Auxetic materials are a class of meta-materials which exhibit negative Poisson’s Ratio (Meta-materials are a class of man-made materials that have been specially and artificially engineered to contain small inhomogeneities which alter the properties on a macroscopic level). They have been known for over a hundred years but have only gained attention in recent decades. They can be single molecules, but more often they consist of an engineered material with a particular structure on the macroscopic level. Auxetic materials can occur in nature, but they are very rare. For example, some rocks and minerals demonstrate auxetic properties as does the skin on a cow’s teats.

Auxetics are created by modifying the macrostructure of the material so that it contains hinge-like features which change shape when a force is applied. If a tensile force is applied the hinge-like structures extend, thus causing lateral expansion. If a compressive force is applied the hinge-like structures fold even further causing lateral contraction.

A common analogy used to describe the behavior of auxetic materials is to consider an elastic cord with an inelastic string wrapped around it. When a tensile force is applied the inelastic material straightens at the same time as the elastic cord stretches, thus effectively increasing the volume of the structure.
Auxetic materials are often based on foam structures and as such they have a relatively low density. It is this open cell structure which can be modified to give the desired properties.

The unusual properties of auxetic materials mean that they are relatively resistant to denting. When an auxetic is hit the compression caused by the impact results in the material compressing towards the point of the impact, thus becoming much denser and resisting the force. Auxetic materials are also more resistant to fracture; they expand laterally as a force is applied and this closes up potential cracks in the material before they start to grow. The main drawback of these materials is that they are often too porous, not dense enough or not stiff enough for load bearing applications and when these properties are adjusted the auxetic behavior tends to be reduced. Applications of these materials therefore often rely on a compromise of properties.

Auxetic materials can essentially be made in two different ways. In the top-down approach everyday polymers are manipulated to give the desired structure and properties. In the bottom-up approach the material is built up from scratch, molecule by molecule, allowing them to be engineered on a very small scale. In both cases the objective is to create a repeating pattern of building blocks or cells which contain the necessary hinge-like features.

There have been many suggested uses for auxetic materials, but most of these have yet to come to fruition commercially. These materials can be difficult to process on a large scale, making industrial manufacture difficult. Some potential areas for use are described below:

Biomedical applications
Many of the materials that are currently used in medical applications can be processed to exhibit auxetic properties. It has been suggested that auxetic materials could be used to dilate blood vessels during heart surgery. A piece of auxetic foam, possibly made from PTFE would be inserted into the blood vessel and then tension applied to this to cause lateral expansion and open out the vessel. Auxetics could also be used in surgical implants and prosthesis and for the anchors used to hold sutures, muscles and ligaments in place.

Filters
Traditional filters can be incredibly difficult to clean, leading to them being thrown away prematurely. A filter made from an auxetic foam could be cleaned much more easily by simply applying a tensile force to open up the pores. Once clean the force can be removed and the filter refitted.

Auxetic fibers
This is one area which does show real potential for exploitation. The key here is the development of a continuous process for developing auxetic materials in the form of fibers. The resulting fibers could be used in monofilament or multifilament form and could be knitted or woven together to make cloth. It has been suggested that these fabrics could be used in crash helmets, body armor and sports clothing where their dent and fracture resistance would be exploited.

Auxetic materials could also be added to metallic materials such as steels to create composites with improved resistance to cracking under shear strain (twisting).

When a machine must provide unusual displacement or speed characteristics, don’t overlook noncircular gears. These oddly-shaped gears can fulfill several types of special motion requirements, and one of them may be the best solution for your application.

Why thease gears?

Various mechanical systems, such as cams and linkages, can provide special motion requirements, but noncircular gears often represent a simpler, more compact, or more accurate solution.

