This is the third in a series on the basics of haptic technology and perception. This installment focuses on thermal feedback. Subscribe to our newsletter to get these delivered straight to your inbox.
Thermal feedback: more than just hot and cold
Most of us understand that thermal feedback is what lets you know that an ice cube is cold and a flame is warm; however, thermal feedback is more than just temperature.
Here’s an example: when you first step into the car on a winter morning, the metal door handle feels a whole lot colder than the cloth seat. Yet, if you measured the temperature of the metal parts of the car and the cloth parts of the car with a thermometer, you would probably find that they are about the same temperature. So why does the cloth only feel slightly cool, while the metal feels cold enough to make you want to quit your job and move to Hawaii?
The key is that your skin can sense not just absolute temperature, but also the flow of heat energy. This is called heat flux. Heat flux depends not just on the temperature of an object, but also on other thermal properties like its thermal conductivity (how well it conducts heat).
Returning to our car example, the metal is much more thermally conductive than the cloth — perhaps by 1000x or more. Thus, the cold metal pulls heat out of your skin more quickly than the cloth. This effect is what makes the car handle feel much colder than the seat. The same effect applies to hot objects. A hot oven mitt is perfectly safe to touch, but a metal pan at the same temperature will give you a severe burn because heat energy flows from the metal into your hand much more quickly.
There are three different ways for heat to move around: conduction, convection, and radiation. All three of these modes of heat transfer must be considered for realistic thermal feedback.
Conduction is the transfer of heat between two objects in direct contact with each other. Conduction is the most important mode of heat transfer for thermal feedback because it occurs any time you touch a virtual object that is hotter or colder than your body temperature.
Convection is the transfer of heat through the movement of fluids. Convection is important for modelling any interaction with virtual fluids — the wind blowing across your skin, or water flowing over your hand.
Lastly, radiation is the transfer of heat through electromagnetic radiation. Radiation is most important for modelling very hot objects, like a fire. A fire radiates enough energy that you can feel it from a distance, long before you get close enough to feel convection or conduction.
Feeling the burn with thermal feedback devices
Like the tactile and vibrotactile devices I discussed in the last two posts, thermal feedback devices ultimately come down to a grid of actuators in contact with the skin. In this case, instead of producing physical movement, these actuators move heat around.
In some ways, thermal feedback devices are easier to build than tactile or vibrotactile feedback devices. While tactile feedback devices require their actuators to be very close together and vibrotactile feedback devices require their actuators to be very fast, thermal feedback devices require neither. Like vibration stimuli, humans are bad at determining the location of temperature stimuli, so relatively few thermal actuators are needed for a given area of skin.
There is one big catch with thermal feedback devices. In accordance with the laws of physics, they can’t simply create heat out of thin air. They can only move heat around. For example, to simulate a cold object, a thermal actuator would pull heat out of your skin. The excess heat must go somewhere besides the actuator or your skin, otherwise everything would just end up the same temperature eventually. The result is that a lot of heat energy needs to be moved around quickly to create convincing sensations of hot and cold. This takes a lot more power than moving around small actuators to create tactile and vibrotactile feedback.
HaptX, our unique haptic textile, uses patent-pending microfluidic technology to produce high quality thermal feedback. We heat up and cool down water in small reservoirs, then carefully control the flow of the water through tiny channels in HaptX to create sensations of hot and cold. Think of the water in the reservoirs like a thermal battery that stores heat energy. As the water flows through the channels in HaptX it exchanges this heat energy efficiently with the wearer’s skin. The water then flows back into the reservoirs where it is re-heated or -cooled. The water is hot enough to convincingly simulate hot objects, but not nearly hot enough to cause burns.
This design allows HaptX to produce convincing thermal effects without requiring too much energy. Paired with sophisticated models of heat transfer in our SDK, HaptX can accurately simulate sensations from the warmth of a fire to the feel of a car door handle on a winter day.
In the next post, we’ll talk about force feedback. May the force be with you!