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 changesas 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

  1. Connect the potentiometer to EX.
  1. A potentiometer only rotates 270 degrees so you must center the potentiometer and the rack to start centered.
  1. After the potentiometer is centered, run the diagnostic program and determine the values and directional information for the potentiometer.
  1. Have the motor turn in one direction until the potentiometer reads a predetermined value, then change direction and have it go the other way.
  1. 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.

  1. 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?
  1. Given an example of a positional control system and an explanation of how it works.
  1. Give two examples where you might find a potentiometer.

Project Lead The Way, Inc.

Copyright 2007

POE – Unit 4 – Activity 4.5j – Closed Loop Control 3– Page 1