Trim servo problem

This is related to Safety Directive SD-00001 for RV-12/12iS - Elevator Trim Servo but not directly related to the installation so I thought I'd start a different thread rather than hijack the other one.

Anyway, after installing the new trim servo the servo gets stuck at the extreme ends of the servo travel. By that I mean that near the extreme ends of servo travel the servo will not move using the cockpit trim switch. The only way I can recover is to disconnect the white & gray servo wires from the harness and use a 9v battery to move the servo back into the operating range. It happens at both ends of the range.

I looked at the RV-12 schematic and the PTRIM servo pos disappears into the AV-50000A Control Module as do the Trim Switch signals.

So far this hasn't been a problem in flight. I only discovered it when I ran the servo to its limits to time the transition. The new servo seems to run slower than than the old servo and I wanted to speed it up with the pot in the Control module.

Anyway, does anyone have a suggestion about what to do here? Since the servo itself seems to operate OK with an external battery at the extremes I assume it's OK and the fault is in whatever control circuitry is in the AV-50000 Control Module? Anyone else had this problem? Any suggestions?

I recommend first adjusting the speed pot to get the specified travel time between full up and full down trim, and then see if the problem still occurs.

Every servo is a bit different in how much internal drag it has. With how slow the PWM circuit is driving the servo, if it hits a point where the internal drag increases a significant amount it can be more than the circuit can handle to get it started again.

Additionally, make sure you didn't over tighten any of the cotter pined bolt locations to the point that it is adding friction to the system. I think the KAI specifys that they should be loose enough that you can rotate the bolt with your fingers but tight enough that there is no end to end play (see details in Section 5).

I had a similar problem, annoyingly on takeoff in what would have been my final leg into Oshkosh last year. Trim motor would happily move in one direction but not the other. Fortunately the RV12 is flyable out of trim so coming back to land wasn't a huge deal, but I had no desire to fly into busy airspace like that. I drove the final leg.

Giving it a tad more juice via the PWM control pot fixed it. I suspect it was probably marginal and a bit of dirt got in it and raised the static friction beyond what the previous PWM setting would do; in hindsight it had been pretty slow prior to stopping entirely.

Weirdly none of the engineers at the Van's booth at Oshkosh suggested this; took a while to figure out.

rvbuilder2002 said:

I recommend first adjusting the speed pot to get the specified travel time between full up and full down trim, and then see if the problem still occurs.

Every servo is a bit different in how much internal drag it has. With how slow the PWM circuit is driving the servo, if it hits a point where the internal drag increases a significant amount it can be more than the circuit can handle to get it started again.

Additionally, make sure you didn't over tighten any of the cotter pined bolt locations to the point that it is adding friction to the system. I think the KAI specifys that they should be loose enough that you can rotate the bolt with your fingers but tight enough that there is no end to end play (see details in Section 5).

Click to expand...

Good call Scott, thanks. One of the castle nuts was over-tightened. Corrected that and lubed the joints & hinges and all is good.

Hi Everyone,

My plane now has over a 1000 hours. I am replacing the trim servo and can’t get the new one to run. The new servo runs on a 9 volt battery. The old servo runs okay.
I have removed the linkages so binding is not an issue.
The next idea is to adjust the pots. How is this being done with all the Avionics harnesses installed on top of the module?
Any ideas would be appreciated. I cannot go back to the old servo as the Clevis is loctited to the shaft. The wires on the new servo are a heavier gauge to the wire from the plane.

Thanks
Julian
RV123016

I'm curious - Is Ray Allen control system PWM? I had this same problem about four years ago and ended up installing Ray Allen 2-speed trim controller. My trim servo would run fine on a 9V battery but very intermittent on the early SN 120058. Zero problems since and still original trim servo at 750TT.

