PLUG 2321322 - Caterpillar

General
This section provides an overview of the features and operation of the 705 Digital Speed Control. Figure 2-1 shows the control block diagram and Figure 4-1 is the plant wiring diagram for reference in the following descriptions.The 705 Digital Speed Control utilizes a 16-bit microprocessor for all control functions, such as computing engine speed, performing the control algorithm calculations, speed ramps, etc. All control adjustments are made with a hand-held terminal/display that communicates with the control via a serial port. The terminal/display is disconnected from the control when not in service to provide security against tampering.The speed sensor contains a special tracking filter designed for reciprocating engines, which minimizes the effects of engine torsionals or irregularities in the gear used for sensing speed. This provides exceptionally smooth steady-state control and allows the control dynamics to be matched to the engine rather than detuned to compensate for speed torsionals. The speed signal itself is usually provided by a magnetic pickup supplying from 1 to 60 Vrms to the control.Failed speed detection automatically reduces the control output to minimum fuel position if the speed signal is lost.The control features a switching power supply with increased spike, ripple, and EMI (electromagnetic interference) rejection. Discrete inputs are optically isolated and capable of rejecting EMI and variable resistance in switch or relay contacts. Analog inputs are differential type with extra filtering for common mode noise rejection. This protects the control from spurious interference and noise which can cause speed and load shifts.The control also provides a 4 to 20 mA tachometer output or a 4 to 20 mA actuator position output for an analog meter or as input to a computer. The offset and span are adjustable for range.Control Dynamics
The control algorithms used in the 705 control are designed specifically for reciprocating engine applications. Control dynamics are varied automatically as functions of both speed and load to provide better performance over the operating range of the engine.The 705 control provides two mappings of control dynamics as a function of speed (see the following sections for more information on maps and tuning parameters). The control automatically maps gain proportional to engine speed. This provides higher gain at normal operating speeds and lower gain at low speeds. This provides a better match with engine control requirements than a constant gain control. A second optional dynamics mapping provides additional stability as speed is decreased. This feature is useful particularly on large, low-speed engines where dead time between cylinder firings becomes an important factor in performance.To provide better transient performance, the control can be operated automatically with two gain settings depending on engine speed error (speed error is the difference between the speed setting and the actual engine speed). During steady-state operation with a constant load, the control uses the base gain setting. This gain is adjusted by the user to a value to prevent the control from responding to minor fluctuations in engine speed, a common problem with gas-fueled, spark-ignited engines. This feature eliminates potentially damaging