Robot Arm

From Science Olympiad Student Center Wiki

Robot Arm
Engineering & Build Event
Forum Threads 2013 2012
2011 (Trial)
There are no tests available for this event
Images Image Gallery
There are no question marathons for this event
This event was not held last year in Division B
Division C Champion Harriton High School

Robot Arm is a Division C national event for the 2012-2013 season, replacing Mission Possible, after being run as a Division C trial at the 2011 National Tournament. The event involves the construction of a robotic arm to grab, lift, and deposit specific items in prescribed locations. The Trial rules can be found Here





In order to have a functioning Robot Arm, it'll need a base. This can be made out of any material, as long as it fits within the designated 30cmx30cm square. Something to keep in mind is that the base needs to be heavy enough to prevent the device from tipping over while the arm is moving objects around.

The base can also contain a motor used to spin the arm. One possible design of a base can be seen to the right:

The Arm

Due to the event being named Robot Arm, one should probably include an arm in their device. If a team wishes to get maximum points then their arm needs to reach about 58 cm (from the center of the designated arm square to the top of the bonus boxes). It only needs to reach 15 to 20 cm outside of the base box if the operator plans on only utilizing the East and West Goals. In order to utilize the North Goal, one needs to build to be sturdy and stiff enough to keep the arm from breaking or flexing, while also keeping it lightweight so that the motors are still able to handle the load.

Your arm will also need to include a number of joints. Most arms will have 3 joints, though some have up to 5 and some only have 1. Each joint must have a motor in order to move it's position, as well as some kind of support for the axis of the joint. Most servo motors are strong enough that with an added bracket, the servo can be both the motor and axis of rotation. When designing your arm keep in mind that the number of motors on your device is a tie breaker.


Almost all hobbyist-made robots utilize the Servo. A servo is a complete package of motor, gearing, position sensor, and control. Originally made for RC vehicles, the servo has been widely adopted in robotics for their small size, high strength, and ease of use. Servo's come in many varieties but for the most part have fairly standardized system with all servos being made to the same size specifications. Categories of size include Micro, Mini, Standard, and Quarter/Giant scales. If you're new to robotics choosing the right servos can be a daunting task given such a broad range of choices. The most important factor when choosing a servo will be its torque rating. For an easy way to find out how much torque you will need at each joint, use this calculator. The main and really only downside to servos is their limitation of rotation. Most servos are limited to 180°, and are further limited to only 90° when using most RC control systems.

Other options for motors include those found in proprietary kits such as VEX or Lego NXT. More advanced builders may prefer to use stepper motors or even build their own motor+gearing+control systems. Though doing so will require much knowledge of both mechanical and electrical engineering.

The End-Effector

The End-Effector is the part of the arm that actually interacts with the objects on the playing field. There are two broad categories of end-effectors; active and passive. For the 2013 season, the rules permitted detachable passive end-effectors, whereas in 2011 and 2012 they were prohibited greatly reducing their utility.


An active end-effector is one which consumes energy in order to function. The most common examples include a claw/gripper and electromagnets. Certainly the most common and arguably most useful active end-effector is the claw/gripper. One will likely want to include one on their arm because, if designed properly, it can pick up all the objects on the field. Designing and building your own gripper is a difficult task given the tight tolerance in construction, so most will find it easier to buy a pre-made gripper or use an assembly kit.

Some may find that they would like to include an active end-effector other than a basic gripper. Doing so can greatly increase competition efficiency and speed by using specialized tools to pick up certain, and often difficult, items. Coming up with an idea for one is mostly left up to your desires and creativity, but the most common example is the electromagnet which can be used to pick up the nails, and possibly the pencils or batteries(2011, 2012).


Passive end-effectors are those that do not consume energy to function. Picking up objects without consuming energy is difficult and there are only a few known ways to do so. The most common are permanent magnets and adhesives. Magnets may be used to pick up any ferromagnetic items such as the nails, pencils(provided that they have the metal bit), and batteries(2011, 2012). Adhesives can be used to pick up any item on the field, though it should be noted that it must be temporary and any damage to the objects could result in disqualification. Many find that tape, such as 3M double-sided tape, works great and does not leave any residue or damage. Though the most important factor, and the reason why passive end-effectors will be valuable in 2013, is that they must be detachable in order to be advantageous. This means that when one drops an object in the goal box, the end-effector goes with it. The most efficient way to manage this is to have several passive end-effectors that an active gripper may grab and use to easily pick up a nail before depositing it, and the end-effector, in the goal box.


