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[Design Briefs]
 Analog Voting Circuit Is More Flexible Than Its Digital Version R. Jayapal  |   ED Online ID #6886  |   December 4, 2003

Majority voting systems are used to protect critical plants and processes. Such systems find applications in chemical, power, nuclear, and aerospace industries. Here, multiple sensors monitor a critical parameter, and the readings of, say, two sensors (which are closely tracking) are taken as correct. But if one sensor fails, the critical parameter is still monitored.

Figure 1 shows a typical majority voting system in which three identical initiators (A, B, and C) give 0 or 1 logic signals, depending on whether a process parameter—such as temperature, pressure, or level—is below or above the trip setting. These logic signals are passed to a two-out-of-three voting circuit. The voting circuit sends a trip signal to the trip system only if two or more initiators are in a trip condition. This voting circuit follows the expression:

Y = AB + BC + CA

To improve reliability, four initiators are used with the two-out-of-three voting circuit, following the expression:

Y = AB + BC + CD + DA + AC + BD

Similarly, for a three-out-of-four voting circuit, the following expression applies:

Y = ABC + BCD + CDA

In a typical high-pressure, high-temperature liquid-level monitoring application, conductivity probes were kept at various levels. Each probe circuit gives a logical 1 output when the liquid level reaches that probe. In view of how critical this measurement is, two-out-of-three logic for tripping was adopted. That is, if the nth probe is set as a trip point, (n − 1) and (n + 1) probes are also monitored. If any two probes go HIGH, the voting circuitry gives a HIGH output for further action, like a trip, etc.

For the same application, some users wanted two-out-of-four voting logic, and others wanted three-out-of-five voting logic, etc. With a microcontroller-based circuit, this meant either rewriting the program each time or writing a program for various logic sequences and selecting the one required.

To eliminate these difficulties, an analog circuit was designed that simplified the selection of the number of initiators and the voting logic (Fig. 2). The first op amp of U1 is a buffer for the voltage adjusted by P1. Potentiometer P1 is adjusted to get 1 V at TP1. U1's second op amp is configured as an inverting summer, while U1's third op amp is an inverter.

Every probe output going to logical 1 closes the switch corresponding to it in U2. Closure of each switch in U2 gives a 1-V increase at TP2. If a two-out-of-three voting circuit is needed, select the appropriate probes through the DIP switch setting. Voting by two probes gives 2 V at TP2. Set the reference voltage at TP3 to under 2 V but above 1 V. Output Y will be 1 if two or more probes output a logic 1. Similarly, if two-out-of-four voting is required, select the four probes with the DIP switch and set the reference voltage at TP3 as 1.5 V.

To generalize, if m is the number of probes selected, and n out of m voting is necessary, select the m probes with the DIP switch and keep the reference voltage at TP3 slightly less than (n × 1) V, but more than (n − 1) × 1 V. With the circuit of Figure 2, m has a maximum of eight, and n m. This circuit can be extended for any voting logic, which can be set or altered even in the field.

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I was really scared while reading this article. Is the author really suggesting using this circuitry for 'to protect critical plants and processes'...'in chemical, power, nuclear, and aerospace industries'?

The all idea of redundancy is based on 'spreading' the criticality among multiple elements. The proposed circuitry makes the results dependant on SEVERAL singular element like U1, P1, P2. The fault of ANYONE of them will cause a failure trip. The circuitry is certainly 'flexible', but this is not really required here.

Has author ever heard of Fault Tree Analysis? Has he ever asked himself how reliable a potentiometer is? Only partial comfort can be provided by the fact that this kind of 'technology' is originated from remote India. It was published in a reputable US journal, though.

Krystian Hirsch -December 08, 2003

I believe that there are a couple of problems with the output transistor circuit.

(1) When U1-14 is at -12V (or close to), the V_EB rating of the NPN transistor is probably exceeded. At least this is true for most common NPNs. The result can be transistor failure, as I recently observed in a computer interface circuit. A reverse protection diode, emitter to base, can solve this problem.

(2) The ouput logic polarity appears to be backwards relative to the text. This could be fixed by swapping U1 pins 12 & 13.

Eliot Mayer -December 12, 2003

<b>Author replies:</b>

1. Comments by Mr.ELIOT MAYOR:<br> This practice of adding a diode to protect the transistor is common, if V(BE) rating is exceeded. If a transistor of sufficient V(BE) rating is selected, this diode can be avoided. In this circuit, output "LOW" is taken as the required trip output. Thanks for the comments.

2. Comments by Mr.KRYSTIAN HIRSCH:<br> He may once again go through the article calmly. I only said, voting systems are commonly used in critical plants. To further educate this commenter, to improve reliability and availability, TMR (Triple Modular Redundant) systems are used. The same circuit can be triplicated and output logic built accodingly, depending on the criticality of application. In such cases even redundant power supplies with diode auctioneering are used to provide reliable power supply. My aim was to give an indicative circuit to add flexibility compared to the digital voting circuit in this design brief. His comments about "Remote India" and "Published in a Reputable US Journal, though "are not in good taste and he may stop imagining that "only he knows everything".

Dr.R. JAYAPAL -January 12, 2004

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