Molded Elastic Sensors: A Unique Measurement Range for New Applications

Humans are increasingly interacting with machines in domains like medical technology, industrial production, and well-being. Machines collaborate and support humans in improving their performance or health conditions. In some cases, the machine is wearable and comes into direct contact with the human body, such as exoskeletons, prosthetic and orthotic devices.

In this instance, the machine and its components adapt to the three-dimensional shape of the human body. Force sensors can aid in making interactions more intuitive, protect humans and their surroundings from misuse, and ensure comfort and safety.

Such sensors should ideally fit perfectly between the machine and the human body, be delicate and skin-friendly, and be capable of dealing with enormous tolerances inside complicated, three-dimensional systems under a variety of environmental conditions.

This will enable consistent measuring settings in which the sensor is constantly exposed to the force interaction between the machine and the body. Furthermore, it prevents the sensor signal from saturating under high environmental or mechanical conditions due to overload, enhancing the reliability of the sensor system.

Electronic Skin

Human skin has an extraordinarily fine and complex sensor system for perceiving touch and forces, allowing individuals to function intuitively and accurately. The mechanical properties of silicone are extremely similar to those of skin. Silicone can be utilized as a mechanical transducer for forces and pressures by appropriately engineering the form and conductivity of the material.

Force sensors based on elastomers were pioneered in the late 1960s1-4 and have been continuously improved throughout the subsequent decades. The sensor mentioned here is based on Fraunhofer Gesellschaft’s work.5

The current silicone sensor is made up of three elastic conductive layers that are placed on top of each other. Each layer comprises three-dimensional flexible spacers that divide the layers when stacked.

The spacers between the layers are formed in the shape of tiny membranes. They operate as elastic springs and hold the layers in place. This results in the formation of a soft, compressible sensing capacitance that can be electronically measured.

When a force or pressure is exerted on the sensor, the conductive layers move in a specified direction toward each other, decreasing the distance between the layers (Figure 1a). This results in an increase in the electrical capacity of the silicone sensor, which the electronics can then measure.

It can also accurately analyze the sensor signal even with high-impedance elastomer material by building the measurement electronics as an AC voltage divider.

When the load is removed from the sensor, the spring elements reset the sensor, and the measurement signal is returned to its original state.

Working principle. (a) Sensor appearance incl. cross-sectional view (b) typical measurement curve.

Working principle. (a) Sensor appearance incl. cross-sectional view (b) typical measurement curve.

Figure 1. Working principle. (a) Sensor appearance incl. cross-sectional view (b) typical measurement curve. Image Credit: Sateco AG

 

A typical capacitive measuring signal from a silicone sensor is shown in Figure 1(b).

Due to the material’s viscoelastic characteristics, some hysteresis is detected. By modifying the material hardness and the shape of the spacers between the layers, the relationship between load force and resultant capacitance can be tuned for the sensor application.

The higher the maximum load that is applied, the harder the material and the stiffer the spacers. Most experience is now available with sensors ranging in length from 10 to 20 mm, which can estimate forces of 10 N with an accuracy far below 1 N.

Tested Durability

When compared to other elastic materials, silicone’s strong elastic qualities allow it to tolerate a very high number of load cycles. The results of a mechanical durability test on a sensor with a diameter of 16 mm and a nominal load of 10 N are shown in Figure 2.

In this test, the sensor was subjected to 500,000 repeated load cycles of 20 N, which is double the nominal maximum load. Additionally, further sensor testing was performed inside a climate chamber based on typical automotive industry standards.

Durability load test of a silicone force sensor.

Figure 2. Durability load test of a silicone force sensor. Image Credit: Sateco AG

Silicone sustains its elastic ability throughout a very wide temperature range, typically −40 °C to +200 °C, and is thus suited for application in harsh weather circumstances. In the climate tests, the temperature is cycled between -40 °C and +85 °C during the cyclic load.

Furthermore, humidity is added up to 90% RH. Under these conditions, silicone sensors were subjected to 200,000 load cycles. Depending on the environmental conditions, aging is observed in the form of drift. A reference sensor can compensate for this drift.

