PDA

View Full Version : Connecting Garmin GAD 27, GSA 28, and trim


Greenley
09-01-2016, 09:36 PM
Looking for info on how to power the GSA 28 AP servos and trim when you have the GAD 27. The GAD 27 shows connecting the trim through it, but what about the GSA 28 servos, they also show connecting the trim to them. The GAD 27 install information makes no mention of the GSA 28 AP servos. I have seen a diagram someone posted showing the AP servos being powered from the GAD 27, and the trim from the AP servos, but no idea if this is right.

g3xpert
09-02-2016, 08:13 AM
Looking for info on how to power the GSA 28 AP servos and trim when you have the GAD 27. The GAD 27 shows connecting the trim through it, but what about the GSA 28 servos, they also show connecting the trim to them. The GAD 27 install information makes no mention of the GSA 28 AP servos. I have seen a diagram someone posted showing the AP servos being powered from the GAD 27, and the trim from the AP servos, but no idea if this is right.

Hello Bill,

Since you have GSA28 autopilot servos and will likely want to use auto-trim, please connect the trim motors to the servos like shown on page 24-10 of the Rev. Z G3X Installation Manual.

This page also shows how to connect the trim switch inputs/outputs on the GAD27 to the pilot/copilot switches and to the servos.

Let us know if you have questions.

Thanks,
Steve

tgmillso
10-11-2017, 02:58 AM
Hi All,
Does anyone know the reasoning behind why the power wires from the autopilot servo to the trim servo are shielded? The 5 core 26AWG wires supplied with the Ray Allen trim servos are not shielded, and nor are the power wires to the autopilot servo, so I'm a little confused as to why just this section of wire is.
Cheers,
Tom.

http://i65.tinypic.com/nnjkp5.jpg

g3xpert
10-11-2017, 05:26 AM
[QUOTE=tgmillso;1210180]Hi All,
Does anyone know the reasoning behind why the power wires from the autopilot servo to the trim servo are shielded? The 5 core 26AWG wires supplied with the Ray Allen trim servos are not shielded, and nor are the power wires to the autopilot servo, so I'm a little confused as to why just this section of wire is.
Cheers,
Tom.

Hello Tom,

As you probably know, the GSA 28 servo doesn't simply turn the trim motor on/off to operate it in different directions using +12Vdc and ground.

It performs speed scheduled trim control where the trim motor is run fast at low air speeds and slower at high air speeds.

The trim motor speed is controlled by varying the trim motor drive voltage, and this is done by varying a high speed pulse width modulated (PWM) signal.

If you don't shield the wires carrying this signal to the trim motor, it can cause interference with other systems in the aircraft. In extreme cases we have even seen it break the COM radio squelch.

Thanks,
Steve

tgmillso
10-11-2017, 05:52 AM
Excellent. Thankyou Steve. This makes perfect sense. I knew there must have been some logical reasoning for it, I just couldn't quite put my finger on it.

Regards,

Tom.

YvesCH
10-12-2017, 08:27 AM
Ouff thank you very much Tom for asking this! I did not realize that there is a requirement for shielded wire and wanted to rivet my turtle deck very soon! It would have been a pain in the a... to do it afterwards!

cheers, yves

tgmillso
10-15-2017, 05:58 AM
Hi Steve. Do you need to run this shielded cable all the way to the trim servo, or can you just terminate the ground at the aft deck for the roll servo and assume that this far back on the fuselage you should be out of the danger zone from an interference perspective, and just run unshielded from the elevator to the trim servo? The only other equipment I have down the back of the aircraft is a shielded LED tail light (the magnetometer is in the wingtip). The reason I ask is that setting up a grounding point in the elevator is going to be much more difficult than on the aft deck.
Cheers,
Tom.

Raiz
11-28-2018, 02:36 AM
I'm never sure whether it's best to resurrect an old, but related thread, or start a new one but I'll give it a try.

Looking at the G3X install manual, a 5 amp breaker is called out for the trim power supply to the GAD 27 and that powers the trim motor via the GSA 28 servo. However, Ray Allen specifies a 1 amp breaker, which makes sense, because the trim motor has 24 AWG wires. The G3X manual also indicates a maximum trim motor current draw of 1 amp.

Does the GAD 27 or the GSA 28 provide some form of current limiting that protects the trim motor wiring or do I need an additional fuse somewhere?

Mark H
11-28-2018, 09:34 AM
You can use a smaller breaker than 5 amps for the trim motors run through the GAD 27. Garmin doesn?t know what servos you will use so they have to specify a maximum current their equipment can handle. I installed a 2 amp breaker and it works great.

g3xpert
11-28-2018, 10:33 AM
One thing to remember is that the trim motor(s) are rarely powered by the GAD 27.

In normal operation the GSA 28 servos are always powered-up, and even when the autopilot is disengaged, the servos are providing speed scheduled variable voltage to the trim motors independent of the GAD 27. The GAD 27 is only providing trim direction signals to pins 11,12 of the GSA 28 the majority of the time.

It is only when the GSA 28 is powered off (a trim bypass relay drops) and non speed scheduled direct drive power is being supplied by the GAD 27 to the trim motor does the size of the GAD 27 trim power circuit breaker make any difference.

