Using Ultrasonic Distance Sensors to Detect Glass Window Surfaces

The demand is growing to apply distance sensing in conditions where glass is frequently part of the environment. This surface can be challenging to “detect” or measure for light emitting sensors, for example, when working under strong ambient light conditions or approaching at angles other than 90 degrees.

The signal can go straight through the transparent glass material in some instances, or be reflected away, which will not supply accurate readings. TeraRanger Duo is designed specifically to combat the problem of measuring “difficult surfaces”.

TeraRanger Duo is a hybrid sensor of two technologies: (1) high performance ultrasound (sonar), and (2) infrared Time-of-Flight (ToF) as found in the TeraRanger One sensor. ToF deals with various material surfaces extremely well (and this can often include glass surfaces, as the report shows), with ultrasound supplying a redundancy measure for the rare situations in which ToF could be suboptimal.

This article will demonstrate the performance of both the optical Time-of-Flight and ultrasound components of TeraRanger Duo when detecting glass surfaces. The purpose of this examination is to observe how well, and in which conditions, Terabee technology works in “detecting” difficult surfaces like plate glass.

These tests were carried out utilizing a TeraRanger Duo* chosen randomly from stock. All ToF distance measurements were performed in the ‘default/precise’ (instead of the ‘fast’) mode.

* Please note: TeraRanger Duo uses TeraRanger One Type A sensor technology for its Time-of-Flight component and is not optimized for use outdoors or in bright sunlight in the same way that a TeraRanger One Type B sensor is.

Test Materials

Two glass surfaces were tested:

  • Metallic coated (tinted) window
  • Terabee control window

The tinted window is an ordinary double-pane float-glass window with metallic coating which is  designed to reduce solar heat, this coating also generates a mirror effect. The control window is a standard double-glazed unit made up of two glass panes which are separated by a hermetically sealed insulating air space.

Terabee control window (left): Metallic coated window (right)

Figure 1. Terabee control window (left): Metallic coated window (right)

Test Setup

The TeraRanger Duo sensor was utilized to calculate the distance to windows in sunny, outdoor conditions at an ambient temperature of 28 °C, with a particularly high ambient light level (115 000 lux). This means that data gathered in the test is highly representative of many real-life outdoor use applications.

Test Setup

Figure 2. Test Setup

Bearing in mind that in applications, often the distance to objects are measured fromdifferent angles, the test is divided into two sections:

  • During Test A, distance values were performed perpendicular (90 degrees) to both of the glass surfaces, measuring from distances of 3, 5 and 7 meters.
  • In Test B (2), distance values were gathered from a 45 degree angle to the glass surfaces, measuring distance from 3 and 5 meters.

To confirm the distance to the glass, a laser distance meter was employed. Each distance measurement was performed over a time period of five seconds. The arrangement of both A and B tests are shown in Figures 3.2 and 3.3.

Test A environment illustration: perpendicular to glass

Figure 3. Test A environment illustration: perpendicular to glass

Test B environment illustration: 45 degrees to glass

Figure 4. Test B environment illustration: 45 degrees to glass

For measuring the distance to the glass window from roughly a 45 degree angle, inthe case of “Terabee control window”, TeraRanger Duo was simply turned and positioned to the mentioned angle. Yet, to test the metallic coated window at a 45 degree agle, a complex environment was discovered where a glass wall acts as the side of a building (Figure 3.4).

Test B (45 degrees): Terabee control window (left): Metallic coated window (right)

Figure 5. Test B (45 degrees): Terabee control window (left): Metallic coated window (right)

Test Results

The graphs below show the results of both test measurements. The distance value is always represented in millimeters in the left vertical axis. Charts are “zoomed” to show precision and patterns for both sensing technologies utilized in the TeraRanger Duo; Time-of-Flight and ultrasound. Lastly, in order to test the ToF element of TeraRanger Duo in highly challenging conditions, the experiment was performed under strong ambient light conditions.

Test A: Perpendicular to Window

Surface: Terabee control window

Figure 6. Surface: Terabee control window

Surface: Metallic Coated Window

Figure 7. Surface: Metallic Coated Window

Figure 8. Surface: Terabee Control Window

Surface: Metallic Coated Window

Figure 9. Surface: Metallic Coated Window

Surface: Terabee Control Window (Note: Ultrasound was unable to attain a reading at this range and in these conditions and is therefore not present in this figure).

Figure 10. Surface: Terabee Control Window (Note: Ultrasound was unable to attain a reading at this range and in these conditions and is therefore not present in this figure).

Conclusion for Test Perpendicular to Glass

When located perpendicular to both window types, test A results show that TeraRanger Duo always detects the glass surface. It also shows that in many instances the Time-of-Flight component of the sensor can do an acceptable job of detecting glass, but in the instances where ToF infrared light pierces the window surface (see Figure 4.2), ultrasound will always recognize the glass surface.

Test B: 45 Degree to Window

Figure 11. Surface: Terabee Control Window

Surface: Metallic Coated Window

Figure 12. Surface: Metallic Coated Window

Surface: Terabee Control Window

Figure 13. Surface: Terabee Control Window

Conclusions for Tests on Windows at 45 Degrees

Test B results establish that detecting window surfaces can be more challenging when measurements are taken from an angle (45 degrees in this case). Figures 4.6 and 4.7 show how ToF signals fail to identify the correct distance of the window surfaces, instead detecting objects which are behind the window glass.

Yet, as can be observed, the sonar technology acts as a redundancy measure for these instances, returning a signal from the glass surfaces reliably (Figure 4.6, 4.7). Note that test B shows a suboptimal approach angle for any sensor technology. An implication of this is, the accuracy of the reading can degrade for both ToF and sonar, as the distance from the window grows (Figure 4.8).

Yet, TeraRanger Duo has been able to clearly detect window surfaces from a 45 degree angle in every attempt, even in these challenging conditions, though in some instances the ultrasound measurement has not converged for the total five second test period.

Conclusion

From different angles and distances, in addition to outdoor sunny conditions, TeraRanger Duo has been able to clearly detect window surfaces in all measuring attempts. At particular angles, when calculating the distance to window surfaces, accuracy can be affected. However, the sonar technology has been proven to recognize window surfaces in cases where ToF has failed to do so reliably.

The results of this test prove the suitability of TeraRanger Duo for cases where glass surfaces will be encountered. A number of different surfaces and targets will be encountered in most real-world applications, and in the majority of cases ToF supplies a longer range solution with much quicker data refresh rates.

The combination of ToF and sonar produces a solution with redundancy for the surfaces - notably plate glass - where ToF alone may not always be the best solution.

This information has been sourced, reviewed and adapted from materials provided by Terabee.

For more information on this source, please visit Terabee.

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