US Sensor Wait Block: The Difference between the Change and Compare Modes

During my semester teaching with the Mindstorms EV3 software, I actually had to call and speak with a LEGO technical support representative because the difference between the ultrasonic sensor change and compare modes was eluding me. The representative was very helpful, and I basically came to understand the simplicity of the settings as the difference between the meaning of two prepositions, 'to' and 'from.' (See EV3 Wait Block- Ultrasonic Sensor.)

This semester, I designed an activity for the students to figure this out for themselves. The activity involved collecting class data in a Google spreadsheet so that the kids could draw the conclusion for themselves. They simply followed the instructions in the LEGO Mindstorms EV3 Education edition software. The kids used the Robot Educator-Basics-Stop at Object tutorial. This tutorial uses the ultrasonic sensor wait block on the "change" mode. The students completed this tutorial from different distances that I tried to control and then they recorded their  data in a spreadsheet shown below.

Stop at Object Change Mode (11 cm)
Start Distance from Object (cm)	How far Robot Moved (cm)	End Distance from Object (cm)
90	11	79
70	12	58
40	15	25
80	12	68
60	10	50
90	12.5	77.5
45	5	39
90	11	79
60	8	49
50	11	39
30	17	13
90	80	10
20	10	30
44	13	31
47	11	36
83	10	73

From this data, the students were able to conclude that even with a couple of outliers, eleven of the data points show movement of 11 cm +/- 1-2 cm TO the object. 

The students then repeated the same experiment using the compare mode instead of the change mode. Their data is shown below. 

Stop at Object Compare Mode (11 cm)
Start Distance from object (cm)	How far Robot Moved (cm)	End Distance from Object (cm)
90	75	15
90	42	48
60	46	14
30	16
70	25	45
50	7	43
90	47	43
90	24	76
45	12	38
15	4	12
15	26	41
30	11	41
37	11	48
17	4	12

This data did not turn out as I had hoped. The third column should have contained end distances FROM the object closer to 11 cm. Some 11 cm were recorded in the second column which could have been placed in the wrong column by students. 

Still, with a couple of demonstrations with the students looking on, they came to the conclusion that the "compare" mode causes the robot to move a certain distance FROM an object. 

All in all, this was a fun exercise to introduce the students to the idea of collecting a set of data, drawing conclusions, and discussing possible sources of error. 

For me, I learned that I need to provide students with more explicit directions on how to record data. 

Thank you for reading.