Overview To prevent seawater from entering the periscope, a protective glass capable of withstanding a certain amount of seawater hydrostatic pressure is installed at the periscope's light inlet. When a submarine uses the periscope for observation in cold climates, its bow window quickly becomes covered with frost. Previously, the only solution was to lower the periscope, rinse it in seawater, and then raise it again for observation. However, this method was neither convenient nor effective, as the periscope would soon become covered again. In extremely cold climates, such as in the Arctic Ocean, the periscope would be severely affected and rendered ineffective. The solution is to install a heating device for the bow protective glass within the periscope. This device de-ices, defrosts, and defoils the periscope window's protective glass, ensuring its normal operation. Temperature Control System The temperature control system consists of a heating conductive film on the periscope window's protective glass, a temperature sensor, and a control circuit. Two AD590M temperature sensors located diagonally opposite the window glass measure the average temperature of the protective glass. The temperature detection circuit converts the sensor output current into voltage, which is compared with the threshold voltage of a dual-limit voltage comparator to display the actual temperature range of the protective glass and control the operation of relays. By switching the operating modes of the normally closed and normally open contacts of each relay, the heating of the conductive film on the protective glass is controlled. A transparent conductive film is coated on the periscope window protective glass. Metal electrodes are plated around the conductive film, and temperature sensors are attached to the four corners of the protective glass. The heating and temperature control device applies a controllable AC power supply to the conductive film through the metal electrodes around the film, heating the periscope window protective glass. Since the conductive film is always exposed to air, it is susceptible to damage and contamination. When energized, the rapid movement of free electrons in the conductive film is accelerated, converting electrical energy into heat energy, thus heating the protective glass. If the heating temperature is too high or the heating time is too long, spot-like film peeling, short circuits, or even breakdown may occur. When the periscope is lowered into water, it may also cause the glass to shatter. If the heating temperature is too low or the heating time is too short, the purpose of rapid de-icing, defrosting, and defogging cannot be achieved. To resolve this contradiction, in addition to appropriately selecting the power of the heating device, automatic adjustment and monitoring of the heating temperature is essential to adapt to the heating requirements under different external temperatures. Temperature Sensor Selection Semiconductor temperature sensors mainly fall into two categories: digital temperature sensors and analog temperature sensors, and each type includes many products. Digital temperature sensors, which can monitor local and remote temperatures, have replaced analog output temperature sensors in many applications. However, analog temperature sensors can generate voltage or current signals that are linearly proportional to temperature, and do not require additional linearization circuits to calibrate the sensor's nonlinearity. Therefore, analog output temperature sensors also have wide applications in situations where digital output is not required, such as the AD590M current output temperature sensor. The AD590M is a monolithic integrated two-terminal current output temperature sensor manufactured by Analog Devices, Inc. Its operating voltage is 4–30V; temperature range is -55–150℃; accuracy is ±0.5℃; it features standardized output and inherent linearity (output current changes by 1mA for every 1℃ change in temperature); the output zero point is the thermodynamic zero point, meaning the AD590M's output current is 0mA at -273℃ and approximately 273mA at 0℃. Its high-impedance current output characteristic makes it insensitive to voltage drops over long transmission lines, thus enabling remote temperature detection. Window Glass Temperature Measurement and Control Circuit The periscope window glass temperature control circuit is shown in Figure 1. [align=center]Figure 1 Window Glass Temperature Measurement and Control Circuit[/align] To accurately measure the temperature of the protective glass, two of the four AD590M sensors located at the four corners of the inner side of the window glass are connected in parallel on the diagonal, with the other two as spares, forming an average temperature sensor circuit. The