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Old Oct 23, 2012, 04:27 PM
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GWS S125 6T Problem

Hey guys,

I am working on a project for my Masters thesis and I have run into a problem involving a GWS S125 6T servo. I have the servo geared to a shaft (picture attached) and I am using it to change the angle of the shaft. The servos works perfect when I use it in a forward direction. When I reverse the servo however the servo turns until it hits its mark and then reverses and travels 5-15 degrees in the other direction. It looks like it is overshooting the angle and then correcting but it is hard to tell. Has anyone seen anything like this before?

I purchased 4 servos for use in the project and 3/4 of them exhibit this problem. I thought maybe it could have something to do with the gearing to the potentiometer. I need a servo that turns at least 540 degrees and has torque comparable to the S125. Is it possible that a S125 2T would behave better?

Thanks!
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Old Oct 23, 2012, 05:24 PM
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Most servos HAVE a mechanical stop molded onto the large plastic gear to stop running too far in either direction.
You should not be able to rotate more than + or - 160 degrees If you have the potentiometer still connected.

Removing the pot causes the servo motor become a F & R drive motor.

You did not reassemble the pot, joy stick & trim lever in the ...neutral... position. That does cause more rotation in 1 direction.

You will not get equal movement if you plug the servo into the throttle channel. Try any position that is a spring centered lever.
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Old Oct 23, 2012, 07:33 PM
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The PWM signals actually dominate all of its turns (3 turns each side) and degrees, while every minimeter the joystick and trim lever on your transmitter does will cause the variation of PWM sent by receiver significantly. The PWM applied for controlling this servo is normally 0.9 - 2.1ms. This 6T servo is digitalized one that applies very precise resolution regarding the steps of each degree you noticed on it, as a result each minimeter of movement you made on the joystick or trim lever will change the steps/resolution quite a lot. It's better to use a joystick or transmitter that has long enough stick travel range for the job you need. A good servo tester or micro controller works better than most R/C transmitter in this regard.

A 2T servo could rotate 1T each direction, that's 360 degrees left side and 360 degrees right side. So the movement you made on the joystick or trim lever makes the 2T servo rotate less sensitive than the 6T one. If the 540 degrees needed is in total, then 2T is good enough, if you meant 540 degrees each direction, probably you need 3T or even 4T one. Be reminded that you normally have to push the stick most and with the help of its trim lever for reaching the turns told by GWS in my experiences. Also, the 2T one might not be the said digital one. You have to refer to its plate sticker to see if it is digital. The travel ends on digital servo could be further changed by using programming card.

Chen
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Old Oct 23, 2012, 07:41 PM
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Hi Chen

My age is really showing.

Sorry VTechGrad. I Should have checked on the fact it is a multi turning servo.

Rich
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Old Oct 23, 2012, 07:53 PM
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Am I correct that 3 out of the 4 servos have this problem ? Does that mean 1 servo works as you thought it would ?

If yes. Send a Email to GWS about the 3 having a problem & brand new. Possibly where you bought them may have had the same problem before and can help you with 3 working units.
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Old Oct 23, 2012, 07:58 PM
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Hi Rich,

No worries, age is not problem at all. I did apply the 6T servo two years ago, that's why I have some ideas about its secret. And your advice does work great on most servos used on R/C planes. VTechGrad might have applied such multi turning servo on his great device, might not be a flying kind.

The said 6T servo actually needs to be controlled by a good servo tester or micro tester if you would like to make it work precisely. If you want it work on your sail boat, the R/C transmitter is good enough. So in my idea the all 4 servos are still good ones, the problem might have been the joystick he applied.

Chen
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Old Oct 23, 2012, 08:01 PM
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I looked them up on the GWS site. Specifications are terrific.
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Old Oct 23, 2012, 08:12 PM
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Yes, the modern technology, the computer chip, makes servos digitalized well. With this help there have been 3T, 6T and multi-T servos available. But R/C plane pilots need cheaper and lighter analog servos instead, and chopper pilots need speedy servos, while these multi-T servos are actually needed by robot users.
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Old Oct 23, 2012, 08:20 PM
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And to think I used to make a sail servo from the largest servo Then glue & bolt a wide grooved wood pulley on top of the servo output wheel to pull the line in & out.

I suddenly feel older.

Rich
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Old Oct 23, 2012, 08:23 PM
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I am old !!

75 this December. All the parts are still working well.

Thank you for all the help over the years. You put the fun in life.

Your friend
Rich
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Old Oct 23, 2012, 08:35 PM
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Hi Rich,

My pleasure ! Building and flying R/C planes make you younger and smarter, no worries at all. My oldest flying buddy is 87, you are much younger than this gentleman.

All the best,
Chen
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Old Oct 24, 2012, 09:47 AM
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Thanks for the advice guys! I am currently in the testing stages of my project but in the actual application the servo will be controlled by a Pololu servo controller. Ill hook it up and see if it makes any difference. It is strange that one of the servos does not be apearing to exhibit the problem.

I would really like to get these servo to work because they have exactly the stats I need. Otherwise I will need to find a different servo to do the job.

Thanks!
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Old Oct 24, 2012, 11:10 AM
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I still would get 3 that work right out of the box. You paid for that reliability.

Rich
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Old Oct 24, 2012, 09:01 PM
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VTechGrad,

If your servo controller could not ease the problem you experienced before, probablly the VR used inside of these 3 servos all need to be cleaned further. The potential dusts or the possible over-applied grease might have affected the VR itself, well, the grease was actually intended for lubricating the gears only. Widely operating the servo arm might be the good way to wipe off the possible dusts and grease from the contact surfaces inside of the VR. It's not hard to open up any standard servo then put it back together, but it's quite hard to do so on this 6T servo.

Chen
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