Boole helps simplify wiring and save money
Edited by Bill Travis
Jean-Bernard Guiot, Mulhouse, France -- EDN, July 8, 2004
To safely observe the positions of two cylinders, you need two signals: one circuit, X, which is open only when both cylinders are in a safe position (two normally closed switches in parallel), and one circuit, Y, which is closed only when both cylinders are in a safe position (two normally open switches in series, Figure 1).
This redundancy enables the detection of errors, such as a short circuit within a cable. The Boolean-logic equations of these two circuits are X=
+
, and Y=A·B. The problem is that most small limit switches are SPDT (single-pole, double-throw) switches, with which you cannot configure the circuit of Figure 1. Using Boolean arithmetic, you can derive X=+=+
, which corresponds to the schematic in Figure 2. You can easily make a circuit equivalent to that of Figure 2 using two ordinary SPDT limit switches (Figure 3).
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The basic concept the author presents is correct, however the boolean logic is incomplete, and the final circuit acutally defeats the fail-safe function of the original circuit.
The problem with the Boolean logic is that it only deals with each output line independently. Actually, the fail-safe requires both lines to be used together with an exclusive OR (or NOR) function to detect a fault.
It is true that X in figure 1 equals X in figure 3, and the same for Y. The difference lies in what happens if a switch fails or the cable shorts. In figure 1, a failed switch would cause either a X=1 and Y=1 or X=0 and Y=0. This is not true for figure 3. Using a truth table for X and Y to create a Z output will clearly show the difference when a switch fails or a cable is shorted or cut.
In otherwords, the suggested circuit (figure 3) is the same as the original (figure 1), only if nothing goes wrong. The reason two switches are used on each cylinder is a fail-safe redundancy. It is highly improbable that both switches will fail at the same time. If you only have one switch, there is no 'fail-safe' redundancy.
Yes, you can save money with the proposed design, but only at the expense
(trade-off) of safety.
I would suggest that you caution your readers to carefully analyze their application and this circuit before implementing it.
Dan Rogers - 2004-23-9 06:37:00 PDT -
re: Boole helps simplify wiring and save money - July 8, 2004 issue. The print version of this does not have the correct figures.
David B. Bley - 2004-22-7 09:44:00 PDT -
The electronic issue has the same circuit for all three figures.
Phillip Milks - 2004-19-7 08:14:00 PDT -
A last-minute mistake on our part put the wrong figures in for this Design Idea; we will post the corrected ones ASAP!
Bill Schweber - 2004-9-7 06:08:00 PDT -
My eyes aren't what they used to be, but it seems that figures 1 ,2 and 3 are identical??
Also what is the point of using the SPDT switch anyway, the whole reason for the redundant circuit is safety, and as the most likely fault is either the roller or the contacts within the limit switch, you gain nothing by putting the redundant circuits in the same switch!?
Bob Turner - 2004-8-7 20:59:00 PDT
