Flexing tiny muscles

Looking for a way to create small, affordable, silent motion, perhaps in a mobile, hand-held device? Then check out the NanoMuscle Motor from US-based NanoMuscle.

If you are looking for a way to create small, affordable, silent motion, perhaps in a mobile, hand-held device, then check out the NanoMuscle Motor from US-based NanoMuscle.

Unlike conventional motors that use electromagnetism as their source of motion, NanoMuscle Motors use Shape Memory Alloys (SMA) to produce movement. Shape Memory Alloys are a class of materials developed in the 1950s that can be fashioned into tiny wires that produce movement when an electric current is passed through them.

These wires behave like a ‘delayed action’ rubber band. In their initial state, they can be stretched out using very little force in much the same way that a rubber band can be stretched by pulling on it by hand. Unlike a rubber band, however, when the external force is removed, SMA wires do not return to their original length. Instead, they remain in the elongated state until an electric current is passed through them.

By carefully controlling the application of current to an SMA wire, the speed of contraction (return to original length) can be controlled, as can the final position. In other words, the SMA wire does not need to return all the way to its original length, but can be precisely stopped at any intermediate length.

Unlike an electromagnetic motor, the force output of a NanoMuscle Motor is independent of its speed. This means that it can be used without a gearbox, therefore reducing overall system size, complexity, and cost.

Electromagnetic motors produce rotary motion, yet over 60 percent of all small motor applications require linear motion. This makes it necessary for system designers to develop complex mechanical systems like lead-screws and rack-and-pinion gears to translate from rotary to linear motion.

In contrast, NanoMuscle Motors produce linear motion directly and thus avoid the need for such complex mechanisms, reducing product size and design time.

Electromagnetic motors produce electrical noise during operation, and their gearboxes produce acoustic noise. In contrast, the smooth, lifelike movement of a NanoMuscle is completely free of both electrical and acoustic noise.

The time it takes for an SMA wire to be extended and then contract back to its original length (cycle time) has historically been measured in minutes. In contrast, NanoMuscle Motors can cycle several times per second making them appropriate for a far wider set of applications.

The stretching and contracting of an SMA wire results in a much smaller displacement than a rubber band. Whereas a rubber band can be stretched by 10% to 20% of its original length, an SMA wire can only be increased in length by at most 4%. It is very difficult to integrate an SMA wire that is fifty micrometers in diameter yet 10 cm long into a real world application. However, a NanoMuscle Motor integrates that 10 cm of SMA wire into an actuator that can contract by 13% of its length, creating a much more versatile package.

Failure of fine SMA strands after only a few hundred cycles is not uncommon. NanoMuscle Motors, however, can be cycled continuously for millions of cycles. A variety of product grades allows designers to match the longevity of the NanoMuscle Motors to their application and budget.

NanoMuscle Motors will consistently move between their fully extended and fully contracted positions over their entire service life.

Over many cycles of extension and contraction, SMA wires stretch out and become slack. In future cycles, instead of pulling on the mechanism to which they are attached they simply take up the slack, making it very difficult to design mechanisms that contain SMA components without some form of a complex tensioning scheme. A NanoMuscle Motor does not have this problem and will contract by it’s rated distance each cycle.

It is also not uncommon to see SMA actuators with force outputs equivalent to a NanoMuscle Motor with current requirements measured in Amps. This high current requirement makes it very difficult to use SMA in battery powered and portable devices. The NanoMuscle Motor, however, was specifically designed for battery powered devices and only consumes milliamps of current. In fact, NanoMuscle Motors are two to five times more efficient than a DC motor solution of comparable size.

Due to competitive pressure, shorter and shorter development cycles are required. So system designers are seeking motion solutions that are easy to integrate and deploy. To meet this need, NanoMuscle provides a number of different ways to interface a NanoMuscle Motor with the host system.

Modern consumer devices are normally controlled by embedded microprocessors. For these microprocessors to produce movement they must connect to a motor via an interface of some kind. Electromagnetic motors consume large currents and can generate voltage spikes, both of which make them unsuitable for direct connection to a microprocessor. A complex system of high current transistors and diodes, referred to as an H-Bridge, is required to connect a digital microprocessor to an analog motor.

In addition, the microprocessor will need some kind of feedback so that it knows when to switch the motor off. For example, when a motor is being used to eject a CD from a CD player, some kind of limit stop detection is required.

The NanoMuscle Digital Interface (DI) embeds a layer of electronics and sensors inside the NanoMuscle Motor and provides control and status feedback in a form that can be directly connected to a microprocessor without the need for any additional external devices. This level of integration produces a lower cost and smaller solution that can be added to a digital system in less time than an electromagnetic equivalent.

The NanoMuscle Digital Interface also relieves the system designer of the need to know anything about the electrical operating characteristics of SMA. The interface will automatically adjust power levels to produce the requested movement, therefore optimizing power consumption, speed, and cycle life.

The Digital Interface has integral end stop detection that can signal the controlling microprocessor when the NanoMuscle Motor is either fully extended or fully contracted. Without these signals, the system designer would have to design limit switches into the mechanism, taking up valuable space, budget, and design time.

The NanoMuscle Digital Interface takes the form of a high-density Flex Circuit connector and is available in a number of configurations to suit individual applications.

A complete motion system created by combining a NanoMuscle Motor and a NanoMuscle Interface is called a NanoMuscle Actuator. This tight integration has many benefits.

For example, a NanoMuscle Motor with an integrated Digital Interface is much smaller and lower in cost than the motor, gearbox, rotary to linear converter, end stop sensor, and H-Bridge assembly that it replaces. Its highly integrated nature allows it to be deployed in a fraction of the time of a traditional electromagnetic motor-based system.

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