Thanks,
Steve

Raiz
11-28-2018, 12:04 PM
Thank you Steve and Mark, I hadn't appreciated how it works. One more question though, is it the 5 amp autopilot breaker (supplying the GSA 28) that limits the current to the trim motor while the GSA 28 is powering it or is there some other mechanism at work?

Ray

Bavafa
11-28-2018, 01:47 PM
One thing to remember is that the trim motor(s) are rarely powered by the GAD 27.

In normal operation the GSA 28 servos are always powered-up, and even when the autopilot is disengaged, the servos are providing speed scheduled variable voltage to the trim motors independent of the GAD 27. The GAD 27 is only providing trim direction signals to pins 11,12 of the GSA 28 the majority of the time.

It is only when the GSA 28 is powered off (a trim bypass relay drops) and non speed scheduled direct drive power is being supplied by the GAD 27 to the trim motor does the size of the GAD 27 trim power circuit breaker make any difference.

Thanks,
Steve

Hi Steve,
I have this setup as well and just began flying so I am at the very early stage of it. One thing that I have noticed when I operate the pitch trim servo all by itself (i.e. Auto pilot is powered off) the trim servo runs in a steady mode as long as I am pressing the switch on the stick but if the A/P is powered on and disengaged, the trim motor does not seem to run steady.

This observation is primary based on the sound that the trim motor makes. with the A/P turned off, I hear the trim motor (Ray Allen) noise continuously but with the A/P powered on it sounds as if it is pulsing rather a stead noise.

One other thing that I have noticed during the initial setup of the auto pilot, if I touch the trim switch on the stick it will disconnect/disengage the A/P, is that a normal operation?

g3xpert
11-28-2018, 01:50 PM
Thank you Steve and Mark, I hadn't appreciated how it works. One more question though, is it the 5 amp autopilot breaker (supplying the GSA 28) that limits the current to the trim motor while the GSA 28 is powering it or is there some other mechanism at work?

Ray

Hello Ray,

Yes, since the GSA 28 servo is providing power to the trim motor, the circuit protection provided for the GSA 28 is sourcing the power.

Thanks,
Steve

g3xpert
11-28-2018, 02:33 PM
Hi Steve,
I have this setup as well and just began flying so I am at the very early stage of it. One thing that I have noticed when I operate the pitch trim servo all by itself (i.e. Auto pilot is powered off) the trim servo runs in a steady mode as long as I am pressing the switch on the stick but if the A/P is powered on and disengaged, the trim motor does not seem to run steady.

This observation is primary based on the sound that the trim motor makes. with the A/P turned off, I hear the trim motor (Ray Allen) noise continuously but with the A/P powered on it sounds as if it is pulsing rather a stead noise.

One other thing that I have noticed during the initial setup of the auto pilot, if I touch the trim switch on the stick it will disconnect/disengage the A/P, is that a normal operation?

Hello Mehrdad,

Congratulations on your new RV-14A. Really beautiful!

When you power off the servos and run the trim motor, it is running full speed at a steady max voltage and there is no speed scheduling which affects the trim motor speed/voltage.

When you run the manual trim with the servos powered, speed scheduling is being applied, which is done with a pulse width modulated (PWM) signal.

Depending on the speed scheduling setup, the trim motor will run slower in some conditions, and this could make it sound different.

There is a limit to how much the servo can reduce the voltage to the trim motor and still have it move, so at some point it even pulses the motor to further reduce speed, so this is something else you might have heard.

If the pilot pushes the trim buttons when the autopilot is engaged (with auto-trim enabled), it assumes the pilot is trying to take over control of the trim motors, and disengages the autopilot.

Let us know if you have additional questions.

Thanks,
Steve

Bavafa
11-28-2018, 02:49 PM
Hello Mehrdad,

Congratulations on your new RV-14A. Really beautiful!

When you power off the servos and run the trim motor, it is running full speed at a steady max voltage and there is no speed scheduling which affects the trim motor speed/voltage.

When you run the manual trim with the servos powered, speed scheduling is being applied, which is done with a pulse width modulated (PWM) signal.

Depending on the speed scheduling setup, the trim motor will run slower in some conditions, and this could make it sound different.

There is a limit to how much the servo can reduce the voltage to the trim motor and still have it move, so at some point it even pulses the motor to further reduce speed, so this is something else you might have heard.

If the pilot pushes the trim buttons when the autopilot is engaged (with auto-trim enabled), it assumes the pilot is trying to take over control of the trim motors, and disengages the autopilot.

Let us know if you have additional questions.

Thanks,
Steve

Thank you Steve, I really appreciate the clarification. This all makes sense, the only puzzling part is that my speed schedule is set to 100% for 70knots but I was on the ground when I was testing this and speed was Zero. I would have thought that it would give me the full speed with the A/P on.

But I wasn't looking at the trim tab since I was in the cockpit so could not verify if the speed was slower, only could hear a pulsing sound.

Lots to learn with the new toys :)

Raiz
11-28-2018, 03:14 PM
Thanks Steve
Ray