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9. The Color Sensor

Grady Koch¹

(1)

Yorktown, VA, USA

The Robot Inventor includes a Color Sensor, pictured in Figure 9-1, that works with light. It can measure the color of objects placed in front of the sensor, as well as the brightness of light that hits the sensor. The Color Sensor includes a circular light on its front surface to provide a source of light for objects to reflect. An exercise in this chapter will work with this brightness reflection function to build a beeping tone generator. Then to use the namesake function of the Color Sensor, the chapter project builds a machine that sorts liftarms by their color.

Exercise: The Brightness Beeper

The Brightness Beeper, shown in Figure 9-2, serves as an introduction to working with the Color Sensor. The Brightness Beeper will play a different sound on the Hub’s speaker depending on how much light is reflected into the sensor. This light will come from the Color Sensor itself.

Figure 9-2
The Brightness Beeper plays a tone that varies with brightness of light

Building the Brightness Beeper

Building instructions for the Brightness Beeper are given as follows. One piece of the exercise is a stack of three T-shaped liftarms that aren’t mounted to the baseplate. Rather, this stack of liftarms remains free to slide around in front of the Color Sensor. Its function is to act as a reflector to bounce light into the sensor.

A quick look at Color Sensor performance can be seen by connecting the Hub to the Robot Inventor app and opening the Hub status screen. As shown in Figure 9-3, an icon indicates the Color Sensor’s state. The drop-down menu next to the icon allows selection among several modes of operation. Selecting Reflect and then placing the stack of liftarms in front of the sensor should result in a percent value that indicates how much of the light from the sensor’s built-in light source is being reflected back. Moving the reflector to various distances in front of the Color Sensor should make the light level percentage change.

Figure 9-3
The Hub status screen will show a measurement when the Color Sensor is connected

Programming the Brightness Beeper

While the level of light measured by the Color Sensor can be viewed in the Robot Inventor app, such as in Figure 9-3, this data can also be put to use. As an example, the light level can be made proportional to the pitch of a tone. When the light level changes, by moving the reflector liftarm stack, the pitch will change. Coding examples can be found as follows.

The Word Blocks Code

Figure 9-4 shows the Word Blocks code for the Brightness Beeper. The block that plays a sound is within a loop that can be terminated by pressing a button on the Hub’s front panel. This means for ending a program has been used throughout this book. The sound-generating block is described in detail following Figure 9-4.

Figure 9-4
Word Blocks code for the Brightness Beeper

Step 3: The play beep block produces a sound on the Hub’s speaker. The Color Sensor’s reading, represented as a percentage, is converted into the MIDI note to play. A higher MIDI value corresponds to a higher pitch, so the beep will get higher when more light is reflected into the Color Sensor. The default duration setting of 0.2 seconds is a good choice to produce a quick beep that will rapidly update as the loop restarts.

The Python Code

The following Python program performs the same task as the Word Blocks code; it plays a beep on the Hub’s speaker at a pitch that varies based on the brightness detected by the Color Sensor. The program begins by importing modules and creating a myhub object, as done throughout this book:

from mindstorms import MSHub, Motor, ...

from mindstorms.control import wait_for_seconds, ...

from mindstorms.operator import greater_than, ...

import math

import hub

from sys import exit

# Create your objects here.

myhub = MSHub()

1 color = ColorSensor('B')

# Program "brightness_beeper". Beep speaker at pitch related to brightness.

while True:

if myhub.left_button.is_pressed() or myhub.right_button.is_pressed():

exit()

2 brightness = color.get_reflected_light()

3 note = 44 + int(brightness * 0.79)

4 myhub.speaker.beep(note,0.2)

An object is needed to represent the Color Sensor, given the variable name color in this case 1. A function associated with color is get_reflected_light(), used to measure the brightness of the light reflected from the sensor’s built-in light source. The measurement result from this function is assigned to a variable named brightness 2. However, this brightness value will be between 0 and 100 percent, which won’t work well to represent a musical MIDI note, as these notes range from 43 to 123. So a mathematical manipulation converts the possible brightness values to corresponding values within the desired MIDI range. This new value is assigned to a variable named note 3. The speaker.beep() function plays this note for a duration of 0.2 seconds 4. After the note is played, the loop runs through again to play another note.

A good start on using the Brightness Beeper is to place the stack of liftarms a couple centimeters away from the Color Sensor. A beep should result that quickly repeats. Moving the liftarm stack closer or farther away will make the beep change in pitch. But if the stack is placed very close, such that the stack almost touches the Color Sensor, the tone goes to a low pitch. This low pitch indicates that not much light is being reflected into the Color Sensor, which happens because the direction of the reflected light is not well aligned to hit the light-sensitive part of the Color Sensor.

