Bio-Inspired Design

Blake Johnson

Abstract— The goal of this paper is to obtain a completeunderstanding of the concepts required in the design of a bio-inspired robot. The general concept and thinking behind theproject, as well as the analysis of the reason behind the successof the project is the essential purpose of this project. Statics andkinematics were the main mathematical base to the entirety ofthe project. The ability to realize how each part was going toreact with one another was the most essential ability to obtainduring the process of the design. The goal of this paper is togo over the design process, and to highlight the entire thoughtprocess of the entire design process.

I. INTRODUCTION

For the third project in MMAE 232 - Design for Innova-tion, it was required that we design a bio-inspired walkingrobot (see Fig. 9). The robot had to use a servo, or a systemof servos, and had to resemble a gait of a biological being.The robot had to be able to walk 4.9m autonomously, andonce it completed the 4.9m walk, we had to pick it up andmove it to the start line to complete its second trial. Thedesign also had to be disassembled, meaning glue could notbe used to hold any of the electronics in place.

II. CONCEPT GENERATION AND EVALUATION

Our four original designs had two that consisted of onecontinuous servo, making it as simple as possible for thedesign process (see figs. 2 and 3). One of them was goingto represent the gait of a duck, and the other represented aseal. The other two designs were a little more difficult(seefigs. 4 and 5). The design with a similar gait plot to a dogwas going to consist of four servos that would move eachleg individually (see gait 3). The design that resembles agorilla was going to move with one continuous servo in thefront with two servos that work individually in the back. Byfurther inspection of both the gorilla and the dog designs,we realized it was going to be much more difficult to makethese two designs. The timing of the gorilla gait was going tobe very difficult to time out. And the dog gait may have hadto use eight servos instead of four in order to work properly.We then did a Pugh chart to determine the strengths andweaknesses of each potential design (see Table 1). We thendecided that the seal gait was going to be the easiest tomimic, and it was also going to require the least amount oftime to design.

III. ANALYSIS

The most important part of the design process was deter-mining whether the system was going to be statically stablein all phases of the entire gait. On top of this, the amountof torque necessary to move the design had to be found aswell. This was found using a free body diagram (see fig. 8).

Fig. 1. Caption minutes after the successful test

TABLE IPUGH CHART

Criteria Weight Seal Duck Dog GorillaMass 3 +1 +1 -1 -1Stability 2 +1 -1 +1 0Posture 1 +1 -1 0 0Speed 1 0 +1 +1 +1Total +3 0 +1 0Weighted Total +6 +1 0 -2

This free body diagram shows the very instant that the bodybegins to lift. Since we made the base so long, we knew thatit would never come completely off the ground. This was adesign feature that allowed us to use two arms to propel thedesign forward. Once the design is lifted off of the ground,the only force that has to be overcome is friction. Since mostof the force will be resting on the arms, we knew that thefrictional force was going to be much less than the forcenecessary to lift. Therefore, the distance (d) from the centerof mass to the reaction force (FR) is 150mm. The distance

Fig. 2. Original drawing of the seal

 

Fig. 3. Original drawing of the duck

Fig. 4. Original drawing of the dog

Fig. 5. Original drawing of the gorilla

from the force necessary to lift (Farm), and the reaction forceis 220mm. If you do a sum of the moments equation, youwill be able to find the force necessary to lift the object offthe ground. The weight of the project including the servo isabout 9.8N:∑

M = −((Fmg) ∗ d) + ((Farm) ∗ d) (1)

0 = −(9.8N ∗ 150mm) + (Farm ∗ 220mm)

You find that Farm = 6.68N. This is the maximum forceallowed to keep static equilibrium. Therefore in order tobreak static equilibrium, you need any force greater than6.68N. In order to find the minimum torque necessary, youwill use this number in the equation used in the torque freebody diagram (see fig. 9). In order to find the minimumtorque required to lift the seal’s body, you will use:

Tneeded = Farm ∗ larm ∗ sin(θ) (2)

Tneeded = 6.68N ∗ 100mm ∗ sin(49)Tneeded = 505Nmm

You find that the torque necessary is about 500Nmm. Thisis well over the maximum torque that the servo can handle.We then decided to use gears in order to increase the amountof torque that the servo can apply to the arm. The gear ratiowe decided to use was 2:1, and we placed the smaller gearon the servo. This would in turn slow down the speed ofthe motor, yet would double the amount of torque that canbe applied to the arms. Now that we found that we need500Nmm of torque to lift the body, the 2:1 ratio will causethe amount of torque that was needed by the servo motor tobe 250Nmm, which is well under the maximum allowabletorque given by the servo.

IV. EXPERIMENTAL RESULTS

The project went relatively smooth, other than the needto cut weight. When the entire design was put together, theservo could not handle the torque necessary to rotate. Wedecided to scrap most of the design that was unnecessary tomotion. We also cut weight off of pieces that were oversizedand unnecessary as well. By the time we got the design tomove, we had cut out everything but the necessary pieces(see fig. 1). As you can see in the figure, we didn’t preciselycut anything off of the project. All that was necessary formotion was the base, to stabilize the design during motion,the gears, the two pieces that held the servo, the two piecesthat stabilized the rod, and the arms, which pulled the designforward. After the weight was cut, the design began to move,but it rotated to the right as it moved forward. So we began toshave down the arms to make sure they were the exact samelength and were only in contact with the floor at the sameexact time, otherwise it was going to rotate in the direction ofthe arm that had less contact with the floor. After these minoradjustments we were able to get the design to perpetuateforward with almost no rotation. Our design walked the 4.9min both directions with no issue. I had more confidence inthis project than the other two we had done earlier in the

Fig. 6. Convex Contact Polygon of the seal

Fig. 7. Gait plot of the seal

         

Fig. 8. Free Body Diagram of the Seal

Fig. 9. Free Body Diagram of the arm

semester. This project actually took less time to design thanthe other two had as well.

V. DISCUSSION

As stated previously, we had issues with the weight ofthe design, and the contact time for both arms, but otherthan this, we had no issues with the design. The reasoningfor the issues with the weight is because we underestimatedthe amount of weight that was going to be involved in thedesign. Then when we cut the arms, which was done becausethey were too long, we had issues with the amount of timethe two were in contact with the ground. After these twowere fixed, the project needed no adjustments. The servowas continuous so there was no timing involved. The projectwent as expected after the minor adjustments.

VI. CONCLUSIONS

This project went as smoothly as it possibly could have.The key to the simplicity of this project had to do with theanalysis done prior to the project being designed. As soonas we figured out what we wanted to do for the project, wemade an effort to achieve simplicity throughout the entiredesign process. This attempt for simplicity made any issuesextremely easy to fix. Any issues that we ran into had noeffect on the rest of the project. This feature in the designprocess is something that we can use in the future. It makesfor less complications, which allows for less time wasted.

VII. MATLAB CODE

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