Question 1

(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 2

(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 3

(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.