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Audio system noise caused by Trutrak servos

B

brettt777

Member
Has anyone ever had any issues with a Trutrak autopilot causing a high pitched whine in the audio system whenever the autopilot is engaged? I just put a Sorcerer in an L-39 and also a Garmin GMA-340 audio panel and whenever I engage the autopilot I get a high pitched whine in the audio system. It's not just a noise; it sounds like an inverter whine only this one is at about 4.4khz. I am positive that it comes from the servos somehow because whenever the pilot is engaged, you can actualy hear this same sound from the servos like there is some sort of high frequency inverter in them. The real kicker of this whole thing is that the sound gets into the audio system through the Garmin GDL-69 audio output to the 340. If that line is not connected, you barely hear the noise and only during radio or intercom transmission. I have relocated audio and data lines, moved this, grounded that, ungrounded the other thing and nothing helps. I have heard this noise one time before when I installed a Sorcerer in another jet along with a 340. That ended up being one of the headset audio jacks touching ground and was easily remedied. This one though, has me pulling my hair out. Even the Trutrak guys were stumped. They had no idea where this noise would even be generated. All the signals going to the servos are either straight DC or ground. This noise is somehow generated in the servos and is feeding back into the audio system, through the GDL-69. If the GDL-69 is not in the circuit, in particular the audio output, no noise. Any input on this would be appreciated. Thanks in advance.
 
we put in a DB-25 filter at the trutrak control-head connector.

however, our problem wasn't audio noise but rather a noticeable autopilot pitch change while transmitting on com1.
in any case, i would certainly try the db-25 filter as it's cheap and the trutraks appear to have a history of some emi/rfi issues.

check http://trutrakap.com/forum/showthread.php?t=43

rgds, bernie
 
flyvans.com said:
we put in a DB-25 filter at the trutrak control-head connector.

however, our problem wasn't audio noise but rather a noticeable autopilot pitch change while transmitting on com1.
in any case, i would certainly try the db-25 filter as it's cheap and the trutraks appear to have a history of some emi/rfi issues.

check http://trutrakap.com/forum/showthread.php?t=43

rgds, bernie
Click to expand...

Well I have tried so many things, including an 8400uf cap across the servo DC line. That made a noticable difference but did not cure it. As I said, this isn't noise getting into the Trutrak, it's noise getting into my audio system from the Trutrak servos. It's a 4.4 khz whine. Yesterday I even went so far as to replace all the servo lines including the DC and ground with double shielded multiconductor cable which was rerouted away from everything. The noise was definitely diminished but still there, especially in the radio sidetone. I just got off the phone with Trutrak and they are sending me some new servos. I am not 100% convinced the servos are the problem but I'm not convinced they aren't the problem either. Bernie, you said Trutraks have a history of emi/rmi issues? Like what kind of issues?
 
brettt777 said:
Well I have tried so many things, including an 8400uf cap across the servo DC line. That made a noticable difference but did not cure it. As I said, this isn't noise getting into the Trutrak, it's noise getting into my audio system from the Trutrak servos. It's a 4.4 khz whine. Yesterday I even went so far as to replace all the servo lines including the DC and ground with double shielded multiconductor cable which was rerouted away from everything. The noise was definitely diminished but still there, especially in the radio sidetone. I just got off the phone with Trutrak and they are sending me some new servos. I am not 100% convinced the servos are the problem but I'm not convinced they aren't the problem either. Bernie, you said Trutraks have a history of emi/rmi issues? Like what kind of issues?
Click to expand...

I think what Bernie is referring to is the RF/pitch change problem. This is a symptom of RF bouncing around inside the airframe and/or getting transmitted back to the panel through wiring into the AP controller. The pitch gyro is quite sensitive to RF energy input. When the controller gets hit with RF, the energy spike skews the gyro's analog output, causing a pitch change.

If I'm off target and someone knows something I don't know, please fill me in! :)
 
One thing about this noise, you can actualy hear an audible high pitched whine coming from the servos when they are engaged. I don't remember hearing this before. Has anyone else noticed this? It is the same exact noise that I hear in my audio system. I actualy measured it on the servo DC line. It was 4.4 khz. Lucas is there some sort of inverter in the servo circuitry that could somehow be feeding back into the DC system?
 
brettt777 said:
One thing about this noise, you can actualy hear an audible high pitched whine coming from the servos when they are engaged. I don't remember hearing this before. Has anyone else noticed this? It is the same exact noise that I hear in my audio system. I actualy measured it on the servo DC line. It was 4.4 khz. Lucas is there some sort of inverter in the servo circuitry that could somehow be feeding back into the DC system?
Click to expand...

I've followed this thread for a few days. I don't get the whine, and a friends aircraft doesn't either. My Trutrak is for altitude, and he has both pitch and roll.

P.S. --- I mentioned the friend's airplane because I have a lot of time in it. Just called him to verify, and he say's it must be something else.....besides the servos.

L.Adamson --- RV6A
 
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One thing about this noise, you can actualy hear an audible high pitched whine coming from the servos when they are engaged. I don't remember hearing this before. Has anyone else noticed this? It is the same exact noise that I hear in my audio system. I actualy measured it on the servo DC line. It was 4.4 khz. Lucas is there some sort of inverter in the servo circuitry that could somehow be feeding back into the DC system?
Click to expand...

