Time-of-Flight, Triangulation, or Field Based approaches

There are many different classes and instances of noncontact ranging devices, but with very few exceptions they are based on one of the following three basic principles:

1. Energy propagates at a known, finite, speed (e.g., the speed of light, the speed of sound in air)

2. Energy propagates in straight lines through a homogeneous medium

3. Energy fields change in a continuous, monotonically decreasing, and predictable manner with distance from their source

The techniques associated with these basic phenomena are referred to as time-of-flight, triangulation, and field based, respectively.

Time-of-Flight

Time-of-flight (TOF) systems may be of the “round-trip” (i.e., echo, reflection) type or the “one-way” (i.e., cooperative target, active target) type. Round-trip systems effectively measure the time taken for an emitted energy pattern to travel from a reference source to a partially reflective target and back again. Depending on whether radio frequencies, light frequencies, or sound energy is used, these devices go by names such as radar, lidar, and sonar. One-way systems transmit a signal at the reference end and receive it at the target end or vice versa. Some form of synchronizing reference must be available to both ends in order to establish the time of flight.

A characteristic of many TOF systems is that their range resolution capability is based solely on the shortest time interval they can resolve, and not the absolute range being measured. That is, whether an object is near or far, the error on the measurement is basically constant.

Triangulation

Triangulation techniques were known and practiced by the Ancients. Triangulation is based on the idea that if one knows the length of one side of a triangle and two of its angles, the length of the other sides can be calculated. The known side is the “baseline.” Lines of detection extend from either end of the baseline to the target point. A surveyor uses a precision pointing instrument to sight a target from two positions separated by a known baseline. Reference notes that the distance to a nearby star may be calculated by observing it through a pointing instrument at 6-month intervals and using the diameter of Earth’s solar orbit as the baseline. Stereo ranging, which compares the disparity (parallax) between common features within images from two cameras, is another form of passive triangulation. It is of interest to note that human vision estimates distance using a variety of cues, but two of the most important — stereopsis and motion parallax — are fundamentally triangulation based.

Active triangulation techniques use a projected light source, often laser, to create one side of the triangle, and the viewing axis of an optical detection means to create the second side. The separation between the projector and detector is the baseline.

A fundamental issue for all triangulation-based approaches is that their ability to estimate range diminishes with the square of the range being measured. This may be contrasted with TOF approaches, which have essentially constant error over their operating range.

Date: 2016-04-22; view: 845