Rockwell Automation 8720MC Drives Integration Manual - Manual (Page 52)

Publication 8720MC-IN002A-EN-P — December 2002
2-20 Commissioning Your 8720MC Analog Drive
complimentary offset value to assure there are no direction reversals
during the orient.
Assuming modulo scaling is selected the complimentary offset value is
defined as the number of resolution units set in Parameter 79, minus
the modulo position value in parameter 51, “Motor Posn Fback”. A
simple rule of thumb is to use the complimentary offset value if the
orient direction is CCW.
Multiple revolution orients can be obtained by having the “Modulo
Value” (parameter 103) a value greater than the “Rot Posn Resolut”
(parameter 79). For example, “Rot Posn Resolut” = 3600, “Modulo
Value” = 7200.
For Example: Assume Parameter 79, “Rot Posn Resolut” = 10,000
counts/revolution; Parameter 103, “Modulo Value” = 10,000 counts/
revolution; Parameter 154, “Orient Options” = CW and Servo Loop
Parameter Group 0 is selected as determined by Parameter 254,
“Actual Param Set”. Initiate an orient by setting parameter 152, “Orient
Request”, to a value of 1. Note that after an orient with zero in
Parameter 150, “Mtr Marker Ofset” and zero also in Parameter 153,
“Orient Angle” the position display, Parameter 51, “Mtr Posn Fdbk” = 0
or 10,000 counts. To determine the required offset correction
manually rotate the motor to the desired angle with the drive disabled.
Assume after doing this Parameter 51 reads 2,500 counts meaning we
rotated cw 2500 counts. For parameter 150 CW offsets have a minus
value. We record the difference as -2500 counts CW by setting
Parameter 150 to -2500. To prevent orient direction reversals it is
always good practice to record the marker offset direction the same as
the desired orient direction. In this example since the selected orient
direction is CW and we do not desire direction reversal during orient
and the orient direction is also clockwise. To achieve the same target
orient position with a CCW orient direction we would set Parameter
150, “Mtr Marker Ofset” to a complimentary CCW offset. This is done
by subtracting the measured offset counts from the number of counts
per revolution, Parameter 79. In this case we subtract 2500 from
10,000 and get 7,500 counts, CCW. For parameter 150 CCW offsets
have a plus value. We now then enter 7500 in Parameter 150, “Mtr
Marker Ofset”. Assuming a CCW orient direction and the motor
standing still, if we request an orient via the digital I/O or Parameter
152, “Spin Orient Req”, the motor will behave as follows:
• Accelerate to “Spin Orient Spd”, Parameter 222, in the CCW
direction using the “Posn Accel Rate”, Parameter 260, until it is
within the velocity speed window, Parameter 157, “At Spd
Window” or Parameter 272, “Speed Window %”. The drive will not
look for the encoder marker until it determines the motor speed is
within the selected velocity window.