point servo & Aux Switches

[aleopardstail]

Hi, on Dingleberry I'm using the pretty common SG90 servos for point control, jammed in 15mm metal C channelling, and for switching points it works. I was wondering, is there an "accepted" way to piggy back some sort of auxiliary contact switch to this arrangement?

 

don't exactly need it here but say for frog polarity switching?

 

I did think of physically mounting a pair of small microswitches to either side of the channelling but was wondering is there a better way to do this? Obviously I can also use the driver MCU to control switches, or for like frog polarity maybe drive a small relay but it would be nice to have some reasonably self contained way to add a few switches.

 

thoughts?

[AndyID]

7 hours ago, aleopardstail said:

Hi, on Dingleberry I'm using the pretty common SG90 servos for point control, jammed in 15mm metal C channelling, and for switching points it works. I was wondering, is there an "accepted" way to piggy back some sort of auxiliary contact switch to this arrangement?

 

don't exactly need it here but say for frog polarity switching?

 

I did think of physically mounting a pair of small microswitches to either side of the channelling but was wondering is there a better way to do this? Obviously I can also use the driver MCU to control switches, or for like frog polarity maybe drive a small relay but it would be nice to have some reasonably self contained way to add a few switches.

 

thoughts?

 

It sort of depends on how much effort you want to put into creating a mechanism that works reliably. You can certainly use a couple of microswitches or even a single microswitch if you prefer. If you'd rather avoid the mechanical arrangement and don't mind a bit more wiring the simplest way to do it is to use a two-pole changeover switch at the control panel. One pole drives the controller and the other pole switches the frog polarity.

 

If you don't mind all the servos having the same throw angle (say 90 degrees) and use a mechanical arrangement to connect the horns to the tiebar you can drive them all from two pulse generators. The control switch selects which pulse drives each servo. An extension of that method is to put a 2-pole changeover relay at the point and control it via a single wire from a single pole on/off switch. One of the relay's poles selects which pulse generator is connected to the servo. The other pole supplies the correct polarity to the frog.

[grriff]

MERG (Model Electronic Railway Group) sell an SG90 mount with a fitting for a microswitch. It is only available to members (£16 p.a) but if you know a member they may have one to try. 

David

[Pannier Tank]

Have you seen :- http://www.dingoservo.co.uk/product-list.html

[aleopardstail]

5 hours ago, grriff said:

MERG (Model Electronic Railway Group) sell an SG90 mount with a fitting for a microswitch. It is only available to members (£16 p.a) but if you know a member they may have one to try. 

David

actually in MERG, hadn't seen that, will have a look cheers :)

[aleopardstail]

13 hours ago, AndyID said:

 

It sort of depends on how much effort you want to put into creating a mechanism that works reliably. You can certainly use a couple of microswitches or even a single microswitch if you prefer. If you'd rather avoid the mechanical arrangement and don't mind a bit more wiring the simplest way to do it is to use a two-pole changeover switch at the control panel. One pole drives the controller and the other pole switches the frog polarity.

 

If you don't mind all the servos having the same throw angle (say 90 degrees) and use a mechanical arrangement to connect the horns to the tiebar you can drive them all from two pulse generators. The control switch selects which pulse drives each servo. An extension of that method is to put a 2-pole changeover relay at the point and control it via a single wire from a single pole on/off switch. One of the relay's poles selects which pulse generator is connected to the servo. The other pole supplies the correct polarity to the frog.

 

already have the driver, going for computer and not panel control, I could also drive a relay reasonably easily but prefer the idea of the mechanical interlock (goal is one switch for a frog and another one or two for feedback on the position), issue is more mounting the switches to be operated reliably - first pass was screwing them to the sides of the channel but the servo horn doesn't touch them, may just have to make a custom mounting - I've got a 3d printer so shouldn't be too hard. was more wondering if there was "accepted wisdom" on solving this

 

:)

[AndyID]

On 05/06/2022 at 05:54, aleopardstail said:

 

already have the driver, going for computer and not panel control, I could also drive a relay reasonably easily but prefer the idea of the mechanical interlock (goal is one switch for a frog and another one or two for feedback on the position), issue is more mounting the switches to be operated reliably - first pass was screwing them to the sides of the channel but the servo horn doesn't touch them, may just have to make a custom mounting - I've got a 3d printer so shouldn't be too hard. was more wondering if there was "accepted wisdom" on solving this

 

:)

 

It's not a bad idea to use position feedback with servos. They are inclined to respond to electrical noise/interference by going bonkers. Your mechanical arrangement should accommodate the possibility that they will full-travel in either direction for no apparent reason.

[aleopardstail]

1 hour ago, AndyID said:

 

It's not a bad idea to use position feedback with servos. They are inclined to respond to electrical noise/interference by going bonkers. Your mechanical arrangement should accommodate the possibility that they will full-travel in either direction for no apparent reason.

the driver software is (currently) set up to drive 16 servos and simulate 32 switches back into JMRI, it wouldn't be all that hard to replace some or all of that with actual switches (reading them is just a few 74hc165 static shift registers and not a lot of code afterall.

 

have noticed one or two going "heeeellllllooooo Mr Endstop"

 

so far have this as a basic 3d printed mount, no switches though

 

likely wouldn't be overly difficult to adapt to have a pair of limit switches low down and swap the control horn for one that sticks down as well as up.

 

will have to have an experiment, the ideal is something monitoring the actual point blade position but the servo horn position is probably next best, pair of small micro switches  and a small circuit board to hold some screw terminals and some pull up/down resistors for position..

 

pondering...

[AndyID]

1 hour ago, aleopardstail said:

the driver software is (currently) set up to drive 16 servos and simulate 32 switches back into JMRI, it wouldn't be all that hard to replace some or all of that with actual switches (reading them is just a few 74hc165 static shift registers and not a lot of code afterall.

 

have noticed one or two going "heeeellllllooooo Mr Endstop"

 

so far have this as a basic 3d printed mount, no switches though

 

likely wouldn't be overly difficult to adapt to have a pair of limit switches low down and swap the control horn for one that sticks down as well as up.

 

will have to have an experiment, the ideal is something monitoring the actual point blade position but the servo horn position is probably next best, pair of small micro switches  and a small circuit board to hold some screw terminals and some pull up/down resistors for position..

 

pondering...

 

As you have a printer you might also consider making the servos ultra-reliable by removing their electronics and just use the motor and the gear-train. "All" you need to do is print a cam that goes on the servo output. The cam acts on a micro-switch that disconnects power at the limits. The input control can be a simple SPDT switch but in your case an inexpensive DPDT relay. One pole would activate the servo and the other pole would switch the frog.

 

I posted a topic about it on here yonks ago but I can't find it now. Let me know if you are interested and I'll provide more details.