These advantages make compliant mechanisms ideally suited for space or aerospace applications, where low weight and no lubrication are desirable [ 2 ]. Compliant bistable mechanisms [ 3 , 4 ] gain their bistable behavior from the energy stored in the flexible segments, which deflect to allow mechanism motion. This approach integrates desired mechanism motion and energy storage to create bistable mechanisms with dramatically reduced part count compared to traditional mechanisms incorporating rigid links, joints, and springs.
As a deflection is applied to the mechanism, it rapidly transitions from one stable position to the next. The force-deflection response for a typical bistable mechanism is illustrated in Fig 1. An optional preload stabilizes the mechanism for lower force inputs. Compliant bistable mechanisms can be used in space applications as switches, latches, or relays, thereby eliminating friction and improving the reliability and precision of those mechanical devices.
Further, the bistable mechanism does not require power to be held in either of its stable positions. Such mechanisms could be integrated into deployment systems as non-explosive release mechanisms. Compliant mechanisms have many advantages for space or aerospace applications and significant performance gains are possible with the introduction of compliant mechanism technology [ 2 ].
Current space-related applications of compliant mechanisms are largely limited to flexures in precision instruments such as optics. Flexures were also used in the wheels of the Mars Science Laboratory and Mars Exploration Rovers to provide suspension. Flexures have also been used to compensate for different coefficients of thermal expansion in different materials [ 5 ].
A compliant hinge providing 90 degrees of rotation was recently developed as a potential hinge for deployable booms on spacecraft [ 6 ] and compliant elements have been proposed for use in deploying booms [ 7 ]. Compliant mechanisms can achieve bistable motion without bearings or friction. They can be designed to provide precise state positions. Compliant bistable mechanisms, such as that shown in Fig 2 , have potential application in space systems as switches, latches, or as an alternative to pyromechanical release devices [ 8 — 11 ].
Bistable mechanisms are flexible devices with two stable equilibrium positions. A pseudo-rigid-body model PRBM [ 12 ] is shown for a generic translating compliant bistable mechanism in Fig 3. The compliance in the mechanism is modeled by the inclusion of torsional and linear springs where. The PRBM is useful for initial design to find bistable configurations; then finite element analysis FEA is valuable to verify and refine the design.
The PRBM gives reasonably accurate deflections and rougher approximations for stress. Compliant bistable mechanisms [ 3 , 4 , 13 — 15 ] take advantage of stable minimum-energy points in their geometrically nonlinear elastic energy curves. These mechanisms are specifically engineered so the energy stored in the deflected mechanism can be quickly released when the device is actuated. This approach integrates desired mechanism motion and energy storage to create bistable mechanisms with dramatically reduced part count compared to traditional mechanisms.
As a deflection is applied to the mechanism, it rapidly transitions from one stable position to the next, as illustrated in Fig 1. Bistable mechanisms have an established history, especially in micro devices [ 13 , 16 — 18 ]. Their application to macro devices, particularly in metals, and for space applications is a subject of current interest. The bistable mechanisms are SMA-actuated, but must be manually reset in the current design. The ability of bistable mechanisms to maintain two distinct positions without requiring external energy has shown promising applications in robotics [ 21 — 23 ].
Bistable mechanisms together with dielectric elastomer actuators were proposed for robotics for planetary exploration [ 24 , 25 ]. They have also been proposed for use in architectural structures [ 26 ] as well as in origami-inspired structures and mechanisms [ 27 , 28 ]. Tape springs have often been used in cube-sats and proposed for other space applications to enable bistability of space structures [ 29 — 31 ]. Bistable composites and laminates have also been developed for active shape control [ 32 — 35 ].
It is desirable to develop bistable mechanisms in metals because metals are more robust than polymers in many situations and can withstand the harsh environments that space imposes. Metals can withstand higher loads than polymers and are less susceptible to creep and stress relaxation. They are also thermally and electrically conductive, which can be desirable for certain applications, including actuation. Release mechanisms have been developed in response to the need to anchor deployables to the spacecraft body for launch and flight, and then to be released on electrical command.
Pyromechanical release devices [ 8 — 11 ] are common release mechanisms in aerospace applications. The have a fast response and are well understood, but are costly, one-shot devices, that apply pyro-shock loads to the spacecraft when fired. Release mechanisms can be divided into pyromechanisms and non-explosive release mechanisms. Pyromechanisms can be further subdivided into 1 separation devices which carry heavy loads, to be released on command , 2 cutters, and 3 pin pullers.