Project: The Color Sorter

The Color Sorter, pictured in Figure 9-5, automatically sorts three-hole-long liftarms by color. Liftarms of various colors are fed into a chute at the top of the machine, and a selected color will be separated into one side of a basket. All unwanted colors go into the other side of the basket. The Robot Inventor set comes with 28 three-hole-long liftarms in colors of black, turquoise, blue, red, yellow, green, and white, so there are plenty of three-hole-long liftarms for the Color Sorter to work with. The Color Sorter incorporates many of the ideas presented in this work to combine a sensor, motors, and mechanism.

Figure 9-5
The Color Sorter places liftarms of a selected color into one side of a basket

Building the Color Sorter

The Color Sorter has four separate components to build: the baseplate, the chute, the pusher, and the basket. Building instructions for each of these four components are l presented as follows. The final assembly step then combines the chute, pusher, and basket onto the baseplate.

The Baseplate

The baseplate serves as a platform and connection point for all the other parts of the Color Sorter, as well as the Hub and one motor. It’s important to zero the motor before its installation in step 10.

The Chute

The chute holds a vertical stack of three-hole-long liftarms to sort. As one liftarm gets pushed into the basket, gravity loads in the next liftarm from the vertical stack. The Color Sensor, also part of the chute, measures each liftarm’s color. The liftarm being measured temporarily rests against the Color Sensor’s face.

The Pusher

The pusher moves a liftarm off the Color Sensor after the liftarm’s color has been measured. As the pusher retracts, the next liftarm in the chute is loaded onto the Color Sensor to repeat the measure-and-push action. The pusher is based on an eccentric mechanism, with a similar design to that built in Chapter 3. The difference here in this application is that the eccentric is driven by a motor, rather than by a manual hand crank. The motor zero position should be verified before installation in step 1.

An illustration displays how a pusher connects to a platform of rectangular frames to build a color sorter.

The Basket

The basket holds the sorted liftarms, with the selected color on one side and other colors on the opposite side. A motor spins the basket to change the side of the basket in which the liftarms are dropped. The two halves of the basket are separated by a wall.

Final Assembly

The final assembly of the Color Sorter involves the attachment of the basket, chute, and pusher onto the baseplate. This placement is described in the following steps. Figure 9-6 shows a map of where the basket, chute, and pusher go, since it can be hard to see the mounting points in the building instructions.

Figure 9-6
Map of placement of the basket, chute, and pusher on the baseplate

Programming the Color Sorter

The program for the Color Sorter should solve the following problem statement:

Load a liftarm onto the Color Sensor by moving the pusher motor. Wait for one second to allow the Color Sensor to take a measurement. If the measured color is the one preselected (red in this example), set the basket motor’s angle position to one side of the basket. For all other colors, orient the basket motor to the other side of the basket. Push the liftarm into the basket by moving the pusher motor. Repeat these steps for all six liftarms loaded into the chute.

The problem statement refers to several tasks involving the selection of an angle position on two motors, namely the pusher motor and basket motor. Angle settings to accomplish the required movements are referenced to a zero position, so it’s critical that both motors were installed in the building steps with the motors in zero position. Chapter 6 described how to set the zero position for motors. The algorithm to solve the problem statement is as follows:

1.
Start the program.
2.
Set the basket motor speed to 25 percent.
3.
Set the pusher motor speed to 100 percent.
4.
Create a loop that repeats six times.
5.
Move the pusher motor clockwise to the 180-degree position.
6.
Delay for one second.
7.
Determine whether the liftarm color is red. If yes, go to step 8. If no, go to step 9.
8.
Move the basket motor to the 180-degree position.
9.
Move the basket motor to the zero-degree position.
10.
Move the pusher motor counterclockwise to the zero-degree position.
11.
Stop the program.

Figure 9-7 shows the algorithm in the form of a flowchart, allowing visualization of the loop that repeats six times. An if conditional is embedded in the loop to decide on the orientation of the basket.

Figure 9-7
Flowchart of the Color Sorter algorithm

The Word Blocks Code

Figure 9-8 shows the Word Blocks code for the Color Sorter algorithm, with each code block corresponding to a block in the flowchart. Most of the program is done by an if conditional nested within a loop that repeats six times. Specific blocks to note are listed following Figure 9-8.

