Drive control and features
59
Position/synchron control features
Position calculation
The actual position of the drive is measured using a position feedback device.
During normal operation, the actual position is calculated by keeping track of the
position change between the current time and the last known position. The position
calculation is non-saturating: after the maximum position has been reached, the
position gets the negative value with the maximum absolute value.
Depending on the machinery used, different types of scaling can be applied to the
position measurement for drive position calculation, such as
• unit (parameter 60.05 POS UNIT)
• feed constant, transforming rotational axis movement to translational (60.06
FEED CONST NUM, 60.07 FEED CONST DEN)
• gears (60.03 LOAD GEAR MUL, 60.04 LOAD GEAR DIV, 71.07 GEAR RATIO
MUL, 71.08 GEAR RATIO DIV)
• axis type (60.02 POS AXIS MODE), and
• resolution of the position calculation (60.09 POS RESOLUTION).
The drive position system range is defined by 60.13 MAXIMUM POS and 60.14
MINIMUM POS. If the actual position is out of this range, a fault (POSITION ERROR
MAX or POSITION ERROR MIN) is generated. The range is supervised in position,
synchron and profile velocity modes.
Hardware limits can be established by parameters 62.05 NEG LIMIT SWITCH and
62.06 POS LIMIT SWITCH. If a limit switch is triggered, the speed reference in that
direction is ramped down along the emergency stop ramp, and only movement in the
opposite direction is allowed. In the homing operating mode, when seeking the home
position, the drive does not use the limit switches; they are however used in some
homing methods to reverse the direction of movement when seeking.
With an absolute type of position feedback, the actual position after a power-up or an
encoder refresh request is calculated on the basis of the measured number of
revolutions of the absolute encoder and its position inside one mechanical shaft.
After this, the actual position is calculated by keeping track of the position changes.
The actual position can be reproduced uniquely and clearly after the next power-up
only if the encoder position has not moved outside its working area.
Example:
With an absolute multiturn encoder and 12 bits for revolution counting (defined with
parameter 91.03 REV COUNT BITS), the encoder position must not exceed 4096
revolutions, or else it will fall below 0 revolutions. If the encoder is run to the position
of -10 revolutions, the position at the next power-up will be 4086 revolutions.
The same situation may occur in a roll-over application with a gear different from a
power of two, because the old and new encoder readings do not give the same
position. This situation can be avoided by activating parameter 91.06
ABS POS
TRACKING.
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