Just a toy worth $ 100-a glass jar with a group of Eam swimming in Osaka, Japan.
Robot fish produced by ex company. Unusually, these plastic fish driven by internal forces swim in the water almost like real fish. They don't contain any mechanical parts: no motor, no transmission shaft, no gear, and even no battery. These fish can swim because their plastic internal organs are bending back and forth, just like fish have their own will. They are the first commercial products based on a new generation of improved electroactive polymers (EAPs), which can move under electrical stimulation.
Decades ago, engineers who made actuators or actuating devices had found artificial substitutes for muscles. In response to nerve stimulation, muscles can precisely control the force they exert by changing their length, such as blinking or lifting barbells. At the same time, muscles also show the property of constant proportion: the mechanism is the same for muscles of all sizes, and the same muscle tissue can give insects and elephants strength. Therefore, something like muscle may be useful for driving equipment that is difficult to manufacture electric motors.
EPAs claims to be the artificial muscle of the future. Researchers have been working ambitiously, hoping to find EPA-based alternatives for many contemporary technologies, and are not afraid to compete with natural objects for their inventions. A few years ago, several people, including Yoseph Bar-Cohen, a senior scientist from the Jet Propulsion Laboratory (JPL) in Pasadena, California, challenged the research group of electroactive polymers to stimulate people's interest in this field: a competition was held to see who could first build an EAP-driven mechanical arm and must win a one-on-one arm wrestling competition with human arms. Then, they began to look for the support of sponsors and gave the winners cash as a reward.
At present, the most promising work may be the ongoing research of Stanford Research Institute (SRI), which is located in a non-profit contract research laboratory in Menlo Park, California. SRI management hopes to set up a company (tentatively named Artificial Muscle Integrated Company) with an initial investment of $4 million to $6 million to commercialize its patented EPA technology within a few months. Even now, SRI still has six R&D contracts. Party A includes American government and companies in toy, automobile, electronics, mechanical products and footwear industries. SRI is trying to bring artificial muscle to market as soon as possible.
What is the goal of this new company? I just want to replace countless motors and many other actuating devices with smaller, lighter and cheaper products and SRI's new actuators. Phillip von Guggenberg, director of its laboratory business development department, pointed out: "I think this technology is a good opportunity to realize the revolution in the field of mechanical actuation. We hope to popularize this technology and make it something you can buy in hardware stores. "
Artificial muscle material
Since the mid-1960s, Bar-Cohen has been an informal coordinator of ever-changing international EAP researchers. Back in its early days, "the electroactive polymer materials I read in scientific papers were not as magical as advertised," he recalled, smiling cunningly. "When I got funds from NASA to study this technology, I had to find out who worked in this field in order to find some inspiration." In just a few years, Bar-Cohen has mastered enough knowledge to help hold the first scientific seminar on this topic, start publishing EAP newsletter, publish EAP website, and write two works on this emerging technology.
In a low research building in the courtyard of the Jet Propulsion Laboratory (JPL), the test bench is filled with prototypes of various actuating devices and testing devices, and Ba-Cohen begins to review the history of this field that he has long known. He said: "For a long time, people have been looking for ways to move animals without motors, because motors are too heavy for many applications. Before the advent of EPAs, the standard alternative technology for motors was piezoelectric ceramics, which was once a hot topic of research. "
In piezoelectric materials, mechanical stress will lead to crystal polarization, and vice versa. Stimulating this material with electric current will deform it; Electricity can be produced by changing its shape.
Ba-Cohen picked up a small light gray plate from an experimental platform and said, "This plate is made of PZT (lead zirconate titanate)." He explained to us that the current caused the piezoelectric PZT to contract or expand, and the amplitude was less than 1% of its total length. The deformation is small, but useful.
In the next room, Bar-Cohen showed a foot-long percussion drill driven by PZT plates, which he was developing with JPL colleagues and engineers from Cybersonics. He said, "There are a bunch of piezoelectric plates in this cylinder. When alternating current is activated, the pile of plates will beat the drill bit at supersonic speed, and the drill bit will jump up and down at high speed, thus drilling into hard rock. " On the other side are piles of stones that have been drilled deep holes.
As an example, the drill shows the effectiveness of making actuators with piezoelectric ceramics, which is really impressive. However, in many applications, the expansion range of electroactive materials is required to exceed 0.0%.
