ECE 110/120 Honors Lab Section : Heated Gloves

Statement of Purpose

The purpose of this project is to create a set of self-heating gloves in order to provide a way in which one effortlessly maintains a consistent temperature of their hands without having to constantly switch between gloves that provide different levels of insulation.

Background Research

This project combines both the hardware and software sides of ECE, allowing the group to gain experience on a multifaceted and universal design. The topic of heat regulation and cooling is such an involved and theoretical discipline that it would be a mistake not to explore it. Heat is and always will be a prime focus of the electromechanical world—and in our project, we have decided to take our first step in the realm of self-heating. Especially in a cold location like Urbana-Champaign, we experience first hand the dangers of going out without proper apparel and a self-heating product would definitely mitigate the risks of frostbite and hypothermia.

Previous groups have implemented self-heating projects as well as projects related to gloves to accomplish a particular task. However, we wanted to create an integrated project where the self-heating aspect of this project was directly related to the glove as an article of clothing.

Heat is produced when electric current meets resistance. The heat is a loss of power over the circuit. Energy does not disappear, it changes from one state, or form, to another. The energy or power lost in a circuit becomes heat. The resistance produces thermal energy, felt as heat. Ie. temperature increases linearly with increased resistance. 

The resistance of a wire can be calculated by the following equation:

R = pLA  ; p = resistivity (datasheet of wire) ; L = Length of Wire ; A = Cross-section Area

The relationship between production energy and dissipating it as heat is described by Joule’s First Law. Joule’s law, in electricity, is a mathematical description of the rate at which resistance in a circuit converts electrical energy into heat energy. A quantitative form of Joule’s law is that the heat evolved per second, or the electric power loss, P, equals the current I squared times the resistance R, or P = I2R

Contrary to ideal electrical circuits with low resistance wires, because this project requires power loss high resistance wires must be used.

Heating elements must be able to withstand the heat that they are required to generate. The elements must also hold up to environmental factors, including moisture, which can cause corrosion. Heating wire has a high resistance and resists oxidation. It is able to withstand a high surface load. Other considerations that make these wires beneficial are its ability to resist sagging and deforming while being light in weight. 

In terms of which element to use there are various options. Nichrome 60, Kanthal A1, Kanthal D, are all wires that have various industrial applications. Another option is using Carbon Fiber (in the form of wire or tape). The main advantage of carbon fiber is resistance to repeated bending. Unlike a nichrome and kanthal wire heater, carbon fiber is very soft to the touch like a normal cloth and doesn't interfere with fabrics. The next best option would be Kanthal D, which is commonly used in households (not extreme heat dependent functions) in dishwashers.

In addition to the resistivity equation, we need to apply Newton’s Law of Cooling and the convection-conduction thermal resistor equations to increase the thermal conductivity of the glove; this will decrease the heat loss to the surrounding environment. Moreover, this will minimize the amount of current that needs to run through the wire and effectively increase the safety of the product.

Based on the equations above, we will work on creating a model or algorithm that takes in certain parameters, inputs, and controls to get the desired heating curve (with a big focus on the extreme temperatures). 

For the battery chemistry, we are going to be using two 18650 Lithium-Ion cells to power our product. Initially, we were researching the strengths and weaknesses of Lithium-Polymer (LiPo) and Lithium Iron Phosphate (LFP) to compare their energy densities, weight, and dimensions. The battery chemistry will remain lithium ion because of its higher energy density and the fact that we have more experience with it—and thus more proficiency in terms of soldering and spot welding the components together. There are some BMS options for a 2s1p configuration and the materials to create a small battery pack with a positive and negative terminal protected with heat shrink.

Discussing some at the cost analysis of the different battery chemistries, Li-Io has a higher energy density; it is also more popular and the industry standard, though it is starting to be replaced by LFP. LiPo on the other hand is more robust as it uses a gel-like electrolyte rather than a liquid and it is supposed to be safer (with a lower chance of thermal runaway), though it has a higher manufacturing cost and a shorter life cycle. Considering the dimensions, the 18650 cells are 18 mm x 65 mm cylinders while the LiPo cells are 30 mm x 35 mm rectangles, so it would actually be better volume wise to use LiPoThere are also some slight concerns with overheating issues for Li-ion batteries in extreme situations.

