Are you really familiar with the use of high-value selectors and low-value selectors?

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Signal selectors are divided into high-value selectors and low-value selectors, and they are commonly used in the construction of safety protection control systems. In this article, CHANGHUI Instruments shares professional knowledge on the application of eight types of signal selectors, which is essential knowledge that those working in the industrial control field must be familiar with and master. Signal selector: yunrun.com.cn/product/1954.html. The function of the signal selector is to choose one from two signals as the output signal. There are four ways to control the output of the signal selector: automatic selection of the higher level, automatic selection of the lower level, selection via external digital/level signals, and selection of one of the input signals. Once the selected signal is determined, it is isolated from the input signals and power supply before being output. A signal selector that automatically selects the high value is called a high-value selector, while a signal selector that automatically selects the low value is called a low-value selector. http://yunrun.com.cn/upload/201805/15/201805150436075704.png Figure 1: Schematic diagram of the high-value selector
http://yunrun.com.cn/upload/201805/15/201805150439515074.png Figure 2: Schematic diagram of the low-value selector
Eight typical applications of high-value selectors and low-value selectors: yunrun.com.cn/tech/1974.html
1. Application of signal selectors in selective control systems
As we all know, a compressor is a device used to transfer gases. It can be driven by steam or by a DC servo motor. Adjusting either the input amount to the steam engine or the terminal voltage (or excitation voltage) of the DC motor can regulate both the output flow rate and pressure of the compressor. Figure 3 shows the schematic diagram of a flow and pressure control system managed by a low-value signal selector. PID regulators are used for both the pressure and flow systems. In Figure 3 of the selective control system, P* represents the setpoint signal for the pressure control system; the output pressure of the compressor is required to be maintained around P*, but it is not allowed to exceed this value. Q* is the given signal for the flow control system; the output flow rate of the compressor is required to be maintained around Q*, and it is also not allowed to exceed Q*. Therefore, the control method involves choosing between the two control systems; whichever controlled variable exceeds the set value is used as a reference, and the regulator of that system sends a signal to adjust the set value of the thyristor speed control system. This is a selective control system. The role of the low-value selector in this system is to manage the functions of the two systems. To illustrate the working principle of the system, let’s assume that P＜P* and Q＞Q*. If the tuning parameters of the regulators in both systems are the same, the output of the regulator in the Q system will be higher than that of the regulator in the P system; this higher value is then blocked by the low-value selector. However, the system does not enter the integral saturation region, ensuring that when Q＜Q* and P＞P*, a smooth transition can take place from the closed-loop control of the Q system to that of the P system, while keeping both pressure P and flow rate Q within their designated values. http://yunrun.com.cn/upload/201805/15/201805150505449965.png Figure 3 Alternative regulation of two control systems (selective control system). 2. Use of a high-value selector as a protection device for detecting excessive signal levels. In nuclear reactors or chemical reactors, temperature control is essential for ensuring the proper operation of the production process. If only one thermocouple is used for monitoring or regulation, it is often considered insufficient from a safety perspective; therefore, several thermocouples are installed in different locations, and their signals are sent to a high-value signal selector unit through temperature transmitters. There are two advantages to doing this: first, it is possible to control things based on the highest temperature, thereby preventing localized overheating ； Secondly, even if one, two, or three thermocouples break, control can still be maintained, **improving the safety and reliability of the control system’s operation. http://yunrun.com.cn/upload/201805/15/201805150533486834.png Figure 4: Multi-signal maximum selection control system. If, in Figure 4, the high-value selector in the high-value selector group is replaced by a low-value selector, a multi-signal maximum selection control system can be established. 3. The signal selector is used to coordinate the sequence of operations in the system. If a single control signal is used to regulate several systems simultaneously, issues related to time allocation and signal matching arise among these systems. Signal matching ensures that the systems can reach a stable operating point, while time allocation guarantees the safety of the production process. In many production processes, clear regulations are established regarding the sequence of actions of the control systems, in order to avoid the formation of by-products or explosions, and even for energy-saving purposes. For example, in the process of burning heavy oil, it is stipulated that when the production load increases, air flow should be increased first and then the amount of oil used; when the production load decreases, the amount of oil used should be reduced