Rockwell Automation 1771-QB Linear Pos. - Manual (Page 32)

Hardware Description
Chapter 4
4Ć3
analog output interface terminals
discrete output terminals
The terminals for these four groups are divided between loop 1 and loop 2. Odd
number terminals are for loop 1; even numbered terminals apply to loop 2.
Terminals 1 through 8 on the module’s wiring arm provide connection points for
the transducer interface. The module is designed to work with the linear
displacement transducers (LDT) listed in Chapter 1.
The transducer interface circuit is electrically isolated from the 1771 I/O
chassis. This protects the 1771 backplane from noise and current surges in the
transducer circuits. The transient isolation exceeds 1,500 volts RMS. The
transducer interface is also isolated from the other module interfaces and
external power supplies.
The module supports a transducer length of up to 15 feet (4572 mm), and can
resolve the signal from the transducer to within two thousandths of an inch with
one circulation. You can achieve a higher accuracy by configuring the
transducer for more circulations. For example, the resolution for 60 inches
(1524 mm) is better than one thousandth of an inch if two recirculations are
used.
Every two milliseconds, the module sends an interrogate signal to the
transducer. The transducer returns a pulse width that is proportional to the axis
position. The maximum pulse width that can be measured without overflowing
the counter is about 1680 microseconds (1.680 milliseconds).
The pulse width returned to the module depends on the transducer stroke length
and the number of circulations. Each doubling of the number of circulations
doubles the width of the gate pulse and the resolution of the position reading.
Doubling the gate pulse length, however, effectively halves the maximum
transducer length supported by the module, because the maximum pulse width
is still determined by the size of the module’s counter. Overflowing the counter
causes a feedback fault. Is is recommended that you configure the digital
interface box for the highest number of circulations that still allows a long
enough stroke length for your application. Increasing the number of circulations
reduces the effect of noise and improves resolution.
Transducer Interface
Determining the Optimum
Number of Circulations