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Dual servo control issue

(IP68) #1

Hi All,

Im building hydrometric river boat for river flow surveys, it will be powered by two azimuth thrusters , each azimuth thruster assembly equipped with servo HS-5646WP mounted upside down and fixed to shaft that should turn T100 thruster left or right. The mechanical setup looks promising so far but I have a problem with RC control for dual servos , they should move synchronized with little or no delays , but instead when the sticks on transmitter moved rapidly Im getting delays and slow response. On the other side they work good when operated individually, please check the video link.


Any thoughts why is it happening ? any help will be much appreciated .


Suitable servo for rotating T200
(Jacob) #2

I’m pretty sure this has something to do with settings in the radio. Many digital radios have options to delay, or slew the pwm output.

See page 62 in the manual, although this might not be exactly the issue with your setup, it will give you an idea of the kinds of settings that can have this effect.


(IP68) #3

Hi Jacob, thanks for your reply , I wish it was something to do with radio controller settings , it would’ve been easy to find but unfortunately I already tried all this mixes and still having same issues, I also tried connecting both servos to Y harness and plug it to one channel in Rx , since it sends ppm signal on one channel it should give synchronized output to servos , but it still does the same thing, I suspect maybe something wrong with Rx or Tx hardware?



(Jacob) #4

How are you providing power? The only other thing that I can think of is voltage sag when both servos attempt to operate is causing the problems.



(IP68) #5

I used Lead Acid 12V 1.2Ah Battery and BlueRobotics 30A ESC (without motors connected to ESC) ,Im pretty sure the battery was fully charged, but this could be a good test to try, I’ll take another power supply and see how is it going.Thanks

(Adam) #6

Hi IP68,

I strongly suspect the issue you are experiencing is due to the servos exceeding the current specification of the BEC on the Basic ESC. The Basic ESC can only supply a maximum of 500mA on its 5V line, which even a single servo is probably close to reaching without any significant load. The no load current of the smaller HS-5086WP micro servo is 280mA at 6V, so your much stronger HS-5646WPs are likely causing the voltage to drop and the behavior you are seeing, even in a bench test.

A visualization of the kind of current spikes than can occur with a servo.

I would recommend powering your servos with an alternate power source than can supply at least a couple amps. It may also be a good idea to pass the power directly to the servos instead of through the receiver, since that can be a bottleneck at higher currents.

I hope this helps!


(IP68) #7

Hi Guys,

I had another go yesterday , it works great now, apparently the solution was in combination of both suggestions, first I’ve changed power suply to 6v 2.3Ah NiMh battery , plugged it directly into Rx without ESC and noticed a lot better performance than before but still a bit of sticking and lagging present , then I found a SPEED setting on the radio it can slow down the output rate when moving sticks rapidly, dropped the rate to half of what it was originally and after that the servos started to respond flawlessly.

Adam ,you bang on the money there, BR basic ESC BEC is too small to run dual servos , however one servo was working OK so I suspect it’s pulling about 500mA (could be wrong , can’t find any info about it), so 2 servos would need at least 1A , I don’t really want to have two separate batteries in my boat as I’m trying to keep weight as low as possible, so Im thinking of getting ESC’s with 2A or 4A BEC , do you think receiver can handle this current, it didn’t seem to be a problem on the test bench ? your thoughts?

Much appreciate your help, thank you!

(Adam) #8

Hi IP68,

Great to hear you solved the problem! An ESC with a 2A or 4A BEC should do the trick, or you can use a standalone BEC to feed all of your 5/6V electronics. I’m not sure what your use case is, but depending on the load the servos will be under, it may still be a good idea to power them directly instead of from the receiver if they are going to be constantly highly stressed. You shouldn’t need another battery for this, simply splitting the power from the BEC, with one line going to the receiver and the the other directly to the servos would certainly eliminate the load the servos may put on the receivers power traces. The yellow signal cables would still run from the servos to the receiver. If the servos aren’t going to be under constant high load, you should be fine with the traditional setup of using the receiver to split the power.

Those assemblies in your testing video looked very cool- can you share what you’re working on?


