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Focus servo drive protocol


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#1 Phil Rhodes

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Posted 03 January 2014 - 05:39 PM

I may have a little electronics project coming up for which I'll need to control something like a focus servo of the type often used with radio remote focus gear.

 

I am aware that there are two generations of this stuff, the earlier analog approach which is functionally equivalent to a PWM driven radio control servo (from which the technology arose), and the more modern approach with digital communications, about which I know next to nothing. Various manufacturers' servos and radio receivers are advertised as being cross-compatible, so there's presumably some sort of de-facto standard at play.

 

As with many de-facto standards in niche industries, googling doesn't turn up any documentation. I have been informally advised that the older PWM types operate on exactly the same protocol as a common radio control servo, that is, 1-2ms duty cycle PWM at about 50Hz, but that's hardly concrete intel and I'm not sure what the situation is with the more modern ones at all.

 

Can anyone point me in the direction of some paperwork on this stuff?

 

Thanks,

 

P


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#2 Chris Millar

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Posted 04 January 2014 - 04:39 PM

hrrrm, ok I don't know remote focus gear at all really.

 

lots of quote-unqoute to follow (so much misinformation and really unfortunate/confusing terminology in the field, and yes a major dearth of solid white papers)

 

However, if the modern implementations use 'digital' RC/hobby 'servo' type 'servos' then the control PWM (as opposed to drive PWM) can be exactly the same as the older 'analog' RC/hobby control PWM signals you mention. What makes them better ( :rolleyes:)  is the ability to go to higher carrier frequencies which allow the higher torques and precision but along with that is the muuch higher current draw, so you shouldn't go 'too high' ( :rolleyes: )or you'll melt the servo drive. On a recent project we went up to maybe 200Hz, but without the little arduino script I cant recall the exact carrier. Basically it was well torquey and fast enough so no further experimentation was required, if I recall but the battery type becomes a consideration as they do need quite high peak current draw capability.

 

This is only relevant if it's the 'hobby'/RC style 'servo' control PWM style 'servos'

 

I'm aware I probably just added to the misinformation on the net  :blink:


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#3 Chris Millar

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Posted 04 January 2014 - 04:48 PM

It's coming back to me now...

 

It's actually not best to think of hobby/RC servos as being controlled by PWM. As the carrier can change the duty cycle will change for the same pulse, if you do this you'll note no change in position. So it's actually more like 'pulse duration controlled' in that the off pulse is ignored (within limits).

 

if you have a maximum of 2ms pulse for one direction (which by the way you can go further than on at least the hi-tec models I've tried) that infers a maximum carrier of 1/0.002 Hz - i.e. 500Hz and 2ms being %100 duty cycle. Great!  but no ... it's just doesn't work ...  200Hz turned out to equate to 0.002 / (1/200) = %40 duty cycle - but as I mentioned it's not a particularly useful number, until everyone decides on a universal carrier that is.  :)

 

Hopefully I'm not bogging the thread down with tangential information.  :lol:


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#4 Phil Rhodes

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Posted 04 January 2014 - 04:52 PM

I'm talking about the control signals - the actual drive is, as far as I'm aware, sorted out internally, based on the resistive feedback. That's the old stuff. From what I've heard, the more modern stuff may use gray-coded tacho feedback, but I'm not sure.

 

P


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#5 Chris Millar

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Posted 04 January 2014 - 05:50 PM

Hrrrm, too many things to define so we're all on the same page and can talk unambiguously.

 

...and the nature of ambiguity means that there will be some who don't perceive there to be an issue with it 

 

I'll keep an eye on the thread and if it goes in a direction that I might be helpful I'll chirp in again  ;)


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#6 Phil Rhodes

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Posted 04 January 2014 - 06:42 PM

I wouldn't hold your breath. Next time my scope and I are in the same room as one of these things, there shall be tinkering.


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#7 Chris Millar

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Posted 04 January 2014 - 07:06 PM

I remember the time I completely desoldered a Tobin crystal unit of a mates without telling him and beeped out the circuit  - bought all the parts and assembled a new one on a bread board - it worked !  quite chuffed as new they were hundreds but I had made it with about $8 worth of parts, and with PCB manufacture pretty cheap nowadays too...

 

Two issues: it stopped working as soon as I had reassembled the proper unit (which was thankfully ok) - my reverse engineering had only been at the level of cloning so even with the circuit right in front of me I didn't have the knowledge to work out what had gone wrong, but at least I learned it was based on a PLL..  and then film kinda fell flat and the market was effectively already flooded with enough units.

 

Good luck though !


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Visual Products

FJS International, LLC

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