Non-explosive release mechanisms are characteristically slow, have lower force output, and are difficult to time. However, they do not cause shock loads like those associated with pyromechanisms. Burn-wire mechanisms, paraffin actuators, and shape-memory metal release mechanisms all have less shock, but have slower actuation times, are more complex, and can be less robust than pyromechanisms. An integral part of designing CMs for space applications is material selection.
For compliant mechanisms, we often consider the strength-to-modulus ratio of a material as a measure of its fitness for compliant applications. For space mechanisms, weight becomes critical as well. Amorphous metals rank highest, followed by aluminum alloys and , and then titanium, Elgiloy, and Inconel see Table 1. Aluminum has the lowest density of all the materials considered. Tantalum is very dense, but it is a refractory metal, highly corrosion resistant, and heat resistant.
Invar, like tantalum, has a low coefficient of thermal expansion, making it ideal for optics. Bulk metallic glasses amorphous metals are a new area of materials research. They can have high fracture toughness, although they are also characterized by a low ductility [ 36 — 40 ]. Metallic glasses are strong due to their lack of defined grain structure, but also have an elasticity comparable to conventional metals. The absence of microstructural defects also improves their resistance to corrosion [ 38 ]. Metallic glasses, with their exceptional yield strain, are an excellent prospect for compliant space mechanisms.
Several iterations of the bistable mechanism design are illustrated in Fig 5.
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Fig 5a shows the basic form of the bistable device. Fig 5b and 5c show the addition of thicker midsections to the flexible beams. Such midsections are common on early bistable devices, but were included primarily because of the limitations on analytical methods; the pseudo-rigid-body model for small-length flexural pivots was originally used to model these flexures [ 12 ]. Such segments may also improve the stability of the mechanism by directing the flexible segments through a more defined motion.
Finite element analysis was used to determine the effect on performance of these thicker midsections. A brief analysis of the effects is shown in Fig 6 and Table 2. The thicker segments increase the bistable actuation force slightly, which helps the mechanism hold its second stable position. However, the stress is also increased with the addition of these thicker segments; the longer the thick segment, the greater the force and stress increase. A bistable compliant mechanism was developed with properties suitable for potential application as a release mechanism in space systems.
An amorphous metal, or bulk metallic glass BMG , was selected because of its properties described earlier. As with other metals, metallic glasses are corrosion-resistant and able to withstand the harsh environment of space. It is also noteworthy that metallic glasses can be manufactured by a process similar to injection-molding for plastics [ 41 ]. This has the potential to reduce manufacturing and labor costs. The composition of the alloy selected for the design was Metallic glasses have a high strength-to-modulus ratio, which is an important characteristic for compliant mechanisms because it means the material will allow a larger deflection before failure [ 12 ].
The mechanism presented in this paper was designed to evaluate the performance differences between metallic glass specifically, Vitreloy 1 and titanium Ti-6Al-4V. The basic design for the bistable device is illustrated in Fig 2. The part was also rapid prototyped from extruded ABS as an early demonstrator of the model see Fig 7.
The design parameters are listed in Table 3. The bistability of the device is irrespective of the thickness of the material, but the actuation force is increased with increasing thickness i. All designs were made for 3 mm thick sections. The shuttle should snap between two stable positions, with the second position bringing the shuttle just into contact with the base. Fig 2 defines the pertinent design parameters that can be changed or optimized to give a feasible design. These models were then analyzed using finite element analysis and prototypes were fabricated and tested.
The mesh, shown in Fig 10 , was created by specifying the number of divisions along each of the lines in the model. The compliant segments required a finer mesh because they will undergo large, nonlinear deflections. The outer geometry goes through less displacement and can therefore have a coarser mesh. The finite element analysis is displacement-controlled, where the total displacement was applied over load steps, depending on which material is being modeled. The performance of the two materials was compared through two controlled designs.
The main design parameters are listed in Table 3. The two mechanisms were manufactured to maintain the same safety factor, or same ratio of the yield strength to the maximum stress. This resulted in a titanium device that was more than twice as long as that of metallic glass mechanism due to the large difference in material properties. The two mechanisms were also manufactured with identical geometries. The safety factor for the metallic glass mechanism is over two, while the safety factor for the titanium mechanism is equal to one. With such a low factor of safety, it is likely that the titanium mechanism experienced local yielding.
To compare performance, the identically sized titanium and metallic glass flexures were tested in a load frame to determine the force-displacement behavior. The results of the test are described in [ 41 ], where it can be seen that the mechanisms exhibit bistability with a clear intermediate instability point. The two materials exhibit roughly the same response because they have comparable stiffnesses, but the strength of the metallic glass is twice the strength of the titanium, thereby doubling its factor of safety.
