Chapter 27: Other Actuator Technologies Flashcards
1
Q
Another means of actuation is derived from pressurised fluids, what can these working fluids be?
A
These working fluids can be either gas such as air in pneumatic systems, or liquids such as hydraulic oil in hydraulic systems.
2
Q
What is the theory of operation of fluid drive systems for pneumatic and hydraulic systems?
A
Pressure differentials are established to create flow, and create useful force and motion in the process.
3
Q
Look at diagram 14, showing a typical fluid actuation system. How does it work?
A
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.
4
Q
What type of actuator is shown in diagram 14?
A
The actuator shown in diagram 14 is a piston that provides linear motion.
5
Q
What is the force available in the actuator shown in diagram 14?
A
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 .
6
Q
Piston actuators can be easily obtained with a stroke, what does this mean?
A
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.
7
Q
Pneumatic systems can survive very large numbers of cycles before wearing out, why is this?
A
This is because of the limited number and simplicity of the moving parts.
8
Q
What is typical pressure for a pneumatic piston actuator?
A
Typically 20psi
9
Q
In contrast to pneumatic systems whose pressure doesn’t usually exceed 100psi, the working pressure of hydraulic systems can be?
A
The working pressure of hydraulic systems can be much higher, 3000psi is typical.
10
Q
What is the consequence that the working pressure of hydraulic systems is usually so much higher than pneumatic systems?
A
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.
11
Q
Hydraulic actuators are capable of creating far greater actuation forces than pneumatic actuators, but what does this mean for the components?
A
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.
12
Q
What is the difference between a solenoid valve and a solenoid?
A
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.
13
Q
What is the theory of operation of a solenoid valve?
A
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.
14
Q
The simplex example of a solenoid valve is a direct acting 2-way valve. How is a direct acting solenoid valve described?
A
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).
15
Q
What is the function of a 2-way valve?
A
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.
16
Q
Diagram 16: Shows a cutaway of a normally closed, 2-way direct acting solenoid valve. On the left, how is the valve shown?
A
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.
17
Q
Diagram 16: what happens when the solenoid is energised?
A
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.
18
Q
Diagram 17: What do the left and right parts of this diagram indicate?
A
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.
19
Q
Similar to how switches are available with different poles and throws, equivalent solenoid valves are available such as?
A
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.
20
Q
What is a 3-way solenoid valve used for?
A
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.
21
Q
Diagram 18: What does this show?
A
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.
22
Q
Diagram 18: What happens when the solenoid is off?
A
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.
23
Q
What is the exhaust port of a 3-way solenoid valve typically open to?
A
The exhaust port of a 3-way solenoid valve is typically open to atmospheric pressure so has the effect of venting the actuator.
24
Q
Diagram 18: What happens when the solenoid is enabled?
A
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.
25
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.
26
What do 4-way solenoid valves allow for?
4-way solenoid valves allow for powered extension and retraction of actuators.
27
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.
28
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.
29
What does diagram 20 show?
Diagram 20 shows a cutaway view of a piloted 2-way solenoid valve.
30
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.
31
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.
32
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.
33
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.
34
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.
35
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.
36
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.
37
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.
38
What is Actuator valve otherwise known as?
Servo valve
39
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.
40
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.
41
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.
42
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.
43
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.
44
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.
45
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.
46
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.
47
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
48
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
49
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).
50
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.
51
How is best to pick a design of servo valve for a given application?
It is best to look at the flow rates and speed of response required. The control electronics that are coupled to the valve's actuators are also a major concern for the designer, and interfacing these components to the rest of a system can be one of the most challenging parts of incorporating a sever valve into your system.
52
What is a linear actuator?
The most common pneumatic and hydraulic linear actuators are based on the principle of pressure acting upon the face of a piston within a cylinder, resulting in the generation of force and linear motion.
53
Diagram 22: Piston based linear actuator; the motion of the piston within the cylinder can be transferred to external components by means of a rod, as shown in diagram 22. How else could this be done?
