Limit switch to control motor

Need some advice..
I am designing a small feed system for lumber grading. This feed system is part of a lumber grading system.
There is a motor drive feeding the individual boards along a horizontal surface using rubber wheels or rollers. The feed motor is started manually with a magnetic switch.
At a certain point the piece of lumber should activate a limit or proximity switch indicating the lumber is correclty against the horizontal surface of the grading component ( very important to correctly grade)
If the board rises above the surface , even as little as 1/3 in., the limit or proximity switch should shut the feed motor down using the same magnetic switch used to start it..(severly warped board, etc..)
I wish to use one limit/proximity switch to do this. The difficult part is that the switch is normally open while the board is being fed through, then becomes closed, then open if there is a problem.
The open switch then needs to shut the feed motor down...
I have been looking at metal roller ball type switches, preferably dust-proof and rugged.
How to perform this trickery?
Norman
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Norman Pirollo wrote:

Sounds like an interesting project.

Is this lumber of the heavy board type? Does it bounce up and down on the rubber wheels or rollers easily?

I will presume this is a motor starter contactor unit where the coil of the contactor is in a circuit that will stop the motor if broken.

A switch between the roller set (one of the standard metal housed types with a sprung probe actuator) would probably do that for you (if the boards are on the heavy side - no good if they are sawn timbers in the domestic sizes - less than 6 by 2).
You will also need another switch that detects that a board is on the conveyor. This can be achieved by a light beam type detector across the conveyor. This should be placed in-line with the under conveyor limit switch and wired such that the motor will still run with no board on the conveyor and when the board is on the conveyor and is lying flat down.

Which it would if in series with the contactor coil. That sort of accuracy is not really a problem, just a matter of being able to adjust the switch position.

I do not think there is a solution with just one switch. Think of the number of system states that you are having to deal with. Draw a truth table for this and you will see the number of independent switched that will be required.

Which may indicate a need for a third switch (and/or maybe a timer) to determine the board's exit from the influence of the system.

See above for types. I think the spiral spring type might be the most suitable. However, only you know how much space you have and the environment you are dealing with so far so your judgement may be best on this matter.

Draw the conveyor plan out, place points for the detection of the product appropriately and then formulate the truth table I suggested. Make a cardboard model if that helps you visualise it (I often do for some of the complex moving machinery tasks I design controls for). Come back with your resultant thinking if you need any firther advice.
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Thank You, will do
On Wed, 17 Nov 2004 20:12:56 +0000, "Paul E. Bennett"

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Good micro PLC application. You will end up needing a few I/O of different types and count on adding one or two in the future.
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mindspringnews wrote:

I didn't think the application (as described) warranted that level of complexity. However, the OP might have other things in mind and he would be best placed to decide that. Especially once he has followed the advice to lay out the plant diagram and to think about the logic of how the whole thing must operate.
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