first and then air flow. This is done to prevent secondary combustion from damaging the equipment. Systems of this kind generally require two signal selectors (one for high values and one for low values) to coordinate the sequence of operations between the systems. Figure 5 shows the composition of the heavy oil combustion control system. When the load requirement increases, the load command I rises. Since I > I1, the low-value selector selects I1 as its output signal. Moreover, because I > I2, the P signal can be passed through the high-value selector to the air volume regulator, causing the air flow to increase first. As the air volume increases, I1 will gradually rise, which in turn leads to an increase in the output of the low-value selector. Conversely, when the load requirement decreases, I＜I1 and I＜I2; the load command first reduces the fuel volume, and only after that does it reduce the air flow rate. The control method for this type of action is called \"increase first then decrease\" or \"decrease first then increase\", and it represents a typical circuit design for instrument systems. Once the adjustment process stabilizes, maintain the relationship I=I1, I=I2. In the diagram, the constant setter and adder are used to adjust the matching of the initial state signals of the two systems, while the multiplier is used to regulate the ratio of fuel to air. http://yunrun.com.cn/upload/201805/15/201805151238082857.png Figure 5: Schematic diagram of the control principle for the heavy oil combustion control system. 4. The signal selector is used to prevent the output of the divider from exceeding its limits. For a divider to function properly, certain conditions must be met; namely, I1 ≤ I2, where I1 is the dividend signal and I2 is the divisor signal. If I2 < I1, the output of the divider exceeds the range of the meter signal. For example, in the case of a divider, when I2 < I1, its output exceeds the upper limit of the meter signal, which is 20 mA. If it is possible for the control signal I2 for the production process variables to be lower than I1, measures must be taken in the circuit to prevent this from affecting the divider. Before the signal selector manufactured by Changhui Instrument Manufacturing Co., Ltd. was put into use, the dividend signal was usually diverted or voltage-divided before being fed into the divider; the degree of diversion or voltage division had to ensure that the dividend was always lower than the divisor signal (as shown in Figure 6). However, after the selector is introduced, a high-value signal selector can be used to prevent the divider output from going out of range, and its circuit configuration is shown in Figure 7. Compared to the former method, the latter can maintain the sensitivity of the divider unchanged under normal operating conditions. http://yunrun.com.cn/upload/201805/15/201805151252390357.png http://yunrun.com.cn/upload/201805/15/201805151259260868.png Figure 6: Using a splitter to prevent the divider’s output signal from exceeding limits. Figure 7: Using a signal selector to prevent the divider’s output signal from exceeding limits. 5. Selecting the intermediate value among multiple input signals: In some manufacturing processes, it is often necessary to choose the middle-value signal among several measurement signals with identical properties as the signal to be used for measurement. For example, in a heating furnace with three temperature zones, three temperature transmitters are installed respectively, providing high, medium, and low measurement values. If the control is based on the highest temperature signal, the average temperature of the entire furnace will be on the high side. On the contrary, if adjusted according to the lowest temperature, the average temperature will be low. Therefore, the most representative of these three temperature measurement points is the signal of the middle value. Figure 7 shows the circuit configuration using the middle value of the three monitoring points. This circuit is composed of two high-value selectors and two low-value selectors. The following discusses it in three cases. http://yunrun.com.cn/upload/201805/15/201805151453348883.png Figure 8: Composition of the circuit for selecting the median value signal among three points. ① When I1＜I2＜I3, the output signals of selector I and selector II are I12=I2 and I13=I1. The output signals of Signal Selector III and Signal Selector IV are I14=I12=12, I0=I14=I2. The intermediate value signal I2 is selected as the output signal. ②I2＜I1＜I3 In this case, the output signals of signal selector I and signal selector II are I12=I1 and I13=I2. The output signals of Signal Selector III and Signal Selector IV are I14=I1, I0=I14=I1. The intermediate value signal I1 is selected as the output signal. ③I1＜I3＜I2. In this case, the output signals of signal selector I and signal selector II are I12=I2 and I13=I2. The output signals of Signal Selector III and Signal Selector IV are I14=I3, I0=I14=I3. The intermediate value signal I3 is selected as the output signal. From the analysis of the above three cases, it can be seen that in all situations, the circuit in Figure 8 always takes the middle value of the three input signals as its output signal. http://yunrun.com.cn/upload/201805/15/201805151510561025.png Figure 9: Circuit for selecting the intermediate values of the five output signals. Based on the principles described above, it is not difficult to construct a circuit for selecting the intermediate values of the five output signals, as shown in Figure 9. This circuit is composed of series-connected selection circuits for two three-point intermediate value signals. The first half selects one signal from the three measurement signals and sends it to the second half, where it is then compared with the other two signals, and another central signal is selected from them. Following the same principle, circuits for selecting intermediate values such as 7, 9, and so on can be constructed. 