Starting Robotic Arms

Read, Watch, & Answer

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Degrees of Freedom (DOF)
The Degrees of Freedom, or DOF, is a very important term to understand. Each degree of freedom is a joint on the arm, a place where it can bend or rotate or translate. You can typically identify the number of degrees of freedom by the number of actuators on the robot arm. Now this is very important – when building a robot arm you want as few degrees of freedom allowed for your application!!! Why? Because each degree requires a motor, often an encoder, and exponentially complicated algorithms and cost.

Also note that a DOF has its limitations, known as the configuration space. Not all joints can swivel 360 degrees! A joint has some max angle restriction. For example, no human joint can rotate more than about 200 degrees. Limitations could be from wire wrapping, actuator capabilities, servo max angle, etc. It is a good idea to label each link length and joint max angle on the FBD (Free Body Diagram).

Screen shot 2013-02-28 at 1.14.05 PMScreen shot 2013-02-28 at 1.14.42 PM
Robot Workspace
The robot workspace (sometimes known as reachable space) is all places that the end effector (gripper) can reach. The workspace is dependent on the DOF angle/translation limitations, the arm link lengths, the angle at which something must be picked up at, etc. The workspace is highly dependent on the robot configuration.

Since there are many possible configurations for your robot arm, from now on we will only talk about the one shown below. I chose this 3 DOF configuration because it is simple, yet isnt limiting in ability.

Screen shot 2013-02-28 at 1.18.20 PM

Now lets assume that all joints rotate a maximum of 180 degrees, because most servo motors cannot exceed that amount. To determine the workspace, trace all locations that the end effector can reach as in the image below.

Screen shot 2013-02-28 at 1.18.42 PM

If you change the link lengths you can get very different sizes of workspaces, but this would be the general shape. Any location outside of this space is a location the arm cant reach. If there are objects in the way of the arm, the workspace can get even more complicated.

These videos may help before answering the questions:
How it’s Made: Robot Arm
Human Arm Anatomy

and, just because its interesting, heres a video of a robot controlling a human arm with electrodes.


1) What are some of the uses of Robotic Arms?

2) What is an end effector?

3) When discussing robotic arms, what are degrees of freedom?
How many Degrees of freedom does the average Human Arm have (from should to finger tips)?

4) How is the workspace of the Human Arm limited?

For the following question, check out a video or two here:
Robot Arms Controlled by Human Arms

5) What applications could this have in our world today?

Next class you guys will play Alchemical Codex of Engineering, and have some questions related to it.
You can check it out if you finish early, or check out the post below this one.


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