Sensing - Carnegie Mellon Universitycmra.rec.ri.cmu.edu/products/teaching_robotc_vex/sensing/forward_… · Sensing In this lesson, you will learn how an Ultrasonic Rangefinder (a.k.a

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Ultrasonic Rangefinder • 1© Carnegie Mellon Robotics Academy / For use with VEX® Robotics Systems

Sensing

In this lesson, you will learn how an Ultrasonic Rangefinder (a.k.a. Sonar Sensor) works, and how to use it to move to within a specific distance of an object.

Ultrasonic Rangefinder Forward till Near

The Touch Sensors (Bumper and Limit Switches) allow your robot to detect physical contact. They allow the robot to keep track of the position of its arm, and can potentially detect walls or other objects in the environment if the robot bumps into them.

The Encoders allow your robot to measure rotation of motors, wheels, and other important parts. Measuring the rotation of these parts can tell you how far the robot has traveled.

We still do not have a sensor that allows the robot to detect objects without physically hitting them. If the robot ever hopes to pick up the mines without knocking them over during the autonomous period, “touchless” detection will be absolutely necessary.

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Ultrasonic Rangefinder Forward till Near (cont.)

The Ultrasonic Rangefinder offers exactly this capability. Using the same physical principle as a bat or submarine’s sonar, the Ultrasonic Rangefinder measures distances using sound. It sends out a pulse of sound, then waits to hear the sound’s echo off of a solid object in the environment. By measuring how long it takes for the sound to bounce back, the sensor can calculate the distance that the sound must have traveled, and hence, how far away the object was that reflected it back.

The Ultrasonic Rangefinder will work in a very similar way to the Encoder program you wrote in the previous section, but instead of measuring the distance that the wheel has turned, it will use the Ultrasonic Rangefinder to measure the distance to the nearest object in front of the arm.

1. Add an Ultrasonic Rangefinder to the front of the robot’s arm. The design shown below will be used as the reference for the remainder of the unit. If you choose to use a design that differs significantly from this one, you may need to adjust accordingly.

1a. Add the sensorAttach the Ultrasonic Rangefinder to the front of the Squarebot 3.0 arm.

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2. Create a new program.

3. Save the new program as “ForwardNear”.

4. Use the Motors and Sensors Setup Menu to configure the Ultrasonic Rangefinder on Analog Digital Port 5, and Interrupt 1.

2. File > NewSelect File > New to create a new program.

3a. File > Save As...Select File > Save As... to save your program under a new name.

3b. Renate programGive this program the name ForwardNear.

3c. SaveClick Save.

4a. Robot > Motors and Sensors SetupSelect Robot > Motors and Sensors Setup to open up the configuration menu.

1b. Connect the OUTPUT cableConnect the OUTPUT cable of the Ultrasonic Rangefinder to INTERRUPT port 1.

1c. Connect the INPUT cableConnect the INPUT cable of the Ultrasonic Rangefinder to ANALOG / DIGITAL port 5.


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4b. Select A/D Sensors 1-8

4c. Set the sensor Name to “sonarSensor”The Ultrasonic Rangefinder is indexed by the number of the Analog/Digital Port it is plugged into, so enter a name for the sensor under Analog/Digital Port in5.

4d. Set the sensor Type to “SONAR”Set the type of the sensor to “SONAR”, another name for the Ultrasonic Rangefinder (because it uses sonar sound waves to measure distance).

4e. Configure the Second Port to “int1”When you select “SONAR” as the sensor Type, a column for the “second port” appears for this sensor. The second port is the other port that the sensor is plugged into, Interrupt 1 (int1).

4f. Click OKClick OK to finish configuring the Ultrasonic Rangefinder.

Checkpoint

The Ultrasonic Rangefinder is now set up and recognized by the program. It will now provide sensor readings as values through SensorValue[sonarSensor]. The values represent distances to the nearest detectable object (the first echo that the sensor hears), in inches. If an object is 6 inches away, in front of the sensor, SensorValue[sonarSensor] will be 6.


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Let us first begin by reviewing the way a forward-for-distance Encoder behavior works, and then adapt it to use the Ultrasonic Rangefinder instead.

The forward-for-distance command below was taken from the early parts of the Encoder lesson, and moves the robot forward until the Encoder accumulates more than 5000 counts of rotation. It does so by using a while loop to repeat basic movement commands as long as the current count is still below the desired target.

while(SensorValue[leftEncoder] < 5000) { motor[port3] = 63; motor[port2] = 63; }

In the case of the Ultrasonic Rangefinder moving until the robot is close to an object (such as the “stem” of a mine), we want the robot to move until the detected distance is below the target distance. Rephrased, the robot should keep running as long as the distance to the object is still greater than the desired distance.

