Requirements and Operating DetailsThe system has to monitor and record data from multiple sensors. These sensors are K-Type thermocouples. Each thermocouple has an operating range of approximately 0°C to 1300°C, which corresponds to an output voltage of 0-55 mV. The low voltage inputs from the thermocouples has to be readable by the system.
Additionally, the system has to be designed to be expandable later and support a large number of probes (and different types of probes in the future that may have different operating voltages).
For more information, see Problem Definition.
Design ChoicesBecause of the small voltage output from the thermocouples there are really only two feasible options of getting the voltage level. The first is amplifying the signal to a larger value while the second is to use a small reference voltage. We do the former because the small reference voltage requires additional components and is not supportive of future additions to the system. That is, using a higher reference voltage allows us to more easily use probes and other sensors that have different voltage characteristics in the future.
A Raspberry Pi was chosen to be the core of the system, as it is widely supported and is Debian-based, so it has a large range of supported software and packages (including Python). Additionally the Raspberry Pi provides easy input/output and support for standard monitors, which allows us to have a touchscreen and keyboard for the system.
For the ADC the MCP3208 was used because the SPI interface is compatible with a Raspberry Pi and it has been documented working. Additionally the MCP3208 provides 12-bit resolution, which allows much better precision over smaller resolution ADCs.
To amplify the signal we use an op-amp in a non-inverting amplifier configuration,which gives an amplification that depends on the resistor values, and is given below.
Because we want the range to be mapped from 0-55 mV to 0-3.3V, we choose a gain of 57. To get this output voltage of 3.3V, standard resistors were used. These resistors gave an easy to achieve gain which was close to our goal.
Based on testing results it was noticed that if one of the inputs is grounded, the other channels perform much more consistently at the high-end voltages. This is likely because of capacitances in the system causing higher voltages than an ideal circuit would create, but further investigation is required to find the actual cause. Although not the most ideal solution, this allows for the circuit to perform much better while still leaving 14 total inputs. As testing showed, with this change in place and calibration performed, all 14 inputs respond correctly to inputs in the range of 0-55mV.
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