Servo current consumption

[GONZO]

The main limiting factor in current carrying capacity is the connector itself, not the cable. Cable gauge becomes a factor when one considers the length of cable run. The internal surface area within the connector is small as are the pins it mates with. For continuous load capability it is a factor of the contact resistance and the connectors abillity to disapate the heat generated. For almost instantaneous current bursts then it would be the contact resistance only up to the point when localised temperature rise would cause the contacts to weld. My own understanding is that servo connectors are rated at 4 A continuous (surge rating unknown). Then we get into the current carrying capabillities of the PCB land that the pin half of the connector is mounted on. On this point it is recomended that the Rx connection strip is feed from both ends from the supply, when running multiple high power digital servos, to distribute the load and provide redundancy/diversity.

I'll just pop down the air-raid shelter now, before the bombs start to faaaaallllllll

[GONZO]

Sorry, gone a bit off topic, from 'Servo current consumption' to 'current carrying capabillities of connectors'

Edited By GONZO on 06/10/2010 11:54:09

[Wingman]

Cheers BEB.

 

In any case it looks like everybody's standard method of powering the radio/servo gear  is more than adequate for what we need.

[Steve Hargreaves - Moderator]

I asked ModelPower about "standard" servo wires......this is what they said....

 

"Standard futaba servo cable
3x 0.14mm2 where as the thinnest we stock is 3x 0.25mm2

Strands 72 x 0.05

Max current 3a

AWG 26

Just to complete the circuit, they do an ultra light cable too

3x 0.08mm2
Strands 19 x 0.07

Max current 1.5A

AWG 28

 

Regards,
James Poll
ModelPower.co.uk"

[Steve Hargreaves - Moderator]

Tim wrote...

I think the PVC version is probably "standard" ?

Incidentally, the data I showed earlier was for 4 servos ( approx .5A ) the single servo loaded was 140m/a, so not sure if you were suggesting in your post above that each servo may be carrying .5A, hence you were talking about total of 2A ??

 

No Tim...all I was trying to illustrate is that the current drawn from the battery is cumulative & that whilst all our servos might not draw that much the cumulative current (that the battery/switch & associated wiring has to cope with) is much higher as it is the sum of the currents drawn by each servo...I use 0.5A for nice round numbers!!!

Edited By Steve Hargreaves on 06/10/2010 12:09:37

[Tim Mackey]

OK back on topic

As requested I did some tests with digital servos.

First picture shows the rig, and the data is of the standard analogue HS311 again, at rest, with no load applied. As before the current draw is 30m/a.

All pictures enlarge when clicked.

Next up we have the same servo static - but loaded with weights and as before, its pulling 140 m/a

Third picture shows the same again but the servo is being moved through its travel.

Current draw has almost doubled to 220m/a - this was a snapshot and did fluctuate between 180 - 250 m/a

 Next up we have a spektrum digital DS821 servo - unloaded and static - as can be seen the current draw is the same.

 

Now things get interesting.... when loaded up with the same weight and left static the current remains the same at 30 m/a - I was surprised at this.

 When weighted and moved through its throw, the current snapshot was 200 m/a. 

 

Approx the same as the analogue.

 I then tried a different digital servo ( tower pro metal gear MG995 ) and got the following

 snapshot of current draw  340 m/a.

 Finally I connected up all three servos, weighted both the digis, and stirred the sticks.

This represents a pretty extreme load - yet the total load showed as just 660 m/a or if you prefer, just over half an ampere ( .5A)

 

I did do other tests also, but results were pretty similar, so for simplicity I have not bothered showing them.

In conclusion, I was surprised that despite drawing more current when actually moving a load, the digitals actually took far less to simply hold their neutral position than the standard analogue servo. It also shows that there can be quite a difference in current draw between two apparently similar digital servos.

 

Right I'm off flying now up the Orme - Im all data'd out.

[Steve Hargreaves - Moderator]

Wow!!! I think I'll stop worrying then.....

 

I've been beating myself up over the size of battery to fit & would it supply enough current to power the servos or should I fit a Lipo & regulator & if I did would a 3A regulator be sufficient etc etc.......

 

Seems a typical 4 cell AA Rx pack will be more than adequate for most "Sports" applications then.....

