Smart Field-Mounted Control Components, Part 1
You are already accustomed to the smart and versatile nature of communicating process controllers. The dumb and obedient field devices have, for some time, been evolving in the same way.
Access to process information and the ability to act on it is becoming faster and easier year upon year. Watch-keeping and actions that once involved a walk and poke round the plant are now being done from the control room. Archived records become readily available for environmental conformity, and process fault and incident analysis. There are impressive case histories of time and cost savings on commissioning, maintenance, asset management, process uptime and record-keeping.
Control components that make this possible have digital electronics and sensing devices built in, and often are called smart or intelligent. These also are marketing words, so judge each case for yourself.
A communications cable leaving the control room calls at each component, enabling you to watch and manipulate its parameters. This has bestowed great benefits onto process diagnostics, preventive maintenance and plant uptime -- and don't forget wiring complexity.
Components under discussion include control valves, power-control devices, temperature sensors, signal converters, transducers and motor drives. Controllers, indicators and PLCs have long had smart features. They too are often remote but can still keep in touch with a control center.
Control ValvesA control valve is not often located where you can see it working. You may see its 4 to 20 mA command signal and an analog position signal in the control room. What more could you want to do without leaving your seat?
- Control the valve position with a digital signal (Modbus, Profibus, etc.).
- Set the stroke (the span and zero positions of the valve).
- Make its travel linear.
- Measure the applied thrust or torque.
- Observe the deadband.
- Measure the speed.
- Count the number of operations.
- Measure and control the flow. (This is usually a more relevant and linear manipulated variable than position.)
- Measure upstream and downstream pressures.
All of these functions can be built into a microprocessor incorporated into a control valve and exploited from the control room while the process is running.
Power ControlMany heat processes are controlled using low cost solid-state contactors (SSRs) operating in the time-proportioning mode. At this basic level, more features may not give you a payback, and you may not even justify a load ammeter. A small but handy step upmarket is a bit of circuitry that compares the coincidence of a control pulse with the resulting load current and gives an alarm upon finding an inconsistency. Possible inconsistencies that would trigger the alarm include pulse present but no load current (which signals a possible load open circuit); no pulse but load current present (which signals a possible short circuit SSR). This says, "You'll soon be making scrap, get out here."
Many rich control features are available that let you grow the SSR into a sophisticated SCR unit. Though they use traditional logic and analog techniques, I rate them smart even before we turn to microprocessor use.
Next month, I'll continue my look at power controls and look at signal conditioners.