Microwave power calorimeter load and overall structure design

1 Introduction

The calorimeter is currently the most accurate method for measuring power. In the design of the calorimeter, the temperature stability of the overall structure is a necessary and sufficient condition for accurate testing. However, within the frequency range below 40 GHz, due to the large volume of the waveguide, the overall volume of the calorimeter increases and the heat capacity increases. In order to ensure the transmission and thermal insulation effect of the waveguide transmission line, a multi-waveguide connection method is adopted, which increases the complexity of the structure and creates certain difficulties for the accuracy of temperature control. Based on the design and simulation of the calorimeter's heat body and the overall thermodynamic structure, this paper adopts fuzzy PID temperature control technology to achieve a technical indicator of a temperature control accuracy of 5% degrees Celsius, which has certain practical significance.

2 Composition and principle of calorimeter:

Calorimeters used as power standards usually adopt a double-load structure. As shown in Figure 5, in a double-load calorimeter, two calorimeters A and B with exactly the same thermal conditions are placed in an insulated container. Among them, calorimeter A is used to add the high-frequency power or DC substitution power to be measured, and the other calorimeter B does not add any power and is only used as a temperature reference for calorimeter A. When a constant power is added to calorimeter A, a constant temperature difference will be generated between calorimeters A and B. The thermopile installed between calorimeters A and B will detect the temperature difference electromotive force. Assuming that high-frequency power and DC power have the same thermal effect on the same calorimeter, the high-frequency power can be measured by the high-frequency-DC substitution method.

Figure 1 Calorimeter model

3 Structural design and thermodynamic simulation of loads

3.1 Theoretical analysis of load

The low-power waveguide matching load consists of a short-circuited waveguide and an absorber installed in the waveguide. In order to obtain greater attenuation, the absorber material is required to have low electrical conductivity and high magnetic permeability. In terms of thermal properties, since the absorber material is mostly composed of insulating materials, its heat transfer characteristics are much smaller than those of metal materials. The volume, surface area, and roughness of the material processing of the absorber will affect the absorption of microwave energy and the heat transfer between the absorber and the waveguide wall. Therefore, the design of the calorimeter load has very high requirements for the shape, material, surface roughness, and processing accuracy of the load absorber.

From the working principle of the calorimeter in the previous section, we can know that the waveguide load mainly plays the role of a calorimeter in the calorimeter, that is, it is required that when it absorbs most of the microwave power, the temperature rise of the entire load has a certain quantitative relationship with the load temperature rise under DC heating, and the microwave-DC power substitution efficiency of the waveguide load is required to be as high as possible. Therefore, in the design, not only the microwave characteristics of the load should be considered, but also the thermal characteristics of the load.

According to the basic working principle of the calorimeter, we know that the heat transfer process on the microwave load is:

1) During the power absorption process, the heat transfer process is:

Figure 2 Heat transfer process of absorbing microwave power

2) During the working process of the heating resistor, the heat transfer process is:

Figure 3. Heat transfer process of absorbing DC power

According to the above analysis of the heat transfer process, in the whole process, the main heat exchanges are conduction heat exchange and radiation heat exchange. Under the condition of air convection caused by gravity, the total heat flux is equal to the sum of conduction heat flux and radiation heat flux.

3.2 Sheet absorber load design and thermodynamic simulation

This structure has a wedge-shaped outer wall of the waveguide and a single-sided inner surface covered with a silicon carbide absorber. The thermal performance of this structure is higher than the first one because the absorber material is less than the first form and the overall thickness is small. The microwave energy absorbed by it will be transmitted to the outer wall of the waveguide in a very short time, and its equilibrium time is shortened. In addition, the DC heating source and the absorber material are attached to the same side of the wide wall of the waveguide. The heating efficiency of the DC source and the microwave source is very high, which can reach more than 90% as mentioned in the technical literature consulted. However, this type of structural load has very high requirements on processing accuracy and also has very high hardness and strength requirements on the absorber material.

The following are the load structure, electromagnetic field and thermal simulation results for this idea:

(1) Electromagnetic field simulation analysis

The load structure is shown in Figure 4:

Figure 4 8mm waveguide load design

The simulation standing wave results are shown in Figure 5:

Figure 5 8mm load standing wave simulation diagram

(2) Thermal simulation of waveguide load is as follows:

The simulation environment is that the absorber cone evenly distributes 10mW power. The temperature distribution of the entire 8mm waveguide load in the stable state shows that the temperature at the highest position is about 0.14 degrees Celsius higher than the corresponding position without adding power.

