Author Topic: Broken servo.. can it be fixed?  (Read 1221 times)

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Offline TheDarkLordTopic starter

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Broken servo.. can it be fixed?
« on: August 27, 2011, 04:48:42 PM »
Well, I finished building the $50 robot, programmed it and all, and while seeing if it worked, I noticed that only one servo changed direction according to the photo resistors' input, the other kept going in one direction no matter what. I realized I probably modified it wrong (I modified it using Admin's method) so I opened it again...however no matter how much Acetone I used, I could not turn the pot again (because I had super-glued it in place). So.. I cut off the wires connecting to the pot to try the two resistors method. But while doing this, the PCB inside the servo broke neatly in half, as you can see in these pictures :(

http://www.tumblr.com/photo/1280/9473925900/1/tumblr_lqlz0nz1Wl1r2r8su
http://www.tumblr.com/photo/1280/9474011761/1/tumblr_lqlz46Tg151r2r8su
http://www.tumblr.com/photo/1280/9474102126/1/tumblr_lqlz7ugMaQ1r2r8su

As far as I can see, only the connections to the motor are severed, so I think it should be easy to fix with some solder. The problem is, I do not know where to solder, as there are no visible copper tracks on the PCB. Any help or guidance here would be greatly appreciated.

Also, in this picture (http://www.tumblr.com/photo/1280/9474185388/1/tumblr_lqlzb9GlRT1r2r8su) , you can see the three wires that were connected to the pot before. After connecting two resistors together, I have to attach them to the wires like on the diagram on page 4 here: http://www.md.kth.se/mip/bilder/servomod.pdf
But I do not know which of the three wires (red, yellow, green) goes to the middle of two resistors. Again, all help here would be appreciated.

By the way, the servo I'm using is the HS 311 standard.  I really need to be able to fix this because I'm on a low budget and can't afford to buy another.
If it would help, I can upload slightly higher quality photos.

Thank you all in advance,
Vignesh R.

Offline waltr

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Re: Broken servo.. can it be fixed?
« Reply #1 on: August 27, 2011, 04:53:58 PM »
Re-take the pictures but this time ensure they are in focus. Blurred pictures are not really any help, smaller is better than blurred.

The pictures in the link on modifying the servo are of more help.
Look at them very closely to see where the motor terminals are attached to the PCB.

Offline TheDarkLordTopic starter

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Re: Broken servo.. can it be fixed?
« Reply #2 on: August 27, 2011, 05:45:53 PM »
Sorry, I used my phone camera the first time.
Here are some better photos:
http://vigneshr.tumblr.com/

Offline garrettg84

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Re: Broken servo.. can it be fixed?
« Reply #3 on: August 27, 2011, 08:13:50 PM »
You might check to make sure you aren't grounding or powering the control lead on accident. If you ground or power the servo control lead it can make it flop to one side or the other. That would be my first guess - confirm this by swapping servo plugs with the one that works....


*Edit*

Sorry, I was assuming you already put it back together. You can use a file to scrape away some of the coating on the pcb to expose the copper and rejoin the traces.
« Last Edit: August 27, 2011, 08:17:37 PM by garrettg84 »
-garrett

Offline waltr

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Re: Broken servo.. can it be fixed?
« Reply #4 on: August 28, 2011, 11:07:02 AM »
Pictures are a bit better and I can now see where the PCB broke.
A cell phone camera can do fine, just don't get too close so the pics are in focus. Then look at the pic before s4ending it, if OOF shoot again.

A little epoxy (hardware store 5 min will work) can put the PCB back together. Then scrap of the green solder mask and solder a short piece of wire across the PCB break to re-connect the motor.

If you want to practice first, find and old piece of electronics that is being thrown away, take it apart and pull the PCB to practice scraping & soldering.

good luck

Offline TheDarkLordTopic starter

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Re: Broken servo.. can it be fixed?
« Reply #5 on: August 28, 2011, 11:17:05 AM »
@waltr Alright, thanks a lot! What about the resistors though? Would you be able to tell me which color wire goes to which part of the resistor? I already scraped off some of the green stuff (using a file like garrett said), will post pictures soon.


