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Methods 3

Method 3: Analysis of Marginal Technologies (Why Standard Sensors Fail)

It is crucial to explicitly rule out lower-cost industrial sensors to prevent "false economy" decisions.

1 - Standard Time-of-Flight (ToF) Sensors

Sensors like the Acuity AR1000 or standard measurement lasers (e.g., Micro-Epsilon ILR1171 ) offer long ranges (up to 100m+) but lack the requisite precision.

TIME-OF-FLIGHT PRINCIPLE

The Time-of-Flight principle (ToF) is a method for measuring the distance between a sensor and an object, based on the time difference between the emission of a signal and its return to the sensor, after being reflected by an object. Various types of signals (also called carriers) can be used with the Time-of-Flight principle, the most common being sound and light.

NOTE: Some sensors use light as their carrier because it is uniquely able to combine higher speed, longer range, lower weight, and eye-safety. By using infrared light we can ensure less signal disturbance and easier distinction from natural ambient light, resulting in the highest performing distance sensors for their given size and weight. Unfortunately we also should avoid infrared sensors for obvious resons !

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Direct ToF sensors send out short pulses of light that last just a few nanoseconds and then measure the time it takes for some of the emitted light to come back. Indirect ToF sensors send out continuous, modulated light and measure the phase of the reflected light to calculate the distance to an object.

  • Specs: Typical accuracy is \(\pm 1 \text{--} 3 \, \text{mm}\).
  • Analysis: This is an order of magnitude worse than the 0.2 mm requirement. They are designed for crane positioning or tank level sensing, not metrology.

2 - Standard Triangulation Sensors

Sensors like the Keyence LK-G5000 or Micro-Epsilon optoNCDT 2300 are the standard for high-precision displacement.

  • The Standoff Barrier: These sensors typically operate at standoff distances of 50 mm to 500 mm. Snippets mentioning "long range" models (e.g., Keyence LK-G long range) typically max out at ranges of 1 meter or have measurement ranges that are large (e.g., \(\pm 500 \, \text{mm}\)) but require the sensor to be close to the center of that range.
  • Geometric Failure: Triangulation requires the laser emitter and receiver to form a triangle with the target. At 3 meters, the triangle becomes extremely elongated. To maintain triangulation, the receiver would need to be physically separated from the emitter by a large distance (e.g., 500mm+), which is impossible when viewing through a "small window." The window frame would block the return path of the laser spot.

COMPARISON

WayCON Long Distance Laser LDI

https://www.waycon.biz/products/laser-sensors/

Minimum stated resolution is 1.8 mm ( NOT SUITABLE )

  • Ranges 0.05...500 m
  • Repeatability max. ±0.3 mm
  • Linearity max. ±1 mm
  • Frequency max. 250 Hz
  • Output 0...20 mA, 4...20 mA, RS232, RS422, RS485, SSI, ProfiNet, EtherNet/IP, EtherCAT

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