Leuze uses artificial intelligence (AI) to significantly improve the measurement accuracy of optical distance sensors for challenging industrial applications. This innovation reduces measurement uncertainty by half without the need for additional computing resources during operation. The solution is based on a neural network. Optical distance sensors with time-of-flight technology (TOF) offer system operators practical benefits. The sensors enable fast, contactless measurement of large distances. They are also insensitive to ambient light and provide continuous distance data in real time. The sensor’s operating principle measure distances by recording the time it takes for emitted light to travel to the object and back. Laser or LED pulses are generally used for this purpose. 

However, time-of-flight technology also has limitations in measurement accuracy: How precise the results are depends heavily on the nature of the object surface. Dark surfaces can weaken the reflected signal. They generate narrower pulses and the echo is detected later. Bright surfaces, on the other hand, generate stronger signals with a wider pulse width that are detected earlier. That means the returning signal is detected at different times depending on whether the object’s surface is light or dark. This can cause measurement errors that must be compensated for.

Polynomial function only automatically adjustable to a limited extent

Until now, mathematical models based on defined algorithms have been used to correct these errors. A correction value is calculated for many different surfaces and distances, which is later applied automatically. This calculation is based on a so-called polynomial function. Polynomial functions offer an efficient solution for stable, continuous error curves. One disadvantage, however, is the limited imaging accuracy in the case of complex factors, such as strongly varying surface reflections. As the model parameters are fixed, the functions cannot automatically adapt to changing environmental conditions.

Neural network as the basis for optical distance sensors

Leuze can count on a much more precise and flexible solution. Instead of working with rigid formulas, Leuze uses a neural network to determine the correction value. A neural network is a form of artificial intelligence that is modeled on the human brain. It consists of nodes (neurons) in three types of layers: the input layer, hidden layers and the output layer. The neural network processes information by passing input data step through these one layer at a time. The neurons weigh their results, summarize them and convert them using functions so that a precise result is produced at the end. A so-called activation function decides how strongly a neuron becomes ‘active’, i.e. what value it passes on to the next layer. This activation function enables the network to learn even complex, non-linear relationships and is not limited to simple calculation patterns. 

Practice-oriented calibration and areas of application for optical distance sensors

The AI solution developed by Leuze uses sample data to learn how brightness and surface texture affect the optical distance sensor’s measurements. This makes it much easier to correct the measured values. The neural network is trained with data consisting of raw distance values and pulse widths as input parameters as well as the corresponding standardized correction values at the output. The training data can be generated from the production process, in which many measured values are collected: for light, dark and differently textured surfaces as well as for different distances. These measured values are communicated to the production facility’s control system. From this, the production facility’s neural network calculates the correction values for the sensor. The sensor then requires no additional computing power during operation – the AI has already ‘learned’ everything.

Five steps for precise values

The Leuze neural network consists of five layers. In each layer, all neurons are fully connected to each other. This means that all information flows into the calculation. A ‘Rectified Linear Unit’ (ReLU) activation function is used. This ensures that the network sets negative counters to zero and only processes positive values, similar to a filter that only lets positive signals through, making the learning process reliable. So, the network works faster and it avoids the computing problems that can occur with other methods. The last layer of the network, the output layer, determines the final correction value. Here, ‘tanh’ (hyperbolic tangent) is used as activation function. This ensures that the calculated correction value is always within a defined range between -1 and +1. The system then converts this value so that it directly indicates how much the sensor must correct the measured distance in order to deliver precise results.

Optical distance sensors in use

Time-of-flight distance sensors with AI-based correction are particularly useful in industrial automation where precise measurement results are essential. 

Typical applications include:                                                                                                         

• Navigation and collision avoidance: On robots and mobile platforms
• Materials handling: Checking positions and distances on conveyor belts
• Quality assurance: Checking distances on workpieces with difficult surfaces
• Automated guided vehicle systems (AGVs): Precise distance control when parking and maneuvering
• Safety applications: Detection of proximity to machines and systems

Summary: Greater precision through AI for optical distance sensors

Leuze is raising the precision of optical distance sensors to a new level with artificial intelligence. Tests have shown that the method's AI-based calibration reduces systematic measurement errors, i.e. the dependence of measurement results on surface and distance, by more than half. Customers benefit from more robust and accurate measurements without any effort during operation, even with difficult surfaces. This makes it the ideal solution for challenging industrial applications.

Benefits at a glance:

• Fewer measurement errors, delivering significantly more precise results
• Flexible use with different sensor types and surfaces
• Learns better from real data, even with strongly oscillating 3D curve characteristics
• No additional computing load during operation
• Future-proof thanks to modern AI

Author

Janina Sanzi
Competence Center Software 
Leuze Electronic