It could also be done with a slide which is called a Rodless cylinder.
54
How can linear actuating cylinders with a rod be constrained?
For cylinders that include rods, they may be unconstrained so that they are able to rotate relative to the cylinder, or they can be constrained so that the rotation is prevented.
55
Why is damping sometimes incorporated into the design of linear actuator cylinders?
Damping may be incorporated into the design of the cylinder to reduce the shock during accelerations and deceleration, otherwise the cylinder can be undamped.
56
In a rodless cylinder, where is the piston located?
In a rodless cylinder, the piston is located inside a cylinder with pressure acting on both sides, so that it can slide in either direction in response to changes in the differential pressure. The piston is coupled to a mounting block either through a slot or using strong magnets.
57
What is the major advantage of slides or rodless cylinders?
The major advantages with slides or rodless cylinders is the compactness. When a traditional piston and rod is fully extended, its length effectively doubles, which can be difficult to accommodate in many applications. The position of a slide however doesn't affect its overall size.
58
What is a Rotary actuator?
Controlled by rotary motion can be created from the pressure differentials available in pneumatic and hydraulic systems in a great variety of ways.
59
What are the simplest and most widely available type of rotary actuator?
The simplest and most widely available type of rotary actuator are called rotary vane actuators, and are based on a rotating version of a piston.
60
Diagram 23: How does a single rotary vane actuator work?
In the simplest case of the single vane actuator, the output shaft is located at the centre of the device, and a vane that serves as the piston rotates through the volume within the cylinder when a pressure differential is established between the two chambers defined by the vane.
61
Diagram 23: How does a double rotary vane actuator work?
In the case of a double vane, two vanes are used and hence the pressure differential acts upon twice the surface area. This results in increased force and speed of actuation at the expense of range of motion.
62
All types of pneumatic and hydraulic actuators have a specific and limited range of motion, what angles is this?
All types of pneumatic and hydraulic actuators have a specific and limited range of motion, with angles from about 45degrees to 280 degrees. High levels of torque are available at reasonably high mechanical efficiencies.
63
Designs that derive rotational motion from linear motion are also widely available, give an example.
The rack and pinion actuator.
64
How does a rack and pinion actuator work?
This type of actuator translates linear motion from one or two piston/cylinder pairs into rotary motion of the output shaft at the centre. These types of rotary actuators allow for more than a single rotation of the output shaft, are capable of providing very high output torque and have high mechanical efficiencies.
65
Give some examples of some other types of rotary actuators?
Helical spline, enclosed piston crank, bladder, helix, scotch yoke, and piston chain designs.
66
What are Grippers?
Grippers are special purpose assemblies of actuators and linkages that are used to grasp and manipulate workpieces. In a limited way, them mimic the human hand's ability to pick u and hold objects.
67
What sorts of grippers are available?
They are available in 2-finger configurations where the finger motion is parallel, angular, angular with a wide range of motion and 4-finger configurations for grasping and centring items, in-and-out motion (called escapement actuators); and toggle grippers that maintain their gripping force after the drive pressure is released.
68
What is a RC servo?
They are really nothing more than a standard permanent magnet brushed DC motor, couple to a low-precision position feedback system. They are easy to use, simple to interface, with readily available and inexpensive. They are commonly used in mechatronic applications.
69
Where were RC servos originally used?
RC servos were originally introduced for position control applications in radio controlled device applications, such as model cars and airplanes. They are commonly used to control steering angles in cars and elevator, rudder and aileron position airplanes.
70
What are RC servos most commonly comprised of?
In their most common embodiments, they are comprised of plastic housing that incorporates a motor coupled to a high ratio gear reduction stage, resulting in low output shaft speed and relatively high torque.
71
RC servos can be considered an alternative to what as a means of implementing position control?
In many cases, RC servos can be considered an alternative to stepper motors as a means of implementing position control, often because of the relatively low torque capabilities of most stepper motors.
72
What must be considered when deciding between a RC servo or Stepper motors?
