Description

Principle

The following error is caused by the control system's velocity, acceleration and jolt delay in the position controller with respect to the calculated command variable in the interpolator. In the interpolator, the feedforward control calculates the theoretical following error tat would result from the current velocity and acceleration in the position controller and, on the basis of this, it determines a velocity setpoint v_feedforward that is added to the position controller's output (Figure 1-1). This additional setpoint can be weighted with a factor.

The expected following errors can be calculated in accordance with the relationship

for constant velocity

and for constant jerk

for constant acceleration. T_a is the mechanical time constant of the drive, K_v the psosition controller gain.

Alternatively, the command velocities and accelerations calculated in the CNC control can be transmitted as additve command velocity and torque values to the drive controller and fed into the appropriate control loops as additive command values (see picture 1.2).

Feedforward control modes

Based on the cause of the following error, the control system distinguishes between the following feedforward control modes:

• Velocity feedforward control and
• Acceleration feedforward control.
• Jerk feedforward control.

Both feedforward control modes can be activated or parameterized independently of one another.

Activation

The P-AXIS-00223 parameter activates feedforward control for each axis or spindle. When the corresponding bits are set, the feedforward modes are selected, and further control information is also activated. The feedforward control mode is selected in the NC program.

Weighting

Feedforward control can be weighted with two axis-specific weighting factors. The entire feedforward control is weighted with the P-AXIS-00228 and P-AXIS-00229 parameters.

Acceleration feedforward control can also be weighted with the substitute time constant T_a, which is determined on the basis of the P-AXIS-00225 and P-AXIS-00226 parameters.

Jerk feedforward control can also be weighted with a factor

which is determined with parameters P-AXIS-00337 and P-AXIS-00338.

According to experience, the entire feedforward control can be weighted with a factor within the range of 0.7–1. At values more than 1, the axis leads and has a detrimental influence on contouring accuracy.

Parameterization for PROFIdrive

In the case of PROFIdrive, the control system currently only supports the telegram 5. PROFIdrive also requires the parameters

• P-AXIS-00092: position sensor increments per motor revolution and
• P-AXIS-00165: time offset of feedforward control setpoints,

which must be set in the axis parameter lists.

In the case of PROFIdrive, with the P-AXIS-00223 parameter it is also possible to select how the calculated feedforward control value is passed on to the drive:

• P-AXIS-00223, bit 9 = 0: The feedforward control value is calculated into the control deviation (XERR – signal No. 25) as a following error.
• P-AXIS-00223, bit 9 = 1: the feedforward control value is included in calculations as the velocity in the velocity setpoint (NSOLL_B – signal No. 7).

Programming

Feedforward control is programmed in the NC program with the modally active commands G135, G136 and G137 [PROG].

Selection of feedforward control

Feedforward control is explicitly deactivated every time the program is started. Axis group-specific activation of feedforward control is programmed with the NC command G135.

Feedforward control weighting

An axis-specific percentage weighting is assigned to the calculated feedforward control quantities via G136. It is limited for all axes to 100%. The weighting factors are reset to 100% after every program start. If feedforward control is activated or deactivated during the NC program, the weighting factors remain set to the values set by G136. For axes for which no G136 has been programmed, the weighting factor remains set to 100%.

It is also possible to enter selection and weighting of feedforward control in a block.

Cancellation of feedforward control

G137 is used to deactivate feedforward control a specific axis group.

Deactivating individual axes

In the case of axes for which no feedforward control is to be implemented after global selection with G135, a percentage weighting of 0% must be specified with G136.

Example

G135 (Selection of feedforward control) 
(für alle Achsen 100%) 
G136 X80 Y95 Z0 (Weighting: in this case the Z axis) 
(has no feedforward control) 

Spindle parameterization

Feedforward control for spindles also is programmed in the NC program with the modally active commands G135, G136 and G137[PROG].

Feedforward control can be programmed for a specific spindle with these commands. The commands must not be used simultaneously with other spindle-specific commands.

Example

S[G135] (Feedforward control activation for S) 
S[G136=80] (Feedforward control weighting in %) 
S[G137] (Feedforward control deactivation) 
S2[G135 G136=90] (Activation with 90% weighting for S2) 
S2[G136=0] (Weighting changed to 0%) 
S1[G135] (Activation with 100% default ) 
(weighting for S1) 

Delay

For calculation of the feedforward values, there is a delay of at least one clock pulse between the interpolator and the position controller.

Example

For a simple travel movement, the following figures show the measured following errors with and without velocity and acceleration feedforward control. Figure 1-3 shows a clear following error during the entire travel movement.

When velocity feedforward control is activated, following error peaks occur only in the acceleration phases; in the constant-velocity range, the following error is compensated entirely by feedforward control (Figure 1-4). The following error can be reduced further in the acceleration phases by means of additional acceleration feedforward control (Figure 1-5).