Chapter 27: Other Actuator Technologies Flashcards
Another means of actuation is derived from pressurised fluids, what can these working fluids be?
These working fluids can be either gas such as air in pneumatic systems, or liquids such as hydraulic oil in hydraulic systems.
What is the theory of operation of fluid drive systems for pneumatic and hydraulic systems?
Pressure differentials are established to create flow, and create useful force and motion in the process.
Look at diagram 14, showing a typical fluid actuation system. How does it work?
The compressor, reservoir and regulator serve to ensure that the pressure and flow characteristics of the working fluid are reasonably well known and constant, so that the actuating solenoid values give the desired result at the actuator.
What type of actuator is shown in diagram 14?
The actuator shown in diagram 14 is a piston that provides linear motion.
What is the force available in the actuator shown in diagram 14?
The force available from the actuator is the product of the working fluid pressure and the piston’s surface area, minus losses from friction.
F=PxA -F1 .
Piston actuators can be easily obtained with a stroke, what does this mean?
A stroke is a linear range of motion. The linear range of motion usually ranges from a fraction of an inch to about 10 in.
Pneumatic systems can survive very large numbers of cycles before wearing out, why is this?
This is because of the limited number and simplicity of the moving parts.
What is typical pressure for a pneumatic piston actuator?
Typically 20psi
In contrast to pneumatic systems whose pressure doesn’t usually exceed 100psi, the working pressure of hydraulic systems can be?
The working pressure of hydraulic systems can be much higher, 3000psi is typical.
What is the consequence that the working pressure of hydraulic systems is usually so much higher than pneumatic systems?
This means that hydraulic systems result in far greater actuation forces, and hydraulic actuators are capable of creating thousands of pounds of force over very long strokes.
Hydraulic actuators are capable of creating far greater actuation forces than pneumatic actuators, but what does this mean for the components?
The components must be capable of withstanding high pressures and loads, and it becomes critical to ensure that working fluids don’t leak as it can be hazardous.
What is the difference between a solenoid valve and a solenoid?
From an electrical point they are much the same, but the difference here is that the solenoid valve is a component within a larger system comprising of an actuator, and within these systems solenoid values are used to modulate the pressure and flow of the fluid that does the real work. The effort required to control the solenoid valve is small compared to the forces and motion that can be generated by the working fluid and its associated actuators.
What is the theory of operation of a solenoid valve?
The theory of operation of solenoid valves is straightforward, a linear solenoid moves a plunger or a spool relative to one or more orifices in order to control the flow of fluid through a valve of the body. The routing of the fluid and resulting actions can get complex.
The simplex example of a solenoid valve is a direct acting 2-way valve. How is a direct acting solenoid valve described?
The solenoids plunger acts directly as the valves metering component, and the force to actuate it must overcome the working pressure in the system to either open or close (depending on if the valve is normally opened or closed- both are readily available).
What is the function of a 2-way valve?
A 2-way valve simply allows or stops the flow of the working fluid from the pressure port to the outlet or actuator port, as shown in diagram 15.
Diagram 16: Shows a cutaway of a normally closed, 2-way direct acting solenoid valve. On the left, how is the valve shown?
On the left the valve is shown in the closed configuration. In this state no current is flowing through the solenoid coil and plunger is held tight against the valve seat by the pressure from port P and the core spring.
Diagram 16: what happens when the solenoid is energised?
When the solenoid is energised, the plunger is pulled up towards the centre of the solenoid coil, allowing flow from the pressure port P to the actuator port A. To do this the solenoid must overcome the force from the core spring and the fluid pressure acting on the plunger.
Diagram 17: What do the left and right parts of this diagram indicate?
Diagram 17 represents the states of the 2-way solenoid valve. On the left side the valve blocks the path between the inlet P and the outlet A ports, and on the right the valve body is in a position that allows flow.
Similar to how switches are available with different poles and throws, equivalent solenoid valves are available such as?
For example a 3-way solenoid valve is analogous to a single port, double throw switch. It controls the flow so that the valve outlet port connected to the actuator is switched between the supply inlet port and the exhaust port.
What is a 3-way solenoid valve used for?
A 3-way solenoid valve is used to pressurize the actuator when the solenoid valve is energised, and to release the pressure and relax or retract the actuator when the valve is de-energised.
Diagram 18: What does this show?
The diagram shows a cutaway view of a normally closed, direct acting 3-way solenoid valve. The inlet port is labelled port P, and the outlet is labelled port A, the exhaust is port E.
Diagram 18: What happens when the solenoid is off?
When the solenoid is off, the plunger is pressed against the valve seat 1 and pulled away from valve seal 2 by a spring at the plunger’s centre. Thus the valve between the pressure port P and the actuator port A is closed, while the valve between the actuator port and the exhaust port E is open.
What is the exhaust port of a 3-way solenoid valve typically open to?
The exhaust port of a 3-way solenoid valve is typically open to atmospheric pressure so has the effect of venting the actuator.
Diagram 18: What happens when the solenoid is enabled?
When the solenoid is enabled, the plunger is pulled towards the middle of the solenoid coil, which reverses the states of valves 1 and 2: the pressure port is connected to the actuator port, and the connection between the actuator port and the exhaust port is closed.
What does diagram 19 show?
Diagram 19 shows a schematic representation of the 3-way solenoid valve, and shows how the outlet/actuator port A is connected to either the pressure inlet port P or to the exhaust port E, depending on the position of the plunger.
What do 4-way solenoid valves allow for?
