MEGA-F  TRAINING

                               Technical Note

                    “My First Mega-F Bode Plot”

                                Written by:

                         Dr. Boaz Eidelberg

                                Optinet Inc.

           Main Mega-F Representative in the USA

                             April 7, 2006

Objective:

The objective of this note is to train Mega-F  users in conducting Frequency Response of close and open loop

servo systems and generate their Bode Plots. Frequency response is very important tool in studying the “EKG” 

characteristics of the positioning system  in order to improve performance by proper component selection, design

practice or tuning. Most of all it is a highly effective tool in maximizing the bandwidth of a positioning system thus

making it more robust. A robust machine is one that rejects high frequency disturbances on one hand and insensitive

to changes in machine parameters. Finally Bode plots provide easy visual means to ensure system stability by observing

the Phase Margin and Gain Margin of the system.  With Mega-F user friendly tools, Frequency response reduces to

the level of a few clicks as demonstrated below.

Training Equipment:

Bental Motor + Encoder

Mega-F Wiz Alg Evaluation Unit

User’s Host PC Computer

Mega-F Test and Evaluation software INSTALLED in Host computer

Plug and Play Cables Connecting Wiz Alg Evaluation Unit to the Host Controller and to Bental Motor / Encoder 

Run Bode Plots  in 3 simple steps:

1. Click on Frequency Response icon
2. Assign values to the plot variables
3. Run the open and close loop Bode plots

Step 1 – Click on the Frequency Response Icon

1.1    Click the “Frequency response” Icon as shown by the arrow in Fig 1. The screen shown in Fig. 2 opens.

                        Fig 1 -  Click Frequency Response Icon

Step 2 - Assign Values to the Plot Variables

2.1 On the right side of the frequency analysis screen enter the desired values of the Plot

For example: “Signal amplitude” =10 counts, “Signal Variable”= reference position, “Input Variable”   = encoder position.

                                                Fig 2 – Bode Plots

Step 3 – Plot Collected Data

3.1  To start plotting click the icon with the green arrow on the upper portion of open screen as shown in Fig. 2.

       The close loop Bode plots of Gain and Phase are then generated

3.2  In the “Tools” menu click “close loop”. The close loop Bode plots of Gain and Phase are then generated

Notice that the plot starts at 1 Hz and ends at 1,000 Hz with Log steps of 0.1. These parameters can change from

plot to plot.

Looking at the Bode plots we can make a few interesting observations, which are typically made for servo controlled positioning systems, as follows:

• the close loop bandwidth is 100 Hz as found in our previous FFT training example for the same tested system at

  http://www.sizingtools.com/MegaF_myfirstfft.htm

• The observed attenuation rate above 100 Hz is about 40DB / Decade ( a frequency multiple of 10 ) as expected for a second order mass system.

• The Gain Margin is 12 DB ( i.e. the gain attenuation of the open loop transfer function at 180ْ  phase angle )

• The Phase Margin is 18ْ  ( i.e. the amount of Phase shift less than 180ْ  at the 0 DB cross over of the open loop gain ) 

• Both Gain Margin and Phase Margin meet the requirements for a stable system.

For more Mega F tutorials in software programming and system analysis please see

• Writing your first positioning motion code  - http://www.sizingtools.com/MegaF_myfirstcode.htm
• Making your first Fast Fourier Analysis - http://www.sizingtools.com/MegaF_myfirstfft.htm

For schedule of our next demo in your territory please inquire with webmaster@optinetinc.com