Yokogawa digitalYEWFLO (DY) - Manual (Page 95)

IM 1F2B4-01-YIA
Page 90
TROUBLESHOOTING
1. Vortex Shedder (Sensor): The shedder bar assembly (sensor) may be integrally mounted to
the amplifier or remotely connected via a special signal cable. In any case, the piezoelectric
crystal signal is amplified by a high impedance preamplifier circuit.
2. Noise Balance: The Noise Balance function maximizes the signal to noise (S/N) ratio by
mixing the two crystal input signals. One crystal measures primarily flow frequency. The
other crystal measures primarily frequencies due to noise. This parameter is factory set, but
on occasion may need to be adjusted after installation. By properly balancing these signals in
a one time setup, the S/N ratio is maximized.
3. Gain: The balanced signal is then amplified by the gain stage. The gain is automatically set
when meter size and application (liquid or gas) are selected. This parameter should not
normally be adjusted, unless directed by the factory.
4. After the gain stage the signal is split. One signal is used to measure frequency, proportional
to flow velocity. The other signal is used to measure amplitude, proportional to fluid
momentum, for Noise Judge calculations.
5. Band Pass and High Frequency (H.F.) Filter: The frequency signal is filtered by both a
Band Pass filter and a High Frequency Filter. Filter settings are automatically set when meter
size and application are selected.
6. The output of the filter stage may be measured by a voltmeter or viewed with an Oscilloscope
at the TP2 and Com2 test points.
7. Trigger Level Adjustment (TLA): The filtered waveform is then converted to a square wave
by comparing its amplitude to the Trigger Level Adjustment (TLA) setting. The resulting
square wave may be viewed at the P and Com2 test points. The frequency of this square wave
is directly proportional to velocity.
8. The flow rate frequency is then input to the CPU for further calculation.
9. Noise Judge circuit: Simultaneous to steps 5 through 8, the Noise Judge circuit measures the
average signal amplitude.
10. The vortex signal is converted from an AC voltage to a DC voltage using a simple
rectification circuit. The Noise Judge circuit then outputs a frequency proportional to signal
amplitude.
11. The Noise Judge frequency is then input to the CPU for further calculation.
12. CPU input: The CPU then uses the frequency inputs to compute actual flow rate, and
perform Noise Judge signal discrimination.
13. Circuit Output: Based on the software flow diagram (shown below) the CPU outputs a
frequency proportional to flow rate. The CPU output frequency may be observed at the CPU
PLS and COM1 test points. This frequency output is then converted based on the output
selection jumpers to either a 4-20 mA current signal, or frequency.
14. Analog Output: If the software and jumpers are set for analog output, the CPU frequency is
in the range of 1000 Hz to 5000 Hz. The frequency to current (f/i) circuit will convert this to
4-20 mA.
15. Pulse Output: If the software and jumpers are set for pulse output, the CPU frequency will
be scaled (or unscaled) according to the software settings. The low level CPU frequency will
be converted to a powered voltage output pulse by the pulse output driver circuit as shown.
Software calculation:
The equations at the end of this section describe how the flow rate is calculated from the input
frequency. These equations are presented for reference only. It is not necessary to become familiar
with them to operate a vortex flowmeter.