Woodward 721, Operation And Calibration Manual, Page: 28 / 52

721DSC for Marine Propulsion Application Manual 26004
Woodward 24
4. Derivative Ratio 1
Derivative Ratio compensates for the actuator time constant. Set this set point to 5 for
ordinary applications.
5. Window Width 1
Window Width is the magnitude (in rpm) of a speed error at which the control
automatically switches to fast response. The control does not use the absolute value of
speed error, but “anticipated” speed error to make this switch. This method provides for
quick switching to the high gain value when an offspeed occurs and early switching to
the low gain value when recovering from the speed transient. This provides smoother
switching than if the absolute speed error was used for the window.
6. Gain Ratio 1
Gain Ratio is the ratio of the Gain setting at steady state to the Gain setting during
transient conditions. The Gain Ratio operates in conjunction with the Window Width
and Gain adjustments by multiplying the Gain set point by the Gain Ratio when the
speed error is greater than the Window Width. This makes the control dynamics fast
enough to minimize engine
- speed overshoot on start - up and to reduce the magnitude of
speed error when loads are changing. This allows a lower gain at steady state for better
stability and reduced steady
- state actuator linkage movement.
7. Gain Breakpoint 1
Gain Breakpoint sets the percent output above which the Gain Slope becomes effective.
It should usually be set just above the minimum load output. See figure 4. The Gain
Breakpoint adjustment is common to both sets of dynamics.
8. Gain Slope 1
Gain Slope changes Gain as a function of actuator output. Since actuator output is
proportional to engine load, this makes gain a function of engine load. Gain slope
operates in conjunction with the Gain Breakpoint adjustment to increase (or decrease)
gain when percent actuator output is greater than the breakpoint. This compensates for
systems with high (or low) gain at low load levels. This allows the gain settings to be
lower at light or no load for engine stability, yet provide good control performance under
loaded conditions.
9. Speed Filter
Set this value to the cutoff frequency found by using the formula below (see figure 5).
This is the roll
-
off frequency for the firing torsional filter. The formula to get the proper
roll - off frequency setting is:
camshaft frequency
= (engine rpm)/60 [for 2 - cycle engines]
= (engine rpm)/120 [for 4 - cycle engines]
firing frequency
=
(camshaft frequenc
y) × (number of cylinders)
Initially set the filter frequency to the firing frequency.