design of the temperature measurement circuit first requires converting current into voltage. Since the current output element AD590M corresponds to a temperature of 1μA/K, a resistor R1 = 10kΩ/2 = 5kΩ is selected. The voltage drop across this resistor is approximately 10mV, which translates to 10mV/K. Capacitor C is used to filter noise. The two ends of the relay coil are connected to the power supply voltage and the collector (c) of the output transistor, respectively. When the current flowing through the relay coil suddenly decreases, an electromotive force (EMF) is induced across it. This EMF, superimposed on the original power supply voltage, is applied between the collector (c) and emitter (e) of the output transistor, potentially causing a breakdown between the collector and e. To eliminate the harmful effects of this induced EMF, a diode is connected in parallel with the relay to absorb it, thus protecting the transistor. Operational amplifier A1 is configured as a voltage follower to increase the input impedance of the signal. A2 and A3 form a window comparator. The non-inverting input of comparator A2 is connected to the upper threshold Vh of the window, and the inverting input of comparator A3 is connected to the lower threshold Vl of the window. The input signal Vi is simultaneously applied to the non-inverting input of comparator A2 and the inverting input of comparator A3. When Vi < Vl, the output of A2 is high, transistors VT2 and VT3 and diode D4 are cut off, and relays KA2 and KA3 are de-energized and released. The output of A3 is low, VT1 and D5 are turned on, and relay KA1 connected to transistor VT1 is energized and energized, closing the normally open contact KA1-1. The 220V AC power supply, through the step-down transformer T, heats the conductive film on the protective glass through the normally closed contact KA3-1 of relay KA3, illuminating indicator LED1. When Vl > Vh, the normally open contact KA2-2 of relay KA2 closes, energizing relay timer KA4. After a 5-minute delay heating period (the delay heating time can be adjusted from 0.1 seconds to 10 minutes as needed), the normally closed contact KA4-1 of KA4 opens, stopping the heating of the conductive film. KA4 is a multi-functional relay timer H3RN-1 with four operating modes: on-delay, interval, flashing off-start, and flashing on-start; the timing range is 0.1 seconds to 10 minutes; the control output is single-pole double-throw; the rated power supply voltage is DC 12V, 24V, and AC 24V; the rated current is 3A resistive load. When Vi > Vh, A3 outputs high, VT1 and D5 are cut off, A2 outputs low, VT2 is cut off, VT3 and D4 are conducting, and relay KA3 connected to VT3 is energized. The normally closed contact KA3-1 of KA3 opens, preventing heating of the conductive film on the protective glass, and indicator LED3 illuminates. To minimize the impact of self-heating on the current-output type temperature sensor AD590M, a lower excitation voltage should be used during debugging. Therefore, the output voltage of the three-terminal regulator LM7805C is used as the excitation voltage of AD590M. This voltage also serves as the reference voltage for the upper and lower threshold levels Vh and Hl of the comparator. This overcomes the influence of power supply fluctuations on the upper and lower threshold levels Vh and Hl, thereby improving temperature measurement accuracy. During adjustment, the surface temperature of the protective glass is measured using an infrared thermometer, and the corresponding output voltage is measured using a digital multimeter. When the temperature is 10℃, the measured output voltage is used as the lower threshold level Hl; when the temperature is 30℃, the measured output voltage is used as the upper threshold level Vh. This process is repeated several times, and the average value of the upper and lower threshold levels Vh and Hl is used to determine Vh and Hl. Conclusion The circuit structure of the temperature control system for protective glass using the AD590M analog temperature sensor is simple, reliable, and easy to debug. Compared with digital temperature sensors as temperature sensors for protective glass, the analog temperature sensor has a wider heating temperature range (upper and lower threshold levels Vh and Hl). In cases where the temperature around the periscope window is high and only defogging is required, it has outstanding practical advantages. Therefore, it has practical application value for temperature detection systems for periscope protective glass that require long-distance transmission. References: 1. Zhang Kunshi et al.: Submarine Periscope, National Defense Industry Press, 1983 2. Ji Bo, Gao Youtang: Circuit Design for Bridge Measurement Using ispPAC10, Modern Electronics Technology, 2004 (17) Editor: He Shiping