Microprocessors in instrumentation systems (case study)

Modern industry needs automatic measurement, decision and control of many variables with accuracy and speed. A microprocessor gives programmable intelligence at low cost, so it is the heart of today's instrumentation.

Case study: boiler (furnace) temperature control in a plant

A thermocouple measures boiler temperature. The signal is amplified, cold-junction compensated and digitised by an ADC. The microprocessor linearises it, compares it with the set point (say 400 deg C), runs a PID algorithm and drives a fuel valve through a DAC. It displays the value, logs it and raises an alarm if the temperature exceeds the limit.

 Thermocouple -> Amplifier -> ADC -> Microprocessor -> DAC -> Valve
       ^                              |   |   |                 |
       |                           Display Alarm PC link        |
       +----------------- Boiler <---------------------------- +

Observations supporting the statement

1. Control algorithms (PID, limit checks) run in software and are easily tuned.
2. One processor monitors many channels by multiplexing, reducing cost.
3. Accuracy is high: calibration and linearisation are done digitally.
4. Alarms, interlocks and safety shutdown happen automatically and quickly.
5. Data logging and communication (RS-485, Ethernet) allow remote supervision.
6. Changes in process need only software changes.

Therefore the processor is indispensable for modern industrial instrumentation.