User`s manual

G-2 Motion Mate™ DSM314 for Series 90™-30 PLCs User's Manual – January 2001 GFK-1742A
G
A = Acceleration/deceleration during the strobe event which is 250,000,000
cnts/sec
2
(assumed to be constant over the entire 250
µ
s period; Larger
acceleration values will increase the amount of error in the calculation)
T
p
= Position sampling period which is 250 microseconds
V
I
= Initial velocity just before the strobe event which will be 0 for this example.
The change in the number of encoder counts (Cnts) for a given amount of time (t) can be
calculated using the following formula:
P
act
= V
I
t + ½ A t
2
Therefore the total number of counts to occur during the sampling period for this example
is approximately 8 counts (actual calculated values is 7.8125) or 0.343 degrees of motor
rotation.
The average velocity for the sample period given the change in position would be as
follows:
V
avg
= Change in counts = 7.8125 cnts = 31250 cnts/sec
Sampling Period 250
µ
sec
The following formula can be used to estimate the strobe position using the velocity
derived above:
P
est
= V
I
t + V
avg
t
Therefore, the error between the estimated strobe position and the actual strobe position is
as follows:
Error = P
act
- P
est
The graph below contains plots of the actual position, the estimated position, and the
resulting strobe position count error for the 250 microsecond sample period. From the
graph, it is evident that the greatest count error occurs in the middle (i.e. at 125
microseconds) of the time period.