A servo is shown in Figure 1 and consists of a spool (two lands connected by a rod) and an outer sleeve (sometimes called a bushing) with flow ports drilled in the sleeve. The position of the spool determines the flow areas and hence controls the amount of flow through the valve. The spool can be positioned in a number of ways. Some of the more common methods are two position solenoid controlled (open/closed), proportional solenoid controlled (position is directly proportional to applied current), mechanical lever controlled and a servovalve arrangement (see section on Servovalves, Hydraulic - Description). Some form of servo type valve is used in almost all of the hydraulic paths to an actuation component.
Figure 1 Basic Servo Showing Flow Geometry and Parameters
Flow through servovalve openings is characterized by the orifice flow equation using a relationship between valve spool position, xv, and flow cross-sectional area, Av. As can be seen in Figure 1, the flow cross sectional area is a function of spool position. Turbulent flow is usually assumed since in most cases the pressure drop across the servo is sufficiently large. The flow rate in a servo is normally controlled by the outlet flow area and hence the outlet flow area is smaller than the inlet flow area.
Servos are either zero lapped, over lapped or under lapped as shown in Figure 2. In zero lapped servos, the width of the land and the width of the flow port are equal. Thus there is only one position for zero flow. This configuration generally results in the tightest control and is commonly selected for high precision servo valves. For over lapped servos, the land width is greater than the flow port width resulting in a deadband in the flow area vs. pressure curve. The means a minimum amount of spool movement (equal to the overlap dimension) must occur before flow will occur. The main advantage of an overlapped servo is the servo is more tolerant to “noise” factors in the control and also to manufacturing tolerances. This occurs at the price of accuracy. For under lapped servos, the land width is smaller than the port width, which results in servo flow at all positions of the spool. The benefits of a under lapped valve is faster response time with a cost of high leakage flows.
Figure 2 Servo Lapping
Servo Flow Characteristics
Using the turbulent orifice equation, the flow expression for flow through servo flow ports is
where is the area of the valve orifice (servo port). The flow area depends on port geometry, which varies with manufacturer, valve type, and spool position. Inspection of the equation (1) indicates that the flow rate varies proportionally with area if the Δp is held constant, and that the flow rate varies with the square root of Δp if the flow area is held constant. Figure 3 shows notional charts of the flow behavior for a servo which are similar to orifice flow graphs.
Figure 3 Flow Rate Behavior for a Servo
The effects of a lapping can be seen in Figure 4. Figure 4 assumes a 4 way servo and illustrates ideal flow curves. In the figure, control flow (flow through the valve port) is plotted against valve position (where Δp is assumed constant) for under lapped, zero lapped and overlapped valve. For a zero lapped valve, the curve goes through the origin. For an overlapped valve, the flow is zero until the valve spool has moved sufficiently to allow flow. For the under lapped valve, there is flow through both directions of the servo yielding a zero flow to the load at the null position. However, as the under lapped valve moves off of null, flow to the load will change quickly (higher gain) and change less rapidly once the spool has moved to the point where flow through one side goes to zero.
Figure 4 Effects of Valve Lapping on Flow
For a two position (open/close) solenoid servo, the flow area is either Amin (which is usually zero) and Amax. Changing between Amin and Amax takes less than 100 milliseconds in most applications (see Figure 5 below).
Figure 5 Flow Rate Behavior for a Two Position Servo
For a mechanically actuated servo valve the flow area will be a function of the mechanical linkage, Flow Area = f(Input, Geometry). Figure 6 shows an example of a mechanically actuated servo.
Figure 6 Mechanically Positioned Servo
Lateral Spool Forces
If an unequal pressure distribution occurs in the small clearance between the spool outer diameter and the inner diameter of the bushing, a lateral force will be applied to the spool. This force can sometimes result in a spool experiencing a “hydraulic lock” condition. Here the lateral force is so great that the spool cannot be moved as long as hydraulic pressure is applied. Minor machining tolerances on the spool outer diameter can result in slightly varying leakage flow across the spool periphery setting up the unequal pressure distribution on the spool periphery. This type of lateral force is commonly minimized by machining grooves around the spool outer diameter since the grooves allow the pressure to equalize around the periphery of the spool. The grooves are perpendicular to the bore the spool rides in (see Figure 7).
Figure 7 Servo Showing Spool Grooves
When selecting a servo, the following factors should be considered
Pressure Rating – make sure servo is rated for your system pressure
Pressure Drop Across the Servo in the Flow Range – this will affect design pressure available to a downstream component and will affect sizing of that component. In some case, an orifice or flow restriction is included in a servo to regulate flow or pressure drop downstream of the servo.
Temperature Rating – servo should be rated for fluid temperatures and applicable environmental temperatures
Spring – A spring is often used in servos to return the servo to the non-powered position. Spring forces should be examined to ensure they are sufficient to return the servo to the non-powered position in all operating conditions. When powering, the solenoid or mechanical linkage must be capable of overcoming the spring forces in all operating conditions.
Actuation Method – What method is used to control servo position (mechanical lever, solenoid, etc.)? This will usually be defined by the overall system layout and design methodology of the hydraulic system. The actuation method also requires analysis. For example, if the servo is controlled by a solenoid maximum and minimum available voltages need to be determined, and a solenoid needs to be selected with is compatible with this voltage range. When a mechanical linkage is used, characteristics of the mechanical system will be important. Characteristics to examine include inertia, friction, maximum applied load, compliance (stiffness) of structure and components, reset forces if a spring in the servo needs to reset a mechanism, etc.
Speed of Actuation Device – this is especially important for 2 position (open/closed) servos. A fast response servo will create pressure waves when it closes. Where a servo is used to shut off flow in response to a detected fault in the system, there is usually a requirement on the time required to close the servo. Many solenoids will open/close a valve in 50-100 milliseconds.
Closed Loop System Interactions – when using a servo in a closed loop system, the closed loop system interactions (both stability and closed loop performance) should be evaluated.
Valve Materials – should be sufficient to pass proof and burst testing, not be susceptible to corrosion and other environmental considerations, and not cause any problems under temperature extremes
Friction – friction of the spool may be important for mechanically actuated servos. For example, mechanical primary flight control linkages will have minimum and maximum friction requirements. Any servo connected to the linkage will add to the friction in the mechanical system. Friction is generally not a lower concern for solenoid actuated servos. (See Friction – Hydraulic Components)
Seals/Clearances – affects overall reliability of the servo. Some servos may not use seals and will maintain tight clearances between spool and housing to minimize leakage across the servo pistons. The design characteristics can be affected by environmental conditions and aging/wear over time.
Failure Modes – the main failure mode are servo valve jamming in any position from full closed to full open and contamination. It may also be possible for a solenoid to not position the servo properly for a given current due to high friction or resistance in the spool or solenoid.
Chattering – servo should be evaluated for potential to exhibit chattering or limit cycle behavior under certain upstream or downstream conditions. This will be a function of the natural frequency of the servo and the damping. This is primarily a concern for proportional servos where the servo can be positioned in any position between open and closed.
See Qualification - Hydraulic Components for discussion on qualification of hydraulic servos and required certification testing.