The output torques of both need to be considered. RC servos tend to have torques at the lower end of the range,
73
How is the position of the RC servo's output shaft measured?
The position of the RC servo's output shaft is measured by a potentiometer and used for closed-loop position control.
74
What is usually attached to output shaft of a RC servo?
A convenient mounting flange.
75
What do low cost and higher cost RC servos have for motors and gears?
Lower cost RC motors incorporate inexpensive, small, brushed permanent magnet DC motors and plastic gears. While higher cost options use brushless DC motors, and some are available with metal gears.
76
True or false: The majority of RC servos are not capable of continuous rotation, instead having a range of motion less than 360 degrees with physicals stops at their limits of travel.
TRUE. Some do allow for continuous rotation or can be modified to achieve it.
77
RC servos require power connections for what components?
RC servos require power connections for the motor and control electronics as well as position command input.
78
What is the position command input usually for RC servos?
The specification for the position command input is common among RC servos, the position command is 0-5V digital pulse whose duration corresponds to the desired angular position. The duration of the pulse varies between 0.5-3ms. with 1.5ms corresponding to approximately half way position. The time between pulses is typically 20-30ms, but wide variation is allowable.
79
Give in example of something that RC servos have been incorporated into?
They have been incorporated into wall-crawling robots, such as Stanford's Stickybot, snake robots and pan-and-tilt camera mounts.
80
What are piezo actuators?
Piezo actuators derive their function from a physical phenomenon called the piezoelectric effect. This is that naturally occurring crystals such as quartz, tourmaline, topaz etc, reacted to the introduction of an electric potential by changing shape, and conversely when subjected to mechanical strains, the materials created an electric potential. These characteristics are exploited in a piezo actuator.
81
How do piezo actuators use piezoelectrically active materials?
The application of electric potential across the material causes it to deform and the resulting motion and forces are used to create an actuator.
82
What are piezoelectric ceramics?
They were developed during ww2, they exhibit much higher piezoacitvity than previously known crystalline materials.
83
Give two examples of piezoelectric ceramics.
Barium titanate and lead zirconate titanate (PZT)
84
What do the responses of piezoelectric ceramics exhibit?
Even these materials don't deform much, and their responses exhibit hysteresis- that is the dimensions of the material at a given voltage are different depending on the voltage history. The displacement from an increasing voltage is different to the displacement from a decreasing voltage.
85
What can be used to compensate for the hysteresis behaviour of piezoelectric ceramics?
Correction factors (open-loop compensation) or independent position measurements (closed-loop feedback control) can be used to compensate for this behaviour.
86
What are piezo actuators used mostly for?
Piezo actuators are used mostly to create motion on a very small scale, though it can be very accurate and repeatable and achieve very impressive resolution. Ranges of motion from nanometres to hundreds of microns are achievable, with subnanometer resolution .
87
What limits the precise control of piezo active elements?
Precise control of piezoactive elements is limited primarily by how well you can control the applied voltage. It can also be limited by electrical noise, component drift, temperature effects.
88
What are some of the impressive characteristics of piezo actuators?
- They can generate very high forces
- They have very high positional resolution
- They have very fast responses
- They don't generate and aren't affect by magnetic fields
- They consume very little power, especially when holding a position
- They have very long service lives
- They can be used in sever environments.
89
What are some limitations to piezo actuators?
- They are relatively expensive
- The maximum range of stacked piezo actuators is very small
- The maximum range of motion of bimorph piezo actuators is less limited but still small
- Piezo actuation elements are very brittle, and do not tolerate tensile or shear loading well.
- Piezo active materials exhibit substantial hysteresis which can be mitigated with open-loop compensating techniques or closed-loop control.
- Relatively high voltage is required to achieve the full range of motion from piezo actuators
- The precision, stability and noise characteristics of the electronics used to apply a voltage across a piezo element ultimately determines how accurately it can be controlled.
90
Where are piezo actuators commonly used?
They are used in fairly exotic applications such as in optics, MEMS, laboratory environments and semiconductor manufacturing.
