Saturday, 13 June 2015

CONTROL SYSTEM SUMMARY



When process control technicians calibrate process control devices they need to know whether the output signals represent the input values and vice verse.  By knowing how to convert from one to the other, the technician will be able to verify his readings against the design specifications.  You will use the information you have learned in this section in the next part and often in later tasks.
Exercise
Use the formula in the lesson sheets to find answers to the following questions.
1)                A temperature transmitter has a range of 200ºC to 700ºC and the output range is 3 to 15 psi.  The input temperature is 300ºC.
a)     What should the output value be?
b)    What is the percentage of span?
2)                A temperature transmitter has a range of 150ºC to 750ºC and the output range is 3 to 15 psi.  The output value is 9.25 psi
a)     What should the input value be?
b)    What is the percentage of span?
3)                A pressure transmitter has a range of 0 to 1000 psi and the output range is 3 to 15 psi.  The output value is 9.25 psi.
a)     What should the input value be?
b)    What is the percentage of span?
4)                A pressure transmitter has range of 200 to 1000 psi and the output range is 3 to 15 psi.  The measured value is 925 psi.
a)     What should the output value be?
b)    What is the percentage of span?

     PLC SYSTEMS

There are various manufacturers of PLC equipment.  They all use different methods for sending data and make their diagrams to different standards.  This means that operating companies do not mix the different types of PLC systems.  You cannot mix PLC systems when controlling a process. 
The most common PLC systems are Allen-Bradley and Modicon which use the ladder diagram method for PLC logic.  This system is explained here as it’s the most common.

LADDER DIAGRAMS

Ladder diagrams show the operating sequence for a PLC system.  They indicate step by step what happens when the system is started, shut-down or operated under an emergency.  You move down the ladder from top to bottom.
 
Figure 6-1   Simple Ladder Diagram for Pump Starting
Figure 6-1 shows a simple ladder diagram for starting/stopping a pump. 

The diagram will be explained as if the system were operated in relays.  Remember the diagram is drawn with contacts in the de-energised position.
1)                CR 1 is the coil of control relay one
2)                The stop button, temperature switch and suction pressure switch are normally closed.  When the start button is pressed CR 1 will energise.
3)                The contacts of CR 1 (located on rungs 2 and 3) change over, from normally open to closed.
4)                CR 1 contact on rung 2 is the hold "ON" contact.  When this has closed the start button can be released.
5)                When the CR 1 contact on rung 3 is closed it energises control relay CR 2
6)                CR 1 is the operating relay for the pump.  When CR 1 operates the supply is connected to the pump motor.
7)                The CR 2 contacts on rungs 4 and 5 change over.  The normally open contact on rung 4 closes to light the run light (PL-R).  The normally closed contact on rung 5 opens and the stop light goes out (PL-S).
8)                The stop button, temperature switch and pressure switch are connected as an AND circuit.  If any one of them is operated then CR 1 is de-energised.  The motor stops and the run-stop lights change over.
The operating supply for the relays on the ladder diagram can be 24V d.c. 110V a.c. or 220V a.c. depending on the system.  The relays themselves are located in the electrical switch room.  The stop/start buttons and lights are located in the control room.  The temperature and pressure switches are located in the field and are hard-wired into the relay system.
A PLC system provides the same sequence to start the pump but the relays are replaced by electronic switches.  The contact switching sequence is set into the PLC by a hand held programmer (pocket programmer).  Normally, the programmer uses the same type of ladder diagram as the relay system, with the same symbols.  A ladder diagram of the required switching sequence must be drawn before the programme can be set into the PLC.
A table to show the common symbols is included as a reference to the Allen-Bradley PLC ladder diagram system.



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