Motor has incorrect Velocity (RPM) when using MAX

[Staffan Erickson]

When using 1-D Interactive in MAX on a PCI-7344, I do not get the the same Velocity (in RPM) that I entered.  The motors are always about a factor of 3x too slow.  Why is this?

[Jochen]

Staffan,

the calculation of the velocity in RPM depends on the settings for Encoder Counts per revolution (for servos) or Stepper Steps per revolution (for steppers).
Pleae make sure to set these values correctly (see screenshots).

Best regards,

Jochen Klier
National Instruments

[Staffan Erickson]

Hello Jochen,

Thank you for your quick responce.  There was no tab for Stepper Conditions in MAX (4.2.0.3001) at first when I checked it today.  The Default Configuration setting was set to Custom.  When I switched to Open Loop Configuration I found the missing Stepper Condition tab on all the Axis and the encoder Stepper Steps per/rev. were all correctly set to 2000.  However, now I have a new problem.  None of the motors are operational.  Using 1-D Interactive on any Axis with appropriate data loaded, the Current Trajectory Data changes yet motor is not turning.  I have reset the system and PC several times, including Initialize several time with no change in outcome.  Not sure what has happend?  Help!

[Jochen]

Staffan,

what type of motor do you have? If you are using a servo and you set the motion settings to "Open Loop Stepper", you shouldn't expect your axes to move, as the board is generating signals at the step and direction outputs now, instead of the analog outputs.
Stepper Steps per revolution is only relevant if you are using a stepper motor.

Jochen

[Staffan Erickson]

Jochen,

I am using a PM motor with a 2000 ppr encoder, not a stepper motor.  When I use the Servo Default Configuration I'm getting RPM values that are 1/4 the actual measured motor RPM's in 1-D Interactive.  My Encoder Settings are set to 2000 counts/rev.  Is this difference due to the motion controller samples at every 4th encoder transition state?  I would have expected to see RPM change when I changed the Encoder Counts Per Revolution from 2000 to 500.  There was no change.  Please clarify discrepancy.

[Jochen]

Staffan,

the motion board uses X4 decoding for the encoder inputs (counting every rising and falling edge of Phases A and B). So if your encoder generates 2000 counts per revolution, the setting should be ok. Please check if this setting is correct by turning the motor by one revolution. Do 2000 position counts equal one revolution?

The test that you have done (changing the Encoder Counts Per Revolution value from 2000 to 500) should result in different velocities, if you define the velocity in RPM. If it doesn't, please make sure that you have save the new setting and initialize the board, before running a move in 1D-Interactive again.

If you still see a discrepancy, I'm wondering if there is a gear box between your spindle (or whatever you move) and your motor.

Jochen

[Staffan Erickson]

Jochen,

I confirmed my encoder setting of 2000 counts and the shaft rotates only 1/4 revolution. 

I confirmed my Encoder changes 2000 to 500 again (using the save-initiate) and there was no change in RPM.

I'm not using any gear box. 

[michaeld]

Staffan,

Can you verify that your encoder's resolution is given as 2000 quadrature counts per revolution, and not in a different form?

The following tip given in MAX when you setup your encoder seems to indicate that your encoder resolution may be different than what it seems.

Tip  If your encoder does not supply resolution in quadrature counts per revolution, determine quadrature counts per revolution by multiplying the encoder resolution in encoder lines or periods by four. Encoder counts per revolution is also referred to as counts per index. For example, a 500 line encoder has 2,000 quadrature counts per revolution.