In an open-loop control system, the control action

depends on the error between reference and output

is based on a sensor measuring the output

Which pair is correctly classified?

Toaster with timer is open loop; air conditioner with thermostat is closed loop

Toaster with timer is closed loop; air conditioner with thermostat is open loop

The role of the feedback element/sensor is to

measure the output c and convert it to b, comparable with r

Compared with open loop, closed loop has

higher accuracy and disturbance reduced by 1+GH, but may become unstable

lower cost and no chance of instability

accuracy that depends on calibration only

full disturbance effect at the output

Asked 2/27
2066 Bhadra (old course) · 8 marks

Open and closed loop systems with block diagram

Short answer

Open-loop systems have no feedback; control action is independent of output. Closed-loop systems measure output, compare with reference, and use the error to drive the plant. Components: reference input, error detector, controller, actuator, plant, feedback sensor, and disturbance input. Feedback gives accuracy and disturbance rejection.

What are open loop and closed loop control systems? Draw the block diagram of closed loop control system and explain the role of each block.

Open-loop control system

A system in which the control action does not depend on the output. The output is neither measured nor fed back.

 r   +------------+  u   +-------+   c
---->| Controller |----->| Plant |------>
     +------------+      +-------+

Examples: electric toaster with timer, washing machine on timer, fixed-time traffic lights. They are simple and cheap but cannot correct errors caused by disturbances or parameter changes.

Closed-loop control system

A system in which the output is measured and fed back to the input, and the control action depends on the error between the desired and actual output. Examples: room air conditioner with thermostat, motor speed control with tachometer, voltage regulator.

                       disturbance d
                            |
 r   +----+  e   +------+ u +---+---+   c
---->| Σ  |----->|Contr-|-->| Plant |----+---->
     +----+      | oller|   +-------+    |
      - ^        +------+                |
        |  b    +-----------------+      |
        +-------| Feedback/sensor |<-----+
                +-----------------+

Role of each block

Reference input $r$ (set point): desired value of the output.
Error detector / summing point: compares $r$ with feedback $b$ and gives error $e = r - b$ .
Controller: acts on the error (P, PI, PID) and produces control signal $u$ .
Actuator: power element (motor, valve, amplifier) that applies $u$ to the plant.
Plant / process: the system being controlled; gives the controlled output $c$ .
Disturbance $d$ : unwanted input acting on the plant (load change, noise).
Feedback element / sensor: measures $c$ and converts it to $b$ , comparable with $r$ .

The overall transfer function is $\dfrac{C(s)}{R(s)} = \dfrac{G(s)}{1+G(s)H(s)}$ , where $G$ is the forward path (controller, actuator, plant) and $H$ the feedback element.

Comparison

Point	Open loop	Closed loop
Feedback	absent	present
Accuracy	depends on calibration	high
Disturbance effect	fully at output	reduced by $1+GH$
Stability	generally stable	may become unstable
Cost, complexity	low	higher
Example	toaster	air conditioner

✓ Answer checked by Claude Opus 5.5 (high effort)

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