Is there a new breed of servo ?

[Andy48]

Posted by Tim Ballinger on 09/03/2020 15:36:27:

While compiling my spreadsheet of likely solutions I eventually found a current consumption quoted . It was for the Corona DS919MG - a 12g servo in the class I am looking for albeit the Torque at 1.7 is a little low compared to others. However the quoted current consumption is 200 -240 mA.
So at 4 servos all waggling its only around 1A which is well within scope of most SBEC or esc/SBEC combinations. I am therefore going to assume all in this class will be similar.

Tim

Is that torque really low? 1.7 -presumably kg.cm means the servo would stall when a load of 1.7kg is placed 1cm from the centre of the arm. Most of my servos only have a maximum distance of 1.5cm throw, so they will clearly handle 1kg. 1kg is probably more than enough to rip a servo arm out of an aileron or rudder on a normal sized sports plane for instance. Indeed it would probably rip the whole aileron out of the wing.

While I have never actually tried calculating it, I suspect the force on an aileron/elevator or rudder on a sports plane going at full wack is far lower than the above figures.

[Tim Ballinger]

Andy,

Not sure I would like to hang 1kg off a servo or control horn either however my comment was in comparison to many others which are quoting @3kg.cm ! and from such tiny form factors.
Sure someone will have tested these numbers are genuine but for me I am using them as a guide to how likely the servo is stall and hence jump from 200mA to around 1.2A.

I just tried querying which Chinese manufacturer produces the 4max mini/ micro servos (which all have these high torque specs )and George’s reply was “They are from a well known Chinese servo manufacturer
So far we have sold 991 of the 175DMG's servo in just over a year “

Presumably without any issues.

Tim

[Erfolg]

The forces have been calculated by Dr David Burton. He did emphasis that there are a number of issues. The first is the velocity of the air over the surface, the second was the width of the control surface and finally the area of the surface.

The general finding was that for many applications, that the forces are relatively modest, compared with the capabilities of some servos.

It will be the extremes where the issues are, pylon racing, F£b types of model, quarter scale and so on.

From memory he used F=0.5*Cl*V^2*Roe*S and then taking moments from the servo and the control arm on the surface.

It was noted at the time that back in the day, models of all types were successfully flown using servos of very modest torque and pretty unreliable radios.

Edited By Erfolg on 10/03/2020 15:30:29

[Braddock, VC]

I bought a couple of those cheap servo testers and link up 3 servos at a time to each, that's 6 all told.

Leave them running when I go in for a cup of tea etc for at least 1/2 an hour.

Any that get hot I complained to HK about and send them back for a credit.

I've sent back about 3 corona early ones and no turnigy ones so I always buy turnigy ones now if I need new servos.

I also have a stock of hitec 422, 322 and 311 s which I use, none of them overheated. I only do i.c. models btw.

FWIW the plastics on both corona and turnigy cases are inferior to the hitec and futaba servos I've had. How do I know?

I recovered 4 lost planes two fitted with either corona or turnigy or a mix, one with futaba and one with hitec and most of the lugs on the first two were broken, water had gotten in and shafted a couple, hitec and futaba worked straight away with no broken lugs or leakage. I still have one plane out in the Lydden triangle, I'll find it one day, if I live that long.

The consensus for losing some of the planes was that they dipped below the horizon and the failsafes cut in stopping the engine. One was structural and one was pilot error, well 2 if you count in I missed the structural damage.

[Martin McIntosh]

You do not say if the Corona and TGY ones were of the same power/size. What do you expect if you leave a servo running for 30min? The power has to be dissipated with heat because there is nowhere else for it to go. Not a fair test at all.

The cases are not meant to be crash proof so if you keep on crashing then expect some gear damage too.

[Nigel R]

If a servo can't operate under no load conditions for 1/2 hour I think braddock is being pretty sensible in rejecting it.

My take... I have trashed a handful of 9g types, assorted brands, in a foam 3d model with a mere 150w up front. That's in the air not hitting the ground. No specific control surface. Thus I wouldn't use 9g on anything larger than a park flyer. I'm also a bit mardy about putting budget brand kit to work in something I've sunk 150 hours of shop time into.

Other opinions are available.

[Jesus Cardin]

For those interested, in short time Futaba will introduce 2 new standar size servos to replace some of their popular and cheap servos:

- Futaba S-U300 to replace S148/S3003/S3005

- Futaba S-U301 to replace S3001/S3151/S3073HV

These two new servos will be cheap standar size plastic gear servos but with full S.BUS capability. This is you will be able to program chanel, angle, speed, reverse movement and a lot of other parameters individually, instead than from the transmitter software.

