gamemasterks

Servo Motor not fully centering anymore

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I don't know if this is normal but I've used my PF Servo Motor for two months now and it doesn't fully center like it used to anymore, the angle is always a little off. Is this normal for a PF Servo Motor after normal use? I didn't use the motor heavily, I just used it occasionally in my Lego RC Car once in a while.

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It is the same situation with every PF servo that I have (4 of them)

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Posted (edited)

I will take a internal picture of my servo later tonight. The problem is probably the angle sensor, the servos don't use the normal angle sensors you see in mindstorms motors and control plus ones, they use a metal contact to tell the motor driver it's positions and the thing is they don't have a "mounting point" for the metal contact, so it can usually move out of it's position therefore giving the sensor false angles.

 

Found a picture! Not mine though.

TechnicBRICKs: TBs TechNuggets 13 - Inside the PF L-Motor and SV-Motorpict

Edited by JoshsTechnicWorkshop
picture

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In my experience the servo motor's center position can be off by a negligible amount. When I use it in a creation I can usually see the output rotating slightly when turning on the battery box (I'm guessing it's to calibrate center position). However, if the dots are too far apart at center position then it's probably an issue with the motor.

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any luck with the factory response?

 

Response from LEGO SHOP

17 November 2017
Jordan, Consumer Services

There is a fix for this! If the motor is not returning to center it is because it has been told that a new direction is actually center. To fix this it is important that the output is center positioned – this is indicated when the 4 dots at the output are aligned. To set the Servo in center position, connect it to an output with power ON but no control (for example on an IR Receiver that is turned ON). I hope this helps you get back to building!

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I have one where if it gets to (approx) the 4th position out of 7 in one direction it'll shoot to the 7th, and stay there. The other side is fine.

I already pulled the model apart and took the motor apart to try and fix it. Solved it for a few minutes then it started doing it again. 

No clue what's actually wrong with it but that's 30€ down the toilet.

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Mine had a similar issues too. One mail to Lego and they sent a replacement by express mail. They didn't even ask me to ship them the defective one. 

I've read a lot of these servos had issues. I think TLG knows it and try to make for a bad part with good service. But it won't work for second hand, so if you need one, I suggest you buy new, directly from TLG, while they still have it available. 

This part dates from 2010, if I'm not mistaken. A shame they didn't issue a revised version in 10 years after so many failures...

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Posted (edited)
1 hour ago, 1980SomethingSpaceGuy said:

Mine had a similar issues too.

Good to hear I'm not the only one, mine exactly does this:

 
1 hour ago, 1980SomethingSpaceGuy said:

I've read a lot of these servos had issues. I think TLG knows it and try to make for a bad part with good service. But it won't work for second hand, so if you need one, I suggest you buy new, directly from TLG, while they still have it available. 

It was bought via BrickComplete, and came brand new in the original packing.

I guess it doesn't cost me anything to ask them. Thanks for the tip!

1 hour ago, 1980SomethingSpaceGuy said:

A shame they didn't issue a revised version in 10 years after so many failures...

It'll never come now: PF is dead.

I'm really hoping the various Chinese makers issue a new version with analog steering, and including their more powerful electric motors.

Edited by amorti

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Thanks to everyone for sharing, I guess the servo motor isn’t that reliable after use. I would normally buy another servo motor from Lego itself but I’ve reached my limit of servo motors that I can order being shipped to my household. Lego sadly in the US limits each person to 3 of each Power Functions components per household which is sad because when I look at the European Lego website, there is a limit of 50 instead of 3 for each component for European customers. You guys are lucky :). Interestingly 2 of my 3 servos have different problems. One doesn’t center fully and the other doesn’t return to center when you let off the IR remote quickly like it used to (See amorti’s post in the topic to see what I’m talking about). I tried various solutions first I tried,

11 hours ago, bonox said:

To set the Servo in center position, connect it to an output with power ON but no control (for example on an IR Receiver that is turned ON).

However this didn’t sadly work, then I tried to correct this with the BuWizz power curves feature (I use BuWizz for my RC MOCs). In theory this should work but in practice the power curve interface isn’t accurate enough to make small corrections, so you either overshoot the servo position too much or too little.

2 hours ago, amorti said:

I'm really hoping the various Chinese makers issue a new version with analog steering, and including their more powerful electric motors.

