BECKHOFF EL7037 - Manual (Page 24)

Product overview
calculated from the phase currents. Via the torque-forming current component i
q
, the torque of the machine
can now be regulated directly, like for a DC machine.
A prerequisite is that the rotor position is available with sufficiently high accuracy. For a stepper motor the
encoder resolution should be at least 4000 increments per mechanical revolution, in order to achieve
adequate positioning accuracy. The minimum encoder resolution also depends on the number of full steps
and can be calculated approximately as follows.
Fig. 14: Calculation of the resolution
Commutation determination for Extended Operation Modes
Because the absolute actual position is not available for incremental encoders, on system start-up there is
no direct reference to the rotor position, which is required for field-oriented operation. Therefore, the
reference between the actual position and the rotor position must be generated at start-up via a commutation
determination process. During this process the rotor is moved forward and back several times up to two full
steps.
Note
Commutation determination
• The maximum current should be set just below the rated motor current.
• During commutation determination the rotor shaft should not be subject to an external
torque. If this condition is not met, the Extended Operation Modes cannot be used.
Control structure
The drive control structure is a cascade control structure with a position control loop and a lower-level speed
and current control loop. If a speed setpoint is specified, the external position control loop can be omitted.
Fig. 15: Cascade control structure with field-oriented control (Extended Operating modes)
Motor dependency
Due to the fact that the control is strongly dependent on the motor parameters, the controller parameters and
motor behaviour itself, field-oriented control is limited to Beckhoff motors. This mode is not supported for
motors from other manufacturers.
Main advantages compared with standard mode
• Low current consumption (almost full load-dependence)
• High efficiency
• Consistent dynamics compared with standard mode
• Step losses are inherently avoided
EL70x7 24 Version 1.0