The surface temperature distribution when the ambient temperature is 35 degrees Celsius and the load does not add any power is shown in Figure 6:

Figure 6 Load temperature distribution under constant temperature conditions

The ambient temperature is 35 degrees Celsius, and 10mW power is evenly distributed on the load surface. The temperature distribution on the load surface is shown in Figure 7:

Figure 7 Load temperature distribution under heating conditions

3.3 Sheet absorber load test results

Figures 8 and 9 are the load processing diagram and standing wave test diagram. The standing waves are all below 1.12, and the standing waves in the high-frequency part are better than those in the low-frequency part. The microwave characteristics meet the use requirements. When the heating power is 10mW, the temperature rise is about 0.1K, and the DC and microwave substitution efficiency can reach more than 80%. The load design meets the overall needs of the calorimeter.

Figure 8 Actual processing diagram of 8mm load

Figure 9 8mm load measured standing wave results

4 Structural design and thermodynamic simulation of calorimeter

4.1 Structural design of calorimeter

Since the calorimeter method for measuring microwave power is based on the DC power replacing the microwave power through the thermal effect on the load, this measurement method places very high requirements on the temperature stability of the overall structure of the calorimeter. In the preliminary design of the calorimeter structure, we adopted a double-load calorimeter structure. In terms of temperature control accuracy requirements, the overall structure adopts a three-layer barrel structure, with a thin-walled aluminum cylinder as the outer layer, a large-mass and large-heat-capacity copper temperature control barrel in the middle, and a copper inner barrel as the inner layer. The surface of the copper temperature control barrel is affixed with a heating resistor film, and the temperature control device controls the size of the heating power through the temperature change of the temperature measuring resistor affixed to the surface of the barrel to achieve a surface temperature stability of less than 0.05 degrees Celsius. The temperature control device controls the size of the heating power by the change between the thin-walled outer aluminum barrel and the outer copper temperature control barrel, and fills with foam plastic to reduce the overall heat exchange with the outside world.

The power input port uses a stainless steel thin-wall waveguide. The main purpose of this design is to reduce the heat loss of the entire waveguide transmission line. A plastic heat-insulating waveguide section is used between the inner and outer copper barrels. The waveguide connection surface and the inner wall of the waveguide cavity are gold-plated. The reason for this is that the heat capacity transmission line has a large mass, while the stainless steel thin-wall waveguide has a low load-bearing capacity and is easily damaged. A stainless steel thin-wall waveguide heat-insulating transmission line is used between the load and the inner barrel cover heat capacity transmission line and the outer barrel cover heat capacity transmission line to ensure that the heat exchange of the inner barrel is not affected by the environment and meets the requirements of accurate temperature measurement.

The structure of the microwave transmission channel is shown in FIG10 .

Figure 10 Schematic diagram of microwave signal transmission channel

4.2 Thermodynamic analysis of the calorimeter structure.

The thermodynamic structure simulation results of the inner and outer copper barrels of the calorimeter are shown in Figure 11:

Figure 11 Thermal simulation diagram of temperature control barrel

The simulation environment of the thermodynamic simulation is as follows: the surface temperature of the copper outer barrel is kept at 35 degrees Celsius. From the thermodynamic distribution diagram, it can be seen that the temperature inside the inner barrel is relatively stable. There is a slight temperature loss between the heat capacity transmission line and the non-metallic insulation waveguide section, but it does not have a great impact on the overall temperature stability. Therefore, from the simulation structure, the thermodynamic structure design of the calorimeter basically meets the requirements.

4.3 Research on temperature control technology of calorimeter.

In the design, considering the temperature control requirements, environment and cost, we determined the basic scheme of using foam plastic as the outer layer filling, multi-layer barrel structure and outer surface temperature control. Fuzzy PID control technology was used in the temperature control technology.

(1) Introduction to fuzzy PID temperature control technology

In the calorimeter temperature control system, we use a fuzzy PID control system. The control system structure is shown in Figure 12:

Figure 12 Temperature control system structure diagram

The control law of the PID temperature controller is:

Since the fuzzy controller is implemented in the system, the calculation of the defuzzifier is required to be relatively simple and will not be discussed here.

(2) Actual temperature control test results

The temperature control data collection time is set to 10 hours, starting from the initial heating, and the target temperature is 22.000 degrees Celsius. Figures 13 and 14 are temperature control data graphs. It can be seen from the figures that it takes 5-6 hours from the start of heating to the temperature being basically stable, and it takes about 8 hours for the temperature change to be less than 5 thousandths of a degree Celsius. From the data in Figures 13 and 14, the temperature control indicators are basically up to standard, meeting the requirements for further calibration experiments on the system.

Figure 13 Temperature control data diagram (the entire temperature control process is 10 hours)

Figure 14 Temperature control data diagram (temperature is basically stable)

5 Conclusion

From the above analysis, the load and the overall constant temperature structure of the calorimeter designed in this paper basically meet the project requirements. However, the design of the calorimeter is a complex and iterative process. After the conclusions obtained through numerical calculations and software simulations are verified by experiments, further optimization design is needed to obtain more accurate test data to meet the calorimeter's accurate requirements for experimental results.