Acceleration feedforward control

Configuration

To apply acceleration feedforward control, first configure acceleration feedforward control in P-AXIS-00223 and the output of the acceleration feedforward control value as an additive command value:

vorsteuer.vorsteuerung   0x202  #0x002: Activation accele-
                                ration feedforward
                                #0x200: Output as additive
                                torque command value

In addition configure the additive torque/current command value in the cyclic process data.

Notice

If the output of acceleration feedforward control is activated as an additive command value and no additive torque/current command value is configured in the cyclic process data, the error message ID 70351 is output.

Torque calculation

With acceleration feedforward control using an additive torque or current command value, the required motor torque is calculated by the equation below.

M = J_tot*a

or

F = m_tot*a

This torque is rescaled to the format used by the motor and then output to the drive.

Since specific knowledge of the mechanical structure of the axis is required to calculate angular acceleration, it is advisable to configure the path resolution of the axis by specifying the parameters P-AXIS-00092, P-AXIS-00233, P-AXIS-00234, and P-AXIS-00362 and P-AXIS-00363.

If a value unequal to 1 is assigned to one of the two parameters P-AXIS-00362 and P-AXIS-00363, the path resolution is calculated by the equation:

Parameter set

The following parameter set results for an axis with a motor encoder resolution of 2²⁰ increments per revolution, a gearbox ratio of 3 and a spindle pitch of 5 mm/rev:

antr.feed_const_num     50000         #feedconstant 5mm
                                      per revolution
antr.feed_const_denom          1      #
getriebe[].incr_per_rev 1048576       #2**20 increments per                                       motor revolution
getriebe[].wegaufz             3      #gearbox ratio: input                                       revolutions
getriebe[].wegaufn             1      #gearbox ratio:
                                      output revolutions

The total motor load (J_tot) is also required as a further parameter.
J_tot is composed of the following:

• Total of motor moments of inertia and
• load torque referred to the motor shaft.

Enter the resulting value in the parameter P-AXIS-00391 in the unit kg*m².

getriebe[].load             0.48      #Total motor load
                                      0.48 kg*m**2

The required motor torque can be calculated using the parameters now available. The following parameters are also required to calculate the torque/current command value output to the drive:

1. Motor nominal torque as reference value for torque scaling in P-AXIS-00392.
2. Torque scaling factor in the parameters P-AXIS-00325 and P-AXIS-00326.

The value for P-AXIS-00392 is contained in the motor data sheet:

antr.acc_reference_value     0.45      #stall torque in Nm

Enter the value which must be output as additive torque command value for the parameters P-AXIS-00325 and P-AXIS-00326 so that the motor outputs its nominal torque. This value depends on the torque scaling used in the drive.

For example, if a percentage weighting is used in 0.1% referred to the maximum current of the drive amplifier, the following results for a maximum current of 3.16 A and a nominal current of 1.58 A for the parameters P-AXIS-00325 and P-AXIS-00326:

P-AXIS-00325/P-AXIS-00326 = 1,58A/3,16A * 1000 = 500

The possible values for P-AXIS-00325 and P-AXIS-00326 would then be, for example:

antr.torque_scale_num            500
antr.torque_scale_denom            1

In addition, the output of the acceleration feedforward value can still be delayed in relation to the position command value; this delay can be set by the P-AXIS-00390 parameter.

Notice

The maximum delay time that can be set is one position controller cycle. If a longer delay time is parameterised, the error message ID 70348 is output and the delay time is set to 0.

The feedforward value to be output can be weighted by the parameters P-AXIS-00225 and P-AXIS-00226. In the following example the feedforward value is 1.2.

vorsteuer.vs_a_faktor      12   # Weighting acceleration feedforward numerator
vorsteuer.vs_a_nenner      10   # Weighting acceleration feedforward denominator

The resulting parameter set is:

vorsteuer.vorsteuerung   0x202  #0x002: Activation accele-
                                ration feedforward
                                #0x200: Output as additive
                                torque command value
antr.feed_const_num         50000     #feedconstant 5mm per                                       revolution
antr.feed_const_denom           1     #
getriebe[].incr_per_rev  1048576      #2**20 increments
                                      per motor revolution
getriebe[].wegaufz              3     #gearbox ratio:
                                      input revolutions
getriebe[].wegaufn              1     #gearbox ratio:
                                      output revolutions
antr.acc_reference_value     0.45     #stall torque in Nm
antr.torque_scale_num         500
antr.torque_scale_denom         1
vorsteuer.acceleration_delay_time 150 # Delay accele-
                                      ration feedforward
                                      against
                                   # position
                                   command value in us
vorsteuer.vs_a_faktor      12   # Weighting acceleration                                 feedforward numerator
vorsteuer.vs_a_nenner      10   # Weighting acceleration                                 feedforward denominator