Anyone who has ever seen inside a personal stereo or simply pressed the aeject' button on a camcorder will be impressed by the miniaturised mechanisms therein. It seems that there is no limit to the desirability of small, powered, devices, whether they are consumer products such as these, toys, or incorporated within less obvious applications such as door locks. While the cost and size of miniature electric motors has continued to drop, there are some tasks for which these are not ideal.

In order to meet customer demand or simply to create a competitive advantage, designers are seeking means of actuation that are small, lightweight, controllable, quiet and nergy-efficient. In addition, they must be reliable, affordable and have an acceptable operating life.
One type of device that appears to meet these requirements is the Nanomuscle, from the
USA-based company of the same name. Both linear and rotary actuators are available, said to be suitable for replacing conventional electromechanical motors and solenoids in many applications, and at a price that makes them feasible for use in mass-produced products. It is claimed that the devices have a nominal lifetime of one million cycles.
Based on shape memory alloys (SMAs), the standard devices incorporate digital electronics to monitor and adjust the motion, making them easy to integrate with a CMOS-based micro controller using a PWM (pulse width modulated) signal.
In simple terms, passing a current through the shape memory alloy causes it to heat up and contract; when there is no current flowing, the alloy cools and can return to its original shape under the influence of another force, typically from a spring.
A Nanomuscle consists of a large number of SMA devices, each of which is only a few nanometres across. These devices are assembled into thin wires with a diameter of approximately 50micrometers. Several of these wires are combined to produce a macro-scale linear motor with a stroke measured in millimetres.
At the bottom of the Nanomuscle scale hierarchy, each nanometre scale device is actually a single crystal of NiTi (nickel and titanium) that has been treated so as to exhibit the shape memory effect.
The Nanomuscle RS-70-CE is a bidirectional rotary actuator that provides up to 60degrees of smooth movement with a constant torque of 12g-cm from an operating voltage of 2.5 to 6.4V (Fig.1). No gearbox or clutch is required, which saves component count, space, and cost. Designed as a complete motion package, the actuator includes position sensors and over-strain protection, and the typical actuation time is quoted as 160ms, though it has to be appreciated that the actuator must be cycled a number of times before it warms up and gives consistent performance, and the return stroke is unpowered.
For linear motion, the company offers the Nanomuscle NM series of single-acting linear actuators that deliver 70grammes of force and a maximum stroke of 4mm (Fig.2). These share the same electrical and control functions as the rotary actuators, but there is no built-in return force and over-strain protection is not provided.
Depending on the application requirements, the company offers a commercial grade actuator and an economy grade version, the latter being suitable where there are less stringent life cycle and environmental tolerances.
Due to the silent nature of Nanomuscles, they are particularly suitable for applications where it is required to synchronise motion with sound or speech.
To help develop such applications more quickly, software is available to run on a range of voice ICs (integrated circuits) that allows synchronised motion to be achieved by simply placing markers in the speech/sound file. This software ensures that the ound-motion synchronisation is maintained over a range of ambient temperatures and battery levels.
Depending on the application, a 4-bit voice IC with a good resolution time can implement this kind of control; for applications that demand higher accuracy, an 8-bit voice IC should be used.

Piezo technology

While miniature actuators based on shape memory alloys are certainly attractive, there are other competing technologies. Devices utilising piezo technology also offer fast, quiet operation with a long life and low power consumption.
Moreover, mass production techniques mean that they are affordable for incorporation within everyday consumer products such as door locks and proportional valves for gas cookers. Other applications with lower production volumes include medical devices, coin vending machines, optical fibre positioning, laser control and robotic grippers.
Servocell is a company that is developing devices based on patented piezo ceramic actuators that are claimed to provide an unrivalled combination of movement and blocked force.
Patented methods of construction also help to keep component costs down.
In addition to offering standard actuators, Servocell has developed a series of modules and mechanisms specifically for the security and control markets. OEMs therefore benefit from easy integration into normal manufacturing processes, made possible by the actuators having sufficient movement, together with the necessary control electronics and mechanical linkages.
For instance, the company's first move into the lock and security market was a piezo-ceramic actuated strike plate mechanism as a replacement for conventional solenoid actuators that require a continuous power supply to maintain the system status. Using the piezo-ceramic technology, signals and power can now be supplied by the same cable.
Another example is the AL1 active latch (Fig.3).
The concept for this grew from a market demand for a low-power release mechanism. While piezo-ceramics was identified as the best actuation technology, a further requirement was for all the control electronics to be contained within the same housing. This has been successfully achieved in the finished device, which can be networked, powered and controlled via a USB port on a computer -- or over a telephone line. Thanks to the small number of moving parts and the solid-state actuator, the AL1 is rated for a life of 1.5million cycles.
For OEMs who wish to utilise piezo-ceramic actuators within their own products, Servocell offers a range of bare actuators and controllers. For example, the P305 double-sided, single-beam actuator gives 0.8mm of movement and a blocking force of 0.6N, all within an envelope measuring 41x16x2.7mm.
For applications where a greater blocking force is required, customers can choose actuators with two, three or four beams, giving a maximum of 2.3N. Evaluation kits are also available or those wishing to try the technology for themselves, or Servocell can assist in the development of custom products.