3. a. PLC\'s run the program using a cycle of reading the inputs, applying these

Question

3. a. PLC's run the program using a cycle of reading the inputs, applying these inputs and updating the outputs. This can lead to some faults occurring. i. Relating to this cycle explain the possible reasons for (10 marks intermittent faults. ii. Some pieces of code fail due to the order of the commands, give an example of this and again explain, relating to the above cycle, why this is. (10 marks b. It is also possible in pneumatic circuits to produce a locked in signal. i. Explain the term locked in signal and give one ii. Explain how such a signal can be avoided. example, with a diagram, of how this can be caused. (5 ma (10 m Discuss the relative merits of the STL style of programming over standard ladder logic. Discuss the difference in the way the two types of programming vary in the way the PLC executes them. (15 m

Explanation / Answer

The first step in PLC troubleshooting is to decide if the problem is internal to the processor or in the I/O system. Problems that can be localized to a specific I/O module or even a specific input or output device are usually external, while internal problems normally result in large groups of failures, globally erratic behavior, or even total failure of the PLC system.

The first thing to check is the integrity of the PLC's power and ground.

If the PLC processor has an AC power source, check the input voltage; it should be within the manufacturer's recommended range. PLC processors actually operate on DC power, so that also must be checked.

Also check the DC supplies for AC ripple, value measured should be well below the manufacturer's specifications. Excess ripple has drastic effects on the operation of the microprocessors and memory devices typically found in PLC processors.

Battery power is often used to prevent a PLC from losing its program during power outages, and battery voltages should be within recommended values.

Causes for erratic processor behavior are electro-magnetic interference (EMI) or radio frequency interference (RFI).

Power, grounding, and interference problems all can cause the corruption of the PLC memory, so the next step is to verify that the program is still correct.

Troubleshooting inputs and outputs

determine the relationship between physical I/O modules and the I/O instructions in the PLC program. This is done by using the addressing scheme for the particular PLC that you are working on, and this scheme differs from one manufacturer to another.

Troubleshooting digital output modules

Output modules are designed to cause some change in the external world in response to an instruction in the PLC processor.

Many different digital output module types are available, with the most common varieties being DC outputs that rely on transistors as switching devices, AC outputs that rely on triacs, and universal outputs that use relay switching.

The power to drive PLC outputs, like inputs, is usually not supplied by the module, so it's important to find out where that power comes from. Once again, there are isolated and nonisolated modules.

the programming device must be hooked up to the PLC, and the address that is associated with the output in question must be determined. The output then can be "forced" ON or OFF internally in the PLC, and the module can be observed for a reaction. If the indicators on the module do not reflect the forced condition, change the output module. If the module is working properly but still does not react to the forcing, the problem again lies in the communication between the processor and the module, and the manufacturer's documentation is your best source for troubleshooting information.

if the voltage is not changing, the problem can most likely be found in the field wiring.

2) The actual logic of the control system is established inside the PLC by means of a computer program. This program dictates which output gets energized under which input conditions. Although the program itself appears to be a ladder logic diagram, with switch and relay symbols, there are no actual switch contacts or relay coils operating inside the PLC to create the logical relationships between input and output. These are imaginary contacts and coils, if you will. The program is entered and viewed via a personal computer connected to the PLC’s programming port.

Consider the following circuit and PLC program:

When the pushbutton switch is unactuated (unpressed), no power is sent to the X1 input of the PLC. Following the program, which shows a normally-open X1 contact in series with a Y1 coil, no “power” will be sent to the Y1 coil. Thus, the PLC’s Y1 output remains de-energized, and the indicator lamp connected to it remains dark.

If the pushbutton switch is pressed, however, power will be sent to the PLC’s X1 input. Any and all X1 contacts appearing in the program will assume the actuated (non-normal) state, as though they were relay contacts actuated by the energizing of a relay coil named “X1”. In this case, energizing the X1 input will cause the normally-open X1 contact will “close,” sending “power” to the Y1 coil. When the Y1 coil of the program “energizes,” the real Y1 output will become energized, lighting up the lamp connected to it:

It must be understood that the X1 contact, Y1 coil, connecting wires, and “power” appearing in the personal computer’s display are all virtual. They do not exist as real electrical components. They exist as commands in a computer program—a piece of software only—that just happens to resemble a real relay schematic diagram.

he pushbutton switch connected to input X1 serves as the “Start” switch, while the switch connected to input X2 serves as the “Stop.” Another contact in the program, named Y1, uses the output coil status as a seal-in contact, directly, so that the motor contactor will continue to be energized after the “Start” pushbutton switch is released. You can see the normally-closed contact X2 appear in a colored block, showing that it is in a closed (“electrically conducting”) state.

If we were to press the “Start” button, input X1 would energize, thus “closing” the X1 contact in the program, sending “power” to the Y1 “coil,” energizing the Y1 output and applying 120 volt AC power to the real motor contactor coil. The parallel Y1 contact will also “close,” thus latching the “circuit” in an energized state:

Now, if we release the “Start” pushbutton, the normally-open X1 “contact” will return to its “open” state, but the motor will continue to run because the Y1 seal-in “contact” continues to provide “continuity” to “power” coil Y1, thus keeping the Y1 output energized:

The solution to this problem is a reversal of logic between the X2 “contact” inside the PLC program and the actual “Stop” pushbutton switch:

Thus we come to a conclusion that if we change the order of the commands that we are supposed to give, it will have a different way of functioning and a vastly different output than what we have considered to be our output. Sometimes, changing the order will also result in a null loop or infinite loop and nothing is executed.

In order to hold or lock an operating conveyor or a similar machine, the cylinder must be locked until asignal for cancelling the lock is received. Therefore, the signal for cancelling the lock should be operated by a normally open type control valve. This is the lock in signal.

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