MGL Avionics User Forum

I am trying to setup up my iEFIS system with the MGL auto pilot servos. The servos are converted to capstan as my control cables have a lot of travel. 7.5” total travel for bank, 5” for pitch and 3” for yaw.

I would like to confirm that I understand the autopilot setup correctly in terms of range of motion for the MGL servos.

The range is 0-4095. As per autopilot manual, pitch and bank servos are limited to 180 degrees of travel . Do I understand the manual correctly that this translates to only 2047.5 units servo range out of the 4095 units available? The problem I have is that my control cables for bank and pitch travel further than the 2047.5 units. If I setup for example full left and right bank to the full travel of the controls it would be greater than 2047.5 units. Eg full left bank at 500 units full right bank at 3500 Units. The servo will than not test properly if setup this way. If I stay within the 2047.5 range it works fine.

The problem is when doing the preflight autopilot test and the controls are set outside the software defined range. Eg the elevator is full down - control column full forward. Let’s say I defined the range from 1000 to 3000. But full travel forward is 500 units. The preflight test then actuates in the wrong direction pitching down instead of up. If I start the test while the elevator is resting between 1000 and 3000 it works fine. This could be potentially dangerous if the AP would be engaged in flight outside the defined control range.

I do not see any reason why the capstan servo should be limited to 180 degrees of travel for bank and pitch.

Is there a possibility to extend the range of motion for the capstan servos via a software update?

Yes the active rotation is limited to 180 degrees (preferably a little less to cater for tolerances).

You must also make sure the position does not go through zero during the travel (you can rotate the arm or drum accordingly be means of the shear screw which has 4 positions).

The 180 degree limitation really only is relevant for the direction of servo travel in case of the drum. The servo will always try and go the shortest distance to a new position - normally that is a very small distance so there is no issue.

The limits you set are not really limits - in fact the autopilot does not care much about them other than using them to figure out which direction to move the servo and the limits are used for your preflight check function.

In other words - if your position exceeds the limits - nothing bad happens, the AP works as usual. You must just make sure that you don't go though zero and you can't do more than one 360 degree rotation. When you engage, you should engage with the stick more or less in neutral position - the AP follows your stick movement even if not engaged so it will take over when you engage at the current position.

The AP does not take the position as some sort of guideline - it only knows relativity. It will move the controls in whatever direction needed to achieve it's desired result - if you engage straight and level for example - it will happily fly a wing low and at whatever pitch angle is needed to maintain straight and level flight - subject only to speed restriction and maximum bank angle limits.

Hello Rainier,

Thank you for the explanation. I have my servos setup as per your explanation above.

Thanks again.

rainier wrote: ↑Mon Jul 10, 2023 1:04 pm Yes the active rotation is limited to 180 degrees (preferably a little less to cater for tolerances).
...
The 180 degree limitation really only is relevant for the direction of servo travel in case of the drum. The servo will always try and go the shortest distance to a new position - normally that is a very small distance so there is no issue.

I had a capstan installation where this 180 limit was a show-stopper. We needed more throw, and had to make a larger drum to get the increased throw at the expense of lower torque.

The software could be modified to allow almost 360 travel without seeking the shortest path through a no-go zone and even accommodate travel through zero.
This could be done in either the servo software itself or as a servo driver in the EFIS between the AP and the outside world.
The servo mechanical position would be normalised to always appear as a well-behaved servo operating in the above 0...below 4095 range.
The servo shortest path seeking would need to be disabled.
No change to the AP software itself.

Setup might require a few more steps than the current simple left limit/right limit settings, but actually with a few smarts in software it could learn the limits and go/no-go zones by simply sweeping the controls from one extreme to the other....done!

As someone once said- it's only software!