Mine is a Classic RV12 with the Dynon D-180. I could not get the new servo to run consistently. Sometimes it would run one direction and not the other. The problem in the old style RV12 is that the magic box regulated the voltage to slow down the motor. and the potentiometers did nothing. So I bypassed the magic box and shelled out cash for a couple of grips and speed control aka potentiometer from Ray Allen. I tapped into the cigarette lighter style power port between the control sticks and it works great. I really like the trim on the stick too. Yes it was a little spendy but turned out fine.

If I were to do it again I probably would not have replaced the motor at all. The original one worked just fine.

The pwm circuit used to control the speed of the pitch trim servo on the RV-12 has a fixed voltage output. This is actually a much better method of controlling a dc motor compared to varying the voltage (which I think is what the Ray Allen speed control does but I’m not sure), because it doesn’t cause a reduction in torque output like a reduced voltage does.
PWM (pulse width modulation) in simple terms is just turning the voltage on and off quickly. The amount of on time vs off time factors into what the actual run speed is, but the motor is being fed full voltage.
If measured with a typical volt meter it will seem to be a lower voltage value because the meter reads the RMS value which in simple terms is the average voltage, over time. If an oscilloscope is used, you would see a pulsing output of very near current bus voltage.

The same pwm circuit is used on all versions of the RV-12. The only difference with the D-180 version is it has a single turn adjusting pot. which does make it more difficult to adjust precisely compared to the later version with a 25 turn pot.

printbr said:

Mine is a Classic RV12 with the Dynon D-180. I could not get the new servo to run consistently. Sometimes it would run one direction and not the other. The problem in the old style RV12 is that the magic box regulated the voltage to slow down the motor. and the potentiometers did nothing. So I bypassed the magic box and shelled out cash for a couple of grips and speed control aka potentiometer from Ray Allen. I tapped into the cigarette lighter style power port between the control sticks and it works great. I really like the trim on the stick too. Yes it was a little spendy but turned out fine.

If I were to do it again I probably would not have replaced the motor at all. The original one worked just fine.

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Make sure that when you replaced the servo you didn’t over tighten any of the hardware as mentioned earlier in this thread.

Hi Scott,
Do you have any ideas what I should try to do? I have a few ideas above which I will discuss with my helpers.
I slid the Dynon screen to one side and that allowed access to the trim pot screw. We tried 3 plus 3 turns clockwise and then 8 turns anti clockwise. No response from the servo. The old servo works fine when we sub it in to check.
Thanks
Julian 120316

Disconnect the servo and connect a small incandescent lamp in its place.
The lamp should be low wattage so that it doesn't overload the circuit.
The "Lisle 28400" test lamp available at auto parts stores might work, but I don't have one to critique.
Don't use a test light that has an internal battery or LEDs.
Watch the brightness of the test light as you adjust the trim speed pot.

rvbuilder2002 said:

Make sure that when you replaced the servo you didn’t over tighten any of the hardware as mentioned earlier in this thread.

Click to expand...

I tried to run the new motor before being attached to anything, In fact I installed and removed it several times and attempted to run it in several different configurations, attached and not attached.

The PWM would not consistently run it no matter what I tried. If I hooked up the old motor it ran fine. So there is something with the new motors that my old PWM did not like. The Ray Allen speed control has been working great now for about 9 months so far.

Thanks for your response and correcting me about the PWM.

J.Coles said:

Hi Scott,
Do you have any ideas what I should try to do? I have a few ideas above which I will discuss with my helpers.
I slid the Dynon screen to one side and that allowed access to the trim pot screw. We tried 3 plus 3 turns clockwise and then 8 turns anti clockwise. No response from the servo. The old servo works fine when we sub it in to check.
Thanks
Julian 120316

Click to expand...

If you have a Skyview airplane with the AV-50000 module, the speed control pot is 25 turns. Because of that, 3 turns one way or the other wont have much effect.
It sounds like your new servo has more internal drag/resistance than the old one.
Try turning the speed pot to max. by turning it 25 turns counter clock wise, and see if the servo runs.