Sir, you can consider Swedish Fall Cone Method to determine the plastic limit of the soil. A reference to this is below -
Wasti, Y. "Liquid and Plastic Limits as Determined from the Fall Cone and the Casagrande Methods," Geotechnical Testing Journal, Vol. 10, No. 1, 1987, pp. 26-30, https://doi.org/10.1520/GTJ10135J. ISSN 0149-6115.
Thank you Mr. Rupam Saikia , but the penetration method does not reflect the physical process of transform of the soil from semi-solid to plastic state.