Emerson EZMotion, User Manual, Page: 104 / 268

92
EZMotion User/Programming Guide
www.controltechniques.com Revision: A8
Load
Inertia Ratio
This parameter is the ratio between the load inertia and the motor rotor inertia. The formula is defined as follows:
In order to calculate the Inertia Ratio, the user must first calculate the load inertia. The load inertia is the inertia of the load reflected
to the motor shaft. Control Techniques offers a free sizing software called CTSize that helps the user calculate the load inertia.
CTSize can be downloaded from the Control Techniques’ website at no charge (www.emersonct.com).
When entering the inertia ratio, if the exact ratio is not known, it is better to enter a lower (more conservative) value than to enter a
“higher-than-actual” value. Higher numbers in the Inertia Ratio amplify the gains of the Velocity and Position Loop parameters
described below.
The motor rotor inertia used in this calculation can be found on the Motor Tab on the Motor/Encoder view in PowerTools Pro or in the
SPMotor.ddf file that is installed with PowerTools Pro software.
Tuning
Velocity Loop Bandwidth
The Velocity Loop Bandwidth is the theoretical bandwidth of the velocity controller. It is important for the motor data file to be
accurate for ideal velocity performance, in particular the Motor Inertia and Motor Current Constant (Ke). Higher Velocity Loop
Bandwidth values will result in better response to change in velocity command, however setting the bandwidth too high can result in
ringing (audible ringing) in the velocity loop. The units for Velocity Loop Bandwidth are Hz.
Position Loop Response
The Position Loop Response directly impacts the stiffness of the position loop. Position Loop Response is effectively the
Proportional gain term for the position loop controller. Higher Position Loop Response values will result in less following error
throughout a motion profile, however setting the response too high can result in instability in the position loop.
Enable Velocity Feedforward
The Velocity Feedforward applies the calculated velocity command directly to the drive’s velocity loop. Enabling the Velocity
Feedforward will generally yield faster velocity response, however the feedforward will introduce some overshoot. It is vital in
applications that require the use of Jog profiles to enable the Velocity Feedforward signal.
Filter
Current Demand Filter
The Current Demand Filter is a first-order low pass filter applied to the current command of the Unidrive SP. The parameter entered
by the user is a time constant, t, in the filter formula. The frequency of the filter, f, can be derived as follows:
The units for the parameter are milliseconds. This parameter is written directly to parameter 4.12 of the Unidrive SP parameter
database.
Module Gains Enable Check Box
Under normal operation, EZMotion calculates Current Loop and Velocity Loop gains based on information provided by the user on
the Motor tab and Tuning view. These calculated gains are sent by EZMotion to the Unidrive SP/Digitax ST on every power-up or
warmstart, overwriting any gain values stored in the drive. In some cases, a user may decide to use their own values for Current
Loop and Velocity Loop gains rather than the values calculated by EZMotion. Therefore, it is necessary to be able to prevent
EZMotion from sending its calculated gain values to the Unidrive SP/Digitax ST.
By default, the Module Gains Enable check box will be selected so that the values calculated by EZMotion are sent to the Unidrive
SP/Digitax ST. To disable this function, simply clear the Module Gains Enable check box.
If the user decides to enter their own gain values, the values are entered using the Unidrive SP/Digitax ST Keypad (or Drive Menu
Watch Window in Power Tools Pro). Once the values are entered using the keypad, the values must be saved so that they are not
lost the next time power is cycled. This is done by navigating to parameter number zero in any menu (MM.00), entering the value
1000, and then pressing the red Reset button on the front of the Unidrive SP/Digitax ST. This will save all drive parameters to NVM.
The gain parameters in the Unidrive SP/Digitax ST written to by EZMotion are as follows:
Velocity Loop Gains
Kp (Velocity Loop Proportional Gain) – 3.10
Ki (Velocity Loop Integral Gain) – 3.11
Current Loop Gains
Kp (Current Loop Proportional Gain) – 4.13
Ki (Current Loop Integral Gain) – 4.14
For more information on the Unidrive SP/Digitax ST gain parameters, please refer to the Unidrive SP Advanced User Guide or the
Digitax ST User Guide .
The Module Gains Enable check box can be seen on the Tuning view as shown in Figure 92.
Simple Tuning Procedure Example
1. Select the desired Drive Type (Unidrive SP only) and Motor Type on the Drive/Encoder view in PowerTools Pro.
InertiaRatio
LoadInertia
RotorInertia
------------------------------------ =
f
1
2
π
t
( )
-------------- =