Sensing System

1.  Thermocouples
2.  RPM Sensor

Thermocouples

      To quantify the efficiency and effectiveness of the Lister engine pre-heating modifications, we need to know temperatures at various points in the oil pre-heating cycle.  Ideally, we would like to manipulate this data on a computer.  Thus, the signal from the thermocouple must be digitized by a data acquisition card.  The input to the DAQ card, however, must be volts.  Therefore, we are using thermocouples to sense the temperature.

      Thermocouples are temperature sensors that output volts when one end differs in temperature from the other.  A thermocouple is created by making connections between two dissimilar wires at both free ends.  If one connected end is heated, a current flows through both wires.  Then, if the loop is broken at a point different from the connected ends, an open circuit voltage is created, known as the Seebeck Voltage.  This voltage is proportional to the difference in temperatures between the two ends. 

      For the purposes of the engine, a pipe-plug probe from Omega.com proved rugged enough and easily suited for our needs.  Although it reads an “in-flow” temperature, it can be relocated easily if necessary.  Specifically, we used the TC-J-NPT-U-72 ungrounded, J-type, pipe-plug thermocouple.   The J-type Iron/Constantine probe provide the most accurate measurements, with a useful temperature range from 0°C to +750°C.  The output of a J-type thermocouple is 8 mV at 210°C and 33 mV at 600°C.
Three Problems:>

• The thermocouple by itself, however, only yields milliVolts, which are too small to be read by the data acquisition card.  The signal must be amplified. 
• Additionally, the millivolts only provide a temperature differential between both ends of the thermocouple.  Thus, if we set one end at 0°C then the voltage output will yield an actual temperature. 
• Lastly, noise in the system can easily disturb the temperature readings.  The signal must be filtered and conditioned to output useful and real values.

Thermocouple Block Diagram




      These problems are all solved by inputing the thermocouple signal into an AD594aq Monolithic Thermocouple Amplifier with Cold Junction Compensationfrom Analog.Com. The final thermocouple circuit is seen below:


Thermocouple Amplification and Cold Junction Circuit



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RPM Sensor

      The rotations per minute, RPM, of the engine are acquired using a self-contained DC-operated Retroreflective Mode Sensor from Banner Engineering A piece of retroreflective tape is applied to the flywheel.  Every time the tape passes the sensor, a signal is outputed in hertz [hz], which is read by the data acquisition card.

RPM Sensor



RPM sensor circuit


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