Of course advantages should be only for Futaba users; others or if you just do not want to tackle with new technology may use the servos as normal, no digital servos, as you take them out of the box.

Edited By Jesus Cardin on 11/03/2020 08:11:47

[Tim Ballinger]

The Futaba S-u300 is certainly at the right price point and up with the tech. Looks like it could move a mountain and is indeed ‘standard’ with regard to size and weight which is well in excess of the type (9 - 20g) I am personally musing over.

Tim

[Gary Manuel]

Posted by Braddock, VC on 10/03/2020 17:35:20:

I bought a couple of those cheap servo testers and link up 3 servos at a time to each, that's 6 all told.

Leave them running when I go in for a cup of tea etc for at least 1/2 an hour.

Any that get hot I complained to HK about and send them back for a credit.

I've sent back about 3 corona early ones and no turnigy ones so I always buy turnigy ones now if I need new servos.

I also have a stock of hitec 422, 322 and 311 s which I use, none of them overheated. I only do i.c. models btw.

FWIW the plastics on both corona and turnigy cases are inferior to the hitec and futaba servos I've had. How do I know?

I recovered 4 lost planes two fitted with either corona or turnigy or a mix, one with futaba and one with hitec and most of the lugs on the first two were broken, water had gotten in and shafted a couple, hitec and futaba worked straight away with no broken lugs or leakage. I still have one plane out in the Lydden triangle, I'll find it one day, if I live that long.

The consensus for losing some of the planes was that they dipped below the horizon and the failsafes cut in stopping the engine. One was structural and one was pilot error, well 2 if you count in I missed the structural damage.

Hi Braddock.

I'm not sure whether you are aware, but the issue of servos burning out when using a servo tester isn't a genuine test of their durability. Servo testers in some cases can actually drive the servo into it's end stops at maximum travel. This effectively stalls the motor for a short time during each cycle. The degree of stalling will depend upon the actual servo tester being used and the make/model of servo being tested.

Your Corona servos (not sure which model you have but SPECS are all similar) have a travel range of 40 degrees per side with a pulse travel of 400µs. What this means is that they centre with a pulse width of 1500µs and reach their minimum and maximum travel limits at 1100µs and 1900µs respectively. HITEC HS422 servos on the other hand, have a travel range of 900µs to 2100µs i.e a pulse travel of 600µs per side.

You do not say what make of servo tester you are using, but THIS recent thread has shown that some servo testers have a travel range of 800µs to 2200µs i.e. a pulse travel of 700µs per side. I dare say that some brands of servo tester have less travel than this particular one.

A typical transmitter / receiver has a pulse travel range of 1000µs to 2000µs (500µs per side). Even this appears to be enough to stall your Corona servos at full travel - so set them up and use them with care!

The Corona servos are clearly much more prone to servo stalling when using a servo tester than Hitec servos are due to their reduced travel range. Returning them to the supplier because you have burnt them out by continually over-driving them therefore appears a little unfair to me. I personally never use my servo tester on the auto setting for this reason.

One way to test whether your servo tester is over-driving your servos is to use the manual control on your tester and SLOWLY approach your limits (ideally with an ammeter in line) and observe what happens to your servo (and meter reading). Are there signs that you can move it even further by hand while at maximum travel? If not, it's reached it's limits and WILL be damaged by putting it on auto.

[Andy48]

A really sensible and informative post. Cheers Gary.

[Martin Harris - Moderator]

Interesting to see another report of casing problems with Corona servos - when I've mentioned this in the past I've been met with accusations of mistreatment/heavy handedness...although as I have not had problems with other makes I haven't been able to rationalise this.

My experience with them has been that they are fine mechanically but those lugs seem to snap off at the slightest provocation - often only becoming apparent when removing them for maintenance or re-use. In fact I did repair some with aluminium mountings epoxied to the cases and they have given great service.

Edited By Martin Harris on 11/03/2020 11:42:32

[Nigel R]

so, to be clear, the corona servos, they have weak lugs that easily snap, and they can burn out when used with a "normal travel range" because they only support 45 deg rotation either side of centre?

[Gary Manuel]

Posted by Nigel R on 11/03/2020 12:19:13:

so, to be clear, the corona servos, they have weak lugs that easily snap, and they can burn out when used with a "normal travel range" because they only support 45 deg rotation either side of centre?

 

The travel range isn't an issue (other than on a servo tester), as long as the travel limits (end points) are set up correctly as you would do with any other servo. I've used Corona servos before and had no problem with the lugs. Maybe some of them have Corona virus (sorry - couldn't resist).

Edited By Gary Manuel on 11/03/2020 12:29:34

[Tim Ballinger]

Gary,

Thanks for that information on servo testers and the potentially adverse interaction with servo travel limits.

