Engineering properties of finger and foxtail millet seeds for development of remote-operated millet planter

Introduction

Millets are among the earliest domesticated cereal crops and are widely recognized for their high nutritional value, climate resilience, and adaptability to marginal agro-climatic conditions (Kiple, 2001) [8] . Globally, millet production exceeds 30 million tonnes annually, with India contributing approximately 38% of the total output across nearly 12 million hectares (FAO, 2023-24; APEDA, 2024) [3,2] . In India, Rajasthan and Karnataka are major millet-producing states, together accounting for a substantial share of national production. Millets such as finger millet (Eleusine coracana), foxtail millet (Setaria italica), pearl millet, and sorghum play a critical role in ensuring food and nutritional security, particularly in rainfed, tribal, and hilly regions. Despite their agronomic importance, millet cultivation continues to exhibit low levels of mechanization, especially during sowing operations (Abhishek Pandey et al., 2025) [1] . The small size, low weight, and irregular shape of millet seeds create significant challenges for achieving precise seed metering and placement when conventional planters designed for larger cereal grains are used. Consequently, traditional broadcasting and non-precision drilling methods remain prevalent, often resulting in uneven plant populations, poor crop establishment, and reduced productivity (Santosh et al., 2024) [14] . Precision sowing with appropriately engineered planters can substantially enhance seed placement accuracy, crop uniformity, and field operational efficiency. The effective design and development of such equipment depend on accurate knowledge of the engineering properties of seeds, encompassing physical, gravimetric, and frictional characteristics. Key parameters such as seed dimensions, geometric properties, bulk density, angle of repose, and coefficients of friction directly govern the design and performance of critical components including seed hoppers, metering devices, and seed delivery systems (Mohsenin, 1986; Sahay and Singh, 2001) [9,13] . A comprehensive understanding of these properties is essential for ensuring smooth seed flow, minimizing mechanical damage, preventing clogging, and achieving uniform seed distribution during planting operations. Consequently, the systematic evaluation of seed engineering properties forms the scientific basis for optimizing planter design and improving the precision and reliability of sowing machinery. Several researchers have documented the engineering properties of millet grains to support equipment design and post-harvest handling operations. Swami and Swami (2010) [16] reported that the geometric mean diameter of finger millet increased from 1.60 to 1.80 mm with rising moisture content. Ramappa et al., (2011) [12] observed bulk density values for finger millet in the range of 731.67-733.60 kg m⁻³. Similarly, Powar et al., (2018) [11] noted that the geometric mean diameter increased from 1.41 to 1.58 mm, whereas bulk density decreased from 749 to 665 kg m⁻³ as moisture content increased. For foxtail millet, Gouda et al., (2019) [7] reported bulk density and angle of repose values of 724 kg m⁻³ and 24.30°, respectively. Gaurav et al., (2021) [6] documented bulk density values for whole minor millets ranging from 527.34 to 738.20 kg m⁻³, with corresponding angle of repose values between 25.02° and 30.04°. Nethra et al., (2022) [10] reported the geometric mean diameter and angle of repose of finger millet as 1.60 mm and 25.25°, respectively. Although these studies provide valuable insights into the physical characteristics of millet grains, most investigations have focused on individual crops or general handling properties, with comparatively limited attention to parameters directly applicable to planter design. With the growing adoption of automation and remote-operated agricultural machinery-particularly suited to small and fragmented landholdings-the availability of crop-specific engineering data has become increasingly critical (Garg and Yadav, 2017; Felix et al., 2024) [5,4] . In this context, the present study was undertaken to determine selected engineering properties of finger millet and foxtail millet seeds relevant to the design and development of remote-operated precision planters.

Materials and Methods

Seed Material

Finger millet (Eleusine coracana) and foxtail millet (Setaria italica) seeds were used for the study. The samples were cleaned manually to remove broken grains, dust and foreign matter prior to testing.

