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Mini memory `muscle` challenges actuators

01 November, 2001

Mini memory `muscle` challenges actuators

A Californian company has come up with a new linear actuation technology that it claims is smaller and cheaper than traditional technologies such as small motors and actuators, and has many other advantages.

The company, called NanoMuscle, is hailing its technology as "the first major change in motor technology for more than 170 years". It predicts that its actuators will replace conventional linear drive systems in applications such as computer peripherals, medical devices, robots and toys.

The NanoMuscle technology is based on shaped memory alloys (SMAs) - materials that can be deformed and then restored automatically to their original shape by heating them or passing a current through them. Although SMA technology has been known about for decades, it has had few practical applications until now because of drawbacks such as its slow response, limited movements, short life and high power consumption.

NanoMuscle has spent millions of dollars on research to overcome these limitations since it was founded by a Scottish engineer, Rod MacGregor, three years ago. It claims that it has succeeded and has produced an electronically controlled form of SMA that is one-tenth the size of an equivalent DC-motor-based actuator, and costs one-twentieth as much.

The paperclip-sized actuators are formed from long, thin strands of a nickel-titanium SMA, attached to a stack of metal plates that slide over each other. Normally, the extension of an SMA wire is limited to just 4% of its length. NanoMuscle claims that its actuator can vary its length by 13%.

Applying a current to the wires allows them to expand or contract at a controlled rate and to stop at any point. Unlike a motor, the output force does not depend on the speed, allowing the device to be used without a gearbox, thus reducing its complexity, size and cost.

NanoMuscle has also developed a technique for embedding electronic circuitry and sensors inside its actuators to feed information directly to a controlling microprocessor. The circuitry adjusts the power levels to provide the required type of movement and to optimise the power consumption, speed and cycle life of the device. End-stop detection is built in, avoiding the need for separate limit switches.

For applications needing particularly precise movements, the actuators can be used with external position sensors to achieve closed-loop control.

Conventional miniature DC motors usually produce low torque at high speeds and therefore need gearboxes to deliver low-speed, high-force movements. If linear movement is needed - as happens in at least 60% of all small motor applications, according to NanoMuscle - complex mechanical systems such as leadscrews or ratchets are required, adding to the size, weight, complexity and cost of the actuator.

NanoMuscle`s actuators do not need gearboxes or other mechanical systems and are said to produce faster linear movements than are possible from miniature motors. The company suggests that the devices are up to five times more efficient than similar-sized motors, and have an energy density 4,000 times greater. An SMA actuator weighing just 1.1g can lift a 140g weight.

The actuators are also more versatile than solenoids, it contends. For example, the position, force or speed of a solenoid cannot be controlled in the same way as an SMA device. Also, solenoids do not produce their rated force equally along the length of their stroke - the available force drops by up to 90% after the first 5% of movement.

Among the other advantages that NanoMuscle claims for its technology are:

• freedom from electrical or acoustic noise;
• low manufacturing costs;
• the ability to cycle several times a second;
• a cycle life measured in millions of operations;
• a guarantee that the actuator will contract by its rated amount on every cycle; and
• the ability to operate on battery power.

Initially, NanoMuscle is producing its actuators with a maximum stroke of 4mm, but it is developing other versions with longer strokes. One way of increasing stroke lengths is to use levers.

The maximum nominal force rating is 70 gramme-force, but higher forces can be produced by applying more power - at the price of a shorter cycle life. The output can also be boosted by using several devices in parallel.

The power consumption of the devices depends on several factors including the load, the ambient temperature, and the speed of extension or contraction. The faster the movement, the greater the power required. The initial version draws about 470mA at 4V.

Smaller versions could be produced for sub-miniature applications without the scaling problems that affect motor-based devices. NanoMuscle envisages its technology filling a gap between microelectromechanical systems (MEMS) and small motor-driven applications.

The company also foresees a variety of volume applications for larger versions of its technology, for example, as disk ejection mechanisms for computers, zoom lenses for disposable cameras, and mirror adjusting systems for cars. A medical company is even looking at using NanoMuscle actuators in penile implants.

But perhaps the largest initial market will be in toys. Two large toy-makers, Hasbro and Jetta, have already signed deals to use the NanoMuscle technology in their products. Hasbro has an option to buy 10 million of the devices, and Jetta has taken a stake in the company.

NanoMuscle expects the first commercial products incorporating its technology to go on sale early next year.

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