Strain Gauge & Piezoelectric Sensors Flashcards
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Read the first 7 Strain Gauge & Piezoelectric Sensors flashcards as text
When multiple load cells are wired in parallel to a single indicator (summing junction), what happens to the combined output impedance?
Answer: It decreases, which may reduce the total signal output
Parallel load cells reduce the combined output impedance; the indicator sees a lower source resistance, and total mV/V signal remains the same but current demand increases.
What is the function of trim resistors installed in a multi-cell summing junction?
Answer: To equalize the sensitivity of individual load cells so all corners contribute equally
Trim (or span) resistors in a summing junction are adjusted during corner loading tests to balance sensitivity differences between individual load cells.
A technician notices the indicator reading drifts upward slowly after placing a load on the scale. Which load cell fault is MOST likely?
Answer: Moisture ingress degrading insulation resistance
Moisture in the load cell lowers insulation resistance, creating a leakage path that changes slowly and causes the reading to drift rather than give an immediate wrong value.
Which specification describes the maximum load a load cell can withstand without permanent change to its metrological characteristics?
Answer: Safe overload limit
The safe overload limit (typically 150% of rated capacity) is the maximum load after which the cell returns to its specified performance; the ultimate limit causes permanent damage.
In terms of strain gauge load cell construction, what does 'self-temperature compensation' (STC) mean?
Answer: The gauge alloy is selected to match the thermal expansion coefficient of the flexure material
STC gauges use a special alloy whose resistance-temperature coefficient is tailored to cancel apparent strain caused by the thermal expansion mismatch of the specific substrate material.
Which NTEP (National Type Evaluation Program) performance class requires the tightest accuracy specifications for load cells used in commercial weighing?
Answer: Class II
NTEP Class II load cells have the tightest tolerances (fewest divisions of error allowed) and are used in laboratory and precision commercial applications.
A piezoelectric force sensor with a charge sensitivity of 4 pC/N is connected to a charge amplifier with a feedback capacitor of 100 pF. What is the output voltage per newton of applied force?
Answer: 0.04 V/N
Output voltage = charge sensitivity / feedback capacitance = 4 pC/N ÷ 100 pF = 0.04 V/N.