--- title: "Data acquisition, conversion and distribution" subject: "Digital Control System" chapter: 1 key: "semester_6/digital_control_system:data-acquisition-conversion-distribution" source: https://www.bctengineer.com/q/semester_6/digital_control_system/data-acquisition-conversion-distribution site: BCT Engineering --- # Data acquisition, conversion and distribution Digital Control System · Chapter 1: Introduction to Discrete Time Control System - Asked in: 2082 Kartik, 6 marks; 2080 Chaitra, 6 marks; 2079 Chaitra, 8 marks; 2078 Chaitra, 6 marks; 2077 Chaitra, 8 marks; 2076 Bhadra, 8 marks; 2073 Magh, 8 marks; 2072 Asoj, 8 marks; 2071 Bhadra, 6 marks (asked 9 times) - Video: [Data Acquisition Systems | Block Diagram | Part-1/2 | Electrical Instruments ( EIM ) | Lec - 85](https://www.youtube.com/watch?v=jRbw2n2PA0E) — Education 4u - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/digital_control_system/data-acquisition-conversion-distribution - Whole chapter: https://www.bctengineer.com/md/semester_6/digital_control_system/chapter-1 ## Question With the help of discrete time control system block diagram, explain the process of data acquisition, conversion and distribution system applied in discrete time system. ## Short answer A discrete-time control system acquires analog plant signals through sensors, amplifiers, anti-aliasing filters, and an analog multiplexer; converts them with S/H and A/D; processes them in a digital computer; then distributes outputs through a register, D/A converter, demultiplexer, and hold circuit to actuators. A clock synchronizes sampling, conversion, computation, and output update within each sampling period. ## Answer A discrete time (digital) control system takes analog signals from the plant, converts them to numbers (**data acquisition and conversion**), processes them in a computer, and sends the result back to the plant as analog signals (**data distribution**). #### Block diagram ```text DATA ACQUISITION Sensor -> Amp -> LPF --+ Sensor -> Amp -> LPF --+--> Analog --> S/H --> A/D Sensor -> Amp -> LPF --+ MUX | v Digital computer DATA DISTRIBUTION | Actuator <- Hold <--+ v Actuator <- Hold <--+-- Demux <-- D/A <-- Register ``` #### Data acquisition The data acquisition system collects analog signals from many points of the plant and prepares them for conversion. 1. **Transducer/sensor:** converts the physical variable (temperature, speed, pressure, position) into a voltage or current. 2. **Amplifier:** raises the low-level sensor signal to the range of the A/D converter (e.g. 0 to 10 V) and gives isolation. 3. **Low-pass (anti-aliasing) filter:** removes high-frequency noise so that the signal is band-limited below half the sampling frequency, avoiding aliasing. 4. **Analog multiplexer:** a set of electronic switches that connects one of many analog channels at a time to the single S/H and A/D, so one converter serves many inputs (time sharing). #### Data conversion 5. **Sample-and-hold (S/H):** takes the sample of the selected signal at the sampling instant and holds it constant while conversion takes place. 6. **A/D converter:** quantizes the held voltage and codes it into an n-bit binary number. The computer reads this number. 7. **Digital computer/controller:** compares the measured values with the set points and computes the control action $u(kT)$ using the control algorithm (PID, lead/lag, state feedback). #### Data distribution 8. **Register/D/A converter:** the output word is latched in a register and converted into an analog voltage by the D/A converter. 9. **Demultiplexer:** routes the D/A output to the correct output channel; a single D/A can serve many actuators. 10. **Hold circuit (ZOH):** keeps each channel's output constant between updates, producing a staircase signal. 11. **Actuator:** (motor, valve, heater driver) applies the control action to the plant. #### Timing A clock (timer) gives the sampling period $T$. In each period the computer selects a channel, samples, converts, computes, and updates the outputs. The whole cycle must finish within $T$. **Example:** in a boiler, temperature, pressure and level sensors feed one multiplexer and one 12-bit ADC; the computer computes valve openings and sends them through a D/A converter and demultiplexer to the fuel and feed-water valves. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is an analog multiplexer used in the data acquisition part of a digital control system? A. To amplify weak sensor signals to the ADC range B. To connect many analog channels, one at a time, to a single S/H and A/D so one converter is shared C. To remove high-frequency noise before sampling D. To convert the digital output back to analog Answer: B. The multiplexer is a set of switches that time-shares one S/H and ADC among many channels, reducing cost. (Key point: Role of analog multiplexer) 2. What is the purpose of the low-pass filter placed before the multiplexer and S/H? A. To hold the signal constant during conversion B. To isolate the plant from the computer C. To band-limit the signal below half the sampling frequency and avoid aliasing D. To increase the number of quantization levels Answer: C. The LPF removes high-frequency noise so the signal is band-limited below fs/2, which prevents aliasing. (Key point: Anti-aliasing filter) 3. Why is a sample-and-hold placed ahead of the A/D converter? A. It holds the sample constant while the conversion takes place B. It routes the signal to the correct channel C. It converts the binary word to a voltage D. It computes the control action u(kT) Answer: A. The S/H freezes the value at the sampling instant so the input cannot change during conversion. (Key point: Why S/H before A/D) 4. In which order does the control output travel in the data distribution system? A. Computer, hold, D/A, register, actuator B. Computer, register, D/A, demultiplexer, hold, actuator C. Computer, demultiplexer, A/D, hold, actuator D. Computer, S/H, D/A, multiplexer, actuator Answer: B. The word is latched in a register, converted by the D/A, routed by the demultiplexer, held by the ZOH and applied by the actuator. 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