Commutation

Setting a proper commutation method is required for BLAC / BLDC motors. Summit servo drives include different strategies to configure the commutation with the highest precision.Different feedback can be used as a commutation reference. The feedback readings are converted into mechanical and electrical angle to know the position of the rotor and then apply the proper excitation to get the maximum efficiency (based on FOC algorithm). 

Usually motor feedbacks are not aligned with the rotor of the motor by mechanical assembly. Therefore, before it can be used as commutation feedback, an alignment sequence is required. This pages describe all the elements that summit products include for getting this alignment. 

Commutation feedback alignment methods

The objective of these methods is to detect the offset between the angle generation and the readings of the commutation feedback. There are three main blocks of methods:

  • Forced. Requires to move the actuator injecting a known voltage or current. Its main use is to align incremental encoders (dig. encoder, sincos, etc) used as reference and commutation, or to align an absolute encoder.
  • Non-forced. An absolute feedback is used as reference to measure the difference between the commutation feedback and system angle. It can be used with any feedback as commutation feedback and an absolute feedback  as the reference feedback.
  • No phasing. No phasing procedure is performed. The alignment must be previously stored in memory or introduced manually for the system to work. Its main targets are absolute encoder (halls, SSI, Resolver 1 pole, BiSS, etc) as reference and commutation feedback

    Before using non-forced and no phasing methods, the absolute feedback must be calibrated (aligned to rotor) using the forced method. Once it is executed, the configuration might be stored into the non-volatile memory and the forced method is not required anymore

The parameters required for aligning properly the system are:

  • Commutation feedback sensor. Selects the type of feedback used for commutation

    Warning

    Only four feedbacks can be mapped simultaneously in all of the feedback sensor parameters

  • Commutation angle offset. Defines the angle difference between the expected angle 0.0 by the drive algorithm and the angle value read from the commutation feedback at this same position
  • Motor pole pairs. The commutation feedback usually gives mechanical angle reference instead of the electrical angle that is the real one needed for proper commutation. This register helps to convert from mechanical to electrical angle.
  • Commutation angle value. It contains the value of the electrical angle read by the commutation feedback once the pair poles have been applied.

Forced methods

There are different methods to align the commutation feedback. Summit product implements the next ones:

Binary search

The objective of this method is to apply the minimum possible displacement to determine the relationship between the generated angle by the FOC algorithm and the read one by the commutation feedback. 

From this point, it is highly recommended to have some knowledge about Field Oriented Control algorithm. Take a look at this application note.

The sequence generates a set of points (angle) inside the current D-Q space. These points are generated using a binary search technique:

  1. The first angle is applied to the middle of the range.

    In the angle world, if the range is 360º, the middle point is 180º

  2. Then the drive reads cyclically the commutation feedback to calculate the distance between the initial angle and the current one.
  3. When a minimum displacement is detected, then the next point is generated.
  4. The next point is calculated as : \( Next \ point = Previous \ point · (1 + 0.5 · (sign(displacement))\)

So the algorithm detects the small movements and looks for the nearest angle to the actual position of the actuator. The next diagram shows an example of how the steps are generated

BinarySearchBasics

Note

The binary search phasing method modifies some of the feedbacks internally to perform the phasing. If the phasing procedure is either interrupted or finished, the feedback configuration of the user is restored.

These are the parameters required to adjust the phasing sequence to the different actuators:

  • Max. current on phasing sequence. Forced methods use the current mode during the phasing sequence. The current is increased slowly until a movement is detected or the value of this register is reached.

  • Phasing timeout. This timeout is used for binary search method and expires if the displacement of the actuator is less than the expected one. If this happens, the next point is generated assuming a positive displacement. 
  • Phasing accuracy. This register determines the number of steps and the minimum distance the drive should detect to consider that the actuator has been moved. The last step is defined as the first angle delta smaller than 3 times the phasing accuracy.

    Example

    Setting a value of 10000 mº means that the actuator must travel 10000 mº in each step and the number of steps will be 4:

    - Last angle delta will be smaller than \( 10000 mº · 3 = 30 º\)

    - First angle delta is 180 º > 30º

    - Second angle delta is 90 º > 30 º

    - Third angle delta is 45º > 30ª

    - Last angle delta is 22,5 º < 30 º

    Therefore, this register is used to adjust the traveled distance based on the limitation of the application or the commutation feedback itself.

    Dig. halls

    In case the system includes digital halls and the motor has only 1 pair pole, the minimum measurable angle delta is 60 º. In this case, the user must set the phasing accuracy to a value bigger or equal than 60000 mº. The algorithm then is modified to generate two steps multiple of 60 º:

    - First delta is 240 º

    - Last delta is 120 º

Non-forced methods

This method uses an additional feedback as reference for align the commutation feedback. This reference must be previously aligned, in order to be used for this purpose, by a forced method.

The result of the forced method is duplicated automatically into the reference feedback parameters.

Once the reference es properly configured and the desired commutation feedback, the sequence of non-forced method is:

  • Wait until device enters in operation enable state
  • Once in enable state, device uses reference feedback as commutation until it is updated
  • The difference between the reference angle and the commutation angle is used to compute the value of the register Angle offset

Example

The system is composed by a BLAC motor with digital encoder and digital halls. Digital encoder is used as commutation feedback and digital halls as the reference. During the non-forced alignment method:

  • The system starts to use the digital halls for commutation until a halls transition is detected.
  • In that moment, the offset between halls and the digital encoder is computed
  • Finally, the device use digital encoder for commutation.

These are the parameters related to the reference feedback:

  • Reference offset. Defines the angle difference between the expected angle 0.0 by the drive algorithm and the angle value read from the reference feedback at this same position.

    It contains the same value than the register angle offset once the forced alignment method is executed.

  • Reference feedback. Selects the type of feedback used as a reference.

    Warning

    Only four feedbacks can be mapped simultaneously in all of the feedback sensor parameters

  • Reference angle. It contains the value of the electrical angle read by the reference feedback once the pair poles have been applied.