Finite element analysis was used to verify and refine the design and to determine the maximum stress in the compliant members. The mesh was refined along the compliant flexures and the motions were examined over steps. The maximum stress in the finite element model was measured and is plotted in Fig The yield strengths for metallic glass and titanium are indicated in the plot. This illustrates the difference in safety factors for the two materials with identical geometries. There is a difference between the FEA results and experimental results for the second stable equilibrium positions.
The FEA assumes a rigid-fixed condition while the hardware necessarily has some elasticity even for the relatively rigid components and connections. This difference is seen most in areas of higher deformations, such as the unstable equilibrium position and the second equilibrium position.
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Fig 12 shows an example of the different stress states in the finite element model during the simulated displacement. The highest stress state Fig 12b occurs partway through the deflection, when the compliant beams are under a high compressive load.
The maximum stress state occurs in the buckled flexures; its location varies along the length of the flexure depending on how the beams buckle. There is a complex interplay between geometry and material properties that affect bistability. Table 4 lists the key parameters for bistability, which are all interdependent. Table 4 summarizes the effect of these parameters on bistability and stress. We defined an improvement in bistability as as increase in the force required to reset the mechanism from its second stable position to its first as fabricated position.
As can be seen from the table, the dimensions l c and t will be driven by the constraints of the manufacturing process and design space; optimizing the design will drive t to its minimum thickness and l c to its maximum allowable length. Compliant bistable release mechanisms can be used as non-explosive release mechanisms at a fraction of the cost and weight of traditional release mechanisms. Compliant bistable release mechanisms would eliminate the challenges of having explosive charges on the spacecraft. They can be compact compared to other alternatives, thereby reducing weight.
They will enable systems to be testable and resettable. Other advantages of compliant bistable mechanisms are that they can accommodate integrated thermal actuation to change state and they only require power to change states, not to maintain state this means much lower power requirements than many alternatives. Bistable mechanisms share many of the same desired characteristics of explosively actuated mechanisms in that they respond rapidly upon triggering, are capable of high force output, and can be designed so that they only require minimal amounts of activation energy to release large amounts of stored energy.
A major advantage of bistable mechanisms is that the actuation is easily reversed for repeated testing and the tested hardware can be flown, which is not possible to do with pyromechanisms. Actuation of bistable mechanisms differs from that of current explosive and non-explosive release mechanisms. While it is possible that a similar method could be used to actuate bistable mechanisms, such as the use of shape-memory materials [ 42 , 43 ] or the heating of the bistable flexures to trigger them into their second position, we also investigated more rapid actuation methods.
Because a small input displacement can actuate a bistable mechanism from its second stable position to its fabricated position, this is one of the simplest methods of activation. A prototype of a magnetically actuated mechanism, shown in Fig 13 , uses a 3AV electromagnetic linear actuator to demonstrate this type of actuation. In the prototype, a push-pull electromagnet was used; however, a device such as a solenoid actuator would be more ideal for the final design due to its greater efficiency.
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The prototype was 3D-printed out of PLA filament. The electromagnet provides a force of 8. The prototype was designed such that the electromagnet would provide just enough force to trip the mechanism into its fabricated position from its second stable position. The prototype in Fig 13 demonstrated the effectiveness of electromagnetic actuation of bistable mechanisms. The benefits of such a system include the fact that much less input force from the linear actuator is required to activate the system than is output by the bistable mechanism; when the mechanism is in its second stable position Fig 13 left , an input force of 8.
This allows a high magnitude force to be preloaded into the deployment mechanism before launch, and requires only a small electrical pulse to release the stored energy almost instantaneously. Two basic release mechanisms, a pin-puller and a cutter, were chosen to evaluate the possibilities of using the developed bistable mechanism as an alternative to pyromechanical release mechanisms.
Due to the nature of the high output force requirements, the frame of the mechanism was closed and additional compliant legs were added to the design to increase the transition force. The prototypes were 3D-printed in PLA. A bistable version of a pin-puller release mechanism is shown in Fig The puller is designed so the bistable mechanism makes contact with the pin only after the legs have moved well past the unstable position. The contact position is such that the bistable mechanism engages with the pin while it is in the high-force region of its displacement.
In its movement from the unstable position to the pin-contact point, the bistable mechanism gains momentum which aids in breaking the pin free from the static friction caused by the loading on it. The bistable mechanism, then in its high force region, would then pull the pin free. The cutting mechanism demonstrator shown in Fig 15 was designed using the same bistable configuration that was developed for the puller. The additional compliant legs and the increased width of the mechanism provide a higher force for cutting wires and cables. The mechanism is designed such that the cutter engages with the wire or cable while in the high-force region of the shuttle deflection.
