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Axis parameters, axis monitoring and axis calibration

TwinCAT axis commissioning

Determination and setting of the most important axis parameters: maximum speed, following error monitoring, end position monitoring as well as dynamics and calibration.

interface

The axis parameters can be accessed via the System Manager.

General axis parameters

A number of additional parameters can be set on the global tab of the axis, which determine the behavior of the axis and control a number of monitoring functions and status signals.

With the Maximum speed you can specify the value to which all specifications for the axis are limited. Here you should get a value of about 90 to 95Set% of the maximum speed of the drive. The remaining modulation is required for the controller to ensure correct positioning.

When using an NC interpreter, the axis is marked with a Rapid traverse speed adjustable control positioned. In this case, no more than 90% of the maximum speed of the axis should be selected.

The Manual travel speeds min and max are freely selectable and depend more or less on the "courage" of the operator. In practice it has proven to be useful to roughly proportion these speeds 1 to 5 adjust. Of course, the values ​​must be less than the maximum speed.

The Homing speed in positive direction only works for axes with incremental encoders. The axis moves at this speed when searching for the calibration cam. The direction is set on the encoder's incremental tab.

The Homing speed in negative direction only works for axes with incremental encoders. The axis moves at this speed when leaving the calibration cam and when searching for the synchronous pulse. The direction is set on the encoder's incremental tab.

The Pulse width positive / negative direction define the distance to be traveled, which is covered by the PLC in the corresponding travel modes when the axis is started. You can find more information on this in the block descriptions of the PLC.

You can set a travel range limitation both downwards and upwards (Software limit switch) and set the position.

For axes with an analog tab (i.e. not for rapid / creep or stepper motor axes), the Following error (Difference between target and actual position) is determined. Monitoring of this value with an adjustable threshold can be activated. There is one Filter time can be specified, which prevents an alarm from being generated if the threshold is briefly exceeded, e.g. when accelerating or braking.

Two independent windows can be placed around the target position of a movement (Target position window). The window width can be freely selected. This makes it possible to generate a signal to the PLC at any distance from the target. One of these windows also offers a filter time. The signal is only generated when the axis has been in the window without interruption for the duration set there.

With fast / creep axes, it is possible to always approach each destination from the same direction and with a minimum travel distance. As a result, greater accuracy can be achieved with asymmetrical axis behavior or mechanical backlash. This behavior can be activated and the route can be specified. The sign of the set distance determines the direction.

With servo axes, the operating mode "Backlash compensation"A mechanical play in the axis can be compensated. The value entered as" backlash "defines the distance by which a destination is crossed. The sign of the value defines the direction in which the backlash is crossed Cross the target only when driving in positive direction. A movement in the direction of smaller positions is not affected. With negative backlash, this behavior is reversed.

Axis dynamics

The values ​​for the acceleration, the delay and the shock adjust. Two options are offered: Direct entry as numerical values ​​or indirect definition via the run-up time and the acceleration characteristic.

The values ​​to be set here are the parameters to be adhered to by the setpoint generator. The drive manufacturer often mentions values ​​that lead to a very hard positioning behavior of the axis if they are set here. The reason for this is that the values ​​mentioned represent the limit values ​​of the power unit or the motor. As a rule, however, the behavior of the machine is then heavily dependent on the load. The result is considerable fluctuations in the following error, which trigger the position controller to intervene. However, the axis can hardly react to the controller output because it is already running at the current limit.

The "correct" values ​​for an axis depend heavily on the desired behavior and the properties of the drive technology and the machine. They can only be determined through test drives. Gradually increased values ​​should be checked by observing the following error when starting and stopping. In practice it has been shown that the numerical value of the jerk should be roughly twice (for machining axes) to ten times (for transfer axes). In the case of an indirect setting, this corresponds to a soft or a hard setting.

Use the TwinCAT Scope to set the values ​​for acceleration, deceleration and jerk. Whether the entered values ​​for the acceleration A + or delay A- really be achieved depends on the critical joltJ ± off, the ratio (A +) 2 / V or. (A-) ² / V, in which V denotes the target speed. In particular, check whether the actual acceleration values ​​can follow the nominal acceleration values ​​(also for the deceleration).

Effects of incorrect cycle time settings

Incorrect cycle time settings, i.e. a difference between the set cycle time (NC task: SAF task: online: cycle sticks in msec) and the real cycle time of the SAF task lead to two errors:

  • The information from the setpoint generator (position, speed, acceleration) does not match the real cycle time, i.e. the speed precontrol is incorrect.
  • The position setpoints of the position controller and the speed setpoints of the speed precontrol are not consistent.

Speed ​​pre-control: When the real cycle time TC_eff is greater than the set cycle time TC_saf this results in a linear expansion of the time axis (scaling TC_eff / TC_saf) which implies a non-linear transformation of the setpoints. Every calculated target speed is (TC_eff / TC_saf) times longer than expected. The integrated, effectively output target speed reaches a target position that is (TC_eff / TC_saf) times larger than the calculated one.
Position control: Since the setpoint speed and the setpoint position do not match (v (t) ≠ dp (t) / dt) the position controller must work against the feedforward control. The resulting behavior depends on the proportionality factor, the size of the incorrect cycle time setting and the controller resources (reference speed). In any case, there is a lag and large overshoots in the phases that are not force-free.

Delay between feedforward control and position control

The delay generator is used to compensate for the effects of a system (hardware / communication) dead time in the P component of a position controller between the output of the speed setpoint and its effect on positioning. Instead of the output speed v_o (t) as a function of the speed pre-control v_g (t) and des P-Proportion K_v (p_g (t) - p_i (t)), in which p_g (t) the position setpoint and p_i (t) the actual position value is,
v_o (t) = v_g (t) + K_v (p_g (t) - p_i (t)) + ...
the position setpoint is increased by the period of time delta issued late
v_o (t) = v_g (t) + K_v (p_g (t-delta) - p_i (t)) + ...

Parameter: delta Lag time in sec> = 0.0

Axis calibration parameters

These settings are only available for axes with Incremental encoders required. In order to adapt the actual value system to the reference system of the machine for these axes, a calibration run is started. A number of parameters and signals are required for this.

If the Calibration run an axis is started, it starts with the one mentioned above Homing speed in positive direction (Global tab of the axis). The actual direction is set on the encoder's incremental tab. The axis stops when the signal bit for the calibration cam is set in its NC-PLC interface. With this cam, a specific pulse can be selected in a rotating encoder system with one synchronous pulse per revolution.

The axis now starts again to leave the cam. She goes with it Homing speed in negative direction (Global tab of the axis) until the signal bit is deleted. Without changing the speed, the axis continues to move until the encoder's synchronous latch has responded. Only now does the axis stop.

The distance between the response of the latch and the place at which the axis comes to a standstill is determined. The sum of this distance and the Reference position (Incremental tab of the encoder) is set as the new actual axis position. This procedure enables an absolute position in the machine to be assigned to the synchronous pulse of a selected rotation of the axis.

The calibration is started in the Axis Online System Manager via .