Project 06/2005
Danaher
Motion
68 Rev
E
M-SS-005-03l
For example, a motor driving a rotating table is often configured as a rotary
axis. In this case, the table units are set up as degrees and the units of the
axis are set to repeat after 360°. In that way, the position repeats after every
rotation of the table, rather than continuing to increase indefinitely. The key
to the rotary mode is setting rollover position (
POSITIONROLLOVER
or
PROLLOVER
) correctly and accurately.
Consider this example where the axis drives a rotary table through a 5:3
gearbox:
Desired units: Degrees
Desired repeat: 360
Gearbox: 5:3
Feedback 2000 line encoder
For this example, first set position units to degrees:
1 degree = 1/360 revolution of table
= (5/3) * (1/360) revolution of the motor
= 2000 * (5/3) * (1/360) lines
= 4 * 2000 * (5/3) * (1/360) counts
= 40000/1080
After setting these units, you must enable the rotary mode and set the
rollover to 360 as follows:
A1.PositionFactor = 40000/1080
A1.PositionRollover = 360
A1.PositionRolloverEn = On
‘Enable Rotary Motion
In this case, the feedback position is always between 0 and 360. You can still
command long moves, say 10,000 degrees. However, when the motor
comes to rest, the feedback is read as less than 360.
When using rotary mode in applications where the motor turns continuously
in one direction, it is important to take advantage of all available accuracy in
the MC. This is because inaccuracy is accumulated over many revolutions of
the motor. For example, we could have rounded A1.POSITIONFACTOR in
the above example to 37.037, which is accurate to 1 part in 1,000,000.
However, after 10,000 revolutions (2000 rpm for just 5 minutes), that 1 part in
a 1,000,000 would have accumulated to a few degrees. This means that if
the table position is 100 degrees, it might read as 97°. In other words, the
table appears to drift.
In the example above, the math is exact until the MC performs the division.
Because the MC calculates with double precision (about 14 places of
accuracy), you should use MC math when calculating ratios that cannot be
represented precisely in decimal notation. In the example, using the full
accuracy of the MC (about one part in 1014) the motor would have to travel
about 3x1011 revolutions to accumulate one degree of error. That is
equivalent to 88 years of continuous rotation at 6000 rpm.
All SERVO
STAR
drives provide interface hardware to accept an external or
auxiliary encoder. This encoder is in addition to the primary position
feedback device. You will need to set up the SERCOS telegram to support
this. External position is frequently used in master-slave operations (gearing
and camming) where the system is slaved to an external signal such as a
line-operated motor.
