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Author Topic: converting servos to act as esc ?  (Read 1731 times)

wadsworthj

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converting servos to act as esc ?
« on: February 02, 2014, 04:16:41 PM »

Hi Peeps.
             I am puzzled, how does one convert a servo, to do the job of an e.s.c.
My next boat will have two of these, which will steer the boat instead of rudders.
Weird !!! HELP!!!!!!      {:-{
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Stavros

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Re: converting servos to act as esc ?
« Reply #1 on: February 02, 2014, 04:38:10 PM »

And what size are the motors in your next build
 
Dave
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Re: converting servos to act as esc ?
« Reply #3 on: February 02, 2014, 05:30:41 PM »

The don't make very good ESC's because they only pulse at a maximum duty cycle of 50%, meaning your motors only receive half the current they should, limiting their power. Digital servos are another kettle of fish.

You can purchase miniature ESC's quite inexpensively these days, and they will do a much better job.

malcolmfrary

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Re: converting servos to act as esc ?
« Reply #4 on: February 02, 2014, 06:35:11 PM »

The don't make very good ESC's because they only pulse at a maximum duty cycle of 50%, meaning your motors only receive half the current they should, limiting their power.
I wish I had known that before I used them getting the full function all those years ago.  The only thing you miss out on is the lack of deadband.  A servo has the exact same logic elements as an ESC, but a couple of components have their values changed to make them more suited to keeping a position rather than providing running control.  No problem using them singly, except that you are limited to the radio battery and half an amp for the motor plus the lack of a positive stop.
Using a couple, they might not respond well to the mixer, "proper" ESCs, being what the mixer was designed for, will be much better.


The modification consists of removing the gearbox from the top of the servo, removing the bit of moulding covering the motor shaft, maybe getting the gear off it, plugging in and setting the "stop" using the exposed potentiometer shaft as a trim.  The result will be fully proportional, BUT it will be best to control the speed of the motor using the trim tabs alongside the sticks as an engine room telegraph.  Full speed appears very early in the stick travel. 
Optionally, the motor can be replaced, but it needs to be a close match to the one in the servo.  The original can be dug out to mount elsewhere. 
If a bigger motor is wanted - either more amps or more volts or both, it is easy to make a higher power output circuit for the servo electronics to drive.  Nowadays, it is actually cheaper to buy a ready made ESC - the result will likely be smaller and lighter.
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Re: converting servos to act as esc ?
« Reply #5 on: February 02, 2014, 09:36:41 PM »

So you managed to get a full 100% current feed from an an analogue servo chip e.g. NE544, M51660L etc.?

I scoped the outputs of several amps, and they all gave 50% duty maximum.

malcolmfrary

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Re: converting servos to act as esc ?
« Reply #6 on: February 03, 2014, 10:05:19 AM »

Looking at the data sheets for those chips, there is nothing to suggest that there is any restriction on the output percentage due to the I/C itself.  Component choice in the pulse stretcher could obviously have an effect.  I just used the servo electronics that were to hand, but there was no noticeable difference between running the motor direct from the battery and at full speed through the control.
I no longer have access to a scope, so not something I can check on these days, but when knocking my own together I used ZN409 I/Cs, and the scope I could get to then did show the full mark space ratios between 0 and 1, as predicted by the datasheet http://webpages.charter.net/mullenmj/Files/zn409%20datasheet%20.pdf   Most of the information on that sheet regards its use as a servo control, but there is a little throwaway note towards the end mentioning the circuit differences required.  Since most of the pre-PIC servo control I/Cs are copyright avoiding clones, it is reasonable to infer that performance will be very similar.
But I still reckon that for differential steering, whether using two throttle sticks or a mixer, purpose designed ESCs are better.  But the cheap 10A ESCs from fleabay are probably best avoided due to their distinctly unequal performance between forward and reverse.
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wadsworthj

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Re: converting servos to act as esc ?
« Reply #7 on: February 03, 2014, 08:13:36 PM »

Hi Everybody.
                       Many thanks for the plethora of info,
but the majority is way above my head, however I take from this that it is simpler
to leave well alone ,and  go for two mini esc, an a y branch cable.
But many thanks for the contributions.
Cheers  John W.     :embarrassed:
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malcolmfrary

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Re: converting servos to act as esc ?
« Reply #8 on: February 04, 2014, 08:19:09 PM »

If you are hoping to steer the boat using the ESCs, a Y lead will do nothing for you.  Each one needs to be plugged into either uts own slot on the receiver, or into a mixer which in turn is plugged into a throttle slot and what would have been the rudder slot.
Have a look in the wiring diagrams thread at the top of "Black Arts"
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