Electro Hydraulic Servo Valve

Project report on Electro Hydraulic Servo Valve

Nandan kumar 2009ME20669 Prince kumar 2009ME20675

Electro Hydraulic Servo valve:A hydraulic servo valve is a servo with a device (either flapper nozzle or jet pipe) used to position the servo. Servo valves are normally used when accurate position control is required. Position control is achieved through a closed loop control system, consisting of command sensor, feedback sensor, digital or analog controller, and the servo valve. Servo valves can be used to control hydraulic actuators or hydraulic motors. The main advantage of a servo valve is that a

low power electrical signal can be used to accurately position an actuator or motor. The disadvantage is complexity and cost which results from a component consisting of many detail parts manufactured to very tight tolerances. Therefore, servo valves should only be used when accurate position (or rate) control is required. A schematic of a servo actuator is shown in Figure 1.The primary components in a servo valve are a torque motor, flapper nozzle or jet pipe, and one or more spools. The flapper/nozzle (alternatively jet pipe) and the spool valve are considered “stages”. A stage provides hydraulic force amplification: flapper/nozzle or jet pipe goes from low power electrical signal to spool Δp and the spool valve amplifies Δp on the actuator. The servo valve shown in Figure 1 is a 2 stage servo valve. Almost all servo valves are 2 stage, but some 3 stage designs exist. A 3 stage servo has an additional spool valve between the 1st spool valve and the actuator. The 1st spool valve provides a spool Δp to the 2nd spool valve. The servo valve shown in Figure 1 uses a flapper nozzle. A servo valve has a hydraulic pressure inlet and an electrical input for the torque motor. The input current controls the flapper position. The flapper position controls the pressure in Chambers A & B of the servo. So, a current (+ or -) will position the flapper, leading to a delta pressure on the servo, which cause the servo to move in one direction or the other. Movement of the servo ports hydraulic pressure to one side of the actuator or the other, while porting the opposite side of the actuator to return. Operation of a servo valve is described in more detail below.

Figure 1 Flapper Nozzle Servoactuator Flapper Nozzle System Flapper position is controlled by the electromagnetic torque motor the torque developed by the torque motor is proportional to the applied current. Currents are generally small, in the milliamp range. A torque motor consists of two permanent magnets with a coil winding attached to a magnetically permeable armature. The armature is part of the flapper piece. When a current is applied to the coils, magnetic flux acting on the ends of the armature is developed. The direction

of the magnetic flux (force) depends on the sign (direction) of the current. The magnetic flux will cause the armature tips to be attracted to the ends of the permanent magnets. This magnetic force creates an applied torque on the flapper assembly, which is proportional to applied current. In the absence of any other forces, the magnetic force would cause the armature to contact the permanent magnet and effectively lock in this position. However, other forces are acting on the nozzle, such that flapper position is determined through a torque balance consisting of magnetic flux (force), hydraulic flow forces through each nozzle, friction on the flapper hinge point, and any spring (wire) connecting the flapper to the spool. As the applied current is increased, the armature and flapper will rotate. As the flapper moves closer to one nozzle, the flow area through this nozzle is decreased while the flow area through the other nozzle increases. The flapper generally rotates over very small angles (~ 0.01 rad) and the gap (G in the figure) is around 0.002 – 0.003 inches. If the gap, G, between the magnet and the flapper end gets too large, the torque motor may latch and become inoperative due to limited available torque from the torque motor. The flapper nozzle consists of the flapper, two inlet orifices (O 1 and O 2 ), two outlet nozzles (n 1 and n 2 ), nozzle backpressure nozzle (n3) and usually a feedback spring. As described above, the torque motor positions the flapper, which in turns controls the flow through the nozzles. The inlet orfices, O 1 and O 2 , are important as they create a pressure volume whose pressure is controlled by the flapper.