Servo systems may also be able to do the job, and they can be programmed to handle changing or complex functional requirements. But they are usually more expensive. Also some companies lack the expertise to solve problems with servo systems. Moreover, noncircular gears do offer limited ability to handle changing functional requirements. For example, you may be able to change an output function by adjusting the phase relationship between two mating noncircular gears.
​
Some of the more common requirements handled by noncircular gears include converting a constant input speed into a variable output speed, and providing several different constant-speed segments during an operating cycle. Other applications require combined translation and rotation or stop-and-dwell motion. Here are a few examples that may stimulate some ideas about your own applications.

Variable speed
Several types of noncircular gears generate variable output speeds, particularly elliptical gears. Other, less commonly used types are triangular and square gears.

Elliptical: 

An ellipse is defined by a set of points such that the sum of the distances from two fixed points on its long axis to any point on the perimeter is a constant. This enables a set of like elliptical gears to run at a constant center distance but deliver an output speed that changes as they rotate. Elliptical gears come in two basic types: unilobe, which rotates about one of the fixed points, and bilobe, which rotates about the center. The speed-reduction (or increasing) ratio of these gears varies from 1/K to K, where K depends on the gear geometry, during each cycle of rotation. Practical values of K range up to 3 for a unilobe and up to 2 for a bilobe gear.

The largest radius of the driving gear mates with the smallest radius of the driven gear so that output speed is at its maximum. As the gears rotate, the radius of the driving gear gradually decreases and that of the driven gear increases, so speed decreases for the first ¼ revolution. Then the speed increases for the next ¼ revolution, etc. These periods of increasing or decreasing speed occur four times per revolution.
Elliptical gears are commonly used in packaging and conveyor applications.

Triangular:

A pair of triangular gears also converts constant input speed into alternating output speed. However, they have three lobes, or high points on the perimeter, rather than the two lobes in elliptical bilobe gears. As a result, triangular gears deliver six periods of speed increase or decrease per revolution, rather than four.

Square:

Gears that are square provide yet another way to produce varying output speed. These gears have four lobes, so they produce eight periods of speed increase or decrease per revolution.
Both triangular and square gears are limited to a smaller range of speed ratios than with elliptical gears.

Constant-speed segments

Where an application requires several constant-speed periods within a cycle, multispeed gears may be the answer. These gears make the transition between speeds by using special function segments on the gear perimeter, usually sinusoidal, between the constant-speed sections. Typically, the input and output gears are different.

Translation and rotation

For applications requiring both translational and rotational motion, certain gears serve as cam substitutes. These cam gears are often used in labeling machines. The cam gear duplicates the shape of a part to be labeled and a cam-following rack carries the labeling device at a constant surface speed. In welding applications, a cam gear simulates the shape of a part to be welded and a follower carries a welding torch at a constant speed to ensure uniform weld application.

Stop-and-dwell motion

Some machines must provide either stop-and-dwell, or reverse motion with a constant input speed. This is achieved by combining noncircular gears with round gears and a differential (epicyclic gear train). Using round gears with different ratios gives such an arrangement the flexibility of providing either stop-anddwell or reverse motion.

Stop-and-dwell motion is common in indexing mechanisms, where gears are used rather than cam or Geneva mechanisms. Reverse motion is required where a transfer device must operate between two locations. Here, a noncircular gear arrangement is usually simpler than a commonly used linkage assembly. Despite their flexibility, such arrangements tend to be expensive.

Typical applications

These examples show how noncircular gears solve manufacturing problems.

Sealing head: 

A heat-sealing device seals the tops of containers on a constant- speed conveyor. It must maintain contact for a short period of time (less than 1 sec) without slippage between the sealing head and container. A traditional approach is to install an indexing device that stops the conveyor while sealing occurs. Or, if the conveyor can’t be stopped, a cam and linkage device or electronic servo is used.

A simple solution involves mounting the sealing head on a crank mechanism driven by elliptical gears which let the head stay in contact with the container momentarily. A varying output speed provided by the gears lets the sealing head follow the container without slippage and then return for the next container.

Rotary cutoff knife: 

A rotary knife cuts material on a conveyor to different lengths while the conveyor runs at a fixed speed. To cut material of different lengths without changing the conveyor speed would require using different knife sizes (diameters) and changing the knife speed.

