Activity 3.1.6 Open and Closed Loop Systems – Fischertechnik

Activity 3.1.6 Open and Closed Loop
Systems – Fischertechnik
Introduction
Many devices function without ever knowing whether they are doing the job that they
were programmed to do. They might run for a specific amount of time or perform one
more function and then stop. For example if you set the clothes dryer to run for 45
minutes, your clothes might be dry or they might not be dry. A clothes dryer is an
open loop system because the process provides no feedback to the device. Newer
clothes dryers possess moisture sensors. The moisture sensors inform the machine
when the clothes are dry, at which point the dryer can stop running. The feedback
provided by the sensor makes this a closed loop system.
Equipment
Procedure
RoboPro software
fischertechnik ® components:
Interface, power supply,
and USB cable
2 mini switches
5 wires 9 building blocks 30 mm
Motor 8 building blocks 15 mm (1 pin)
2 lamps 3 rack and pinion 60 mm
Photocell Motor rack gearbox
Phototransistor
In this activity you will design an open loop program to control a motor driven on a
track. Then you will design a closed loop system utilizing feedback from both analog
and digital inputs to control the motor.
1. Build the support system and track pictured below.
2. Add the motor and gearbox as shown below, and then wire the motor to M1 on
the interface. Slightly loosen the gearbox from the motor so that you can freely
adjust the motor. Before tightening the motor, center the gearbox over the middle
building block.
Project Lead The Way, Inc.
Copyright 2010
POE – Unit 3 – Activity 3.1.6 Open and Closed Loop Systems – Fischertechnik – Page 1

3. Write a program that will shuttle the motor back and forth 1.5 seconds each way
20 times. Write a description next to each function block to explain what happens
at each block. Save the program.
4. Run the program and observe the final resting place of the gearbox. You may or
not have to look closely, but the gearbox probably did not stop in the center.
5. Is this an open loop or closed loop system?
6. Where do you think the gearbox would have come to rest had you set the
program to cycle 120 times?
7. What problems and inefficiencies might this process cause if it were used in an
assembly mechanism designed to run 24 hours per day for several days?
Instructor’s Signature ______________________ Date __________
8. Place a mini switch on both ends of the device as pictured below. Wire the
switches to I1 and I2. Write a program that will shuttle the motor back and forth
five times based on feedback from when the switches are pressed. Write a
description next to each function block to explain what happens at each block.
Save the program.
9. Is this an open loop or closed loop system?
10. How precise would this device be if it were designed to cycle 500 times?
Instructor’s Signature ______________________ Date __________
Project Lead The Way, Inc.
Copyright 2010
POE – Unit 3 – Activity 3.1.6 Open and Closed Loop Systems – Fischertechnik – Page 2

11. Remove the mini switches. Make the modifications as seen below. Add the
smaller building blocks to the ends of the gearbox and motor. Use the larger
building blocks to create extensions from the track on each end. Add covered
lamps to the extensions using smaller building blocks on one side of the track.
Using the same attachment method, place a phototransistor across from one
lamp and a photocell across from the other. Note that the phototransistor sensor
is polarity sensitive therefore it will only function when positive and negative
leads are correctly attached.
12. Write a program that will shuttle the motor back and forth five times using the
phototransistor and photocell for feedback. Write a description next to each
function block to explain what happens at each block. Save the program.
Demonstrate the successful program for your instructor.
Instructor’s Signature ______________________ Date __________
Conclusion
1. Describe an open loop system that has not already been given as an example.
Could the system benefit from feedback? Justify your answer.
2. Describe a closed loop system that has not already been given as an example.
Describe the feedback utilized, and describe why that type of feedback is utilized.
Project Lead The Way, Inc.
Copyright 2010
POE – Unit 3 – Activity 3.1.6 Open and Closed Loop Systems – Fischertechnik – Page 3