(IP68) #9

Thanks Adam, that sounds good. Im working on autonomous river survey boat , mainly used for river flow gaugings using ADCP sensors. Those assemblies on the bench are part of the azimuth thrusters , they will be mounted on the trimaran, each on its own pontoon.

(Spally) #10

Hi guys, I know this is late but I have had a lot of experience with the BR thrusters and mounting them on steerable mounts. (See the old thread on Azimuth Mounts). I have broken many Servos, BEC’s and T200’s because of the steering.

Firstly, turning the T200 whilst under load or high RPM creates a fit bit of centrifugal force on the Stator tube. Due to the design, the magnets create the rotating mass. It is like spinning a bike wheel and trying to rotate it just by the axel, it is difficult. I can report that the T200’s are now up to the task. I have been providing feedback to Rusty and the boys and they have improved the strength of the Stator tube to handle the extra load put on them.

Servos. Well I have killed my fair share of them. Forget the hitec WP units, they aren’t up to the task. Once again, stationary with no rpm, they turn fine. Under load the force required is greatly increased. I now use Some of the higher rated Savox servos (30kg+) to do the job. Then that causes another issue…

BEC. I use the Castle Creations 20A BEC. The BEC in the ESC can’t handle the current required. I found this out early on when doing a few turns, it would stop responding to turns. Stopping the boat gave me back my steering after a few seconds. Once again, the torque required to turn the motors at speed creates great load, maxing out the Servos and then the BEC.

Yes, I have gone overkill on the BEC & Servos but that is better than waiting for the wind to blow it home.

(IP68) #11

Hi Spally, thanks for the heads up, that’s very interesting to know .
I have been using only T100s so far and managed to kill them all by pushing too hard running in the rivers, I heard that BR working on upgrading the T100s design and make them more robust to high speed and loads, unfortunately they still underpowered for my applications so I have two new T200 ordered and I’m looking forward to run them in the same conditions.
I didn’t get to use my azimuth assembly in the water yet, first I went for differential thrust control due to weight restriction on small boat and it works good if thrusters run at 3/4 power, this leaves room for steering by speeding up one thruster and slowing down the other but when the boat gets in to fast flow of a stream its becomes hard to control , worst of all its slowing down in order to steer .Now T200 have double thrust power so Im quite keen to try that on them.
My second boat is too big for differential thrust so Im planning to use azimuth thrusters , Ive seen your assembly and was very impressed by the design and quality of 3D print, if you seen my youtube clip in the first post I have slightly different design, my servos positioned upside down and fixed directly to the shaft, Im not a mechanical engineer but I think it may ease of some force applied on the shaft moreover Im using the boat in the river so if I the river flow runs one direction and the thruster working against it so the forces on the servo will have to be equal ? will be interesting to find out in practice.
Have you tried to add gear box to your servos , this can reduce the amount of power you need to apply to the systemv and reduce were and tear ?


(Mark Langille) #12

Have any of you tried slight vector mounted thrusters to help with the turning capabilities? ie with a slight offset such as is done on the Blue ROV2?

For the servo power issue I would suggest running a separate dedicated BEC vs the BEC in an ESC. You can typically get them at high amperage as well as 6V that most digital servos support vs 5V.

(Spally) #13

Just watched your video. Up until now, every time I clicked on the link it said is was a private video and I couldn’t watch it. Nice assembly. Are you running them through the hull or off the back like an outboard? I am trying to keep the profile low on my boat so mounting the servos above the unit is not possible. Since I am basically using a tube inside another tube, there is no seal between them. This allows water between the tubes which acts as the lubricant. The tops of the tube are above the waterline so no water gets in. My boat is sold for commercial use so making it solid and reliable as possible is my priority. I did consider using chain with a large sprocket on the mount and a small one on the servo but the high torque servos and dual linkages have proven to work well.

Here is a like to the Accura Brochure https://www.dropbox.com/s/eiia5pa9cmsdbnl/ACCURA%20Flyer%20single%20v11.pdf?dl=0

I am looking forward to seeing your Servos mounted and in action.