Time of Flight Measurement

This article aims to explain how the time of flight laser sensors operate, which are used for longer distance measurements.

The basic principle of time of flight is easy to explain simply, but in practice, for accurate measurements, it is not as straightforward as it seems. The laser beam is projected from the instrument and reflected from a target surface to a collection lens. This lens is typically located adjacent to the laser emitter and focuses an image of the spot on a linear array camera (CMOS array).

Put simply therefore, the time for the light to be sent and to return can then be used to determine the distance it has travelled based on the speed of light. Modulated beam systems also use the time light takes to travel to a target and back, but the time for a single round trip is not measured directly. Instead, the strength of the laser is rapidly varied to produce a signal that changes over time. The time delay is indirectly measured by comparing the signal from the laser with the delayed signal returning from the target.One common example of this approach is “phase measurement” in which the laser’s output is typically sinusoidal and the phase of the outgoing signal is compared with that of the reflected light.

Phase measurement is limited in accuracy by the frequency of modulation and the ability to resolve the phase difference between the signals, so some modulated beam rangefinders work on a range-to-frequency conversion principle, which offers several advantages over phase measurement. In these cases, laser light reflected from a target is collected by a lens and focused onto a photodiode inside the instrument.

The resulting signal is amplified up to a limited level and inverted, and used directly to modulate a laser diode. The light from the laser is collimated and emitted from the centre of the front face of the sensor. This configuration forms an oscillator, with the laser switching itself on and off using its own signal. The time that the light takes to travel to the target and return, plus the time needed to amplify the signal, determines the period of oscillation, or the rate at which the laser is switched on and off. This signal is then divided and timed by an internal clock to obtain a range measurement.

The measurement is somewhat nonlinear and dependent on signal strength and temperature, so a calibration process is performed in the sensor to remove these effects. Modulated beam sensors are typically used in intermediate range applications, for distances from a few centimetres to tens of meters on uncooperative targets. With cooperative targets, like reflectors, the range can be extended to several hundreds of meters.

Techni Measure Limited would be pleased to offer advice on which sensor would be the best choice for any given application. Shiny targets can present a problem, but there are solutions, and we would be pleased to offer a trial demonstration when possible.

April 2015


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