Case study: existing vs proposed system

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

(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.

 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):

 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

/* 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.