Limit Switches

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Originally Posted by Robin Hewitt

If you want a microswitch to set the zero point, rather than spot a whoopsy, make sure you always use the same transition. They have huge hysteresis.

I prefer an opto slot. They usually move from high to low in a fairly straight line as the beam is interrupted. Reading it is slightly tricky because logic gates have big hysteresis, so, you add a comparator and get no hysteresis at all. Switches perfectly in either direction.

Any cable will do, if worried about interference, twist it :D

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Your going to have to brake it down for me please because some of these words i know nothing about:

1, Zero Point
2, Transition
3, Hysteresis
4, Opto Slot
5, Logic Gates

1, Zero Point: A base point for the machine to measure it's tool position from. A limit switch could simply tell the software it had better stop now because what comes next ain't gonna work :D

2, Transition: The switch closes or the switch opens. A logic gate switches low-high or high-low.

3, Hysteresis: The difference, distance-wise, or volt-wise between the transitions

4, Opto Slot: An electronic gizmo that tries to shine a beam of light across a small gap and tells you when it can't do it because something is in the way.

5, Logic Gates: TTL, CMOS, AND's, OR's, NAND's, NOR's, XOR's, inverters, flops etc.

Robin, complete as your answer was I think a tad more explanation might be needed. Here's my interpretation...

Zero Point & Transition
This is using a switch to tell the CNC computer that the tool is at some known XYZ co-ordinate in space e.g. 0,0,0. The problem with using switches is that the transition point where the switch changes from open to closed depends on mechanical pressures and so suffers from issues similar to backlash. Approching the transition point from the same direction at the same speed aids accuracy and repeatability.

Hysteresis describes the variation in switching point of a device (electronic or mechanical) between transitions in one direction versus another. A typical example is an electronic switch might be that it comes on at say 3 volts but doesnt go off until the voltage drops below say 0.7 volts. Another example is a magnetic reed switch which switches on when the magnet is say 0.1" away but doesnt switch off until it is moved 0.3" away.

Opto Slot
The use of a light beam to indicate a transition by detecting light coming through a slot (or not). Capable of a very high accuracy, using laser light, down to a thou or better.

Logic Gates
Electronic switches that always work on the binary on/off principle where binary 1 = ON = say 3volts and binary 0 = OFF = 0.7volts, the point being that the actual voltage is immaterial, anything over 2v is ON, anything under 1v is OFF and the system is not allowed to remain in the in-between state. Logic gates are the underlying building blocks of computers, implementing boolean algebra between 2 or more logic signals e.g. the 2 input 'AND' gate implements the equivalent of the truth table:
Input 1 AND Input 2 IS Output
OFF AND OFF IS OFF
ON AND OFF IS OFF
OFF AND ON IS OFF
ON AND ON IS ON

while a 2 input 'OR' gate implements
Input 1 OR Input 2 IS Output
OFF OR OFF IS OFF
ON OR OFF IS ON
OFF OR ON IS ON
ON OR ON IS ON

A third gate, the NOT gate implements the invert function on one input:
NOT Input1 is Output
NOT OFF IS ON
NOT ON IS OFF

From these three fundamental building blocks any computing function can be implemented (actually only the NOT gate and one of the other 2 is needed - can you see how?). Indeed a modern PC is no more than hundreds of millions of these devices wired together.

A logic gate can be implemented in any suitable technology (including mechanically - think of a double in-line clutch - both must be engaged for drive = AND gate) but all modern systems are based on low-power CMOS technology or its variants.


Hope this helps...

Wow guys thanks alot for your posts i understand everything now. Hoping to get my machine to the point of "moving" htis weekend, ill update my build log with the pics and so on. Thanks again guys.