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const tSensors sonarSensor = (tSensors) in5;

task main(){ wait1Msec(2000); bMotorReflected[port2] = 1;}

5. Add task main and base code.

5. Add this codeAdd task main, an initial delay, and the motor reflection command before proceeding with the rest of the program.

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task main(){ wait1Msec(2000); bMotorReflected[port2] = 1;

while() { motor[port3] = 63; motor[port2] = 63; }}

6. Add the basic structure of a move-until behavior.

6. Add this codeThe basic structure of the behavior is a while loop containing moving-forward commands. The (condition) will determine how long this loop lasts, and is left blank for now.


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7. Choose the (condition) based on the Ultrasonic Rangefinder. The (condition) should make the movement commands repeat as long as the distance to the target is still above the desired value.

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while(SensorValue[sonarSensor] > 3) { motor[port3] = 63; motor[port2] = 63; }}

7. Add this codeThe (condition) should be true as long as the Rangefinder’s value is above 3 inches. This will make the robot move forward as long as the Rangefinder does not detect any objects within 3 inches.

8. Download and Run.

8. Go to Compile and Download Program

Checkpoint

Your robot will run forward until it is within 3 inches of a detectable object. Not all objects are detectable, however, and the “area” of detection is only in front of the Ultrasonic Rangefinder itself. These limitations are inherent to the technology, but the sensor does seem to be able to detect the mines, as we had hoped.


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9. Upgrade this behavior to go straight while there is no obstacle nearby, rather than just run forward.

9a. Robot > Motors and Sensors SetupSelect Robot > Motors and Sensors Setup to open up the configuration menu.

9d. Press OKConfirm the new sensor configuration.

9c. Set the “Type” of both sensorsIdentify the sensor attached to both in2 and in3 as “Rotation” sensors (Encoders sense rotation).

9b. “Name” the sensorsAssign the name “leftEncoder” to the sensor in port “in3” (A/D input 3). Name the “in2” (A/D input 2) sensor “rightEncoder”.

9e. File > SaveSelect File > Save to save your program.

9f. File > Open and Compile


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void moveStraight(){ if(SensorValue[leftEncoder] > SensorValue[rightEncoder]) { motor[port3] = 50; motor[port2] = 63; } if(SensorValue[leftEncoder] < SensorValue[rightEncoder]) { motor[port3] = 63; motor[port2] = 50; } if(SensorValue[leftEncoder] == SensorValue[rightEncoder]) { motor[port3] = 63; motor[port2] = 63; } }}

9h. Highlight codeFind the void moveStraight() function and highlight it.

9g. Open MineArmStraight Instead of re-entering the moving-straight code, we’ll take a copy of it from the Encoders program.

9i. Copy this codeSelect Edit > Copy to put the highlighted code on the clipboard.

9j. Select File > Open and Compile


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9k. Open ForwardNearReturn to the Forward Till Near Ultrasonic Rangefinder program.

9l. Place cursor herePlace your cursor between task main and the AUTO code.

9m. PasteThe moving straight function is now declared, and available for use in this program.


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const tSensors rightEncoder = (tSensors) in2;const tSensors leftEncoder = (tSensors) in3; const tSensors sonarSensor = (tSensors) in5;

void moveStraight(){ if(SensorValue[leftEncoder] > SensorValue[rightEncoder]) { motor[port3] = 50; motor[port2] = 63; } if(SensorValue[leftEncoder] < SensorValue[rightEncoder]) { motor[port3] = 63; motor[port2] = 50; } if(SensorValue[leftEncoder] == SensorValue[rightEncoder]) { motor[port3] = 63; motor[port2] = 63; }}


SensorValue[leftEncoder] = 0; SensorValue[rightEncoder] = 0;

while(SensorValue[sonarSensor] > 3) { moveStraight(); }}

9o. Modify this codeReplace the generic motor-forward commands with a call to moveStraight();.

10. Download and Run.

10. Go to Compile and Download Program

Checkpoint

Your robot now runs straight until its Ultrasonic Sonar Rangefinder detects an object within 3 inches, in its forward field of view. You can change the stopping distance by changing the 3 inch “cutoff” point.

9n. Add this codeAdd these commands to reset the values of the encoders to 0.


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This is the second sensor you have programmed to use a “cutoff” value. The Encoder program ran a movement behavior until the encoder count exceeded a certain value, and this new program runs a movement behavior until the Ultrasonic Rangefinder distance goes below a certain value.