One solution is to drive the knife with elliptical or multispeed gears. Changing the input gear speed with respect to conveyor speed changes the length of material that is cut. Adjusting the phasing between gears relative to the cutoff point causes the knife to match the conveyor speed.

Conventional robots and machines are made of rigid materials that limit their ability to elastically deform and adapt their shape to external constraints and obstacles. Although they have the potential to be incredibly powerful and precise, these rigid robots tend to be highly specialized and rarely exhibit the rich multifunctionality of natural organisms.

However, as the field of robotics continues to expand beyond manufacturing and industrial automation and into the domains of healthcare, field exploration, and cooperative human assistance, robots and machines must become increasingly less rigid and specialized and instead approach the mechanical compliance and versatility of materials and organisms found in nature. As with their natural counterparts, this next generation of robots must be elastically soft and capable of safely interacting with humans or navigating through tightly constrained environments. Just as a mouse or octopus can squeeze through a small hole, a soft robot must be elastically deformable and capable of maneuvering through confined spaces without inducing damaging internal pressures and stress concentrations.

In contrast to conventional machines and robots, soft robots contain little or no rigid material and are instead primarily composed of fluids, gels, soft polymers, and other easily deformable matter. These materials exhibit many of the same elastic and rheological properties of soft biological matter and allow the robot to remain operational even as it is stretched and squeezed. Because of the near absence of rigid materials and its similarities to natural organisms, soft robots may be considered a subdomain of the more general fields of softmatter engineering or biologically inspired engineering. However, whereas these existing fields can be defined by their scientific foundations in soft-matter physics and biology, respectively, the emerging field of soft robotics remains open and free of dogmatic restrictions to any constrained set of methods, principles, or application domains. Instead, soft robotics represents an exciting new paradigm in engineering that challenges us to reexamine the materials and mechanisms that we use to make machines and robots so that they are more versatile, lifelike, and compatible for human interaction.

The promise of soft robots is perhaps best realized in environments and applications that require interaction with soft materials and organisms and/or the artificial replication of biological functionalities. For example, whereas industrial robots typically handle metals, hard plastics, semiconductors, and other rigid materials, medical robots will primarily interact with soft materials such as natural skin, muscle tissue, and delicate internal organs. Likewise, biologically inspired robots for field exploration and disaster relief will often encounter easily deformable surfaces like sand, mud, and soft soil. To prevent the robot from penetrating into the surface and causing damage or mechanical immobilization, the forces transferred between the robot and surface must be evenly distributed over a large contact area. This requires compliance matching—that is, the principle that contacting materials should share similar mechanical rigidity in order to evenly distribute internal load and minimize interfacial stress concentrations.

One measure of material rigidity is the modulus of elasticity, or Young’s modulus—a quantity that scales with the ratio of force to percent elongation of a prismatic bar that is stretched along its principal axis. Young’s modulus is only defined for homogenous, prismatic bars that are subject to axial loading and small deformations (< 0.2% elongation for metals) and thus has limited relevance to soft robots and other soft-matter technologies that have irregular (nonprismatic) shape and undergo large elastic or inelastic deformations. Nonetheless, Young’s modulus is a useful measure for comparing the rigidity of the materials that go into a soft robot. Most conventional robots are composed of materials such as metals and hard plastics that have a modulus of greater than 109 Pa = 109 N/m2. In contrast, most of the materials in natural organisms, such as skin and muscle tissue, have a modulus on the order of 102–106 Pa. That is, the materials in natural organisms are 3–10 orders of magnitude less rigid than the materials in conventional robots. This dramatic mismatch in mechanical compliance is a big reason why rigid robots are often biologically incompatible and even dangerous for intimate human interaction and rarely exhibit the rich multifunctionality and elastic versatility of natural organisms.

