The engine speed signal is usually obtained from a magnetic speed sensor mounted in close
proximity to the teeth of a ferrous ring gear that is driven by the engine. The frequency of the speed
sensor signal is proportional to the engine speed. The speed control unit will accept any signal if the
frequency is proportional to engine signal, and in the frequency range of the speed control unit (1K
to 7.5K Hz.). The speed sensor is typically mounted in close proximity to an engine driven ferrous
gear, usually the engine ring gear. As the teeth of the gear pass the magnetic sensor, a signal is
generated which is proportional to engine speed. The signal strength must also be within the range
of the input amplifier. An amplitude of 1 to 120 volts RMS is required to allow the unit to function within
its design specifications. The speed control unit has an input impedance of 20K-ohms between the
speed sensor input terminals. ("C" & "D"). Terminal "D" is connected internally to the battery negative.
Only one end of the shielded cable should be connected.
When a speed sensor signal is received by the controller, the signal is amplified and shaped by an
internal circuit to form constant area pulses. If the speed sensor monitor does not detect a speed
sensor signal, the output circuit of the speed control unit will turn off all current to the actuator.
The summing point of the speed sensor and the speed adjust control is the input to the dynamic
control section of the governor. The dynamic control circuit, of which the gain and stability
adjustments are part, has a control function that will provide isochronous and stable performance for
most engine types and fuel systems.
The speed control unit circuit is influenced by the gain and stability performance adjustments. The governor
system sensitivity is increased with clockwise rotation of the gain adjustment. The gain adjustment has a non-
linear range of 33:1. The stability adjustment, when advanced clockwise, increases the time rate of response
of the governor system to match the various time constants of a wide variety of engines. The speed control
unit is a PID device, the "D", derivative portion can be varied when required.(See Instability section.)
During the engine cranking cycle, STARTING FUEL can be adjusted from an almost closed, to a nearly full fuel
position. Once the engine has started, the speed control point is determined, first by the IDLE speed set point
and the SPEED RAMPING circuit, After engine speed ramp- ing has been completed, the engine will be at its
governed operating speed. At the desired governed engine speed, the actuator will be energized with sufficient
current to maintain the desired engine speed, independent of load (isochronous operation).
The output actuator current switching circuit provides current to drive the actuator. The output transistor is
alternately switched off and on at a frequency of 300Hz. which is well beyond the natural frequency of the
actuator, hence no visible motion from the switching results. Switching the output transistors reduces its
internal power dissipation for efficient power control. The output circuit can provide current of up to 10amps
continuous at 25°£C for 24VDC battery systems. The actuator responds to the average current to position the
engine fuel control lever.
In standard operation, the speed control unit performance is isochronous. Droop governing can be selected by
connecting terminals K & L and the percent of droop governing can be varied with the droop adjustment
control. The droop range can be decreased by connecting Terminals G and H.
The speed control unit has several performance and protection features which enhance the governor system.
A speed anticipation circuit minimizes speed overshoot on engine startup or when large increments of load are
applied to the engine.
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