Installation guide

Servo Drive 251
Ki:
Integral gain.
The reciprocal (1/Ki) of this term is
multiplied by the
sum
of the position error over time.
The effect of Ki is thus time related, and affects the
steady state error. The higher Ki, the longer it will take
for the controller to “integrate out” any steady state er-
ror. The effect of Ki is seen mostly at constant speed
(including standstill). Ki is NOT required for stability,
and generally has a de-stabilizing effect on the system,
especially if it is too low. If Ki is TOO LOW the sys-
tem may oscillate slowly and wildly back and forth like
a washing machine. Ki is required, though, if the sys-
tem must achieve a very low steady state error (within a
few counts).
The general range for Ki is 10 to 70. Ki less than 10
may lead to wild, low frequency oscillations. If steady
state error is not a consideration, Ki may be set to zero.
Ki is often disabled during motion to reduce overshoot
at the end of the move.
Kd:
Derivative gain
. This term is multiplied by the encoder
velocity at any point in time. Generally, raising Kd will
reduce overshoot in the move response, however, Kd is
the term most susceptible to “digital instability”. This is
where the quantification effects of the digital encoder
feedback in conjunction with too high a Kd cause the
system to “buzz”.
The general range for Kd is 5 to 20. Kd less than 5 usu-
ally leads to an unstable system, Kd >20 usually leads
to “buzzing”.
Kvff:
Feed forward velocity gain
. This term is multiplied by
the commanded velocity to contribute to the output
torque command. It has no effect on general stability,
and may be set to as high as 100% to reduce position
error during motion. Too high a Kvff causes undue
motor heating.
Generally, Kvff should be set between 50 and 100.
Kaff:
Some controllers have a Kaff term. This term is multi-
plied by the commanded acceleration to contribute to
the output torque command. This term only takes effect
to reduce the error during acceleration and deceleration.
Generally Kaff is less than 4. Most applications will
run fine with Kaff set at zero.
9.1.1.6.1 - Adjustment based on auto
tune calculation
It is usually desirable to use the auto tuning gains as a
starting point for further adjustment. If the system is un-
stable at given bandwidth, the bandwidth may be lowered,
and the auto tuning run again. If the move response at this
lower bandwidth is unacceptable, the following procedure
may be attempted.
Set bandwidth to 25 Hz and calculate gains. Then:
1) Update gains and energize system .
2) If the system “buzzes”, cut Kp in half, and lower Kd
by 25%.
3) If the system no longer buzzes, check your move
response.
4) If the move response over shoots too much, or the
system buzzes sometimes, then lower Kp until the
buzz goes away and the overshoot is acceptable.
5) Check your move response, and set Kvff to between
50-100%. This should reduce the error during the
move, and may also improve the overshoot.
6) If the response is well behaved, but sluggish, raise Kp
in increments of 2 until acceptable response is
achieved. If ever the system “buzzes” Kp must be
lowered again.
7) Verify proper response.
8)
The system should now be stable and well behaved.
9.1.1.6.2 - Full Manual Adjustment
Although it is much more involved, the servo can be
tuned “from scratch”. The trick here is to be very
patient
and methodical.
Make sure to record each change and its
resultant effect on the response. In step 1 the
measure
system gain
button is used to determine proper encoder
direction. Step 2 is not used at all. Step 3 is used to enter
and update the servo gains. Step 4 is used to enter the
move profile parameters and execute a move response.
Make sure to Update Gains after each adjustment so they
take effect. You can use the example response screens at
the end of this procedure as a guide.
CAUTION! Motor
instability can cause severe vibration or sudden
movements. Insure that appropriate safety measures
such as mechanical limits are employed to prevent
dangerous movements of the motor and load.
1) Click
measure system gain
.
Caution! the motor
will move suddenly during this process.
This will
verify that the encoder direction is correct for the
servo to run properly.
2)
If the encoder direction is found to be reversed, then
quit the auto tune screen immediately and enter the
Configuration. Select the encoder folder and change
the encoder direction to the opposite of the present
setting. Save the configuration information, compile
and download the modified project.
3) Re-enter the servo tuning screen and set Ki, Kvff, and
Kaff to zero.
4) Together, set Kp to a low number, say 5, and Kd to a
mid-range number, say 10.
5) Update the gains and see if the motor is stable by
moving the load slightly by hand (if this is safe). Be
ready to
shutdown
if the motor oscillates.
6) If the motor is stable and does not vibrate, raise Kp
by 2.
7) If not, lower Kp by 1. Repeat until the motor is sta-
ble.
8) Once Kp is as high as it will go and still be stable,
reduce Kp by
50%
to provide some stability margin.
9) Now try your move response.
10) If the move is stable but overshoots severely, lower
Kp slightly. Slight overshoot is o.k. at this point.