Kits generally fall into two categories; self-made and pre-made. Self-made kits are those which are not already assembled and often will require you to come up with your own design for assembling the included pieces into an arm. Pre-made kits are those that come already assembled and ready for use and were not intended to be disassembled and modified by the manufacturer. Pre-made kits must be modified in some way to comply with the rules, whereas self-made should not need to be.

Examples of self-made kits:

  • Lynxmotion
  • VEX
  • Lego Mindstorms

Examples of pre-made kits:

  • OWI (specifically, the OWI Edge robot arm)


There is a wide variety of methods to controlling your arm, ranging from simple plug-and-play systems to fully automated computer control. You can also choose between wired and wireless, though in most cases wireless with be much more exspensive.

Toggle Switches

This is by far the simplest and cheapest way to make a control system. Just use switches hard wired to simple DC motors and a battery. Flip the switch to turn a motor on or off.

Remote Control

Using an RC system is usually rather simple, but can be exspensive ranging from under hundred to several hundred dollars. For the inexperienced, visiting a local hobby shop and talking to the experts there can be a great way to learn about your options and they can often provide a good deal on a used system. The basic RC system comes with a transmitter and a receiver. You can plug a battery and servos into the receiver and use the controls on the transmitter to control the position of the servos. You can also use an Electronic Speed Control(ESC) to throttle a DC motor. The only downside to using an RC system with servos is that most limit the servo's range to 90°; some systems can be programmed to fully extend the range, usually to 180°, however these systems usually cost more.


The use of a microprocessor really opens of the possibilities and at around $100 isn't enormously expensive. However, to fully utilize, it requires knowledge in both programming and electronics. With a microprocessor, you can use a variety of input devices, including laptops, joysticks, buttons, and completely custom built controls. If you really want to be fancy, you can program the arm to do the entire event by itself, with or without sensors to correct errors. To use a microprocessor you will need a control board, such as the Lynxmotion Bot Board II or the Arduino, and the actual microprocessor, such as the BASIC Atom family.

Master-Slave System

One particular type of contolling system which is dependent on a microprocessor is the master-slave system. With this, you build a "Master" arm, an arm identical to the actual robotic arm(called the "Slave" arm), except where there are motors in the slave arm, you put potentiometers in the master arm. You then wire the potentiometers through the microprocessor. As you physically manipulate the master arm, the microprocessor will determine a position from the potentiometers. It will then match this position with the motors in the slave arm so that the slave arm mimics the position and movement of the master arm. It is a relatively easily used and incredibly efficient control system.


This competition is about 30% design and 70% practice. You could have a design that could win at nationals, but without practice, that means nothing. The best way to practice is to put yourself in the situation you would be in at a competition. To do this, you'll need an arena to practice in. Printable layouts can be found here. After practicing for a while, you should develop an efficient plan for the order you will move the objects, as well as how you will move them. A finalized version of this is part of the documentation you will hand in to the event director on the day of the competition.


The bulk score for this event is determined by how many of which objects are placed in which goal box:

  • 10 points are awarded for one of each type of item in the bonus box
  • 4 points for each goal box that contains a ping pong ball
  • 3 points for each pencil in the North goal box, 2 for West or East
  • 3 points for each nail in the East goal box, 2 for North or West
  • 3 points for each PVC pipe in the West goal box, 2 for North or East

Points are also awarded for other factors:

  • 1 point for any item in the North zone(not in a goal box) at the end of the run
  • 5 points for each goal box that never lied completely sideways during the run and at least one object is in the North zone(can be in a goal box)

For the 2012-2013 season, a bonus height score was added to minimize ties and add a new feature to the event. At the end of the run, but before time stops, the arm may lift a ping pong ball as high as it can. Though the end-effector used to lift the ping pong ball must have previously been used to score points. The event supervisor will measure the height after time has been called. 0.02 points will be given for every centimeter above the ground.


  • For each missing document, the team is docked 10% of their score, max of 30% penalty
  • For each incomplete document, the team is docked 5% of their score, max of 15%
  • 1 point is docked for each missing or incorrectly identified item during check-in


According to the 2013 rules tie breakers are as follows:

    1. Shortest competition time
    2. Least number of motors
    3. Lowest labelled circuit voltage