Activity 1.2.8 Emergency Stop (VEX®)
Introduction
An emergency stop, e-stop, is a feature that many machines have to enhance safety
by providing a system to rapidly stop a machine. The emergency stop feature can be
activated remotely when the machine is operated from a remote location. The most
common configuration is a clearly marked button located within a safe area close to
the machine. This button can be pressed by a human to stop the machine in an
emergency.
In this activity you will apply a multitask program to provide an emergency stop for
an operation.
Equipment
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Computer with ROBOTC software
CIM VEX® testbed
PLTW ROBOTC template
Activity 1.2.7 Multitasking Emergency Stop presentation
Procedure
1. Form groups of four and acquire your group’s CIM VEX® Kit under your teacher’s
direction.
2. Connect the CIM VEX® testbed Cortex to the computer.
CIM VEX® Testbed
3. View the Activity 1.2.7 Multitasking Emergency Stop presentation to understand
multitask programming.
4. Open the PLTW ROBOTC template. Click File, Save As, , and then select the
folder that your teacher designated, and name the file A1_2_7_eStop.
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Computer Integrated Manufacturing Activity 1.2.8 Emergency Stop (VEX®) – Page 1
5. In this activity the emergency stop task runs concurrently and ends all tasks if the
e-stop button is pushed. If an ultrasonic sensor reports a value of 10 inches or
less then both motors run at quarter power. Otherwise both motors turn at full
power.
6. Confirm that the Motors and Sensors Setup window reflects the inputs and
outputs to be used. Note that additional motors and sensors that are physically
attached may be configured; however, these are not required to be configured.
Click OK to close the window.
Cortex Wiring Diagram
7. Use the program as the main body of the program.
task e_stop()
{
while(true)
{
if(SensorValue(e_stopBtn) == 1)
{
stopAllTasks();
// ends the program and all tasks including task main.
}
wait1Msec(10);
// prevents the current task from using majority of
available CPU capacity
}
}
task main()
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Computer Integrated Manufacturing Activity 1.2.8 Emergency Stop (VEX®) – Page 2
{
startTask(e_stop);
simultaneously
// initiates the e-stop task which will run
while(true)
{
if(SensorValue(sonar) <= 10)
{
motor[rightMotor] = 32;
motor[leftMotor] = -32;
}
else
{
motor[rightMotor] = 127;
motor[leftMotor] = -127;
}
wait1Msec(10);
majority of available CPU capacity
// prevents the current task from using
}
}
8. Save the program, power on the Cortex, compile, and download the program. If
you have any errors, check with your instructor to troubleshoot your program.
9. Test the program and troubleshoot until the expected behavior has occurred.
Save the program.
10. Describe the behaviors observed.
11. Document what this program could look like as pseudocode simple behaviors.
12. Modify the program to use the limit switch connected to digital input 1.
13. Follow your teacher’s direction and either print the programs or submit it
electronically with this activity.
Conclusion
1. Describe any challenges that you encountered while developing the program.
2. Describe applications for which an emergency stop could be used.
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Computer Integrated Manufacturing Activity 1.2.8 Emergency Stop (VEX®) – Page 3