ELEC 242 Lab

Experiment 7.2

Plant Characterization

One of the traditional terms for the system being controlled is the plant. Our plant is the board with the motor-gearbox assembly on it, along with the motor amplifier.

Part 1: Potentiometer Calibration



Step 1:

 Using the "Controller 1" program, run the drum fully counterclockwise. Adjust the number of turns of cord on the spool so that about 4 inches is hanging down.

Step 2:

 With a yardstick, measure the height of the hook above the floor. Note the value of Vact.

Step 3:

 Run the drum fully clockwise and repeat the above measurements.

Question 2:

 Find the expression for vs. x , the distance above the floor. Assuming the diameter of the drum is 0.9 inches, find the expression for vs. shaft angle .

Part 2: Step Response

Our standard techniques for characterizing transfer function are the step response and the frequency response. For reasons which will become apparent, trying to measure the frequency response of the open loop system is quite a challenge. Let's start by trying to find the step response.


Step 1:

 Use the "Controller 1" program or your fingers to turn the drum fully clockwise. If you use the Controller program, be sure to stop it when you have finished.

Step 2:

 Load the "Step Response" Labview program. Set the amplitude to 5 V and the duration to 1.5 sec.

Step 3:

 Run the program. The step response should be a ramp with a curved beginning. This is what we expect, since in the steady state, the motor is running at a constant velocity which means the angle is increasing at a constant rate.

Remark:

 We've been characterizing motors in terms of their velocity behavior. We can find the velocity be differentiating the position, but because the position signal is fairly noisy, this produces a very noisy velocity estimate. However, with a little filtering, we can get a curve that is at least recognizable.

Step 4:

 Load and run the "Velocity Step" Labview program.

Question 3:

 The program uses an exponential curve fitting routine to estimate the damping coefficient . How does its estimate compare with yours?