Reaction of plastics to electricity
Bar-Cohen tells us that polymers that change shape in response to current can be divided into ionic and electronic types, and their advantages and disadvantages are just complementary.
The working principle of ionic electroactive polymers (including ionic polymer gels, ionic polymers such as metal composites, conductive polymers and carbon nanotubes) is electrochemistry-that is, the movement and diffusion of positive and negative ions. They can be directly driven by batteries, because even a single-digit voltage can make them bend greatly. The disadvantage is that ionic EAP usually must be wet, so they should be sealed in a flexible thin layer. Another major disadvantage of many ionic electroactive polymers is that "as long as the current is switched on, the material will move continuously," Barcohen pointed out, adding: "If the voltage exceeds a certain value, electrolysis will occur, which will cause irreparable damage to the material."
On the contrary, electroactive polymers (such as ferroelectric polymers, dielectrics, electrically insulating rubber and electrostrictive grafted rubber) are driven by electric fields. They need a relatively high voltage, so they will produce an uncomfortable electric shock. However, in return, electronic EPA can respond quickly and transmit powerful mechanical force. They don't need a protective layer and can maintain a certain position with a small current.
SPR artificial muscle material belongs to electronic EAP type. Its successful development has gone through a long and tortuous road, which is somewhat accidental and can be called a classic example of whimsical technological innovation.
Charge the rubber
Ron Pelrine, head of the SRI team, said: "After signing a contract with Japan for the Japanese micro-machine program, Stanford Research Institute began to study artificial muscles from 1992." He used to be a physicist, but now he is a mechanical engineer. Japanese officials are looking for a new micro-driver technology. Several SRI researchers began to look for an actuating material with similar characteristics to natural muscles in mechanics, stroke (linear displacement) and strain (displacement per unit length or unit area).
"We looked at many promising activation technologies," Pelrine recalled. However, they finally chose electrostrictive polymers, which Jerry Scheinbeim of Rutgers University was studying at that time. The hydrocarbon molecules in this polymer are arranged in a semi-lattice, and this crystal array has piezoelectric-like characteristics.
When in an electric field, all insulating plastics (such as polyurethane) will shrink in the direction of the power line and expand in the direction perpendicular to the power line. This phenomenon is different from electrostriction and is called Maxwell stress. Pelrine said: "This phenomenon has long been known, but it has always been regarded as a very troublesome effect."
He realized that polymers softer than polyurethane are easier to squeeze under electrostatic attraction, so they can provide greater mechanical strain. By testing soft silica gel, SRI scientists quickly proved that its strain is between 10 and 15%, which is very satisfactory. After further research, this figure can be increased to 20 ~ 30%. In order to distinguish this new actuator material, silicone and other softer materials are named as electrically insulating elastomers (also known as electric field activated polymers). )
After identifying several promising polymer materials, the team focused on developing specific equipment application details for the rest of 1990. At that time, the new external financial support and research direction of the SRI research team were provided by the Advanced Research Projects Agency (DARPA) and the Naval Research Office of the US Department of Defense, and its director's main interest was to use the technology for military purposes, including small reconnaissance robots and light generators.
As rubber began to show greater tension, engineers realized that electrodes must also be expandable. Ordinary metal electrodes cannot be extended unless they are separated. Pelrine mentioned: "At first, people don't have to worry about this problem, because the strain provided by the materials they studied is only about 1%." Finally, the research team developed a flexible electrode based on filling carbon particles in rubber matrix. He pointed out: "Because electrodes and plastics expand together, they can maintain an electric field between the whole active area." SRI applied for a patent for this concept, which was one of the keys of artificial muscle technology later.
Pelrine was anxious to show us. He took out a 15 cm square thing that looked like a photo frame, and the plastic clips on both sides were tight because of expansion. "Look, this polymer material is very malleable," he said, and pressed a finger into its transparent film. "In fact, it is a kind of double-sided adhesive, and the price of a large roll is very cheap." On both sides of the middle clip are black and nickel-sized electrodes, which are connected by wires.
