Usage:
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 jiggle of the actuator and fuel system. The control automatically increases gain by an adjustable ratio when a speed error exceeding an adjustable window occurs. Operation with base gain is restored once the control senses the return to steady-state speed.
The control can also compensate for nonlinear fuel systems and changes in engine dynamics with load. The control dynamics are mapped as a function of actuator current (actuator current is proportional to engine load). This provides optimal dynamics and smooth steady-state operation for all conditions from no load to full engine load.
The control also provides two complete sets of dynamic adjustments which are externally selectable. The two sets of dynamics are provided for use where engine operating conditions change, such as in systems with clutched-in loads or dual fuel sources.
The control provides speed droop as a function of control actuator output for additional stability, if desired. Speed droop reduces the speed reference proportional to the load on the engine.
Fuel Limiters
The 705 Digital Speed Control provides a Start Fuel Limiter to limit overfueling or flooding during start-up. The limiter is set to provide the desired maximum rack position during starts. The control will reduce the fuel when the speed set point is reached as required to control engine speed, but will not exceed the start limit.
The control provides a 4-20 mA/1-5 Vdc input for fuel rack limiting based on turbo boost pressure. When used, the two-slope limiter helps prevent overfueling and subsequent smoke emission. The 20 mA setting provides a maximum load limit if the fuel rack limiter is not used.
The control provides optional two-slope torque limiting as a function of engine speed if the turbo boost pressure signal is not available.
Speed Reference And Ramps
The 705 control provides local control of the speed reference with discrete inputs to issue raise and lower speed commands. For remote speed setting, the control allows for a 4 to 20 mA/1-5 Vdc input which is used to vary the speed reference. This section describes the operation of each of the speed reference and ramp functions and their relation to each other. Read this section carefully to be sure your switchgear sequencing provides the proper operating modes.
The control provides Idle, Lower Limit, Rated, and Raise Limit set points along with Accel and Decel Time and Raise and Lower Rates for local and remote operation. Accel Time determines the time required for the engine to ramp from idle (low idle) to rated (fast idle) speed. Decel Time determines the time required for the engine to ramp from rated speed to idle speed. Raise and Lower Rates determine how fast speed is increased or decreased by the Raise and Lower command inputs and the remote reference.
Raise Rate 1 and Lower Rate 1 are in effect whenever the speed reference is between the lower limit and the rate breakpoint, and the Raise/Lower Rate 3 contact is open. Raise Rate 2 and Lower Rate 2 are in effect whenever the speed reference is between the rate breakpoint and the raise limit, and the Raise/Lower Rate 3 contact is open. Whenever the Raise/Lower Rate 3 contact is closed, Raise Rate 3 and Lower Rate 3 are in effect.
The Idle Speed set point is provided for engine start-up and/or cool down speed. Idle speed may be set equal to or less than the rated speed set point. Idle is independent of the Lower Limit set point and may be set to a lower speed. When Idle is selected (Idle/Rated switch in Idle position with contacts open), Remote, Raise, and Lower inputs are all disabled. Idle speed cannot be changed except through adjustment of the Idle Speed set point.
When Rated Speed (high or fast idle) is selected by closing the Idle/Rated switch contact, the speed reference is determined by the status of the Local/Remote switch. If Local is selected when the Idle/Rated switch contacts are closed, the speed reference will ramp from low idle to fast idle (rated) speed, based on the Accel Time set point. Closing either the Raise or Lower contacts (or the Remote contacts) while ramping from idle to rated results in immediate cancellation of the idle to rated ramp.
After acceleration to rated speed is completed, the Raise and Lower commands increase and decrease engine speed based on the Raise and Lower Rates. The Raise and Lower Limits determine the limits of these commands.
If Remote is selected after the engine reaches rated speed, the control will ramp speed to the reference value set by the remote speed setting milliamp input based on the Raise or Lower Rate. The remote speed setting operates from 4 to 20 mA (1 to 5 Vdc). The values of the 4 mA and 20 mA Remote Reference set points must be set between the Raise and Lower Limit set points. The 4 mA Remote Reference set point may be set to a lower or higher speed than the 20 mA set point, providing for either direct or reverse-acting remote speed setting.
If Remote is selected when the Idle/Rated switch contacts are closed or during the idle to rated ramp, the speed reference will ramp to the speed reference value determined by the milliamps on the remote speed-setting input, based on the Raise Rate set points.
Remote speed setting inputs between 0 and 4 mA (0 and 1 Vdc) are treated as the minimum of 4 mA (1 Vdc). Between 4 and 20 mA (1 and 5 Vdc), the control determines the required speed reference based on a straight line between the 4 mA Remote Reference and 20 mA Remote Reference set points. If a difference is detected between the current speed reference and the remote reference computed from the mA input, the current speed reference is raised or lowered at the rate determined by the Raise or Lower Rates to bring the speed reference into agreement with the remote speed reference. The remote reference will not increase speed over the Raise Limit or lower it below the Lower Limit.
When remote reference is selected and the remote input is failed (0 mA/0 Vdc), the speed reference ramps to the 4 mA (1 Vdc) value. This also means that if the Idle/Rated switch is changed from idle to rated and Remote is selected with an input of 0 mA/0 Vdc, speed will ramp to the 4 mA (1 Vdc) setting. Speed will remain at the 4 mA/1 Vdc setting until the remote input is increased to a value greater than 4 mA/1 Vdc, at which time speed will ramp based on the Raise Rate set point to the remote speed setting. (If Local is then selected under these conditions, speed will remain at idle until the Idle/Rated switch is changed to idle and back to rated to restart the ramp, since selection of the remote input cancels the accel ramp.)
When the current operating mode is Rated and either Local or Remote, switching to idle results in immediate switching of the fuel limiter to the start fuel limit, and ramping engine speed to idle based on the Decel Time set point.
Low Idle Droop
The control provides low-idle droop to reduce or prevent engine speed undershoot when quickly reducing speed to low idle. Low-idle droop increases the speed reference when the rack moves below the low-idle breakpoint.
Power Up Diagnostics
The Power Up Diagnostics feature is provided to verify the proper operation of the microprocessor and memory components. The diagnostics take about four seconds after the control is powered on. A failure of the test will turn off the output of the control. If diagnostic testing is successful, the CPU OK indicator on the control cover will light.
Figure 2-1. Block Diagram