The author has taken a broad approach to the study of fluid power control, spanning a period of nearly forty years. He set up a new fluid power laboratory and developed new courses in fluid power and condition monitoring which he taught along with control theory, fluid mechanics and dynamics. He has published more than 170 papers and 4 books on fluid power and has mentored many MSc and PhD students.
He received the degree DSc for his contributions to fluid power and became professor of fluid power. He has also undertaken a variety of consulting projects for the fluid power industry ranging from component analysis and design to hydraulic control strategies for high power forging presses. He has also been active as an expert witness on fluid power issues ranging from system failures to patent litigation.
He was awarded the Bramah Medal of the Institution of Mechanical Engineers and received an honor from the Japanese Fluid Power Society. Power fluid control systems can be deployed in environmentally demanding applications and increasingly with alternative fluids to pure mineral oil. This book specifically examines the application of electrohydraulic valves in control systems, an extremely important part of fluid power.
Electrohydraulic valves, direction and pressure relief types
Also shown is the positive displacement pump power supply with a relief valve to both set the supply pressure and to protect the system when flow is not required by the servo drive. It should be pointed out at this stage that the term "electrohydraulic" is not strictly limited to servo valves, as it can be applied to directional control valves, proportional pressure relief valves, or any fluid power device that uses an electrical input signal to change its fluid power output function. However, in the context of control, electro-hydraulic valves are used here to indicate that the output flow rate can be varied proportionally to the electrical input signal.
Applying a voltage, either ac or dc, to each of the solenoids creates an electromagnetic force that throws the coil to the end of its stroke thereby opening the appropriate flow gates. An off-load pressure relief valve (PRV) is another form of electrohydraulic component, and a two-stage PRV is shown as figure 1.3. A pilot pressure can also be applied to the valve to cause it to open at a minimum pressure and this is useful for sequencing operations.
When the set force, equal to the set pressure, is overcome in the first stage, the poppet attempts to open by a very small amount, allowing a smaller pressure to be applied to the top of the coil of the second stage, which has a much weaker spring, allowing the flow rate of the system to pass through the valve and return to the tank. A two-stage valve is less sensitive to vibrations from the moving parts compared to a single-stage valve, and is aided by the damping hole at the top of the second stage. An electro-hydraulic relief valve is shown in Figure 1.4, where in this case the system pressure can be set directly via a variable input voltage to the "proportional solenoid", i.e. the solenoid is designed to produce a force apparently proportional to the applied current.
Applying an increasing voltage to the solenoid causes an electromagnetic force that tries to move the valve to the left, increasing the resistance to flow through the cone valve/restrictor and thus the pressure at the first stage of the valve. The operation of the second stage coil is then as described for the valve shown in Figure 1.3. The hysteresis is dominated by the electromagnetic component of the valve, so that if the direction of the applied voltage changes, the pressure does not respond immediately until there is a small change in the voltage threshold.
These features demonstrate the importance of understanding the required application performance and then selecting the right proportional pressure relief valve. Fortunately, or perhaps due to good design, the non-linear region for the first valve is within a low-pressure range that is unlikely to be needed for the major part of the duty cycle, except perhaps for pressure relief.
![Figure 1.1 Linear and rotary servodrive units [www.star-hydraulics.co.uk]](https://thumb-ap.123doks.com/thumbv2/1libvncom/9201424.0/12.892.140.769.124.609/figure-linear-and-rotary-servodrive-units-star-hydraulics.webp)
Electrohydraulic valves, the servodrive concept
For the closed-loop position servo shown in Figure 1.6b), if the required position voltage exceeds the actual position voltage from its position sensor, then a net error in the servo valve voltage causes it to supply flow to the actuator. A good design will result in the desired position being achieved in an optimal way, but the key here is to note that increasing the gain of the servo amplifier to a higher and higher value will eventually create an unstable closed-loop behavior due to the dynamic effects of the system, which is clearly undesirable. An electronic controller is required to receive the various electrical signals and then form the signal that is sent to the servo valve.
It is a combination summing junction/servo amplifier and receives the command signal and the feedback (subtracted) signal for closed loop control, and sends the resulting error voltage, converted into a current, to the servo valve coils. The basic amplifier boards used in most applications have several analog input and output channels and represent low cost, high quality approaches to closed loop control. Click the ad to read more Click the ad to read more Click the ad to read more Click the ad to read more Click the ad to read more Click the ad to read more.
The controller in Figure 1-7 is the Moog MSC I and is best suited for applications where one or two actuators need to be controlled through an analog interface. The PSC is set up by connecting a PC and using the Moog Axis Control Software (MACS). Once a program is loaded onto the card there is no need to maintain the connection to the PC as the PSC contains its own microprocessor and storage facilities.
However, it may be necessary to maintain a connection in order to change a setting, monitor operation, or monitor control parameter values. The PSC can be configured to perform advanced closed-loop control with up to two nested control loops for each control axis. Commands can be entered directly from the computer or saved to an ASCII text file and loaded in blocks from the computer to the PSC.
The Moog MSC II controller is best suited for applications where multiple axes need to be controlled via a fieldbus interface, and also uses the MACS programming software. Typical applications are closed-loop control with or without profile generation of multiple electrical or hydraulic axes.
Servovalve operation and design
An input current to the coils of the electromagnetic torque motor creates a deflection of the flap which then creates a pressure difference of 3D±3E. Application of a small current, of the order of mA, then creates an electromagnetic field along the axis of the armature, and the magnetic poles generated react with the poles of the permanent magnets. This causes a very small rotation of the armature/flap and the very small displacement, [Q, at the end of the flap is used to generate a pressure difference applied across the coil to move it.
Then consider how this difference in pressure can occur; the two methods discussed here are flap/nozzle and jet. The flap/nozzle stage is the earliest method of creating differential pressure, which is achieved by using a pair of very small diameter restrictors or orifices on either side of the flap. This resistance bridge is similar to a voltage potential divider and is shown in Figure 1.11.
The combined effect is that a pressure differential of 3D±3E is created and then sensed across the coil. By careful selection of restrictor diameters and clearance on either side of the flapper, a reasonably linear relationship between pressure differential and torque motor input current can be designed over a good operating range. The normalized characteristic is usually as shown in Figure 1.12 and is determined using real servovalve data.
The maximum differential pressure generated at maximum valve displacement, for the design parameters selected here, is 80% of the supply pressure. Then consider using a nozzle instead of a flapper/nozzle to generate the differential pressure, as shown in Figure 1.13. Consequently, the two receivers, or diffusers, will capture only part of the total available flow rate due to their finite widths.
The analysis of this type of device is difficult due to its complex practical form, but the differential pressure recovery characteristic will be similar to that shown in Figure 1.14 and recalculated using the actual servo valve data. Peak pressure recovery may be less than a flap/nozzle device, but the stated advantage of the spray tube is that it is less susceptible to fluid particle blockage compared to the flap/nozzle approach.
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Servovalve flow characteristics, critically-lapped spool
To understand the flow rate characteristic, it is necessary to consider the steady state flow rate through each port. To determine the flow characteristics in practice, the outlet ports are connected via a manually adjustable restrictor valve. Click on the ad to read more Click on the ad to read more Click on the ad to read more Click on the ad to read more Click on the ad to read more Click on the ad to read more Click on the ad to read more Click on the ad to read more Click on the ad to read more Click on the ad to read more Click on the ad to read more.
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