Thermocouples are widely used in automotive industry for temperature testing at product design and EOL (End of Line) stages and for permanent monitoring afterwards. However, with the increasing voltage levels in Emobility thermocouples possess many risks in product design and testing stages. Some of them are safety related risk and have potential to be life threatening for employees (research and test engineers).
Thermocouples are based on the principles, that a small voltage (in milli-volts) gets produced when a junction (joint) of two dissimilar metal wires is cooled or heated. The generated voltage signal is measured at the other end of the metal wires and calibrated for the temperature range. During the temperature testing the junction end is put on the test object and voltage is being measured at the other end to estimate the temperature of test object.
Now imagine if the two metal wires are put on the 1000 V Battery or Motor for temperature testing and somehow the test engineers touch the wire by mistake. Even worse how accurate will be the milli-volt signal when it passes through an environment where there is high electric and magnetic field.
Without getting into more technicality let us take a closer look on the common reasons why thermocouples have failed in Emobility testing and should be replaced with intrinsically safe sensors.
Common reasons why thermocouples have failed
It is very obvious and common sensical thing to find out the safety risk of using thermocouples in electric vehicle testing, especially when test engineers have to adjust sensing location such as finding the right hot spot or reach to the measuring points that are not easily accessible (Inverters, Stator Windings, Battery Cooling duct). Thermocouples are subject to creating a short circuit and electrocution risks to the employees.
With the Emobility going towards 1000V and even higher 2500V (for commercial vehicles), thermocouples are highly susceptible to noise. The milli-volt signal requires lot of isolation under such large voltages and even then, the signal is not clear enough to measure accurate temperature.
3. Larger Size
Thermocouples are not suitable for Emobility applications that has very tiny space for putting sensors such as Charging Points, Invertor IGBTs, Battery Inter-cell temperature measurement etc. It has been observed that thermocouples are less responsive and inaccurate (up to 20 to 30 Deg Celsius) if they are not put directly on the charging points.
4. Non-Linearity Over The Range
Though the thermocouples are calibrated for a certain range, they still need complex compensation algorithm to maintain linearity over the range since they are being used at harsh conditions (High electric, chemical and magnetic fields) and different test environments.
5. Longer Response Time
Faster and accurate measurement is critical for Emobility during the performance, life cycle and abuse testing. Not only the accuracy, thermocouples are also limited by the response time requirements of Emobility applications such as detection of Thermal Runaway Issues, Charging Point Temperature, Stator Winding Temperature etc.
6. Poor Repeatability
Thermocouples are made of two dissimilar metallic wires and susceptive to material purity which varies from batch to batch and manufacturer to manufacturer. This variation creates calibration issues resulting into non-repeatability of accurate measurements. Further the chemical composition of metal changes with time especially if they are put into chemical environment like Batteries.
Solution of Fiber Optic Sensor
With the currently available technologies Fiber Optic Temperature sensors stand out clearly to be the most suitable sensors for Emobility applications at higher voltages (250V+). The major benefits of Fiber optic Temperature sensors are:
1. Safety: Fiber optics are safe – highest dielectric strength, ~1pC, tested up to 1500kV
2. Noise: Sensor are immune to electric, chemical and magnetic environments. Being used without any isolation, in applications that has 1500kV+ voltage, up to 25 Tesla magnetic field and chemical environment ranging from 0 to 14pH without any interference to the sensory readings.
3. Size: Ultra small sensors (Diameter of up to 0.4mm) to fit into tiniest spaces.
4. Linearity: Fiber optic sensors transmit light signals through glass, the purest form of silica. The sensors are linear and does not need any compensation and special algorithms.
5. Response Time: Fiber optic temperature system has a milliseconds response level. Response rate varies between 1ms to 200ms depending on the type of monitor selected for the application.
6. Repeatability: Fiber Optic temperature sensors are very stable and repeatable over the entire range without getting influenced by and external fields.