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Power Control Unit, Hydraulic - Description

Hydraulic power control units (typically referred to as PCUs) are used to position primary (or secondary) flight control surfaces. PCUs are a single assembly (i.e., single part number) that combines individual hydraulic components into an assembly. PCUs perform two critical functions: (1) they position the flight control surface in response to mechanical or electrical commands, and (2) they represent a principle structural element capable of withstanding flight loads and providing protection against flutter (unsteady, aerodynamic loads). The requirements of these two functions make PCU design a challenging task.

At the heart of PCU is a servovalve and an actuator (servo actuator). The servovalve can be controlled by a flapper nozzle, jet pipe, solenoid, torque motor or mechanical linkage. Some PCUs contain only a servo actuator. However, PCUs often contain other components to meet performance and failure mode performance criteria. Other components typically found in PCUs are shutoff valves, pressure relief valves, input filter, check valves, and compensator. Another component that is often part of a PCU is one or more servos whose control is based on some performance criteria. These servos can be positioned electronically or by hydraulic pressure (where loss of hydraulic pressure allows a spring to position the servo).

A simple PCU is shown in Figure 1.



Figure 1 Simple PCU Diagram


This PCU shows the pressure input going through a filter to a shut off valve. If the shut off valve is open (as shown), then pressure is applied to a solenoid operated servovalve. When closed, the SOV ports pressure to return. The servovalve controls actuator position. (Typically, actuator position is measured by a LVDT on the actuator piston and fed back to a controller that provides closed loop position feedback – see position control system.) The pressure relief valves will bleed off pressure if the pressure in the actuator chamber exceeds a certain level. Pressure can increase above acceptable limits due to external loads or thermal expansion of the fluid.

As shown in Figure 1, PCUs are built from other hydraulic components, which are packaged into an assembly. An understanding of PCU behavior and characteristics can be ascertained by understanding each component in the system and how it interacts with other components. An analysis or simulation model can be built using models for the individual components to better understand behavior and performance characteristics (see Power Control Unit, Hydraulic – Equations).

A more complex PCU is shown in Figure 2.



Figure 2 Flight Surface PCU


In Figure 2, the hydraulic fluid flows through a filter and check valve before going to the servovalve. The servovalve is positioned mechanically via a linkage from the cockpit. The servovalve positions the actuator. Connecting the input link to the actuator piston rod provides feedback and helps stabilize the PCU. Under normal operation, flow to and from an actuator chamber is through the servovalve. If actuator chamber pressure exceeds a maximum allowable level, then fluid is drained through the pressure relief valves at the bottom of the schematic. These settings will be roughly 20% above nominal system pressure. The return line pressure relief valve (top right portion of Figure 2) provides a constant backpressure to the actuator and servo. This helps minimize variance from system back pressure to improve PCU performance. In addition, should the main system hydraulics fail, the return line relief valve will help maintain sufficient back pressure to push fluid through the anti-cavitation check valves (overcome spring force) as the actuator moves in response to airloads. The relief valve setting will be above system reservoir pressure setting, generally in the 100-200 psi range. The compensator acts like an accumulator to help maintain a constant backpressure on the system under small disturbances. The main function of the compensator, however, is to store sufficient fluid to prevent cavitation after a main hydraulic system failure. The compensator must therefore be sized to have sufficient volume so that there is enough fluid to maintain fluid in the actuator chambers after considering worst case leakage losses over the length of the longest possible flight.

A third example is shown in Figure 3. This PCU is similar to the PCU shown in Figure 2, except it is now electrically actuated and has a mode control valve or mode control servo that is also electrically actuated. The control valve is spring loaded to the center position but has two solenoids to push the servo to the left or right. The three modes would be normal (center position), damped bypass (left position) and blocked (right position). These are 3 common modes used in flight surface PCUs (a 4th mode would be undamped bypass). In normal operation, the PCU operates similarly to the PCU in Figure 2, except that the servovalve is electronically controlled instead of via a mechanical linkage. In bypass mode, solenoid 1 would be energized which would push the mode servo to the left. This would connect both chambers of the actuator through a damped orifice which would limit the flow rate between the chambers. The effect of the damped orifice is to significantly dampen the actuator so that movement is allowed but at a rate such that the actuator is insensitive to flutter. In blocked mode, hydraulic fluid would be trapped in the actuator chamber which would hold (lock) the actuator in the current position. The pressure relief valves, compensator and anti-cavitation check valves would function the same as described for the PCU in Figure 2. In this PCU example, both solenoid1 and solenoid 2 would be controlled by a digital controller. They would be two position solenoids (energized or unenergized). The solenoid on the servovalve would be an infinitely positioned solenoid where position is proportional to applied current. As mentioned above, but not shown in the Figure 3 PCU example is a bypass mode. A PCU that has a bypass mode connects both actuator chambers through a fluid path without an orifice – if the orifice is removed in left side of the mode valve this would indicate a bypass mode. Bypass mode allows hydraulic fluid to flow freely between the actuator chambers. Bypass mode is normally seen on flight surfaces with 2 or more PCUs connected to the surface. An actuator will be put into bypass mode when the actuator controller senses a failure, which allows the other actuator(s) to control the surface with minimal resistance from the failed actuator. Damped bypass mode (bypass with a orifice) is used when it is acceptable for the surface to float at a controlled rate and be insensitive to flutter forces. Later examples show PCUs with bypass and damped bypass modes.



