Page 310 - Programmable Logic Controllers, Fifth Edition - Mobile version

useful for detecting intermittent problems, either hardware-
or logic-related. By tracking the status and time between
status changes, you can detect different types of problems.
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Bat
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13.8 Preventive Maintenance –
The biggest deterrent to PLC system faults is a proper Bat
preventive maintenance program. Although PLCs have –
been designed to minimize maintenance and provide
trouble-free operation, there are several preventive mea-
sures that should be looked at regularly. Figure 13-17 CPU backup memory battery.
Many control systems operate processes that must be
shut down for short periods for product changes. The fol- replaced, it must be replaced with exactly the same
lowing preventive maintenance tasks should be carried type of battery module.
out during these short shutdown periods: • Stock commonly needed spare parts. Input and output
• Any filters that have been installed in enclosures modules are the PLC components that fail most often.
should be cleaned or replaced to ensure that clear • Keep a master copy of operating programs used.
air circulation is present inside the enclosure. To avoid injury to personnel and to prevent equip-
• Dust or dirt accumulated on PLC circuit boards ment damage, connections should always be checked
should be cleaned. If dust is allowed to build up on with power removed from the system. In addition to dis-
heat sinks and electronic circuitry, an obstruction of connecting electrical power, all other sources of power
heat dissipation could occur and cause circuit mal- (pneumatic and hydraulic) should be de-energized before
function. Furthermore, if conductive dust reaches someone works on a machine or process controlled by a
the electronic boards, a short circuit could result PLC. Most companies use lockout and tagout proce-
and cause permanent damage to the circuit board. dures, shown in Figure 13-18, to make sure that equip-
Ensuring that the enclosure door is kept closed will ment does not operate while maintenance and repairs are
prevent the rapid buildup of these contaminants. conducted. A personnel protection tag is placed on the
• Connections to the I/O modules should be checked power source for the equipment and the PLC, and it can
for tightness to ensure that all plugs, sockets, termi- be removed only by the person who originally placed the
nal strips, and module connections are making con- tag. In addition to the tag, a lock is also attached so that
nections and that the module is installed securely. equipment cannot be energized.
Loose connections may result not only in improper
function of the controller but also in damage to the
components of the system.
• All field I/O devices should be inspected to en-
sure that they are adjusted properly. Circuit boards
dealing with process control analogs should be
calibrated every 6 months. Other devices, such as
sensors, should be serviced on a monthly basis.
Field devices in the environment, which have to
translate mechanical signals into electrical, may
gum up, get dirty, crack, or break—and then they
will no longer trip at the correct setting.
• Care should be taken to ensure that heavy noise- or
heat-generating equipment is not moved too close to
the PLC.
• Check the condition of the battery that backs up the
RAM memory in the CPU (Figure 13-17). Most
CPUs have a status indicator that shows whether the
battery’s voltage is sufficient to back up the memory Figure 13-18 Lockout/tagout devices.
stored in the PLC. If a battery module is to be Source: Photo courtesy Panduit Corporation, www.panduit.com.

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