25.4 DESIGN CASES

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25.4.1 Oven Temperature Control

Problem: Design an analog controller that will read an oven temperature between 1200F and 1500F. When it passes 1500 degrees the oven will be turned off, when it falls below 1200F it will be turned on again. The voltage from the thermocouple is passed through a signal conditioner that gives 1V at 500F and 3V at 1500F. The controller should have a start button and E-stop.

Solution:

Figure 399 Oven Control Program with Analog Inputs and Logical Outputs

25.4.2 Water Tank Level Control

Problem: The system in Figure 400 will control the height of the water in a tank. The input from the pressure transducer, Vp, will vary between 0V (empty tank) and 5V (full tank). A voltage output, Vo, will position a valve to change the tank fill rate. Vo varies between 0V (no water flow) and 5V (maximum flow). The system will always be on: the emergency stop is connected electrically. The desired height of a tank is specified by another voltage, Vd. The output voltage is calculated using Vo = 0.5 (Vd - Vp). If the output voltage is greater than 5V is will be made 5V, and below 0V is will be made 0V.

Figure 400 Water Tank Level Controller

Figure 401 A Water Tank Level Control Program