Delta RMC101 - Manual (Page 883)

Appendix D: Status Field Reference
D-21
using a 12-bit (4000-step) digital-to-analog converter (DAC), which will generate a ±10,000 mV
output in steps of 5 mV. The internal drive calculations are done to 14-bit resolution. This
additional resolution is used to dither the least significant bit of the output, giving additional
resolution.
In many applications, the full ±10 V range is not used, and drive output beyond that range may
even be unsafe. For this reason, the Limit Drive (L) command can be issued to limit the drive
output range to be smaller than ±10 V.
Note: This field is unused on 12-bit analog channels or 16-bit analog channels that do not
include channels 0 and 2. That is, on 16-bit analog modules, only channels 0 and 2 have drive
associated with them. On auxiliary pressure/force channels, the drive is only affected by the
Open Loop command; when controlling pressure/force, the position axis's drive is used.
Note: Software adjusts the null. There are no hardware nulling adjustments on the motion
control module. Null drive will be near 0 volts.
Why Bother?
If the DRIVE tries to go below -10,000 or above 10,000, an overdrive condition has occurred.
D.2.9 Actual Speed
The Actual Speed is the calculated speed at which the axis is currently moving, as computed
from the change in the Transducer Counts status register. The Actual Speed is in position units
per second.
This value is filtered to reduce the effect of quantizing errors. In firmware dating 19991216 or
newer, the Actual Speed is filtered by applying a low pass Infinite Impulse Response (IIR) filter
with a 7 ms time constant. In earlier firmware, the filter consists of summing the change in
position over the last four time periods and dividing by four time periods.
What is Quantizing Error?
Before the position of an axis can be used by a digital controller such as the RMC, its real-world,
infinite-resolution, analog value must be converted through a transducer into an integer number of
transducer counts. The loss of position resolution that occurs when truncating to integers is
referred to as quantizing error.
Quantizing error affects the accuracy of positions, but it also affects the accuracy of speeds even
more significantly. The following example demonstrates the problem quantizing errors pose to
computing the Actual Speed.
The simplest way to compute the Actual Speed would be to divide the change in position units
since the last sample by the sample time period, as shown below:
ActSpd
= x(n) -
x(n-1)
T
where: x(n) is the position sampled this control loop
x(n-1) is the position sampled last control loop