For control system migration, there are various technical details that need to be analyzed and resolved during the installation and startup preparation process. In addition to software configuration and development, many signal details also need to be considered when connecting the software to the process.
At a high level, particularly in the early stages of project scope definition, it is useful and appropriate to analyze control system migration based on the number of common types of inputs/outputs (I/O), including analog inputs (AI), digital inputs (DI), analog outputs (AO), digital outputs (DO), and soft tags. After the early stages of scope definition, and before finalizing the budget and beginning detailed design, the project team should consider signal details that may affect project cost and schedule.
Signal considerations during the migration from PLC to DCS control system
Compatibility between new and old systems
There are many signal details to consider during a migration project. This is especially important if the project is based on retaining existing field instruments.
The control system may require a mix of single-ended and isolated DO modules to ensure compatibility with existing field signals, including simple solenoid valves and start/stop signals that are directly cabled and powered by existing electronic control loop transformers. The control system may also require a mix of single-ended, differential, HART-enabled, and isolated AI modules to ensure compatibility with existing loop-powered transmitters, self-powered transmitters, HART transmitters, and known ground loop issues.
The new control system may require several different DI and DO module types to be compatible with existing 24Vdc and 120Vac discrete signals, unless the project is designed and intermediate relays are installed. The aforementioned changes in I/O modules will result in significant changes in I/O density (the number of channels per module), which, for a given number of I/Os, will further affect the space required for physical panel cabinets and the basic structure of the I/O rack.
To ensure compatibility between existing field signals and the I/O modules offered in the new system, or to avoid procuring different types of I/O modules for only a small number of unique field signals, signal isolators or conditioners may be required. The input impedance of the DIs in the new control system may be higher than in the existing system, making them more vulnerable to induced voltages generated on long-distance cables.
Unlike dry contacts, existing solid-state, discrete-output field device signals typically require a minimum current to conduct and turn on. Therefore, if the input impedance of the DI module in a new control system is too high, a step-down resistor is needed. Besides field instruments, solid-state output signals are also encountered when there are hard-wired DOs ranging from control systems such as small unit-level programmable logic controllers (PLCs) to integrated monitoring systems such as distributed control systems (DCS) in a factory.
As part of the migration project, the plant may also install discrete field instruments with Namur sensors, which may be incompatible with all DI modules in the new control system. Furthermore, if a new Namur sensor is introduced into an existing plant, maintenance personnel should be trained on the operation of the Namur signal, which, unlike a simple dry-contact position switch, requires an excitation voltage to function. The Namur sensor may also have a contact action opposite to that of an existing simple dry-contact position switch and require reversed control system logic.
Additional requirements
As part of a migration project, a plant may want to change its motor control loop (MCC) from two-wire control (single maintenance signal) to three-wire control (requiring an additional DO at any time) to enhance safety or for other process reasons related to recovery from power outages.
If the MCC in the old factory can use a 480Vac signal (using two power supply phases) for motor start, stop, and operation confirmation signals, it almost certainly requires the use of an intermediate relay in the new system, unless the existing motor starter is replaced.
Existing actuators (AOs) used to drive a pair of split-range control valves may need to be implemented as two separate AOs in the new system to ensure sufficient voltage drop or to provide an intuitive operating interface on the human-machine interface (HMI). The plant may be using outdated thermocouple temperature transmitters whose 4-20mA output is no longer linearly related to temperature changes; these transmitters may still work in existing, conventional control systems, but will be incompatible with the new control system.
The plant may have field transmitters that use old, primitive fieldbus technology that has never been widely adopted and is therefore not supported on new control systems, but provide critical process data to operators on existing legacy control systems.
While many newer control systems have this built-in DO module functionality, some 24Vdc DO modules still require a reverse-biased diode at their output. This is to prevent excessive voltage generated by the energy released from a collapsing magnetic field on the induced load, which could damage electronic components. The excitation voltage of the AI in newer control systems may differ significantly from that in existing systems, potentially leading maintenance personnel astray in their troubleshooting efforts.
