Download Activity 4.5j – Closed Loop Control 3 Purpose Develop an

Activity 4.5j – Closed Loop Control 3
Purpose
Develop an understanding of a closed loop system using a potentiometer as a
feedback device.
Gain further knowledge in the understanding of analog sensing devices.
Understand the concept of position control.
In this exercise you will use a potentiometer to control the back and forth
motion of a motor and gearbox on a rack.
Equipment
Computer Interface
Potentiometer
Rack
Wires
Gearbox
Procedure
A potentiometer is a type of variable resistor. Resistors are used to control the
values of voltage and current in an electronic circuit. Resistors can have a
fixed resistance value or one that can be changed; hence they are generally
classified as “fixed” or “variable” resistors.
Variable resistors are used when it is necessary to frequently change the
value of resistance in a circuit. These devices have a sliding/rotating contact
arm that is in contact with a strip of resistive material. A potentiometer has
three contacts. The resistance between the center shaft and the two outside
contacts changes as the shaft of the potentiometer is rotated
In Lucky Logic, the interface measures the resistance between the center and
one of the outside contacts of the potentiometer. It reports this as an analog
value between 0 – 1028. The potentiometer is connected to either EX or EY.
Procedure
1. Set up your Fischertechnik parts according to the system diagram.
System Diagram
2. Connect the potentiometer to EX.
3. A potentiometer only rotates 270 degrees so you must center the
potentiometer and the rack to start centered.
4. After the potentiometer is centered, run the diagnostic program and
determine the values and directional information for the potentiometer.
5. Have the motor turn in one direction until the potentiometer reads a
predetermined value, then change direction and have it go the other way.
6. You must use the VARIABLE and COMPARE function block for this
program.
System Pictured
Rack and building blocks move back and forth against stationary
potentiometer w/gear. As rack moves potentiometer rotates giving
new readings.
7. Once the program is functioning correctly, demonstrate its operation to
your teacher, obtain their initials and print out a copy of your program.
Instructors initials:________________
Conclusion
1. What is the feedback in this system?
2. Given an example of a positional control system and an explanation of
how it works.
3. Give two examples where you might find a potentiometer.