--- title: "Case study: existing vs proposed system" subject: "Instrumentation II" chapter: 10 key: "semester_5/instrumentation_ii:cs-2074asoj" source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/cs-2074asoj site: BCT Engineering --- # Case study: existing vs proposed system Instrumentation II · Chapter 10: Case Study - Asked in: 2074 Asoj, 2+3+5+2 marks - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/cs-2074asoj - Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-10 ## Question Answer the following questions with regard to your case study: (a) Discuss the main architectural differences between the existing system and the proposed system. (b) Does your proposed system use a microcontroller or a microprocessor? Justify your choice, and make a neatly labeled block diagram of your proposed system. (c) In your proposed system, explain in detail the interfacing process of peripheral devices with the microcontroller or microprocessor in terms of data format, data rate, data converters, communication protocols, timing diagrams, and handshaking signals. (d) List the technical drawbacks present in your proposed design. ## Short answer Architecture: existing manual/thermostat, analog gauges; proposed closed-loop microcontroller with PT100, ADC, RS-485 Modbus, SCADA. Microcontroller chosen for on-chip ADC/timers/UART, low cost, deterministic. Interfacing: PT100→amplifier→12-bit ADC (0.037°C/LSB) with EOC; flow timer; keypad scan; LCD 4-bit E strobe; RS-485 9600 8N1; outputs opto-isolated. Drawbacks: noise, drift, firmware, training. ## Answer **Assumed case study:** a milk pasteurisation and packaging section of a dairy plant (HTST process: raw milk tank, heat exchanger, steam-heated holding tube, cooling, filling). The existing system uses analog panel indicators, bimetal thermostats, a manual steam valve and operator logging. (If your own visit was to another plant, replace the process names; the structure of the answer stays the same.) #### (a) Architectural differences | Point | Existing | Proposed | |---|---|---| | Control | Manual / thermostat | Closed-loop digital (microcontroller) | | Sensing | Analog gauges, bimetal | PT100/level/flow sensors + ADC | | Actuation | Manual valve, push-button | Relay/SSR driven valve, pump | | Monitoring | Visual, logbook | LCD + PC logging, alarms | | Communication | None | RS-485 (Modbus) | | Flexibility | Fixed | Programmable set-points | #### (b) Microcontroller or microprocessor A **microcontroller** (e.g. ATmega328/ARM Cortex-M) is chosen. It has an on-chip ADC, timers, UART, GPIO and flash/EEPROM, so few external parts are needed, and it gives lower cost and power and a deterministic response. The task also has few I/O points. A microprocessor would need external memory, ports and an ADC, which increases cost and complexity. ```text PT100 --> Amp/SigCond --+ Level sensor ----------+--> MUX/ADC (12-bit) Flow sensor ---(pulse)-+ | Keypad ----------------+---> [Microcontroller] <--> LCD 16x2 | ARM/AVR + flash | Opto + SSR/relay <----------+ watchdog, timers +--> RS-485 | | (Modbus) Pump / steam valve / alarm +--> EEPROM log PC / SCADA Power supply: 24 V + 5 V + UPS ``` #### (c) Interfacing of peripherals - **Temperature (PT100):** analog 0-5 V after the amplifier; 12-bit ADC (resolution 150/4096 = 0.037 C); data format: binary right-justified, scaled in software; sampling at 10 samples/s; anti-alias RC filter. Timing: start conversion, wait for the end-of-conversion (EOC) flag or interrupt, then read the data. - **Flow sensor:** a pulse train counted by a timer/counter input (frequency is proportional to flow). - **Level switch:** digital input through an optocoupler with a pull-up. - **Keypad:** 4x4 matrix, scanned on I/O ports, debounced for 20 ms. - **LCD 16x2:** 4-bit data mode with RS and E handshaking signals (E pulse at least 450 ns). - **Relay/SSR (pump, valve):** port pin -> optocoupler -> transistor driver -> relay with a flyback diode. - **PC link:** RS-485 transceiver (MAX485), 9600 baud (data rate), 8N1, half-duplex, direction pin (DE/RE) controlled by the MCU; Modbus RTU frame with CRC, register values sent as 16-bit words; one frame of 8 bytes needs 8.3 ms. - **Handshaking:** ADC SOC/EOC, LCD E strobe, RS-485 DE/RE and master-slave polling. **Timing diagrams** (H = high, L = low): ```text ADC conversion (SOC/EOC handshake) SOC ____/-----\_________________________________ EOC -----------\____ converting ____/------------ (polarity depends on the ADC) DATA ---------------------------------[ valid ]-- ^ MCU reads the 12-bit result after EOC LCD write cycle (4-bit mode) RS,RW ====< stable >========================== (set up before E rises) E ______/---------\__________________________ (E high >= 450 ns) D4-D7 ======< valid data >================== (latched on the falling edge of E) RS-485 transmit (one byte, 8N1) DE/RE ____/-------------------------\______ (high only while sending) TXD ----\ start |D0 D1 ... D7| stop /------- (1 bit = 104 us at 9600 baud) RXD <- master listens for the slave reply ``` ```c /* Assumed: 8-bit MCU, ADC on channel 0 (PT100 amplifier), relay on PORTB.0 */ unsigned int read_adc(void){ /* 12-bit conversion */ ADC_START(); while(!ADC_DONE()); /* start, wait for EOC */ return ADC_DATA; /* 0..4095 */ } void control(void){ float t = read_adc() * 150.0 / 4096; /* temperature in C */ if (t < 72.5) STEAM_ON(); /* open steam valve */ else if (t > 74.0) STEAM_OFF(); /* hysteresis control */ wdt_reset(); /* watchdog */ } ``` The control band (72.5 to 74 C) stays above the 72 C minimum holding temperature. #### (d) Technical drawbacks of the proposed design A single controller is a single point of failure; it is sensitive to noise and power cuts; memory and processing power are limited; calibration drifts; RS-485 is half-duplex and slower than industrial Ethernet; custom firmware needs maintenance; and it has no certified safety rating, which a PLC has. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the main architectural change from the existing to the proposed system? A. Manual or thermostat control becomes closed-loop digital microcontroller control B. Digital control becomes manual control C. Sensors are removed and operators estimate temperature D. The valve is replaced by a push-button Answer: A. The existing system relies on gauges and manual valves; the proposed one senses, computes and actuates automatically. (Key point: Existing vs proposed control) 2. Why is a microcontroller chosen over a microprocessor? A. It has a much faster clock and large external memory B. It needs external ADC, ports and memory C. It has on-chip ADC, timers, UART, GPIO and memory, giving lower cost and power for a task with few I/O D. It does not need firmware Answer: C. On-chip peripherals mean few external parts, whereas a microprocessor would need external memory, ports and ADC. (Key point: Why a microcontroller) 3. How is the RS-485 PC link handled by the MCU? A. Full-duplex with no direction control B. Parallel 8-bit bus with strobe C. Using the LCD E strobe D. Half-duplex via MAX485 with the DE/RE pin controlled by the MCU, Modbus RTU with CRC Answer: D. RS-485 is half-duplex, so the MCU must switch the transceiver between transmit and receive using DE/RE. (Key point: RS-485 interfacing) 4. In the sample code, steam turns ON below 71.5 C and OFF above 73.0 C. What is this called and why is it used? A. Hysteresis (on-off) control, to avoid rapid switching around the set-point B. PID control, to remove steady-state error C. Open-loop control, to ignore sensor readings D. Interrupt handling, to read the ADC Answer: A. The gap between the two thresholds prevents the valve from chattering around a single set-point. 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