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Tactile sensing technology replicates the capabilities of human skin, namely biological cutaneous touch. It enables artificial touch sensing in devices or products. Tactile sensors are mainly used in robotics, automotive and touchscreens, but can also be found in consumer products. They are embedded in surgical devices, mechanical tools, robots, handheld devices and automobile brakes and tires for safety and quality control. They are based on the principles of resistive and capacitive sensors. They involve touch and force sensing to determine pressure, texture, spatial resolution and other target object parameters. They function similar to the neuroreceptors in human skin. Combined with real-time intelligent signal processing, they can effectively mimic biological tactile sensory perception.
The human skin is one of the principal sensory organs and can sense a variety of environmental stimuli. This sensory perception is increasingly being mimicked and harnessed for technological purposes and this field is broadly called haptics. Tactile sensing is a subset of haptic technology that mimics or is modeled after the human sense of physical touch. Tactile devices are fitted with tactile sensors that mimic receptors in the human skin. The sensors detect feedback stimuli, just like the human sub-cutaneous sensors do. Tactile sensing is principally used in robotics, medical imaging, consumer products, security systems and automotive domains. Tactile sensors can augment visual and auditory sensing in devices. This technology is widely spread as it is applied to devices such as basic touchscreens.
Simply put, tactile sensing is defined as a haptic or cutaneous touch-based interaction with certain elements in the environment, which provide a measurable reaction or feedback. The interaction is in the form of physical touch, and the response by the targeted element is to provide a measure of pressure, vibration, temperature or texture. This response data is processed by an PCB to which the sensor is connected.
The human skin, especially on the hands, has a complex network of tactile sensors or mechanoreceptors to detect external parameters such as pressure, vibration, texture, temperature, degree of hardness, shape, size, and pain or damage. Tactile sensing technology has replicated these sensors so that these parameters may be artificially detected and processed by robotic and other automated devices.
Tactile sensing took birth within the domain of robotics in the 1970s. Research began with a survey of 61 human mechanoreceptors and their stimulus-detection properties conducted by Knibestöl and Vallbo. The first significant application of the technology occurred when Clippinger integrated this technology into a prosthetic hand capable of detecting and processing tactile feedback. It has been increasingly integrated into robotic devices since the 1980s, thanks to reduced costs and advances in material science, intelligent signal processing, and easier ways to integrate tactile sensors into robotic systems. Nowadays tactile sensing has found it’s way in many products that people use on a daily basis.
One of the most advanced areas of sensor technology, particularly in automation and robotics, is vision sensing. Cameras can be made durable, damage-resistant, small, and flexible enough to overcome any physical integration challenges. Vision sensors can identify and compute an object’s location, shape, and color, and with advancements in this technology, they can also remotely measure size, density, etc. through 3-D reconstruction of received data.
However, not all environmental stimuli can be detected by vision alone as it is a remote sensing technology. Some stimuli can be detected only through physical interaction with the target object, especially if some areas are hidden from cameras. Sensory feedback such as hardness, texture, vibration and other force-sensing parameters can only be detected through tactile sensing. An object’s response to a change in force, particularly its stiffness, and any shear or slip detection can be measured by tactile sensing alone.
Tactile sensing enables the detection and measurement of an object’s size, shape, weight, texture, temperature, shear, and response to a change in force stimuli. While some of these criteria can be detected by advanced vision sensors, in-depth data about these attributes can only be gathered by tactile sensing. A few criteria such as force-sensing can be performed by tactile sensors alone.
Tactile sensing has a decided edge over plain vision, auditory and other sensing technologies. While vision is limited to object perception, tactile sensing enables object handling and opens up vast possibilities with respect to interaction. This is particularly crucial in robotics, to provide intelligent automated devices a stronger capability of learning through interaction.
In clinical imaging, tactile sensors can measure the deformity of a surface when pressure is applied and translate it into 3-D imaging to determine an object’s response to a change in pressure. In ergonomics, tactile sensors are used to map pressure responses on a specific surface of the human anatomy and improve the design of the product. This has significant potential in prosthetic design. Tactile sensors are used in robotic devices that perform intricate tasks where dexterity and precision are required.
Tactile sensing technology is based on the principle of transduction. A transducer converts a signal from one form of energy into a signal in another form. A tactile sensor is an electromechanical transducer, which converts a mechanical stimulus into an electrical signal, and feeds it to a controller that processes and interprets it to generate data. This method replicates biological tactile sensing, where mechanoreceptors in the skin perceive external stimuli and convert them into electrical signals, which travel via the neural pathway of the central nervous system to the brain, which in turn interprets them.
Tactile sensing operates on a variety of transduction methods:
Capacitive
Piezoresistive
Piezoelectric
Optical
Magnetic
Binary
Of the above, capacitive and piezoresistive are the most commonly used principles in the design of tactile sensors.
Capacitance is the ability to store an electriccharge. A parallel plate capacitor consists of two electrodes separated by aninsulating medium.
