloaded BEC) that drops its voltage when a large amount of power is demanded from it. This demand is usually always servos making large movements - made worse if there are several servos moving at once - i.e. you trying to recover the plane from the servos not moving to their commanded positions!

When the voltage drops too low the receiver will effectively crash and restart - this is the brown out. So when it restarts, the receiver will then need to lock back on-to the channel that the transmitter is on. How long this takes depends on the way that the 2.4GHz frequency hopping is implemented. On Hitec gear it sits on a channel and waits a full transmission cycle before trying another channel. So for Hitec there are 21 channels it hops on in a transmission cycle, each being hopped onto every 21ms - so a full cycle of all 21 channels takes 441ms. If there is noise on that particular channel, it won't be able to lock and will then have to try another channel - thus taking longer to lock on.
Hence given the scenario that someone makes a large stick input, the receiver will receive this signal, and tell the servos to drive to that position. As its a large movement, the servo will put the motor at full power to try and move there as quick as possible. This full power requires a lot of current, so this is demanded from the battery. The battery can't give that amount of current and so the voltage drops. The voltage drops below which the receiver can operate, crashing (effectively turning off the power to it). This stops the servo driving its motor and then the voltage rises again and so receiver can restart. The receiver then starts up, locks on and then receives the next valid position. However as the servo has stopped been driven, it hasn't moved to its final demanded position. Hence the servo then gets told to move a large movement, it puts the motor in full, drops the voltage and this cycle all starts again - usually until the plane becomes one with the ground.

4. On the later Hitec software versions, they removed scan mode. In its place they put a 're-tune' mode. So if the site your at has a lot of interference (which can be seen from the jerky movements in Point 1 above), you can tell your transmitter to retune itself and move those 21 channels into a free, clear part of the 2.4GHz spectrum.
This works well and is fine, but you must rebind all your receivers after. If you don't, the channels that the receivers expect to listen on will be wrong. When retuning, you may get some of the channels to be the same as last time. So the receiver will try locking onto that channel, hopping to the next expected channel that the transmitter is on.
However as its been retuned, the next channel the receiver is expecting to receiver on, is not what the transmitter is actually transmitting on. The receiver will then fail to receive a valid signal, change into failsafe mode and try the initial channel lock-on search again. It will then lock onto the same channel, receive a valid packet, move the servos, hop to the next (but wrong) channel and the whole cycle repeats.

5. Use of servo extension leads ex MPX Newsletter July 2011

Servo leads often have to be extended due to the installed position of the servos.
Where this becomes necessary in models with wingspans greater than 2.5 m the conductor cross-section plays a crucial role in terms of the servos’ actual performance.

Typical calculation:
2 m cable with 0.25 mm² conductor cross-section, resistance approx. 400 mOhm (including connector), current drain of a High-Volt (HV) servo in normal use approx. 2A -> U=R * I = 0.40Ohm * 2A = 0.8 V
This means that the voltage drop along the extension lead is 0.8 V per wire; positive and negative wires together produces a drop of 1.6 V. In these circumstances a High-Volt (HV) servo used with a 2S LiPo battery is not operating on 7.4 V, but on only 5.8 V.
The servo can therefore never even approach its specified performance for 2S LiPo operation!
2 m cable with 0.5 mm² conductor cross-section, resistance approx. 100 mOhm (including connector), current drain of a High-Volt (HV) servo in normal use approx. 2 A -> U=R * I = 0.10Ohm * 2A = 0.2 V
In this case the voltage at the servo is still 7.0 V!
This simple calculation clearly shows what an immense influence the correct choice of extension lead can have on the real-world performance of servos.
================================================== ======

This is a fairly lengthy post, but hopefully its clear enough and not full of technical jargon to be understandable! If anything isn't clear, let me know and I'll try rewording it to make it clearer.

In a nut shell, if you have jerky moving servos when making large movements either when doing a range-test or in the air, DO NOT FLY AND/OR LAND IMMEDIATELY!
One last thing, I've seen mentioned in other forums and said down my local field that you don't really to range test 2.4GHz. - This is completely not true, in-fact as the requirement to keep to the correct frequencies is even greater than 35/72MHz, its even more important.

If an antenna becomes loose or damaged, if part of the RF circuitry is damaged/faulty, or vibrations (from Infernal Combustion engines) causes components to rattle around, loosing contact, 2.4GHz kit may still be able to receive the signal at close range - however as soon as you take off, bye-bye airframe...

So please, please range test all planes, at the start of the flying session as the minimum. If its a precious airframe (to you or anyone else) and especially if its IC, I'd take the 30-60 seconds (at full/half/low throttle - there may be resonating at different throttle positions) to do a quick range test to be sure. As described in the above points, I'd recommend large control movement on several control surfaces, to allow you to see any problems. Happy flying Cheers, Si."

ex Hitec USA Support Forum Sticky:
Aurora 9, AFHSS Spectra Modules, Optima Transceivers, Minima Receivers
& Telemetry
- FAQ & Undocumented Features
- Mixes, Setups, Tips. {Individual Links often updated}

Alan T.
Alan's Hobby, Model & RC FAQ Web Links (quick search = Ctrl+F)
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Last edited by A.T.; Nov 13, 2012 at 03:33 AM. Reason: add #5 - DIY calculate extension lead guage
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