32.2.1 Troubleshooting

After a system is in operation it will eventually fail. When a failure occurs it is important to be able to identify and solve problems quickly. The following list of steps will help track down errors in a PLC system.

1.Look at the process and see if it is in a normal state. i.e. no jammed actuators, broken parts, etc. If there are visible problems, fix them and restart the process.

2.Look at the PLC to see which error lights are on. Each PLC vendor will provide documents that indicate which problems correspond to the error lights. Common error lights are given below. If any off the warning lights are on, look for

electrical supply problems to the PLC. HALT - something has stopped the CPU

RUN - the PLC thinks it is OK (and probably is) ERROR - a physical problem has occurred with the PLC

3.Check indicator lights on I/O cards, see if they match the system. i.e., look at sensors that are on/off, and actuators on/off, check to see that the lights on the PLC I/O cards agree. If any of the light disagree with the physical reality, then interface electronics/mechanics need inspection.

4.Consult the manuals, or use software if available. If no obvious problems exist the problem is not simple, and requires a technically skilled approach.

5.If all else fails call the vendor (or the contractor) for help.

32.2.2Forcing

Most PLCs will allow a user to force inputs and outputs. This means that they can be turned on, regardless of the physical inputs and program results. This can be convenient for debugging programs, and, it makes it easy to break and destroy things! When forces are used they can make the program perform erratically. They can also make outputs occur out of sequence. If there is a logic problem, then these don’t help a programmer identify these problems.

Many companies will require extensive paperwork and permissions before forces can be used. I don’t recommend forcing inputs or outputs, except in the most extreme circumstances.

32.3 PROCESS MODELLING

There are many process modeling techniques, but only a few are suited to process control. The ANSI/ISA-S5.1-1984 Piping and Instrumentation Diagram (P&ID) standard

plc software - 32.4

provides good tools for documenting processes. A simple example is shown in Figure 32.1.

control valve

Figure 32.1 A Process Model

The symbols used on the diagrams are shown in the figure below XXXXXXXXXXXXX. Note that the modifier used for the instruments can be applied to other discrete devices.

plc software - 32.5

Discrete Device Symbols

Instruments

field mounted

panel mounted

unaccessible or embedded

auxilliary location, operator accessible

Controls

Computer Function

PLC

Shared Display/Control

Figure 32.2 Symbols for Functions and Instruments

The process model is carefully labeled to indicate the function of each of the function on the diagram. Table 2 shows a list of the different instrumentation letter codes. XXXXXXXXXXXXXXXXXXXXX

Table 1: ANSI/ISA-S5.1-1984 Instrumentation Symbols and Identification

LETTER	FIRST LETTER	SECOND LETTER


A	Analysis	Alarm

B	Burner, Combustion	User’s Choice

plc software - 32.6

Table 1: ANSI/ISA-S5.1-1984 Instrumentation Symbols and Identification

LETTER	FIRST LETTER	SECOND LETTER


C	User’s Choice	Control

D	User’s Choice

E	Voltage	Sensor (Primary Element)

F	Flow Rate

G	User’s Choice	Glass (Sight Tube)

H	Hand (Manually Initiated)

I	Current (Electric)	Indicate

J	Power

K	Time or Time Schedule	Control Station

L	Level	Light (pilot)

M	User’s Choice

N	User’s Choice	User’s Choice

O	User’s Choice	Orifice, Restriction

P	Pressure, Vacuum	Point (Test Connection)

Q	Quantity

R	Radiation	Record or Print

S	Speed or Frequency	Switch

T	Temperature	Transmit

U	Multivariable	Multifunction

V	Vibration, Mechanical Analysis	Valve, Damper, Louver

W	Weight, Force	Well

X	Unclassified	Unclassified

Y	Event, State or Presence	Relay, Compute

Z	Position, Dimension	Driver, Actuator, Unclassified