SG90 Servo information
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These units are widely faked including the label so performance varies. The units I purchased most recently had digital innards, it would appear the genuine analogue SG90 is unlikely to be still available, all comments apply to this digital version. Unfortunately they are indistinguishable externally but the interior can be viewed through the rear case, if it has a single SO8 chip & SOT23 device with no large capacitors it is the digital version, this picture is the top view of the pcb inside-
I fear there are several versions of this digital pcb but the final key is the chip being marked AA51880 data here
This servo draws a peak current of just under 1 amp when moved and about 35mA idle.
I only use this servo over 90 degrees of movement, pulse width from 1 to 2mS, I have found some do not operate reliably at the extremes of 180 degree movement using pulse widths of 500uS & 2.5mS.
The idle non-moving current can become very large under some circumstances, if there is a fixed load like a spring trying to move the shaft to some other position than that commanded or if the servo is unable to reach the commanded position due to some obstruction. Either of these situations can rapidly heat the internals of the servo and maybe your power supply.
A way around this that I employ is to switch of the servo command impulses several seconds after the movement.
Noise immunity is fairly poor and can lead to instability particularly with long connecting leads, especially if they are of small cross section such as servo extension leads. I combat this by using large gauge (0.8mmsq) bus wires for 0V & +5V that feed each servo at it's connector, the peak current (and therefore voltage drop) in the 0V bus is also reduced by using a large (1000uF) capacitor at each servo and a small (6R8) series resistor to +5V.
These servo's are very tolerant of frame rate, I tried all the way from 10mS to 250mS and they worked happily (it does not affect the servo speed as rumoured for the analogue ones) but the large frame rates do have an interesting effect near the end of the commanded stroke, they first overshoot slightly and on the next pulse reach final position, it maybe they always do this but at higher frame rates it's unnoticeable.
I have so far seen no evidence of power up twitches or any kind of jitter. Certainly if you have an oscilloscope check the control micro for software induced jitter, this if present will simply be acted upon by the servo. I also found any momentary supply glitches below 4.2V would upset it violently (sometimes a full stroke) so proper decoupling/filtering and large gauge wire helps to eradicate this source. If the problem persists it may be crosstalk from other servo channels or traction wires in the same cable bundle, a 10nf capacitor from signal to 0V at the servo may help or in extreme cases of crosstalk reducing the drive of the processor by adding a 100R resistor at it's output (before the long wire).
To be continued
Last edit: by fourtytwo
Roger OO DC Steam
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I use servos on one section of my layout but admit, I've never been inside one…………………..
Your comments about small section servo control cables and noise limitation might well be valid on mine. I am quite surprised by how much noise the things actually make when operating - I'd been led to believe they were virtually silent as opposed to both Tortoise motors and the clunky solenoids. My longest run from the controller is around 1.5 metres - I use (I think) 22 SWG Futaba type connecting cable. It could of course, be the cheapo servos - SG90 purchased from Hobby King some time ago. (I've just noticed how much the price has gone up since I bought mine !)
Using the Megapoints controllers, I haven't noticed any kind of twitching - except on power-up and whilst electrically centering them in programme mode when I'm guessing power is supplied continuously. I think in "normal" mode, once they've completed their programmed run, power is cut.
'Petermac
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Trying to follow along :thumbs
Cheers
Matt
Wasnie me, a big boy did it and ran away
"Why did you volunteer ? I didn't Sir, the other three stepped backwards"
"Why did you volunteer ? I didn't Sir, the other three stepped backwards"
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'Petermac
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Edit, Peter I get no twitching on power up at all, even without any extra decoupling. I know nothing of your controller but I hazard a guess it is supplying the 5V for the servo's. Does your power up twitch still happen with just the native servo length lead ? and does it still happen if you remove the orange wire (signal) from the connector and leave it floating ? also does it happen if only one servo is connected to the controller ?
Last edit: by fourtytwo
Roger OO DC Steam
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Reading Dave Fenton's notes (Mr Megapoints himself), I see there is a different servo he recommends if "twitch" on power up is to be avoided so presumably, the problem (if it is a problem) lies with the servos I use. To be honest, that initial twitch isn't a problem for me so I'm not about to change them all ………………
When I bought them, from Hobby King in the States, they only cost me around £1 each - UK duty almost doubled the price !!
The twitching stops after probably 2 seconds, if that, then they're still. My "noise" is just the common, electronically uneducated, audible noise which might be baseboard related although it doesn't seem to matter if the baseboards are chipboard or Sundeala. My preferred surface is ply but, where the servos are installed, the boards (old) just happen to be either Sundeala or chipboard ……………..
'Petermac
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TBH Peter AFAIK there are many variations (clones) of the SG90 so I may just be lucky and maybe Dave recommends a servo that has less or even no clones so they are all guaranteed to operate the same. I guess usually a suppliers stock are the same variety so perhaps the process is buy one, test it and if ok buy the rest quickly hoping for the same variation.Reading Dave Fenton's notes (Mr Megapoints himself), I see there is a different servo he recommends if "twitch" on power up is to be avoided so presumably, the problem (if it is a problem) lies with the servos I use.
Roger OO DC Steam
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Roger OO DC Steam
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That aside, It is excellent work here and your technical expertise has me in awe but with no aspiration to follow down that path. I've enough on my plate already with things I can get my mind around.
Last edit: by Colin W
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Regards
Roger
Roger OO DC Steam
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However, I know what a pain it is to replace the point in the middle of a complex junction when all are joined with insulated joiners so now mainly use either Tortoise or servos…………………..
'Petermac
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Roger OO DC Steam
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Will it be a Havoline smell or a Castrol smell ………….. :hmm
'Petermac
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