Laser sensors

Back to Basics tutorial: Lasers can be used in non-contact distance sensors. In industrial machinery or for plant-floor applications, triangulation-based laser displacement sensors are used most often. Laser-based triangulation displacement sensors combine high resolution and comparatively long ranges. See diagram and sensing primer.

01/01/2007


Machine Control, Instrumentation

Principle of triangulation: As the target moves across the laser sensor field of view, left to right in the diagram, the change in the distance between sensor and target flattens the angle of the returning light from A to B. As the position of the beam on the receiving array changes, so does the measurement.

Lasers can be used in non-contact distance sensors. In industrial machinery or for plant-floor applications, triangulation-based laser displacement sensors are used most often. Laser-based triangulation displacement sensors combine high resolution and comparatively long ranges.

 

Sensor manufacturers generally offer versions with 10 µm resolution and ranges to 1 m.

 

Triangulation technology (see diagram) uses a laser light source to project a well-collimated beam onto the target. Light reflected by the object passes through a lens that focuses the reflected beam onto a receiver. Changing the distance between sensor and target changes the angle of the returning light and the position of the beam on the receiving array, typically a charge-coupled device (CCD) line. The CCD signal feeds a microcontroller, which provides measured values as output using analog signals.

 

To limit signal noise, laser measurement sensors perform internal sampling, sometimes called integration or averaging. During sampling, the device averages multiple readings for smoother, more accurate output. Integrating more samples creates higher resolution, but increases the measurement time.

 

Faster decisions, response

 

Advanced sensing arrays and fast microprocessors speed response times. Some sensors project a laser line, rather than a laser point, bringing in more data to smooth over and tune out irregularities.

 

Author Information

Stephen Petronio is photoelectric product manager with Baumer Electric.

 

Primer on sensing

To understand performance of various sensor technologies for different applications, it can be helpful to review terms used to describe sensor measurements, suggests sensor manufacturer Baumer Electric.

 

Resolution corresponds to the smallest possible distance change that causes a detectable change in the output signal.

 

Repeat accuracy is defined as the difference of measured values in successive measurements.

 

Linearity is the deviation from a proportional linear function (straight line). It is given as a percentage of the upper limit of the measuring range (full scale).

 

Reaction time is defined as the time required by the sensor’s signal output to rise from 10% to 90% of the maximum signal level. For sensors with digital signal processing it is the time required to calculate a stable measured value.

 

Temperature drift : Ambient temperature changes cause measured values to drift. Temperature drift is nearly proportional to temperature change.



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