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Rationale for microprocessor; open-loop MBI; DMA

Short answer

Rationale: microprocessor adds programmable intelligence, multi-channel handling, accuracy, and communication. Open-loop is better when the process is predictable (stepper positioning, traffic lights) or when feedback is difficult/delayed (timed heating) because it avoids sensor cost and loop stability problems.

What is the rationale behind using a microprocessor in instrumentation systems? Describe two situations where an open-loop MBI system performs better than a closed-loop MBI system, illustrating your answer with block diagrams.

Rationale for using a microprocessor

A microprocessor makes instruments programmable, accurate and intelligent. It replaces fixed hardware logic with software, handles many channels, performs calculations (linearisation, PID, filtering), takes decisions, stores and displays data, and talks to other computers, all at low cost and size.

Situations where open loop is better than closed loop

  1. Process is predictable and the output follows the input exactly (e.g. stepper motor positioning, traffic signals, washing machine timer). The result is known from the commands. A feedback sensor and ADC would add cost and complexity without improving accuracy.
  2. Measurement is difficult, slow or noisy, or feedback could cause instability (e.g. a drying or timed heating process, a fast process where the sensor delay is too long). Open loop is faster and always stable, since there is no loop to oscillate.
 Open loop (stepper motor positioning)
 Required      +----+  step pulses  +--------+  +-------+
 position ---> | uP |-------------->| Driver |->| Stepper|-> shaft
 (keypad)      +----+               +--------+  +-------+

 Closed loop (needs extra blocks)
 Set ->(+)-> uP -> DAC -> Actuator -> Process -+
        ^ -                                    |
        +---- ADC <- Conditioner <- Sensor <---+

Open loop gives lower cost, simpler software and no stability problems; it is chosen when disturbances are small and accuracy needs are moderate.

✓ Answer checked by Claude Sonnet 5.5

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