Tactile Sensor: 3 Important Factors Related To It

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Image Source: Richard Greenhill and Hugo Elias of the Shadow Robot Company, Shadow Hand Bulb large, CC BY-SA 3.0

The Subject of discussion: Tactile Sensor and its Function

Types of Robot Sensor

What is a Robot Sensor?

A robot interacts with its environment with the help of various sensors which measure physical quantities. Sensors work on the principle of transduction, where energy is transformed from one type to other one. A controller processes the sensed data to enable action from the robot. Sensors also monitor the condition of a robot.

tactile sensor
Sensors in a typical Robotic Arm image credit : pixabay

Classifications of Robot Sensor

The first type of classification is as follows:-

  • Proprioceptive (PC): Sensors which provide a ‘sense of self’ to the robot. They measure internal to the robot system values, for example, joint angle, wheel position, battery level, et.
  • Exteroceptive (EC): Sensors that provide information about the external state, such as observations of the environment and the objects in it.

The second type of classification is as follows:-

  • Active (A): Sensors which work by emitting energy, for example, radar-based.
  • Passover (P): Sensors which receive energy passively example camera.

Following are the types of sensors based on typical use:-

class

Tactile sensor and its function

What does ‘tactile’ mean?

  • Something that is designed to be perceived by touch.
  • Something is perceptible by touch or apparently so, tangible.
  • Something that is connected with the sense of touch.
  • It is synonymous with palpable, touchable, touch, tangible, haptic, real, tactual, physical, substantial, visual and texture.

What is a tactile sensor?

Based on physical contact with the environment, a tactile sensor measures information. The touch sensor’s architecture is derived from the biological sensing of cutaneous touch that can detect sensations arising from various mechanical stimulations, temperatures, and pain (although sensing pain is bit uncommon in artificial tactile sensor). In robotics, security systems and computer hardware, tactile sensors are used.

SynTouch BioTac
Bio-tactile Sensor in human arm prosthetic; Image Source: anonymous, SynTouch BioTac, CC BY 3.0

Vision is often asserted as perhaps the most significant human sensory modality that underestimates the role of touch. Of course, losing the capabilities provided by touch will lead to devastating impairments in posture, locomotion and limb function, object property retrieval, and any physical contact with the environment in general.

Psychophysical experiment has revealed that the human haptic touchs are rich in textures, shapes, hardness, and temperature for interactions, discovery, manipulation, and object property extraction. Innumerable types of receptors, for example mechanoreceptor (pressure and vibration), thermoreceptor (temperature) and nociceptor (pain and damage) register this information distributed with variable density acted on the body and situated in the different area of the skin.

Human hands have an exceptionally high mechanoreceptor density, one of the most advanced areas of the body for providing precise tactile feedback.

This field has progressed from the year 1970s and minimal incorporation of these systems into robots, though tactile sensing was comparatively ignored during the premature age of automation. In comparison, significant developments in tactile sensor technology were seen in the 1980s, followed by a decline in production costs.

Progress has been made in sensor materials, design and manufacturing technologies, and in the methods of transduction for incorporation into different robotic platforms. Different types such as capacitive, piezo-resistive, piezo-electric, magnetic, inductive, optical and strain gauges were the key tactile sensing technologies are developed at this period, enabling the efficient production of particular devices for object shape, texture, force and temperature detection.

Technologies involved in Tactile Sensing

Tactile sensor technologies are defined by transduction used to correctly translate stimuli from the external world to an intelligent device. This types of sensor utilized in robotics are focused on the methods of capacitive, piezoresistive, optical, magnetic, binary and piezoelectric transduction listed in the following sections.

Capacitive sensors-

By measuring the capacitance variation from an applied load over a parallel plate type capacitor, tactile sensors centred on capacitive transduction work. The capacitance is connected to a similar plate capacitor separation and field, which uses an elastomeric separator to ensure enforcement. Capacitive sensors can be produced in petite sizes, allowing their construction and incorporation in small spaces, such as palms and fingertips, into dense arrays. In terms of better-sensitivity, drift-stability, less-temp sensitivity, small power consumptions and the sensing of natural or tangential force, this technology also provides various advantages. Significant hysteresis is one of the fewer limitations.

Piezoresistive sensors-

As force is applied, this transduction method tests changes in a touch’s resistance. Piezoresistive sensors are usually manufactured or made of piezoresistive ink in conductive rubber and stamped with a pattern. When no contact or stress is acting on the sensor, a maximum resistance value will be formed. Conversely, the resistance to touch decreases with increased pressure or stress. Its extensive dynamic range, durability, decent overload tolerance, economically comparable price and production capability in tiny sizes are the benefits of this technology. Disadvantages include reduced spatial resolution, the complexity of cabling multiple sensor components individually, sensitivity to drift and hysteresis.

