National Instruments NI-Motion - Manual (Page 35)

Chapter 3 Tuning Servo Systems
© National Instruments Corporation 3-7 NI-Motion User Manual
Ga
Ga is the Amplifier Gain.
Kt
Kt is the Torque Constant of the motor. Kt is represented in Newton Meters
per Amp.
1/J
1/J represents the motor plus load inertia of the motion system.
Ke
Ke represents the conversion factor to revolutions. This may involve a
scaling factor.
Dual Loop Feedback
Motion control systems often use gears to increase output torque, increase
resolution, or convert rotary motion to linear motion. The main
disadvantage of using gears is the backlash created between the motor and
the load. This backlash can cause a loss of position accuracy and system
instability.
The control loop on the motion system corrects for errors and maintains
tight control over the trajectory. The control loop consists of three main
parts—proportional, integral and derivative—known as PID parameters.
The derivative part estimates motor velocity by differentiating the
following error (position error) signal. This velocity signal adds, to the
loop, damping and stability. If backlash is present between the motor and
the position sensor, the positions of the motor and the sensor are no longer
the same. This difference causes the derived velocity to become ineffective
for loop damping purposes, which creates inaccuracy in position and
system instability.
Using two position sensors for an axis can help solve the problems caused
by backlash. As shown in Figure 3-3, one position sensor resides on the
load and the other on the motor before the gears. The motor sensor is used
to generate the required damping and the load sensor for position feedback.
The mix of these two signals provides the correct position feedback with
damping and stability.