handbook of modern sensors

7
Position, Displacement, and Level
“…If you keep moving in that direction,
we always can displace you to such a position
that is not embarrassing to the level of your wisdom”.
—Julius Caesar to his senator
The measurement of position and displacement of physical objects is essential for
many applications: process feedback control, performance evaluation, transportation
traffic control, robotics, and security systems—just to name the few. By position,we
mean the determination of the object’s coordinates (linear or angular) with respect to
a selected reference. Displacement means moving from one position to another for a
specific distance or angle. In other words, a displacement is measured when an object
is referenced to its own prior position rather than to another reference.
A critical distance is measured by proximity sensors. In effect, a proximity sensor
is a threshold version of a position detector. A position sensor is often a linear device
whose output signal represents a distance to the object from a certain reference point.A
proximity sensor, however, is a somewhat simpler device which generates the output
signal when a certain distance to the object becomes essential for an indication.
For instance, many moving mechanisms in process control and robotics use a very
simple but highly reliable proximity sensor—the end switch. It is an electrical switch
having normally open or normally closed contacts. When a moving object activates
the switch by physical contact, the latter sends a signal to a control circuit. The signal
is an indication that the object has reached the end position (where the switch is
positioned). Obviously, such contact switches have many drawbacks, (e.g., a high
mechanical load on a moving object and a hysteresis).
A displacement sensor often is part of a more complex sensor where the detection
of movement is one of several steps in a signal conversion (see Fig. 1.1 of Chapter
1). An example is a pressure sensor where pressure is translated into a displacement
of a diaphragm, and the diaphragm displacement is subsequently converted into an
electrical signal representing pressure. Therefore, the positions sensors, some of which