Optical sensors-

Using state-of-the-art vision sensors, optical sensors work by transducing mechanical touch, friction, or directional motion into light intensity or refractive index changes. A downside is that light emitters and detectors (e.g. CCD arrays) need to be included, resulting in increased size.

Magnetic sensors-

Via the use of Hall effect, magnetoresistive or magnetoelastic sensors, this technology operates by detecting changes in magnetic flux caused by an applied force. By measuring variations in the voltage produced by an electrical current passing through a conductive material submerged in a magnetic field, Hall effect sensors work. Hall effect sensor is utilized also for detection of an artificial whisker’s multi-directional deflection. Magnetoresis and magnetoelastic sensors recognize changes in magnetic fields induced by the use of mechanical stress.

Better sensitivity, extensive range, small hysteresis, linearity and robust in nature are different advantages of this type of sensors. They are vulnerable to magnetic interference and noise, however. The physical size of the sensing system and the need to work in non-magnetic environments restrict applications.

Binary sensors-

Contact switch capable to detect discrete on/off event triggered by mechanical contacts to be detected. The simplicity of design and development of this type of sensor has made it possible to incorporate it into an extensive range of robotic systems. Contacting devices that go beyond a necessary binary code can be built. This sensor technology’s primary drawback is the lack of resolution, restricting applications to issues such as touch or collision detection.

Piezoelectric sensors-

An electrical charge proportional to the force, pressure or deformation applied is generated by piezoelectric sensors. The key disadvantages of this sensing technology are limitations on dynamic measurements and temperature susceptibility. However, because of their higher sensitivity, high-freq. responses and various types as per applications, such as plastic, crystal, ceramic and polyvinylidene fluorides, they are ideal for measuring vibrations and are commonly used (PVDF).

Hydraulic sensors-

This is a sort of actuator that transforms pressure of fluid to a mechanical motion is used in hydraulic technology. Recent industrial and medical applications need hydraulic-based microscopic servomechanisms, referred to as microactuators, to detect stress and measure strength. Micro-hydraulic structures have been designed to produce a low-power, precise and robust flow sensor. Composed of a biomimetic hair-like system, this sensor enables flow to be converted into hydraulic pressure, providing a wide range of measurements and high sensitivity.

Based on the micro-hydraulics sensing technology, force sensor arrays, close to the human fingertip scale, we can attain higher sensitivities. Manufactured with a stereo-lithography technique, these low-cost force sensors provide robust touch data and high spatial resolution, suitable for ideal for skin-alike sensing.

Design Criteria of Tactile Sensor

The human hand is a fine example of a design with a wide range of sensors that support different touch forms. It would be desirable to achieve an artificial design which can mimic the human hand. The standard guidelines for the creation of tactile sensors as presented by Dargahi and Najarian (2004), with consideration for limitations and possibilities of sensors, are summarized below: 

design

Notable Applications of Tactile Sensor

The development of robust, flexible and adaptable robots to study perception and safe interaction with environment including humans, has given a place of significance to the various kinds of tactile sensors in robotics. This has led to the continuous development of tactile sensor technology in different robotic platforms that strive to study/recreate perception ranging from fingertips to arms to torso. Some notable applications/works involving tactile sensors are listed below:

  • Use of robotic fingertips fitted with piezoelectric tactile sensors to recognize object properties like texture, shape and hardness while performing procedures like pushing, sliding, squeezing, etc.
  • Design of prosthetic hands with tactile sensors to mimic the natural motion and detect contact.
  • iCub is a fine example of a new humanoid which is equipped with tactile sensors on body surface like fingers, arms, torso, etc. to investigate perception and interaction.
iCub
iCub Robot; Image Source: Niccolò Caranti, ICub – Festival Economia 2018 2, CC BY-SA 4.0
  • PUMA robot, which is used for investigation of perception and control approaches, is fitted with a planar tactile sensor array to extract object edge and orientation. This varieties utilize tactile images and geometrical form. A related technique, focused on geometrical moments, was able to explore and identify the shape of different objects using a KUKA arm with planar tactile sensors.
  • Implementation of rolling and enclosing exploration procedures for robust object recognition has been carried out using tactile fingertips of five-fingered robotic hands.
  • Integration of tactile sensors into biomimetic robots to understand how tactile sensing works in animals. Demonstration of stimulus perception such as texture, contact distance, direction and speed by using whiskered robots.
  • Development of artificial antennae with pressure and force sensors to explore ants and cockroaches’ behaviour by modelling contact.
  • Use of tactile sensors in underwater robotics (artificial whiskers to mimic the perceptual capabilities of seals) to measure speed and understand fluid motion, angle and wake detection has been carried out.

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