control & instrumentation engineering

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CONTROL AND INSTRUMENTATION ENGINEERING LABORATORY Automatic Washing Machine Aim: The aim of this exercise is to develop, implement and test a ladder program for the automatic washing machine application. The description of the automatic washing machine application is given below. There are three sessions to complete the task, during the last session you should show the working program to your tutor. Hardware description: The Automatic Washing Machine module illustrated in Figure 1 comprises a screen-printed panel carrying lamps and switches to simulate the actions of this typical household appliance. Associated electronics are mounted on the open board and a connector for the interconnecting ribbon cable is mounted at the top. Figure 1. Layout of the Automatic Washing Machine module There are nine controls (listed in Table 1) on the panel. The start button to the right represents the user control that starts the washing machine. The four switches at the top of the panel represent various alternate functions that the user can choose and which are selected before starting. The other controls represent sensors which, in a real washing machine, would respond to some conditions and events, but on the module are controlled by the user. The PLC input lines connected to the switches need to be identified. There are fourteen lamps, controlled by sending an appropriate signal to the relevant output line from the PLC. The lamps are listed in Table 2. The PLC output lines that control lamps need to be identified. The three lamps to the left of the motor represent three different running conditions of the main motor, which might in a real machine be brought into operation by three relays (electromechanical or solid-state) controlled precisely as you would control the lamps. The pump on lamp likewise represents an auxiliary motor which drives the pump which empties the washing machine. The hot on and cold on lamps represent solenoid-operated water valves, which admit water to the machine when energised, and shut off the water supply otherwise. The locked lamp represents a further solenoid that locks the door so that the machine cannot inadvertently be opened when it is full of water. The other lamps represent indicators that you could find on a real washing machine’s control panel. Table 1. Switches on the panel Table 2. Lamps on the panel Program specification: The following are sequences that the washing machine is required to follow. The time periods are rather short so that the simulated machine will progress through its sequences (or ‘cycles’) more quickly than a real one, but still slowly enough so the user can observe and interact without difficulty. Start sequence: 1. If START is pressed and DOOR is open, there is no response. 2. If START is pressed and DOOR is closed, the START and LOCKED lamps turn on. Wash sequence: Hot wash program with Half Load not selected 1. Switch on the HOT VALVE ON and TUB FILLING lamps. 2. About one second later, HOT lamp turns on to show tub water temperature. As the filling continues, the TUB HALF FULL lamp turns on when the associated button (representing a liquid level sensor) is pressed. Subsequently, the filling is completed when the TUB FULL button is pressed and this is indicated by the corresponding lamp. When the filling is complete the HOT VALVE ON and TUB FILLING lamps are turned off. 3. In the event of either liquid level sensor failing to operate, a time out should occur: stop the filling, do not proceed with the sequence and indicate failure. Normally the tub should be half-full after 5 seconds of filling and full after another 5 seconds of filling. 4. Next, three wash cycles take place. Each cycle consist of two seconds of SLOW FORWARD, half a second pause, two seconds SLOW REVERSE and then half a second pause. 5. At the end of the wash the tub is emptied. The PUMP is switched on, after 2 seconds the TUB FULL lamp is turned off, after another 5 seconds the TUB HALF FULL lamp is turned off, finally after another 5 seconds the HOT and PUMP lamps are turned off. 6. The tub is now filled in the same manner as before (points 1-3), except that cold water is used this time, so that the COLD VALVE ON and COLD lamps are turned on instead of the HOT VALVE ON and HOT lamps. 7. Rinse: this is like a wash (point 4), except that only two cycles instead of three are required. 8. At the end of rinse the tub is emptied (as in point 5), except that the COLD lamp is turned off. 9. A fast forward spin is executed for 5 seconds. During the spin the pump should be on. 10. The sequence fill cold, rinse, empty and spin should be executed 3 times 11. The washing machine finally stops with all lamps turned off. If at any time the vibration sensor is triggered the sequence should stop, all valves, pump and motor should be turned off and a failure should be indicated. Note that the above description could raise several question marks, but this was done on purpose to mimic a real description received from a business client. Therefore, your task is also to complete the above description with you own suggestions and assumptions. Hint: a flowchart could be helpful to locate such unclear parts. Assessment: DEMONSTRATION: (50% of the total assignment mark) There are three sessions allocated to complete the task; during the third session (on 31st March 2014) you should demonstrate your working program. Note that you are allowed up to two attempts to demonstrate a working program, so you should test the program yourself before demonstrating it. During the demonstration you will be asked one or two questions about your program to verify your understanding of its operation. REPORT: (50% of the total assignment mark): 1. Every person should submit an individual report. 2. List all assumptions you made and describe your algorithm using a method of your choice (e.g. flowchart, timing diagram etc.). 3. Include a screenshot of your ladder program (this can be identical for all group members). Make sure that it is readable; use more than one page if necessary. 4. Explain which parts of the ladder program correspond to different steps of your algorithm. 5. To get more than 70% for the report, you need to ensure a coherent structure of the ladder program and use subroutines where appropriate. 6. If your program did not work correctly during the demonstration discuss how the errors were rectified. If your program did work correctly during the demonstration then there is no need to discuss this. 7. Be concise; there’s no page limit for the report, since your program may be very long and may not fit in 10 pages, but the format should be similar to your traffic lights control report, i.e. focus on your algorithm and implementation rather than on background. 8. Please submit your reports through the Turnitin link in Blackboard (in folder Labs). The submission deadline is 28th April 2014.