TTX Series DC Actuator Installation & Maintenance Instructions |
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The 4-20 mA input and output circuits are isolated from each
other and all other circuits in the actuator, allowing unrestricted
placement in any 4-20 mA loop. The isolated parts of the
circuits are “loop powered” meaning that each circuit uses a
small amount of power as voltage drop from the 4-20 mA loop
and uses that to generate the internal power supplies needed.
4.14 4-20 mA INPUT
As with the voltage analog input, the 4-20 mA input may be
used as a position, velocity or current command. It can also be
used for a Velocity Override feature to dynamically moderate
velocity during a move.
The 4-20 mA input circuit requires a minimum of 11 V of
available loop voltage to operate. There is a fixed 5.6 V drop
plus 258 Ohms of loop resistance. The circuit reaches full
accuracy when the input reaches 3 mA. Except during power-up
and power-down, keep the signal presented to the input circuit
between 3 mA and 21 mA. Loss of Signal settings are generally
set a little below 4 mA and a little above 20 mA. If there is too
little voltage to power all devices in a 4-20 mA loop, the actual
current will reach a maximum below the intended current. The
Tritex TTX080 input channel will continue to report the actual
current, if over 3 mA.
When the 4-20 mA loop is unpowered, the software reads an
off-scale low value near 2 mA, not 0 mA. A signal with current
above a value near 22 mA will also read as 2 mA. Therefore, it
is important to consider what should be done when an out-of-
range signal is encountered.
The 4-20 mA input circuit is calibrated at the factory for
accuracy and should not be recalibrated without proper
equipment. Often what is desired is to match the source value
before converting it to a 4-20 mA signal to the value used in
the actuator. Rather than re-calibrating, this can be adjusted
using scaling in either of two places using Expert software.
The analog input is not used directly, but is converted to a 0%
to 100% value on the IA4 Option Board page, Input Setup tab.
This is the best place to make small scaling adjustments. The
value is scaled again into position, velocity, or current units
where used
4.14.1 4-20 mA Input Connection
Figure 24
–
Connection to High Side Controller
Figure 25
–
Connection to Low Side Controller
4-20 mA Input Specifications
Description
Specification
Input Range
3 mA to 21 mA
Voltage drop at 20mA
11V max
Input Accuracy
±0.1% of 4-20 mA span
Input Resolution
>14 bits over 4-20 mA span
Update Rate
0.5 ms
4.15 4-20 mA OUTPUT
The 4-20 mA output channel, like the voltage Analog Output
may be configured for position, velocity, or current monitoring,
and can be dynamically switched between two sources. Details
for configuration are in the Expert Software Manual.
The 4-20 mA output is an isolated two wire circuit requiring an
external loop power supply. The Tritex TTX080 circuit requires
a minimum of 8 V to operate and can tolerate up to 30 V. The
best approach to determining whether a particular loop will
24V Com
+24V
External Controller
4-20 mA Output
+ 4-20 mA Input
- 4-20 mA Input
TTX080
IA4 Option
Power
Supply
+
_
AD
Converter
Isolation
To CPU
