CSC 126 Robotics

Synthesizing RoboLab Concepts 

Objectives

Tools and Parts Needed


In this lab, you will be asked to put together concepts discussed in our previous labs and use the new NXT wheel encoders in order to synthesize and apply these concepts in more challenging programming situations.  

We will need some new icons which are not currently in your RoboLab menus, so copy and install the patch from \\Academic\Academic\Mathematics & Computer Science\PearceJ\Resources

In this lab you will be exploring the improved motors, which unlike the motors for the RCX contain wheel encoders.

The NXT Motors
NXT motorNXT motpr gearsContained in the new Lego Mindstorms NXT kit are three newly designed servo motors. The new NXT motor delivers high torque because of its internal speed reduction gear train. Because of that, it also turns slowly and the efficiency is somewhat reduced. Although this motor could physically be connected to RCX with a compatibility cable, it is not recommended for use on a RCX because the high current it consumes is too much for RCX current output.

Even more importantly, these new motors include rotation encoders, which tell the NXT the position of the shaft with an amazing 1° accuracy. This is an important new feature because the RCX motors had no built in encoders, so using "time passed" was one common but inaccurate method for applying current, but using "time" in this way is not an accurate way to determine how much a wheel has turned. The NXT encoder wheel and optical fork provide a very accurate rotation sensor function to the NXT motor, which is very useful for such things as determining how much the wheel has turned, and can be used for computing distances traveled or number of degrees rotated.

Using the NXT Rotation Encoders

A wheel (or wheel shaft) encoder sensor detects precisely how many times a motor shaft turns by having an encoder sensor monitored as the wheel shaft turns.
To drive a robot with two motors straight or to turn a precise amount, use the motor sync iconNXT synch and the NXT SMART rotational encoder NXT SMARTicons.

The motor sync icon tells the "follower motor" to follow whatever "the controller motor" does. The following program will drive straight forward 7.0688 inches, and turn  360 degrees and then stop:
           Using Smart

In this example, the motor power icon turns on Motor A (and thus Motor C, because it is following Motor A). The SMART rotational encoder controls Motor A's movement to the specified angular position. Motor C follows along, exactly in sync with Motor B.

Once you have synced two motors, they will only respond correctly to commands for the controller motor. If you want to control the follower motor on its own again, you must turn off the synchronization. To do so, use the motor sync icon without any wires attached to it: NXT synch.

Driving the NXT Straight
To drive the NXT straight, we should synchronize the wheels and then drive forward.  The formula for the circumference of a circle is Diameter x PI.  The NXT wheels are 2.25 inches in diameter, so we can find the distance traveled with two wheels synchronized in 1 rotation of the wheel as 2.25 inches x PI = 2.25 x 3.14159... = 7.0688 inches.  You may use this number to compute distances traveled as long as you understand that each revolution of the wheel is 360 degrees.  

Turning the NXT
To turn the NXT, we can drive just one of the wheels forward.  The wheel-base of the NXT is 4 inches, so if only one wheel is turning, the NXT will drive in a circle which is 8 inches in diameter.    We must convert this full circle to degrees, so  we use the fact that there are 360 in a full circle, which is 2 x PI.
(8 inches x PI)  x  (360 / (2 x PI)) = 1440 degrees on one wheel for a full 360 degree turn of the robot.

Are these values going to be perfect?  No. Because of wheel slippage on the floor, we may not see a perfect response, and the floor surface will make a difference.  Even so, this is an improved methodology over the RCXs.

Working with Values in Containers

We have learned that a programmer use the variable to store data or any information by assigning values. As you know, in RoboLab, we store data in container by filling it with the data. In the diagram below, the red container is set to the value of 14.  Simple math can be used to store data and change the values that are already stored. There are other mathematic operations like subtraction, division, and addition. So one can use any math and also any value to use with those operations. In the diagram on the right, the number 12 is being subtracted from the red container. Therefore, if this is run after the above command, the value of the red container is now 2. (14 - 12 = 2)

In this lab, you will need to some kind of arithmetic technique with containers in order to determine how far the robot will turn....  

In addition, remember that you can use the Clicks Container, Clicks Containter which allows the robot to count and store the number of clicks since the Clicks Container was last reset.  By using this, the robot can count the number of "clicks" made on one touch sensor.  One can use this for example, to count clicks made on one touch sensor while another another touch sensor is not pressed.  Just like any other container, it is important to zero it before using it.  Remember that we used the following code segment to allow the engineer to enter a count:

Counting clicks


One really nice feature of the NXT is that the display of the sensors will happen automatically when they are used.

Team Roles

Decide which of you will fill each of the following roles for this lab: Note that it is expected that each member of your team to remain in his/her role for the whole lab.

Your Task

In this lab, you will create a program which allows the engineer to enter a number between 2 and 8.  (You may do this with two touch sensors set up with one as a "counter", and the other as a "stop" to record the number, as we did in an earlier lab.)

