Edington Junction

[stephenfreeman]

They can be - if the servo control board keeps sending signals then yes they are. Early versions of Servo4 were like this (high propensity to destroy a turnout/burn out the motor), hence one of the improvements was the option to stop sending the signals.

[davebradwell]

You're not getting this Stephen. The accepted definition of a servo requires that it has feedback so that when the output arm reaches the position described by the input signal the motor will be switched off - it doesn't need to move any more. The input position signal is repeated, I believe, every 20mS in an analogue servo. As long as each pulse is a repeat of the last signal the thing will not move so there is no need to switch off the signal. If you are stalling the servo then that is a fault and it is trying to move beyond the range you expect. I've been trying to tell you that the link to your tiebar should be able to absorb this condition to avoid damage, either to the tiebar or the servo which is not intended to be stalled.

Having eventually found an explanation of what the hobby calls a digital servo then I can't see why anyone would choose them for a layout. The advantages of faster response time and less deadband just won't make a difference and are more than overcome by the increased cost and noise when stationary.

DaveB

[Alan Turner]

Having eventually found an explanation of what the hobby calls a digital servo then I can't see why anyone would choose them for a layout. The advantages of faster response time and less deadband just won't make a difference and are more than overcome by the increased cost and noise when stationary.

Analogue servos are better for points because you can turn off the signal and the servo will not then move. Digital servos don't work like that and so will continue to move even when the signal is turned off.

You are quite right that any linkage to a servo requires that it has some form of "spring" action so that the servo never sees a hard stop and is always able to get to where it thinks it is going.

The reason why servos were found to be burning out was precisely because people were putting rigid links between the servo and point, which caused the servo to continually try to get to where it thought it needed to be and failing. A servo in this condition draws about 1 A so clearly that is not going to last long. The cludge that was incorporated in the MERG Servo4 was to turn the signal off but that only works for Analogue servos. Hence the advice to use analogue on points.

regards

Alan

[davebradwell]

That makes more sense but the fudge of turning the power off by removing the signal is a lousy one because it prevents the servo holding its position and there's nothing to stop it being moved by external forces. Not good.

DaveB

[John Donnelly]

My layout uses analogue servos with MERG Servo4 boards to control them. No issues with twitch on start up at all although the other South Pelaw layout I'm involves with that uses MERG CBUS instead has every servo twitch every time you switch the layout on.

Whist I use MERG products, I really have no idea how the boards work, all I can say is I've never had an issue with the servos being moved by external forces once in position.

John

[Alan Turner]

That makes more sense but the fudge of turning the power off by removing the signal is a lousy one because it prevents the servo holding its position and there's nothing to stop it being moved by external forces. Not good.

DaveB

Believe me they hold their position very well. It takes quite a bit of force to move a servo.

regards

Alan

[Serjt-Dave]

Hi All. First of all my apologies for not responding to the many helpful comments on my issue with servos etc.

It's not been a good start of the year for me and my family having my fathering-in-law passing away and having to go back to work. So not been able to do to much modeling etc.

Taking on board what everyone is saying. It's not a good idea to replace my analogue servos with digital ones but instead replace the fixed link between the servo and switch blades actuator with a link with a loop in it. Also adding a 10k resister between the positive and signal leads.

I've attached an image of one of my operating boards that carry all the gubbins that works the turnout. Although I like having the ability to being able adjust this link, I think it would be better and easier to make/fit a one piece link with a loop. Would I use a spring wire to make this link?

Adding the 10k resister to the mix. Would I solder the resister to the lead on the servo?

Again thank you very much for your help in this matter.

All Best

Dave

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