Tam Valley Depot Servo Controls

[C855B]

Hmm... dual links? Good luck with that. Given all the loose tolerances, and materials and adhesives that tend to flex, the likelihood is zero that any adjusted "perfect" linkage will stay adjusted (or perfect). I'd save myself a lot of grief and just spring for the separate controller and servo.

[N_DaveS]

Yeah, you're right about that. I already have the right number of servos so it's a matter of getting more controllers.

On the other hand it's much less of an engineering challenge so the work should go quicker overall.

Thanks for the insights!

[Big Windy]

On Tam Valleys site it states you can use his y cable and control two servos,so it can be done.

[C855B]

On Tam Valleys site it states you can use his y cable and control two servos,so it can be done.

I stand corrected. You're absolutely right. I'm now curious how this can work.

[peteski]

Won't work. Servos are a feedback thing - the controller makes movement decisions based on what comes back from the servo's position sensor. Having two position sensors telling it what to do would be loads of fun. 

Many years when I was into RC helicopters, servos did not send feedback back to the controller. They only had 3 wires: Power, ground and control.  The control signal was an input to the servo. The controller provided a PWM signal to the servo, directing it to go to a certain position. All the feedback was done inside the servo itself.

Just as an example, if the controller provided a control signal with 50% duty cycle then the servo would center itself.  75% would make the servo to travel to one extreme throw and 25% to the other extreme and so on.  That way Y-cables worked just fine when driving multiple servos.

[C855B]

Ah. Thanks. More info here: http://pcbheaven.com/wikipages/How_RC_Servos_Works/

[N_DaveS]

I actually did get the Y-cables to do this, and I am pretty sure I double-checked with TV as to the feasibility. Hmm, is that a guinea pig suit hanging on that hook? Perhaps I should try it on for size...

[Ian MacMillan]

Yes you can do it and I have done it on my last layout. The PITA is making sure that both throw far enough each way, because the decoder is only remembering one set of throw distance.

[N_DaveS]

The PITA is making sure that both throw far enough each way, because the decoder is only remembering one set of throw distance.

Definitely a PITA. I'll need to make sure that the linkages attached to each turnout's throwbar are the same length to ensure the same throw distance.

When you set up the crossover servos, did you wire up just one servo, set the throw distance on the decoder, then wire up the second servo to the Y-cable? It seems like you wouldn't want the decoder having to choose between two sets of feedback on setup.

[C855B]

... The PITA is making sure that both throw far enough each way, because the decoder is only remembering one set of throw distance.

Aha! That's what I thought, although it was more like, "How do you resolve the different endpoint calibrations?" The answer is: you don't, really. Thanks for the confirmation that I'm not going completely crazy... crazy... crazy... crazy...

[peteski]

Aha! That's what I thought, although it was more like, "How do you resolve the different endpoint calibrations?" The answer is: you don't, really. Thanks for the confirmation that I'm not going completely crazy... crazy... crazy... crazy...

True. Since both servos will be feed the same control signal, they will travel in unison. Same amount of throw, same direction.

[Ian MacMillan]

It seems like you wouldn't want the decoder having to choose between two sets of feedback on setup.

Yes only set up one.  The second one you can make fine adjustments by moving the arm or using the bend in the music wire.