Palin unscrewed the power control knob. Immediately, the pair of black circular electrodes began to expand, and the diameter increased by a quarter. When he turned the knob back to its original position, the electrode immediately contracted to its original state. He grinned and repeated the operation several times, explaining: "Basically, our equipment is a capacitor, that is, two parallel charging plates, which are electrically insulated. When the power supply is turned on, positive and negative charges are respectively accumulated on the opposite electrodes. The electrode plates attract each other and squeeze the insulating polymer in the middle, and the area of the polymer is expanded. "
Although several promising materials have been identified, it is really a challenge to achieve acceptable performance in practical equipment. However, a series of breakthroughs made by the group in 1999 have aroused great interest from the American government and industry. Through observation, it is found that the properties of polymer materials will be greatly improved by pre-stretching before electrical activation. Roy Kornbluh, another member of the team and engineer, recalled: "We started to notice a sweet spot, and then we could get the best performance. No one knows the exact reason, but the pre-stretched polymer can improve the breakdown strength (resistance of current passing between electrodes) by 100 times. " The magnitude of strain increased by electrical activation is similar. Although the reason is not clear, Pei Bingqi, a chemist at SRI, thinks: "Pre-stretching can locate the molecular chain along the plane expansion direction, and the material makes it more difficult in this direction." In order to obtain pre-stretching effect, the actuator equipment of SRI adopts external support structure.
The second key discovery benefited from the researchers "testing every extensible material we know, which we call Edison method," Pelrine told us happily. In order to find a suitable material for electric lamp filament, Thomas Edison made systematic experiments on various materials. In my family, in order to prevent my toddler from taking things everywhere, we lock the refrigerator with a door lock made of polymer material. As the children grow up, we no longer need to lock anything, so we took the lock away. Because it is made of extensible material, I decided to test its strain performance. Surprisingly, its performance is excellent. " It is not difficult to trace the source of the lock and analyze its composition. Finally, this mysterious polymer "turned out to be polyacrylic rubber, which can provide great strain and energy output, and the linear strain is as high as 380%." These two developments enabled us to begin to apply electrically insulating rubber to actual actuator equipment. "The researcher said.
Artificial muscle dreams come true
The general research methods of SRI group are flexible, including many designs and even different polymers. As Pei Bingqi said: "This is a piece of equipment, not a piece of material." According to Pelrine, the team was able to produce activation effects with different polymers, including acrylic and silicone. Even natural rubber can have a certain effect. For example, in the extreme temperature environment in the outer space, it is best to use silicone plastic for artificial muscles. It has been proved that this material can work in a vacuum environment of-100 degrees Celsius. For applications requiring greater output force, more polymer materials may be needed, or multiple devices may be connected in series or in parallel.
Von Guggenberg, a member of SRI, estimated: "Since electrically insulating rubber can be bought in stock, and we only use a few square feet of material in each device at most, actuators will be very cheap, especially for mass production."
The voltage used to activate the electrically insulating rubber actuator is relatively high, usually 1 to 5 kV, so the device can work at very low current (generally speaking, high voltage means low current). The actuator can also use thinner and cheaper wires and can be kept relatively cold. Pelrine said: "When the electric field stops and current flows through the gap, higher voltage will produce greater expansion and stress."
Cohen Blue commented: "High voltage is a problem, but it is not necessarily dangerous. After all, fluorescent lamps and cathode ray tubes are high-voltage devices, but no one will worry. The bigger problem is that mobile devices need high voltage, because batteries are usually low voltage, so extra transformer coils are needed. " In addition, at Penn State University, Zhang Qiming and his team have been trying to reduce the activation voltage of some electrostrictive polymers by combining them with other substances to form compounds.
When asked about the durability of electrical insulating rubber, Von Guggenberg admitted that more research was needed and confirmed a "reasonable sign" that they should continue to work for a long time before they can be used commercially. "For example, the equipment we run for a customer can generate 5 ~ 10% strain and cycle 100000 times." The other equipment can produce 50% area strain, and the number of cycles is 1 10,000.
Although the artificial muscle equipment is much lighter than the corresponding electric motor (the density of polymer itself is close to that of water), SRI is still trying to reduce its mass by reducing the necessary external pre-strain equipment. For example, Pei is experimenting with chemical methods to eliminate the need to use relatively heavy frames.