There are various materials used to build soft robots. Electroactive polymers (EAP) are often used as artificial muscles. Whole robots are usually made of multiple of these muscle-like actuators. EAPs are a branch of polymers that can contract and bend under applied voltage. They have lot of features that suit them for soft robotics. They can be made in any shape, are elastic, low weight and have large actuation strain. Also, very useful is an ability of EAPs to sense how much stretched they are. This is important when calculating (or ‘sensing’) the shape or the state of a robot’s body.

Other kind of soft actuators are the pneumatic artificial muscles (PAMs). These are basically soft tubes with inflexible fiber mesh reinforcement in the wall of the tube or on its surface. When air is pumped in, the tube expands in its diameter and shrinks in longitudinal axis because the fiber cannot stretch. There is also an extensor PAM actuator in which the mesh is placed differently, so the tube extends only in longitudinal axis when pressurized.
​
controlling soft robots is rather hard. First part of the problem is to determine the actual position and shape of a robot. In case of hard robots it is quite easy to determine its shape, as we know where the DoFs (joints) are. We only need to know the angle of each joint and then calculate the position. Soft robots have infinite number of DoFs, however, there is always a finite number of sensors and actuators (muscles) in their bodies. Therefore it is impossible to have information about the state of each of the DoFs. Possible solution is determining the position of robot’s body parts from outside, using visual information, when the robot would have camera to see the actual situation. Next part of the problem is determining what actions the robot should do to get in the desired positon. To determine the shape of a robot from internal and external forces, multiple physical models have to be employed, namely models for solid and fluid mechanics, kinematics, electro-mechanics, thermodynamics and chemical kinetics. Simulating behavior of a continuum is, however, complicated problem demanding a lot of computational power. Sensing, control and path planning are main problems of today’s soft robotics.

Q: What is a multi-axis sensor? 
A: A multi-axis sensor is one that can measure forces happening in more than one plane as, for example, measurements in the x and y directions. Some multi-axis sensors can measure not just directional forces but also moments, rotational forces about an axis.

Q: How many axes can multi-axis sensors typically measure? 
A: Multi-axis sensors can measure up to six axes. A six-axis device would measure x, y, z directions and moments.
Q: What applications measure forces in more than one planner direction? 
A: Many such applications try to determine a vector load that must be described in terms of x, y, and z positional coordinates. Similarly, multi-axis sensors help resolve a direction or gauge inputs in multiple directions.

Q: What types of outputs can be expected? 
A: Depending on the application and the magnitude of the force, typical outputs are in units of mV/V analog or they are converted to a digital output that follows a standard protocol such as FireWire or CANbus. The mV/V electrical output refers to sensor excitation at the rated load, torque or pressure. For example, the voltage output of a 2 mV/V load cell at 100 lb-rated capacity using 10-V excitation will be 20 mV at 100 lb or 0.2 mV for each pound of applied load.

Q: When do we use multi-axis sensors instead of multiple single-axis sensors? 
A: A multi-axis sensor can be smaller, and occupy a smaller space envelope than multiple single-axis sensors. Moreover, its connection scheme can be simpler as well. These factors tend to reduce material costs.

Q: What choices are there in term of size and capacities? 
A: It is possible to find multi-axis sensors able to detect loads of only a few grams. It is also possible to find units that can respond to loads of several thousand pounds without being crushed.

Q: What is crosstalk in multi-axis sensors? 
A: When a load is applied in only one direction and there is an output in the other axes, there is said to be crosstalk between the channels. Crosstalk levels are part of the technical specs for multi-axis sensors. They are given as a percent of the channel output. Crosstalk interdependencies among force and moment axes can be compensated mathematically.

Q: How is nonlinearity defined for multi-axis sensors? 
A: Nonlinearity is the maximum deviation of the calibration curve from a straight line drawn between the no-load and rated load outputs, expressed as a percentage of the rated output and measured with an increasing load.

Q: What is multi-axis sensor hysteresis? 
A: Hysteresis is the maximum difference between the transducer output readings for the same applied load. One reading comes from increasing the load from zero and the other from dropping the load from the rated output. Hysteresis is usually measured at half the rated output and expressed in percent of rated output.