Figure 2 Flapper Nozzle Geometry

Referring to figure 2, for the flapper nozzle to control flow in a linear manner, the relationship Must be maintained. This relationship implies that the circumferential area created by the flapper distance to the nozzle must be smaller than the nozzle diameter, such that the circumferential area controls flow and not the nozzle diameter. In this way, the flow area varies linearly with flapper position. Also, the torque motor materials, windings and overall design features lead to accurate control of torque such that small movements of the flapper are possible. This leads to accurate control of the pilot spool, which in turns provides accurate control of the actuator. The goal of the flapper and nozzles is to control the pressure acting on both sides of the pilot spool. When the flapper is in the neutral position, the nozzle flow areas are equal and the pressures P n1 and P n2 are equal. When the flow areas and inlet nozzle pressures are equal, the flow forces through each nozzle keep the flapper centered in the neutral position. For a zero

lapped pilot spool valve, there would be no flow into or out of the actuator chambers. As the flapper moves towards one of the nozzles, the outlet flow area is reduced for this nozzle. Outlet flow area increases for the other nozzle. For example, looking at Figure 1 let the flapper move towards the n1 nozzle. This will reduce the outlet flow area and the pressure P n1 will increase. At the same time, the outlet flow area at the n2 nozzle will increase and the pressure P n2 will decrease. A delta pressure Δp = P n1 – P n2 will occur across the pilot spool piston and the pilot spool will displace to the right. High pressure fluid will then flow to the P A actuator chamber while the P B actuator chamber is ported to return. Depending on the size of the flapper and nozzles, the Δp across the pilot spool is limited in magnitude (200-300 lb range for medium size aerospace applications). Most servo valves incorporate a feedback spring (wire) between the pilot spool and the flapper. This wire is shown as a dotted blue line in Figure 1. Examining Figure 1, if the flapper moves to the left, the Δp on the pilot spool moves the spool to the right. The feedback wire will then pull the flapper back towards the neutral position. Hence the feedback wire provides a stabilizing force to the flapper and helps improve stability and response of the flapper system. This same affect can be done electronically by putting a feedback sensor (usually a linear variable differential transducer) on the pilot spool. The output of the sensor is fed back electronically to reduce the current command and allow the flapper to move back to the neutral position. Jet Pipe Another method to control the pilot spool is to use a jet pipe configuration. The jet pipe is an alternative to the flapper nozzle system; however, a similar torque motor is used to control the jet pipe position. A schematic of a jet pipe servo actuator is shown in Figure 3.

Figure 3 Jet Pipe Servo actuator

The jet pipe converts kinetic energy of the moving fluid into static pressure. When the jet pipe is centered between the 2 receiver holes in a receiver block, the pressure on the servo is equal.

However, when the jet pipe is rotated toward one of the receiver holes, the pressure at this receiver hole is greater than the other receiver hole, thus creating a load imbalance on the servo. Figure 4 shows a schematic of the jet pipe illustrating how pressure varies between the receiver holes as the jet pipe is rotated.

(a) Jet Pipe Centered

(b) Jet Pipe Rotated to Right

Figure 4 Jet Pipe Operation The stagnation pressure at the tip of the jet pipe is given by Bernoulli’s equation as

This is the stagnation pressure at the midstream of the flow and would represent the maximum pressure given by the jet pipe. From the center of the jet stream, the pressure drops off as shown in the Figure 4(a). In the Figure 4(a) configuration, the pressures on both sides of the servo are equal (Δp=0). In Figure 4(b), the jet pipe has been rotated to the right. This has the effect of increasing the pressure on the right side of the servo and reducing pressure on the left side. The servo will then move to the left. As a general rule, movement of the jet pipe is sufficiently small such that the differential pressure will vary linearly over the range of jet pipe travel. Optimization of nozzle performance is done by experimentation. There is a relationship between the nozzle diameter and receiver hole diameters, which usually must be developed through testing. Also, the distance from the nozzle exit to the receiver is important (L = 2 D n has been suggested in literature) as well as the distance between the receiver holes. In general, the receiver holes should be as close together as possible, to keep P 1 and P 2 as high as possible. It is desirable to keep receiver holes as large as possible to prevent contamination issues. The goal of a jet pipe design is to achieve the necessary maximum Δp across the pilot spool and maintain tight position control (no different from a flapper nozzle design). The biggest advantage of the jet pipe valve over the flapper valve is less sensitivity to contamination. Jet pipe orifices are generally larger than flapper nozzle orfices at the expense of more leakage flow. A clogged flapper nozzle orifice will cause a servovalve to go hardover in one direction. A jet pipe valve will generally fail neutral or operate sluggish if the inlet nozzle plugs. However, both configurations are still used today and both have proven to be reliable and accurate in service.