Commercialization of products
After developing the basic principles, the SRI research team immediately continued to study a series of application concepts:
Linear actuator (linear actuator). In order to make what they call a spring coil, engineers wrap several layers of prestrained layered electrical insulating rubber sheets around the spiral spring. The tension spring supports circumferential pre-strain, while the longitudinal pre-strain of the rubber membrane keeps the spring in compression [see the illustration on page 48]. Electricity makes the thickness of rubber film compress and relax simultaneously in the longitudinal direction, and the equipment is elongated. Therefore, the spring drum can generate a strong force and a large stroke in the compression package. Kornbulu reported that automobile manufacturers showed interest in this equipment, hoping to use it to replace many small motors in automobiles, for example, in the position control of electric seats and the valve control of high-efficiency silent engines.
Bending roller. Using the same basic spring drum, engineers can connect electrodes and make two or more different parts that work independently around the circumference of the drum. Electrical activation of a component can lengthen one side of the cylinder, so that the whole cylinder bends to the other side [see illustration on page 48]. The device based on this design can complete many complex actions that traditional motors, gears and linkage devices can't. Its possible uses include steerable medical catheters and so-called snake robots.
Push-pull actuator. Groups of electrically insulating rubber diaphragm pairs or spring cylinder pairs can be arranged in a "push-pull" structure, so that they can interact to respond in a more linear way (one input produces one output). The shuttle voltage from one device to another can change the position of the whole device group back and forth; Activating two devices at the same time can fix the device group at an intermediate point. In this way, the actuator can work like biceps brachii and triceps brachii which control the movement of human arms.
Loudspeaker. Stretch the electrical insulating film on the frame with the opening. The diaphragm rapidly expands or contracts according to the applied electric signal, so it can emit sound. This configuration can be made into a portable and inexpensive speaker, and its vibration medium includes a driver and a sound panel. The current design shows high performance in the intermediate frequency and high frequency range. However, this loudspeaker configuration is not optimized as a woofer, although it works well in the low frequency range [see the illustration on page 49].
Pumps The design of the electrically insulating rubber diaphragm pump is similar to that of a low-frequency speaker, but the engineer only adds a fluid chamber and two one-way check valves to control the flow of liquid. Artificial muscle is very suitable for providing power for microfluidic pumps, for example, in laboratory-on-a-chip equipment that is highly valued by medicine and industry.
Sensor. As far as its nature is concerned, all SRI electrical insulating rubber equipment will have capacitance changes when it is bent or stretched. Therefore, it is possible to manufacture a sensor that is compliant and works at low pressure. Kornbluh said that the SRI team is negotiating with a car manufacturer to use this sensor to measure the tension of seat belts. He said that the sensor can also be used to measure the tension of fabrics or other materials, such as optical fibers, belts or clothes.
Surface features and smart surfaces. If the polymer is imprinted with electrode patterns, various shapes can be carved on the surface as needed. This technology can be used to actively camouflage the fabric, change the reflectivity as needed, or be used as a device to make "braces" to improve the aerodynamic drag characteristics of the wing surface [see the illustration on page 50].
Generator. Because this material can act as a soft capacitor, it can be used to manufacture generators and energy collectors with variable capacity. DARPA and the US military invested in the development of a "heel" generator, which is a portable energy source that soldiers or other battlefield personnel can use instead of batteries to provide electricity for electrical equipment. Using this equipment, which is still in the development stage [see the picture on this page], an ordinary person can take one step per second and generate about 1 watt of power. Von Guggenheim-Berg said that this concept has attracted the interest of footwear companies. Similarly, this device can also be installed on a backpack belt or a car suspension device. In principle, this method can also be applied to wave generators and wind power generation equipment.
SRI researchers recently tested a more radical concept "polymer engine". Propane fuel burns in the combustion chamber, and the pressure generated by the combustion products will deform the electrically insulating rubber diaphragm, thus generating electric energy. This design may eventually lead to the emergence of high-efficiency micro-generators of centimeters or less.
However, it will take time for a truly marketable product to appear. Von Guggenberg pointed out: "At present, we are making all turnkey equipment, which can be handed over to engineers so that they can study them, accept and appreciate this technology. I hope that in the future, all engineers can consider this technology when designing new products. "
Bar-Cohen said that he was deeply impressed by the progress made by SRI in actuator technology. But success also brings some problems: arm-wrestling competition. He joked: "I had hoped that in about 20 years, someone could develop a mechanical arm comparable to human arms." Now SRI says they want to build one, and we haven't raised the bonus yet! "