I hope so too because when PF dies, I’m still using it for most of my RC MOC’s. However, I really hope they make good versions because when I heard about the Chinese Monster Motor (Clone of Buggy Motor) I heard many stories of people’s motors just burning up. 

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Just tested my spare servo motor and found it was off centre by about 3 degs.

With battery power OFF connected it directly to the battery box and then switched power back ON.

Instantly the servo self centred itself and with all dots aligned OK.

Tested with PF switch, PF remote & PF train remote and each time returned to centre OK.

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41 minutes ago, gamemasterks said:

 I’ve reached my limit of servo motors that I can order being shipped to my household. ... 2 of my 3 servos have different problems. 

Seems like Lego owes you two replacements?

Otherwise get three shipped to your neighbor or your mom. Easy.

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I saw some YouTube video once that suggested opening up the motor and cleaning off the semicircular contacts with a pencil eraser (and then blowing off the eraser dust). I have not tried this personally, but you could give it a shot!

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I tried that. It's probably a fault in the circuit board itself, because it was only a few hours of play old and the tracks were clean, nothing damaged.

Edited by amorti

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14 hours ago, amorti said:

I tried that. It's probably a fault in the circuit board itself, because it was only a few hours of play old and the tracks were clean, nothing damaged.

I have found with many of my motors that the insulation on the wires at the points where they enter either the motor or the plug can wear down, causing servos to malfunction, and normal motors to stop operating. If this is the problem, trying different wire positions, and pressing the wire into the motor will likely make a difference. When this happens to me, I will open up the motor, remove the wire, cut out the bad portion of the wire, and re-solder it. However, when I try this on servos, I usually fail because of the four leads in a small space on the circuit board; perhaps a more skilled solderer would have better luck. Of course, you may have a completely different problem...

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I'm pretty sure it's something else. The servo worked for an hour or so of play, then hit and won't work right again. The circuit which senses the position is kinda janky, relying on multiple sliding connectors each of which runs into a PCB with resistors and whatever else. I think one of those tracks has failed or a component wasn't right from the beginning.

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On 6/30/2020 at 4:31 PM, amorti said:

mine exactly does this

Had strictly identical symptoms.

On 6/30/2020 at 7:34 PM, Doug72 said:

Instantly the servo self centred itself and with all dots aligned OK.

Yes, it sometimes stops a bit off center, but resets (power OFF/ON) correctly when in this position. Same when powered off on the normally functioning side: it gets back to center when the power is restored. However, on the side from which it does not recenter (release RC control) automatically, tho, it doesn't reset either. 

I also noticed it seems to go further on the "dead" side. This would point to something like it doesn't get the signal to stop, runs all the way to the physical limit and stops on torque/tension limitation. It then fails to run the recenter logic.

Since they didn't ask me to ship back the defective one, I might try to open it, figure out how the stop signal at 90 degrees is supposed to happen and investigate. But I fear that if a team of professionals couldn't solve it in 10 years, it might prove too hard to fix. 

When TLG stops selling them at all, we might get third party alternatives, certainly less cheap...

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Hi again everyone,

I contacted TLG and they said that replacement Servo Motors would be shipped to replace the broken Servo Motors I have. Also, does anyone know of any good third party Lego Servo Motor clones? I seen some on YouTube but I don’t know where to get them. I’m asking because I don’t want to buy another Servo Motor only to have it break after the last ones did.

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The clone servos are all junk. They operate digitally, but even if you're using Lego digital controls they're no good. If they get an instruction to turn left, they have to turn a full 90° then return to the centre. This is horrible for drivability of any larger or faster model, although you can get away with it on small stuff especially if they only have a limited steering angle anyway.

They're often stronger and faster than Lego servos which isn't a benefit, as they'll rip apart a lot of steering racks especially if it is built to block the steering physically before 90°

Edited by amorti

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10 hours ago, amorti said:

The clone servos are all junk. They operate digitally, but even if you're using Lego digital controls they're no good. If they get an instruction to turn left, they have to turn a full 90° then return to the centre.

Then what should I do? TLG wont be there to constantly replace Servos after Power Functions dies at the end of the year. Therefore, I can’t have Servos that just keep on breaking. 

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