I’ve never felt the need for a servo tester so I may be missing out but on the other hand I may have saved some grief should I have used one for reliability/ endurance testing .

Tim

[Martin Harris - Moderator]

Posted by Gary Manuel on 11/03/2020 12:27:23:

Posted by Nigel R on 11/03/2020 12:19:13:

so, to be clear, the corona servos, they have weak lugs that easily snap, and they can burn out when used with a "normal travel range" because they only support 45 deg rotation either side of centre?

 

The travel range isn't an issue (other than on a servo tester), as long as the travel limits (end points) are set up correctly as you would do with any other servo. I've used Corona servos before and had no problem with the lugs. Maybe some of them have Corona virus (sorry - couldn't resist).

Nice one Gary.

My radio has a larger pulse width range than the majority (750 - 2250 mS) and there have been no operational problems with the Corona servos I've used - and why would there if the servos are set so that they operate within their limits?

The only servo I've had burn out in use was a fairly well regarded Hitec HS225MG bought from the most reputable of sources.

Edited By Martin Harris on 11/03/2020 13:16:08

[Andy48]

Posted by Tim Ballinger on 11/03/2020 12:42:48:

Gary,

Thanks for that information on servo testers and the potentially adverse interaction with servo travel limits.

I’ve never felt the need for a servo tester so I may be missing out but on the other hand I may have saved some grief should I have used one for reliability/ endurance testing .

Tim

I always use a servo tester when installing servos in models. I use a digital one, set to the mid-point for setting up, and always test full throw for my transmitter in either direction. This is usually more than accurate enough to maiden will very little trim needed.

[Andy48]

Posted by Nigel R on 11/03/2020 12:19:13:

so, to be clear, the corona servos, they have weak lugs that easily snap, and they can burn out when used with a "normal travel range" because they only support 45 deg rotation either side of centre?

Ideally you should not take a servo beyond 30 degrees either side of centre, because of the geometry.

[Martin McIntosh]

Just removed some Corona DS 939MG`s from a model I no longer want (Peacemaker) and they happily move from 700 to 2300 ms on my HK LED tester.

[Gary Manuel]

Posted by Martin McIntosh on 11/03/2020 16:42:34:

Just removed some Corona DS 939MG`s from a model I no longer want (Peacemaker) and they happily move from 700 to 2300 ms on my HK LED tester.

That's good. They perform better than their specification (as should all servos). Some won't of course.

Edited By Gary Manuel on 11/03/2020 17:26:25

[Tim Ballinger]

Since we are on servo testers, well everyone else seems to have one and I’m still not sure what extra it gives me.

Admittedly I have to hook up a Rx but I can center servos, check limits and run repeated cycles with software on my Tx. Not that I ever run repeated cycling as I have never experienced an example of one failing after only a small amount of use.
Still feel I am missing something here. Is there some numerical feedback as to precision of centering or travel limits that the standalone servo tester provides?

Tim

[john stones 1 - Moderator]

I bought a servo tester, seemed a must have.

Sat in the drawer and never been used.

[Martin McIntosh]

For a long time I could not see the sense in buying one but now it is the first thing out when installing servos and retracts, especially servo driven ones. The 1500ms is a very accurate centre and corresponds to my Tx`s.

Certain electric retracts I bought would not respond to some Rx`s but they work fine on the tester. (HK MCR`s).

Much easier than connecting a Rx and setting up the Tx to get the control surfaces initially at centre.

[Don Fry]

I with Martin. Mine is taped to its battery. Plug it into the servo you are working on, set up the servo. Then when you fine tune to the transmitter, it's a small job. The control arms are correct. Centres are known and can be transferred.

But I still think, watching a servo slam from end to end is an abuse of machinery. By all means load it, but short of these pilots who can write their names on the ground, by strapping a marker on the rudder, its an unfair test.

Years ago, I was tasked with trialing Leyland Metros as a Panda car. Daft idea. I broke one every week, gearbox or engine broken. Same use as a servo tester on auto.

[john stones 1 - Moderator]

I'm still a meh, tend to have my TX/RX at hand anyway, when installing servos, can't check the flappy/waggly bits without em.

[Gary Manuel]

I'm also with Martin. I have a servo tester and use it regularly while building. It allows me to set up a complete wing including servo arm positioning, control rod length and check for good movement throughout the servos full range. I can then lay this aside knowing that I just need to tweak sub trim and travel on the tranny later - I know already that geometry is ok. Same goes for other servos.

Also handy at field for diagnosing faults i.e. why isn't that servo moving? Is receiver duff or is it the servo?

What I don't do is whap the tester onto auto mode and go for a cuppa!