Thermal conductivity and diffusivity of an object/material, rather than the absolute temperature, has often been utilized as a means of tactile sensing.

One of the most common issues associated with thermal sensing, has been the long time response of most thermal sensors, which makes the tactile sensing feel less life-like and can cause delays and more complicated problems. While miniature temperature sensors measuring the object surface temperature with short time delays are commercially available, tiny sensors determining heat conductivity of a material are still lacking [Kron03].

Previous thermal sensing units have often included the use of a reference temperature that is always above or below any subject temperature [Caldwell92, Monkman93]. The drawbacks of this is that if the device encounters any materials existing at the same temperature of the sensor, it will not be detected.

The thermal conductivity of a material can also be difficult for a sensor to indicate when the surface texture varies. The gaps between the sensor and material are filled with air, which can cause inaccurate readings from the sensor.

Another issue that has been noted with thermal sensors, is the inability for thermal sensors to discriminate between two similarly thermal conductive materials, such as steel and aluminum. While users will be able to detect the difference between a block of wood and a block of steel, it is very difficult for them to differentiate between a block of steel and a block of aluminum using thermal sensing alone [Jones03]. It is believed that in combination with pressure/force sensors and slip sensors, a user may be able to distinguish between such materials.

Subjects may respond more to variations in heat capacity than thermal conductivity when discriminating between materials. This later hypothesis can only be verified by using materials that span a greater range of heat capacities [Jones03]

Human Thermal Sensor Characteristics [Caldwell96]

The basic behaviour of thermal sensors can be summarised as:

1. The surface of the finger is at about 32°C under 'normal' conditions but can vary over a large range.
2. The reaction time for cold sensations for a temperature drop of greater than 0.1 deg C/sec, is 0.3-0.5 sec.
3. The reaction time for hot sensations with a temperature rise of greater than O.1 deg C/sec is 0.5-0.9 sec.
4. Thermoreceptors can sense rates of change of temperature as small as 0.01 "C/sec (0.6"C/min), i.e. relative temperature sensing is accurately measured.
5. For small areas of skin the temperature range that skin can adapt to is from 2OoC-40"C i.e. most of the absolute range is adapted to, but it cannot be accurately gauged.
6. Below 20°C there is a constant cold sensation (full adaption does not occur) which gives way to cold pain below 3°C.
7. Above 40°C there is a constant hot sensation that gives way to burning sensationipain above 48OC.
8. Measurements have shown that the rate of heat penetration through the cutaneous tissues is in the range 0.5 to 1.0 mm / sec depending on vascular conditions at the time.

This places the cold sense receptor at about 0.15mm below the skin surface, and the warm receptor at about 0.3mm beneath the surface

In his thesis, Steven Lawther, from the University of Salford, states that while there has been research to develop thermal sensing technology, not many researchers have known how to use the thermal information and use it for a tactile display.

ThermalSensorTechnology

References

• D.G.Caldwell & C Gosney, Enhanced Tactile Feedback (Tele-taction) using a Multi-Functional sensory System, ICRA (1) 1993: 955-960:
• G.J Monkman, P.M Taylor, Thermal Tactile Sensing, in IEEE Trans. Rob. Automat, vol 9, no 3, June 1993,313-318:
• D. Siegel, I. Garabieta, J.M Hollerbach, An integrated tactile and thermal sensor, in Proc. IEEE Int. Conf. Robotics:
• Kron, A.: Schmidt, G.,Multi-fingered Tactile Feedback from Virtual and Remote Environments, Haptic Interfaces for Virtual Environment and Teleoperator Systems, 2003. HAPTICS 2003. Proceedings. 11th Symposium, 22-23 March,16- 23
• T. Someya: Conformable, flexible, large-area networks of pressure and thermal sensors with organic transistor active matrixes, Published online before print August 17, 2005, Proceedings of the National Academy of Sciences of the USA
• D. Taddeucci: An Approach to Integrated Tactile Perception, Proceedings of the 1997 IEEE Intemational Conference on Robotics and Automation, Albuquerque, New Mexico - April 1997
• Yamamoto, A.: Control of Thermal Tactile Display Based on Prediction of Contact Temperature, Robotics and Automation, 2004. Proceedings. ICRA '04. 2004 IEEE International, 1536- 1541 Vol.2
• Russell, R A, Processing data from a robot thermal sensor. 1985 Conference on Computers and Engineering; Hobart (Australia); 25-27 Sept. 1985. pp. 123-125. This paper describes a new tactile sensor designed to provide information about the material constitution of objects held by a robot manipulator. The sensor is modelled on the thermal touch sense which enables humans to distinguish between different materials based on how cold or warm they feel. Some results are presented and methods of analysing the sensor data are discussed. Could not find an online PDF
• Russell, R A, Thermal sensor array to provide tactile feedback for robots. International Journal of Robotics Research. Vol. 4, no. 3, pp. 35-39. 1985 This paper describes a new type of tactile sensor array designed to provide sensory feedback for a robot manipulator system. It is modeled on the thermal touch sense, which enables humans to distinguish between different materials based on how "cold" or "warm" they feel. The essential parts of the sensor are a heat source and a thin layer of silicone rubber with a 2 X 10 array of miniature thermistors embedded in its surface. Some results are presented that domonstrate the ability of thermal sensors to produce images of touched objects and to discriminate the different materials used in their construction.

• Jones, L: Material Discrimination and Thermal Perception, Proceedings of the11th Symposium on Haptic Interfaces for Virtual Environment and Teleoperator Systems (HAPTICS’03)

• Benali_Khoudja.pdf:

• Caldwell96.pdf: