An Adaptive Magnetorheological Fluid-Based Robotic Claw with an Electro-Permanent Magnet Array

Round 1

Reviewer 1 Report

The authors reported the holding performance of an adaptive magnetorheological fluid (MRF) based robotic claw operated with the electro-permanent magnet (EPM) array was addressed. Their contact stiffnesses could be changed by activating the EPM array in three different operation modes. Three different diameter objects were tested as target objects, and the dynamic and static holding force, the dynamic and static holding coefficients, and the controllable dynamic and static holding forces were determined to evaluate the holding performance of the MRF-based robotic claw. By using the concepts of holding force and holding force density, the holding performance of EPM array and traditional electromagnetic (EM) the MRF-based robotic claw is compared. The results show that EPM array MRF-based robotic claw was very feasible to grasp a wider range of objects and could produce a better specific holding force. It seems to be a comprehensive and worth-learning work. Therefore, I recommend its publication after a minor revision and explanation. 

1)     What causes the fluctuation of magnetic density magnitude in Figure 3?

2)     The two images in the upper part of Figure 8 are somewhat unclear and can be replaced with clearer images.

Author Response

actuators-2738419

Title: An Adaptive Magnetorheological Fluid-Based Robotic Claw with An Electro-Permanent Magnet

First, we would like to thank Reviewer #1 for your valuable comments on our manuscript. We sincerely answered Reviewer #1’s comments below each question. Our answers are presented in blue-colored italic text.

Reviewer #1
Comments to the Author (black-colored text)

1) What causes the fluctuation of magnetic density magnitude in Figure 3?

Magnetic density measurements in Figure 3 were obtained from several electro-permanent magnet (EPM) positions within the EPM array. Each EPM featured an AlNiCo magnetic core with a coil winding. However, the magnetic properties of the AlNiCo magnetic cores and the coil windings in the EPM array were not perfectly the same in practical conditions. These disparities can give rise to fluctuations in the magnetic density magnitude in Figure 3. This aspect has been explicitly elucidated in the revised manuscript.

2) The two images in the upper part of Figure 8 are somewhat unclear and can be replaced with clearer images.

According to the reviewer’s comment, the two images in the upper part of Figure 8 were replaced with clearer images in the revised manuscript.

We hope that the reviewer will agree that we have made an appropriate revision to this manuscript and that it is now worthy of publication.

Reviewer 2 Report

The manuscript of Choi et al. describes the design, construction, and characterization of an adaptive robotic gripper based on magnetorheological fluid (MRF) and magnetorheological elastomer (MRE). The main novelty of the work is the use of electro-permanent magnets (instead of classical electromagnets) for tuning of grippers' mechanical properties. The investigations are well-described and clearly presented. Nevertheless, besides a demonstration that such a system indeed functions and generates gripping forces in the tens of newtons range, the presented results are very specific for the selected design and materials used for the gripper as well as for the selected testing objects. In particular, the use of testing objects with different shapes, compliance, or surface properties such as roughness and adhesion would definitely strongly affect the measurement results. Therefore, the general value of the reported findings is quite low.

In addition to this, I have the following specific suggestions:

1)      It is very unusual to “promote/advertise” a specific measurement instrument (testing machine from Instron) in the abstract of a research paper.

2)      The description of the work principle of EPMs is unnecessary. Nowadays, these are commercial devices. One can find extensive explanations of them in Wikipedia (https://en.wikipedia.org/wiki/Electropermanent_magnet) . Also, a description of ferromagnetic hysteresis is unnecessary, as this is basic textbook physics.

3)      Fig. 2a is not clear. There are two different descriptions of the big grey rectangle area. One would appreciate knowing what the size of a single EMP was, and how exactly they were spatially positioned and tuned. What exactly was the I(t) dependence of the switching electric pulses, and whether all 16 EMPs were used in the same state (off, LR, SR), or maybe some combinations were used? The present text says that …high-frequency current pulse in the kHz range was used… – which is completely qualitative, and is also not clear if this means AC or DC current…

4)      Fig 3 – it is not clear which four locations of the magnetic pole were tested. And also not why only 4 and not all 16 were probed.

5)      Error bars are missing in all the graphs shown in the manuscript.

6)      In Figures 4 and 6 schematic drawings of the setups would be welcome because it is difficult to resolve the details in the photographs where many black parts appear.

7)      In Fig. 5 please describe in what steps the displacements changed.

8)      In Fig 6 and the corresponding measurements, I wonder why the cylindrical object was not oriented vertically.

9)      The info on the masses of 3 different testing cylinders is missing.

10)   The comparison given in Fig 11 is very artificial. It is not clear why in one case, the Mass and the other the volume was considered. It is also not clear why for the EPM-driven gripper the M/V of the EPM array was also taken into account, and why for the EM-driven gripper this was not taken into account. What is the value of such a comparison, and what does the claim that one of those is “better” than the other mean? Isn’t it logical that EM-driven grippers are the only choice in case of dynamical gripping needs, while EPM-driven grippers are more energetically efficient when an on-off operation is needed?

x

Author Response

Author's Response Manuscript ID:

actuators-2738419

Title: An Adaptive Magnetorheological Fluid-Based Robotic Claw with An Electro-Permanent Magnet

Array

First, we would like to thank Reviewer #1 for your valuable comments on our manuscript. We sincerely answered Reviewer #1’s comments below each question. Our answers are presented in blue-colored italic text.

Reviewer #2
Comments to the Author (black-colored text)

1) It is very unusual to “promote/advertise” a specific measurement instrument (testing machine from Instron) in the abstract of a research paper.

According to the reviewer’s comment, “the Instron material testing machine” in the abstract was

replaced with “a benchtop material testing machine” in the abstract of the revised manuscript.