The momentum gained in the travel from the unstable position to the engagement position aids in the initial cutting. As the blade cuts through the material, it meets the mandrel in a preloaded position that maintains high force between the blade and mandrel, thus ensuring a complete cut. The 3D-printed prototype release mechanisms demonstrate potential applications for bistable mechanisms.
To be used in space, the bistable release mechanisms would be manufactured in bulk metallic glass or titanium. Finite element analysis reveals that if the same design used in the pin-puller and cable cutter prototypes was fabricated in BMG rather than plastic, forces of over N 90 lbf could be achieved, ensuring that these mechanisms can be tailored to meet a variety of load requirements. These mechanisms could be easily incorporated into current space vehicles and satellites without requiring major modifications.
The students designed Doggo to be open source. Anybody can download the plans and build their own Doggo robot. Each motor has an encoder used to track the motor angle. The trend in legged mobile robots seems to be for development to start in university research laboratories. As researchers gain knowledge, they start spinoff companies, and then eventually team up with industrial partners for broader sales distribution.
Agility Robotics , located in Albany, Oregon is a good example. One of the co-founders is also a professor at Oregon State University. Those robots also require that the customer unload the package from the robot. Digit, on the other hand, can drop off the package all by itself if no one is home. They might already be in your grocery store, and before long you could get a text message saying that your pizza is at your front door—delivered by a mobile robot, every inch of its progress monitored by its encoders.
It is my goal to make this blog as informative, engaging and as accurate as possible. If you ever have some additional or contrary information, please contact me directly and I will be glad to make any appropriate corrections in a future post. Previous Post. Sponsored by TechBriefs This link leaves usdigital. Published in Posts on Monday, June 24, This post continues our discussion of the various classifications of encoders identified previously.
One of the most common classifications used for encoders is whether their architecture is incremental or absolute in design. This refers to the type of output the encoder emits, or what information is being provided by the encoder. This post will begin our discussion of incremental output and our future post will continue that discussion. Later, we'll have a discussion of absolute encoder output and make some comparisons between incremental and absolute encoders. The most common encoders are incremental encoders , for two main reasons.
First, the information supplied by an incremental encoder is sufficient for most applications. The second reason is just a matter of economics and simplicity: it is much more cost effective to manufacture an incremental encoder than an absolute encoder. A very rudimentary form of an incremental encoder is shown above. It only involves a disk with one slot, an LED, and a photo detector. The detector provides an output each time it "sees" the LED.
The first piece of information that can be determined by an incremental encoder's output is distance. As the above disk rotates, each time the slot is aligned with the LED and detector, the detector produces an output. The disk has rotated through an angle of mechanical degrees. A controller can use this information to calculate distance traveled in a system. Each time the cart's wheels rotate, a pin on the axle engages a cog on a system of cogwheels that keep track of total distance traveled. A second piece of information which can be determined by an incremental encoder's output is velocity , or speed of movement - the encoder essentially acts as a tachometer.
As the disk in our first drawing above rotates, each time the slot is aligned with the LED and detector, the detector produces an output. The number of outputs in a minute would be the RPM or speed of the disk. Although in the rudimentary drawing of an encoder at the beginning of this post you will be able to determine if a full rotation has taken place, for most of the degrees, movement can take place without that movement being reported.
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The disk rotates without a change in output until the slot is reached. To resolve that issue, a disk can be used with multiple slots as is shown in the picture above from a blog post by Aditya Prasad. Please note in this specific design, it would work as shown with an optical sensor or with a magnetic sensor as described earlier in the same post. Now with 15 slots and 15 teeth, the optical sensor will provide an output while the disk turns through 12 mechanical degrees and a slot is in front of the sensor. During the next 12 degrees, a tooth will block the light, and the optical sensor will provide no output.
Most applications require an output when the movement is much less than 12 degrees so disks are made with much finer increments. The technical term used to define the size of increments used is resolution. We will discuss resolution in more detail in a future post but for now, we will define it as the number of outputs provided by the encoder based on the number of lines or windows on the disk.
To illustrate the resolution in everyday or at least Thanksgiving Day terms, think of resolution as the size of the pieces of a pie. Neither high resolution nor low resolution is better but the resolution should match the need. Speaking of pies, if one is extremely hungry, the pie on the left would be the best choice. However, if one is trying to limit their caloric intake, other than not eating the pie at all, a smaller piece would be the best choice.
