Shed some light – race timing part 2

By | 08/06/2021

The main idea of a beam breaker system is that a light source sends electromagnetic energy (photons) to a detector. If something is between source and detector, that something will block the electromagnetic energy. The detector then does not detect the light. A subsequent system then uses the output of the detector ("I see light" or "I don't see light") to do something (e.g. sound the bell for a shop or lap a timer for a racing system). The light does not need to be visible to the human eye, as long as it is electromagnetic energy that can be blocked by, e.g., a tyre or a human body, I’ll call it light.

A light sensor device outputs a signal how much light it detects. For beam breaker systems, this signal should be binary ("Yes" or "No"). There are numerous light sources and light detectors, all with their characteristics. A pair of light source and light sensor must be selected that work well together.
For light sources, the main characteristics are:

  • The amount of energy on each frequency (the light spectrum) it emits,
  • Diverging or non-diverging beam
  • Switching characteristics,
  • The total amount of power emitted.

For light detectors, the main characteristics are:

  • Spectral sensitivity (i.e., how strong they respond to certain electromagnetic frequencies (= colours)),
  • Specific filtering characteristics and signal preprocessing,
  • Type of output (binary or analogue).

For the physical setup of a beam breaker: light source and light detector can be opposing each other so that the light source shines its light towards the detector, and can be on the same side, where the light is reflected through a mirror or another object. Furthermore multiple beams can be used to increase accuracy, and the beams can be horizontal, vertical, or any other direction. Systems can also use projection lenses or moving mirrors to create full coverage of a whole (virtual) beam breaker plane; even small objects could be detected with this setup. (However, for correct operation, this would require many detectors, or a camera system).

I can see clearly now

The unicycling event is scheduled to be held outdoors, hopefully on a nice, bright, sunny summer day. This means that there will be a lot of light,. All that light that may let the detector “think” it sees the light from its source, even when the source is blocked by a competitor (i.e., the line is crossed). This is a common problem in any system that does communication over some distance: how to distinguish the real signal from other signals (also known as noise). Various methods to reduce or eliminate noise problems are in use in communication lines, such as:

  • Shielding: make sure as little as possible noise can enter the communication line (e.g., shielded wires, fibre optic cables)
  • Balancing: make sure noise entering on the line carrying the signal, is also entering another line carrying the inverted signal, so the receiver can subtract the two received signals end thus eliminate the noise (e.g., balanced audio, RS422/RS485 serial communication etc.). Having the pairs of wire twisted gives an additional cancelling of noise per twist.
  • Filtering: make the receiving side very specific for certain signals (e.g., radio sets, channel selectors)
  • Modulation: transform the signal using some (higher frequency) carrier signal (most long-distance communication systems use this).

Because there is no cable involved for light beams, the balancing option is not in scope for beam breaker systems.

Shielding

Shielding can be done by e.g., placing the receiver under a cap (shielding sunlight) or at the end of a tube (shielding sunlight and other light sources). The cap or tube can be painted matte black on the inside to further absorb noise, i.e., prevent the noise entering from an angle reflecting on the inside of the shield.

Filtering and modulation

For a “on/off” light signal, the simplest type of modulation is a pulsating signal with a fixed frequency. Because of the pulsating signal, this can be filtered and detected as a signal “over” noise in the same light spectrum.

An example: many infrared remote controls for hi-fi and television sets etc. send binary (series of 1 or 0) commands using this type of modulation. A single bit is a series of pulses (a burst) followed by a “silent” time (a gap). A complete command sent has many bursts and gaps, in which the length of the burst determines if it is a 1 or a 0 bit in the command. Because of the bursts of pulses and the gaps, and the pulse frequency, a very effective filter can be built.

Other posts on Timing:

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