Determination of Physical Properties 2.2.1 Size

The length, width and thickness of randomly selected twenty seeds from each millet type were measured using a digital vernier caliper with an accuracy of ± 0.001 mm. The average values were recorded (Mohsenin, 1986) [9] . These parameters are fundamental for designing the seed metering mechanism. They determine:  Size and shape of metering cells, holes, or grooves  Clearance between seed plate and housing  Selection of appropriate metering type (e.g., inclined plate, vertical roller, or vacuum disc)  Prevention of seed cracking and multiple seed pickup For very small seeds such as millets, dimensional data help achieve single-seed metering, which is critical for precision sowing.

Geometric Mean Diameter

The geometric mean diameter was calculated using the relation (Mohsenin, 1986) [9]: Where, D g = Geometric mean diameter, (mm) L = Longest intercept (Length), (mm) B = Longest intercept normal to L (Width), (mm) T = Longest intercept normal to L and B (Thickness), (mm) This property represents the effective seed size and is widely used for:  Designing orifice diameter of seed plates  Determining seed tube diameter to prevent bridging  Selecting sieve sizes for seed cleaning prior to planting  Establishing optimal pickup hole size in pneumatic planters Typically, the metering hole is designed slightly larger than the geometric mean diameter to ensure smooth release without allowing doubles.

Bulk Density

Bulk density was determined using a 100 ml measuring jar. The mass of seeds occupying the known volume was recorded and bulk density was calculated as the ratio of mass to volume (Mohsenin, 1986) [9] . It also helps estimate the total planter weight distribution for stable operation.

Determination of Frictional Properties 2.3.1 Angle of Repose

Angle of repose is the angle between base and slope of the cone formed on a free vertical fall of grains on to a horizontal plane. It was determined by following the procedure described by Sahay and Singh (2001) [13] . The apparatus consisted of a hopper filled with millet and a circular metal platform supported on three legs. Grains were allowed to flow freely from the hopper onto the platform, forming a heap on the circular iron plate. Using the measured height and diameter of the natural grain heap, the angle of repose as shown in Fig. 1 was calculated with the given formula.

Where, θ = Angle of repose, (°) H = Height of heap, (mm) r = Radius of heap, (mm) Typically, hopper wall angles are designed 5-10° steeper than the angle of repose to ensure continuous seed movement.

Coefficient of Friction 2.3.2.1 Co-efficient of internal friction

The coefficient of internal friction represents the resistance created when grain kernels move against one another. To determine this, a wooden box measuring 10 × 10 × 10 cm³ was fastened with a cord that passed over a pulley and connected to a weight pan. A thin layer (1 cm) of millet was spread over the surface. Weights (W₁) were gradually placed in the pan until the empty box began to move on the millet surface. The box was then loaded with 200 g of millet (W) and the process was repeated by adding weights until the filled box started to slide, at which point the required weight (W₂) was noted and the average value was recorded (Shivabasappa et al., 2012) [15] .

Co-efficient of external friction

The coefficient of external friction refers to the resistance encountered when grains slide over a horizontal surface in contact with a wall as shown in Fig. 2. For millets, this property was measured using a mild steel test surface. A box measuring 10 × 10 × 10 cm³ was connected to a cord that passed over a pulley, with a weight pan attached to the opposite end. Weights (W₁) were gradually added to the pan until the empty box began to move. The box was then loaded with 200 g of millet (W) and the procedure was repeated by adding weights until the filled box started sliding. The weight (W₂) required to initiate sliding of the filled box was recorded (Shivabasappa et al., 2012) [15] .

Where, W 1 = Weight to cause sliding of empty box, (g) W 2 = Weight to cause sliding of millet filled box, (g) W = Weight of millet inside box, (g)

Friction of Millet Seeds

This parameter affects interaction between seeds and construction materials (steel, aluminum, plastic) and is used for:  Selecting hopper and seed tube materials  Designing surface finishes to improve flow  Reducing power requirement in mechanical metering systems Materials with lower friction coefficients promote smoother discharge and improve metering consistency.

Results and Discussion

The engineering characteristics of millet seeds, including size, geometrical mean diameter, bulk density, angle of repose and coefficient of friction, were evaluated using standard testing methods as explained above. The results of these measurements are summarized in the Table 1 below.

