A very intresting read I found about servo's and BEC.

[rmdesignworks]

Im all for the test Charlie however one parameter is off...the horn/linkage to the scale..unrealistic to our installations...a more advanced linkage is needed to see real world numbers for how we use the servos....1" servo horn as you listed...linkage to second arm to replicate the knuckle and O.D. of our most typical comp tire...so the second linkage should be 2.75" long...the second arm should pivot parallel to the servo horn as in the best mounting situations we use...the arm will pivot at one end and the linkage form the servo horn will attach 1" from the pivot point...then a second linkage attaches to the opposite end of the second arm to the scale...this will replicate a Rover pushing against a rock which is the most typical resistance we encounter. One number that should be considered as well is the lateral weight bias of a rig as it is pushed sideways by a servo in steering actions. As I am thinking now it may be better to use a proportional pivot table with load springs and a variable source of scrub and applied weight values so the servo can really be shown to work for the desired torque vs. load vs. input/output vs. amps. and voltage. I actually think the pivot table would be much more valuable for delivering real world numbers and values.

 

[Duuuuuuuude]

I think you're over thinking it.

 

[Duuuuuuuude]

lemme 'splain...

Electric motors make their most torque and pull the highest amps when they are stalled. Once they start moving both torque and amp load start to drop.

All that Charlie needs to find during his test is supply voltage, voltage drop, stall torque, and stall amperage. With that information we can determine how hard we are pushing our gear.

All that other stuff you mentioned would really only give us data for when the servo is cycling, which would be less stressful on the system compared to a full on motor stall.

 

[rmdesignworks]

I think you're over thinking it.

No more of an extreme for this question than to draw imaginary lines through the link geometry of a rig to find roll centers etc...just different applications of the science.

 

[rmdesignworks]

Electric motors make their most torque and pull the highest amps when they are stalled. Once they start moving both torque and amp load start to drop.

All that Charlie needs to find during his test is supply voltage, voltage drop, stall torque, and stall amperage. With that information we can determine how hard we are pushing our gear.

Agreed...however....the knowing the power consumption during a set parameter of cycles would give a better understanding of how the system surges and why. Based on the age old question..."Do I really need a bec?"

For those of us that know certain opening data the rest is common sense and learned base line knowledge but for those who dont understand how our electronics work and why Rx's stall and glitch and why internal bec's are crap for the most part...this would be the "ah ha" moment as it were.

 

[Duuuuuuuude]

Agreed...however....the knowing the power consumption during a set parameter of cycles would give a better understanding of how the system surges and why. Based on the age old question..."Do I really need a bec?"

For those of us that know certain opening data the rest is common sense and learned base line knowledge but for those who dont understand how our electronics work and why Rx's stall and glitch and why internal bec's are crap for the most part...this would be the "ah ha" moment as it were.

The only reason you would care about what is going on within the cycle is if for some odd reason you achieved something greater than stall torque and stall amperage, but its not going to happen.

If you know what the maximum amount of current drawn is, then, in this case, that is damn near all you need to know. As long as the equipment you're running can handle that then you're golden, if not, you're eventually going to go "poof".

Geometry doesn't matter, tire compounds don't matter, none of it matters. A servo will only do what a servo can do. If it provided 400oz at 1" on 6v, then that is what you'll get. You could attach either a feather or a 3/4 ton pickup at that 1" mark, and the servo will still only provide 400oz.

I think what you've got rolling around in your head is how that torque is transmitted through the steering system, which really has no impact on this test at all.

 

[Charlie-III]

OK guys.......relax a bit.

I'm looking for a few items here.

1-Do a selection of servos (7955, 7954, 7950, BLS197V?) provide ~listed torque at a specified voltage.

2-What kind of amps do these pull at stall.

Soooo......the 1" arm ~90* to the digital scale should get me a reasonable "Xinoz" @ "Y volts". If my numbers are fairly close to listed spec for idle & moving, I will assume stall is reasonably accurate as well.
I realize I will need to allow for arm arc as I'm moving the scale under load.

The stall amps may/may not exceed the output of the BEC causing them to fry. Curious to see what I get.

A stable PS (better than a BEC) should help here.
Yes, I can track volts either at the servo connection or at the PS output. If there is a voltage drop in the servo wires or connection, it should be similar for all servos I test.

 

[JohnRobHolmes]

Sounds like a great test with a simple test bed to build! I would agree that measuring the voltage at the servo would be a useful measurement. Measuring at the servo lead or shunt would be good enough.

You shouldn't have to account for the movement of the servo arm if it is just stall torque. I would assume your load cell doesn't deflect more than 5 thou, maybe 15 thou at the most.


