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TB045 Rev1 Autotune Parameters and What They Mean to You
See TB044 for more information on Kv1 and Kg
Overview
Autotune automatically sets the following parameters Kg Kv1 Ka Max rate Accel/Decel time and Dead
Start This document will describe what these parameters are and what they mean
Kg
Kg is called the Gravity Constant It indicates an imbalance in an axis A high Kg value means that the motor
is working harder to move the axis in one direction than the other Typical Kg values range from 5 to +5 for X
and Y axis and 10 to +10 for Z axis
Causes of high Kg
1 A bed mill with an incorrect weight on the counter balance
2 A loose gib See TB037
3 A failing support bearing
In version 6 0 a drag plot will show you the drag in both directions the "gap" or difference between the two
lines in the plot is the Kg value
Kv1
Kv1 describes the amount of current it takes to slow jog the axis By knowing the amount of current it takes to
slow jog the axis we can know the amount of friction or mechanical resistance that axis has Common values
for Kv1 on a knee mill are 10 20 on the X 12 22 on the Y higher because it carries the weight of the X and
3 10 on the Z
Software
1 Version 6 0 You can see a plot of Kv1 over an entire axis by doing a drag run "Drag" in the PID screen
and then plotting it
2 5 27 and earlier Slow Jog the machine and observe the PID OUT parameter in the PID screen and take a
visual average of the numbers you see being displayed You may get different results depending on the
direction you are jogging See Kg
Note
Remember that a Kv1 of 10 on a machine equipped with a 15 amp drive is not the same as a machine
with a 12 amp drive To figure out how much current is required to slow jog the machine take Kv1 and divide
by 128 and then multiply the result by the amp rating of the servo amplifier Example Kv1 = 35 Servo amp is
a 3T or 15 amp drive see Table 1 for turn to amp conversions therefore 35/128 = 2734 multiplied by 15 = 4 1
amps! Way too much power to slow jog an axis
Table 1 Relation of current sensor turns to drive amps 2T 25 amp
3T 15 amp
4T 12 amp
5T 9 amp
6T 6 amp
Ka
Ka describes the current necessary to accelerate the axis to its maximum velocity at the acceleration rate set
by autotune
Example
If Ka is 54 Max rate is 330 and accel time is 245 seconds it takes 54/128 times 15 amps = 6 3 amps of
continuous current for 245 seconds to accelerate that axis to 330 inches per minute
Accel
Accel is the time for the axis to reach maximum velocity It is set by autotune to be as fast as possible
Sometimes a customer may desire a slower rate For example an accel rate of 1 second is very fast and the
machine will appear to be "jerky" You can change the accel rate to a slower setting but Ka must be manually
changed at the same time If autotune produced a 1 second accel time and a Ka of 60 and you want to
increase the accel time to 2 seconds you would manually double the accel value and divide the Ka value by
2 The new values would be an accel of 2 and a Ka of 30 This will produce a noticeable difference
Max Rate
Max Rate is 80% of the maximum physical speed on the axis It is set at 80% to compensate for
environmental changes For example without this on a hot day when the lube is "loose" autotune might find a
value of 400 IPM In a few months it may be very cold in the machine shop and the lube will be like molasses
Now the control will try to run at 400IPM and unable to will most likely give a position error message
Deadstart
Deadstart has to do with direction reversal of an axis Autotune does a good job of setting the Deadstart The
deadstart usually never has to be changed on a Milling machine Sometimes very light wood routing tables
with very low resistance and low inertia can benefit with a Deadstart change along with other "hand tuning "
Call in if you have this case
AC Brushless Systems
When performing an Autotune on an AC Brushless machine for the first time you should set the Autotune
Move Distance paramters 95 through 98 for a short distance then increase until the Max Velocity levels off
Start with 0 5 inches and then increase the parameter number until the Max Velocity does not increase
signifigantly Autotune will overshoot this distance by 2 times so if the distance is set to 1 inch expect the
movement to go out to 3 inches Care should be used in setting the starting point for autotune so not to crash
the machine
If the machine has low friction and high mass the drive may stop due to overcurrent If this happens the max
velocity and the jog rates may be set to zero by the autotune Reset these numbers to the previous values and
move back to the starting position Decrease the travel distance parameter and try again
Document History
Rev1 Created on 1998 06 05
by #001