Electrohydraulic Valves...
ELECTROHYDRAULIC VALVE APPLICATIONS Moog Inc. was the founded in 1951 by William C. Moog, inventor of the Electrohydraulic Servovalve. His creation heralded a new era in precision control. It also spurred the growth of Moog to become the world leader in design and manufacture of electrohydraulic control products and systems. During the past decade the company has extended its control expertise into Servo-Proportional Valves, Servo Electronics and Direct Drive Valves.
ELECTROHYDRAULIC VALVE SELECTION GUIDE Moog offers the broadest line of Electrohydraulic Valves on the market today. Our product line consists of Servovalves (Mechanical and Electric Feedback versions) and ServoProportional Valves (Direct Drive and Two Stage ServoJet® versions). Servovalves typically utilize a ISO10372 mounting pattern and are nearly always zero lapped or axis cut (no mechanical deadband). Servo-Proportional Valves utilize an ISO4401 mounting pattern and may have a mechanical deadband.
HOW TO SELECT A SERVO OR PROPORTIONAL VALVE DETERMINE THE REQUIRED VALVE FLOW RATE AND FREQUENCY RESPONSE KEY PARAMETERS FOR SERVO OR PROPORTIONAL VALVE SELECTION a) In order to compensate for unknown forces, size the actuator area to produce a stall force 30% greater than the desired force to the supply pressure available. Supply Pressure Servovalve and ServoJet® Valves are intended to operate with constant supply pressure and require continuous pilot flow to maintain the hydraulic bridge balance.
Dynamic Response WL WL TYPICAL BODE PLOT OF DYNAMIC RESPONSE 4 250 225 FL Ø FL = mWL WL = weight of load (lb) m = coefficient of friction FL = mWLcosØ (lb) 0 200 175 -4 150 125 -8 -12 -16 Force Due to Acceleration The forces required to overcome inertia become very large in high speed applications and are critical to valve sizing.
ELECTROHYDRAULIC VALVE PILOT STAGE AND SPOOL ACTUATION TECHNOLOGIES NOZZLE FLAPPER TORQUE MOTOR DESCRIPTION An electrical command signal (flow rate set point) is applied to the torque motor coils and creates a magnetic force which acts on the ends of the pilot stage armature.This causes a deflection of armature/flapper assembly within the flexure tube. Deflection of the flapper restricts fluid flow through one nozzle which is carried through to one spool end, displacing the spool.
TYPES OF SERVO SYSTEMS POSITION SERVO SYSTEM SERVOAMPLIFIER A load positioning servo system is comprised of a Servo, ServoJet® or Direct Drive Valve, actuator, position feedback transducer, position command generator, and a Servoamplifier. A typical linear position servo system using a double-ended piston is shown to the right (a rotary position servo system can be created by substituting the appropriate rotary components). The valve’s two output control ports are connected across the load cylinder.
GENERAL TERMINOLOGY Per SAE ARP 490 See Moog Technical Bulletin No. 117 for a complete discussion of Closed Loop and Valve terminology and test techniques. ELECTRICAL Input Current – The electrical current to the valve which commands control flow, expressed in milliamperes (mA). Rated Current – The specified input of either polarity to produce rated flow, expressed in milliamperes (mA).
Valve Pressure Drop ÆPV – The sum of the differential pressure across the control orifices of the valve spool, expressed in psi or bar.Valve pressure drop will equal the supply pressure, minus the return pressure, minus the load pressure drop, [ÆPV = (PS – R) – ÆPL]. HYDRAULIC Control Flow QV – The flow through the valve control ports to the load expressed in in3/sec (cis), gal/min (gpm), or liters/min (lpm).
HYDRAULIC CHARACTERISTICS Rated Flow: See Figure 1. page 9. Frequency Response: Servo or Proportional Valve frequency response will vary with signal amplitude, supply pressure, and internal valve design parameters.The typical response varies with supply pressure as expressed by the change in frequency of the 90˚ phase point, as shown in figure 2. Note that Direct Drive Valve response is independent of system pressure.
PERFORMANCE CHARACTERISTICS PERFORMANCE CHARACTERISTICS Flow Gain: The no-load flow characteristics of Servo or Proportional Valves can be plotted to show flow gain, symmetry and linearity.Typical limits (excluding hysteresis effects) are shown in Figure 4. Linearity: The nonlinearity of control flow to input current will be most severe in the null region due to variations in the spool null cut.
ELECTRICAL CHARACTERISTICS INTRODUCTION Moog’s many electrohydraulic valve designs employ a number of different electrical connections. Mechanical Feedback Valves utilize the simplest electrical connections, while Electrical Feedback Valves can be more complex with different command signals, supply voltages and techniques to monitor actual spool position being employed.
