A 13 x 13 square millimetre tri-axial taxel is presented which is suitable for some medical applications, for instance in assistive robotics that involves contact with humans or in prosthetics. Finite Element Analysis is carried out to determine what structure is the best to obtain a uniform distribution of pressure on the sensing areas underneath the structure. This structure has been fabricated in plastic with a 3D printer and a commercial tactile sensor has been used to implement the sensing areas. A three axis linear motorized translation stage with a tri-axial precision force sensor is used to find the parameters of the linear regression model and characterize the proposed taxel. The results are analysed to see to what extent the goal has been reached in this specific implementation.
Tactile sensors have increasing presence in different applications, especially in assistive robotics or medicine
and rehabilitation. They are basically an array of force sensors (tactels) and they are intended to emulate the human skin.
Large sensors must be implemented with large area oriented technologies like screen printing. The authors have
proposed and made some piezoresistive sensors with this technology. They consist of a few layers of conductive tracks
to implement the electrodes and elastomers to insulate them, on a polymer substrate. Another conductive sheet is placed
atop the obtained structure. Pressure distribution in the interface between this conductive sheet and the electrodes has a
direct impact on the sensor performance. The mechanical behavior of the layered topology with conductive tracks,
elastomers and polymers must be studied. For instance, the authors have observed experimentally the existence of
pressure thresholds in the response of their sensors. Finite element simulations with COMSOL explain the reason for
such thresholds as well as the dependence of the pressure distribution profile on the properties of the materials and the
geometry of the tactel. This paper presents results from these simulations and the main conclusions that can be obtained
from them related to the design of the sensor.
This paper presents results from a few tactile sensors we have designed and fabricated. These sensors are based on a
common approach that consists of placing a sheet of piezoresistive material on the top of a set of electrodes. If a force is
exerted against the surface of the so obtained sensor, the contact area between the electrodes and the piezoresistive material
changes. Therefore, the resistance at the interface changes. This is exploited as transconduction principle to measure forces
and build advanced tactile sensors. For this purpose, we use a thin film of conductive polymers as the piezoresistive material.
Specifically, a conductive water-based ink of these polymers is deposited by spin coating on a flexible plastic sheet,
giving as a result a smooth, homogeneous and conducting thin film on it. The main interest in this procedure is it is cheap
and it allows the fabrication of flexible and low cost tactile sensors. In this work we present results from sensors made with
two technologies. First, we have used a Printed Circuit Board technology to fabricate the set of electrodes and addressing
tracks. Then we have placed the flexible plastic sheet with the conductive polymer film on them to obtain the sensor. The
result is a simple, flexible tactile sensor. In addition to these sensors on PCB, we have proposed, designed and fabricated
sensors with a screen printing technology. In this case, the set of electrodes and addressing tracks are made by printing an
ink based on silver nanoparticles. There is a very interesting difference with the other sensors, that consists of the use of an
elastomer as insulation material between conductive layers. Besides of its role as insulator, this elastomer allows the modification
of the force versus resistance relationship. It also improves the dynamic response of the sensor because it implements
a restoration force that helps the sensor to relax quicker when the force is taken off.
This paper presents a thermopneumatic actuator to build large tactile displays as well as a smart activation circuitry to study and improve its performance. Since the main drawback of large tactile screens in the market is their cost, this proposal is intended to reduce the price because of the simplicity of the actuator and the potential low cost assembling. A small display with 4 x 4 taxels and 2.54mm of distance between centres has been built to show the viability of the proposal. Furthermore, a smart actuation strategy is implemented where the heater element (a diode) is also used as sensor in a feedback control loop that improves the dynamic response. Such strategy consists in sensing the voltage drop in the diode to measure its temperature, thus it can be heated up quickly without being destroyed because power supply is decreased once the target temperature is reached. We have measured rise times around 2 seconds and fall times around 4 seconds, while the maximum force and stroke are above 10grams (0.1N) and 1mm respectively. The obtained results are good, specially to implement a large tactile screen. Power consumption is high, but it could be lower if latching mechanisms are used to keep the taxel active without power supply.