ABB 086444-005 Channel input module

KVFF is the velocity feed forward gain. In the absence of velocity feed forward (KVFF = 0), the commanded velocity is proportional to the position (following) error. This means that the actual position will lag the commanded position by a value proportional to the speed. The error will be smaller for larger values of KPP. The following table gives a feel for the following error magnitude.

The following error can easily

 exceed one complete motor revolution. In many electronic gearing applications, such following errors are not acceptable (real gears don’t have following errors!) Also, stepper systems don’t have such errors. Feed forward takes advantage of the fact that the PC830 DSP knows the frequency of the encoder or step inputs and hence knows how fast the motor should be going at a given instant. All or part of this velocity can be added to the velocity command to reduce following error. If KVFF is set to 100 (%), then the steady state following error reduces to zero.

Overshoot 

Setting KVFF equal to 100% can result in position overshoot. Somewhat lower values may be required if this is a problem. KVFF set to 70%-80% typically achieves the fastest step response with no overshoot. However, setting KVFF to less than 100% will give steady state following error when running at constant speed.Definitions for the terms used in the equations above are shown on the following page.

                                                                        

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