Discussion Forum

Hello! I'm designing a piezoresistive tactile sensor using COMSOL 5.5 which consists of an array of microdomes sandwiched between two rectangular blocks. The microdomes are arranged in a ring and surround a central cylinder, whose height is smaller than the radius of the microdomes, and all the structures are made of PDMS-CNT which is a piezoresistive material. I deliberately designed it as such so that when a small vertical force is applied on the top rectangular block, only the microdomes will be deformed. Only when a larger vertical force is applied on the top rectangular block will the cylinder come into contact with the block and be deformed. My objective is to optimise the geometrical parameters and material properties so as to create two separate pressure sensing ranges for the microdomes and the cylinder.

However, as I am new to COMSOL, I have encountered some problems with my simulation and was hoping that the support team could assist me in resolving them:

1. I followed the guide for contact pair generation on the COMSOL blog and in the COMSOL tutorial and did the following: -Created two contact pairs, one between the bottom boundary of the top rectangular block and the top boundary of the cylinder and the other between the bottom boundary of the top rectangular block and the top boundaries of the microdomes -Applied a prescribed displacement instead of a boundary load for better convergence -Applied a ramp factor to the prescribed displacement and gradually increased it via an auxilliary sweep for better convergence -Used penalty and adhesion for the contact pair

I used prescribed displacement for better convergence but my objective is to analyse the pressure-deformation relationship for the microdomes and the cylinder. I have tried applying a temporary weak spring on the rectangular blocks and the cylinder to stabilise them since they are not in contact at the start of the simulation, but the solution doesn't converge when I apply a boundary load. May I know if there are some settings I am unaware of that can be configured to allow for better convergence?

2. I read in the blog that it is possible to find an indirect relationship between the prescribed displacement and the applied force, through analysing the contact pressure, but there are no tutorials available. If it is not possible to find the pressure-deformation relationship directly, could you teach me how to establish this relationship?

3. I am unfamiliar with the post-processing portion of COMSOL and was wondering if you could teach me how to analyse the maximum possible deformation of the domes and the corresponding maximum force applied to give rise to this deformation so I can establish the upper bound of the pressure sensing range of the microdomes.

4. I am also aware that the deformation may not vary linearly with pressure, so I would like to find out what the linear range is.

5. I would also like to establish the minimum pressure that will "activate" the cylinder, that is, cause it to come into contact with the top rectangular block.

6. Lastly, may I know how to determine the maximum pressure that my entire structure can withstand? I've tried carrying out other simplified simulations and noticed that when large boundary loads are applied and the scale factor is 1, the structure just appears completely flattened, which is unrealistic. May I know how to determine the physical saturation threshold?

7. In my current simulation, I have yet to take the piezoresistivity of the material into account. I started with just the pressure-deformation analysis because I am still relatively new to COMSOL. That being said, I have tried exploring the piezoresistivity module and am not sure how to set up the circuit properly. Could you provide me with some guidance on this matter?

8. I am also unsure of what the piezoresistivity matrix in COMSOL means; I've tried googling but the only search results are related to COMSOL. May I know what the inputs for this matrix are?

I am so sorry for bombarding you with many queries but I have exhausted multiple simulation software, explored multiple versions of COMSOL and sought help from every avenue possible over the last two months and still have seen no progress. I would be extremely grateful if you could kindly assist me in resolving all my issues as I really love COMSOL's user interface and hope to be able to learn how to use it properly.