Design, Development and Calibration of Isometric Vertical Leg Strength Measuring Device
2.3 Requirement of Leg Strength Measurement Device
should be set at 5th percentile of maximal strength value of male workers. For example, the sickle is used by both male and female workers with characteristic pull / sawing mode of operation for more than 5 min. Therefore, 30% of 5th percentile value of pull force with right hand in sitting posture for female workers was considered using 30% criterion. Gite et al. (2009) recommended pull force with right hand in sitting posture 12 N for Indian population. On the other hand, for the tractors mainly operated by male workers, required working strength of males should only be considered for design of various controls in the tractor, such as clutch, brake, steering wheel and gear lever. These controls are operated frequently for short durations (less than 5 min) and therefore, required effective force may be calculated to be 30% of the 5th percentile of male strength value. The baseline information on various tools and equipment provided by Patel et al. (2014) would be helpful for design or design modification of agricultural tools – pertaining to the executed working force fairly within safe limit.
2. Safety of the equipment ensures user’s safety during the measurement of isometric vertical strength. Furthermore, participants should perceive the system to be stable and safe to demonstrate true maximum strength exertions.
3. Comfort of use makes the equipment more users friendly and provides the user with a feeling of better acceptance and effortlessness.
4. Use-reuse flexibility assures convenient resetting / calibration of equipment between consecutive trials, so that data collection of all the measurements of same person is possible with short rest pause period, thus facilitating data collection.
5. System calibration entails the equipment to be factory calibrated so as to compensate for gravitational force and any dead load of device itself; and isolate this from the user exerted force components.
6. Accuracy of the device makes it free from random and systematic errors (except inbred mechanical error negligible or nearest to zero) to ensure precision of measurements across different trials.
7. Cost of the instrument should be economic enough and thus affordable to the local working population, without compromising in precision of measurement.
2.3.2 Development of the device
The process of product development consisted of two phases (Fig. 2.2) – ideation and prototyping. The ideation phase entailed about research, brain storming, concept generation and concept refinement of selected ideas; whereas prototyping phase comprised of visualization, concept detailing, test and refine prototype, finally leading to product development.
Fig. 2.2 Product development process followed in present study
Test and Refine Prototype Product
Development Brainstorm
Concept Generation
Concept Refinement
Concept Details Ideation
Prototyping
2.3.3 Conceptualization
Based on the available information and research inputs gathered therefrom, various ideas were hypothesized to explore possible means to measure isometric vertical leg strength. Thus the objectives of the isometric leg strength measurement in standing posture came up with several approaches to satisfy the parametric requirements. A series of sketch explored the concepts / options for product outlook, working principle, manufacturing and assembling feasibilities. Some of the competent thoughts for the proposed isometric vertical force measuring device were depicted in Fig. 2.3.
(a) (b) (c) (d)
(e) (f) (g) (h)
Phase – I
(A) (B) (C) (D)
Phase - II
Fig. 2.3 Schematic diagram of some final concepts for isometric leg strength measuring device (phase – I: Some feasible solutions; phase – II: selected solution)
2.3.4 3D digital prototype of the device
Conceptualizing any device prototype (like an isometric force measuring device) need several multidimensional inputs to be considered. In addition to the factors viz.
versatility, safety, comfort, use-reuse flexibility, system calibration, accuracy and cost, feasibility of manufacturing with locally available resources also behold pronounced importance. Taking this into consideration, a leg force measuring device was hypothesized to meet the possible needs of every user. The ‘mechanical design’ option in Delmia Human Modeling Software (v.5.19) was used to create 3D CAD model of hypothesized force measuring device as shown in Fig. 2.4. The leg force measuring device comprised of two vertical posts, a base frame to support vertical posts, a middle frame with foot rest attached for application of force during experiment. The load cell was bolted between the frame attached for foot rest and the vertex of the main frame. One end of the base frame was supported with rod through nuts appended for stability of the device. Due effort was put to make the leg force measuring device compact and portable.
(a) Isometric view (b) Front view
Fig. 2.4 Isometric vertical leg force measuring device ‒ 3D CAD model
2.3.5 Creation of digital human models and rendering for range of adjustment Review of literatures revealed serious lack of normative database for Assamese agricultural workers, except Dewangan et al. (2005, data of 40 male farmers). So manikins (digital human models, to decide the fitting range of adjustment) were created
using Indian national anthropometric database for agricultural workers, where Smallest, average and largest dimension of agricultural workers were represented by 5th, 50th and 95th percentile manikins respectively. The effective range was demarcated by 5th percentile female manikin and 95th percentile male manikin. The knee joint angle of 5th (female) and 95th (male) percentile manikin was adjusted approximately about 1500 and 900 in order to get maximal promising range of adjustment as shown in Fig. 2.5. The investigations and design reiterations using DHM registered the maximum capable range of adjustment to be 150 mm for efficient operation by everyone (from 5th to 95th percentile of body dimensions) measured.Therefore, suitable mechanisms for adjustment for regulating height of isometric force measuring device were essential in order to analyze at different knee-joint angles viz. 90°, 120° and 150°, with right and left legs.
5th percentile female 95th percentile male Fig. 2.5 Manikin of 5th and 95th percentiles at 1500 and 900 knee angles
Based on the results attained from manikins, various dimensions of leg force measuring device was finalized. Different views and dimensions (scale 1:5) of isometric vertical leg force measuring device is shown in Fig. 2.6.
(a) Isometric view (b) Front view
(c) Side view (d) Top view
Fig. 2.6 Different views and dimensions of isometric vertical leg force measuring device (Scale 1:5; all dimensions are in mm)
2.3.6 Finite element analysis
The Finite Element Analysis (FEA) is a numerical method to find an estimated solution by dividing any region into small sub-regions. The solution within each sub-region that satisfies the governing equations can be reached more simply than that required for the entire region. FEA was used for isometric force measuring device in order to predict
deflection and stress distribution accurately for the relevant components, to make sure that, it could be operated safely. The tetrahedral element type was used in this study for the FEA analysis. The load / force applied for all the components were 1000 N. The number of elements was 8200, 180000 and 16000 for middle frame, main frame and upper clamp respectively as shown in Fig. 7. The total deflection, von mess and tensile yield were found to be 0.1127 mm, 92 MPa and 370MPa for middle frame, 0.910 mm, 102 MPa and 370 MPa for main frame, 0.214 mm, 301.11 MPa and 370 MPa for upper clamp respectively, as shown in Figs. 2.7 ‒ 2.10. Since the proposed design satisfied the criteria, the design was considered as ‘safe’.
Fig. 2.7 CAD model/meshed model for upper clamp, middle frame and main post
Fig. 2.8 Static loading condition FEA results for upper clamp
Fig. 2.9 Static loading condition FEA results for mid clamp
Fig. 2.10 Static loading condition FEA results for main frame