Experimental Analysis of Accuracy and Precision in Displacement Measurement Using Millimeter-Wave FMCW Radar

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript (ID: applsci-3476171) conducted experiments on the displacement measurement accuracy of millimeter wave FMCW radar. Millimeter wave radar is suitable for various industrial applications, and these radars must detect extremely small displacements at the sub micron scale.
1. The literature related to millimeter wave radar, sensors, sub micron scale detection, single-chip microwave integrated circuits, etc. in the past 5 years has not been fully read and discussed. Please update the references。
2. How to ensure the validity of data for phase estimation algorithms in displacement monitoring? The results here need to be compared with other results.
3. How is the validity of the data determined for Figure 9? What is the basis for determining the spacing of burst orders?
4. For Figure 8, the overall presentation is good. Is there a standard model as a reference for experimental debugging?

Author Response

Thanks a lot for your feedback and let me respond to the comments.

Comment 1. The literature related to millimeter wave radar, sensors, sub-micron scale detection, single-chip microwave integrated circuits, etc. in the past 5 years has not been fully read and discussed. Please update the references.

Response1. Understood and I will update the reference. By the way, do you concern either term of past researches (i.e., more than 5 years) or much wider research coverage within 5 years? Please let me know your point?Comment 2. How to ensure the validity of data for phase estimation algorithms in displacement monitoring? The results here need to be compared with other results.

Response 2. The equation 8 shows “Δφ” (phase) is composed by “d” (displacement) in accordance with popular radar signal processing technique. However there I didn’t mention the equation provide the displacement out of phase data. Do you mean luck of such words?

Comment 3. How is the validity of the data determined for Figure 9? What is the basis for determining the spacing of burst orders?

Response 3. The figure 9 shows the case of just one burst for clear view, but as you mentioned some burst data (e.g., 16 bursts) should be displayed for validity view point. I realized “burst order” and “chirp order” are not explained clearly comparing real waveform and I will update it.

Comment 4. For Figure 8, the overall presentation is good. Is there a standard model as a reference for experimental debugging?

Response 4. That’s critical point indeed and there is not standard model today, because measuring sub-micron order displacement is the best-in-class accuracy comparing other measurement technique like LiDAR. Thus it can be hard to be golden reference. Furthermore environmental vibration noise (e.g., from 50/60Hz power / fundamental structural vibration) is difficult to be cancelled completely. So the why I made loop-back-waveguide and it distinguished fundamental accuracy.

I will revise my manuscript in accordance with your feedback and please give me a period to do it.

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript deals with development and testing of a millimeter-wave FMCW radar sensor with the application of sub-micron displacement measurement. The Authors introduce a new measurement approach that combines phase extracting methods and regression analysis based calibration in order to get the minimal displacement measurement. The experimental testing involves two kinds of tests: a loopback waveguide test that has been performed with the intention of measuring the stability of the phase of the sensor and an industrial shaker test that has been performed with the intention of simulating real environment vibrations. The study has an application in fields such as condition-based maintenance and structural health monitoring that demand an increase in the use of non-contacting precise displacement measurements.
The manuscript is properly formatted and written; it has a good theoretical foundation with derivations of the phase-based displacement estimate and a description of experimental configurations. The results are explained with ease through figures and tables and the claimed accuracy will be a contribution toward the state of the art. The discussion of the intrinsic MMIC offset and gradient error and the proposed calibration technique through high-pass filtering are valuable supplements. The overall contribution of the work toward the state of the art of the field of radar-based displacement sensing, pushing the limits of the resolution of FMCW radar toward the micrometer scale, justifies the acceptance of the manuscript with a few minor areas that could be improved.

These comments relate to minor revisions:

• The Authors have to illustrate competing technologies, i.e. fully photonics (see, e.g., org/10.3389/fphy.2022.785650, doi.org/10.3389/fphy.2023.1215160, 10.23919/IRS57608.2023.10172457, https://doi.org/10.1364/OL.537710, https://doi.org/10.3390/mi14071296 ) with a performance comparison.