[GONZO]

Tims tests show that ,in general, we over specify the capacity of our Rx battery packs. But, it is imperative that the pack used has a low internal resistance so that the pack voltage does not drop excessively when the current consumption goes up. DVM's do not show sharp negative spikes in the voltage. If Tim has (I think it unlikely - cost more than his modelling) a high speed storage scope it could be set (trigger set close to 2.4gHz Rx drop-out voltage) to monitor the +ve line. This, I would think, could be very informative. The importance of not over specifying the capacity lies in the fact that as the capacity of Ni-mh cells is pushed up (around the 2000mAh plus) the internal resistance increases and the cells ability to deliver current decrease. The changes in the cells internal construction to increase the capacity in the same dimensional constraints (thinner electrodes and insulation) also renders the cell less robust and more susceptable to failure from mechanical vibs/impact. This is fine in a Tx application where the current is constant and low (use Eneloop to get over self discharge probs) but I have started to move to A123 1100mAh two(2) cell packs (HERE and HERE) for Rx applications and A123 2300mAh two(2) cell packs (HERE) for more demanding Rx applications. A123 cells have low internal resistance and can deliver large currents; are physically robust and can be fast recharged at high currents. I comend the cells to the house!

Ah well, time to get back down the air-raid shelter.

[Biggles' Elder Brother - Moderator]

Thought you might be interested to learn that a theoretical calculation bears out Tim's results. Let's take a pretty standard analogue servo - the good old  S3001. Futaba's specs tell us the speed is 0.28sec/60 degrees and Tim is hanging a 1.5lb weight from it. Assuming it can still operated at its rated speed at this condition we can work out the power the servo needs:

 

Power=Torquexspeed

 

I.5lbs is 0.68Kg which gives us - 0.68x9.81= 6.67Newtons for the weight.

 

Assume the lever arm is 1cm then the torque the servo is generating to hold this weight steady is:

 

Torque=6.67x0.01 Nm = 0.0667Nm

 

Now for the speed. 0.28 secs for 60 degrees is equivalent to one revolution in 1.68secs. So multiplying by 2pi gives us the rotational speed of the servo in rads/sec - which is

 

1.68x2x3.14 = 10.55 rads/sec.

 

So the power expended by the servo is:

 

Power=Torquexspeed = 0.0667x10.55 = 0.7W

 

The servo wont be 100% efficient - so the electrical power in will be a bit more than this - say 0.85W. The speed and torque data for the servo was based on Futaba's figures for a 4.8v supply - so the current required by the servo under these conditions would be:

 

Current=Power/volts = 0.85/4.8 = 177mA. Not bad! That in broad agreement with Tim's experiment - amazing!!

 

Let's look at another condition - max torque output. If you take the servo's max torque of 33 oz-in as the condition after doing all the conversions and messing about you come out with the servo would expend 2.47W which at 4.8 volts is a current of about 510mA.

 

Using my previous calculation of the likely torque in moving an aileron on a sports model at a flying speed of 30mph - see here - you'll find the torque required is about 0.12Kg-cm which is 0.0011 Nm. Using the method above this would require a power of 0.12W, say 0.15W to allow for the efficiency factor, which at 4.8 volts gives us just 31mA!

 

So to summarise. For an S3001 powered at 4.8volts the theoretically predicted current draw would be:

 

Under Tim's test conditions: 177mA

Under maximum spec'ed torque: 510mA

Under typical sports flight conditions: 31mA

 

So the absolute maximum you could draw from 4 servos assuming they were all delivering max spec torque at the same time (very unlikely indeed!) would be about 2000mA. My batteries could do that for an hour!

 

While a typical in flight current draw from 4 servos would be of the order of 120mA.

 

Peanuts really!

 

BEB

[David Ashby - Moderator]

Posted by GONZO on 06/10/2010 16:07:26:

Tims tests show that ,in general, we over specify the capacity of our Rx battery packs. But, it is imperative that the pack used has a low internal resistance so that the pack voltage does not drop excessively when the current consumption goes up. DVM's do not show sharp negative spikes in the voltage. If Tim has (I think it unlikely - cost more than his modelling) a high speed storage scope it could be set (trigger set close to 2.4gHz Rx drop-out voltage) to monitor the +ve line. This, I would think, could be very informative. The importance of not over specifying the capacity lies in the fact that as the capacity of Ni-mh cells is pushed up (around the 2000mAh plus) the internal resistance increases and the cells ability to deliver current decrease. The changes in the cells internal construction to increase the capacity in the same dimensional constraints (thinner electrodes and insulation) also renders the cell less robust and more susceptable to failure from mechanical vibs/impact. This is fine in a Tx application where the current is constant and low (use Eneloop to get over self discharge probs) but I have started to move to A123 1100mAh two(2) cell packs (HERE and HERE) for Rx applications and A123 2300mAh two(2) cell packs (HERE) for more demanding Rx applications. A123 cells have low internal resistance and can deliver large currents; are physically robust and can be fast recharged at high currents. I comend the cells to the house!

