Worksheet Answer Key: Tree Measurer
Projects > Tree Measurer
Maroon = exact answers
Magenta = sample answers
Construct: Test
Questions:
Caliper
Reading
Reading #1
Reading #2
1492
1236
1. Subtract to find the difference between the first and second readings. Record your
answer below.
Students should find this by subtracting the second reading from the first reading.
For our sample values, this would look like:
Difference = Reading #1 – Reading #2 = 1492 – 1236 = 256
Difference Between
Reading #1 and Reading #2
256
2. Measure a third time. Are the readings consistent?
They should not be consistent. Students should record their third measurement on their worksheet.
They should then compare the measurement to the first two. It is unlikely that the third measurement will
be similar to either of the first two, unless the student took care in repositioning the arm to start at the
same place each time. In general, the three measurements will be quite varied. This finding should lead
students to question the validity of their measurements, because the object dimensions have not
changed, but the measurements have, and by a large percentage.
© Copyright 2006 Carnegie Mellon Robotics Academy
Designed for use with the LEGO® MINDSTORMS® Education NXT Software and Base Set #9797
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Construct: Revised Device
Questions:
Revised
Caliper
Reading
Reading #1
Reading #2
1027
1093
3. How far apart were the first and second readings now compared to before? Subtract
to find the difference.
Students should find this by subtracting the second reading from the first reading.
For our sample values, this would look like:
Difference = Reading #1 – Reading #2 = 1027 – 1093 = -66
Difference Between
Reading #1 and Reading #2
66
In our examples, the difference between the first two readings was 256 degrees, while the difference in
the next set was only 66 degrees. This shows that the second readings were much closer in line with
each other than the first. This can be attributed to the increased accuracy provided by starting the caliper
arm at the same point each time. Further readings would continue to verify this.
© Copyright 2006 Carnegie Mellon Robotics Academy
Designed for use with the LEGO® MINDSTORMS® Education NXT Software and Base Set #9797
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Contemplate
4. Why did the measurer give inconsistent readings when you had to reset it manually?
Because the arm started in a different position for each measurement. The caliper measures by
determining how much it must close before it hits an object. If it is not reset to the same place before it
starts to close, then it will close a different distance every time it tries to measure the same object, thus
resulting in a huge disparity between readings.
5. What is the purpose of adding the second Touch Sensor?
To give the caliper arm a point from which to start. The second Touch Sensor is used as a “stop” to
tell the arm when to stop opening. This gives the arm a specific starting position to use for every
measurement, so that it can give repeatable measurements. When this second Touch Sensor is pressed,
the Rotation Sensor resets itself to zero and the caliper arm begins to close.
6. The motor can turn the arm, but the arm cannot turn the motor. Why is this? Try it for
yourself and investigate.
The worm gear stops the arm from turning the motor. When using a worm gear, motion can only be
transferred in one direction, and cannot be transferred back in the other. This is why it is necessary to use
the 20-tooth grey gear located on the arm to close or open the caliper.
In more technical terms, the worm gear applies force to the spur gear perpendicular to the spur gear’s
axis of rotation, which makes the spur gear rotate. If the spur gear tries to transmit force in the other
direction, it will apply force to the worm gear along the worm gear’s axis of motion instead of
perpendicular to it, which will make the worm gear want to slide along its axle, rather than rotate around it.
© Copyright 2006 Carnegie Mellon Robotics Academy
Designed for use with the LEGO® MINDSTORMS® Education NXT Software and Base Set #9797
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7. Summarize how the caliper works. Explain parts of the programming as well as the
physical design of the mechanism.
Students may answer this question in any number of ways. They may choose to answer with a text
description or bullet points describing parts of the robot and program, or with a sketch or diagram of the
robot and the program. Here are some examples:
•
Robot uses Motor A to open arm until Touch Sensor on Port 2 is pressed.
Touch Sensor
on Port 2
Wait For Touch Sensor on
Port 2 to be pressed
•
Motor A resets Rotation Sensor.
•
Robot uses Motor A to close arm until Touch Sensor on Port 1 is pressed.
Touch Sensor
on Port 1
Wait For Touch Sensor on
Port 1 to be pressed
•
Robot takes reading of Rotation Sensor on Motor A and displays it on NXT screen.
© Copyright 2006 Carnegie Mellon Robotics Academy
Designed for use with the LEGO® MINDSTORMS® Education NXT Software and Base Set #9797
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8. The sensor at the end of the moving arm always stays perfectly perpendicular to the
stationary arm, even though it is mounted on a piece that is rotating. What geometric
structure allows this to happen?