Size

The physical dimensions of finger and foxtail millet seeds were measured, with finger millet seeds having a length of 1.70 ± 0.20 mm, width of 1.45 ± 0.06 mm and thickness of 1.33 ± 0.05 mm, while foxtail millet seeds measured 2.24 ± 0.20 mm in length, 1.53 ± 0.13 mm in width and 1.23 ± 0.10 mm in thickness.

Geometrical mean diameter

The geometrical mean diameter of the selected finger and foxtail millet seeds was measured to be 1.54 ± 0.03 mm and 1.48 ± 0.06 mm.

Bulk density

The bulk densities of finger and foxtail millet seeds were measured as 708.2 ± 11.52 kg m -3 and 724.07 ± 12.87 kg m -3 , respectively.

Angle of repose

The angle of repose, which indicates the flowability of seeds, was measured for finger and foxtail millet seeds. The values recorded were 31.85 ± 3.25° for finger and 27.08 ± 0.60° for foxtail millets, reflecting their relative ease of movement during handling.

Coefficient of friction

The frictional properties of finger and foxtail millet seeds were determined to understand their flow behavior. The coefficient of internal friction was found to be 0.61 ± 0.02 and 1.25 ± 0.02, while the coefficient of external friction measured against a surface was 0.49 ± 0.01 and 0.44 ± 0.02.

References

1. Pandey A, Chaudhary S, Shukla P, Upadhyay A, Chouhan P, Kumar A, et al. Mechanization of millet cultivation: status, technological innovations and future directions. International Journal of Research in Agronomy. 2025;8(5):383-395.
2. Agricultural and Processed Food Products Export Development Authority (APEDA). Millet export. 2024. https://apeda.gov.in/apedawebsite/SubHeadProducts/eCatal ogue_Millets_export.htm
3. Food and Agriculture Organization (FAO). Millets 2023. 2023-2024. https://www.fao.org/millets2023/en
4. Felix OA, Raphael BS, Kehinde AO, Joshua AAO. Design and implementation of robotic controlled beans planter using mobile app. Trends in Applied Sciences Research. 2024;19(1):112-125.
5. Garg R, Yadav R. Agricultural robotics: a comprehensive review. Journal of Robotics. 2017;2017:1-15.
6. Gaurav A, Pradhan RC, Mishra S. Comparative study of physical properties of whole and hulled minor millets for equipment designing. Journal of Scientific and Industrial Research. 2021;80(8):658-667.
7. Gouda GP, Sharanagouda H, Udaykumar N, Ramachandra CT, Nagaraj N, Ananda N, et al. Studies on engineering properties of foxtail millet (Setaria italica). Journal of Farm Sciences. 2019;32(3):340-345.
8. Kiple KF. The Cambridge world history of food. Vol. 2. Cambridge: Cambridge University Press; 2001. p. 761-777.
9. Mohsenin NN. Physical properties of plant and animal materials. 2nd ed. New York: Gordon and Breach Science Publishers; 1986.
10. Nethra V, Someswara Rao C, Edukondalu L, Ashok Kumar A. Engineering properties of finger millet (Eleusine coracana L.) grains. International Journal of Science and Research. 2022;11(7):1476-1478.
11. Powar R, Aware V, Shahare P, Sonawane S, Dhande K. Moisture-dependent physical properties of finger millet grain and kernel. Journal of the Indian Society of Coastal Agricultural Research. 2018;36(1):48-56.
12. Ramappa KT, Batagurki SB, Karegoudar AV, Sharnakumar H. Physical properties of finger millet (Eleusine coracana). International Journal of Agricultural Engineering. 2011;4(1):13-15.
13. Sahay KM, Singh KK. Unit operations of agricultural processing. New Delhi: Vikas Publishing House; 2001.
14. Santosh, Devanand M, Vijayakumar P, Anand N. Engineering properties of foxtail millet seeds for development of planter for millet crop. Journal of Scientific Research and Reports. 2024;30(10):236-241.
15. Shivabasappa S, Roopa Bai RS, Sridevi S, Sharanagouda H, Udaykumar N. Engineering properties of finger millet (Eleusine coracana L.) grains. International Journal of Agricultural Engineering. 2012;5(2):178-181.
16. Swami S, Swami S. Physical properties of finger millet (Eleusine coracana). International Journal of Agricultural Engineering. 2010;3(1):156-160.