4-way solenoid valves allow for powered extension and retraction of actuators.
What issue do Piloted solenoid valves overcome that isn’t possible with direct acting valve solenoid?
For some applications requiring high pressure and/or flow rates, the resulting forces on a direct acting valve solenoid’s plunger becomes very large. Increasing pressure or surface area requires ever-larger solenoids, which eventually reaches a practical limit with either size or the amount of current required to generate the necessary forces. This is where piloted solenoid valves can help.
What do Piloted solenoid valves do?
Piloted solenoid valves cleverly harvest the pressure of the working fluid itself to create the forces needed to open and close. Piloted valves employ a small solenoid valve, called the pilot valve, to control the application of the working fluid’s pressure across a diaphragm at the main valve orifice. The diaphragm incorporates perforations that allow a small amount of fluid to flow to the pilot valve.
What does diagram 20 show?
Diagram 20 shows a cutaway view of a piloted 2-way solenoid valve.
Diagram 20:When the pilot valve’s solenoid coil is not energised, the plunger is pressed against the valve seat by the plungers spring and the pressure of the working fluid. In this case, is the valve open or closed?
In this case, the valve is closed.
Diagram 20:When the pilot valve is closed, where does the pressure from the working fluid introduced at the high-pressure port act to hold it closed?
When the pilot valve is closed, the pressure from the working fluid introduced at the high-pressure port acts on the top of the diaphragm at the main valve orifice, holding it closed.
Diagram 20: What else is in place to ensure the valve stays closed when the pilot valve’s solenoid is not energised?
A compression spring also applies downward force at the top of the diaphragm to ensure that the valve stays closed when the pilot valve’s solenoid isn’t energised.
Diagram 20: What happens when the piloted valve’s solenoid is energised?
When the piloted valve’s solenoid is energised and opened, a small amount of fluid is allowed to flow, which is sufficient to pressurize the underside of the main valve’s diaphragm. A pressure drop occurs across the orifices in the diaphragm and the pilot valve as fluid flows through the pilot valve. The resulting pressure acting on the top side of the diaphragm is lower than the pressure acting on the bottom side of the high pressure port and this causes the valve to open. In this way, a small solenoid can be used to control a large pressure and flow.
What must be hooked up correctly in a piloted solenoid valve to ensure the valve functions properly?
Piloted valves have a high pressure port and a low pressure port, these must be hooked up correctly for the valves to function correctly.
Direct acting solenoid valves can have what hooked up in either orientation?
The high pressure and low pressure ports can be hooked up in either orientation.
Can solenoid valves be used at intermediate positions, or is it a switch between on and off only?
Solenoid valve are not intended to be operated at intermediate positions, they can be switched on and off only.
What can the Servo valve do that the Solenoid valve can’t?
The servo valve can allow flow of a working fluid to be metered and adjusted somewhere between fully on and fully off.
What is Actuator valve otherwise known as?
Servo valve
What industry are servo/actuator valves commonly used?
Servo/actuator valves are commonly used in process control industries. They are not frequently used to control the flow of working fluids to standard pneumatic or hydraulic actuators.
What does the name ‘servo valve’ indicate about the valve?
The name servo valve indicates closing a feedback loop around a parameter of interest, usually the position of the valve element that meters flow, and implementing closed-loop feedback control to achieve fast and accurate response of the valve.
Diagram 21 shows a classic feedback controlled block diagram representation for a servo valve. What other parameters can be fed back and controlled?
Other parameters that can be fed back and controlled with a servo valve include pressure and flow rate, as well as the velocity or acceleration of the valve’s metering components.
Do servo valves need to function under automatic feedback control?
Servo valves do not necessarily need to function under automatic feedback control, they may also be run open-loop (without an automated controller). In this case the ‘actuator valve’ term is more suitable because as a result it is simply a metering valve whose metering element is controlled by an actuator, rather than manually.
What is one popular design of a servo valve?
A popular design for a servo valve is similar to a solenoid valve, with a solenoid plunger coupled to the valve’s metering component.
How does a servo valve designed similar to the solenoid valve work to get intermediate positions?
In this type of application, the solenoid is arranged to act against a relatively stiff spring which provides the return force needed to return the valve to its de-energised state. Activating the solenoid with 100% rated current will fully actuate the valve (either fully open or fully closed depending on the valve type), while supplying it with values between 0% and 100% will cause it to move to a corresponding intermediate position.
What is PWM used for ?
Pulse With Modification (PWM) is typically used to achieve intermediate levels of current, and is created by rapidly pulsing the supply voltage on and off while varying the percentage of the time the voltage is on. This is usually more efficient and convenient than creating a continuously variable voltage supply for the solenoid.
What is the duty cycle for a servo valve?
This is ther percentage of the time that the signal is ‘active’, in this case the current is flowing through the solenoid.
Other than the solenoid design for the servo valve, what other designs are available?
- Dual solenoid valves
- Gear motor servo valves
- Torque motor servo valves
What is a dual solenoid valve?
This is a design of a servo valve in which one solenoid actuates the valve’s metering components in one direction, and the other pushes it back in the opposing direction
What is a gear motor servo valve?
Gear motor servo valve in which a DC or AC motor’s output shaft is geared down to increase the torque and reduce the speed to a range of appropriate for this application. The position of the valve’s metering component is measured directly with a potentiometer or LVDT (linear variable differential transformer).
What is a torque motor servo valve?
This is a servo valve in which an arrangement of opposing rotary solenoids is used to rock a lever arm and actuate a valve’s metering components.