Three Position Limit Switch

[NicSacco]

I am looking for some sort of switch, much like a regular limit switch, that would be able to send a signal to my plc to shut a system down. This can easily be accomplished by a normal limit switch, However, I need the same switch to keep count of positioning. I believe a switch with three positions would work but I am unable to find one.

Any suggestions?

 

[Mandrake22]

Are you looking for the three positions to be physically different from each other? Postition 1=off, Position 2=PLC shutdown input and Position 3=postioning count input? If so, I don't know of one either. Is your "keep count of positioning" function an input to the PLC? If so, this might be an 'opportunity" to learn about rotary encoders!

 

[AeroPD805]

Maybe I'm over simplifyng this, but why not set 2 switches side by side?
Or, utilize a mechanical counter?

 

[steveh]

Why not let the PLC keep track of the count? Each time the switch is made, increment a counter.

Steve

 

[quark]

NicSacco!

That is what a Mobrey Switch does for a boiler. This switch has a float which raises and lowers with the level of water in the boiler drum. A magnetic material attached to the float is being sensed by a set of magnetic switches at various levels and give out the signal and control action.

What is your application?

Regards,

Repetition is the foundation of technology

 

[tapetotape]

everybody is making sense but all the answers are application dependant. more specifics on the process are needed.


gravity is our friend

 

[NicSacco]

I'm sorry, I should have provided more detail.

I have a quater turn actuator that turns a shaft 360 degrees over 9 positions. This would make it turn 40 degrees with each turn. The problem I am having is getting the actuator to stop at exactly 40 each time. The first function of the roller limit switch would be top mark the home position, It would then have to mark the other 8 stations with a different signal. I would have to know which directin the shaft was and if it was slipping.

Two Limit Switch
Would work but there is not enough room.

Looking into the other idea's.

Thanks

nic

There is no prosthetic for an amputated spirit

 

[testy]

How accurate does it have to be? Sounds like a job for a servo motor with the above mentioned rotary encoder. If it is a pnuematic actuator they are notoriously inaccurate.

Don't go skatin through the buffalo herd.

 

[ve7brz]

I agree with testy. You won't get reliable position accuracy with a limit switch. A rotary encoder or a stepping motor is your only guarantee of accuracy. These methods will also give you your start and end positions if they're started from a known position.

 

[AeroPD805]

Without knowing the load on your roller, it's hard to
say which motor to use. Although, there are some rather
large stepper motors on the market.
For accuracy [I agree with the others] I'd use an optical
rotary multi-channel encoder with indexing pulse.
If you go with a stepper, you'll need to electrically
align the index pulse with a phase/step of the motor
for repeatability.
If you had more space a less expensive microswitch &
cam setup might be the ticket.

 

[Mandrake22]

NicSacco,
Are you using a "conventional" sized limit switch now? If you are, there are much smaller variants available that you might be able to squeeze in. Micro Switch, for example, has a model 924CE that has a body size of 1.58" wide x 2.9" lg.

 

[sancat]

Nic, I cant exactly understand your design, but is it possible to extend the shaft and mount a cam, that will act the limit switches?
Another possibility would be to build a limit switch, by means of a spring to push forward a small shaft, on one end it will have your roller, and on the other, the necessary smaller diameters to actuate when pushed back two very small limit switches that are transversal to the shaft. In that way, the stress of the shaft will never affect you switches. I dont know if my explanation is good, but is there a way to send a draft with this forum?
sancat

 

[tapetotape]

Obviuosly everyone agrees that a limit switch is not really intended for, nor will it ever provide, what you need.
From your description I still can't tell much about your process, you could be rotating a part mounted on a machining surface on the end of the quarter turn shaft or you could be machining shafts. I don't know how far out of alignment you are or how often it occurs.
A rotary encoder will certainly improve your process reliability and up time but can be expensive to retrofit and may not be a good choice if it can't be protected from process waste, cutting oil, etc..
Since you have now mentioned that you are looking into other solutions I would strongly urge you to return to basics. By this I simply mean that you should take care to separate cause from effect and bear in mind that your essential problem is positive location and not necessarily motor control.
Remember that servos, encoders, prox. sensors etc. are usually methods of overcoming the inability to utilize a positive mechanical locator or stop.
It has been my experience that a cam arrangement provides excellent repeatability on rotating parts.
If there is a machining head that lowers to a part and your rotation alignment is pretty close then it may be possible to use a locator pin.
What I'm getting at is that one of the biggest mistakes I encounter when I'm troble shooting process' is that people tend to chase symptoms instead of root cause.


gravity is our friend