ELECTRICAL FEEDBACK VALVE ELECTRICAL CHARACTERISTICS Supply Voltage: An electrical feedback always employs an on-board position transducer and often times has the valve control electronics on-board.Thus Electrical Feedback Valves require a supply voltage. Supply voltages for some models are 24 VDC (19 VDC min. and 32 VDC max.), while others require ±15 VDC (±3%). 6+PE Electrical Configuration: Moog offers up to three configurations of electrical connections for its Electric Feedback Valves.
ELECTRICAL CHARACTERISTICS DIRECT DRIVE SERVO-PROPORTIONAL VALVES 6+PE Electrical Configuration Valve Connector Mating Connector Cabinet Side Function Voltage Command 0…±10 VDC Current Command 0…±10 mA +24 VDC (22 to 28 VDC) Supply A B ^ (0 V) Supply/Signal Ground C Current Command +4…+20 mA Not Used D Input Command Valve Flow 0…±10 VDC Input Resistance = 50 k½ 0…±10 mA Load Resistance = 200 ½ +4…+20 mA Load Resistance = 200 ½ E Input Inverted Command Valve Flow 0…±10 VDC Input Resistan
ELECTRICAL FEEDBACK SERVOVALVES 6+PE Electrical Configuration Valve Connector Mating Connector Cabinet Side A B C Function Current Command Voltage Command Supply +15 VDC ±3%, ripple < 50 mVpp Supply –15 VDC ±3%, ripple < 50 mVpp ^ (0 V) Supply/Signal Ground D Input Command Valve Flow 0…±10 VDC Input Resistance = 10 k½ 0…±10 mA Load Resistance (diff.) = 1 k½ E Input Inverted Command Valve Flow 0…±10 VDC Input Resistance = 10 k½ 0…±10 mA Load Resistance (diff.
NOZZLE FLAPPER SERVOVALVE OPERATION TORQUE MOTOR Upper Polepiece ➣ Charged permanent magnets polarize the polepieces. ➣ DC current in coils causes increased force in diagonally opposite air gaps. N ➣ Magnetic charge level sets magnitude of decentering force gradient on armature.
Operation Operation Valve Responding to Change in Electrical Input Valve Condition Following Change N N N N N S S S PS PS T S S PS PS T T PS PS PS DPL A T PS Flow to Actuator A B B OPERATION ➣ Electrical current in torque motor coils creates magnetic forces on ends of armature. ➣ As feedback torque becomes equal to torque from magnetic forces, armature/flapper moves back to centered position. ➣ Armature and flapper assembly rotates about flexure sleeve support.
a SERVOJET® SERVO-PROPORTIONAL VALVE OPERATION SERVOJET® PILOT STAGE OPERATION ➣ The ServoJet® pilot stage consists mainly of torque motor, jet pipe, and receiver. ➣ An electrical command signal (flow rate set point) is applied to the integrated position controller which drives the valve coil. ➣ A current through the coil displaces the jet pipe from its neutral position.
DIRECT DRIVE SERVO-PROPORTIONAL VALVE OPERATION LINEAR FORCE MOTOR VALVE SPOOL ➣ A linear force motor is a permanent magnet differential motor. ➣ Spool slides in bushing (sleeve) or directly in body bore. ➣ The motor consists of a coil, pair of high energy rare earth magnets, armature, and centering springs. ➣ Bushing contains rectangular holes (slots) or annular grooves that connect to supply pressure PS and tank T.
PRACTICAL CONSIDERATIONS WHEN LAYING OUT ELECTROHYDRAULIC CONTROL SYSTEMS 1.Power Units Pumps: Constant supply pressure is preferred with minimum variation. Use accumulators with variable displacement pressure compensated pumps. Fixed displacement pump: constant pressure with use of accumulator is an option. – If more than one critical system is fed from one pump, isolate each system with check valves and accumulators (avoids cross-talk).
7.Servoamplifier – The dynamics of the analog electronics are always better than the Servovalve and spring-mass system.Therefore, they can be neglected. – Some digital systems, however, lack the level of dynamics that are needed. In order to see if this is a problem, check the following: (i) That the update rate of the PLC is a maximum of 20 times faster than the frequency of the valve.
ROUTINE MAINTENANCE FOR SERVO AND PROPORTIONAL VALVES 7. There are two considerations in filtration for Servo and Proportional Valves. 7.1 Particle Contamination: Larger particles from approximately 40 microns and upwards can lodge in the Servovalves’ pilot stage filter screen. Particles smaller will generally pass through. This is a last chance filter and is not intended as a system filter. See page 20 for filtration details. 7.
11. Test Equipment. It is difficult to troubleshoot a closed loop system to isolate which components are faulty.The simplest way to check a valve is to use a valve tester. Moog offers valve testers for its valves. Model M040-119 is for Mechanical Feedback Valves, while our M040-104 Series is for both Electrical Feedback Valves with integrated electronics and Mechanical Feedback Valves.
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