All of the illustrations and examples have been focused on a single output as is further illustrated in the above drawing. The drawing is indicative of what that single output might look like on an oscilloscope. The bottom of the drawing represents an output of zero volts. The top of the drawing represents an output of five volts. The complete drawing is showing the changes in output or cycles as the disk rotates. When the sensor sees the LED, the output goes high 5 volts ; when the disk via the line prevents the sensor from seeing the LED, the output goes low 0 volts.
One complete electrical cycle starts when the output goes high and ends just before it goes high again. One electrical cycle is electrical degrees. For every mechanical revolution, the number of electrical cycles will be equivalent to the resolution or lines and windows on the disk.
An encoder with CPR will have lines and windows on the disk and, of course, produce a high and low output times for each rotation of the disk. NOTE : Unfortunately, there are some vendors who use the term CPR to mean counts per revolution which, depending on the vendor, that number can be twice as many or four times as many as the resolution. This will be discussed in more detail in a future post.
One drawback with incremental encoders is that with power cycling, there is no memory as to where the disk is, as all of the lines and windows on the disk are identical. If you have ever been lost, you know this feeling where your surroundings might look the same in every direction. Essentially the position of the disk in relation to the sensor is lost. We will show in our next post how we can use a search operation, like the search dogs shown above, to figure out where the disk is in its rotation.
Although we are able to calculate both distance moved and velocity from a single encoder output, one other piece of information provided by incremental encoders is direction of travel. Our future post will continue this discussion and explain how direction can be determined. Source for photo detector graphic - reviseomatic.
Published in Posts on Monday, June 10, What do an organ, a black widow spider, and a scale have in common? It has always been intriguing to me how companies came into existence. US Digital's founding is an exclamation to the saying - "necessity is the mother of invention". David Madore, the founder of US Digital, worked at the time as a design engineer for a medical ultrasound imaging company.
The equipment had many knobs on the front panel for potentiometers. The company wanted to upgrade the design to use optical encoders to improve operation. In doing research they could not find a good source suitable for the application. The ones they found were overpriced, had long lead times and poor specifications.
In , David Madore, made his first encoder to meet this need. And as they say, the rest is history. BEI's history dates back to the 's.
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Many of us have heard of Baldwin pianos or organs but are unaware of how they fit in with encoders. Dwight Hamilton Baldwin was a minister and a singing teacher in schools who also opened a music store in Cincinnati, Ohio in Instead of just distributing the keyboard instruments, he set out to make "the best piano that could be built". World War II interrupted their operation as the company participated in making wings and other aircraft parts. A more detailed account of their history is available if you are interested.
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After the war, Baldwin started using electronics in the development of keyboard instruments. The goal was to use this technology in an organ to replicate the sound of pipe organs in European cathedrals. The engineers at Baldwin came up with a way to use optically encoded glass discs to reproduce the organ tones. The codewheel transcribed the original organ tones into etched glass - in opaque and transparent segments so that when the disk turned, it created an alternating pattern of light and dark.
Photodiodes were used to translate this into an electronic signal sound familiar? Sidebar - this development by Baldwin was not the first use of photo-electricity with a spinning glass disk to create musical tones. In the U. Army Signal Corp contracted with Baldwin to develop optical encoders, realizing that the company's technology could help in the pointing and tracking for radar antennas.
In Baldwin made their first experimental optical encoder. In , Baldwin's research resulted in an bit optical encoder which was the first optical encoder used in space. The following year, they produced the first optical encoder with an LED light source which was used in space as was highlighted in our post, "Who Made the First Optical Encoder". That same year the electronics division was incorporated as Baldwin Electronics, Inc.
The Gurley enterprise was established in but changed to W. Gurley in as the brothers, William and Lewis, both engineering graduates of Prensselaer Polytechnic Institute in New York teamed up to create products with technical innovations. The Gurley brothers had many different interests but most related in one way or another to measuring things - from electrical current to pressure, to weights and distances and angles.
In their factory the brothers created different departments, with each department making different components and then final assembly taking place in still another department after all components had been made. The area of technology by Gurley which is of most interest to those of us in the encoder field is the surveying field and their designing of theodolites - an optical instrument used for measuring angles.
The crosshairs used in the late 's for these surveying tools was the spider web filament from a black widow spider. The spider web filament was impregnated into the glass of their surveying instrument eyepieces. Gurley and other surveying equipment companies, actually had black widow spiders in their employment to provide them with the material they needed to create the crosshairs.
This technology was also used in the war effort in both world wars. One of the numerous industries developed in the US following the bombing of Pearl Harbor was the special defense plants or spider ranches supplying the spider silk for everything from bomber sights to periscopes and telescopes.