A spatial resolution can be realized by using multiple sensors at the same time, for example, to detect the direction of the force.

Proven Manufacturing Processes

The silicone sensor’s base material is an electrically conductive solid silicone with a hardness of 60 Shore A. Carbon-based particles are used to increase conductivity. The crosslinked solid silicone combination has a conductivity of more than 0.10 Siemens/cm and is widely accessible commercially. The sensor is shaped under high pressure and temperature using a compression tool.

The compression process enables the processing of highly filled, very viscous silicone material and three-dimensional shaping in just one step. The molding process produces both the spring components that support the individual sensor layers and the overall shape of the completed sensor. The crosslinked silicone part is removed from the mold and folded into the three-layer configuration that is shown in Figure 3.

The electrical connection of the sensor can be created using a connector that is bonded to the conductive silicone. In contrast to printing, compression molding allows for the creation of true three-dimensional free-form surfaces. This opens up new possibilities for product design, such as ergonomic use.

Assembly process of the silicone sensor composed of molding and printing.

Figure 3. Assembly process of the silicone sensor composed of molding and printing. Image Credit: Sateco AG

Application Examples

The silicone sensor has a strong elastic deformability in the force or pressure direction. As a result, it can be easily incorporated into soft materials like textiles and foams. Due to its elasticity, it can provide mechanical preload to the system. This facilitates assembly, particularly in curved and intricate constructions. As a result, the silicone sensor is appropriate for use in automated processes.

For mobile robots and portable exoskeletons, weight is very crucial. Object detection can be assisted by force sensors on grippers. According to research, robots that must recognize unknown objects require “tactile” features such as object rigidity in addition to image recognition.

As the silicone sensor is analogous to a soft spring, it can be utilized to accurately assess the stiffness of objects.

Force sensors in robots and exoskeleton feet assist in maintaining balance. The sensor is useful in medical technology for controlling prostheses and sensing pressure points on the soles of the feet and pressure bandages. One recent example relates to a wearable neuroprosthesis that helps people with limited mobility move their paralyzed limbs.6

As shown in Figure 4, silicone force sensors are incorporated into the sole of a shoe to offer data on walking behavior. An AI-based algorithm that drives a system of neuromuscular electrostimulation electrodes then evaluates this data.

Smart show for AI-driven neuroprosthesis. The location of the force sensor inside the sole of the shoe is indicated by green and blue dots.

Figure 4. Smart show for AI-driven neuroprosthesis. The location of the force sensor inside the sole of the shoe is indicated by green and blue dots. Image Credit: Kurage AG

As a result, the system is capable of duplicating functional movements in a personalized and safe manner, compensating for deficiencies in the movement apparatus’s sensomotoric performance. The sensor’s flexible form allows for excellent sensor integration in the shoe, and force distribution on the foot can be monitored across a wide area.

As a result, a dependable signal is produced, allowing for the high force resolution required by the AI algorithm to function properly. The neuroprosthetic device is currently undergoing clinical testing.

Samples of silicone sensors, comprising evaluation electronics and software for viewing measurement data, are readily available for testing (Figure 5).

Silicone sensor starter-kit composed of sensor, electronics and graphical user interface.

Figure 5. Silicone sensor starter-kit composed of sensor, electronics and graphical user interface. Image Credit: Sateco AG

Summary

The industrialization of silicone force sensors is currently underway, and orders for a few thousand parts have already been fulfilled. Sateco has demonstrated various examples of robotics and medical technology applications that warrant the use of soft silicone sensors.

References

  1. National Research Development Corp., German Patent DE 1,916,496, March 1969
  2. Uniroyal, US Patent US 3,875,481, April 1975
  3. Semperit, German Patent DE 2,800,844, January 1978
  4. Key Concepts, European Patent EP 286,747, April 1987
  5. Fraunhofer Gesellschaft, European Patent EP 2,698,616, August 2013
  6. Kurage, Lyon, France, URL: www.kurage.fr

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

For more information on this source, please visit Sateco AG.

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