Servo won't center

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andrea.taras
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Servo won't center

Post by andrea.taras » Fri May 11, 2018 12:50 am

Hello again guys, maybe the title can be a little deceiving, but I'm having trouble centering the trim of the servo board that I just connected to my small DC motor. I'll explain better: I recently bought some budget-friendly digital HV servos so that I could use their boards as ESCs on my new robot. I've done this before with super cheap regular 6V servos and it's worked wonderfully.
So tonight I was following the same procedure, I pulled the guts out of the servo, I wired it up, I centered the potentiometer and all was fine. I then cut off the potentiometer and replaced it with 2 2.2K resistors. When I turned everything back on, the motor started accelerating like crazy and I thought it was just a matter of setting the TX's trim, but I had no luck with that, as even when the trim was all the way up, the motor was still slightly accelerating and never coming to a complete stop. I thought I had wired the resistors wrong, but that wasn't the case, as I still had control over acceleration (when you cut off the potentiometer without soldering the resistors the motor will just spin uncontrollably and won't react to the radio signals). I then wired up the potentiometer and centered it again. It centered fine. As soon as I removed the potentiometer and soldered the resistors on the board again, though, the motor started accelerating again like it was never centered. I can't find a way out... what am I doing wrong? Am I missing something? Is there any way I can center it bypassing the potentiometer, other than the transmitter's trim? Thanks in advance
Andrea

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Shakey
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Re: Servo won't center

Post by Shakey » Fri May 11, 2018 6:07 am

Not every servo uses a 50-50 split for the resistors, I suggest centering the potentiometer, carefully disconnecting it without adjusting it and measuring what resistance values each side of it is.
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peterwaller
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Re: Servo won't center

Post by peterwaller » Fri May 11, 2018 7:24 am

The other option is to leave the potentiometer in circuit, if it is not too big, then you can adjust it to give no drive when the sticks and trims are centered and then just lock it with a small blob of glue.

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andrea.taras
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Re: Servo won't center

Post by andrea.taras » Fri May 11, 2018 8:50 am

peterwaller wrote:
Fri May 11, 2018 7:24 am
The other option is to leave the potentiometer in circuit, if it is not too big, then you can adjust it to give no drive when the sticks and trims are centered and then just lock it with a small blob of glue.
The problem is that I was hoping to save some grams by removing it, considering that across the section it's as big as one of the small brushed motors we use (but only 4mm thick). However that would definitely be an option on the table now, if I can't find another way out.

Shakey, that I did not know! I'll measure it and get back with what I find out

EDIT: Nope, it's precisely 50:50. However I just noticed that its resistance is of 4.7K, so maybe it's got something to do with the fact that I'm only replacing it with 4.4k total resistors? because I have no clue what else it might be. In the past I've used the same resistors to replace a 5.1K potentiometer and it's worked fine, however that was on a regular 6V analog servo and not a HV digital one.
Andrea

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andrea.taras
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Re: Servo won't center

Post by andrea.taras » Fri May 11, 2018 11:16 am

andrea.taras wrote:
Fri May 11, 2018 8:50 am
so maybe it's got something to do with the fact that I'm only replacing it with 4.4k total resistors?
Scratch that, I wired two more for a total of 8.6k according to my tester and it still isn't centering
Andrea

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Shakey
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Re: Servo won't center

Post by Shakey » Fri May 11, 2018 1:15 pm

For £1 more than those servos cost you could just buy a vex29?
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Alex Shakespeare - Team Shakey / Nuts And Bots / Team Nuts:
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andrea.taras
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Re: Servo won't center

Post by andrea.taras » Fri May 11, 2018 1:33 pm

Shakey wrote:
Fri May 11, 2018 1:15 pm
For £1 more than those servos cost you could just buy a vex29?
That would be the smart way to go, but not the fun way lol :wink:
Andrea

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andrea.taras
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Re: Servo won't center

Post by andrea.taras » Wed May 16, 2018 5:30 pm

Okay, in the end I decided to go with an even cheaper (and lighter) DasMikro, which should be at my doorstep in a few days, hoping the customs won't have anything to say about it :)
Andrea

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