Closed loop Heat Temperature Thermostat Heater Voltage to Heater

Transcription

1 EET 273 Spring 2017 HW1 1. Control Terminology For the following 3 systems, explain which element of the system corresponds to which control element. An open loop system may not have all of the elements listed. a. Heating system in your house or apartment. b. Street lights that are designed to turn on at dusk. c. A hair dryer Open/ Closed Loop Process Variable (PV) Setpoint (SP) Controller Final Control Element (FCE) Manipulated Variable (MV) Heating System Closed loop Heat Temperature Thermostat Heater Voltage to Heater Street Lights Hair Dryer Closed loop Light ON when its dark out, OFF when its light out Open Loop Heat Temperature NA due to open loop Light switch Light bulb Power/ Current to light bulb Heating element NA due to open loop

2 2. Given the basic feedback control system below, write the equivalent simplified system in a single block, using the given transfer functions for G and H. Remember that the equivalent TF for a closed-loop system is given by: TF = G / (1 + GH) a. G = 10,000 H = 1/10 TF = 10,000 / (1 + 10,000*(1/10)) TF = 9.99 b. Repeat for the following values of G and H: G = 100,000 H = 1/20 TF = 100,000 / (1+ 100,000*(1/20)) TF = a. Given your results for parts a and b, what relationship do you notice between your simplified TF and H? The TF is roughly equal to 1/H. The larger G gets, the closer the TF value will get to 1/H. This means if we have a system with a very large G, we know that we can control it very well by selecting an appropriate value of H. b. How much does the value of G seem to affect the simplified TF? A lot, or a little? Very little. If G is very large, it has little effect on the TF. As G goes to infinity, the TF goes to 1/H.

3 3. An alternative to the conventional schematic diagram in AC power control systems is the ladder diagram. In this convention, the hot and neutral power conductors are drawn as vertical lines near the edges of the page, with all loads and switch contacts drawn between those lines like rungs on a ladder: As you can see, the symbolism in ladder diagrams is not always the same as in electrical schematic diagrams. While some symbols are identical (the toggle switch, for instance), other symbols are not (the solenoid coil, for instance). Re-draw this ladder diagram as a schematic diagram, translating all the symbols into those correct for schematic diagrams.

4 4. Given the following wiring diagram, create the proper ladder diagram.

5 5. Explain the operation of this ladder diagram, and create a truth table with A and B as inputs, and the indicator lamp as the output. What type of logic circuit does it represent? A: This circuit represent an XOR function. When either input A or B is conducting, the output is ON. When both or neither of the inputs are conducting, the output is OFF. A B Output

6 6. Safety is a paramount concern in electrical systems. Generally, we try to design electrical circuits so that if and when they fail, they will do so in the manner safest to those people working around them, and to the equipment and process(es) controlled by the circuit. One of the more common failure modes of circuits having wires strung through metal conduit is the accidental ground, or ground fault, where the electrical insulation surrounding a wire fails, resulting in contact between that wire and a grounded metal surface. Suppose an accidental ground were to occur at the point shown in this ladder diagram: What would be the result of this fault? Hint: you will need to know something about the L1/L2 power source in order to answer this question! What would be the result if the L1/L2 power connections were reversed? A: If L1 is connected to + and L2 is connected to Neutral (GND), then a ground fault in this location would cause the relief solenoid to be grounded on both sides, which would result in it being non-operational. This is much preferable to the case where L1 and L2 are swapped, in which a ground fault would result in power being applied to the load.

7 7. The following ladder logic diagram (for a steam heater control) contains a serious mistake: This is a mistake I ve seen many students make. Explain what the mistake is, and draw a corrected version of this relay circuit. Why do you suppose this is a common mistake for students to make when sketching a ladder logic diagram? Despite it being in error, there is a certain logic to it. It can be easy to forget that a relay coil is actually a load, and should not be connected in series with other loads. Often, it is easily to become preoccupied with the loads in the system that are giving us the output we are interested in (lights/motors/etc.), and forget that relay coils are also loads. If a real circuit were wired in this manner, what would it do? How would it behave? There would be a voltage drop across CR1 and the Green indicator, which means neither full would be getting the full L1-L2 voltage. This would likely mean the relay coil doesn t turn on, or the light doesn t full illuminate. If a real circuit were wired in this manner, how could you diagnose the nature of the problem using a multimeter? There are several ways. You could measure the voltage across each of the devices in series, and see that you are not getting the full L1-L2 voltage across them. You could also check that each load is grounded, by looking for 0 ohms of resistance between each load terminal and L2. In this case, CR1 does not have a direct connection to ground.

8