The underlying equation for the capacitance is:
Where,
C = Capacitance in Farads
ε = Permittivity of dielectric (absolute, not relative)
A = Area of plate overlap in square meters
d = Distance between plates in meters
In a capacitive tactile sensor, the medium is often an elastic material. Capacitive sensors can detect normal touch and shear force. When touch occurs between the device and the target object, the resulting change in capacitance is measured and transmitted to the controller. Tactile sensors are part of an array to glean stimuli from a larger surface. Capacitive sensors can be designed to be minimal in size and as a result, be part of a denser array for richer data. They are also more resistant to temperature, consume less power, and maintain signal stability during long-term drift. They can be susceptible to hysteresis error, where the output varies with the amount of pressure, due to a degree of inelasticity. They are also sensitive to electromagnetic noise and the elastic medium loses its effectiveness over time due to wear. They are ideally suited for use in touchscreens and surgical robots.
When two materials come in contact and a mutual pressure is applied, there is a change in the electrical resistance of the surfaces.
Resistive sensors are made of conductive material such as conductive rubber, ink, silicone, or foam, and convert applied pressure to a resistance change. This change in resistance can be measured using Ohm’s law:
Where,
R = Resistance in Ohm
V = Voltage in Volt
I = Current in Ampere
This allows the device to detect contact and measure the applied force or pressure. Resistive sensors have the advantage of being low-cost and can be integrated into small micro and nanoscale devices. They provide a quick response, are durable, and can handle high loads. However, they are susceptible to signal drift and hysteresis. They are commonly used in wearable devices and are not often used where high accuracy or precision are key requirements.
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Tactile sensing technology has found uses invarious fields such as switch buttons, touch-displays, medical surgery,automotive, handheld devices, engineering and others, where robotics areemployed to augment human capability through artificial touch sensing. It isused where object perception and manipulation through physical contact arenecessary and can be performed more efficiently and safely by a device ratherthan a human.
In the field of medical surgery, devices that are equipped with tactile sensors allow surgeons to access smaller and less accessible parts of the surgical site. This makes minimally invasive surgery a strong and safe alternative. Surgeons rely on a perception of pressure between the surgical tool they grasp and the object of surgery. A robotic tool equipped with tactile sensors can perceive this criterion and instantaneously transmit the sensation to the surgeon’s fingertips. In addition to tool contact pressure, sensors can recognize surface texture and duration of contact period. They can also perceive tactile criteria like the stiffness, size, and shape of visually inaccessible subcutaneous biological structures. In effect, the tool acts as a remotely controlled yet realistic extension of the surgeon’s hands and can perform surgical procedures on smaller targets, in smaller spaces and possibly with greater precision.
Another medical application of tactile sensors is in prosthetics, where they are capable of mimicking the touch sensing capabilities of human skin to make the prosthetic limb as realistic as possible for the wearer. Neuroprosthetics are artificial limbs that have the added capability of sensation and is currently limited to artificial hands or fingertips. Flexible and small tactile sensors are embedded in the elastomer material that covers the prosthetic limb and mimics human skin. The sensors can perceive the presence of an object, directionality for spatial recognition and force as well. A challenge still remains in order to bridge the gap between the sensory capabilities of bionic skin and human skin. Ongoing research is tackling the challenge of designing sensors that can accurately judge slippage and weight so that the grasping functionality of prosthetic limbs may be improved.
Tactile sensing is used to enable new interaction possibilities for the driver with the car. Through tactile sensors surfaces can be digitized and replace classical buttons. Furthermore, the integration of the sensors enables the creation of human machine interfaces on soft materials within the automotive interior such as the seat. The steering wheel being one of the components where human and car come together is highly interesting point to measure the tactile interaction. Furthermore, tactile sensor can also be used to car parameters such as measuring tire tread behavior, a crucial component of vehicle health, quality and accident avoidance. A tactile sensor array is affixed on a surface and measures tire pressure at high speeds, as a vehicle travels over it. A digital image of the pressure map is generated for further analysis of tire performance and potential issues and for the optimization of the tire tread design. During quality control, various tactile pressure sensors embedded within the vehicle measure brake performance and various pressure points within a moving vehicle to help with motion control.
Tactile sensors are used in human-machine interfaces or HMIs such as touchscreens. The ubiquitous touchscreen is seen everywhere these days from kiosks in the travel and entertainment domains, to handheld devices in retail and hospitality to our smartphones. It is also an interface used in the medical and automotive fields, for quicker, easier and safer task completion. Touchscreens are based on capacitive tactile sensing. Tactile sensor arrays are embedded behind the transparent, non-metallic screen of an HMI. When a finger touch stimulus occurs, the resulting change in capacitance is interpreted by the touchscreen controller as a specific command and appropriate feedback or action is initiated.
In robotics, tactile sensors predominantly augment the intelligent humanoid’s visual and auditory capabilities. This compounds its object recognition, manipulation and exploration capabilities. Tactile sensors are embedded in robotic hands or fingertips to help the device judge the target’s firmness, texture, shear, size, shape, presence and location. Based on this data, the device can make decisions to manipulate the object in terms of grasp, push, pull, slide over and so on. This technology is most useful in small spaces that may also be visually inaccessible, such as clinical examination or palpation, to inhospitable environments where remote manipulation is required, such as on spaceships. Tactile sensing robots are used in surgery, industrial, mechanical repairs, and prosthetics among others.
Tactile sensors have found their way into all types of handheld devices to measure tactile interaction. These could be power tools, smart home devices, gaming controllers, or other consumer products. They enable seamless design, an additional safety layer, manual button replacement, and more reactive and intuitive products.
