Servos

[Chris Theron]

Some interesting info from a South Africa mailing list

Regards

Chris Theron

----- Original Message -----
From: "John Monk" <jsmonk at csir.co.za>

 I did find the following article that I thought worth sharing with the
group. It come from Brian Taylor in Aus.
>
> In a recent job I got to test 50 servos from several manufacturers (Hitec,
> JR, Futaba, GWS, Volz, FMA) for a UAV manufacturer.
>   Perhaps this summary will help.    I set up a 100 hour test with the
> servos cycling +/- 45 degrees once per two seconds lifting a 1 kg load on
a
> 10 mm moment arm.  In normal flight conditions there would be many small
> deflections rather than the continuous sweeping I used in this test.  The
1
> kg.cm torques are fairly representative of flight loads on a small UAV or
> RC model.
>  All tests were done at 5.00 volts with each servo having its own 1 amp
> power supply and a 1000 uF cap to supply a short time peak current a bit
> over an amp.  Note that digital servos can pull considerably more than one
> amp when stalled but running in this test they drew less than one amp.
>
> 1.  There is NO true standard in the hobby RC servo industry except that
> 1500 uSecs was the centre position on ALL servos tested.
> 2.  1000 to 2000 uSecs works with EVERY servo tested.  900 to 2100 or 800
> to 2200 worked with SOME analog servos but caused almost instantaneous
> destruction with newer digital servos.  Digital servos draw 4 or 5 times
> more current in their quest to get to the set point faster.  If the
> internal gearboxes, or a binding linkage,  prevents this, they go into
very
> rapid meltdown internally.
> 3.  Just because your new fancy transmitter lets you dial in 120% throws,
> do NOT assume your servos will comply.  Digital ones may die.
> 4.  Rotation angle is not standard, even within one maker's range.  1000
to
> 2000 uS can be +/- 45 degrees, +/- 60 degrees or even +/- 90 degrees.
> 5.  All nylon gearboxes typically have lowest backlash and remain tight
> over 100+ hours of cycling.  Metal gearboxes wear substantially over 100
> hours.
> 6.  Metal shaft/metal gear servos like the Futaba micro servo are very
> robust but the metal shaft conducts interference into the pot and the
> servos chatter with nearby RF interference - bad news on a UAV with
> on-board transmitter until you fit ferrites & bypass caps.
> 7.  Hitec servos, in general,  have motors far too powerful for their
> gearboxes and instantly strip gears if linkages bind.  Other servos may do
> this but Hitec stood out as poor in this regard.  Volz failed in the
> shortest time under load tests from electronic failures, not gearbox
> troubles.
> 8.  Ball bearing servos performed no better than servos with the output
> shaft just rubbing on the plastic case.  Observed case wear on the no
> bearing Futaba and GWS servos was negligible.
> 9.  Driving the servos with 50 Hz refresh rate gave 100% of makers specs
> for response time and torque.  Driving faster (only went to 60 Hz) did not
> improve response times.  Going down to 25 Hz refresh rate worked for all
> servos tested but holding torque and response rates suffered.
> 10.  Lowest power with highest speed was to drive the servos at 50 Hz rate
> until into position then drop the refresh rate back to 10 Hz.  Only works
> for lightly loaded servos however.
> 11.  Price was absolutely unrelated to lifetime.  The most expensive
(Volz)
> failed first (all three of a sample of 3 at 5, 22 and 35 hours).
> 12.  Cheaper servos have more backlash when new and tended to have highest
> backlash at end of test.  Backlash was very small in every servo tested
and
> your linkages are guaranteed to have more slop than the servos.
> 13.  Digital servos have a genuine 1000+ steps between 1000 and 2000
> uSecs.  Analog servos gave 500+ steps from cheapest to most expensive.
> 14.  How long do servos last??   Unless you physically stress them by
> manually moving the output arms, you can be almost certain to get 75
> continuous hours.  That is probably plenty for normal RC hobby flying but
> for UAV use I would suggest replacement at 50 hours maximum.  Your mileage
> will vary depending on loads, vibration (the wiper on the feedback pot can
> gouge a pit into the track in high vibration), power supply voltage and
> current limits, temperature extremes, moisture ingress, etc, etc.
> Lifetime could be as low as 10 hours if you insist in pulling the full
> rated torque and loads out of the servo with every movement.
>
> >>> "Evan Shaw" <newint at mjvn.co.za> 06/09/03 02:17PM >>>
> I'm in the market for two nice strong micro servos for my Prodij. The
JR371 that I have in there at the moment strip gear much to often for my
liking and I'm sick and tired of replacing them.
> Any suggestions and prices??
>
> Cheers
> Evan
>
> >>> "Evan Shaw" <newint at mjvn.co.za> 06/09/03 02:17PM >>>
> I'm in the market for two nice strong micro servos for my Prodij. The
JR371 that I have in there at the moment strip gear much to often for my
liking and I'm sick and tired of replacing them.
> Any suggestions and prices??
>
> Cheers
> Evan
>
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