Commissioning a stand-alone machine project based on control with a programmable logic controller (PLC) and a human-machine interface (HMI) has many facets to the successful implementation of the equipment that the control system is controlling. Some basics are needed to be sure that when power is applied the equipment response is as expected and as designed.
The assumption has been made that the project is an automated machine that is built in-house.
When the project is built, there is a responsibility by the design and fabrication team to test the system, per the customer design. Once the machine has been put through its paces, it is deemed ready for shipping to the customer as it has met the operational requirements of the design.
Customer visits will vet the operation to make sure that it meets their specifications. To get there, commissioning tests and steps need to be followed.
There are many “departments” of consideration—power, control (PLC), software, I/O connections, peripheral devices, such as network switches, variable-frequency drives (VFDs), field wiring and safety systems.
There is a control panel with the power components installed such as relays, starters, and/or VFDs. There may be a line filter, control transformers and an uninterruptible power supply (UPS). Device fusing would be present.
While a test plan can be helpful, they are often overlooked due to the time and cost of preparing. Normal power system checks are done with the system drawings, and the physical point-to-point wiring is metered and checked off on the panel drawings. This includes all power connections to all hardware in the system.
Power fusing has to be considered since installation is required before the next step—applying power.
Some might do I/O checks with point-to-point metering at this point, but it may not be the most efficient. Applying system power will now allow for device power-up checks such as PLC status, VFD status and peripherals.
Most of the start-up effort will go into the interfacing with the real world. In this modern age, peripheral devices will be connected with an Ethernet network in the panel. Once power has been applied, the connectivity of these devices can be checked using Internet control message protocol (ICMP) commands, such as ping.
Now that the connectivity has been verified and the power and status lights have been interpreted, we have to get down to the business of the control system. Typically, the machine’s I/O would be wired to terminal blocks whose in-panel side would be wired to the I/O modules of the control system. With the PLC in program mode, inputs would be checked first.
The benefit of testing the connected inputs with power on is that, for example, a blocked photo sensor will change state, and that change can be monitored on the I/O module itself. This tests the field wiring from the sensor to the terminal blocks and then to the I/O module directly. The rest state can then be identified to be sure the correct polarity of the sensor has been wired. Each input state change is then recorded to document the procedure.
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This also checks for correct power supply wiring, which, in dc circuits—NPN/PNP devices—can prove very helpful.
Checking outputs is a bit trickier due to machine motion. It is advisable to check outputs using the force function in the PLC system to energize each output and monitor the end device such as a relay or starter. The outputs will actually turn on the device to prevent motion; the motor/VFD disconnect should be turned off. Once the end device has been validated, you could then turn power on to the device to check rotation. Be sure however to remove the coupling from the motor to the end mechanics. Once rotation has been confirmed the coupling can be put back on.
VFD control can be done manually to check the system for rotation and speed control.
The last piece of the puzzle is the system evaluation using the designed software component. The HMI component should be checked first. Each screen would have interface components, which would need to be checked. A motor-start function from the HMI would need to be checked in the PLC program/data table directly. Data in the PLC can be manipulated to engage the output interface components such as “motor on” indicators.
The focus is now set on the control algorithms in the PLC to check sequencing, interlocking and control strategies, which allows the machine to operate as intended. The sequencing can be checked in manual to make sure each device operates as intended.
Only when the previous steps are completed successfully would the machine be ready for automatic operation. This should also test use cases for safety interruptions, sensor failure, output device failure and sequencing failure, such as identification that a dial table didn’t return to center.
Success is driven by attention to detail. Don’t miss any step or take for granted that someone else checked it out. It’s up to you.