Ah well, time to get back down the air-raid shelter.

 I use 'em all the time now too 

[Martin Harris - Moderator]

Posted by Steve Hargreaves on 06/10/2010 14:06:37:

 

Seems a typical 4 cell AA Rx pack will be more than adequate for most "Sports" applications then.....

 Always has been!

[Steve Hargreaves - Moderator]

Gonzo, you make an excellent point & this drop in voltage under load is what has prompted my misgivings about using AA cells. I think the A123 is an excellent solution but I worry about the higher voltage.....I know that people say that a 2 cell LiFE is equivalent to a fully charged 5 cell pack but I think this only tells half the story.....a fully charged 5 cell pack is probably over 7 volts but this will fall back pretty quickly under load to around 6 volts or so.....the A123, due to its excellent voltage stability, won't & will probably stay close to 7 volts no matter what load you place on it (within reason..)

 

I worry that this higher voltage will have a detrimental effect on the Rx & particularly the servos many of which state a maximum of 6 volts.....

 

Don't get me wrong I like the idea of the A123 cells & think it the right way to go....

 

As a user, care to comment??

[Steve Hargreaves - Moderator]

Posted by Biggles' Elder Brother on 06/10/2010 16:07:42:

 

Now for the speed. 0.28 secs for 60 degrees is equivalent to one revolution in 1.68secs. So multiplying by 2pi gives us the rotational speed of the servo in rads/sec - which is

 

1.68x2x3.14 = 10.55 rads/sec.

 

So the power expended by the servo is:

 

Power=Torquexspeed = 0.0667x10.55 = 0.7W

 

 Not that I am questioning you BEB...well alright I am ()  but it would seem from the above that a faster servo uses less power..??

 

If we do the maths with a faster servo...say 0.12s for 60 degrees we get 1 revolution in 0.72secs which equals 0.72 x 2 x 3.14 = 4.52 rads/sec

 

Power therefore equals 0.0667 x 4.52 = 0.3W....

 

Surely this can't be correct or have I missed something??

[Tim Mackey]

Posted by Steve Hargreaves on 06/10/2010 16:43:23:

Gonzo, you make an excellent point & this drop in voltage under load is what has prompted my misgivings about using AA cells. I think the A123 is an excellent solution but I worry about the higher voltage.....I know that people say that a 2 cell LiFE is equivalent to a fully charged 5 cell pack but I think this only tells half the story.....a fully charged 5 cell pack is probably over 7 volts but this will fall back pretty quickly under load to around 6 volts or so.....the A123, due to its excellent voltage stability, won't & will probably stay close to 7 volts no matter what load you place on it (within reason..)

 

I worry that this higher voltage will have a detrimental effect on the Rx & particularly the servos many of which state a maximum of 6 volts.....

 

Don't get me wrong I like the idea of the A123 cells & think it the right way to go....

 

As a user, care to comment??

 

Well as I use the 1100mahr A123 cells I will give my tuppence worth. They are good in many applications but there are warnings out there about the somewhat higher voltage level of two cells as raw power for radio gear. Its not ( generally ) the Rx that complains, but some servos are NOT recommended for 6V+ operation.

I did make this point in my latest article in the special edition of the mag.

2s A123 cells when fully cooked are at 7.2V, and due to their high current capability together with a LSD ( low self discharge), will "sag" very little from this. Even the nominal figure ( 6.6V ) is slightly above the recommended maximum of some servos.
People ( including me in the past ) have cited the freshly charged level of 5 cell nickel pack as also being higher than recommended...however this level drops very quickly as soon as they are connected and used.

Now I am not scaremongering and admit to using 2s a123 on several models with no ill effects on the servos YET.  Often an electronic device will perform perfectly well for a period of time despite being overpowered EG: in my tests yesterday I accidentally hooked the 3s lipo direct to the Rx and analogue hitec servo, and operated it all for a minute or two before I realised .

All appears fine, but sometimes it takes a while before overstressed components fail.

Soooooo -  although I may well use the a123s - the usual disclaimer applies, and YOU the end user must decide for yourself if your servos will be consistently happy with the extra power. 

I for one feel a responsibility to give sound advice whenever I can, so in other words ....dont blame me if you use them and fry your servos !

[GONZO]

Voltage of A123 cells straight off charge is 7.2v but falls very rapidly to 6,6v (3.3v/cell) without load. The discharge curve is vey flat (much flatter than Ni-mh), so much so that battery checkers are no use. Ni-mh cells start at a simillar voltage but after an initial drop continue on a gradual decline and pass through the A123 voltage of 6.6v as they discharge.