Figure 9-8
Word Blocks code for the Color Sorter algorithm

Step 2: The set speed to block assigns the speed of the basket motor. Without this block, a motor will assume a default speed of 75 percent, which could cause jerky motions for the basket. A slower speed will move the basket smoothly.
Step 3: Another set speed to block assigns the pusher motor’s speed. Here, the maximum speed is desired to give the liftarms a firm shove into the basket.
Step 4: The repeat block sets up a loop that repeats six times, then stops. Loops have been used throughout the programs of this book that run indefinitely, but this loop only runs through a specified number of repetitions. Six repetitions are used here, because the Color Sorter holds six liftarms to be sorted.
Step 5: The go to position block sets the pusher motor’s angle to the position specified. This motion retracts the axle connected to the eccentric so that a liftarm drops from the chute onto the Color Sensor.
Step 7: The if then else block presents a condition—it checks if the liftarm seen by the Color Sensor is red. If the red condition is met, then the blocks within the first set of yellow bars will run. Otherwise, the blocks of the else section within the second yellow bars will run.
Step 8: The go to position block sets the basket motor’s angle to the position specified to collect liftarms that correspond to the selected color, red in this case. The basket has two halves, with one dedicated to the selected color and the other half for all other colors.
Step 9: Another go to position block sets the basket motor’s angle to the position specified to collect liftarms that are not the selected color. In other words, any liftarm that is not red will go into the side of the basket that corresponds to this motor position.
Step 10: The next go to position block works with the pusher motor, as opposed to the basket motor of the prior two blocks. This action on the pusher motor shoves the liftarm into the basket.

The Python Code

The Python code for the Color Sorter is listed as follows:

from mindstorms import MSHub, Motor, ...

from mindstorms.control import wait_for_seconds, ...

from mindstorms.operator import greater_than, ...

import math

import hub

from sys import exit

# Create your objects here.

myhub = MSHub()

1 basket = Motor('A')

2 color = ColorSensor('C')

3 pusher = Motor('E')

# Program "color_sorter". Sort liftarm colors into two halves of basket.

4 for x in range(6):

5 pusher.run_to_position(180,'clockwise',100)

wait_for_seconds(1)

6 measurement = color.get_color()

if measurement == 'red':

7 basket.run_to_position(180,'shortest path',25)

else:

basket.run_to_position(0,'shortest path',25)

pusher.run_to_position(0,'counterclockwise',100)

exit()

There are three objects to name in the program: the basket motor given the variable name basket 1, the Color Sensor given the variable name color 2, and the pusher motor given the variable name pusher 3. Loops commonly used in this book kept running indefinitely, but here a loop is set up that only runs six times, using a for loop 4. Six repetitions are used here because the Color Sorter holds six liftarms at the beginning of the program. The sorting routine, described as follows, thus gets run six times.

A set of statements runs inside the repeated loop. These load a liftarm onto the Color Sensor, take a color measurement, and decide to which side of the basket the liftarm should go. To load a liftarm, the pusher is retracted with the run_to_position() function 5. After a pause, the Color Sensor measures the liftarm’s color and assigns the result to a variable named measurement 6. An if conditional selects one of two options depending on the detected color. If measurement matches the value of red, the basket motor goes to a position of 180 degrees using the run_to_position() function 7. The second option, in the else block, occurs if the measurement isn’t red. It presents the other half of the basket. After the basket has been positioned, a motor function shoves the liftarm into the basket.

Using the Color Sorter for the first time involves loading three-hole-long liftarms into the chute with the holes of the liftarm facing the operator, as pictured in Figure 9-9. If liftarms are loaded in the wrong orientation, then the liftarms will get jammed at the bottom of the chute. Six liftarms can be loaded in the chute, with the sixth liftarm sticking up a little out of the chute. Activating the program will have all the liftarms get placed into one of the two halves of the basket. Red liftarms will go in one side, and all the other colors will go into the other side. While red liftarms were used in this example case, other colors can be selected by the setting in the if then else block.

Figure 9-9
Liftarms should have holes facing the operator for loading into the chute

Summary

This chapter explored the use of the Color Sensor, which can measure the color of an object placed in front of the sensor. In addition to measuring color, the Color Sensor can also detect the brightness of light reflected back into the sensor from a light source built into the sensor. Color Sensor data can be viewed in the Robot Inventor app to give a quick look at measurements. An exercise built a device that beeps a tone on the Hub, with the tone changing depending on the brightness of light reflected back into the sensor. Then a more complex machine was built using color detection that sorts liftarms by their color. This project also involved motors that moved liftarms through the machine to sort them. The program for the Color Sorter involved the use of a loop that runs through a specified number of iterations, rather than running indefinitely.

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Table of Contents for 9. The Color Sensor

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9. The Color Sensor

Exercise: The Brightness Beeper

Building the Brightness Beeper

Programming the Brightness Beeper

The Word Blocks Code

The Python Code

Project: The Color Sorter

Building the Color Sorter

The Baseplate

The Chute

The Pusher

The Basket

Final Assembly

Programming the Color Sorter

The Word Blocks Code

The Python Code

Summary

Table of Contents for
9. The Color Sensor