A slightly more 'modern' tilt sensor is the spirit bubble which relates to a vertically-curved calibrated
glass tube and forms the basis of the ubiquitous metre-long builders' level and the plate bubble found on a variety of surveying equipment and other equipment as a rough levelling up tool prior to further more exact alignment using other methods such as accurate spirit bubbles or modern electronic methods; more about which later.
These days, the role of tilt sensors has blossomed first with the mercury switch that when tilted , completes a circuit when the mercury covers the two electrodes in a tiny glass tube. This switch is used mainly for security movement alarms. More recently the application that has the most impact on our lives is the digital tilt sensor or the liquid capacitive inclinometer (LCI). This sensor technology is, as the name implies, based on electrostatic capacitance that varies with the attitude of the sensor with respect to gravity. A tiny amount of liquid dialectic sits in a conductive etched bowl and it has a conductive 'lid which makes it into an electronic capacitor.
The amount of conductive surface varies with the area of dialectic liquid in contact and this varies the capacitance and simple sensitive electronic circuitry measures this change. The capacitance is calibrated into angular change with respect to gravity. These tiny orientation sensors are arranged in different ways, in various numbers as arrays and linked to measurement, monitoring or feedback electronics to serve an enormous number of applications.
These electronic tilt sensors are the same as any other type, both ancient and modern, in that they need to be accurately and firmly fixed within the equipment body they serve to realise their true potential. For example in surveying, the laser level, theodolite, and optical level needs to be roughly levelled on its tripod with a plate bubble before the self-levelling electronics, with their feedback mechanisms come into play. On the other hand, the humble builders level now has an LCI that is firmly fixed and the level calibrated at the factory to give more than adequate accuracy of its digital readout over its one-metre length.
One particular clever application is Honda's prototype Magic Wand Omnicycle that incredibly, stands upright when you sit on it. This uses the LCI as part of its H.O.T. system to provide a low footprint personal transport for the future. The rather similar Segway personal transport but using accelerometers for attitude control, has been on sale for a couple of years and is gaining market penetration both as a fun personal vehicle as well as a disabled person transport.
Tilt sensor technology has now produced incredibly accurate monitoring systems, tilt meters and associated products and devices to detect nano-radians as in this guise, it is very much used to monitor ground movement and seismic activity. For example in 1996, the Japanese Geological Institute set up a whole network of 600 geodetic reference stations to research the possibility of finding a way of predicting earthquakes in this high risk area. These used AGI's tilt sensors, GPS software and met stations to give a real time picture as well as providing the earthquake alarms for the Country.