The XMP firmware also supports mixed commutated and non-commutated motion on the same controller card. XMP controllers provide two DACs per axis allowing for on-board sinusoidal commutation for all axes supported.
Each XMP axis (Motor object) has a primary DAC channel and an auxiliary DAC channel. These channels provide ‘A’ and ‘B’ phase sinusoidal signals that are typically phase shifted by 120 degrees. (XMP controllers also support other phase angles). The third commutation signal is generated internally within the servo amplifier by balance loops in the power stage.
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Servo loop without controller commutation
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| Servo loop with Sinusoidal Commutation |
Torque Ripple
Motors rotate due to the torque produced by two interacting magnetic fields. The maximum torque is produced when the magnetic vector of the rotor is at 90 degrees to the magnetic vector of the stator.
With sinusoidal commutation, the force created by the electromagnetic vector is smoothly rotated along, so that the stator vector remains at a 90-degree angle from the rotor vector. In Hall commutation, the stator magnetic vector is almost always misaligned with the rotor, causing an inconsistency in the amount of applied torque (torque ripple). Torque ripple can cause many problems, including mechanical wear, vibration, noise, and deficiencies in servo performance.
Open- and Closed-loop Control
Sinusoidal commutation provides smooth, accurate motion for both open and closed-loop systems. For open-loop control, the magnitude of the stator’s vector is held constant while the direction is set to the desired position of the rotor. This makes for very simple and stable control, since no sensor is needed to determine the true position of the rotor. Closed-loop control uses knowledge of the actual position of the rotor to control the magnitude and direction of the magnetic vector generated by the stator.
With XMP controllers, it is also possible to change between open- and closed-loop control. While in open-loop mode, the command position can be reset (to the actual position) without incurring motion, since updating the commutation pointer occurs only from the trajectory calculator. Alternatively, the actual position can be reset to the command position so that a negligible error signal is generated. Closed-loop control can then be invoked without causing the motor to jump.
Phase Finding
Successful motor commutation requires initialization in which the armature’s field vector is determined from either the feedback system or Hall sensors. With incremental encoders, the location of the field vector is unknown at power-up and must be determined relative to a reference position. The feedback system and motion controller then track the field vector position for all subsequent moves. During these moves, MEI’s XMP calculates the 90-degree (stator) current phase advance required for closed-loop operation, while the magnitude of the current vector (or Stator Vector) is determined by the error signal and PID algorithm. Three techniques that can be used for initial armature phase finding are stepper, dither, and Hall sensor initialization.
Commanding motion with sinusoidal commutation is essential if you want smooth motion at any speed. MEI has designed the XMP series motion controllers to operate exceptionally well both in terms of hardware and software when utilizing sinusoidal commutation. It ensures a minimization of wear and tear on machines as well as being able to greatly reduce torque ripple.
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