Modifying MG90 servo boards?

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Hogi
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Joined: Sun Dec 30, 2012 6:47 pm
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Modifying MG90 servo boards?

Post by Hogi »

Hi guys. I've found a couple of really cheap sellers on ebay of the MG90 servo and I was thinking of ordering a few post AWS to mod into ESCs for an antweight. I know the basic principle of servo board modification into ESCs is to remove the potentiometer and replace it with some resistors but I would like to answer some of the uncertainties I have before ordering anything as I wouldn't want to dive in too quickly and get things wrong. Anyway, the questions I have are:

.Is the MG90 modifiable? I chose these because they were cheap and they work really well as weapons servos but would they work just as well as ESCs for geared motors?
.After removing the potentiometer, where exactly do I solder the resistors?
.Would they run a 2 cell lipo?
.Where along the set up would I solder in the female port for the battery to plug into?
.What kind of resistors would I use?

Link: http://www.ebay.co.uk/itm/1Pcs-10Pcs-MG ... qMSwR0P94A

Thanks guys. All info appreciated.
Dan. :)
Daniel Jackson.

Team Hectic.

Many antweights

Super antweights: territorial.

Fleaweights: fleadom fighter, gaztons.

Featherweights: hectic (under construction)
AntRoboteer
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Re: Modifying MG90 servo boards?

Post by AntRoboteer »

1) I use the SG90 variety which are a bit cheaper but should be absolutely perfect according to previous experience with this type of servo. If you can get SG90s, they can be a better option because you will have a bunch of gear trains spare but if you want to have spare metal gears and can afford to spend a little more then go right ahead with the MG90s; they should be fine.

2) Solder the resistors to the potentiometer wires that are left over from the removal of the potentiometer

3) I wouldn't even chance it to be honest. A nice 6V regulator is what we use in order to protect the boards from too much voltage. They become VERY unstable above 6V indeed.

4) What you will be left with when you have finished the modification will be a board connected to a standard servo plug. You would then plug the battery directly into the receiver (make sure your battery is regulated to 6V) and plug the boards in as if they were servos

5) We use 2.2k resistors but we have found that even from board to board of servos in the same batch, the resistance ratio you need varies. Think of the resistors being in a ratio. If you have 2.2k on one side and 2.2k on the other, the ratio is 1:1 and hence the servo board will see a centre position in a certain place. However, the motor may not actually centre in that position and hence you may need to change the ratio of resistances to move the 'centre position' into a position where using trims you can stop the motor continually rotating. To get the correct ratio, we measure the ratio of voltages between the centre wire and each of the outer wires. We then know the ratio of resistances to use. Note that you can combine resistors in series to achieve larger resistances (for example for use in a 3:1 ratio setup) or in parallel to reduce their resistance (for example if a really nasty ratio comes up like 2.5:3.75 or the like). You can use combinations of 2.2k resistors in order to achieve whatever ratio of resistances you like. Just make sure you get plenty.

Really sorry for the wall of text above. If you need clarification on anything, just say and I'll try to explain it a little better. It practically comes down to how a potentiometer works: providing 2 resistors in a potential divider arrangement.

Hope that all helps. We have a lot of experience now using these setups and they have so far proven to be rather good performers albeit only really at 6V. You could be daring and go a little higher in terms of voltage but frankly I would avoid it.

Be noted that this is a terribly tedious process so make sure you have a couple of hours spare! :D

Good luck! :)
Hogi
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Re: Modifying MG90 servo boards?

Post by Hogi »

Thanks very much that's very helpful. I'll get on and order some soon and have a go at it. :)
Daniel Jackson.

Team Hectic.

Many antweights

Super antweights: territorial.

Fleaweights: fleadom fighter, gaztons.

Featherweights: hectic (under construction)
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UserFriendly
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Re: Modifying MG90 servo boards?

Post by UserFriendly »

Hi Dan, I recently followed Antroboteer's method above recently (on the SG90s), and got it to work (ran into the issue of the centre position problem, - still yet to be rectified!). It works surprisingly well, my biggest problem has been doing mixing properly on the transmitter instead...
Regarding resistors:
Image
Andy B - Team UserFriendly
Reading Cybernetics Graduate 1996
Trying not to take Antweights too seriously. I think I'm failing.
First AWS was 46.
Hogi
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Joined: Sun Dec 30, 2012 6:47 pm
Location: basingstoke

Re: Modifying MG90 servo boards?

Post by Hogi »

Thanks. I recently figured out how to mix on the transmitter for my featherweight build. I use an orange TX which uses a very similar interface to the spectrum. I found that when switched to rc plane mode, you get features on the menu called mix 1 and mix 2. I found that by setting mix 1 to mix aileron to elevator +100% and then setting mix 2 to mix elevator to aileron -100% I could get the channels to mix how we would desire them to mix to drive the robot. Not sure if that's any help but maybe you can make something of it. I will have a go at modding some servo boards after the AWS. I did a search for a 6 volt regulator and I couldn't find anything useful. Where do you guys get yours from?
Daniel Jackson.

Team Hectic.

Many antweights

Super antweights: territorial.

Fleaweights: fleadom fighter, gaztons.

Featherweights: hectic (under construction)
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Re: Modifying MG90 servo boards?

Post by UserFriendly »

We have a maplin store nearby, I was in there on Sat. They had really low stock, so I ended up buying a number of different ones - different amperages etc.

http://www.maplin.co.uk/p/ts7806cz-1a-p ... case-aw69a
Andy B - Team UserFriendly
Reading Cybernetics Graduate 1996
Trying not to take Antweights too seriously. I think I'm failing.
First AWS was 46.
AntRoboteer
Posts: 441
Joined: Mon Mar 03, 2014 7:01 pm

Re: Modifying MG90 servo boards?

Post by AntRoboteer »

We use the L7806CV models. If you want UK delivery, 5 can be found here for £1.69:

http://www.ebay.co.uk/itm/Voltage-Regul ... X4PuyNccHw

1.5A output as well which is great. You can of course get bulk packs from China to get better value.
Hogi
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Joined: Sun Dec 30, 2012 6:47 pm
Location: basingstoke

Re: Modifying MG90 servo boards?

Post by Hogi »

Thanks. Sorry to sound like a real noon but where and how do I solder those in?
Daniel Jackson.

Team Hectic.

Many antweights

Super antweights: territorial.

Fleaweights: fleadom fighter, gaztons.

Featherweights: hectic (under construction)
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Joined: Wed Feb 18, 2015 3:52 pm

Re: Modifying MG90 servo boards?

Post by UserFriendly »

Like this. obviously, work out which pin is the input and output, as it's not clear on this pic. (also it's probably a 6V output you want, not 5v)

Image
Andy B - Team UserFriendly
Reading Cybernetics Graduate 1996
Trying not to take Antweights too seriously. I think I'm failing.
First AWS was 46.
Hogi
Posts: 1002
Joined: Sun Dec 30, 2012 6:47 pm
Location: basingstoke

Re: Modifying MG90 servo boards?

Post by Hogi »

Thanks very much for the wiring diagram. :)
Daniel Jackson.

Team Hectic.

Many antweights

Super antweights: territorial.

Fleaweights: fleadom fighter, gaztons.

Featherweights: hectic (under construction)
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