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Using encoders with line scan cameras

Use of a rotary encoder

Industrial line scan cameras capture the image line by line.
Using an encoder, it is possible to record the image not only at a fixed frame rate, but also synchronously with the distance travelled. This means that the image is acquired in relation to the distance rather than the time.

In this way it is possible to produce distortion-free images that have the same image scale in the direction of travel as in the transverse direction of the belt.
The use of a rotary encoder is almost always unavoidable when using line scan cameras.

Use of a rotary encoder

  • Rotation generates encoder pulses
  • Depending on the type, up to some thousand up to 32,000 pulses can be generated per revolution.
  • In the configuration software of the line scan camera or frame grabber, factors can be used to set how many encoder ticks should correspond to one pixel.
  • Programmable encoders offer more flexibility. They allow the number of ticks per revolution to be specified, eliminating the need for later conversion in the vision system environment.
  • The signals are transmitted as differential or TTL signals. When selecting an encoder, take into account the interfaces of your frame grabber card or line scan camera.

Use of a linear encoder

  • A non-contact read head of a magnetic sensor generates pulses.
  • An adhesive magnetic tape at a reading distance of a few millimetres represents the opposite side of the system.
  • Depending on the type, a resolution of up to 0.001 mm is possible.
  • Programmable types are also available which generate a definable number of ticks per distance.
  • A differential or TTL signal can also be selected as the signal.

Principle of an incremental A/B encoder

Normally a scan should only be taken when the conveyor is moving forward. However, if a conveyor belt is repeatedly stopped, it may move backwards for a short distance. Again, the use of an encoder helps: the direction of movement can be determined from the sequence of signals from the encoder.

Good line scan camera products are able to count these backward steps and only start capturing images again when the counter is reset by the next forward movement.

In addition, quadrature encoders with A/B signals provide four ticks rather than a single pulse. This gives them a much higher resolution.

Typical problems with encoders

If the image is too dark, the camera will often increase the exposure time per line. However, above a certain value, the image appears distorted in the direction of transport, although the encoder is designed to eliminate this effect.

The cause of this phenomenon is that the camera's exposure time is now longer than the encoder's time, the system is effectively 'over-triggered' and lines are lost during recording. These missing lines result in image compression. An example: At a typical line frequency of 18 KHz, i.e. 18,000 lines per second, the maximum exposure time per line is 55 microseconds!

Possible solutions

  • Reduction of the belt speed.
  • More light again enables short exposure times at the same tape speed.
  • The use of a better camera with larger sensor pixels or a dual-line or TDI sensor increases the light sensitivity enormously
  • Use of particularly fast lenses

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