A transducer is a device that
Answer: A
In measurement the conversion wanted is usually from some physical quantity into an electrical one, because electrical signals can be amplified, transmitted and recorded conveniently. The device producing that conversion is the transducer.
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An active transducer is one that
Answer: B
A thermocouple, a piezoelectric crystal and a photovoltaic cell all deliver an electrical output from the energy of the quantity being measured. A passive transducer such as a strain gauge merely changes an electrical property and needs a supply to reveal the change.
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Which of the following is a passive transducer?
Answer: C
A strain gauge changes its resistance and nothing more; a bridge and a supply are needed before any signal appears. The other three are self-generating and therefore active, which is the standard division of transducers.
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A primary transducer is distinguished from a secondary one in that it
Answer: D
In a Bourdon tube fitted with an LVDT, the tube is the primary element converting pressure into displacement and the LVDT is the secondary converting displacement into voltage. Many measurements involve such a chain of two conversions.
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A digital transducer produces an output that is
Answer: A
A shaft encoder and a turbine flow meter with a pulse pickup are examples: the output is a count or a code rather than a magnitude. No analogue to digital conversion is needed afterwards, which is a considerable advantage in a digital system.
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A potentiometric displacement transducer converts displacement into
Answer: B
The wiper of a resistance element is moved by the quantity to be measured, so the resistance in circuit and the fraction of the supply appearing at the wiper both follow the displacement. It is simple, gives a large output and needs no amplification.
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The chief disadvantage of a potentiometric transducer is
Answer: C
A sliding contact wears, generates noise and eventually becomes unreliable, and the friction it adds loads the mechanism being measured. Its large output and simplicity are real merits, which is why it survives where a limited life is acceptable.
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The gauge factor of a strain gauge is defined as the ratio of
Answer: D
Written out it is \( (\Delta R/R)/(\Delta L/L) \), a dimensionless number of about two for a metallic gauge. It combines the change of dimensions with the change of resistivity under strain, and it is the figure that turns a resistance reading into a strain.
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The gauge factor of a metallic strain gauge is typically about
Answer: A
Around two is the value for the copper nickel and nickel chromium alloys used for bonded foil gauges. A semiconductor gauge reaches a hundred or more through the piezoresistive effect, at the cost of poor linearity and a large temperature coefficient.
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A semiconductor strain gauge has a much larger gauge factor than a metallic one because of
Answer: B
Straining a semiconductor alters the mobility of its carriers, so the resistivity changes by far more than the geometry does. That gives gauge factors above a hundred, but the effect is nonlinear and strongly temperature dependent, so a metallic gauge is preferred for accurate work.
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