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Author Topic: Continuous rotation - Acoms servos  (Read 4818 times)

andrewh

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Continuous rotation - Acoms servos
« on: February 07, 2008, 12:45:17 PM »

I use servo drivers and motors as propulsion for small models, and have done for years. 

I recently bought a four of Acoms servos from Howes models to convert to drive motors for a paddleboat - pic attached if I can find it

Overall a sweet and smooth servo (especially for the money)
Very limited travel in standard form (but room inside to add resistors to the pot and extend rotation)

It turns out that the servos I got are VERY suitable for modification and by far the easiest I to modify.

the part numbers are AS16 and AS17, and the construction makes the potentiometer easy to remove. 
There is a lot of room in the case for fixed resistors if required (I used a 5K pot and mounted it externally)

The output gear needs to have a rib clipped off to allow full rotation, but this is easy

Happy to provide a photopictorial if it would help
andrew
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Martin [Admin]

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Re: Continuous rotation - Acoms servos
« Reply #1 on: February 07, 2008, 01:02:49 PM »

Happy to provide a photopictorial if it would help

Yes Please  O0
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andrewh

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Re: Continuous rotation - Acoms servos
« Reply #2 on: February 07, 2008, 01:18:51 PM »

Martin - it shall be done!

I bought the servos at the Midlands Exhibition because the little paddler is a bit anaemic (each paddle has a servo driver and a tape drive (flat) motor).  It turned out that I got three of one type (pale blue final shaft) and one with a white shaft.  I am using one of each type as a pair and can't see any difference

I also have a rowing boat design hatching in my mind - I have some geometry which will (should/might) produce a long elliptical puuuuuuuull rather than the circular "dab" that most mechanisms seen to generate.

words and pix when I get a moment and the camera

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Stavros

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Re: Continuous rotation - Acoms servos
« Reply #3 on: February 07, 2008, 10:23:37 PM »

Definatly can we have some servo photos as they are being converted prety please O0


Stavros
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Capricorn

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Re: Continuous rotation - Acoms servos
« Reply #4 on: March 05, 2008, 03:43:15 AM »

Andrew, interesting to see this, I finally discovered the usefulness of servo innards.  I had thought they would only provide on/off, forward/backward, but they do have speed control!  Many more potential uses for them now (for me), thanks.

Question, the futaba servo (S148) I used is a bit jittery, any fix for that?  I see you have some sort of coiled wires on a stick, is that the antenna or something else?

Cap

(photo of soon to be micro speed control for 1/350 scale Tamiya Fletcher)
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malcolmfrary

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Re: Continuous rotation - Acoms servos
« Reply #5 on: March 05, 2008, 11:54:06 AM »

I have almost always traced jitteryness in servos back to a questionable power supply.  Servos work by generating a voltage from the control pulse coming from the RX, and use this to decide whether the motor should run, how fast and in which direction.  It does this by comparing the generated voltage with a reference which is a proportion of the supply voltage.  Any variation here will change the answers and cause unexpected responses.  As the supply for the motor and the electronics is common, motor activity puts an extra load on, and any weakness in the supply (battery, wiring, plugs etc) will change the supply, and therefore the reference voltage.
The servo electronics are just an old-fashioned ESC with a very narrow deadband.  The brave and nimble fingered can change some components to increase the deadband and convert into a mini-esc.  I choose to live with the narrow deadband.  If you need to run motors at a higher voltage, there are manufacturers out there who make small ESCs.
Digital servos are another story.
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andrewh

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Re: Continuous rotation - Acoms servos
« Reply #6 on: March 06, 2008, 01:43:38 PM »

Fellow Mayhemers

I will get a how-to to you a soon as possible - the servos I modified have disappeared into my son's robot so are not available for retro-photography! 

Cap - I don't know the whole answer to jittering - I have found that some servos are unpleasant when converted if I run them at 6V (rather than 4.8).

I feel that malcolmfrary is right.  If this is the case, then some hefty capacitors should smooth the voltage as seen by the electronics - easy and safe to try.

They do have a very narrow deadband, and a rather narrow proportional band but what do we expect for GBP4??

For little plastic conversions - ideal

It may be worth bearing in mind that the motor can be changed for any other little motor with low current draw - examples might be tape drive motors, servo motors with longer shafts, N20 mabuchis or any pager motor  (micro servos use 4 and 6mm pager motors anyhow)

Another opportunity is that big servos - like 1/4 scale ones -  use a 380 motor and a controller capable of (guess) 5 amps more or less continuous.  This is what I use as Aux drive motor on my Thames barge. 
So you still have the narrow deadband but they can uproot BIG trees.

andrew
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Capricorn

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Re: Continuous rotation - Acoms servos
« Reply #7 on: March 07, 2008, 02:20:44 AM »

Sounds right to me, maybe it's the same thing but the signal from the radio is a square wave or some such thing, and it's the length of the wave that is read and tells the servo to turn one direction or the other, so maybe it's the same as if the power source is weak.  I do know that the radio antenna can affect it quite a bit, run the antenna along the servo wires and you get much chatter, but pull it away, in a different direction and that diminishes the chatter. 

I'm not sure with this small plastic model what will happen with the antenna, there sure won't be room to stretch it out full length inside, especially not away from the servo leads, it's going to be a bit massive to have outboard.  I know antenna length is important for good radio operation, I guess I'll find out how important.  I'll have to experiment (it will be tough to take the nippers to the antenna on a new micro reciever but what the heck), maybe a single strand of uninsulated wire from the bow over the mast to the stern?  Maybe trail a thin wire in the water?  Probably get caught in the propellers, we'll see.  Welcome any suggestions on that (the boat is only about 13" long).  Cap
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barriew

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Re: Continuous rotation - Acoms servos
« Reply #8 on: March 07, 2008, 11:17:22 AM »

  Welcome any suggestions on that (the boat is only about 13" long).  Cap

Coil it up! You don't need massive range if the boat is only 13" - you wont be able to see it if it goes too far from you :D

Barrie
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Capricorn

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Re: Continuous rotation - Acoms servos
« Reply #9 on: March 07, 2008, 12:58:07 PM »

Thanks Barrie, I will try that first  :).

Cap
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andrewh

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Re: Continuous rotation - Acoms servos
« Reply #10 on: March 07, 2008, 01:04:06 PM »

cap,

I've put some antenna thoughts on your DE thread, but Barrie is right!

I use fine copper wire as aerial wire - recovered from a transformer!  I chop off the makers wire about 2 inches from the case, and use a  small plug and socket to connect the fine aerial wire.

Ideally run it vertically - mast rigging or some such!  run the rest around the decks.  Some people coil the wire round a wood stick to keep it tidy.
andrew
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