91
What is a stacked /serial piezo actuator?
This is one of the most common and straightforward configurations of distinct class of piezo actuators. This type of actuator adds the combined deformations of many piezo elements together to result in an increased range of motion. They are available in simple laminated assemblies and also in protective housing.
92
What is a bimorph/bender piezo actuator?
This construction stacks two piezo elements side by side and applies the voltage differentially across them (one element gets bigger as the other element gets smaller) to cause it to bend.
93
In addition to general purpose actuators, what have bimorph actuators been used to create?
Bimorph actuators have been used to create small fan elements. Though expensive relative to conventional fans, they consume little power, deliver high flow velocities and volume flow rates have long service lives and allow designers to target 'hot spots' with very precisely aimed air steam.
94
What is a piezo motor?
This is another major category of piezo actuator. Within this category there are two approaches to achieving motion; inchworm and ultrasonic motors.
95
Diagram 24: What do inchworm drive actuators do?
Inchworm drive actuators use a finger or multiple fingers to push and pull either a side element (for linear motion) or a rotor (for rotational motion). Essentially, the fingers 'walk' the plunger or rotor in successive grasp-push-release actions to create pseudo-continuous motion.
96
What is the inchworm motion similar to ?
The motion resulting from the inchworm has a discontinuous characteristic, similar to the operation of a stepper motor, but with finer resolution.
97
What are the range of motion like for inchworm piezo motor?
The inchworm piezo motor approach allows for large ranges of motion and each step can be controlled precisely via analogue electronics to provide very high resolution positioning . Rotational actuators are capable of rotating continuously. These piezo motors are capable of jogging at high feed rates, such as 100mm/s and can provide impressively high actuation forces- as high as hundreds of newtons.
98
Diagram 25: What do ultrasonic motors do?
Ultrasonic motors employ an entirely different approach to inchworm motors to create relative motion between a stator and the moving rotor or slide. In these devices, a wave is induced in the piezo actuation elements. One side of the piezo elements is attached to the stator, and the other side is pressed up against the rotor or slide. By controlling the characteristics of the waves induced in the piezo component, relative motion between the stator and the rotor or slide is created. Actuation forces are transmitted from the piezo element to the rotor or slide through friction.
99
when a travelling wave is induced in a piezo beam element, will its points of contact with the moveable surface have ellipitcal or linear motion?
which will result in relative motion between the moveable surface and the piezo
elliptical motion
100
TRUE or FALSE
the process of a travelling wave induced in a piezo beam element, which results in relative motion between the moveable surface and the piezo occurs at high frequencies
TRUE
| the process occurs very quickly usually in the tens of kilohertz
101
what type of motion does the ultrasonic motors have
unlike the inchworm drive approach, ultrasonic motors have smooth continuous motion
102
a piezo buzzer is a device that produces sound waves, this occurs by actuation of the piezo element
does this process require much power?
NO
| it only requires minimal power (tens of milliwatts)
103
what attributes do piezo actuators have that makes them a good match for creating sound waves
the material has fast response times allow for the production of sound frequencies from below 100Hz to 10kHz
piezo materials small deformations are well suited for the task of vibrating a diaphragm to produce sound waves
104
Diagram 26:
| what is the construction of a typical piezo buzzer element
a thin ring of piezo material is sandwiched between two electrodes that allow a voltage potential to be applied across the top and bottom of the ring. The piezo element is bonded to a metal plate on one side and a thin diaphragm on the other side
105
how does the piezo actuator technologies produce sound waves
by applying a driving voltage across the piezo material and causing it to deform
because of the way that the material is constrained by the metal plate and the diaphragm, the assembly assumes concave and convex shapes depending on the polarity of the applied voltages causes the buzzer element to vibrate back and forth as it alternates between a concave and convex shape and this vibration produces pressure variations in the surrounding air
106
TRUE or FALSE
| only a small amount of current is required by most piezo buzzers in the range of a few milliamps.