Servo The servo part of the valve is exactly the same as any servo or spool valve. The function of the servo is the same for either a flapper nozzle or a jet pipe servoactuator. The relationship between flow and Δp through the servo valve is governed by the orifice flow equation. Servo position is determined by a force balance on the spool, which includes the Δp created from the flapper nozzle or jet pipe, friction forces, spring forces and flow forces acting on the spool. For a complete description of a servo, see Servo, Hydraulic – Description. When the spool is in the neutral position, the servovalve is in the null position. In some applications, a compression spring is installed on each side of the servo to help keep the servo centered. In other applications (spoiler panel servovalves, for example), a spring is installed in one side only which will push the servo in one direction. For flight spoilers the spring would bias the actuator to the retract position. So, in the absence of electrical commands, the spring pushes the servo towards the retract command position allowing hydraulic fluid to flow to the retract chamber. The applied current required to overcome the spring force and return the servo to the null (no flow) position is referred to as the null bias. The null bias current will drift in service due to changes in supply pressure, operating temperatures, wear and other factors. Good servovalve design practice is to keep long term null bias shifts to within ±3% of rated current. Servovalve Flow Characteristics Plots of typical flow characteristics of a servoactuator are shown below. As stated above, flow characteristics are determined by the servo. Therefore, the orifice flow equation describes the flow characteristics. Several figures are shown below to highlight the behavior of servovalves. These figures represent a zero lapped servovalve, which are the most common in aerospace applications. Servovalves with overlapped or underlapped spools will have different flow characteristics around null (for further explanation see Servo, Hydraulic – Description).

Figure 5 shows the relationship between control flow and load pressure. The shape of the curves is determined by the orifice flow equation,

(1) where i / i max is the applied current expressed as a ratio of maximum current, ρ is the density, Δp is the pressure drop through the servo flow ports and q is the flow rate through the servo. The servo valve will have a higher gain around null then at the end of the spool travel.

Figure 5 Servo Flow Characteristics – Control Flow vs. Load Pressure

Figure 6 shows the how control flow varies with input current. The relationship is linear for a constant Δp across the servo, as can be seen from equation (1). Also shown in Figure 6 is the effect of pressure drop through the servo. Similar to Figure 5, flow increases for higher Δp. Figure 6 represents an ideal servo, with no hysteresis or friction effects.

Figure 6 Servo Flow Characteristics – Control Flow vs. Input Current BIBLIOGRAPHY . http://www.daerospace.com/HydraulicSystems/ServovalveDesc.php

http://www.me.gatech.edu/mechatronics_course/IntroMech/Hydraulic%20Servo%20Valve%20Construc tion%20plus%20Moog.pdf

1. How is higher flow rate achieved in an electro hydraulic servo valve? Ans:-

In order to achieve higher flow rates, a two or three stage servo valve is used. In this case the torque motor controls the first stage valve that actuates the spool on the second stage. The first stage valve is typically not a spool valve but either a flapper nozzle valve or a jet pipe valve. 2. Why is feedback system used in a multi stage servo valve? Ans:-

. A small flapper motion creates an imbalanced pressure in one direction or the other on the ends of the spool of the second stage. Hence the spool will tend to move in response to this imbalance and allow flow Q L to the actuator. Since continued imbalance in pressure would quickly move the spool to its limits of travel, a form of feedback connects the motion of the spool to the effective displacement of the flapper. 3. Illustrate the two different types of feedback used in a multi stage electro hydraulic valve? Ans:-

Two common forms of feedback are : a. Direct position feedback which moves the nozzle with the spool. Thus the equilibrium position of the spool is 1:1 with the position of the flapper. b. The force feedback arrangement in which a feedback leaf spring applies a force to the flapper to restore equilibrium. 4. Why is the fixed upstream type orfice necessary in the servo valve? Ans:-

The “fixed upstream orifice” in both types of valve is important to allow the pressure on either end of the spool to be below the supply pressure. 5. Why is the differential current Δi supplied by an amplifier in the input? Ans:- .

The differential current Δi is typically supplied by an amplifier to avoid excess loading of the interface to the computer or controller.