Continuing my discussion of smart controls, in this issue, I’ll finish my discussion of smart power controls and look at signal conditioners. Access to process information and the ability to act on it is becoming faster and easier year upon year. 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.
Power ControlAs I noted last month, many rich control features are available that let you grow the SSR into a sophisticated SCR unit. Some smart features of SCRs include:
- Soft start.
- Control by voltage, current and true power, and transfer between these modes.
- Voltage, current and power limiting and monitoring.
- Line-voltage monitoring.
- Ability to receive measurements over the analog control line.
- Heat sink temperature alarm.
- Partial load failure alarm.
- Phase-imbalance monitoring and control.
With communicating SSRs and SCRs, access to these features over your Ethernet link enables remote adjustment, fast fault diagnosis, automatic or operator intervention, and recording of load and process behavior.
Signal ConditionersA typical signal conditioner may come with millivolt input and voltage output with three-way (in/out/ground) isolation, adjustable gain and offset, high input impedance and a robust low impedance at the output. A current in/current out isolator also would offer adjustable gain and offset. It would have near zero input impedance (current sink) and high output impedance (current source).
You find appropriately ranged models buried in flow transmitters or thermocouple heads. Other functions include inversion; thermocouple/milliamp with isolation; millivolt, volt and milliamp signal scaling; custom linearizing; voltage-to-frequency (V/f) and frequency-to-voltage (f/V) conversion; math functions; and relay or logic alarm outputs.
Many models now are field-configurable with respect to magnitude and type of input and output signal. On typical models, this can be done by manual adjustments and DIP switches or, alternatively, by a PC. LCDs are available showing the process signal and other parameters to aid field configuration.
Or, using an extra module, you can -- with a standard browser -- calibrate and configure some models and view all the sensor’s data directly. You can automatically trigger e-mail messages when process variables or maintenance-based performance parameters exceed preset limits.
For all that you can do from your chair, don’t forget: The best fertilizer is the farmer’s foot.
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