Position compensation

In the following sections, the dynamics and positioning specified by the coupling to the respective master will be referred to as basic dynamics and basic positioning. Position compensation is an additional positioning that is additively superimposed on the basic positioning. Over a certain distance (compensation length L), additional positioning is carried out by a desired distance (compensation difference D). The aim of slave axis position compensation is generally spatial synchronization with other axes.

Position compensation may be aborted via a compensation stop (continuous). 

Position compensation of slave axes is a variant of master axis position compensation.

WARNING

If the compensation difference D is negative (deceleration), (double) motion reversal is possible.

General notes:

  1. D may be negative. In this case, the overall positioning lags behind the basic positioning.
  2. Compensation is complete once the distance L±D has been travelled.
  3. Compensation is a linear path control, i.e. during compensation the slave axis will follow all the changes in basic dynamics (e.g. override 0.0).
  4. During compensation, axis compensation start is locked, i.e. there is no compensation iteration.
  5. End of path during axis compensation stops the compensation.

Special notes for slave axes:

  1. If a slave axis is coupled to a virtual axis as master axis, the slave axis can only start and carry out a compensation if the velocity filter time (of the virtual axis) is large enough for the axis not to change direction randomly.
  2. Position compensation is possible independently on all axes of a cascade (including master axis).
  3. Within a cascade, compensation is passed on to the subsequent slave axes. In case of negative (relative to the axis direction) compensation, movement reversal may occur.
  4. In case of several compensations within a cascade, the compensation velocities will be added to each other, and the accelerations will be multiplied to each other.
  5. In case of uncoupling of a slave axis during slave or master axis compensation, or of a control axis between master axis and the slave axis to be uncoupled, the basic dynamics of the master axis is maintained and the velocity and the position is carried out continuously differentiable, i.e. the compensation velocity at the uncoupling time is maintained, and the compensation is never completed.
  6. Compensation takes place independent of the slave axis coupling factor.

Algorithm

After checking the compensation start parameters, a calculation will be carried out to check whether compensation of the difference D over distance L is feasible with the compensation start parameters. If this is not the case, the maximum possible compensation difference is calculated and activated for D. In any case, the parameters are chosen in such a way that compensation occurs over the whole compensation length. The setpoint generation of the compensation is a non-jerk-limited 3-phase-algorithm (trapezoidal velocity profile), whereby different ramp slopes are possible.

Position compensation 1: 

Compensation: slave is making up fully. Master start from 0.0 mm to 10,000.0 mm with V = 2000.0 mm/s. Slave start position -500.0 mm, coupling factor 1.0. 

Compensation: A+ = 500.0 mm/s/s, A- = 1000.0 mm/s/s, V = 300.0 mm/s, VP = 2000.0 mm/s, D = 500.0 mm. L = 5000.0 mm.

Position compensation 2: 

Compensation: slave slows down. 

Master start from 0.0 mm to 10,000.0 mm with V = 2000.0 mm/s. Slave start position 500.0 mm, coupling factor 1.0. 

Compensation: A+ = 500.0 mm/s/s, A- = 1000.0 mm/s/s, V = 300.0 mm/s, VP = 2000.0 mm/s, D = -500.0 mm. L = 5000.0 mm.

Start parameters

Position compensation: start parameters

Start parameters

Meaning and boundary conditions

Compensation length L

Distance, over which additional positioning occurs (L > 0.0)

Compensation difference D

Additional travel distance (D ≠ 0.0)

Basic process velocity VP

Velocity, to which the velocity increase refers (generally axis target velocity) (VP > 0.0)

Velocity increase VK

Permissible velocity increase / reduction relative to the basic process velocity (VK > 0.0)

Acceleration AK+

Required acceleration increase relative to the global axis acceleration (AK+ >0.0)

Deceleration AK-

Required deceleration increase relative to the global axis deceleration (AK- > 0.0)

 

Notes on start parameters:

  1. The compensation difference D may be positive (making up) or negative (deceleration). If D < 0.0, motion reversal is possible.
  2. The compensation refers to a basic process velocity   VP, which is generally reached by starting the master axis to a target velocity with an override of x%, so that: v = x /100. It is assumed that the master will have reached this velocity when compensation is requested.
  3. If the preset basic process velocity VP does not coincide with the amount of the real current local process velocity v, the following applies. If VP > v, the actual compensated distance (absolute) will be less than the required one (or the one calculated at the start). If VP < v, the actual compensated distance will generally be equal the required one (or the one calculated at the start). This means that for stop or override 0.0, the actual compensated distance (absolute) will be less than the required one (or the one calculated at the start).
  4. The values AK+ and AK- are increases in path acceleration/path deceleration values, i.e. if D > 0.0, compensation starts with the acceleration increase AK+, whilst, if D < 0.0, compensation starts with the deceleration increase AK-.

Special notes for compensation within slave cascade.

During simultaneous compensation of all axes, let A_m+ be the acceleration of the cascade master axis, AK_m+ the compensation acceleration of the master axis, and AK_sk+ the compensation acceleration of the k-th slave axis, 1 ≤ k ≤ n. Then the following applies ( Π is the product of all k)

A_m+ × AK_m+ Π AK_sk+ ≤ A_max+


where A_max+ is the acceleration of the master axis that would be movable without slaves. (If an axis does not take part in the compensation, AK_sk+= 1 is to be used for its acceleration in this equation). Analogous considerations apply for A-.

Compensation stop

The function stop terminates the compensation of an axis, if the axis is in compensation mode, but not in the braking phase of compensation, by suitably moving the (temporal) phase limits of the 3-phase algorithm, recalibrating the length of the compensation length L, recalibrating the differential speed and replacing the compensating distance D at the end the compensation by the actual compensated distance.

Interfaces

Compensation start

System Manager: compensation parameters

System Manager: start compensation

PLC function blocks: compensation start

Compensation stop

System Manager

PLC function blocks