Take a charger to the field and recharge after a flight and note how much you put back in mAh. Do this for a number of flights and you will soon get a feel for how many flights you can do on a fresh set of cells( bet its more than you do on a flying session). As to the Rx and servos coping with the voltage; all my Futaba/Hiteck/Jeti and GWS 35mHz Rx are OK; all my Spektrum 2.4gHz Rx are OK (of course). Servos:- (analogue) S3001, S3004, S3010, S9001, 148, 30M,HS645 etc; (digital) HS5645, SC-0251, HD9150, MG996r, haven't found any yet that burst into flame and let out the magic smoke. But, as always, check for yourself. I doubt any sport size servo would not be OK.

I think I'll just put my tin hat on for this one (I'm feeling brave)

Tim, you got there before me!

Edited By GONZO on 06/10/2010 18:42:35

[GONZO]

As an additional bit of info I last used a 4 cell pack back in the mid 1980's. I've been 5 cell Ni-mh since then and now going to A123 cells. My Futaba 33m's and Skyleader SRC-1's are still OK but not used.

[Tim Mackey]

Gonzo said...

"Voltage of A123 cells straight off charge is 7.2v but falls very rapidly to 6,6v (3.3v/cell) without load".

Not in my experience it doesnt - it stays closer to 7V for while.

Anyway, not looking for an argument, people should make their own mind up as to their suitability as suggested.

We are now straying somewhat off topic here.

[David Ashby - Moderator]

Posted by GONZO on 06/10/2010 18:39:00:

Voltage of A123 cells straight off charge is 7.2v but falls very rapidly to 6,6v (3.3v/cell) without load. The discharge curve is vey flat (much flatter than Ni-mh), so much so that battery

 

 I've found this too. Sorry Tim, off topic

[Biggles' Elder Brother - Moderator]

Posted by Steve Hargreaves on 06/10/2010 16:56:38:

Posted by Biggles' Elder Brother on 06/10/2010 16:07:42:

 

Now for the speed. 0.28 secs for 60 degrees is equivalent to one revolution in 1.68secs. So multiplying by 2pi gives us the rotational speed of the servo in rads/sec - which is

 

1.68x2x3.14 = 10.55 rads/sec.

 

So the power expended by the servo is:

 

Power=Torquexspeed = 0.0667x10.55 = 0.7W

 

 Not that I am questioning you BEB...well alright I am ()  but it would seem from the above that a faster servo uses less power..??

 

If we do the maths with a faster servo...say 0.12s for 60 degrees we get 1 revolution in 0.72secs which equals 0.72 x 2 x 3.14 = 4.52 rads/sec

 

Power therefore equals 0.0667 x 4.52 = 0.3W....

 

Surely this can't be correct or have I missed something??

 

 

No Steve you're not - but I am

 

Did the above in a hurry - and made a rather silly mistake! To calculate the servo speed you should of course divide 2pi by the time for one revolution - not multiply - doh!

 

So the servo equivalent speed is actually:

 

Speed=(2xpi)/1.68 = 3.7rads/sec.

 

Putting that in gives:

 

Tims test: 62mA

Max test: 216mA

Typical test: 12mA 

 

Assuming approx 80% efficiency (and that this time I've done my sums correctly!)

 

Sorry about that! "Senior moment"

 

BEB

[Steve Hargreaves - Moderator]

OK Thanks BEB....I thought I was cracking up......

 

Maybe the faster speeds of digital servos is the main reason for the increase in current draw....

 

H'mmm these A123s do seem the way to go....it'll have to wait a bit until I upgrade my charger though....my current (geddit!!!) charger only does LiPos....

[Tim Mackey]

There's a few threads ( old and new ) on this subject, and coincidentally, a new thread today involving current demands on retract servos. A few days ago, I hooked up my eagle tree data logger to my latest project - the Pilatus PC9 1.8metre job.

The setup is..

 

Nosewheel - HS 75BB retract servo

Main Wheels - ditto. Both of these are on Y lead, and powered by a 2s LiFe pack.

 

Nosewheel steering - TP MG995 high torque metal gear digital.

Elevator - DS821 Spektrum digital

Rudder - HS85BB

Ailerons - 2 x HS65HB

These 5 servos are powered off a 6V UBEC

 

Maiden flight will have data recorded, but decided to do a static test also for comparisons.

 

Because of the way its all hooked up, I di the tests sparately as follows.

Nosewheel retraction only, then main gear, then nose and main gears retraction.

Then all three wheels were operated and stalled by jamming both main wheels.

Then logger was re-wired to record all the flight controls, and nosewheel steering - this is the data that will be recorded in flight.

The following 6 charts show the current consumptions involved......

