Rockwell Automation 1395 Digital Reference Adapter Board - Manual (Page 51)

Chapter 6
StartĆUp
6-5
Analog Inputs – Each analog input has two separate sets of gain and offset
adjustments. The first set adjust the gain and offset of the input signal
before it reaches the analog to digital converter (A/D). This makes it
possible to adjust the input signal so that it uses the full range of the A/D
converter. The following steps are general guidelines for adjusting the
analog input parameters:
1. The first step is to determine if an analog offset should be applied to the
input signal. Generally an analog offset would be used when the input
signal has a built in offset that is constant. For example, if the input
signal has a range of 5.0 volts to 5.5 volts, then an analog offset of –5
volts would cancel out the built in offset leaving a resulting output of 0
to 0.5 volts.
2. The second step is to determine whether adjustment of the analog gain
parameters is required. Best A/D resolution is achieved by having the
maximum voltage present at the input to the multiplexer as close to
10VDC as possible. In the example in
Figure 6.2 , a gain value of 8
would bring the signal level up to an acceptable level. Table 6.A lists the
values that can be entered into the Analog Programmable gain
parameters and the resulting multiplication value.
Table 6.A
Analog Programmable Gain Parameter Multiplication Values
Parameter Value
0
1
2
3
4
Multiply Input by:
1
2
4
8
16
The digital gain and offset parameters modify the output of the analog to
digital conversion before it is transferred to the Drive. An example
parameter setup is shown in
Figure 6.3 . In this case, analog input # 1 will
be used to provide the Drive with a speed reference. A potentiometer is
connected to the terminals for analog input #1 and has a range of 0 to
+/– 10V DC. The Analog In 1 Fast parameter (Param 401) has been linked
to the Velocity Reference fast parameter (Param 154) in the Drive. To setup
the pot to control speed to 100% of base speed in both directions the
Digital Scale parameter (Param 550) allows a range of –4096 to +4096, or
100% base speed in both directions. If the user wanted a range of
+/– 2 times base speed, the digital scale value would be set to 4 (base
speed = 4096, 2 times base speed = 8192, 2048 times 4 = 8192).