Travel limit switch placement for rotary turntable 1

[ewans]

Hello all,
Looking for some help with a problem that has me stumped...

I've got a rotary turntable that travels ±200° and I'm trying to locate inductive proximity switches and targets around the circumference of the turntable to provide CW and CCW limit signals. Ideally, I would like to provide 2 discrete signals, one for CW limit and one for CCW limit. However, it would be acceptable to provide a single output signal to indicate a limit, but not know which limit. I would like to use 2 or 3 proximity switches and some combination of targets, and would like to avoid requiring any software logic to evaluate the outputs.

So far, I've tried countless combinations of switch and target placements, but have not been able to find a configuration that works. Everything I've tried will provide an output signal at the correct limit when travelling in one direction, but with the direction reversed, it will provide a second incorrect output. In other words, it seems that I would need to include the direction of rotation in some form of logic circuit (i.e. "look only at switch A when travelling CW, and look only at switch B when travelling CCW").

Any chance I'm missing something obvious? Any help is appreciated.

Regards,
Ewan

 

[quark]

Are your limit switches stationary and position limiters moving?

You may solve your problem if you can make the limit switches moving and limiters stationary. That is fix the limit switches to the turntable back to back.

 

[IRstuff]

Why do you think you can get away with a single switch? You're traveling more than +/-180°. It's inevitable that you would be passing the 200° limit switch at 160° in the opposite direction.

However, since you know which direction you're going, you should be able to the limit switch output at 160° because you know you have to pass two 160° positions to get from -200° to +200°.
The sequence would be:
-200
-160
+160
+200

TTFN

FAQ731-376

 

[kingnero]

Can you not place these (cams, or what you might have fabricated) at a different position (in height)?
So that both cams are only seen or detected by their appropriate proximity sensor?

 

[ewans]

Thanks to all for the replies so far... much appreciated.

quark,
My goal is to have the proximity switches fixed to the moving turntable with stationary targets. This is the configuration I've been trying to make work, so far unsuccessfully.

IRstuff,
I don't think I can get away with a single switch... I think it would require minimum 2 switches because of the 40° travel overlap. I was hoping to avoid the need to include rotation direction feedback for the limit switch function.

kingnero,
Yes, I can have targets that only actuate a specific switch. The issue is that the switch would see it's dedicated target at 160° when traveling in the opposite direction. In other words, if approached in the CW direction, the switch would see the target at the 200° limit, but when the rotation reversed, the same switch would trigger at 160° when approached in the CCW direction.

 

[MikeHalloran]

You have to put each switch at a different radius, so they can't see each other's targets, unless you have some other way to distinguish the targets from one another.


Mike Halloran
Pembroke Pines, FL, USA

 

[IRstuff]

What you take away in intelligence in the controller must be made up in hardware. If you don't want your controller to fuss with directionality, then you have to add hardware that distinguishes the directionality and the occurences, since each limit switch is reached twice in a cycle.

TTFN

FAQ731-376

 

[ewans]

Hi IRstuff,
Thanks for your follow-up, and for confirming that the direction of rotation needs to be evaluated, either by hardware or software... this is the conclusion I had come to (albeit reluctantly).
Ideally, I would like to avoid a software solution, and can envision mechanical levers or links that could be used with an additional switch to provide an indication of rotation direction. I think the directional switch and limit switch could be wired in series to perform "AND" logic.

Any chance you could suggest a simple hardware solution for determining directionality?

Regards,
Ewan

 

[MintJulep]

is ±200° a total of 400° of rotation or a total of 200° of rotation?

What has to happen when you hit a limit? Do you need to stop? Do you need to change direction? Something else?

 

[ewans]

Hi MintJulep,
From a center home position, the turntable travels 200° CW or 200° CCW, for a total travel range of 400° end-to-end.

When the turntable hits either the CW or CCW end limit, it needs to stop.

Regards,
Ewan

 

[MintJulep]

What makes it go in the opposite direction after it reaches either end?

 

[MintJulep]

Regardless, I also come to the conclusion that if you are limiting yourself to limit switches then you need to know the direction of travel.

The attached simple relay logic circuit will reverse the direction of rotation at each end of travel.

It may not be bullet proof if the cycle is interrupted.

 

[byrdj]

this should give you a single indication of at either stop
with the wheel at mid rotation, make 0 TDC
place targets on wheel at +20 (a) and -20 (b)
place switches around wheel at -160 (A) and +160 (B)

wire the logic for stop when you have A AND B
when going CW, you first will get a/B make at 140, and then a/A AND b/B at 200.

when going CCW, you first get b/A at -140, and then b/B AND a/A at 200.

to indicate which stop, you need a C switch with a two contacts NO/NC in series with the A B series
place the target at 180 (c)
place the swith at -20 (C)

Going CW C will make at a stop and wire the NO to indicated
Going CCW C will not make at a stop and wire the NC to indicate

 

[tr1ntx]

If you used 2 limit switches and 2 targets, where, like you said, you use "AND" logic where both must be triggered, that should work. See sketch.

 

[tr1ntx]

I was doing my sketch while byrdj was posting. I think we're talking about the same thing: put a target & switch at 160 deg and another set at 200 deg, then stop when they're both triggered.

My sketch is a wmf file. I don't have a pdf writer.
Here's an AutoCAD 2004 file.

 

[ewans]

Hi byrdj and tr1ntx,
Thanks to both for your replies... appreciated.
Unfortunately, I'm pretty sure with the switch and target placement you proposed the following occurs:
- when going CW, you get a/A and b/B at 180°, not 200°
- when going CCW, you get b/B and a/A at -180°, not -200°

Do you agree, or am I missing something?

Regards,
Ewan

 

[byrdj]

in the words of Roseanne Roseannadanna "Never mind"

 

[IRstuff]

Even with the switches at different radii, each switch will be activated once at 40° away from the previous limit before the system actually reaches its opposing limit.

However, it might be possible to built "hoods" that cover up the opposite direction target when crossing 0°, i.e.,
assume two switches and corresponding targets, one labeled CW, and other CCW. So assume that we start at -200° and travel CW. The CCW target is exposed, and the CW target is hooded. As the system crosses -160°, which is the CW limit, the target is hooded, and there is no activation. As the system crosses 0°, the CW target is unhooded, while the CCW target gets hooded. Then at +160°, the CCW target is hooded and does not activate its switch, and the system continues on to +200°, where the CW limit is reached.

TTFN

FAQ731-376

 

[zekeman]

You can do it with sequential logic as follows"

Let A be the 0 degree position
B a position, say +10 degrees
B' a position , say -10 degrees
C= 200 degrees
C'=-200 degrees
To get the stop signals:
You start with 2 S/R latches, L1 and L2
S1=A ,R1=B
s2=A, R2=B'
Now 2 AND gates:
G1=L1*C'
G2=L2*C

where the A,B, B', C, C' are the proximity pulses
Now on any reversing signal, C to A,or C' to A,then the A pulse alone causes the stop.
Now going CW from A to C, the B pulse resets L1 and the stop signal is from G2=L2*C
Similarly , going CCW from A to C', the B' pulse resets L2 and the gate G1 provides the stop signal.
This is the logic. Not shown is the "enable" on L1 and L2 to release the stops, if necessary.

 

[MikeHalloran]

Put the targets on a disc that rotates at half the turntable speed.

Mike Halloran
Pembroke Pines, FL, USA