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Quadrupedal locomotion and navigation is one of the most common methods utilized by land-based animals. Despite this, roboticists underutilize this configuration and most often substitute it with hexapodal and bipedal designs. This project explores the unique challenges faced in constructing robotic quadrupeds.
In Greek mythology, Chiron was the most intelligent and civillized of all the centaurs. The R1 was named as such to highlight its limb structure.
NOTE 1: This instructable is designed to increase the knowledge of a robot hobbyist. One needs to be familiar with servos, basic electronics, soldering, and basic machining skills, such as drilling, filing, and cutting.
NOTE 2: The pictures of the robot with an upper body (arms and eyes) will likely be integrated into an R2 or R3 version as the upper body made the weight of the robot too great for the robot to stand and walk. Ultimately the robot needs stronger (higher torque) leg servos.
~This instructable is designed and written by Griswald Brooks and Emily Rose Berliner~
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Signing UpStep 1Leg Design 1
Understanding of force and torque is critical to leg design. Initially these concepts were overlooked and the robot could not stand, let alone walk. Principles discussed here are the most basic kind and do not take into account the issue of dynamics, momentum, and inertia.
Basic Leg Torque
When designing a legged robot, one must consider the number of legs that will support the robot while walking. In the case of this quadruped, the minimum number of legs on the ground at any given time will be three.
After determining the number of legs, one must take the total weight of the robot and divide it by this number. This will be the amount of weight each leg (and most importantly each knee joint) must be capable of supporting.
Total robot weight must be known. Usually this will be determined by taking the weight of each component and adding them together. This can usually be found on parts data sheets.
NOTE 3: The conundrum encountered with summing the weight of the parts is that one cannot know what parts are needed (and therefore their weight), until one know how much torque is required. One must estimate weights for parts (heavier is always better), and then calculate torques. After this starting point, the figures can be recursively corrected until one lands on numbers that are comfortable (and affordable). Never underestimate the torque required. Changes in angle and speed will change the torque requirement and it is therefore always better to err on the side of caution.
The basic torque formula is:
Torque = Force x Moment Arm
Force is any push or pull on an object.
Torque is rotational force.
Moment Arm is the length through which a force or torque is applied. ***insert chart
One can determine required Knee Servo Torque by finding values for Force and Moment Arm.
Given the femur length of 5 inches and a total robot weight of 6.3 pounds, force is equal to 33.6 ounces and (Moment Arm is equal to femur length)***?.
Torque = 33.6 oz x 5 in
Torque = 168 oz in
Therefore the Knee Servos must have a torque of at least 168 oz in or more for the robot to stand and walk.
Intuitively one might think that the hip joint plays a role in the getting up process, but through experimentation (by disabling the hip joints) it was proven that only the knee joints are required to lift the robot. This also means that the hip joints cannot be used to aid in the getting up process, but later in the walking process it was seen that hip joint strength is still required to prevent the entire robot from buckling.
The formula:
Torque = Force x Moment Arm
is a simplified leg torque/force equation, but it is sufficient for the robot hobbyist and represents the maximum required torque for robot legs. A more complete formula can be seen here:
http://www.robotshop.us/robot-leg-tutorial.html
Moment Arm/Femur length can be shortened to increase the resultant force produced. This can be a "quick" fix if higher torque knee servos cannot be aquired or total robot weight cannot be reduced further.
***recommendations for types of servos
Basic Leg Torque
When designing a legged robot, one must consider the number of legs that will support the robot while walking. In the case of this quadruped, the minimum number of legs on the ground at any given time will be three.
After determining the number of legs, one must take the total weight of the robot and divide it by this number. This will be the amount of weight each leg (and most importantly each knee joint) must be capable of supporting.
Total robot weight must be known. Usually this will be determined by taking the weight of each component and adding them together. This can usually be found on parts data sheets.
NOTE 3: The conundrum encountered with summing the weight of the parts is that one cannot know what parts are needed (and therefore their weight), until one know how much torque is required. One must estimate weights for parts (heavier is always better), and then calculate torques. After this starting point, the figures can be recursively corrected until one lands on numbers that are comfortable (and affordable). Never underestimate the torque required. Changes in angle and speed will change the torque requirement and it is therefore always better to err on the side of caution.
The basic torque formula is:
Torque = Force x Moment Arm
Force is any push or pull on an object.
Torque is rotational force.
Moment Arm is the length through which a force or torque is applied. ***insert chart
One can determine required Knee Servo Torque by finding values for Force and Moment Arm.
Given the femur length of 5 inches and a total robot weight of 6.3 pounds, force is equal to 33.6 ounces and (Moment Arm is equal to femur length)***?.
Torque = 33.6 oz x 5 in
Torque = 168 oz in
Therefore the Knee Servos must have a torque of at least 168 oz in or more for the robot to stand and walk.
Intuitively one might think that the hip joint plays a role in the getting up process, but through experimentation (by disabling the hip joints) it was proven that only the knee joints are required to lift the robot. This also means that the hip joints cannot be used to aid in the getting up process, but later in the walking process it was seen that hip joint strength is still required to prevent the entire robot from buckling.
The formula:
Torque = Force x Moment Arm
is a simplified leg torque/force equation, but it is sufficient for the robot hobbyist and represents the maximum required torque for robot legs. A more complete formula can be seen here:
http://www.robotshop.us/robot-leg-tutorial.html
Moment Arm/Femur length can be shortened to increase the resultant force produced. This can be a "quick" fix if higher torque knee servos cannot be aquired or total robot weight cannot be reduced further.
***recommendations for types of servos
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