PCI-7358 with external commutation for a brushless servo motor with 5 pole pairs

[W.A.Kaan]

I am using a PCI-7358 motion controller card in combination with an UMI-7772 to control two brushless servos. The PCI-7358 creates the commutation for the servo’s, by using onboard commutation.

The physical encoder counts per revolution are 2048.

One motor has 4 pole pairs and the other motor has 5 pole pairs.

By using the X4-encoding the encoder counts per revolution are 8192.

When I calculate the Encoder counts per electrical cycle the result for the motor with 4 pole pairs is 2048 and the result for the motor with 5 pole pairs is 1638.4

I am using the equation:

Encoder Counts per electrical cycle = 2 * (encoder counts per revolution / number of poles)

Th motor with 5 pole pairs causes some trouble, because I have to fill in 1638 or 1639 for the variable Encoder counts per electrical cycle in MAX. (No broken numbers are allowed)

Every rotation this causes an error of  2 counts, so there is an problem with the Following Error, and the motor can start heating up.

Does anyone know how to solve this problem (except for replacing the encoder by a part with 2000 encoder counts per revolution) in MAX

 

Wim Kaan

[Jochen]

Wim,

thank you for using the discussion forum. We are looking into the issue that you have brought up. Please be patient.

Jochen Klier
National Instruments Germany

[ChadE]

Hello Wim Kaan,

Unfortunately, there currently is no way to correct for this in the NI-Motion API or in MAX.  There are however a couple of options:

1. You could replace the encoder as you mentioned with one that divides down as a whole number.  Though this is relatively inexpensive.  I realize that it can be difficult and sometimes impossible depending on the motor that you are using.  For example, some motors have a housing that covers the encoder, etc.
2. You could replace the drive you are using with one that supports the motor and does the commutation onboard.  We find that many of our customers go with this option because the prices for drives with and without commutation are very similar. 

Kind Regards,