2) The description of the work principle of EPMs is unnecessary. Nowadays, these are commercial devices. One can find extensive explanations of them in Wikipedia (https://en.wikipedia.org/wiki/Electropermanent_magnet). Also, a description of ferromagnetic hysteresis is unnecessary, as this is basic textbook physics.

To achieve three distinct operating modes—passive mode, short-range (SR) mode, and long-range (LR) mode—the construction of each EPM in this study involved using an AlNiCo magnetic core and a coil winding, intentionally excluding the use of permanent magnets that typically initiate a biased magnetic density. Consequently, each EPM within the EPM array in this study was designed to generate zero magnetic field, north-pole magnetic field, and south-pole magnetic field upon receiving an applied current input. This design deviates slightly from conventional EPMs in the field, which are typically engineered for each pole to produce bipolar magnetic fields (i.e., both north-pole and south- pole magnetic fields). In conventional EPMs, a zero magnetic field involves the utilization of permanent magnets. From Figure 1, we want to illustrate the process of achieving zero magnetic field and a north-pole (or south-pole) magnetic field from each EPM of the EPM array used in this study. Notably, the revised manuscript retains the explanatory content of Figure 1, aiming to elucidate the working principle of EPMs used in this study, including the corresponding magnetic hysteresis (B-H) plot. We believe this clarification in Figure 1 will be beneficial for readers and have therefore retained it in the revised manuscript.

3) Fig. 2a is not clear. There are two different descriptions of the big grey rectangle area. One would

appreciate knowing what the size of a single EMP was, and how exactly they were spatially positioned

and tuned. What exactly was the I(t) dependence of the switching electric pulses, and whether all 16

EMPs were used in the same state (off, LR, SR), or maybe some combinations were used? The present

text says that ...high-frequency current pulse in the kHz range was used... – which is completely

qualitative, and is also not clear if this means AC or DC current...

The EPM array employed in this study is currently undergoing the process of securing a US patent. Consequently, specific details regarding the size and shape of each EPM, as well as the explicit parameters of the applied current pulse, cannot be disclosed at this stage. But, as pointed out in the reviewer’s comment #2, the underlying concept of the applied current pulse aligns with conventional EPM technologies reported previously. It is also evident that utilizing an AC current input with bipolar polarity is impractical for EPM technologies, given that a unipolar current is necessary to induce a magnetic pole in the coil winding. On the other hand, the size and shape of each EPM may be reasonably inferred from the schematic and dimensions presented in Figure 2 of the revised manuscript. Additionally, to achieve the three designated operating modes (passive, SR mode, and LR mode), three combinations of magnetic poles within the EPM array were utilized. But, these combinations cannot be explicitly detailed at the moment. Nevertheless, these configurations may also be reasonably inferred from the explanation provided in Figure 3 of the revised manuscript. We appreciate the understanding of the reviewer and request acceptance of our current limitations in providing these specific details.

4) Fig 3 – it is not clear which four locations of the magnetic pole were tested. And also not why only 4 and not all 16 were probed.

The EPM array comprises 4 x 4 EPMs, totaling 16 EPMs. Notably, the magnetic pole patterns in the 1st and 2nd rows of the EPM array were identical to those in the 3rd and 4th rows. Thus, the 1st and 2nd magnetic pole patterns were measured and presented in Figure 3. This point was added to the explanation of Figure 3 in the revised manuscript.

5) Error bars are missing in all the graphs shown in the manuscript.

The reliability test of the holding force measurement of the MRF-based robotic claw was not conducted in this study. As a result, the holding force measurement was not repeated for the same objects in this study. Your understanding of this situation is greatly appreciated.

6) In Figures 4 and 6 schematic drawings of the setups would be welcome because it is difficult to resolve the details in the photographs where many black parts appear.

According to the reviewer’s comment, the schematics of the testing setups were added to Figures 4 and 6 in the revised manuscript.

7) In Fig. 5 please describe in what steps the displacements changed.

The displacement was increased at a rate of 0.1 mm/s. This point was added to the revised manuscript.

8) In Fig 6 and the corresponding measurements, I wonder why the cylindrical object was not oriented vertically.

In this study, since the cylindrical objects were fixed to the load cell of the testing machine, the orientation of those objects did not much affect the holding force measurement of the MRF-based robotic claw. Additionally, the horizontal orientation of those objects was more advantageous for constructing the testing setup.

9) The info on the masses of 3 different testing cylinders is missing.

As answered in Question 8, since the cylindrical objects were fixed to the load cell of the testing machine, there was no need to consider the masses of those objects.

10) The comparison given in Fig 11 is very artificial. It is not clear why in one case, the mass and the other the volume were considered. It is also not clear why for the EPM-driven gripper the M/V of the EPM array was also taken into account, and why for the EM-driven gripper this was not taken into account. What is the value of such a comparison, and what does the claim that one of those is “better” than the other mean? Isn’t it logical that EM-driven grippers are the only choice in case of dynamical gripping needs, while EPM-driven grippers are more energetically efficient when an on-off operation is needed?

In aerospace and other engineering fields where stringent mass and volume constraints are imperative, there is a pronounced demand for devices that are both lighter and smaller. In light of this requirement, we aimed to ascertain the superior choice. In the case of the EPM-driven gripper, we employed a specially designed, in-house, small-sized current amplifier tailored for the EPM array. Consequently, the mass and volume of this in-house current amplifier were factored into the computation of specific holding force and holding force density. Conversely, for the EM-driven gripper case, a general-purpose, large, and hefty power supply was utilized. Consequently, the mass and volume of this power supply were not factored into the determination of specific holding force and holding force density. These points were clearly explained in the revised manuscript.

We hope that the reviewer will agree that we have made an appropriate revision to this manuscript and that it is now worthy of publication.