EET 273 – 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.

  1. Heating system in your house or apartment.
  2. Street lights that are designed to turn on at dusk.
  3. A hair dryer

Open/
Closed Loop / Process Variable (PV) / Setpoint (SP) / Controller / Final Control Element (FCE) / Manipulated Variable (MV)
Heating System
Street Lights
Hair Dryer
  1. 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 = 10,000H = 1/10

b.Repeat for the following values of G and H:

G = 100,000H = 1/20

  1. Given your results for parts a and b, what relationship do you notice between your simplified TF and H?
  1. How much does the value of G seem to affect the simplified TF? A lot, or a little?

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 ofthe 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.

  1. Given the following wiring diagram, create the proper ladder diagram.
  1. 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?

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?

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 logicdiagram? Despite it being in error, there is a certain logic to it.

• If a real circuit were wired in this manner, what would it do? How would it behave?

• If a real circuit were wired in this manner, how could you diagnose the nature of the problem using a multimeter?