 

First, nosewheel up and down - peak current @ 370m/a

Next main wheels ditto - peak current @ 570 m/a

Next was all three wheels cycled once - Peaked @ 760m/a

Next I cycled all wheels 6 times - peaked @ 570 m/a ( interesting ? )

Then all wheels but both mains stalled by jamming on going "up" - peak @ 1.2 Amps

Finally all 5 flight and nosewheel steering servos - peaked @ 1.5A.

This was due in the main to the drag of a stationary nosewheel being steered. With no steering effort, and just flight surfaces being moved, current averaged 400 -750 m/a

[Tim Mackey]

Just stripped the gears on the HS75BB mains servo

Perhaps this should be in "tip of the day" -

Dont hit the gear switch accidentally when the model is standing on its legs

[Steve Fish]

Hi Guys,

 

Don't wish to barge in on a private conversation as such but am really interested in the great work in this thread. This is because I am embarking on 50cc sized petrol powered aerobats for the first time and so power supply to higher torque metal geared digital servos is an imperative decision. I noticed a couple of comments about being off message with respect to power packs: I disagree as voltage drop, pack resistance and therefore pack choice is inextricably linked to servo current draw. Why else would you want to measure servo current draw?

Whilst I submit that most fliers are clubmen of sport flyer 50 size planes, larger models are becoming increasingly common in my experience.

I was particularly interested to see talk of LiFePO4 2s Rx packs used unregulated instead of NiMH for the points very clearly stated above: high current capable, low internal resistance, fast charge capable, not to mention fairly light.

The problems most of us have in making power pack decisions, surrounds, imho, the poor information provided by servo manufacturers that seems limited to physical dimensions and torque at 4.8v and 6.0v, (for non- high-voltage servos). Of course this is all very nice for a regulated voltage supply. But in the real world where keeping-it-simple means no voltage regulator, this does not happen. In the real world, we don't tend to talk in terms of 4.8v supply (?4x1.2v ie a nearly flat 4cell pack), 6v supply (?5x1.2 v ie nearly flat 5cell pack), but in terms of 4 cell and 5 cell NiMH packs, 2S LifePO4 and so on because we recognise that the safe useful voltage supplied by these packs is neither 4.8v or 6v. I do agree however that 4.8-6.0v is the useful range of a 4 cell pack.

 

I would really prefer to see data from servo manufacturers on the Vdne (VOLTAGE do not exceed!) and Vmin (Voltage Minimum) to reflect the range of power packs available, as well as, say, current draw versus torque curves. Perhaps this should be a petition we as modellers should make to the radio manufacturing world.

Or have I missed something here?

[Tim Mackey]

Private conversation ? - absolutely not Steve.

Not really missed anything St6eve, and as you say, we are all fudging along to some extent with limited manufacturers data. I think the main thing I was trying to get across was that in most cases , we tend to over worry, and over engineer our radio supply - me included BTW.

The average club hack sport flyer will almost certainly be quite happy on standard analogue everyday servos - something like the ubiquitous S148, and after all, those of us old enough, will remember flying our models on these ( and also some pretty large models too ) on nothing fancier - all driven by a simple 4 cell 700m/a NiCd pack, 'cos there wasnt anything alternative!

It makes sense of course that these days as better gear is available, models are certainly larger ( although size alone is not the main factor in servo choice of course ), and battery technology has moved on hugely that we seek the best we can afford to use.

I like using LiPo and regulators simply because I am totally geared up for Lipo use in everything, and the charging regime is so simple, plus of course they can provide the power I need, and duration to fly all day or longer if needed.

I do use LiFe packs in one or two models but confess to still being slightly wary of the "fresh off the cooker" voltage being a tad high for many servos.

One thing I do notice is the growing availability of high voltage servos - obviously driven by the increasing popularity of lithium power supply. The thing that bugs me though is the stupid price premium we are asked to pay for them - I mean how much more can it cost them to use components rated for a volt or two more than the norm!

Im sure eventually they will become the norm, and prices will drop to the levels we see now for standard voltage ones.

[Tim Mackey]

Just to revive this ever present debate, here's the inflight data from my PC9s maiden flight.

5 servos ( retracts powered by separate unmonitored pack )

2 x minis 1 x analogue standard size, and 1 x digital standard size fairly high torque ).

1 x V-high torque steering servos for nosewheel

Clicking will show full size.

First three minutes were static on the ground whilst photos were taken, with the odd blip of the controls, including nosewheel steering whilst the wheels were stationary.

( thats the 1.17 amp peak at 60 seconds or so )

As you can see, the maximum current drawn during the 6 minute flight was only 800m/a, and the average was far lower.