Appendix D
Common Questions
SC-2345/2350 Carrier User Manual
D-2
ni.com
I am taking measurements from a strain gage mounted on a motor, and
I am concerned that voltages from the motor might damage my
E/M Series DAQ device. What can I use for isolation?
In this case, to make an isolated-strain measurement, use two SCC
modules.
•
Use an isolated analog output module to provide excitation.
•
If remote sense is desired, use a third module such as an SCC-AI
XX
module. Use an isolated analog input module with appropriate range
to monitor the excitation voltage (remote sense the voltage). For
example, use the SCC-AI03 to perform remote sense if you are using
10
V
EX
. If you are using 5
V
EX
, use the SCC-AI04 to perform remote
sense. Use the remote sense voltage measurements to scale the strain
measurements. Use the remote sense configuration option. Refer to
the
NI-DAQmx Help
, click
Search
,
and enter
remote sense
for more
information about remote sensing.
•
To measure the return voltage from the strain gage, use the appropriate
range isolated analog input module
.
For example, if the strain gauge
returns a ±2 mV signal/
V
EX
, then the maximum signal output is
±20 mV. Use the SCC-AI07, which has a ±50 mV range, and the gain
from the E/M Series DAQ device to completely utilize the A/D range
and achieve maximum accuracy with isolation. Isolation amplifiers
inherently contain noise, therefore NI recommends averaging a buffer
of measurements for maximum accuracy. Use the offset adjust screw
potentiometer on the SCC-AI07 to perform an offset null.
Which modules should I use for a feedback system? I want to send an
AO signal to an external object, read an AI signal from the object, and
adjust the AO based on the AI signal. When a certain condition is
reached in the AI, I want a DO module to turn on an alarm or LED.
Can you tell me how to configure and wire this system?
For the AO, use an SCC-CO20 isolated current output module, or design
and build a circuit on the SCC-FT01 that produces the type of output signal
you need. Use socket J17 or J18 for the analog output and control the output
using AO 0 or AO 1 on the E/M Series DAQ device. Refer to the
SCC-CO20 Isolated Current Output Module User Guide
for instructions on
wiring the module to the external circuit. For the AI, the module you use
depends on the type of signal you are reading from the external object
(circuit). For example, plug an SCC-AI
XX
module into socket J1 and the
module sends one signal to AI 0 and one signal to AI 8 on the E/M Series
DAQ device. Refer to the
SCC-AI Series Isolated Analog Input Modules
User Guide
for instructions on wiring the module to the external circuit.
Plug an SCC-DI01 module into socket J9, and it is controlled by