Offline Soeren

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Re: Broken servo.. can it be fixed?
« Reply #6 on: August 28, 2011, 11:35:27 AM »
Hi,

As far as I can see, only the connections to the motor are severed, so I think it should be easy to fix with some solder. The problem is, I do not know where to solder, as there are no visible copper tracks on the PCB. Any help or guidance here would be greatly appreciated.
[...]
By the way, the servo I'm using is the HS 311 standard.


Referring to [url=http://this photo]this photo [/url] it seems that there's a narrow trace broken as well (going from the 15k SMD resistor, but I cannot make out where it continues on the part of the PCB still on the motor.
I opened a HS-311 standard servo to check it out, but the PCB layout is very different from your, as you can see


One thing they do have in common, despite the differences, it the feedback (or whatever) which includes two 15k resistors and a small cap (in the right side of the photo).

You need to repair this track too, as it must connect where it does originally (or it won't work, or will work eradically).

You can repair it like this:
Desolder the smallest part of the PCB from the motor.
Glue the two parts with Cyanoacrylate glue (Super glue/10 second glue)
Put some strong tape on the back side of the PCB to give it a bit more strength (don't break it again in the process).
With a small and preferably round bellied knife, scrape the lacquer (jut a 4..5mm pad on each, enough for a proper joint) off of the traces that are to be joined. Clean the spots with a glass brush if you have one, or use a small pad of emery paper (at least 800 grit, 2000 grit if you have).
Tin the pads lightly... Very lightly.
Use some wire for connecting up the traces. For the wide motor traces, it will make it easier if you make a "V" shape and solder it in two places at one of the sides.

When it's tested and found to be working, you could enhance the job by a layer of epoxy glue over the break line on the solder side.
Regards,
S°ren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline TheDarkLordTopic starter

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Re: Broken servo.. can it be fixed?
« Reply #7 on: September 02, 2011, 05:29:20 PM »
Thank you everyone for your help! I got around to fixing it today, and it works!
The only thing is that the pulse required to make it stop spinning seems to have changed (probably because I replaced the potentiometer with two identical resistors).. So would any of you be able to guide me into how I could find out what the value to make it stop spinning is?

Thank you once again  ;D

Offline Soeren

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Re: Broken servo.. can it be fixed?
« Reply #8 on: September 02, 2011, 06:33:47 PM »
Hi,

The only thing is that the pulse required to make it stop spinning seems to have changed (probably because I replaced the potentiometer with two identical resistors).. So would any of you be able to guide me into how I could find out what the value to make it stop spinning is?
How much have it changed?
If it's in the interval 1495Ás to 1505Ás (i.e. a 5Ás tolerance on the 1.5ms) you can consider it centered OK. If not, the exact pulse duration will be helpful in estimating the new values.

Or, use the proven method of using a trimmer resistor of roughly twice the value for one of the branches/legs from the center point and adjust until it's dead on. Then remove/desolder the trimmer (carefully, so as to avoid changing the position of the wiper) and measure the value of the trimmer. Finally, select the nearest E-12 row resistor (or use two in parallel for pinpoint accuracy).
Regards,
S°ren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline TheDarkLordTopic starter

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Re: Broken servo.. can it be fixed?
« Reply #9 on: September 02, 2011, 06:42:11 PM »
How much have it changed?
If it's in the interval 1495Ás to 1505Ás (i.e. a 5Ás tolerance on the 1.5ms) you can consider it centered OK. If not, the exact pulse duration will be helpful in estimating the new values.


I am actually unaware.. I uploaded this program (http://www.societyofrobots.com/downloads/hold_servo.hex) to the micrcontroller (which is on the $50 robot board) and the servo just keeps spinning in one direction. I've already soldered two resistors to the leads which (before) connected to the pot as I mentioned in my earlier post. I'd like not to change that if possible. Rather, I was wondering if any modifications could be done to the code itself to make the servo behave how I want? By code I mean the $50 robot code for the Photovore.

Thanks,
Vignesh R.

 


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