Figure 3 Flight Surface PCU with Electric Mode Control


Mode valves can be controlled electrically or hydraulically. Manual (or linkage) control of mode valves is not used in PCUs. The fourth example of a PCU shown in Figure 4 illustrates a PCU with a hydraulically actuated mode valve. In this example, hydraulic pressure will put the mode valve in normal position and loss of hydraulic pressure will allow the spring to push the mode valve to the right, which would be damped bypass mode. So, this PCU has two modes: normal and damped bypass. Note the PCU in Figure 3 has 3 possible mode settings. The configuration in Figure 4 could be used where there is a single PCU installed on a surface. In the PCU shown, when hydraulic power is lost, the PCU automatically switches to damped bypass mode to provide surface damping and flutter protection. The pressure relief valves, anti-cavitation check valves and compensator function as before. In some PCUs where hydraulic pressure is used to position the mode valve (servo), an electrically controlled, two position servo will be used to control hydraulic pressure to the valve. This is shown in Figure 5, where the two position, three way control valve has been added.



Figure 4 Flight Surface PCU with Hydraulic Mode Control



Figure 5 Flight Surface PCU with Electrical / Hydraulic Mode Control


The above examples of PCUs show a natural evolution of a PCU which are variations of the same basic scheme. The PCUs shown in Figures 1 through 5 are more representative of commercial aircraft PCUs, although there still is a wide variation in PCU designs between manufacturers. A different type of PCU is shown in Figure 6, which is more representative of a military application.

The PCU in Figure 6 shows a triplex redundant electrical, dual redundant hydraulic PCU. Not shown is the dual redundant electronics that control the servo and contain the fault detection logic. LVDTs (linear variable differential transducers) are shown notionally on the dual tandem servo and the actuator ram. In normal operation, pressure from P1 and P2 are ported though the dual tandem servo to the actuator. Position control is maintained using the dual tandem servo. The two bypass valves are in the position shown under normal operation. The two mode valves are powered open (valves close without power). Response to failure scenarios is as follows:



Figure 6 Flight Surface PCU with Dual Redundant Command Inputs


The above examples show several different types of PCU designs. The examples are by no means a thorough coverage of the possible options and ways to mechanize a PCU. The examples are provided to assist you in gaining an understanding of some basic configurations of PCU operation.


Power Control Unit Qualification

See Qualification - Hydraulic Components for discussion on PCU qualification and required qualification testing.

In addition to environmental qualification, PCUs will undergo significant functional testing to validate all operating modes and that responses to failures are as required in the component specification. This functional testing can be done under environmental conditions, such as at temperature or at altitude.


FMEAs & Safety Analysis

Due to the numerous components installed in a PCU assembly an analysis of failure modes and effects is an important analysis to complete. The failure modes and effects analysis (FMEA) should examine all single potential failures associated with each component in the PCU. The effects of the failures on PCU performance and operation should be established. Failures to consider should include part jams, part runaways, excessive leakage, burst failures, loss of hydraulic pressure, loss of electrical connection, misleading sensor inputs, loss of sensor inputs, failed check valves, relief valves stuck closed or open, clogged filters, high friction levels, etc.

The failure effects within the PCU will then need to be examined at the airplane level and will also be used in fault trees.

When creating the FMEA, latent failures (undetectable failures) should be identified. Latent failures need to be analyzed separately to ensure they don’t create a hazardous condition or put the actuator in a position where an additional failure would cause a hazardous/catastrophic condition on the aircraft. An example of a latent failure would be a pressure relief valve failed to the closed position (i.e., won’t relieve pressure when required).


Fault Detection

Fault detection and response can be done in the controller electronics (using appropriate sensors and logic) or though mechanical means. An example of a mechanical response to a failure is shown in PCU example of Figure 6. When hydraulic pressure is lost, a bypass valve automatically slides to the bypass position. The controller may or may not know this has occurred. However, the controller could be notified by either monitoring system pressure or through a switch located in the bypass valve.

Electronic fault detection will be more comprehensive and more sophisticated. Examples of items that the electronic controller will monitor include

Electronically failures are checked on power up BIT (PBIT) or through continuous BIT (CBIT), where BIT is built in test. To minimize nuisance failures, most detection routines utilize a threshold (must be outside of a range before flagging a fault) and persistence (must remain outside range for a given time period). Threshold and persistence are used primarily due to circuit noise and performance variation over temperature and loads. Multiple controller channels (separate circuit cards and power supplies) are used for control and monitoring. Monitoring (fault detection and isolation) may be done within a control channel or within a separate channel. The controller and monitoring logic as well as the method for distributing decision making within the processors is called redundancy management.

The benefit of using multiple channels for redundancy management of a PCU is illustrated in Figure 7.



Figure 7 Triplex Redundant Controller Channel Comparison Scheme


Figure 7 shows 3 compare monitors in the electronic controller. Each compare monitor looks at the output command from two channels. If the channel outputs agree, then the compare flag is set to, say, 1. If the channels disagree, then the compare flag is set to, say, 0. A truth table (or other means for voting logic) exists in the voter. When the voter detects a failed channel, the votor will remove power from the failed channel. With triplex controller voting logic as shown in Figure 7, operation can continue with a failed channel. However, when operating with 2 channels any miscompare between the 2 remaining channels will shut the system down. For a quadraplex (4 channel) system, the voting is similar but more complicated with the additional channel. Using a quadraplex controller, operation can continue with 2 failed channels.

Note that all failures that are not detected either mechanically or electronically are considered latent failures. Latent failures are generally detected until either a specific functional test is run by maintenance personnel or during a teardown inspection of the unit. Items checked through PBIT are latent for one power down/power up cycle.


Functional Test Procedures

Functional test procedures will be done before, after and, in some cases, during environmental and operational tests. The Functional Test Procedures will contain any critical performance criteria that the actuator must meet – such as maximum friction levels, velocity, rate, loads, etc. In addition functional testing will need to validate all modes of operation, mode switching, operation under failure, etc as defined in the PCU specification. For complex PCUs, which contain many features and modes, functional testing is a significant test activity.