PROACT DIGITAL SPEED CONTROL SYSTEM FOR MODELS I & II

Description of Operation

Usage:

General

This section provides an overview of the features and operation of the ProAct Digital Speed Control and Actuator system. Figure 1-6 shows the actuator outline. Figure 1-4 is the plant wiring diagram for reference in the following descriptions. Figure 1-3 is a schematic cutaway view of the ProAct II actuator.

The ProAct 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.

A Personal Computer may also be used to program the control system. Contact Woodward Governor Company for information about connecting a computer to the control system.

The operating program is adjusted through seven menus accessed through the hand held terminal display. Details of these seven menus are contained in Chapter 5 of this manual.

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.

The speed signal itself is usually provided by a magnetic pickup supplying an A.C. signal from 1 to 60 Vrms to the control. The frequency (in Hz) is proportional to engine RPM.

The control features exceptional spike, ripple, and EMI (electromagnetic interference) rejection. Discrete inputs are optically isolated and capable of rejecting EMI and variable resistances 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 chasis should be bolted to a good ground to ensure effective EMI/RFI protection.

An auxiliary ±2.5 volt input is provided to interface with Woodward Load Sensors to provide isochronous load-sharing operation.

Control Dynamics

The algorithms used in the ProAct control are designed specifically for reciprocating engine applications. Control dynamics vary automatically as functions of both speed and actuator position to provide better performance over the entire engine operating range.

Alternate Dynamics

The ProAct control provides two complete sets of dynamic adjustments which are externally switch selectable. The two sets of dynamics are provided for use where engine operating conditions change, such as in systems which use two different fuels, clutched-in loads, and electrical power generation where the unit may be operated stand alone and paralleled with an infinite bus.

Each set of dynamics provides different gain mapping, stability, compensation, gain ratio, and gain window settings. This allows instantaneous changes in control for engines which operate with different fuels or have load type changes which require different dynamics.

Gain Ratio/Window Width

Figure 2-1. Window Width for Gain Ratio

Gain Ratio is the ratio of Gain setting during transient off speed conditions to the gain setting at steady state. Speed Gain Ratio operates by multiplying the Gain set point by the Gain Ratio when the speed error is anticipated to be greater than the Window Width. This allows a lower gain at steady state for better stability and reduced steady-state actuator movement. See Figure 2-1.

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-fuel, 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 speed error exceeding an adjustable window occurs, or is anrticipated to occur based on measurements of the instantaneous rate of change of the entire engine. Operation with base gain is restored once the control senses the return to steady-state speed. The Window-Width speed is a ± value, centered around zero speed error. (See figure 2-1.)

Variable Dynamics

The control is designed to compensate for nonlinear fuel systems and changes in engine dynamics with load. The control gain is mapped as a function of actuator position.

Four break points work with four gain settings to map the actuator against expected non-linear conditions. This provides optimal dynamics and smooth steady-state operation for all conditions from no load to full engine load. The four different response rates are achieved by the creation of four different Gain settings. Gas engine installations will usually require all four gain settings for different fuel flows, especially if the actuator is direct coupled to the butterfly. Most diesel applications will use only one or two of the gain settings with the breakpoints of the other settings moved up out of the way (set to 100%).

Fuel Limiters

Start Fuel Limit

Figure 2-2. Start Fuel Limit

The ProAct Digital Speed Control provides a Start Fuel Limiter to limit overfueling or flooding during start-up. The limiter is set to provide the desired 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 start fuel limit is removed when the engine speed reaches the start speed set point.

The start fuel limit is combined with a ramp that will increase the start fuel limit at a constant rate. This ramp is designed to allow for easier starting of the engine during various temperature conditions which may require an increased fuel such as a cold start.

Maximum Fuel Limit

This programmable limit to actuator position is in place when rated speed is selected. This is the maximum actuator position setting allowed for steady state full load.

Transient Overfuel

Figure 2-3. Transient Overfuel Exceeds Maximum Fuel.

This feature allows the user to set the Maximum Fuel Limit near the rated engine horsepower. The Transient Overfuel will allow exceeding this Maximum Fuel Limit for a tunable percentage for a tunable time. This ensures good transient load acceptance while maintaining safe steady-state-horsepower limiting.

Torque Limit

Figure 2-4. Torque Limit Slopes

A two-slope torque limiter is provided for mechanical drive -variable speed applications. The torque limiter provides a maximum fuel position determined by current engine speed to limit overfueling. The torque limiter is compared with the maximum fuel limit and the lowest is used as the actuator position limit.

Speed Reference And Ramps

The ProAct control provides discrete local control of the speed reference with switch inputs to issue raise and lower speed commands. For remote speed setting, the control permits a 4 to 20 mA 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 switch gear sequencing provides the proper operating modes.

The control provides an Idle/Rated discrete input with tunable Idle and rated speed settings. Raise and Lower inputs will raise and lower the speed reference at preset rates.

