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Stall current

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Old 02-17-2007, 01:53 PM
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BillS
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Default Stall current

Stall current

Does anyone know where the stall current can be found for Futaba servos? Below stall is the current linear with torque on standard analog servos?

Thanks.

Bill
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Old 02-17-2007, 04:43 PM
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Default RE: Stall current

I tested some Futaba servos. The results are available on www.servotestsite.com
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Old 02-17-2007, 07:31 PM
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Default RE: Stall current

I was curious so I plotted the current Vs the torque for the 9152:
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Old 02-17-2007, 10:10 PM
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Default RE: Stall current

dirtybird,

Thanks for the very good information. Looks like one might generalize that torque is reasonably linear relative to current as long as the extremes at both ends of the range are avoided.

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Old 02-18-2007, 10:04 AM
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Default RE: Stall current

Actually I think it is linear at the bottom end and trails off at the high end where things start to saturate. The hook on the bottom end of my curve is caused by the function formula I used to plot it.
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Old 02-18-2007, 10:58 AM
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Default RE: Stall current

dirtybird,

You have a tremendous amount of effort in servo evaluation. You already know that servos are expensive and most guess at the requirements and overspend on servos both in torque and number of servos per surface.

Have you evaluated boost tabs and their effectiveness at reducing servo loads? The proponents suggest that servo load might be reduced by as much as 85% with a passive tab. While the 85% number is probably overly optimistic the concept seems sound from an engineering perspective and uses the slipstream to counterbalance the load against the servo.

I am attempting to measure the effectiveness of a simple boost tab by measuring the servo current with and without tab. If it works $3 in parts to replace $100 additional servo expenditure seems like a good approach.

Thanks.

Bill


I often wonder why we commit a lifetime of knowledge to such mundane activities.
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Old 02-18-2007, 12:51 PM
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Default RE: Stall current

a note here on the power to the servos --
The tests shown were from a regulated power supply -as it should be
Any test needs specific inputs or the test is no test at all.
Many battery setups in models are way off providing the power delivered from the regulated supply .
Recently a modeler compalined and suggested a particular servo was putting out no where near the mfg's advertised torque - he was apparantly not aware of the battery-vs- power supply tests.
I have been testing various battery setups -to see what would do the best job of deliving constant voltage UNDER LOAD . So far , two 123 batteries are neck in neck with 4300 ma NiMh cells of extremely low impedance . NO regulated setups were tested .
both put out over 6 volts -- the 123's deliver 7.2 and th NIMH about the same (about ,because fresh charge is a little higher and they settle down to about 6.5-eventually .
both will HOLD the voltages even under prety good loads - If you stall the servos tho -watch out!! the 123 cells will be limited only by line restrictions -they can put out 60 times the rated ampere . the big much heavier pack ( NIMH) also will fry things.
Regulatee setups such as LIIons with higher input outputs will put out only what the cells used and the effective regs wil handle
If the cells will put out- say 50 amps and the servo can grab say - 40 amps and the regulator is designed for 30 amps --guess what can happen?

this is only a example -using a non real world setup but the idea I wanted to present is that all the components must be capable of handling the total power the servos(s) -in your particular setup, can grab.
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Old 02-18-2007, 02:09 PM
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Default RE: Stall current

Dick, I am sure that you are correct but operating at a much higher level than my investigation. The servo used is an S-148, the batteries are 4 cell NiCad and the stall current is 600ma.

I am testing to determine if boost tabs reduce the relative torque required by the servo to hold a surface deflected.

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Old 02-18-2007, 08:25 PM
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ORIGINAL: BillS

Dick, I am sure that you are correct but operating at a much higher level than my investigation. The servo used is an S-148, the batteries are 4 cell NiCad and the stall current is 600ma.

I am testing to determine if boost tabs reduce the relative torque required by the servo to hold a surface deflected.

Bill
The S148 is a good servo but its no load deadband is poor. (About 13 us) I would think you should be working with a coreless such as the 9202 or even the sport digital(3151). I would think the deadband would be critical to what you are trying to do
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Old 02-18-2007, 08:31 PM
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Default RE: Stall current

ORIGINAL: BillS

dirtybird,

You have a tremendous amount of effort in servo evaluation. You already know that servos are expensive and most guess at the requirements and overspend on servos both in torque and number of servos per surface.

Have you evaluated boost tabs and their effectiveness at reducing servo loads? The proponents suggest that servo load might be reduced by as much as 85% with a passive tab. While the 85% number is probably overly optimistic the concept seems sound from an engineering perspective and uses the slipstream to counterbalance the load against the servo.

I am attempting to measure the effectiveness of a simple boost tab by measuring the servo current with and without tab. If it works $3 in parts to replace $100 additional servo expenditure seems like a good approach.

Thanks.

Bill



I often wonder why we commit a lifetime of knowledge to such mundane activities.
I have not worked with boost tabs on a model.
I worked on the MD-11 fuel system for awhile. I was surprised to learn that the MD-11 had a back up manual controlled boost tab system so that the pilot could fly the airplane after a complete power failure.

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Old 02-18-2007, 10:22 PM
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Default RE: Stall current

That’s interesting about the MD11. My friend flew one for years.

I tested a rendition of boost tabs against NO tabs today and it appeared that in the 40/50 mph range there was about a 15% reduction in current to hold the servo in location. At 20/30 mph there was almost no current reduction. At 60 mph things started to appear interesting and it initially appeared the hold current was not increasing with speed. However the wind was 15 to 18 mph and readings were all over the map. Further testing will wait on a calmer day.


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