Circuit Description; Primary Signals - ABB CK-MAG 10D1477 Instruction Manual

10D1477 MAGNETIC FLOWMETER INSTRUCTION MANUAL

5.0 CIRCUIT DESCRIPTION

5.1 Primary Signals

The magnetic flowmeter body houses two signal electrodes with their associated buffer amplifiers
and the flux producing magnet coils, as shown schematically in Figures 5-1 and 5-2. All primary
intraconnection wiring is terminated at the CMF/BUFFER pc board located in the base of the meter
housing.
The electrode signals developed by the primary are of such a high impedance that they cannot be
directly processed by the signal converter. They are first conditioned by buffer amplifiers. The
respective outputs of these buffer amplifiers are then used to drive guard shields fixed over each
electrode and present in the cable system going to the electrodes.
Zero offset in the primary is removed by additional circuitry in the CMF/buffer assembly. A portion of
the reference voltage is summed with the buffered signal from electrode #2. Both this signal and the
buffered signal from electrode #1 are then passed to the signal converter for further processing.
The CK-Mag primary connections include those for regulated power supplies from the converter to
operate the buffering and zero circuitry described in the previous paragraphs. Refer to the signal
converter instruction manual for interconnection wiring.
The primary provides two output signals to the associated signal converter; one, a buffered elec-
trode signal that contains the flow rate information, and two, the reference signal which is propor-
tional to the magnet excitation current. Theoretically, this reference signal is proportional to the flux
density in the metering section. The reference voltage is derived across a precision "constant meter
factor" (CMF) resistance network that is connected in series with the magnet coils. Changes in
magnet drive voltage, which cause a variation of flow signal, will simultaneously cause a proportional
variation of the reference voltage. The circuitry will provide an exact ratio and thereby provide
immunity to power supply variation. The magnet coil drive circuitry is contained in the signal
converter.
The (gated) magnet driver operates at a frequency that permits magnetic flux in the primary to reach
a steady state level during the last 50% of each half period of magnet excitation. By using sampling
techniques, the flow (differential mode) signal is measured only during the intervals that magnetic
flux is constant dφ = 0 .
dt
The MAG-X design concept (sampling technique), provides the meter with extremely good zero
stability.
A thorough discussion of signal converter operation is provided in the instruction manual supplied
with the signal converter.
5-1