Snillen Spekulerar

[wrep]

"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.