EXPERTISE

Selecting the best possible measuring speed

"What is the measuring speed of your system?"


This is one of the most frequent questions our customers ask us. No great surprise - after all, the time factor is of crucial importance in measurements of this kind. The effects of an improperly ajusted measuring speed are not necessarily evident at first glance. This is why we briefly summarise the most important aspects.


Measuring speed is no device parameter


Fundamentally, measuring runs with the SPACETEC scanners can be made at any speed. But speed has a substantial impact on the quality of the results.


These two recordings differ only in their measuring speed, all other parameters are the same. Data processing ensures that both images are shown true to scale. But the information gaps generated by the higher speed (on the right) cannot be bridged.

The outcome: small objects become blurred, the lettering on the marking bolt (clearly legible on the left) cannot be deciphered.


Optimum measuring speed

The optimum measuring speed is the speed at which the resolution of the recording in the direction of travel is as good as in transverse direction. If the speed is greater, detail will be lost; if it is lower, no additional information is gained. This aspect is particularly relevant when image recordings in high quality are desired (e.g. as the basis for crack charting).




Optimum measuring speed
The resolution is the same in both directions. The image recording has a balanced side-to-side ration.


Higher measuring speed
The resolution in the direction of travel is lower than along the circumferential line. This creates information gaps. The image recording appears "squeezed". Geometric adjustment generates an image with reduced resolution, as shown above



Examples of measuring speed selections

The best possible measuring speed depends on the purpose for which the survey is to be used. We discuss this point with our customers and recommend the most suitable speed after weighing up all relevant aspects.

Example 1: The objective is a clearance survey. Small protrusions a few centimetres in size do not matter. A measurement cross-section every 10 centimetres is deemed to be sufficient. Detailed image recordings are not required.

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The standard resolution of 2,500 pixels is adequate.
There is no need to observe the optimum measuring speed.

For the SPACETEC TS3 this means:
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Mirror rotational speed 300 Hz
Measuring speed approx. 100 km/h
In our experience, this situation is not the rule, but the exception.



Example 2: An image recording also showing cracks and fissures is to be made as part of the acceptance tests for a new tunnel construction.
- High resolution of 10,000 pixels required
- The optimum measuring speed must be maintained: the advance per scan line must comply with the point distance along the circumferential line.

For the SPACETEC TS3 this means:
- Mirror rotational speed 200 Hz
- Measuring speed approx. 2 km/h

In most cases, the measuring speed will be in-between these two extremes - typically about 5 km/h.



Finally, a comparison of different scanners in use for the image and profile recordings of an entire subway network with a tunnel system 100 km in length. Due to the capacity utilisation of the tracks and lines, the time window available per shift was 4 hours. The comparison clearly shows how the performance data of the selected device impacts on the measuring speed and therefore on the time and effort required for the measurement survey.

SPACETEC TS3 SPACETEC TS2 Comparison model
Mirror rotational speed

250 Hz

90 Hz

33 Hz

Optimum measuring speed

4,5 km/h

1,6 km/h 0,6 km/h

Measuring time for 100 km 23 h   65 h 172 h
Number of measuring shifts 6 17 43



Device parameters influencing the measuring speed:

Resolution:
Resolution means the number of individual dots (pixels) per scan line. A scan line corresponds to a closed circumferential line or to a full rotation of the scan mirror.


Typical values for tunnel scanners are:

Standard resolution: 2. 500 Pixel
Medium resolution: 5. 000 Pixel
High resolution: 10. 000 Pixel

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The more stringent the quality requirements involving the image recording, the higher the resolution has to be.
The higher the resolution, the lower the optimum measuring speed



Cycle rate: Describes the number of measuring points which the system is capable of recording per second. Typical cycle rates for standard scanners are between 500,000 and 625,000 Hz. The SPACETEC devices are at the higher end with a maximum cycle rate of 2 MHz.

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The given cycle rate and the resolution result in the maximum mirror rotational speed.



Mirror rotational speed: Due to the mechanical bearing and the power of the drive, there is an upper limit to the mirror rotational speed. Typical values for standard devices are 33 Hz or 50 Hz. The SPACETEC scanners are also leading in this field with maximum rotary speeds of 90 Hz (TS2) and 300 Hz (TS3).

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The mirror rotational speed and the speed of travel result in the advance per scan line.
The latter determines the resolution in the direction of travel.

Summary: Thanks to their high cycle rate and mirror rotation speed, SPACETEC scanners are capable of recording substantially  more pixels per time unit than similar units. This means: high resolution images can be recorded at relatively high speed without any "data gaps".
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