Electronic Design

Analog Voting Circuit Is More Flexible Than Its Digital Version

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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