7.4: Arm Design

As part of the design of a competition robot, it is possible a designer would use an arm similar to the ones shown above to pick up objects and dump them in a goal.  Students should apply the lessons learned in this unit to the following example:

1.  Determine the total weight of the object manipulator designed in Unit 6.

2.  Determine the weight of one game object for the game described in Unit 5.

3.  Assuming a simple arm system where the arm is 0.25m long, calculate how many VEX 393 motors would be required to hold the arm stationary with the object manipulator and game object at its tip.

4.  Assume the 393 motors cannot draw more than 2.5 amps before their internal circuit breakers will trip; calculate how many motors are required for this application, then calculate the rotational speed of the arm.


Reviewing the above example, one could say that this is not a practical design because of how many motors it takes.  One variable the designer can change to require fewer motors to accomplish the same task is the length of the arm. A shorter arm would place less torque load on the motors as described in the section on Torque above.  This is known as mechanical advantage or leverage.  A short arm is not practical for many designs – are there other ways to utilize mechanical advantage so that less power can be used to do the same amount of work?  In the next unit, designers will see how this is possible through the use of gear ratios.