Robotics/Sensors/Pressure Sensors

The ability to measure pressure is very beneficial to the subject of robotics. The density of the atmosphere changes with altitude. If the air pressure is known then the altitude can then be established. If the robot contains fluids under pressure (for example, hydraulic systems), pressure sensors can be used to detect faults in the system. Pressure sensors can also be installed in a compressed tank of gas to measure the amount of gas left.

Measuring Pressure

Analogue Methods

A Bourdon gauge

A Bourdon gauge uses a coiled tube which causes a rotation of an arm connected to the tube as it expands due to pressure increase. However, this is not particularly useful in Robotics unless you design a way to get voltage to vary with respect to needle displacement.

Typical foil strain gauge. The gauge is far more sensitive to strain in the vertical direction than in the horizontal direction. The markings outside the active area help to align the gauge during installation.

Electric Methods

• Resistive (Strain Gauge) - As the object is deformed, the foil is deformed, causing its electrical resistance to change. This resistance change, usually measured using a Wheatstone bridge, is related to the strain by the quantity known as the gauge factor. Foil strain gauges are only useful to detect large pressure changes and are not very precise over small changes.

• Capacitive - The deflection of the piston is often one half of a capacitor, so that when the piston moves, the capacitance of the device changes. This is a common way (with proper calibrations) to get a very precise, electronic reading from a manometer, and this configuration is called a capacitive manometer vacuum gauge.

• Piezoelectric/Piezoresistive - For measurements of small strain, semiconductor strain gauges, called piezoresistors, are often preferred over foil gauges. A semiconductor gauge usually has a larger gauge factor than a foil gauge. Semiconductor gauges tend to be more expensive, more sensitive to temperature changes, and are more fragile than foil gauges.