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Distance measurements: laser or ultrasonic?

In this article we discuss the differences between the ultrasonic- and laser-based measuring principles, the challenging applications that emphasise those and an overview of possible solutions.
Ultrasonic versus laser distance measurement

Automated distance measurement

In many processes there is a demand for automation in the form of distance measurement. This is to gain more control and insight into the process. This benefits accuracy, reliability and efficiency.

Examples of this are:

  • A truck that backs up towards a loading bay
  • Determining the position of a gripper towards a container
  • Determining the distance of a ship to a quay, mooring and berthing
  • Level measurement in a bunker
  • Level measurement in a tank

These are all nice applications, but which sensor is the right choice? What do you base this choice on?

In this article we discuss the differences between the ultrasonic- and laser-based measuring principles, the challenging applications that emphasise those and an overview of possible solutions.

Wat is laser light?

Laser is light, but very intense. This is because the light particles in laser light are much closer together than in regular lighting. A characteristic of laser sensors is that laser sensors are made both in the visible spectrum and in the invisible spectrum. That means there are infrared as well as visible red lasers. Laser light can be used to accurately measure very small but also very large distances. Methods that are applied here are: “Time of Flight” and the “phase shift”.

Lasers have to follow the same rules as normal light which makes the advantages and disadvantages somewhat similar. Advantages are, but not limited to, a high speed, very high precision and a sharp focus on small surfaces. When it comes to measuring size, lasers are applicable over very small as well as very large distances.

The disadvantages of the use of light are the susceptibiliy to ambient light, the (limited) reflectivity or too much relief on the measurement/detection surface and objects that let through light. These can all form an obstacle for the emitted light of the sensor.

What is ultrasonic sound?

Ultrasonic soundwave

Ultrasound is a region of the sound spectrum. In other words, vibrations that travel through the air. What characterizes ultrasonic sound is that it cannot be perceived by the human ear.

The frequency, or number of vibrations that this sound makes per second determines whether it can be perceived by the human ear. In nature, this sound spectrum is used by dolphins and bats, for example. Specifically, to detect, communicate and determine distances to something.

The use of sound brings advantages but equally disadvantages. The advantages consist of: the imperviousness to color, gloss and transparency of objects, very good action on solids and liquids and no hindrance on objects with coarser or on the contrary finer structure. As for disadvantages, ultrasound has a smaller range and a very high susceptibility to sound-absorbing materials or surfaces such as foams and textiles.

What are the applications that require laser light or ultrasonic sound?

The aforementioned fundamental description of ultrasonic sound and laser light help in translating these properties to usable practices. Below are a number of industrial applications where ultrasonic sound or laser light provide a solution.

Measurement of transparent objects

Transparent objects such as a plastic bottle have the property of letting through light. If a laser would be used to measure the distance to the bottle it would encounter issues because of the lack of a reflection of the laser light that has to be received by the sensor. An ultrasonic sensor, however, is a very suitable solution for this: the emitted sound does not go through the packaging and thus is reflected. This allows the measurement to be performed, so the sensors can determine the distance to the bottle.

High accuracy

Sometimes a measurement has to be performed on surfaces that are very variable in shape. Think of tire profiles that have to be measured from a distance. The use of an ultrasonic sensor is less suitable here, because the greater the distance, the less precise the measurement. Laser light, on the other hand, has the necessary precision to reach and thus measure these openings. This allows the tire profile to be determined down to micrometers, so that wear or product errors can be traced.

Measurement on different colors

In the packaging industry, materials and product packaging come in all kinds of different colors. Not every color has the same reflectivity, which means that one color reflects more than another. We often notice this when wearing a black shirt on a summer day: we experience the sun even hotter than someone with a white shirt. This is due to the degree of reflection. The laser light, just like any kind of light, is subject to this because not every color reflects an equally good reflection back to the sensor. An ultrasonic sensor therefore is prefered which is insensitive to the color of a package.

Measurement on a very large scale

The range is perhaps the most important factor when it comes to a (distance) measurement. Ultrasonic sensors reach up to 8,000 mm (or 8 meters) and thus can be used within most small and medium distances. The laser sensors start at a maximum measuring range of 10 cm and are available up to even a 3,000 meters (or 3 km). Distance lasers can be used for small, medium and (very) large distances. Think of a level measurement in a silo.

Sound-absorbing materials

In the textile industry, all kinds of materials are used from cotton to wool and from synthetic materials to unprocessed fabrics. Materials with an open structure often have a sound-absorbing property. This ensures that when measuring with an ultrasonic sensor, the sound vibrations are reflected less or not at all to the sensor. It seriously affects the measuring range. The laser is a common solution here because it can perform error-free measurements on these types of materials, without significant influence on the measuring range (apart from the color).

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