After this number has been entered and recorded, your robot should drive around and around and around following a polygonal pattern on the floor given by the number entered by the engineer.  For, example, if the engineer enters 3 clicks, the robot should drive around and around in a triangular pattern.  As another example, if the engineer enters 4 clicks, then the robot should drive around and around in a square.  In general, the pattern driven by the robot should be the appropriate one of the following patterns:

regular polygons.
The robot should then continually drive in this pattern on the floor over and over until....
the engineer indicates that the robot should stop.  At which time the robot should immediately stop where ever it it and beep.  This "stop" indication must be done with a sensor.  I suggest that you use one of the two touch sensors.  This command to stop must be detected by the robot at any time, which means you will want to use event handling

Background Geometry Needed to Compute the Robot's Turn Angle

When the robot makes each turn in the polygon, how far does it need to turn?   Consider, the following images of a selection of the regular polygons:

. . .exterior angles. . .
Mathematicians call angles colored in red in the above images "exterior angles" of the polygon.  If you imagine the robot driving straight along the side as it goes counter-clockwise around the polygon, you should see that the "exterior" angle is exactly the angle which the that robot needs to turn through in order to be able to drive along the next side.  

So, our next question is how big is the exterior angle of a regular polygon?
 
Finding the size of the each exterior angle of a polygon is actually pretty simple.  It is a theorem from geometry that for all polygons, the sum of all of the exterior angles is always equal to 360º. Note that each polygon has the same number of exterior angles as it has sides. Since all the exterior angles in a regular polygon are equal in measure, to find the measure of each exterior angle that our robot must turn, we just need to divide 360º by the number of exterior angles.   Thus, the turn our robot must make is just 360º / n, where n is the number entered by the engineer.    Isn't that a surprisingly simple computation??
An observation from a student in one of the sections:  
"This makes so much sense!  If the robot is going to drive around a triangle, it has to turn 1/3 of the way around each time. So, in general, it has to turn 1/n of all of the way around each time. And, you show us how to make it turn all the way around..."


With that Mathematical background, you should be ready to design your algorithm...

You will need to decide what sensors you will need and where they will be connected.

Next, use PseudoCode to design a program which does the following:
  1. Robot detects the number entered by the engineer, and the indication that it should begin driving...  
  2. Robot computes the amount of time it will need to make the appropriate turns.  As you construct your algorithm, be sure that your robot will do something appropriate if the engineer enters less than 2 or more than 8 clicks.  If you are working after class on this lab, you might consider programming the robot not to move at all if the engineer enters only 0 or 1 clicks.
  3. Robot drives around and around and around in the appropriate polygonal pattern.  The polygon's sides should each be 2 feet long.  Note that the computations for the sides lengths and the turn angle should be computed as best as you can compute it... the instructor understands that the side lengths and the turning will not be perfect due to wheel slippage... just do your best to get the robot to drive in a regular polygonal pattern by using a correct computation.  Note that the corners of your polygons should be pointed, not curvy--this is most easily accomplished by stopping one wheel while the other wheel drives forward.  
  4. When the robot detects the engineer's "stop" indication, at which time the robot will immediately stop where ever it is and beep.  Note that your robot should drive forever if the engineer never enters the indication to "stop."  Hint: This should be accomplished as an event because it can happen at anytime, but do not add this event in to the code until you get everything else working!!!
Next, implement this algorithm in RoboLab:
  1. Be sure to include explanatory comments to your RoboLab code by using the Edit Text icon: edit text icon  In particular, it is required that you add the following comments:
  2. Finally, modify your pseudocode, robot, and RoboLab program to improve your robot's performance. 
  3. Answer the questions in your Lab Report.

Your Lab Report

All lab reports should be self-contained and should contain all of the following information at the top: As usual, for this lab, your team will submit two files: the Lab Report entitled yourteam-L10 and the RoboLab program entitled yourteam-L10. For the report, your team should discuss the answers to each of these questions. Using correct spelling and good grammar, this lab should then address the following questions:
  1. Your Pseudocode: Include the final version of your pseudocode in your Lab Report.
  2. Your Robot: Briefly describe which sensors and actuators need to be attached to which ports for your implementation. 
  3. Computation: Explain your computation and how you arrived at this algorithm.
  4. Error Handling: Discuss what your robot will do if the engineer enters too few or too many clicks.
  5. Your Success: In a paragraph or so, describe how to use your program and whether or not your robot functions as desired. If not, what goes wrong and under what conditions?
  6. NXT vs RCX: Describe how this lab would have been easier or harder on the RCX and why.
  7. Comments and Suggestions: Write a paragraph that summarizes your team's reaction to RoboLab and to this lab. If there are any problems you encountered or any questions that remain, please ask! Also, be sure to include any suggestions you have for how this lab could be improved.