Question 2 As a process control engineer in a NESTLE milk production factory. You are required to design a fluid storage tank control. Figure 2 shows fluid storage tank with temperature and level sensors and controls. In this system, the digital logic controls the fluid in the storage tank. The purpose of the logic is to maintain an appropriate level of fluid by controlling the inlet and outlet valves. Also, the logic must control the temperature of the fluid within a certain range and issue and alarm if any of the level of temperature sensor fail. Basis system operation: The outputs of the system control logic the fluid input, fluid output, and fluid temperature. The control logic operates an inlet valve that allows fluid to flow into the tank until high level sensor is activated by being immersed in fluid. When the high-level sensor is immersed, the control logic closes the inlet valve. The fluid in the tank must be maintained within a specified temperature range as determined by two temperature sensors. One temperature sensor indicates when the fluid is too hot, and the other indicates when the fluid is too cold. The control logic turns on heating element if the temperature sensors indicate the fluid is too cold. The control logic keeps the outlet valve open if the low-level sensor is immersed, and the fluid is at the proper temperature. When the fluid level drops below the low-level sensor, the control logic closes the outlet valve. Operational requirements: The maximum and minimum fluid levels are determined by the positions of the level sensors in the tank. The output of each sensor is HIGH when it immersed in the fluid and is LOW when not immersed. When the high-level sensor is LOW, the control logic produces a HIGH and the inlet valve opens. When the high- level sensor output is HIGH, the control logic produces LOW and the inlet valves pg. 3 EEB1233 Combinational & Sequential Circuit Design Nov 2021 closes. The fluid must be within a specified temperature range before the outlet valve opened. One sensor produces a HIGH when the temperature is too hot, and the other temperature sensor produces a HIGH when the temperature is too cold. The control logic produces a HIGH to turn on a heating element when too cold conditions is indicated. Otherwise, the heating element is turned off. When a too- hot condition is indicated, an alarm is activated. When the low-level sensor produces a HIGH output (indicating that it is immersed) and when the output of both temperature sensors is LOW (indicating a correct temperature), the control logic opens the outlet valve. If the low-level sensor output goes to LOW, the control logic closes the valve. If the control detects a failure in any of the sensor or a too-hot condition, and alarm is activated. A level-sensor failure is indicated when the high-level sensor is active, and the low-level sensor is not active. A temperature sensor failure is indicated when both are active at the same time. Table 1 summarizes the inputs and outputs of the system. Inlet valve LH High-level sensor VINLET LL Alarm Control logic and interface Low-level sensor Heating clement Temperature sensors Outlet valve VOUTLET TCTH H +7 Figure 2 pg 4 EEB1233 Combinational & Sequential Circuit Design Nov 2021 Table 1 INPUTS TO CONTROL LOGIC Variable Description Lu High-level sensor Low-level sensor 724 High-temp sensor TC Low-temp sensor Active level HIGH (D) HIGH d) HIGH (1) HIGH (1) Comments Sensor immersed Sensor is immersed Temperature too hot Temperature too cold OUTPUTS FROM CONTROL LOGIC Comments Variable VNET VOUTLET H Description Inlet valve Outlet valve Heating element Alarm Active level HIGH HIGH (1) HIGH (1) HIGH (1) Valve open Valve open Heaton A Sensor failure or too hot condition a) Draw the truth table for the system. (5 marks) b) Draw k-map to simplify the logic circuits. (5 marks) c) Draw the logic circuits. (5 marks) END. Pg. 5
