In our March 2014 issue of UnderControl, the discussion focused on when it made sense to upgrade industrial control systems. The reasons to upgrade include obsolescence, expensive/unavailable spares, requirements to standardize on a control platform, and the requirement for increased functionality. In this issue, the discussion will be about the information needed to properly specify upgrades for various control components. Whether you perform the upgrade in-house or not, gathering all the pertinent information is of value to the project’s success. And gathering the info before ‘disaster’ happens, can be significant.
PLC System Upgrades 1) The most important information required for a PLC upgrade is the detail about the machine sequence. The preferred form of this detail is the current ladder logic listing. With this listing, the engineer has a relatively easy job of converting the program to the new platform. Even a somewhat outdated listing is better than no listing. If the logic listing is not available, a detailed document or flow chart of the sequence is required. 2) The detail about the I/O should be provided in the form of an I/O schematic or listing. Also, specify if the I/O will be different on the upgraded system, such as converting 120VAC devices to 24VDC. This has been popular because of the added safety measures it provides. 3) Finally, define the operator interface. It could be the old-fashion but functional push-button/pilot-light interface or an HMI (also referred to as a touch screen interface). List all the values and status bits that the operator will need. Values could be motor amperage, part count, defect count, tank level, etc. Status bits could be pump running, cycle in-progress, alarm failure, etc. If specifying an HMI, provide a rough lay-out of the pages and pertinent detail like screen size, communication scheme, and whether color or black/white is required. Temperature Controller Upgrades 1) An often over-looked detail when upgrading a temperature controller is how it interfaces with the rest of the control system. What signals it receives from and provides to the rest of the control system is important to duplicate the functionality of the old controller. Usually this information is available on the schematic. When a schematic is not available, written detail about the process sequence may be sufficient. 2) Provide the detail of any programmable functions that are required, for example ramp-and-soak. Determine if there are multiple ramp-and-soak profiles and define how the operator will select and activate the different profiles. 3) Provide the temperature sensor type. This is usually the thermocouple type with J and K being the most common. 4) Provide the type of output, either relay contact or process signal. For relay outputs, determine the amperage draw required for the device that will do the work (for example a gas solenoid valve). Process signals are commonly 4-20 ma because of its relative immunity to electrical noise, but can be 0-10V or 1-5V. 5) Determine if the controller needs to supply output information to a chart recorder or other monitoring device. 6) Retrieve the PID control parameters from the old controller as this will save re-tuning efforts. Servo System Upgrades 1) A good starting point is to get all the detail concerning the old servo controller/amplifier (many times combined into one package) and the servo motor. Detail includes manufacturer, part numbers, torque rating, supply voltage, amperage, and mounting specifics. 2) Provide information about the motion, whether it is indexing, continuous, or a programmed point-to-point scheme. Good information includes motor rpm, accel/deccel profiles, and dwell time. 3) Provide the specifics about the load including the required gear-reduction, weight of the load, and any friction on the load. A good video of the cycle can be of great value to the designer who will be specifying the new system.