These “cutoff” values – called thresholds – are important in robot decision-making. Thresholds are values that set a cutoff point in a range of values, so that even though there are many possible values for encoder counts or distances, every one of them will fall either below the threshold or above it. This division of the many possible values into two distinct categories (above and below the threshold) allows the robot to make a definite decision about how to proceed for any value it may encounter.


threshold

below above

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Ultrasonic Rangefinder values above the threshold of 3 inches made the program continue looping and moving forward. Values below the threshold caused the robot to stop. The threshold sets the point at which the robot’s behavior will change, because it marks the point at which the (condition) in the while loop (or if-statement) will change from true to false, or false to true, and thus change which lines of code will run.

The robot is now set and ready to run for any object the Ultrasonic Rangefinder might see… but what happens when it doesn’t see anything at all? The sound waves have a limited range before the echo is too soft for the sensor to pick up. In addition, some materials or surfaces can actually deflect the sound waves away from the sensor, preventing it from hearing the echo. What happens then?

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SensorValue[rightEncoder] = 0;

while(SensorValue[sonarSensor] > 3) { moveStraight(); }}

11. Making sure that your robot is turned on and plugged in, open the ROBOTC Debugger and Devices windows. Run the program.

11a. Robot > DebuggerGo to Robot > Debugger to open the Program Debug window.

11b. Robot > Debug Windows > DevicesOpen the Devices window so that you can monitor the Ultrasonic Rangefinder values.


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11c. Pick up the robotPick up the robot so that it doesn’t drive away.

11d. Run the program

12. Point your robot’s sensor away from any nearby objects, or toward a very soft object, like a sweater or cushion. Observe the value that the sensor gives through the ROBOTC debugger.

12a. No SignalOrient your Ultrasonic Rangefinder so that it will have difficulty getting an echo back from whatever is in front of it. This may involve aiming it at something very far away or at something soft (which absorbs sound).

12b. Observe sensor valueThe sonarSensor will show a value of -1 if it is unable to measure distance for any reason.


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13. Run your straight-till-near program in a place where the robot will get a “-1” sonar reading.

13a. Position the robotPlace your robot so that it faces down a very long hallway, or toward a sound-absorbing object.

13b. Run the robotSwitch the robot off and back on to run the program.

13c. Observe robot behaviorThe robot stops immediately, as if it were close to an object.

Checkpoint

Why would the robot act like it was close to an object when the exact opposite was true? The SensorValue of an Ultrasonic Rangefinder is returned as -1 when there is no object in range. Consider how your robot is making its decisions:

while (SensorValue[sonarSensor] > 3) { moveStraight(); }


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The robot will move straight as long as the Ultrasonic Rangefinder detects an object farther than 3 inches away.

Object at 6 inSensorValue[sonarSensor] is 6, therefore SensorValue[sonarSensor] > 3 is true. The loop will continue looping.

Object at 2 inSensorValue[sonarSensor] is 2, therefore SensorValue[sonarSensor] > 3 is false. The loop will end. The next portion of the program causes the robot to stop.

No objectSensorValue[sonarSensor] is -1, therefore SensorValue[sonarSensor] > 3 is false. The loop will end. The next portion of the program causes the robot to stop.


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14. Change the (condition) for continuing to move forward so that the robot will run while the object is detected farther than 3 in OR too far to detect.

End of Lesson

The fine tuned code makes the robot move straight forward whenever it is far away from an object, even if it is so far away that it cannot detect it. You now have all of the tools you need to score additional points during the autonomous period of the Mine Removal Challenge. Now, use what you’ve learned to create your own final competition program!


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const tSensors rightEncoder = (tSensors) in2;const tSensors leftEncoder = (tSensors) in3; const tSensors sonarSensor = (tSensors) in5;

void moveStraight(){ if(SensorValue[leftEncoder] > SensorValue[rightEncoder]) { motor[port3] = 50; motor[port2] = 63; } if(SensorValue[leftEncoder] < SensorValue[rightEncoder]) { motor[port3] = 63; motor[port2] = 50; } if(SensorValue[leftEncoder] == SensorValue[rightEncoder]) { motor[port3] = 63; motor[port2] = 63; }}


SensorValue[leftEncoder] = 0; SensorValue[rightEncoder] = 0;

while(SensorValue[sonarSensor] > 3 || SensorValue[sonarSensor] < 0) { moveStraight(); }}

14a. Modify this codeAdd a second part to the condition, using an OR (||) connector.

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Sensing - Carnegie Mellon Universitycmra.rec.ri.cmu.edu/products/teaching_robotc_vex/sensing/forward_… · Sensing In this lesson, you will learn how an Ultrasonic Rangefinder (a.k.a