Ultrasonic Sensors

Innovative ultrasonic sensors for human-robot collaboration

Humans and robots are already close colleagues in many workspaces. This opens up the possibility of optimally complementing the cognitive and flexible capabilities of humans with the precision and endurance of robots and achieving maximum productivity. However, a prerequisite of this human-robot collaboration (HRC) is a high level of safety. Previous sensor solutions cannot realize comprehensive intelligent coverage of the environment. Micromechanical ultrasonic transducers (MUT) represent an innovative and effective further development in this respect, which can open up new areas of application thanks to their compact design and power efficiency.

Safe interaction of humans and robots using sensor solutions.

Currently available sensor solutions and their hurdles

Currently available sensor systems for environment recognition either detect the direct contact between human and machine by means of direct or indirect force sensors or the approach by means of 3D cameras or capacitive sensors. The main problem of these systems is on the one hand the lack of redundancy of different sensor principles. This can lead to false detection. For example, a capacitive sensor alone cannot detect bodies with low dielectric constant (e.g. plastics). Furthermore, sensor systems usually cover only a limited area of the machine. Coverage of all zones (near and far) would be desirable to guarantee complete detection of all bodies in the environment. Additionally, the setup of the sensors is often complex and needs a good knowledge of the machine environment. Therefore, safe environment detection in MRK requires integrated, networked, redundant and intelligent detection sensors.

 

What are ultrasonic sensors?

Ultrasonic sensors are already used in numerous application areas for environmental monitoring and object detection: Parking assistance in automotive technology, presence detection and level measurement in factory automation, and the development of mobile robots are just a few examples. Distances in the range of a few centimeters to a few meters are covered. The measurement method is based on the transmission of ultrasonic waves and the detection of echo signals from the environment. Quantitative statements about distances and movements of obstacles within the sensor range are derived from the transit time and amplitude of the reflected signal as well as a frequency shift.

Compared to optical and capacitive methods, ultrasonic methods are characterized by the fact that distance and motion detection can also be used in dark or opaque environments and requires less detection effort due to longer signal propagation times. The lower environmental impact of humidity, air pressure as well as particles on the performance as well as the absence of ionizing or intense optical radiation also enable reliable detection in harsh environments. Currently, ultrasonic sensor systems are mainly realized by leaded piezoceramics and composite compounds using precision mechanical manufacturing processes. However, the increasing complexity of measurement and inspection tasks in MRC requires an increasing degree of sensor miniaturization and the local integration of intelligent data processing in real time, for example in online monitoring systems or the embedding of reactive gripper systems. In addition, this approach is limited in terms of resolution capability, especially in the near distance range, as well as RoHS compliance. Accordingly, these sensor systems will not meet the requirements of the next robot generation.

The micromechanical ultrasonic transducers (MUT) based on MEMS developed by Fraunhofer IPMS are a promising sensor approach to overcome these hurdles. These miniaturized systems benefit from reliable manufacturing processes in CMOS technologies, which enable cost-effective and RoHS-compliant production of sensors in high volumes. MUTs are manufacturable for a wide range of ultrasound frequencies, enabling application-specific ranges and resolving power. Sensor solutions can be manufactured in single-channel structures as well as in arbitrary two-dimensional array structures. The latter enable the application of imaging techniques for environmental monitoring. In combination with on-chip sensor control and embedded AI, intelligent MUT systems will be available in the future for integration into sensor networks for multimodal environmental monitoring and enable collaborative interaction between autonomous robot systems and humans.

Ultrasonic sensors for tomorrow's production technology

Industrial mass production places a very high value on automation and process control. This is the only way to ensure that the products are qualitatively suitable for sale. Moreover, it not only prevents potential hazards to the facility, but also saves production costs by producing fewer defective parts. In light of this, and in view of the Industrie 4.0 strategy, more sensitive, accurate and cost-effective sensors are an important building block in production technology. In addition, miniaturization is playing an increasing role due to its advantages in terms of portability, possible modularization of designs, networking and integrability. MEMS technology enables the realization of these goals and at the same time is able to cover the requirements of the industry due to the reliability of ultrasonic sensor technology.

The ultrasonic sensors developed by Fraunhofer are ideal for industrial process control.
© Fraunhofer ISIT
MEMS ultrasound array for air-guided ultrasound.

How do ultrasonic sensors work and what are the advantages?

The three Fraunhofer institutes (ENAS, IPMS and ISIT) develop ultrasonic transducers for a wide range of applications that can generate high-frequency sound waves with a high sound pressure. The core of the ultrasonic sensor technology of the Fraunhofer Alliance are MEMS transducers, which are manufactured using semiconductor fabrication processes. The devices are driven either capacitively or piezoelectrically, so that when an alternating electrical voltage is applied, a mechanical force is generated that causes the transducers to vibrate. The components thus generate high-frequency oscillations that locally and alternately compress the surrounding medium to produce sound waves. The stronger the oscillation, the more intense the ultrasonic wave. Highly efficient ultrasonic sensors can be realized with the powerful transducers developed by Fraunhofer.

A typical feature of MUTs is the use of resonance to generate even stronger vibration amplitudes. This occurs at a certain excitation frequency, which is mainly dependent on the geometry of the transducer and the material properties. However, the excitation frequency also affects the range of the sound wave, since at very high frequencies the sound is strongly absorbed by the medium, and the intensity decreases. With the MEMS-based sound exciters, the excitation frequency can be easily adjusted to the customer's requirement, and thus an optimal design of the devices can be found for the customers. Due to the compact geometries and low weight of the MEMS sound exciters, the Fraunhofer-manufactured transducers are easy to mount in various setups. Moreover, thanks to established semiconductor manufacturing processes, they can be easily integrated with the necessary drive electronics.

What are the development potentials of ultrasonic sensors?

The Fraunhofer ultrasonic transducers offer other advantages for manufacturers of measuring equipment. If different components are arranged close together to form an array, a whole range of new applications opens up. In this way, for example, the positions of objects can be reliably mapped in three-dimensional space by correlating the signals received with different propagation times with the position of each individual transducer (beam forming). On the other hand, it is also possible to align the transmission of an ultrasonic wave so that it hits exactly one targeted point in space (Beam Steering).