[NSRCA-dist7] Servo replacement / wear - how to tell

Thu May 22 15:30:17 AKDT 2008

I just changed my elevator servo, more as a test to see if I could tell if a
new servo was 'better'. I have flown the same elevator servo since the start
of the A-Shadow; 8411SA. I notice no difference 'on the ground', but have
not flown the new servo yet. Not sure how many flights I have on the servo,
but it's a lot. 

I will put the old servo on a long arm and run the servo test and look for
any issues at center. They may tell something.

sc

  _____  

From: nsrca-dist7-bounces at lists.nsrca.org
[mailto:nsrca-dist7-bounces at lists.nsrca.org] On Behalf Of Troy A. Newman
Sent: Thursday, May 22, 2008 9:41 AM
To: 'CA, AZ, HI, NV, UT'
Subject: Re: [NSRCA-dist7] Servo replacement / wear - how to tell

See answers below:

There was some of the typical RC discussion at Hemet - and one topic that
came up, as it usually does, is servo wear and replacement. With today's
digital servos, I have the following questions about digital servo
replacement;

1) what are the symptoms of a digital servo that is starting to wear out?
    - poor centering ?     YES 

        - does this equate to a louder 'buzz' at center , NO Less buzz. The
pot is worn and the servo think its at center so it doesn't try to drive to
center.

        - is the only way to tell this to put a servo on some sort of
protractor and look at the centering?   NO run the Servo test feature in
your TX. If your radio doesn't have one then get a  JR MatchMaker device
about $20. You can plug the servo into it and cycle the servo at various
speeds. You will see hesitations and stopping at center with a worn pot.
This is where the servo spends most of its time and vibes will wear on the
pot at this "center point". A protractor device can help you see it, but if
its bad enough to need replacing you can see it with your naked eye. The
servo will stop at center as it passes through. A new servo will just pass
right through at the same speed with zero hesitation and no speed change.

        - does thee servo draw more current?   No they draw less current.
The way a servo works is the servo sees its position and it sees the
commanded position to go to. This information is given to the servo by a
feedback pot. A small potentiometer in the servo that is attached to the
output gear. If the servo is already there very little juice is required to
maintain this position. As loads increase on the flight surfaces like
sitting on the ground the weight of the aileron will create a load that is
trying to move the servo off its commanded position. So you get the buzz.
The servo is giving power to drive back tot he commanded location. Digital
servos sample this pot position like 5 times more often than an analog
servo. Each time the position is sampled a small little jolt of power is
applied to drive the servo back to its commanded location. This is why
digital servos offer better holding power and centering. They tell the servo
to go to its commanded position more often, and have less deadband in the
middle where it says that as close as I can get it.  If the servo pot is
worn then the servo thinks that a wide area in the worn spot on the pot is
good it then stops trying to drive the servo to the commanded position. It
gets it close and thinks its there. This problem Goes back to the poor
centering.  So it stops 0.1deg short or goes 0.05degs long.  As the pot wear
gets worse the dead spot gets larger and the error gets more. Yet the servo
still thinks its at the commanded location. This is not servo deadband...but
the worn pot can play havoc with the servo and its deadband.

2) What tests can be performed either 'in the plane' or 'outside the plane'
to determine if a servo needs to be replaced?
    - In plane testing would be ideal, but we must compare against a 'new'
servo, so maybe this is not practical. 

The servos can be tested inside the plane...A pointer or throw meter device
can be used to see the servo slow or hesitate at center. Using the servo
monitor and the servo test function in your TX  the servos will cycle slowly
and you can see the hesitations. The MatchMaker device is a servo tester and
servo driver. It works very well for this function. 

Tony Frakowiak taught me that when the servos start to fade away the model
will not lock on nearly as well. I didn't believe him until I flew the model
both ways. It can make a huge difference.  Another thing is that as the pots
wear the end point adjustments will wander. So your models TX setup will
change over time. Aileron differential can suffer and even mixing value will
suffer as the models usually have small percentages of mix. The servos are
trying to operate in the "worn spot" Where I see the biggest problem is on
4/8pt roll elements they are just not crisp and clean.

yes even the Flower powered models will suffer from this problem. There is
vibration, it may not be as bad as the Glow powered models but it is still
there and the pots and servos will wear. I flew Electric for a year and
still saw servo changes with time.

Another factor is the gear trains will wear a little. Metal gears much worse
than nylon. As the gear trains wear the servos will fight to get to
commanded location and the gear slop will constantly over shoot to commanded
location. So the servo will get more pot wear and it can use more current in
this situation. The constant overshoot will have the servo fighting itself.
This can also happen with the worn pots. Turn on the model and one surface
just constantly oscillates. You either have a problem with your linkages or
the servo is worn. By the way its common with poor rudder cable setups to
get this problem on rudder servo in just a few flights. This is usually not
the servos fault but operator error and his cable setup is not good. A poor
pull pull setup can literally eat a rudder servo alive.

This is a reason to use the JR "SA" servos these servos have nylon gears and
perform better in the vibration situations.

Horizon Hobby can replace the gears and pots for you. Its usually a fee of
under $20 depending on which servo it is. When you consider a $100 servo the
fee to make it like new again is small. 

Troy Newman
Team JR

  _____  

From: nsrca-dist7-bounces at lists.nsrca.org
[mailto:nsrca-dist7-bounces at lists.nsrca.org] On Behalf Of Scott
Sent: Thursday, May 22, 2008 8:05 AM
To: dist7 at nsrca.org
Subject: [NSRCA-dist7] Servo replacement / wear - how to tell

There was some of the typical RC discussion at Hemet - and one topic that
came up, as it usually does, is servo wear and replacement. With today's
digital servos, I have the following questions about digital servo
replacement;

1) what are the symptoms of a digital servo that is starting to wear out?
    - poor centering ? 
        - does this equate to a louder 'buzz' at center
        - is the only way to tell this to put a servo on some sort of
protractor and look at the centering?
        - does thee servo draw more current?

2) What tests can be performed either 'in the plane' or 'outside the plane'
to determine if a servo needs to be replaced?
    - In plane testing would be ideal, but we must compare against a 'new'
servo, so maybe this is not practical.

Any thoughts on the above would be considered.

Thanks

sc

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