Analog vs. Digital Process Control and Fault Monitoring
Typically, analog designs provide minimal process and fault information because only the most critical information is monitored. In most process heating applications operating under power control, the power delivered to the load is the only parameter controlled and monitored. The process control system will send a signal (typically 4 to 20 mA DC) to set the desired zone output power. In addition, a transducer (also with 4 to 20 mA DC output) will be used to monitor output power. Both the output setpoint control signal and output transducer signal are sent, via wire connections, between the power control panel containing the SCR controllers and the process control system. Through these individual wires, the control room operator sets the desired output and monitors the results.
In addition to monitoring and controlling specific parameters such as output power, analog equipment uses generic fault indications such as zone failure or power controller failure. These single fault indications have multiple meanings. They could be defined as SCR overtemperature, fuse blown, SCR overcurrent shutdown, load failure or other malfunctions.
To further understand the problem, the control room operator must send service or maintenance personnel out to the control panel location to determine exactly what is causing the generic fault indication. With analog technology, each additional fault requires control relays, wiring and digital inputs on the process controller. If each piece of information is to be displayed in the control room, the cost of showing each fault can be substantial, especially for multizone systems.
If money were no object in the world of analog power control technology, one could monitor all process information, including zone voltage, current and power, as well as separately annunciate all of the individual zone faults. But, because the costs of equipment, installation, wiring and maintenance are significant, many times only the most critical functions are monitored.
The Digital DifferenceWith digital SCR power controllers, all parameters and faults directly related to each power controller (or each zone of control) are monitored, and many are controllable. These parameters include:
- Input and output phase-to-phase voltage.
- Input and output per phase current.
- Input kVA and output kWA.
- Power factor and kW/hr.
Furthermore, single-phase and three-phase digital power controllers monitor a range of faults, including:
- Input high voltage.
- Frequency out of tolerance.
- Overcurrent shutdown.
- SCR overtemperature.
- Phase loss.
Parameters that are controllable by digital SCR power controllers include output voltage, current and power. This level of functionality is driven by the nature of the digital design.
In an analog design, each additional parameter to control or monitor current flow means more circuitry and higher equipment costs. With digital power controllers, that paradigm does not exist because the microprocessor's software dictates the unit's functionality.
Achieving Local ControlTraditionally, manufacturers or processors have used relatively simple methods to achieve local control of SCR power controllers. On/off pushbuttons, control potentiometers and analog meters are provided locally and often are located on the enclosure door of the panel containing the specific SCR power controller. As with other aspects of an analog design, monitoring devices -- and therefore functionality -- are kept to a minimum due to the incremental costs to add features.
Analog designs typically transfer process and control information through twisted-pair control wire connections carrying control signals (commonly 4 to 20 mA DC signals) and other relay-type contacts representing faults and alarms. In some process heating applications, for example, each SCR power controller zone accepts a 4 to 20 mA DC control signal for its pow-er setpoint and sends a 4 to 20 mA DC signal proportional to output power back to the process controller.
In addition, an analog SCR power controller accepts relay contacts for on/off control and provides a relay contact for zone fault. This configuration means that, per zone, there are up to two sets of twisted-pair control wires and three sets of wires for the relay contacts. These wires must be run from the power panel to the process controller in the control room. Multiply those numbers by the total number of power control zones the application requires, and you can see that a lot of wire, conduit and labor are expended to achieve a minimal control level.
In contrast, digital SCR power controllers are capable of continuously monitoring and controlling a multitude of parameters and faults. Using a connectivity package such as Device-Net or Ethernet, digital power controllers transmit key information between each power controller and the central process controller. A single network cable replaces the multiple runs of control wire previously required for each control zone.
In many applications, a digital network connection can replace more than 100 analog control wire connections between the process controller and the power panels. The savings in labor and materials can be significant. The use of digital technology allows for independent, remote operation of SCR power controllers and eliminates calibration and hardware considerations that formerly constrained the ability to precisely monitor and control electric current.