Design approach for an advanced multi-channel pyrometer for bulk oven processes

. Industrial processes such as smelting and sintering require stable and precise temperature control of furnaces. To achieve this, accurate temperature measurements are required. Pyrometry allows for contactless measurement of bulk materials and is particularly suitable for high temperature applications. One of the main in ﬂ uences on the accuracy of pyrometric measurements is the knowledge of the emissivity in the spectral measurement range. To reduce this dependence, two-color pyrometers or multi-color pyrometers can be used. With this in mind, the Institute of Space Systems (IRS) is further developing their existing pyrometer technology by designing an advanced multi-channel pyrometer for bulk oven processes in a joint venture with Stange Elektronik GmbH and New Generation Kilns Grün GmbH. The design approach is explained here and the considered methods of achieving emissivity independent temperature measurements are examined.


Introduction
The Institute of Space Systems (IRS) at the University of Stuttgart has extensive experience in the field of pyrometry; particularily with the PYREX system on board the European Space Agency (ESA) mission EXPERT, which was designed to acquire data on the heat flux through a ceramic thermal protection system during atmospheric entry.[1] Further developing the IRS pyrometer technology, a new advanced multi-channel pyrometer is designed.The pyrometer shall measure temperatures of bulk materials in a range of 100 • C to 1750 • C where the measurement error shall not exceed 1-2 K. Six measurement channels shall be used.Additionaly, the system shall operate independent of the emissivity of the bulk material.These requirements are reflected in the various design aspects.

Optical Design
The signal path of each pyrometer channel consists of collimator optics, an optical fiber, a combination of interference filters and a photo diode.Of those components, the filters and diodes have the largest impact on the resulting signal strength and its wavelength dependence.It is decided that the diodes are the same for all channels, while the filters can be interchanged between channels.This interchangeability is necessary so that the wide range of temperatures that is required can be measured accurately.Also, it enables two-color-and multi-color pyrometry approaches for emissivity independent measurements.
To decide on the final combination of optical components, many different combinations must be tested in order to find an optimum and to fulfill the requirements.To save on prototyping costs, a simulation software is created.The simulation software, "PyroSim", is created using the Python language and the Qt5 graphical user interface library.It simulates the whole signal path from the radiation source through the optical components to the voltage output of the pyrometer.Component types are implemented as as independent modules and are all based on the same code.This makes it easy to add more components if required, without changing the main calculation logic.Specific components are saved as text-based .csv-tables.Components can easily be added to the software database using the data provided in their data sheets.The pyrometer is composed of one sensor head per channel and a base unit.The sensor heads are collimator optics to capture the thermal radiation of the measurement object and focus it into the optical fiber which connects the sensor heads to the base unit.The base unit houses all other optical components as well as the electronics (See Figure 2).The most prominent feature of the unit is the "filter-revolver".It is a disk with six holders for (multiple) interference filters which is mounted on the shaft of a stepper motor.It sits between the optical fibers coming in from the sensor heads and the photo diodes.The motor allows the disk to spin so that the filters can be interchanged between the different channels.

Electronic Design
There are four printed circuit boards inside the base unit: A sensor board, a sensor acquisition board, a micro controller board and a power supply board (see Figure 2).The sensor board houses the photo diodes.On the sensor acquisition board, the photo current of the diodes is converted into a voltage using different resistors.Depending on the photo current, different amplification circuits can be selected for each channel using relays.The micro controller board houses a ATSAMD51N19A-AU micro controller.The controller is fed the analog voltage output of the amplification circuit and calculates a temperature based on them.Lastly, the power supply boards supplies power to all other boards as well as the stepper motor.

Emissivity Independent Temperature Determination
Emissivity independent temperature determination can be achieved using two-color-or multi-color pyrometry approaches.In two-color or ratio pyrometry, it is assumed that the emissivity of the measured material is constant and independent of the wavelength.This assumption is not valid in general and may lead to large errors [2].Thus, a multi-color approach can be a better choice.Here, a polynomial dependence of the emissivity on the wavelength is assumed.The different measurements at different wavelengths or "colors" are matched to a temperature using different fitting methods.This approach is much better suited for the task at hand, as the measurement accuracy for multiple materials with varying emissivities is better [2].

System Test And Calibration
Finally, the whole system has to be tested and calibrated.
A source of thermal radiation as well as a reference thermometer are required.For the source, a cavity radiator that approximates a black body radiator with an effective emmissivity ϵ BB > 0.99 is used.For the reference, the LP3 linear pyrometer developed by KE Technology GmbH.The LP3 is accurate, but emissivity dependent.This setup, in conjunction with pyrometer prototypes, is used to validate the simulation software and to create a calibration curve.

Figure 2 .
Figure 2. Scheme of the pyrometer system design