While drives are capable of regulating a constant speed with zero error, they always lag a ramp or changing speed reference. Inertia compensation is used to force a drive to follow a ramp without a lag.
Drives which change speed while producing web require Inertia Compensation (IC). IC provides the torque necessary to accelerate a roller so it precisely matches the S Ramp. IC can be accomplished in several ways.
All methods rely on accurate knowledge of the acceleration. Modern controllers and drives include S Ramp functions which provide the acceleration signal as well as the ramp output. IC calibration involves repeated accelerations and deceleration without web although the procedure is different in each case.
The simplest method is to multiply the acceleration signal by a speed factor in the drive controller and sum this speed force to the S Ramp for each drive. IC by speed forcing is not considered best practice.
Another method is to multiply the acceleration signal by a torque factor. The torque force is sent to the drive in addition to the speed reference. In the drive, this torque force is summed with the drive's regular torque signal. This method is best practice. During commissioning the torque factor is adjusted to minimize variations in the drive's torque reference.
Finally, the drive itself can determine provide it's own IC. The internal IC is set during it's auto-tune procedure. While internal IC is automatic, it may be the most difficult to set correctly.
It is important to know the IC method to be used and ensure other methods are disabled as they will interfere with one another.
Details on each of these methods of IC will be presented in future blog entries.