PLC Troubleshooting
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PLC Troubleshooting Training Video. Find faults In Just Minutes!
This article details the PLC troubleshooting training video below, which demonstrates how to use a PLC to troubleshoot complex equipment in just minutes. Engineers may point out that there are more complicated faults than those shown in the video below. This is particularly true in new equipment's design and start-up phases, but less so for PLC troubleshooting and maintenance personnel.
5 Facts from a PLC troubleshooting and maintenance perspective:
- PLCs, not PACs, control most automation.
- Most automation controllers are programmed in Ladder Logic.
- Most equipment faults are external to the PLC. For example, limit switches and relays.
- Most equipment faults are discrete. (Should be on, but it is off. Or vice versa.)
- When tracing ladder logic, starting with the symptom-related output and tracing backwards is usually faster than starting with an input.
The PLC troubleshooting training video below shows that troubleshooting complex equipment can be done in just a few minutes. After reviewing the video, we’ll provide more details. At the bottom of this article, we’ll give some advice for troubleshooting PLCs themselves. Should that rare occasion occur, as PLCs are highly reliable.
In the video above, we display the three relevant rungs on a single screen for presentation purposes. Each rung was located in a different subroutine in the real-world PLC program we used in class. This Automated Guided Vehicle (AGV) PLC program consists of thousands of rungs, with over 30 subroutines. However, with PLC software search, it is as simple as shown in the video. No matter how big or complex the ladder logic program is.
Note: PLC Troubleshooting and maintenance electrical troubleshooting are virtually the same when the PLC is programmed in ladder logic.
PLC Troubleshooting Training Video Enhanced Transcript:
The symptom?Using the joystick, the Automated Guided Vehicle will not move forward in manual mode.
The first decision is whether to start the ladder logic trace with the relative input for these symptoms [Auto/Manual Mode, Start push button, Joystick, etc.]. Or do you want to start with the relative output [Motor Forward]? For this example, we choose the latter.
Note: In the video, a contact is highlighted green to indicate the condition is being met. The contact is said to be ‘true’.
After using the PLC software’s search, cross-referencing, or schematics if available, we see that the real-world output address to move the smart car forward is “O:2/6”. We also immediately see that the PLC output is not true. The PLC is not telling the vehicle to move.
We begin tracing backwards through the ladder logic to find the root cause. In most cases, tracing backwards will eventually lead to a real-world input that prevents the vehicle from moving. At that point, you would grab your meter and perform standard electromechanical troubleshooting.
Tracing back (to the left), we see the rung output is not true because input from internal bit B50/0 is not true. Use the PLC software’s search or cross-referencing tool to find the internal output bit B50/0 to see why it is not true.
As you trace left from the B50/0 output, you see that “Forward MS Auxiliary”, address B3/398 is not true, causing the rung/output not to be true. You can find the rung that turns B3/398 on and off using the PLC software's search or cross-reference feature.
As you trace left from the B3/398 output, you see the e-stop bits are true, the plugging timer, and all the manual operation branch bits, including the joystick bit, are true. You see the ‘Run’ bit B3/2 is not true, resulting in the rung/output not being true.
So, you search for the output rung controlling the B3/2 input. On that rung, you see real-world input I:1/0 is not true. As it is a real-world input, you are now done with ladder logic trace. Before grabbing your meter for standard electromechanical troubleshooting, press the ‘START’ momentary push-button while monitoring the input on your computer. This way, you can verify whether the PLC is not receiving the input. You never know, the operator could have forgotten to press the button, in which case the input would become true when the button is pressed.
That’s it; troubleshooting using a PLC is that simple, especially with the RSLogix 500 PLC brand. It typically takes only a few minutes, regardless of the equipment's complexity.
Troubleshooting PLC:
Although it may be rare, if you find yourself needing to troubleshoot the PLC itself and make maintenance decisions, such as choosing spare parts, consider the following essential physics.
Physics of Troubleshooting Examples
(In order of most likely to fail to least likely.)
Attribute
Electrical Control Example
PLC Control Example
Mechanical
External to electrical control
External to PLC control
Electro-mechanical
Ice Cube Relay
Relay Output Card
High current
Motor Starter
Triac Input Card
Low current
Sensor
Analog Input Card
Solid-state
Solid-state Timer
PLC Processor
The environment can be an exception to the general rules. The environment can change the basic order above. A couple of examples, in a foundry, the metal dust causes the backplane to fail more frequently. In the machining industry, penetrating fluid entering sensors causes input cards to fail more often than output cards because the liquid makes the input card draw more current than it is designed to. In wire manufacturing, there is a lot of high voltage insulation testing going on; CPUs fail more often because the control voltage for PLC does not have a line filter in line with it. etc.
Other variables that affect the failure rate are improper design, like not allowing the 20% extra when calculating the relay card or power supply current, etc.
Above and beyond physics, what is a more probable cause of failure than even Mechanical causes? Human Error (sometimes Root Cause Analysis must be performed to reveal this fact.)
Need to practice your electrical troubleshooting? Try our free onlineIndustrial Electrical Troubleshooting Simulator Game.
Just starting out? Try ourTroubleshooting Training Certificate Coursewith real-world simulations.
Related Courses:
Automation Technician Training Bundle
Related Article:
More Fault Finding and Troubleshooting Examples
Electromagnetic Induction Applications for the Instrumentation Technician
Electrical and Instrumentation Technicians - Troubleshooting FAQ
