Setting up the AKD to Close Position Loop on an External Encoder

Setting up the AKD to Close Position Loop on an External Encoder

The AKD drive can accept secondary feedback from an external encoder.  The encoder can be mounted directly to the load or geared to the load in order to get better positioning precision.  The AKD uses this encoder signal to close the position loop, while the velocity loop and commutation are based on the primary feedback device in the motor.

Related drive parameters:

PL.FBSOURCE defines which feedback device is used to close the position loop.  For secondary feedback (external encoder), select “Feedback 2”.

DRV.EMUEMODE defines the mode for the Emulated Encoder Output.  While using an external encoder as secondary feedback, you cannot use the EEO.  So set it to “Input”.

FB2.SOURCE defines the hardware input that will be used to receive the encoder signal.  Set this to “X9”.

FB2.MODE defines the type of encoder signal.  Set this for “Input - A/B Signals”.

FB2.ENCRES defines the number of encoder pulses (post-quad) per revolution of the load.  You must take into account the line count of the encoder and any mechanical gearing ratio between the encoder and the load.  So FB2.ENCRES will be set to the number of encoder lines (pre-quad) x 4 x (gear ratio between encoder and load).  Then you must define the user units in terms of the load.  When programming a move of X number of units, the motor will move however far it needs to move to get the external encoder to move the correct number of pulses (to move the load X distance).  The user units will be in terms of physical distance that the load moves.

Example:

Say you have a motor driving a nip roller.  The nip roller has a ratio of 10 inches per rev.  An encoder wheel rides on the material such that a 4096 line external encoder turns 1 revolution in 8 inches (or 512 encoder lines per inch).

Motor to load ratio does not matter.  The material will be fed the correct distance regardless of any nip roller slippage.

External encoder to load ratio = 512 lines per inch

Set FB2.ENCRES = 4096[lines per rev] x 4[counts per line] / 8[inches per rev] = 2048 pulses per inch

User units can be set to inches of load distance.  In Workbench, set the position units to Custom and set the ratio to 1 unit = 1 rev (meaning 1 inch = 2048 external encoder pulses, based on FB2.ENCRES).

Now the motion task or Basic program position move can be programed in user units and the external encoder will be used to close position loop.  PL.FB will be the user unit value corresponding to the distance the external encoder moves.  The motor will move as far as it needs to in order to make the external encoder (and load) move the correct distance.

DO NOT set the external encoder count resolution based on the mechanical ratio between the motor and load.  It should only be based on the ratio of external encoder counts per load distance.

When commanding motion via a fieldbus, use the appropriate fieldbus position scaling rather than or in addition to the drive's user units.