I'm fairly curious as to the BEC voltage under load. When I originally did the testing for Castle, I logged everything and found voltage swing at 6v to be negligible.

My old test bed:

Castle BEC test 100lbs by Holmes Hobbies - YouTube
<iframe width="420" height="315" src="http://www.youtube.com/embed/32SNVXklxxs" frameborder="0" allowfullscreen></iframe>

 

[DogSpoon]

Why test servos ourselves when the manufacturers have likely already done it. Can't we just write some emails and ask them to publish specs? And why stop at servos? RC is stuffed with un-specified components. Can anyone find a spec sheet telling how much current a sidewinder can handle or even how many winds in just about any CC motor? Specs are so scarce in RC, usually the best you can do is throw a bunch of components together and hope for the best.

 

[JohnRobHolmes]

Why? Because it is interesting. Manufacturers don't specify stall amperage often, either.

 

[Charlie-III]

A few thoughts regarding some posts here related to the tests.

1-I am looking for some of the extremes "comp servos" will do powerwise, sorta puts to bed, "Do I need a BEC?".;-)8)

2-Looking for why some of us have killed CC BEC's, is it a bad BEC or is the servo killing them?

3-Doing a test with the whole linkage can be a slippery slope. There are many angles involved, different tires, different grip surfaces, different weights, etc.
-Are you running a long horn, or short.
-Long linkage or short.
-Stock knuckle arms or longer/shorter.
-Is the horn connection point above/below/even with the linkage at neutral
The list goes on.:roll:

4-JRH, I'm using a decent digital scale, thus there is travel in it. I would love to do a loadcell, but not in my current budget. Since there is travel I want to make sure the arm angle is correct (90* to scale surface) at max force. I do not yet know how much travel there is though.:roll:

5-Whether I measure voltage at the PS output or closer to the servo, it's all relative (I feel) since each test will use the same PS, wiring & connections. Any voltage drop should be very close, thus a moot point.8)

Likely I will test at listed voltages, common would be 6 & 7.4. Whatever I test at, values will be listed.

 

[Charlie-III]

Why test servos ourselves when the manufacturers have likely already done it. Can't we just write some emails and ask them to publish specs? And why stop at servos? RC is stuffed with un-specified components. Can anyone find a spec sheet telling how much current a sidewinder can handle or even how many winds in just about any CC motor? Specs are so scarce in RC, usually the best you can do is throw a bunch of components together and hope for the best.

So, if it's on a spec sheet it must be so?? Just ask in the "MaxAmps thread"!:shock:;-)

Why? Because it is interesting. Manufacturers don't specify stall amperage often, either.

True, some list specs (but no procedure) but most don't post "worst case" which to me is stall."thumbsup""thumbsup"

 

[Hardline]

Not that it's a good substitute for proper testing, but couldn't we just put an inline fuse between the battery and CC BEC? You could play with it at smaller fuse values to see what blows and what doesn't, and put an 8-10 amp fuse in for normal use so that those rare electrical "events" don't cause smoke.

 

[Charlie-III]

Not that it's a good substitute for proper testing, but couldn't we just put an inline fuse between the battery and CC BEC? You could play with it at smaller fuse values to see what blows and what doesn't, and put an 8-10 amp fuse in for normal use so that those rare electrical "events" don't cause smoke.

I guess you could fuse it, but I don't think we know if it's the input side or output side. So maybe 2 fuses (people complain about not enough room now.....)

If we find the servo's are overdrawing the 10A BEC, it would guide people to use the CC BEC Pro.

 

[Hardline]

I guess you could fuse it, but I don't think we know if it's the input side or output side. So maybe 2 fuses (people complain about not enough room now.....)

If the bec receives servo demand for excess current, it draws that current from the battery. One fuse takes the battery out of play. Why do you need a second?

 

[winnerone23]

I'm watching Charlie! Were watching...

 

[Charlie-III]

I'm watching Charlie! Were watching...

Yep, me to.

I did some basic work, but off on work trip for at least a week. More to come.

BTW, great job @ Rausch the other week. It was good to see you there. I'm making some stuff for Calvin, I "may" have material to do 2 sets (you should know what I mean).;-)

 

[fast540]

ive got some large fast planes that run lots of servos on a ccbec with no issue and while they dont stall the servo like a crawler blowback on a big 160mph plane is pretty crazy

 

[Stregone]

Why? Because it is interesting. Manufacturers don't specify stall amperage often, either.

It also keeps 'em honest.

 

[strongwind]

yes,it is very interesting,thanks for your sharing