Sensor, actuator and a block diagram with two feedback loops

Problem statement

a) (0.8 marks) State the difference between a sensor and an actuator.

b) (1.2 marks) Simplify the block diagram and obtain the transfer function $Y/R$.

[Figure: the input $R$ enters a first summing point, from which the output $Y$ is subtracted (direct feedback, no block). The output of that summing point enters a second summing point, from which the output of block $H$ is subtracted. Next come blocks $P$ and $G$ in a chain; the output of $G$ is $Y$. Block $H$ takes the signal between $P$ and $G$.]

Source: Madrid university entrance exam (PAU, Spain) · 2025-2026 · Ordinary session · Technology and Engineering · Question 5.2. Official exam set by the public universities of the Community of Madrid, to whom it belongs; translated from Spanish and reproduced with its source in order to explain it for teaching purposes.