Process Control and Instrumentation

Question 1

(a) Explain briefly what the following terms mean in the context of process control. You may cite examples or draw diagrams to illustrate your answers.

i. Open Loop System.
ii. Closed Loop System.
iii. Self-regulation System.
iv. Analog-to-Digital Conversion.
v. Ideal sensor time response.

(b) Construct a block diagram of a refrigerator control system. You need to definite each block in terms of the refrigerator components. If you do not know the components, look them up in the internet.

Question 2

(a) i. A sensor has a transfer function of 0.5 mV/°C and an accuracy of ±1%. If the temperature is known to be 60°C, what can be said with absolute certainty about the output voltage?

ii. The same sensor in (i) is used with an amplifier with a gain of 15 ± 0.25 and displayed on a meter with a range of 0 to 2 V at ± 1.5% FS. What is the worst-case and rms uncertainty for the total measurement?

(b) A controller output is a 4 to 20 mA signal that drives a valve to control flow. The relation between current and flow is Q = 45 [I - 2 mA] ½ liter/min. What is the flow for 12 mA? What current produces a flow of 162 liter/min?

(c) Objects with 2-cm width are placed on a high speed conveyor belt in a manufacturing process that operating in a 30-ms PLC scan time. When an object passes through a light beam, it provides a high input to the PLC. What is the highest speed of the conveyor to be sure the object is detected?

Question 3

(a) A liquid-level control system linearly converts a displacement of 2 to 3 m into a 4 to 20 mA control signal. A relay serves as the two-position controller to open or close an inlet valve. The relay closes at 12 mA and opens at 10 mA. Determine:-

i. The relation between displacement level and current.
ii. The neutral zone or displacement gap in meters.
(b) A temperature sensor has a span of 20°C to 250°C. A measurement results in a value of 55°C for the temperature. Specify the error if the accuracy is:-

i. ±0.5% full-scale (FS).
ii. ± 0.75% of span.
iii. ±0.8% of reading.
(c) A temperature sensor has a transfer function of 5mV/°C with an accuracy of ±1%. Find the possible range of the transfer function.

Question 4

(a) Define the variables in the system in Figure 1 that constitutes the process load.

Figure 1

(b) Analyze each of the following control systems and determine whether they have self-regulation, what the process load would be and whether a process lag would be expected.
i. Air-condition system.

ii. A water level-control system as shown in Figure 2.

Figure 2

(c) Figure 3 shown an elevator employs a platform to move objects up and down. The objective is that when the UP button is pushed, the platform carries something to the up position. When the DOWN button is pushed, the platform carries something to the down position.

Figure 3

The following hardware specifications define the equipment used in the elevator:-

Output elements
- M1 = Motor to drive the platform up
- M2 = Motor to drive the platform down
Input elements
- LS1 = NC limit switch to indicate UP position
- LS2 = NC limit switch to indicate DOWN position
- START = NO push button for START
- STOP = NO push button for STOP
- UP = NO push button for UP command
- DOWN = NO push button for DOWN command
Hint: NC denotes "Normally Closed" and NO denoted "Normally Open".

The following narrative description indicates the required sequence of events for the elevator system:-

1. When the START button is pushed, the platform is driven to the down position.

2. When the STOP button is pushed, the platform is halted at whatever position it occupies at the time.

3. When the UP button is pushed, the platform, if it is not in downward motion, is driven to the up position.

4. When the DOWN button is pushed, the platform, if it is not in upward motion, is driven to the down position.

Prepare a ladder diagram to implement the control function based on the information given above.

Question 5

(a)

Figure 3

Figure 3 shows a pictorial view of an oven, along with the associated input and output signals. All of the inputs and outputs are two-state variables, and the relation of the states and the variables is indicated. Construct Boolean equations that implement the following events:-

1. The heater will be on when the power-switch is activated, the door is closed and the temperature is below the limit.

2. The fans will be turned on when the heater is on or when the temperature is above the limit and the door is closed.

3. The light will be turned on if the light switch is on or whenever the door is opened.

(b)

i. Develop the control system of an automatic coffee-vending machine. Insertion of a coin and pushing of buttons provides a paper cup with coffee that can be black, with sugar, with cream, or with both. Describe the features of the machine as a discrete-state system.

ii. Prepare a flow chart to describe the event sequence of the coffee-vending machine above.

Semiconductor Device Physics

Question 1

(a) Sketch the crystal structure for simple cubic structure and body-centered cubic structure.

(b) (i) Calculate the number of atoms per unit cell in the simple cubic structure.

(ii) Determine the maximum fraction of the unit cell volume that can be filled by hard spheres in the simple cubic structure. Assume that a is the lattice constant.

(c) The diamond lattice structure is the basic building block in semiconductors such as silicon (Si).

(i) Sketch the diamond structure.

(ii) From the sketch, describe and calculate that the number of atoms in one cell of a diamond structure.

(iii) Calculate the density of atoms in the silicon in terms of atoms/m3.

Question 2

Briefly explain the difference between diamond structure and zincblende structure. Sketch the zincblende structure manually. Give an example of the compound with zincblende structure.

Question 3

(a) Sketch and label the energy bandgap diagrams for a semiconductor and an insulator.
(b) Determine the number of electrons per unit volume in m-3 for gold metal.

Question 4

(a) (i) Explain the difference between a direct band gap semiconductor and an indirect band gap semiconductor. Give an example of a direct band gap semiconductor and an indirect band gap semiconductor.

(ii) Sketch and label the E-k diagram of a direct band gap semiconductor and an indirect band gap semiconductor.

(iii) Find the wavelengths of radiation needed to create electron-hole pairs in intrinsic silicon (Eg = 1.124eV) and gallium arsenide (Eg = 1.42eV).

(b) (i) Write the expressions of carrier concentrations, n and p in terms of intrinsic carrier concentration, ni, intrinsic Fermi level, Ei and Fermi level, EF .

(ii) If n = 1x10¹⁶ cm^-3 for Si at 300 K, find the difference between EF and Ei in eV.