Why then did we choose Li-ion over LiPO? The obvious issue is that we could not find a simple way to connect two LiPo cells in series and solder a BMS to them; there are ways to connect the LiPo cells with solder or crimping techniques, but with the size of the wire, this is a challenge. The other issue is that we are creating a heating model based solely on running current through our wires, and if the battery capacity is not large enough, it will take more effort for the battery to continuously output the charge. As Li-ion has more options for higher current thresholds and usage, it ultimately comes down to the risk of being unable to produce the heat necessary to warm the glove.

It is necessary to control the amount of heat flow warming the glove, and this is accomplished by changing the resistance of the circuit. Since the glove is self-heating, the physical characteristics of the resistance network will be constrained using a microcontroller that integrates the hardware and software in the glove.

To implement this algorithm, we'll use the readings from each of the pins of the temperature sensor as values inputted into the C program written on the Arduino, and then add if-else statements to control resistivity using a potentiometer—this will regulate the amount of current flowing through the heating wires. The system will optimally be connected through small breadboards located in the glove. The input from each of the pins from the temperature sensor will be stored as floating-point numbers in the program. The specific program holding the C program will be dependent on the calculations implemented when calculating how much heat needs to be distributed throughout the glove on the heating wires. Regarding the reading of temperature from the pins of the sensor, we may need to add extension parts resulting from various complications stemming from the hardware of the sensor.

A possible algorithm to implement on the microcontroller is using PID theory. In this case, the process variable that is to be controlled is temperature, and the actuator output would be the resistance provided by the potentiometer. We can implement the algorithm set into place, which is quite common and is utilized as industry standard for multiple temperature control apparatuses. In our case, the set value would be the optimal internal temperature of the glove, the compensator would be the algorithm implemented to change the temperature, the actuator output would be the heating wires that would control heating, the system to heat would be the glove, and the process variable would be the internal temperature. Tests would have to be run in order to figure out how much changing the resistance would change the temperature of the glove. With these findings, we would find the optimal value of the proportional gain value that will change how much heat will be provided to decrease the summation of the error term. Based on the PID algorithm, which would calculate the difference between the actual internal temperature and the external temperature, we would also run tests with the glove to fully implement this proportional gain value. Based on how well the glove reacts to changes from changing the resistance in the potentiometer, we would need to change the derivative response to change the magnitude of the response.

Block Diagram / Flow Chart

System Overview

The apparatus that we will build in this project is a self-heating glove. Heating wires will be distributed throughout the glove, powered by a lithium-ion battery power source. The voltage and current, as well the internal temperature of the glove will serve as inputs to the microcontroller. Based on the inputs, the microcontroller would compute an output of how much to increase or decrease the resistance based on the difference of the temperature from the ideal temperature that was previously set. The potentiometer would control the resistance which would thus control the current flowing through the heating wires, and by extension, the internal temperature.

Parts (Temporary)

Possible Challenges

For this project, the big priority is safety as the glove temperature has to be moderated to a precise range or else the glove will either be too hot (posing a danger level to the wearer) or too cold (in which case the glove doesn’t fulfill its purpose). To get to this acceptable range, there will be a lot of testing: doing preliminary calculations on how the current and internal resistance of the wire affect the thermal flow.

Even before the testing stage, implementing a setup that allows for the optimal thermal release is going to be a challenge because there are spatial and weight restrictions that need to be correlated with the mitten dimensions. Furthermore, choosing the correct wire gauge, type, and plating and then scrutinizing whether the internal resistance of the wire can create the necessary heat will definitely be the key locus of this project—we might have to add additional thermistors to increase the resistance. The next obstacle with this part of the project is figuring out how to actually take apart the mitten to position all the hardware, and how to attach it to the glove.

Afterwards, creating the algorithm that takes the input from the temperature sensor and commands the microcontroller to control the discharge of the battery pack is the software component of this project. Since we are going to be basing the glove heating off of internal temperature, the algorithm will have to be coded to discern the small changes from the inputs and accurately alter the resistance to manipulate the current in the wire to modulate the heat flow.

With all of these elements to consider, there shouldn’t be a compromise on comfort or utility: ensuring the water-resistance of the design for operation in snowfall and limiting the bulk of the hardware—because the greater the weight, the more cumbersome it is for the user to wear the gloves— is a concern.

References

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