© Copyright 2006 Carnegie Mellon Robotics Academy 1/4

Teacher Answer Key: Right Face! Introduction to Mobile Robotics > Right Face! Construct

Observations: 1. What happened when you ran the program? The robot’s left motor, C, began spinning and the right motor, B, did nothing, resulting in the robot making a clockwise turn around the right wheel. 2. Which motor(s) spun? The program at this point only runs the motor plugged into Port C, which is the left motor, if the building instructions were followed exactly. 3. What direction did each motor spin? The motor spun in the ‘forward’ direction, because this is the default setting on the Motor Block in the program. 4. Did the robot’s body turn to its left or its right? The robot’s body turned to its right. If you were looking straight down on the robot from above, this would be a clockwise turn. 5. About how much did the robot’s body turn, relative to a full turn? The robot’s body turned somewhere between 90 and 180 degrees. Students can estimate the amount of turn to be approximately halfway between the two, or 135 degrees. If students are estimating, this represents approximately one third of a full turn. If they are calculating, their calculations will depend on the number of degrees that they thought the robot turned. If they used 135 degrees, then the robot will have made 3/8 of a full turn. 6. This behavior is called a “swing” turn. Around what point does the robot swing? The point under the right wheel. If students followed the instructions exactly, the robot should indeed have made a partial arc as it ran, with the center of the arc being located under the right wheel, which was stationary. That is the point around which the robot ‘swung.’ Contemplate

7. Write a brief one or two sentence description of what each icon in the program “SwingTurn” does.

Block 1: Turns on Motor C forward. Block 2: Makes sure Motor B is stopped. Block 3: Waits for a rotation of 720 degrees on Motor C. Blocks 4 & 5: Turn off motors B and C. 8. The robot started at position A on the diagram shown here.

It then turned in place until it reached position B.

© Copyright 2006 Carnegie Mellon Robotics Academy 2/4

i. Can you tell which direction it

turned to get to this position? Explain why or why not.

No, it is not possible to tell which way the robot turned to get to position B. It could have taken any of an infinite number of paths. Students may think they ‘know’ which way it turned, but they are most likely answering based on suppositions about the shortest path from A to B and the idea of doing the least work possible to get from A to B.

ii. Suppose the robot turned to its left to reach position B. What fraction of a full turn did it make to get from A to B? To get to B from A by turning left (counterclockwise) the robot would have made three-quarters (3/4) of a full turn, or 75%, if it is assumed to have turned in place.

iii. Suppose the robot turned to its right to reach position B. What fraction of a full turn did it make to get from A to B? To get to B from A by turning right (clockwise), the robot would have made one-quarter (1/4) of a full turn, or 25%, if it is assumed to have turned in place.

9. Consider the effects of some additional factors.

i. How do you think different wheels will affect the robot’s ability to turn? Does it matter? Yes it matters. Differences in wheel characteristics (diameter, width, tread, material, etc.) will have a noticeable effect on the turning ability of the robot. The most obvious is that wheels of different diameters will make the robot go different distances for the same amount of degrees the motor spun. Other differences, as in tread, material, width, etc., will affect the amount of friction between the robot and the surface, and may induce slipping. Slipping will cause the robot to go less far per motor degree turned than it would have if the wheels had gripped the surface better, resulting in less overall motion.

ii. Does the surface on which the robot is turning matter? Yes, the surface makes quite a difference. It is the friction between the surface and the wheel that gives the robot something to push off of when the motors spin. If there is less friction, say the robot is traveling on ice, then the wheels may slip, and some of the motor rotations will be wasted and produce no overall motion. If the surface is high friction, for example carpet, then the robot may grip the surface better than it did in the original trials, resulting in a different distance traveled once again.

10. The robot in the given program turns right by moving its left wheel forward while holding

its right wheel stationary. i. Could you also turn right by holding the left wheel stationary and running the right

wheel in reverse? Yes. This is a very useful method of turning in some situations. ii. Write the program to make the robot do the backward-right turn proposed in Part

(i). The program below shows Motor B moving backward, with Motor C immobilized, for 720 degrees. Note that the Wait For block is waiting for 720 degrees on Motor B in the backward direction, as the rotation sensor counts degrees moving forward as positive degrees and degrees moving backward as negative degrees.

© Copyright 2006 Carnegie Mellon Robotics Academy 3/4

Continue: Left Turn

Answer the following:

11. What program blocks are different between the left turn and original right turn behaviors? To change their programs, students will have had to swap the motors that are running (now Motor B should go forward and Motor C should stop). If they have done this, the programs still will not work. This is because of the Wait For block, which is waiting for Motor C to turn 720 degrees. However, in the left turn program, Motor C is not moving. So students will also have to reconfigure the Wait For block to look for rotations on Port B instead.

12. Could a left turn also be done with the backward-moving wheel idea from question 10?

Program your robot to make the backward-left turn. Yes, this can be done. The program below shows Motor C moving backward, with Motor B immobilized, for 720 degrees. Note that the Wait For block is waiting for motion in the backward direction on Motor C,

© Copyright 2006 Carnegie Mellon Robotics Academy 4/4

Continue Point Turn

Answer the following:

13. Describe the difference between the motion of a swing turn and a point turn. The only difference between the two motions is that in the Swing Turn, Motor B is immobilized, and in the Point Turn, Motor B is going backwards. This makes a difference in the radius, or “tightness” of the turn that the robot makes for each program. The swing turn is very wide and is rotating around the stationary wheel, whereas the point turn is an in-place turn, rotating around a point in between the two wheels.

14. Describe a situation where:

i. A swing turn is more useful than a point turn. A swing turn is more useful than a point turn in instances where you actually want to turn around a specific point, like a corner. It is possible, especially with the Taskbot model, to have a point turn take up too much space (as the robot rotates, it has to swing its entire front section around the two back wheels, which can produce a pretty large arc, especially if it has a sensor attachment on the front, too). Swing turns may be more appropriate for navigating tight mazes, because the robot can rotate around one of the maze walls without having to take up the space needed for a point turn. Swing turns are also appropriate when you want the robot to travel as it is turning. By varying the speeds of the motors, you can produce swing turns that turns around different points than the point under one of the wheels. The robot can make a very wide arc, so it can change its position dramatically as well as change the direction that it is facing. ii. A point turn is more useful than a swing turn. Point turns can be more useful than swing turns when the position of the robot it more critical. For example, if you know that the robot needs to move forward to a certain point, and then to move 5cm to the left of that point, a point turn will allow the robot to turn to the left without compromising its position. For robot models that are more compact than the Taskbot, like the REM, point turns can be used when space is at a premium. If the robot had, for instance, tank treads on either side rather than a caster wheel and two large wheels, then it could turn in place under its own footprint, and not bump anything as it changed direction. This is a key feature when workable area is limited. Robots that find land mines, for instance, can turn in place, because they know the ground they’re on is safe. It would be more dangerous for them to cover new ground while trying to turn, because they could potentially hit a mine. There are a myriad of other possible reasons suggesting why to use one turn rather than another. In some cases, the choice is clear, in others, it comes down to the roboticist’s preference.