• Despite being generally well written, a more thorough check for grammar and typographical errors is suggested; for example, at line 47 there is a full stop followed by a comma.
• In Section 1, could be advantageous to give an overview of alternative contactless vibration measurement techniques (e.g., laser interferometry or FMCW ladar approaches) to better highlight advantages and limitations of the proposed system.
• In Section 2,  the Authors should provide more details on why the specific 77-81 GHz frequency range was chosen over other possible radar frequencies, and how this choice impacts the displacement measurement capabilities.
• The discussion of equation (8) would benefit from additional context about the fundamental physical limitations of displacement sensing using radar in the millimeter-wave band.
• In section 3, the Authors should elaborate on why the loopback waveguide approach provides a more accurate baseline than traditional calibration methods.
• For the industrial shaker experiment in section 3, additional details about how the alignment between the radar and shaker was maintained would improve reproducibility.
• In section 4, the discussion of MMIC instability during the first 192 chirps could be expanded to include the physical causes of this behavior and potential mitigation strategies.
• The statistical analysis methods used to determine the 3-sigma precision should be more clearly specified, including the underlying assumptions.

Author Response

Thanks a lot for your feedback and let me respond to the comments.

Comment 1: The Authors have to illustrate competing technologies, i.e. fully photonics (see, e.g., org/10.3389/fphy.2022.785650, doi.org/10.3389/fphy.2023.1215160, 10.23919/IRS57608.2023.10172457, https://doi.org/10.1364/OL.537710, https://doi.org/10.3390/mi14071296 ) with a performance comparison.

Response 1: Thanks your advice and I didn’t see photonic technology indeed. I will review those works and let me consider to add such comparison at introduction section.

Comment 2: Despite being generally well written, a more thorough check for grammar and typographical errors is suggested; for example, at line 47 there is a full stop followed by a comma.

Response 2: Understood and I will revise it.

Comment 3: In Section 1, could be advantageous to give an overview of alternative contactless vibration measurement techniques (e.g., laser interferometry or FMCW ladar approaches) to better highlight advantages and limitations of the proposed system.

Response 3：Understood and let me consider proper words to highlight the feature with radar.

Comment 4: In Section 2,  the Authors should provide more details on why the specific 77-81 GHz frequency range was chosen over other possible radar frequencies, and how this choice impacts the displacement measurement capabilities.

Response 4: I looks at feasibility for commercial use case and I adopter commercial MMIC that covers ISM band. A few past researches adopted extremely wider bandwidth as introduced in Table 1 for much better performance, but it will be never used for commercial products. Anyway I will mentioned the why in Section 2.

Comment 5: The discussion of equation (8) would benefit from additional context about the fundamental physical limitations of displacement sensing using radar in the millimeter-wave band.

Response 5: I got similar comment from another reviewer and I will add more stories about equation (8) because I just mentioned “Δφ” but not mention how it’s important for displacement measurement at the manuscript, which should be improved.

Comment 6: In section 3, the Authors should elaborate on why the loopback waveguide approach provides a more accurate baseline than traditional calibration methods.

Response 6: Understood I will add the benefit in Section 3.

Comment 7: For the industrial shaker experiment in section 3, additional details about how the alignment between the radar and shaker was maintained would improve reproducibility.

Response 7: Understood let me try to detail it. However any shaker test may be affected by environmental vibration noise , thus I made the experiment at industrial commercial lab that was mentioned at line 172 as ISO17025 compliance lab. Anyway I will mention such background and so on.

Comment 8: In section 4, the discussion of MMIC instability during the first 192 chirps could be expanded to include the physical causes of this behavior and potential mitigation strategies.

Response 8: Understood it and I assume that such instability is caused by MMIC temperature over continuous chirp TX signals. I will add more discussion and data (i.e., MMIC temperature data) to explain the instability.

Comment 9: The statistical analysis methods used to determine the 3-sigma precision should be more clearly specified, including the underlying assumptions.

Response 9: As mentioned above, I looks at feasibility for commercial use case and they will want always 3-sigma precision as well as other measurement equipment. I should add such background words why 3-sigma was adopted.

I will revise my manuscript in accordance with your feedback and please give me a period to do it.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The Authors have revised the manuscript. However, as reported in the previous mail, the Authors have to illustrate competing technologies, see, e.g. Chip-scaled Ka-band photonic linearly chirped microwave waveform generator. Frontiers in Physics, 10, 785650, 2022.

Author Response

Thanks you a lot for your feedback. I will look into the specific technique (i.e., Chip-scaled Ka-band photonic linearly chirped microwave waveform generator and likely others) much more, and try to improve the section.