Trio Avionics Autopilot Servo Oscillation Question

24 Aug 2005 Trio Avionics Autopilot Servo Oscillation Question

Do I need to do anything about a 2-3 Hz joystick oscillation that does not damp, caused by activating the aileron servo while the airplane is on the ground not in motion?

Thanks, guys,

-Bill Boyd

From: Jerry Hansen
Subject: RE: servo oscillation

Bill, that phenomenon is caused by the very narrow “deadband” in the Navaid servo. It is seen in some, not all, aircraft with fairly massive counterweights in the ailerons. It also varies with how free your controls are – i.e. if you have a little friction in the system it will probably damp on its own (not that I recommend putting friction into your control system). It can be eliminated or reduced by opening up the deadband area of the servo (an internal potentiometer adjustment), but his will also affect the responsiveness of the servo somewhat and reduce its performance characteristics. If you can tolerate the stick shake on the ground, I would say to leave it alone and get the better performance in the air.

What is actually happening here is common in classic servo systems that are “underdamped”, often for better response. When you turn the servo on (on the ground where there is no airload on the ailerons) the servo will move the ailerons to the neutral position. Because of the mass in the ailerons and the rapid movement of the servo it will reach the neutral point but the mass will cause it to overshoot a small amount. This will generate an error signal causing the servo to rapidly correct in the opposite direction (again with a small amount of overshoot). As this process repeats, you get the oscillation that you mention.

The narrow deadband does give the servo the ability to react quickly (as in turbulence) and to input very small corrections to the controls for extremely fine tracking control.

We can give you directions on how to reduce or eliminate the effect if you want. However, as mentioned, It may reduce the performance some in the air. Because there is no need to turn the servo on while on the ground, except for the preflight check, you shouldn’t find it to be a big problem in normal operation. It will definately not harm the autopilot system in any way.

If you’d like more information, please let us know.

Best Regards,

Jerry Hansen
Trio Avionics
http://www.trioavionics.com
Phone – 619-448-4619