SUMMARYThe goal of our project was to design a robot that would climb stairs in a skillful, calculated and efficient manner. Climbing a set of stairs is not a new challenge for robots, in fact it is often a common obstacle used by designers to showcase the robot’s skills. Another aspect of our challenge was to use two LEGO NXT robots (the brick/brain) in cooperation with each other.

BACKGROUNDBiped robots are most often the ones equipped to climb stairs. Their function is modeled after human movement. We also researched all-terrain robots that are designed to plow over various obstacles with brute force. The smaller scaled robots, such as ours, usually use the brute force method and are not able to skillfully climb stairs. With the specifi-cations of the LEGO NXT kits we decided a biped robot would exceed their ability.

The motivation for this project was the sheer physical challenge presented by the specifications of the NXT kits. The design stems from a crab-like motion with two independent legs that lift a center body up the stairs. We were interested in using Bluetooth technology to communicate between both bricks.

TEAMLauren Miles is a Visual Communication Design Senior with no previous program-ming experience. An interest in interaction design and technology lead her to join the class. So much of design is intuitive, the chance to work on a project that gives the satisfaction and problem solving of a right answer was welcomed.

THESTAIR CLIMBER

University of Washington

Exploration in Designing Behaviors

Winter 2007Andrew Gawronski is a Computer Science Senior with experience teaching programming to computer science introductory level students. His interest in visual languages for education moti-vated him to try this experimental course and explore working with students of other disciplines.

Chris Schlechty is a Computer Science Senior. He likes robots; he hates stairs. He thinks robots that could climb stairs would be cool. Chris is interested in human-computer interaction and robotics.

THE PHYSICAL PROCESSWe have created a robot that works in complete cooperation with another robot to climb a set of stairs. The design is modeled with a crab-like body and uses the communication method similar to a common fire truck.

The Stair Climber consists of two legs that connect to a central body composed of the NXT bricks. We engineered a system that consists of two LEGO NXT bricks that control different parts of the Stair Climber. One brick controls the front legs and the other controls the back. In a prone position, the robot drives with both legs extended parallel to the base of the body until it encounters a step with its touch sensor mounted on the front leg.

The Stair Climber’s two drive arms allow the robot to evenly lift and lower its large

and heavy body from both sides. On each side, there are two motors which lift the drive arm; at the end of the drive arm is a third wheel that moves the robot forward. When the robot encounters a stair, with its touch sensor, the robot raises its front arm and places it at the top of the step. Once the front wheels are at the top of the step, the robot begins to push down, applying pressure on the stair and lifting the body. Simultaneously, the back arm and wheels drive forward and lift up the back half of the robot. After the center of the robot is partially lifted, the robot drives forward to align its body over the next step. Once on the step it raises the back arm and proceeds onward. The program loops to look for another step to repeat this process.

THE TECHNOLOGY As mentioned above, the two logic bricks of the Stair Climber control different

functions; one controls the back half and the other the front. In order to achieve synchronous execution of tasks, we must communicate constantly via Bluetooth technology to let the other logic brick know at what point in the process it is. It works much like a fire truck in that the front and back drivers communicate with each other to navigate the truck. Due to the sheer size of the NXT bricks the programming of this robot requires quite a bit of coordination to manage the difficult angles each motor needs to move. In order to achieve synchronous execution of tasks, we send messages between the two logic bricks and they cannot move on in the program until they have been received. This ensures that both logic bricks are at the same point in the program.

THESTAIR CLIMBER

The Stair Climber starts in a still position. It drives forward until it hits a step with the touch sensor. Then it backs up and raises the front arm.

With the front arm raised the Stair Climber drives forward until the front wheels are on the step.

With the top wheel on the step, the motors near the body push down and therefore lift the body to a designated height.

Once at the desired height the drive motors (attached to the wheels) drive forward pulling the back of the body onto the stair.

Once the body is safely on the step, the back arm lifts to clear the step, while the front wheels drive forward.

The end position is with the Stair Climber completely horizontal on the next step. The program loops and starts looking for another stair.

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Hardware:

2 x NXT Controller Brick

6 x NXT Servo Motors

1 x NXT Touch Sensor

Various LEGO Pieces

MASTER/FRONT PROGRAM

SLAVE/BACK PROGRAM

THESTAIR CLIMBER

MASTER

SLAVE

Send message to SLAVE to drive forward.

Initialize all motors. Send message to SLAVE to go.

Drive forward until touch sensor pressed.

Send message to slave to back up.

Back up and lift front arm.