The four-bar linkage, which is the set of four beams
attached by pins that comprise the moving arm,
provides this capability (see picture on right). In a fourbar linkage where each pair of opposing links are
equal in length, the two links of the pair will always be
parallel. This means that the top link, the one on which
the Touch Sensor is mounted, will always be parallel
to the bottom link, which is mounted to the NXT, thus
keeping the Touch Sensor perpendicular to the other
arm at all times. This structure is used in some vehicle
suspensions, as it ensures that the wheel is always
perpendicular to the road surface, while being able to
move vertically to maintain contact.
9. A robot is a device that can sense, plan, and act. Do you consider this caliper device
to be a “robot?” Why or why not?
This is up to the student to decide. The stronger argument is for the fact that the caliper device is a
robot, because it can sense, plan, and act. However, it is also possible to argue that the device is not a
robot, based on qualifying the above criteria. Students may make either argument, so long as they justify
their response with an explanation. Some of the arguments may be along these lines:
It is a robot:
• It can sense – it uses Touch Sensors to know when to stop opening or closing, and a Rotation
Sensor to tell how much it has closed.
• It can plan – the program tells it what to do next, once something (like a Touch Sensor being
pressed) has occurred.
• It can act – the robot has motion, meaning that one of the arms of the caliper moves.
It is not a robot:
• It cannot “sense” anything from its environment. It does not get feedback from the outside world,
besides using a Touch Sensor as a switch to stop the arm when it is touching an object. The
object might be considered a part of the system when it is between the caliper’s arms, because it
was knowingly positioned there by a human operator. The caliper does not use any other input
from its environment.
• It does not have a plan. The program is a set of actions that the robot must follow, rather than a
way for the robot to make decisions based on inputs. This is more like a simple machine than a
robot. It just goes about its business, regardless of what is going on around it.
• It does not act. The caliper arm can open and close, but only in response to a direct command
(pressing the orange button to run the program) from a human. The robot does not move itself,
nor does it act to respond to outside influences, such as someone holding the caliper arm open.
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Continue Section Notes
Continue 1: Add Sensor
The photo shows how students can add a Light Sensor to the arm of the Tree Measurer. The example
program shows how to modify the program to check that an object is in place before starting the
measuring behavior.
The Light Sensor is using the “Generate Light” option which is the default configuration. That way, the
Light Sensor turns on its own red light so that the sensor is checking the reflection of light from the object.
In the program, the Wait For Light Sensor Block is added to the program after the arm is in place and the
program is waiting for the object to be inserted. In the example, the Wait For Light Sensor Block is
configured for less than 15%.
Students will have to adjust the Light Sensor Block settings to suit the classroom conditions. Refer them
to the video example View Sensor Values in Basics: NXT Menus. Choose the Reflected Light option in
the View menu on the NXT. This robot shows the Light Sensor attached to port 3.
Students can build up the back of the gripper to provide a “wall” so the light reflects from it when no object
is in place. The reflection from the wall can serve as a baseline reading to compare with any other
readings. It may increase reliability.
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Designed for use with the LEGO® MINDSTORMS® Education NXT Software and Base Set #9797
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Continue 2: Add Feedback
The solution shown in the program adds Display Blocks and a Sound Block to provide more feedback to
the user of the Tree Measurer robot. The first Display Block is positioned after the robot caliper arm is in
position and ready to measure. The Display Block shows the words “Place Object.”
The second Display Block is positioned to show the word “Measuring…” once the object is detected. The
text is positioned to appear in the top of the NXT display so that the numerical reading can appear below
it once the measuring is complete. The third Display Block is the original one from the program. However,
it has been configured so that it does not clear the display. That way, the numerical reading shows along
with the words “Measuring…”
A Sound Block is positioned at the end of the program. This one is configured to say “Good Job!” when
the program is over but any sound file can be chosen.
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Designed for use with the LEGO® MINDSTORMS® Education NXT Software and Base Set #9797
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Continue 3: Store Data
Using the File Access Block, students can save their sensor data to a text file on the NXT and then
upload it to their computer. If they have access to a spreadsheet or other software, they can analyze the
data or convert it to another form.
The LMSTreeFile example adds the File Access Block to perform two functions. The first File Access
Block writes the Rotation Sensor data to a file called TreeMeasures. The second File Access Block closes
the TreeMeasures file.
Notice that the Rotation Sensor Block is still wired to the Number to Text Block so that the current value
that is read is displayed on the NXT. If students wire the File Access Block to the Number to Text Block,
the first data point in the file will be displayed, rather than the last so the NXT will not show the current
value even though it is collected by the NXT.
To use the data file, students must first upload the TreeMeasures file from the NXT to the computer. A
complete explanation of uploading is shown in the video found in Advanced: File Access Block, Upload a
File.
© Copyright 2006 Carnegie Mellon Robotics Academy
Designed for use with the LEGO® MINDSTORMS® Education NXT Software and Base Set #9797
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