10 exam-style questions with answers and explanations, straight from our 1,030-question bank. Tap an answer to check yourself. When you're ready, take the scored version in the free practice test.
These 10 free FST questions are organized by exam domain, so you can see how each part of the Fundamentals of Sensor Technology blueprint is tested. Reveal the answer and explanation under each question.
After washdown, an external capacitive level switch reports liquid present in an empty plastic hopper. It passed empty and filled checks before cleaning. Supply and wiring are correct. A film of water now covers the wall at the sensing face; the intact sensor and connector are rated for this cleaning process. The next maintenance step should be to:
Correct answer: A - Remove the water film, then verify both empty and filled switching states.
A failed three-wire, normally open PNP proximity sensor is replaced with a normally open NPN model. Both are rated for 24 V DC. The replacement's detection indicator responds to a stationary target, but the PLC input never turns ON. The input common is connected to 0 V, and the wiring matches the replacement's pinout. What change restores operation without changing the input common?
Correct answer: B - Fit a normally open PNP sensor with the required ratings.
An ordinary retroreflective photoeye counts cardboard cartons but misses mirror-finished packs of the same size. Each opaque pack completely blocks the path to the reflector. In a stationary test, covering the reflector with matte black card produces a blocked indication; placing a mirror-finished pack in the sensing position restores the clear indication. Why does the glossy pack escape detection?
Correct answer: D - The pack reflects emitted light back to the receiver, imitating the reflector.
On an approach test, a proximity sensor switches ON at a 6.0 mm target gap. On withdrawal, it switches OFF at 6.8 mm. The target is brought in to 5.0 mm, then moved back and held at 6.4 mm. The output remains ON. How should this result be judged?
Correct answer: C - Accept the observed state; the target has not crossed the release point.
Five consecutive distance readings are 50.38, 50.39, 50.38, 50.40, and 50.39 mm for a target fixed at 50.00 mm. The reference uncertainty is 0.01 mm, and test conditions remain unchanged. Which performance description matches these results?
Correct answer: A - Good short-term repeatability with a positive bias at this distance
At a parts station, raw input E is 1 when the nest is empty and 0 when a part is present. Input C is 1 when clamping is confirmed, and F is 1 when a process fault exists. The inspection command must be true only when a part is present, clamping is confirmed, and there is no fault. Choose the required Boolean expression.
Correct answer: B - NOT E AND C AND NOT F
Matte plastic trays pass 90-110 mm in front of a sensor. A bright machine panel lies behind them, 300 mm from the sensor. No device or reflector can be mounted opposite the sensor. The trays provide an adequate optical return, and all candidates have compatible electrical ratings and speed. Which choice best distinguishes the trays from the panel?
Correct answer: A - A background-suppression photoelectric sensor with its distance cutoff set to 150 mm
A conveyor presents one metal flag every 100 ms. At the installed gap, each flag is detectable over 6 mm of travel; conveyor speed is 1.5 m/s. The inductive sensor is rated for 100 Hz but requires at least 6 ms of continuous target presence to guarantee switching ON. The controller captures every output pulse. Does the sensor specification guarantee detection of these flags?
Correct answer: B - No; the flag is present for 4 ms, shorter than the specified 6 ms ON requirement.
A linear pressure transmitter is configured for 0-12 bar and 4-20 mA. With a reference pressure of 9.0 bar applied, both its local display and a calibrated gauge read 9.0 bar. The loop current measured at the controller is 16.0 mA, but the operator display reads 12.0 bar. What should be checked before adjusting the transmitter?
Correct answer: C - Controller scaling, because 16.0 mA correctly represents 9.0 bar on the transmitter's range
At a U.S. packaging plant, an emergency stop has halted a horizontal pneumatic pusher. Its air supply remains connected, and compressed air is trapped in the actuator. An authorized technician must loosen a sensor bracket inside the pusher's travel. Before reaching into that area, the technician must:
Correct answer: D - Follow the machine's lockout procedure, control stored air, and verify isolation.
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