I certainly can't speak for Trutrak but I know some Servo systems use PWM (pulse width modulation) to control torque or current supplied to the Servo motor windings. Could be the 4.4 khz is the PWM frequency they are using to control Servo torque. You would be able to physically hear this coming from the Servo motor and it could also cause noise on the supply voltage feed.
 
brettt777 said:
One thing about this noise, you can actualy hear an audible high pitched whine coming from the servos when they are engaged. I don't remember hearing this before. Has anyone else noticed this? It is the same exact noise that I hear in my audio system. I actualy measured it on the servo DC line. It was 4.4 khz. Lucas is there some sort of inverter in the servo circuitry that could somehow be feeding back into the DC system?
Click to expand...

The amp that is used on the servos now is our high torque amp model. Previously, it was only used on the high torque servos. We now use it on all servos. It has a switcher on it. That is what is generating the high pitch whine you can hear. However, we have not had anyone report that the noise is showing up in the audio system without having an issue in the audio system itself. Let's try the new servos you're receiving today and let me know what happens. Thanks Brett!
 
647jc said:
I certainly can't speak for Trutrak but I know some Servo systems use PWM (pulse width modulation) to control torque or current supplied to the Servo motor windings. Could be the 4.4 khz is the PWM frequency they are using to control Servo torque. You would be able to physically hear this coming from the Servo motor and it could also cause noise on the supply voltage feed.
Click to expand...

Our servos do not use a PWM signal anywhere.
 
servo audio noise

Assuming that other users are not getting this noise, a servo swap may be the most logical first try.

4.4khz is still audio and too low to radiate, so it is likely it is conducted through the power wiring, and somewhere along the line coupled to other equipment.

Twisted power and ground wires may solve the problem.

The large capacitors mentioned may not solve the problem. filters should include an inductor (choke) in line with the power line and close to the noise source to block noise. in general the inductor MUST have a current rating high enough for the load, and with that parameter satisfied, the largest inductance you can fit. Then the capacitor plus goes to the junction of the inductor and the noise source power input. the other side of the cap goes to the noise source ground. the other side of the inductor goes to the power supply.
 
L.Adamson said:
I've followed this thread for a few days. I don't get the whine, and a friends aircraft doesn't either. My Trutrak is for altitude, and he has both pitch and roll.

P.S. --- I mentioned the friend's airplane because I have a lot of time in it. Just called him to verify, and he say's it must be something else.....besides the servos.

L.Adamson --- RV6A
Click to expand...

When you say you don't get the whine do you mean you don't get it in the audio system or you don't hear an audible whine from the servos?
 
TrutrakTech said:
The amp that is used on the servos now is our high torque amp model. Previously, it was only used on the high torque servos. We now use it on all servos. It has a switcher on it. That is what is generating the high pitch whine you can hear. However, we have not had anyone report that the noise is showing up in the audio system without having an issue in the audio system itself. Let's try the new servos you're receiving today and let me know what happens. Thanks Brett!
Click to expand...

I just go the servos so I will let you know shortly. In the past when we ordered servos for an L-39, one of them had to be a high torque servo. So all of them are high torque now? Has anything changed in the electrical design of these servos in the last year or so? Maybe I need to go through the trouble of removing an older unit off another plane just to plug it in and try it. I would think that if I have something so amiss in my audio system that it picks up noise somehow filtering back from the autopilot servos, I would notice other problems as well.
 
No, they are not all high torque servos, but they all use the high torque servo amp now. Previously, there was a different amp for the standard servos.

Nothing has changed electrically that would affect anything. The switcher on the high torque amp makes the audible noise. The standard servo amp did not have that on the board.

I hope this helps some.
 
I have two sets of Zulu head sets. I hear the wine in only one of them. I also hear electrical noise when I am taxing the alternator like when I have a full load. But only in one of my zulu's
Go figure....
 
Check grounding from the engine to the airframe. Be sure the MB antenna, if installed, has a good ground plane and be sure everythign going to the GMA340 is using shielded wiring
 
Noise issue when Gyro is switched on

Hello all, I hope that you are well.

My RV-8A is working great except for one little item. After startup, I start switching on systems (Avionics, Flap Trim, Lights, GYRO, etc.)

Everything is great until I get to the Gyro and then I get a high-pitched whine in my headsets, through the audio panel. If I turn off the gyro switch, the sound immediately goes away. Any suggestions?

Thanks in advance.

Best regards,

Bruce Spears
RV-8A - N922ES
Houston, TX
 
If by "Gyro" you mean an electric, motor driven turn coordinator, directional gyro or artificial horizon, then the motor in the unit is probably causing interference on the power line that's being picked up and amplified thru your audio panel.

Putting a noise filter in line with the power wire to either the gyro, or the audio panel would probably clear it up. FAA-approved power line noise filters are rather pricey ($85-150) but since it's an experimental aircraft, you can probably get one from a local automotive stereo retailer for under $20 that's small, compact, lightweight and will perform just fine. Something like this:http://www.installergadget.com/servlet/the-19172/SCOSCHE-ES004-Single-dsh-Stage-Noise/Detail

If you put it in line with your electric gyro's power, it'll stop the noise from getting out onto the ship's wiring. If you put it inline with your audio panel's power line, you'll just stop the noise from coming into the audio system.
 
I've had two-axis TT APs in two plastic airplanes (generally worse for noise, etc. than metal planes). No servo noise ever, but had the pitch problem in both and the filters cured it.
 
Thanks for the reply and yes, I mean the electrically driven attitude indicator. So, to be clear, you are recommending a noise filter on the audio panel? I just want the high pitched whine removed from my headsets. Thanks.
 
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