24
Rev. 1.0
0006302011
SSM24C User Manual
+86-400-820-9661
5 Tuning - Sampling
Being a servo motor, the SSM Integrated Motor employs sophisticated algorithms and electronics
for controlling the torque, velocity and position of the motor to achieve a well behaved operation of
the load.
Because the motor encoder continuously tells the drive what the motor is doing, the drive can
alter the current applied to the motor until the motor achieves the desired operation. This is called
“closed loop control.”
One of these loops controls the amount of current going to the motor. This loop requires no
adjustment other than specifying the maximum current the motor can handle without overheating.
The SSM employs two control loops for the actual motor motion. The first is a Velocity Loop which
is designed to control only the speed of the motor. The second is a Position Loop that controls the
count position of the motor. As shown in the drawing below, the Current Loop is contained in the
Velocity Loop, and the Velocity Loop is contained in the Position Loop. Knowing this is important
as good Position Loop control requires first tuning the Velocity Loop. As mentioned above, Current
Loop tuning is not required as it is already optimized for the motor.
5.1 Velocity Control Loop (V Loop)
The Velocity Control Loop is designed to operate the motor in a velocity-only type of servo
control. This means that it can control the speed of the motor but cannot cause the motor to
follow a command position. The Jog commands available in the drive can use this loop only for
operation, the advantage of this is good stability even with very high inertia loads. The JM (Jog
Mode) command is available to set this feature or it can be configured when selecting the
Velocity
Control Mode. Selecting the
Speed Only
control type setting causes the Velocity Loop alone to be
used in the various Jog or Velocity control functions. JM2 (Jog Mode 2) does the same.
Position
over time
control type adds in the Position control loop for precise position control during the
move and when stopped, JM1 (Jog Mode 1) also configures this setting.
The
Velocity Control Loop
has four terms that can be configured for optimum performance with
a given load. This loop can be set and tuned independent of the Position Control Loop. These
control terms are described below.
5.1.1 Gain: The Velocity Proportional Term (VP)
The simplest part of the Velocity Loop is the proportional, or VP, term. The drive applies current
to the motor in direct proportion to the error. For example, if a motor is not moving, and the shaft
is turned by hand or some other force, the drive will increase the motor current until the motor
returns to “0” speed. The faster the motor is moved from “0” velocity, the more the opposing torque
will increase. The VP term (also called VP gain) governs how much torque will be applied for a
given amount of velocity error (Vn). In general, more load inertia or load friction, requires more
torque and therefore a higher VP gain. The torque provided by the VP term is:
T = VP * Vn
5.1.2 IntegGain: The Velocity Integral Term (VI)
In the previous example, applying the VP term alone will not result in perfect velocity control. If