Q: How are multi-axis sensors mounted? 
A: Because multi-axis sensors measure both moments and forces, they are sensitive to being at even a slight angle to the mounting surface. So mounting procedures must prevent even slight misalignments. Mounting surfaces must be rigid enough so that they do not warp. The general rule is that the thickness of the connection elements should be about one-third that of the transducer height. (It’s best if the contact surface deflects less than 0.005 mm under load.) The surface must also be paint-free and made of steel with a minimum hardness of 40 HRC. The stainless steel measuring body (mechanical interface) of the transducer has a minimum hardness of 42 HRC. Surface flatness should be better than 0.05 mm and surface roughness ≤ Ra 1.6. Ideally, the surface should be ground. The transducer should be centered on the structural elements and aligned using positioning pins. The angle error or the positioning tolerance should be kept below 0.1°. Finally, multi-axis sensor mounting screws should be tightened down diagonally in sequence up to the full tightening torque to keep the sensor lying flat on the mounting surface.

​Information and expertizes have spread all over the internet in form of videos and articles, showing how easily anyone can print the products simply through CAD modeling and pushing the print button. But, if you are into design and manufacturing, you’re probably guessing that there’s more to it.

And you’re right; there are few important things to know more about the additive manufacturing technology, if you’re seriously planning to develop products through this technology in near future.

​Software technology plays a crucial role in additive manufacturing process, and is constantly evolving to compliment the advancing 3D printing capabilities of the printer. The use of software spans across the additive manufacturing lifecycle right from sourcing ideas, designing the model and providing formatted data to printers, to monitoring and managing the printing process. It serves as an important link, enabling interfaces between computers and printers to allow entire 3D printing ecosystem to function efficiently.

In general, printing a part require 4 different software to ensure accuracy, reduced cost and quality. Also, special software packages are also required to convert solid geometries to lattice structures along with tools to perform simulations.

The journey from idea to the additive manufactured final part begins with sourcing the model either through rapidly growing 3D libraries or through 3D scanning techniques or through custom designs. This is followed by the design process where an exact 3D model of the product is prepared that will be printed, considering proper dimensions and geometrical accuracies. The next step is to optimize the geometry using software to suit the printing process and reduce material wastage. Finally, the software is also required to actually print the part and monitor the process to make print runs successful.

​Unlike conventional modeling processes, developing 3D models for additive manufacturing requires a different approach. One of the important things to consider is the resolution of the geometry; too high resolution will consume more time to load and simultaneously print the part. On the contrary, a low-res model would develop prototype with poor quality. Moreover, the geometry to be developed is also material-sensitive.

As an example, products to be printed in plastic would require the dimensions of the holes in the design to be resized, as they would expand or contract during heating and cooling applications. Also, the geometry to be printed has to be thoroughly inspected for any open spaces and must be converted to “watertight”, to avoid errors in printing.

​Design optimization is a key to successful and cost effective utilization of additive manufacturing techniques. The purpose of using this advanced manufacturing technology is to reduce development cost and time, and thus, reducing material wastage during printing process remains a vital strategy. This requires the design engineers to understand the process of printing that requires rafts and supports to build the desired structure. Engineers have to identify a design that uses minimal support material that can also be removed easily once the part gets printed. Also, the design engineer must identify a balance between part density, strength and surface finish, and accordingly choose the material, printing speed and the used additive manufacturing technolgy.

To summarize, Right from sourcing the model, through designing and optimizing to monitoring printing process, the role of software is important across every step in the prototype or product development. Modeling methods require a good knowledge on the printing process, material to be used and the printing technology adopted.

Moreover, the need to optimize the design and apply lattice structures instead of solid geometry should be considered to reduce material consumption while maintaining the required strength of the product being developed. Successfully implementing rapid prototyping to gain the benefits of cost reduction and faster development schedules require manufacturers and engineers to adopt right processes and software technologies.

As such, 3D printing technology isn’t as easy as it seems. There’s so much more to it than simply creating a CAD model and pushing the print button.