TRUE
| in the range of a few milliamps
107
what is an SMA
Shape Memory Alloy
exhibits remarkable mechanical characteristics that results from changes in the materials crystalline structure caused by temperature changes.
108
what can SMAs do
with careful processing these materials can be configured to return to a predetermined shape at a specific temperature, often with substantial force.
109
how is the SMA "taught" to the alloy
by constraining it in that shape while the temperature is raised to a point where annealing occurs. after the material has been annealed and cooled below a threshold temperature, it may be deformed into new shapes and when the temperature is raised to the threshold temperature, it will very forcefully reconfigure itself and return to its default shape
110
what is the best known material that exhibits the SMA behaviour
a mixture of nickle and titanium, sometimes called NiTi, this material was developed by the Naval Ordinance Laboratory which was dubbed Nitinol and is one of the best performing SMAs
111
what are some other SMA other than Nitinol
Many other alloys exhibit the shape memory effect, with widely varying characteristics and costs
Copper-Aluminium-Nickle ( Cu-Al-Ni), Copper-Zinc(Cu-Zn), Copper-Tin(Cu-Sn), Gold-Cadmium(Ag-Cd), Cobolt-Nickle-Aluminium (Co-Ni-Al), Cobolt-Nickle-Gallium (Co-Ni-Ga) and Nickle-Iron-Gallium (Ni-Fe-Ga)
112
what is the mechanism of action of SMA
a phase change of the of the molecular structure of the material. this isnt the typical solid-to-liquid phase change, the material remains a solid throughout the process. Rather the phase occurs within the crystalline structure of the alloy and the different phases (Martensite vs Austensite) have different properties
113
at low temperatures what is the structure entirely made out of
Martensite
| which resembles a series of connected parallelograms
114
what occurs to the SMA above the transition temp
the crystal structure changes to Austenite which is cubic in shape
115
what determines the transition temperature
it is a material property that is determined by the ratio of the constituent elements of the alloys.
116
what is the austensite start temperature (As)
the temperature of the material where the transition to austenite begins
117
what is the austensite finish temperature (Af)
the temperature that the transition is complete
118
what affect does raising the temperature above Af
this will have no effect on the materials structure until high temperatures near the annealing temperatures are reached. returning the material to the annealing temperature will "retrain" the material
119
what happens if the Af temperature is reached and then the material temperature is decreased
the structure will begin transforming back into martensite at the martensite start temperature (Ms)
120
what happens if the temperature is further decreased in the "cooling phase"
the fraction of martensite continues to to increase until the material is compromised entirely of martensite
as cooling occurs the SMA maintains its trained shape, though the crystalline structure is changing . the temperature at which this transformation is complete is called the martensite finish temperature (Mf)
121
how do SMA actuators work
SMA actuators exploit the forces generated as the fraction of austensite increases and the material does everything it can to return to its annealed shape. The SMA can be conveniently heated by causing current to flow through it, though any means of raising the temperature ill have the desired effect.
A common way to build SMA actuators relies on a small diameter wires that are trained with default length. after they have been stretched so that they are longer than their default length, they are heated by flowing an electric current through them. turning the current on heats the SMA and causes it to contract forcefully and turning off the current allows it to cool and relax. an external force is required to stretch the material as it cools, otherwise the material will remain essentially in its trained shape after it returns to martensite
122
what is the speed response for all SMA dominated by
the time it takes to heat and cool the material
| in general this is quite slow compared to other actuators
123
what is the best best response time you can expect from a SMA actuator
about 1 Hz
124
TRUE or FALSE
| the forces that SMAs are capable of generating are small
FALSE
| the forces the SMAs are capable of generating are very large
125
what could be an everyday use for an SMA actuator
an automatics, non-powered opening and closing of skylights in a greenhouse. when the temperature inside the greenhouse exceeds the austensite start temp, SMA actuator opens the sky light to vent the hot air and cool off the greenhouse. when the temp decreases below the martensite start temp the actuator closes the vent
