vkargin

I need to read the displacement of the control surfaces for some static analysis during flight.
I can't add potentiometers to the control surfaces due to lack of space.
I can convert output of the servos to control surface deflections.
So, how can I obtain servo displacements(output of servos) ?

Gulliver

Put the servo on a "Y" connection. Connect the other branch if the "Y" to a pulse width analyser. The pulse width will be proportional to the displacement of the sevo.

Campgems

I'm not clear what you are asking. Are you trying to get a comparison from stick movement to actual movement?


If that is the case, stick movement other that at full mechanical travel and thumbs off center is not very precise, so measureing the surface movement at some deflection other than these three positions would not result in much useful data.

Given the premis that the servo is powerful enough for the job at hand, bench measurements would equal in flight measurements would they not?

Don

tkilwein

Look at the Eagle Tree Seagull system to see if it will work.

vkargin

The Y-cable and "Eagle Tree Seagull" system seems to be sufficient solutions. It seemed to me that I will be measuring the signal send to the servo, not the actual response of the servo. Time delays and saturations will be ignored by this method, won't they?
However,I believe this method will be accurate for static flight analyses.
Don, I am not interested in stick movement. I want to log servo measurements during the flight. I have to make bench measurements to get a relation between actuator deflection and servo reading but I need to log positions of actuator on flight to find trim conditions of the aircraft, anyway.
Thank you for your responses, you were very helpful.

Volkan

iflyj3

My Feedback: (7)

If you are going to measure the pulse sent to the servo by the receiver you might as well measure the pulse from the transmitter as they will be the same and save the down link problem. If you are using high powered digital servos, you would be very close. Weak analog servos, not so much, as they may not move to the actual desired position due to air load.

A feed back device on the surface is the only true way to measure of what you are wanting. To accomplish this with out another device you could interface to the feed back pot in the servo. Yes, you would have to have a custom circuit to do this.

vkargin

Thank you Don. You are very helpful.
I have one question about what you said. Are you sure analog servos have worse positing than digitals because the load on them change?(except saturation cases)
I am just a beginner on these subjects but if the servo is using a PI controller, steady state error should be compansated by the integrator(I) term moving the servo to its actual location at any condition.
In a futaba document, I read the resolution(sensitivity) of digital servos are higher which increases the position accuracy. You can find the document below.
www.futaba-rc.com/servos/digitalservos.pdf

Volkan

iflyj3

My Feedback: (7)

Are you sure analog servos have worse positing than digitals because the load on them change?

Absolutely, in addition they are not as tight at rest as the digitals are on the bench or in flight.

The dead band of the older so called analog servos is greater that the new digitals. That means you can have a small change of the transmitter stick that will not cause the servo to move. In sport flying that is not real important, but if you are interested in absolute surface position it is.

The new digital servos are using microprocessors for control whereas the old analog servos do not.