6. The signal selector functions as a switch and a sampler; the low-value selector and the high-value selector can be used as switches or samplers. Figure 10 uses a low-value selector as a switch, with the setter specifying only two signals: 0 (the zero level of the instrument signal) and 1 (the upper limit of the instrument signal). When IR is 0, the I1 signal is blocked ； When IR is 1, the I1 signal is active. Figure 11 is the exact opposite: when IR is 0, the I1 signal is active ； When IR is 1, the I1 signal is blocked. Figure 12 uses a pulse source in place of a constant value device; the output signal of this pulse source varies periodically between 0 and 1, with signal I1 turning on and off alternately, thereby generating a periodic signal with the same period as the pulses. http://yunrun.com.cn/upload/201805/15/201805151526471454.png Figure 10: Using a low-value signal selector as a switch. http://yunrun.com.cn/upload/201805/15/201805151528224459.png Figure 11: Using a high-value selector as a switch. Figure 12: Periodic signal output. 7. Signal selectors are used to form signal queuing circuits. Some production processes require it to be known which of the measured signals is high, which is low, and which is at an intermediate value. This requirement can be met through the queueing line in Figure 12. The entire circuit is composed of six signal selectors; the highest signal is output by selector III, the lowest signal by selector IV, and the intermediate signals are output by selector VI. The circuit in Figure 13 can be divided into three components: I and III form a circuit for selecting the highest value, II and IV form a circuit for selecting the lowest value, while I, II, V, and VI constitute a circuit for selecting an intermediate value. http://yunrun.com.cn/upload/201805/15/201805151621548609.png Figure 13: Queuing circuits for the three input signals. 8. The signal selector is used as a floating limiter. For safety reasons, many production processes require that certain regulated variables or process variables be limited. Such limiting can generally be achieved by a limiter, but when the limiting value needs to be varied based on another variable, a signal selector must be used to form the limiting system. Figure 14 shows the structural principle of the high-limit floating limiter. The upper limit of the operating signal is Ix; when I1 < Ix, I0 = I1. However, when I1 > Ix, I0 = Ix, and even if I1 continues to increase, I0 will not rise any further and remains at the value of Ix. Since Ix is variable, the circuit configuration is called a high-floating limiter. Figure 15 shows the structural principle of the low-limit floating limiter, when I1 is below the value of Ix. Figure 16 shows the structural principle of the high and low limit floating limiter, which is composed of 14 and 15 connected in series. http://yunrun.com.cn/upload/201805/15/201805151350088030.png http://yunrun.com.cn/upload/201805/15/201805151359438903.png Figure 14: High-limit floating limiter. Figure 15: Low-limit floating limiter. The outputs of the high- and low-limit floating limiters can be represented as shown in http://yunrun.com.cn/upload/201805/15/201805151413246178.png Figure 16: High- and low-limit floating limiters. Floating limiters have many applications in improving the production efficiency of equipment and ensuring safe operation. For example, an important condition for the safe operation of a compressor is to ensure that it does not experience surge. If the inlet flow rate of the compressor is maintained above a certain value, surge can be avoided; however, fixing the inlet flow rate does not facilitate the full utilization of the compressor’s efficiency. A relatively effective method is to vary the minimum suction flow rate based on the outlet pressure; the composition of the control system is shown in Figure 17. The area enclosed by the dashed lines in the figure represents the various components of the function; these are implemented using an adder, with the operational relationship being IΣ = I0 + αIP. In this formula, I0 represents the initial current, which denotes the minimum flow rate required by the compressor when P = 0 ； α is the proportionality constant. IΣ is the lower floating limit signal for the high-value selector; under normal circumstances, the flow control system keeps the control valve installed on the return pipeline at a relatively low opening degree (for example, 90%). When abnormal operating conditions occur, the suction flow rate does not meet the stability requirements of the compressor; in such cases, a floating limit signal takes over from the regulator to control the return flow valve. The opening of this control valve increases, thereby raising the return flow volume and bringing the suction flow rate within a safe operating range. With the widespread use of PLC and DCS systems, it has become convenient to carry out signal selector functions in control systems through programming. This does not mean that signal selectors are outdated; as professionals in industrial control, it is essential to have a thorough understanding of their principles and applications. This knowledge will give you an advantage when designing control schemes and configuring and programming the DCS or PLC systems used to implement those schemes.