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SafeTouch Temperature Sensor

Olivier Musca, Rachel Andersen, and Matthew Tessier

The goal of our project is to develop a heat sensing device which could be implemented in an artificial limb to alert the user when their prosthetic is in contact with a heat source capable of damaging his or her prosthetic. The purpose of wearing prosthetic limb is to allow the user to have the independence that may not have due to missing limbs. Such independence is exemplified in everyday tasks such as cooking meals. A task like this introduces opportunities for damage to the arm from high temperature sources such as stove tops, ovens and any other cooking equipment.

By nature the robotics in the latest prosthetic technology, these devices can be highly sensitive to heat, as well was very expensive to repair or replace, Since the wearer of a prosthetic cannot feel heat in the absent extremity, it becomes essential to provide a warning system when critical temperatures have been reached.

While we are emphasizing the safety of the prosthetic, it is important, it is important to realize that damage to the prosthetic, although undesirable, is preferred to injury to the person. This quality suggests that the warning system itself should never place the wearer's person in danger, which translates to a passive warning system., such as a buzzer or a light, as opposed to a physical motion of the arm moving away from the heat source. This is why SafeTouch chose to use both the buzzer and the LED alerts.

Many designs for heat sensors include several components which, in the space allotted by a prosthetic, seem large and unnecessary. Safetouch has simplified the design for cost effectiveness and to adhere to spatial constraints. Our final design does away with potential problems inherent in the potentiometer and the bulkiness of the relay. Vibrations and other movement that prosthetics are more than likely to encounter could cause the screw adjustment of  the potentiometer to turn in time and throw off the calibration and therefore the sensitivity of the sensor. Furthermore, it is undesirable to have an inexperienced user self-adjusting the temperature threshold as this setting should be as specified by the prosthetic manufacturer.

The temperature sensitive component in the SafeTouch system is the thermistor. The thermistor is essentially  a resistor whose resistance decreases with increasing temperature.

An operational amplifier is used as a comparator. Using resistors and the thermistor as voltage dividers, the voltage input from the positive side will exceed the input of the negative side of the amplifier when the resistance of the thermistor reaches a specified temperature. When this happens, the output voltage of the amplifier will jump from close to zero to close to the power supply voltage, allowing enough voltage for the LED and buzzer to turn on. In other words, when the thermistor is heated to a critical temperature, the alert will be set off.

The SafeTouch system is a simple and cost effect way to protect costly prosthetics from damage due to extreme heat. This project was done in the hopes of helping those whose independence relies on the use of prosthetic limbs.