Campbell Scientific QD1 Incremental Shaft Encoder Interface - Manual (Page 6)

QD1 INCREMENTAL ENCODER INTERFACE
2
4.1 POWERING THE QD1
21X and CR7 - 5 VDC must be supplied to the
QD1 continuously. When used with the 21X or
CR7 dataloggers, a Continuous Analog Output
(CAO) port may be programmed to power the
QD1. CAO channels are located on the panel
of the 21X and the 725 Excitation Card of the
CR7. Instructions to send 5 V out the CAO port
should occur before the measurement
instruction, and must be repeated at least every
minute. This implies a maximum program
execution interval of one minute. If the CAO
voltage is not refreshed every minute, the
voltage level may sag below the minimum QD1
operating voltage.
CR10 - When used with the CR10, the QD1
may be connected to one of the two 5 VDC
terminals located on the CR10WP Wiring
Panel.
4.2 MEASUREMENT
Output from the QD1 is measured with I/O
Instruction 3 (Pulse Count), using the high
frequency option. Two pulse count channels
must be measured, one for "up" counts and one
for "down".
The Multiplier to be used depends on the
number of counts per revolution of the shaft
encoder and the circumference of the attached
wheel, as shown below.
MULTIPLIER = WHEEL
CIRCUMFERENCE/(COUNTS/REVOLUTION)
Engineering units logged by the datalogger are
the same as those used for wheel
circumference.
An Offset of zero should be entered in the
Pulse Count I/O Instruction. Section 5
discusses entering an offset to set the
measurement equal to a reference elevation.
4.3 PROCESSING
Processing Instructions are used to determine
the stage from the measured up and down
counts. The new stage is calculated by adding
the existing stage to the up counts and storing
the sum back in the existing stage location.
The down count value is then subtracted from
the sum, and the result is stored back in the
stage location. This routine minimizes the
number of Input Locations required to calculate
stage.
5. ENTERING THE REFERENCE
OFFSET
The offset, which sets the stage value equal to
some reference point, should be entered after
the system has been installed and the float is
resting on the water surface. Enter the *6 Mode
and advance to the stage Input Location. Press
the "C" key to enable the *6C function, allowing
a value to be entered into the stage Input
Location. Key in the reference offset and press
the "A" key to view the offset value now located
in the stage Input Location. Refer to the *6
Mode in the datalogger manual(s) for additional
information on the *6C function.
If changes to the datalogger program are made
and the program is recompiled in the *0 Mode,
all *6C entries are erased. THE REFERENCE
OFFSET SHOULD BE ENTERED AFTER ALL
PROGRAM CHANGES HAVE BEEN MADE.
6. ON-SITE CHECK-OUT
Testing is recommended to ensure that when
the float rises or falls, the correct pulse counting
channel is incremented. While viewing the
stage Input Location in the *6 Mode, rotate the
shaft in the direction corresponding to a rise in
stage. Wait for the next program execution and
confirm that the reading increases. If the
reading decreases, simply exchange the input
leads to the pulse count channels.
7. RANGE VS. RESOLUTION
If recorded data are to reflect the resolution of
the encoder, consideration must be given to the
datalogger output resolution as well as the
reference offset. Table 7.1 shows minimum
and maximum values stored in the datalogger
using low and high resolution, and associated
memory consumption.