The Idle Speed set point is provided for engine start-up 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 Speed Reference and Raise and Lower inputs are disabled. Idle speed cannot be changed except through programming the Idle Speed set point. The Idle Speed set point can only be changed when the engine is shut down.

When Rated Speed is selected by closing the Idle/Rated switch contact, the fuel limit is set to the Maximum Fuel Limit set point value or the Torque Limit, whichever is less for the current engine operating speed. The speed reference selected at this time is determined by the status of the Enable Remote switch. If Remote reference is not selected (the Remote reference switch contacts are open) the speed reference will ramp from low idle to rated speed, based on the Acc 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. The Raise/Lower ramp rates will take over, depending on whether Raise or Lower is selected.

The Raise and Lower commands ramp engine speed based on the Raise and Lower Rate set points. The Raise and Lower Limits determine the limits of these commands. If Enable Remote is selected (and Rated Speed is selected) the control will ramp speed to the reference value set by the remote speed-setting milliamp input (at the Raise or Lower Rate). The remote speed setting operates from 4 to 20 mA. 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.

Figure 2-5. Remote Speed Reference. 4 mA = 1000 rpm. 20 mA = 2000 rpm.

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/Lower Rate set points.

Remote speed setting inputs between 2 and 4 mA are treated as the minimum of 4 mA. Below 2 mA, the remote input is considered failed. Between 4 and 20 mA, the control determines the required speed reference based on a straight line interpolated 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 Rate 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, nor change speed faster or slower than the Raise Rate/Lower Rate respectively.

When remote reference is selected and the remote input is failed (less than 2 mA), the speed reference remains at the current value. The speed reference can only be changed in this situation by increasing the remote reference above 2 mA or by opening the Remote Enable switch.

When the current operating mode is Rated, switching to idle results in ramping engine speed to idle based on the Decel Time set point.

Droop/Isochronous

Figure 2-6. Droop Speed Setting

The droop/ isochronous switch allows selection of either type of governor operation. If droop is selected the ProAct control will hold engine speed according to a droop schedule installed in the program.

The droop schedule is based on a full 75 degrees of actuator rotation between minimum and maximum fuel. If less than 75 degrees of rotation is used the amount of droop is reduced proportionally. Thus a control programmed for 5% droop will actually only have 2.5% droop if only 37° of actuator rotation is used, from no load to full load. Using only 37° of actuator travel will require programming 10% droop for an actual 5% droop curve (66% of travel is recommended).

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 ten seconds after the control is powered on. A failure of the test will turn off the output of the control. The ProAct control will not increase actuator signal from zero until diagnostics are complete.

The Proact Actuator

The ProAct actuator is a limited-angle rotational torque motor designed specifically for the control of engine fuel. The torque motor is a "run-hold" device. It responds to a fuel-position error at full speed until the position feedback causes the electronic control to change the current signal to hold position. This characteristic makes the actuator extremely fast and at the same time extremely accurate and "stiff" in engine-fuel control.

The ProAct II uses a four-pole torque motor design to provide 2.0 lb-ft of torque (2.5 ft-lbs. work) with 6 amps, 24 volt input at steady state, and 4.0 lb-ft of torque (5.0 ft-lbs. of work) with 12 amps, 24 volt input during transient.

The actuator holds a steady state position by its resistance to move from a desired location. Any force to move the actuator from a desired rotational position is immediately met with a force needed to hold or achieve the desired position.

The actuator is equipped with internal stop springs which allow rotational overshoot of 3° in each direction. The internal spring stops are necessary to halt the rotation of rotor and load inertia without damage to the actuator. This possible over-rotation of 3° in both directions must be considered when designing linkage or connecting to the butterfly shaft.

Figure 2-7. ProAct Output Travel

The terminal shaft on the actuator provides 0.500-36 (inch) serrations. The output shaft is connected to the butterfly valve or fuel control shaft either directly through a zero backlash flexible coupling^* or through an attached lever and linkage. Installations should attempt to use as much of the actuator rotation as possible to utilize as much of the actuator's work capability as possible.

^*When a flexible coupling is used care must be taken to assure that the maximum coupling misalignment is not exceeded and that the coupling is sized properly for the loads.

Return Spring

The ProAct actuator has an internal return spring designed to move the actuator toward minimum fuel in case the electrical control should fail, or power is removed. Spring scale may not be enough to move the engine to shutdown.

Show/hide table

The fuel system should be equipped with a spring return to minimum fuel capable of moving the fuel control in case of failure in the ProAct system, the connections between the ProAct actuator and the fuel control, or loss of electrical power. The return spring should be of sufficient force to return the fuel system to minimum fuel upon loss of actuator control, but should not limit the actuator's ability to properly control the engine under all operating conditions.

The Feedback Device

The ProAct uses a brushless, magneto-resistive position sensor. The position feedback signal to the digital control is responsible for the accurate positioning of the actuator.