Receive message to move arm down

Move arm down Send message to SLAVE to lift up

Continue to lift front

Receive message from the SLAVE that back is up

Drive forward and send message that program is over

Receive message from MASTER to go then drive forward

Receive message from MASTER, then drive in reverse

Receive message from MASTER to drive forward

Drive forward Send message to MASTER to move arm down

Receive message from MASTER lift back end

Lift back end and drive forward

Wait and then lift back arm up off the stair

Send message to MASTER to drive forward

Receive message to stop all motors

Stop all motors

The following programs, created in Mindstorms, controls each end of the robot. The first diagrams show the actual programs created to control the robots. The next diagram shows the links of communication between the 2 logic bricks. All communication was done with Bluetooth to let the different bricks know at what point in the program they were.

COMMUNICATION PATH BETWEEN MASTER & SLAVE

THESTAIR CLIMBER

PROCESS & RESULTSWe had a fairly smooth process with dramatic progress in the beginning and a slower refinement period to complete the project. When the concept was developed, Lauren’s original idea was to have a

“reverse slinky” style robot that could flip its body to make it up the stairs. The original project concept was to include a second that could carry a ball and have the two robots work together to make it up the stairs with the ball. We realized that physics simply would not let a robot somersault up stairs, and that was when the crab-like design was created for the Stair Climber.

The design required the use of six LEGO motors for the different joints and for pure strength. Since each LEGO brick can only control three motors (without access to a motoplexor) it was deter-mined that we would need to use two Mindstorms bricks. The two different bricks would act as the cooperation element required for the project.

We decided the most important part of the task was creating the Stair Climber and after that was done we would look at the other robot and the ball situation.

Construction of the Stair Climber went quickly. There was a strong physical concept to start with and small modifica-tions were made to help balance and leverage as we began programming and saw the actual behavior of the Stair Climber.

It took a little while to adapt to using the Mindstorms program and underestand-ing the Bluetooth functionality. After that, the next step was to make sure that the motors would have enough torque to lift the heavy bricks into the air. We did some simple programming tests to make the robot lift its middle section .

The programming process after that was a tedious effort to balance the weight between the front and the back of the Stair Climber. It was difficult to lift the front and the back simultaniously without causing the angle of one leg to increase to the point where it was too drastic for the other motors to lift the body. For example, when the front leg rests on the stair and is ready to start the lift, if the back leg is not slightly elevating the middle section, both joints located next to body are at greater

EVALUATIONWe felt very successful in the completion of this project. The communication between the two robots was such a strong level of cooperation, we were completely satisfied with how the concept changed from the original idea.

It was interesting to collaborate with other people from different majors and see how differently designers and computer science majors think. For example, Lauren would think up different ways to build the robot with out really knowing the physics behind it would work; and Andrew and Chris would push to complete the path we were on before veering off to new ideas. It seemed that while the computer science majors were confident in making our robot work, the designer was constantly asking if this was the best way to make it work. In the end, it made sense to stick with our original design; had we veered off to try different physical designs we would have had a nice variety of design directions, but an unlikely chance of having the time to make any of them complete the task at hand. Had this been completely a design exploration project that would have been a valid process, but being that this project was also to introduce a behavior

angles than their respective motors have the power to turn against. It was a series of small changes to perfect the timing and Bluetooth communication between the two bricks.

Eventually the Stair Climber made it up one stair. However, this is a “Stair Climber” robot, not a “Curb Climber” robot so the next step was to loop the program so that the robot could go up multiple stairs. The challenge we encountered was that the program we had written did not allow the robot to reset itself after one step; when it would loop to go up the next step because it wasn’t in the exact start position as before the timing and calculations would fail to work. We resolved this problem by adding a few commands at the end of the program, to kill all the motors and put both the front and the back legs in their natural state before looping.The only other thing that we encountered was that the Stair Climber for some reason could not climb three stairs in a row. At first we thought it was an issue with the stairs we built, that maybe the top step was off in measurement, but if we started the Stair Climber on the second step it would go up the top step no problem. We determined that the motors for some reason could not pull enough energy from the battery to repeat the process more than twice. But if we restarted the program it could repeat the first two steps again without recharging.

through programming it was important that we got to that stage. It was a positive experience for all three group members each gaining interest and experience in an unfamiliar area.If this project were to continue there would be two directions to take. One would be to incorporate the second ball-carrying robot, from our original concept, with the current build; and the other direction would be to explore further more efficient ways for the Stair Climber to work. The final design of our Stair Climber had to have a specific size of stair to work, it would be nice to expand this project to develop a structural model that could ascend a variety of stair dimensions.