BECKHOFF KL3351 - Manual (Page 6)

Description of functions
6 KL3351
Description of functions
The KL3351 analogue input terminal permits direct connection of a
resistance bridge. The bridge voltage, U
D
, and the supply voltage, U
ref
, to
the bridge are digitised with 16 bits resolution, and are transmitted over an
electrically isolated channel to the supervising automation system. The
input channels are available in the form of two 16 bit values for further
processing. The resulting measurement can be calculated from the
formula: measurement = U
D
/ U
ref
. Precise acquisition of the supply voltage
along with the bridge voltage compensates for long-term and temperature
drift.
Output of the measurement The bridge voltage between +U
D
and –U
D
is output with a resolution of
500 nV/digit, so that 10 mV corresponds to an output value of 20000 in the
data bytes D0 and D1 of channel 1.
The supply voltage for the bridge is output with a resolution of 500 µV/digit,
so that 10 V corresponds to the output value 20000 in the data bytes D0
and D1 of channel 2.
The resolution can be adjusted by means of gain register R38.
Example A load cell was connected to the KL3351 in an experimental test bed.
The load cell was fed with the terminal’s 5 V supply voltage (U
V
), while the
reference voltage (U
ref
) and the bridge voltage were captured with the U
ref
and U
D
channels respectively.
Technical data for the load cell:
5 kg
=
2 mV/V
The weight is calculated as follows:
Weight = U
D
/ U
ref
* 5/0.002 [kg]
Bridge supply If the internal voltage supply is overloaded (I_load > 20 mA), it is
recommended that a KL9505 or a KL9510 be used.
LED display The four LEDs indicate the operating state of the associated terminal
channels.
Green LEDs: RUN
On:
normal operation
Off:
Watchdog-timer overflow has occurred. If no process data is
transmitted to the bus coupler for 100 ms, the green LEDs go out.
Red LEDs: ERROR
Error LED1: outside the acceptable range for the signal voltage (U
D
), or
broken wire
Error LED2: outside the acceptable range for the reference voltage (U
ref
)