Adding function to materials

Steven Bowns of Technology Futures examines the potential impact of plastic and printed technologies for electrical and electronic systems and finds out about...

The use of printed electronics is increasingly gaining popularity within engineering industries. The technology's low fabrication cost makes it ideal for applications looking to remove cost from traditional electronic integration and installation.

However, the technology has moved well beyond lost-cost/high-volume applications and many of its early limitations have been overcome by material breakthroughs and innovations. As a result printed electronics is increasingly being used for much more sophisticated applications, such as Unmanned Aerial Vehicles (UAVs) as part of their electrical actuation and control systems.

Printed electronics typically uses conductive, semi-conductive or insulating inks printed on to thin, flexible substrates such as Kapton, polyester or polyimide. The term 'plastic' electronics is often used because many of the substrates are plastic or organic.

The technology is usually associated with high volume applications such as RFID tags, photovoltaic cells and printed circuits for consumer electronics. Until recently, it has not been generally considered for aerospace applications, but now that a weight advantage can be so clearly seen, the technology's potential for application in aerospace and other sectors has expanded dramatically.

The most obvious printed and plastic electronics application is in replacement of traditional cabling. But, more interestingly, it can also be considered for high-functionality elements such as motor encoders, servo feedback devices and motor control circuits. The technology is extremely light and flexible which offers some additional and unusual, features and benefits.

Plastic electronics has allowed military UAV designers to avoid the cost, weight and mechanical-electrical constraints of traditional cable harnesses, electronic enclosures and connectors. In some instances, the flexible electrical laminates may be simply embodied as layers within composite structures. This is also being used by some Formula One teams.

One area of particular interest where printed electronics is proving its worth is in sensor and motor control systems for servo actuation and feedback. Such systems are common in aircraft systems like flight control surfaces, intake ducts, brakes, throttles, and undercarriage controls.

Traditionally, these systems use an electric motor, motor encoder, gearbox and a transformer-based servo feedback device such as a linearly variable differential transformer (LVDT). Whilst such transformer-based feedback devices offer precision and reliability, they are often as bulky and heavy as the motor, and often the level of precision is far greater than is actually needed. A 300mm stroke LVDT might be a 25mm diameter cylinder, 500mm long and weigh 1kg, whereas its printed alternative is just 3mm high, 330mm long and weighs just 25g.

As many aircraft systems require duplex or triplex redundancy, a duplex or triplex LVDT roughly doubles or triples the original weight and volume. A printed device simply uses more layers of printed tracks to form an isolated second or third electrical system.

In a simplex system, the weight reduction is typically greater than 95%, whereas in a duplex or triplex system the weight reduction is greater than 99%. On an individual device the net effect is modest. However, more than 50 such devices may be used in an aircraft, so the net effect is often quite significant.

There is a technology cluster in Cambridge, starting from the Cavendish Laboratories in the 1990s, where some of these technologies were developed. These are now being successfully commercialised into printed devices by companies such as Zettlex, which produces rotary, linear and 2D sensors as part of printed actuator control systems. Its position and speed sensors are used in both fixed and rotary wing military aircraft, as well as a raft of other applications in the industrial, medical, and oil and gas sectors.

While these devices seem flimsy and delicate, the printed forms are rarely used in a 'naked' state. Instead, they are usually completely embedded in a resin or bonded to a host mechanical structure such as an aileron, wing spar, gearbox or even seat.

The printed feedback devices do not need precise alignment of the stationary and moving parts, so further mechanical elements such as guides, bushes, bearings and seals can be eradicated, producing further weight savings.

The weight advantage is further increased due to the eradication of cabling between actuators and centralised control units. Traditionally, these centralised units receive signals from the servo feedback device and motor encoder and, in turn, transmit the required power to the motor.

Such signal and power lines might typically require 14 individual wires per actuator. Compare this to the printed approach, where the intelligence is distributed to each of the actuators. The modest amount of software for actuation can also be embedded into the printed sensor's control circuit.
There is no need to transmit power and signals for computation to the centralised unit since the necessary computation is carried out at the actuator itself. Only command signals and power are transmitted from a control unit to an actuator. Since the command signals can be provided over the power lines, only a 2-wire bus is required.

In time, it is likely this technology will be adopted by the civil airline sector, as well as much broader applications in medical, automotive, oil and gas, rail and other industries. The potential for weight saving will be the driving force, with engineers looking for any method to increase fuel efficiency as a result of weight reduction. The greater the inflation of fuel price, the greater the pressure for adoption of printed technologies. It is also likely the technology transfer opportunity is something both OEMs and component manufacturers will focus R&D efforts on in coming years.

Steven Bowns

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