Travel limit switch placement for rotary turntable
Travel limit switch placement for rotary turntable
(OP)
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
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





RE: Travel limit switch placement for rotary turntable
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.
RE: Travel limit switch placement for rotary turntable
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
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RE: Travel limit switch placement for rotary turntable
So that both cams are only seen or detected by their appropriate proximity sensor?
RE: Travel limit switch placement for rotary turntable
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.
RE: Travel limit switch placement for rotary turntable
Mike Halloran
Pembroke Pines, FL, USA
RE: Travel limit switch placement for rotary turntable
TTFN
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RE: Travel limit switch placement for rotary turntable
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
RE: Travel limit switch placement for rotary turntable
What has to happen when you hit a limit? Do you need to stop? Do you need to change direction? Something else?
RE: Travel limit switch placement for rotary turntable
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
RE: Travel limit switch placement for rotary turntable
RE: Travel limit switch placement for rotary turntable
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.
RE: Travel limit switch placement for rotary turntable
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
RE: Travel limit switch placement for rotary turntable
RE: Travel limit switch placement for rotary turntable
My sketch is a wmf file. I don't have a pdf writer.
Here's an AutoCAD 2004 file.
RE: Travel limit switch placement for rotary turntable
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
RE: Travel limit switch placement for rotary turntable
RE: Travel limit switch placement for rotary turntable
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
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RE: Travel limit switch placement for rotary turntable
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.
RE: Travel limit switch placement for rotary turntable
Mike Halloran
Pembroke Pines, FL, USA
RE: Travel limit switch placement for rotary turntable
Are the targets mounted on the shaft or the turntable?
RE: Travel limit switch placement for rotary turntable
What happens at 360 deg of turntable? Doesn't see 0 degrees.
RE: Travel limit switch placement for rotary turntable
Half-speed disc rotates 200 degrees.
Mike Halloran
Pembroke Pines, FL, USA
RE: Travel limit switch placement for rotary turntable
RE: Travel limit switch placement for rotary turntable
Many thanks for your replies... much appreciated.
Regards,
Ewan
RE: Travel limit switch placement for rotary turntable
Yeah it was too close to lunch time and I was rushing. I mirrored the CCW sketch to the CW case and forgot to consider the switches not shown. I realized it during lunch, but then I got real busy with a production problem and didn't have a chance to say Oops!
A half-speed disc or putting switches at different radii would work, but I think it can work with switches at the OD. Let me ponder.