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Dramatic reductions in voltage and power requirements are making tiny piezo motors and drive systems an interesting option for portable, low-power medical devices. As their efficiency is not reduced by miniaturisation, they are effective in power ranges lower than 30W. Kenji Uchino and Safakcan Tuncdemir, of ICAT, Pennsylvania State University, investigate why piezo motors are becoming more and more attractive.

Patients who use battery-operated, wearable and portable medical devices (PMDs) want them to function for longer periods without frequent charging or battery replacement. In addition to long battery life, users favour safe, inexpensive, comfortable and high-performance miniature devices.

PMD manufacturers need components to meet the following end-user requirements:

  • Longevity – longer battery life
  • Small footprint and comfortable – small and lightweight, silent, wearable, easily foldable and bendable
  • Inexpensive – inexpensive price, maintenance and/or replacement, and easily available
  • Effective – high performance and to function even undersevere loading or
  • Extreme conditions
  • Safe – no toxic material, electric shock or dangerous heating.

Ultrasonic motors

An ultrasonic motor (USM) consists of two main elements: a vibrating piece (called a ‘stator’) and a moving piece (‘slider’), which are connected through frictional contact. Final output motion is seen on the moving piece while the piezoelectric material causes the vibration on the stator. The principle is analogous to circus ‘dish-spinning’: the rod is ‘wobbling’ (not rotating) but the dish spins continuously.

Typical electro-mechanical materials in a USM are piezoelectric ceramics that generate strain (mechanical deformation) in response to the applied electric field. What drives the piezoelectric material, and thus the USM, is a drive circuit capable of providing input energy in high-frequency (typically 40-40kHz, which is beyond the human-audible frequency and called ultrasonic) signal schemes

USMs are viable for micro-systems because of their well-known merits.

They are power dense and efficient even at smaller scales, hence USMs generate less heat than the equivalent miniature electromagnetic (EM) motors. That’s roughly 20 times more power for the same size motor at 1W level, or 1/20 smaller in volume and weight for the same power.

They suffer no interference by magnetic fields and don’t interfere with other components even in cramped spaces. Thus, the USM is suitable for magnetic resonance imaging (MRI) system applications.

They are direct drive and so do not require gearbox or similar speed-reduction attachments to the main body because of high torque/thrust owing to the contact/friction principle. This helps obtain cost-effective solutions in simple mechanical structures and also leads to acoustical silence and higher efficiency.

In addition, recent advancements related to the piezoelectric USM field make these ‘nice little actuators’ more attractive for PMDs.

Close interaction

The interaction of USM components is so close to the other constituents of the larger system that any small improvement in one subsystem indirectly affects the others. For example, in the case of battery-operated PMDs, the drive circuit’s capabilities are limited by the stored energy of batteries, which inhabit the same space as the rest of the device’s components. More power-dense ceramics and multi-functional designs can save vital space, which enables the use of larger batteries that in turn deliver longer battery life.

In terms of power (electrical) and geometric (mechanical) limits, there is a high level of inter-dependence between the slider, stator and drive circuit of the USM and the battery. These sub-systems have complex combined effects on USM performance, rather than each affecting the USM performance independently. Conventional analysis considers these subsystems as independent conjugates of USMs. In order to better respond to end-user requirements for design solutions that utilise USMs, it is necessary to expand the analysis of USMs to a system-level perspective. For example, development of a new type of piezo-ceramics may lead to improvement in multilayer ceramic manufacturing technique, which causes reduction in the layer thickness of multilayer ceramics. Multilayers with thinner layers can be driven with lower voltages, which can be used in the human body (~3V). This enables drive circuits to be more compatible with digital control circuit boards, which then eliminates the need for a booster inverter amplifier (typically up to several tens of Volts). Consequently, the efficiency of the drive circuit is enhanced, while the USM with its driver can be packed into smaller-volume packages.

As described in the scenario above, a development in materials technology enhances the electrical characteristics of the USM, which finally results in efficient devices in smaller packages.

Key developments

1.Advanced piezoelectric materials

Continuing development of piezoelectric materials in recent decades has yielded advanced materials with a higher mechanical quality factor and higher maximum vibration velocity. These advanced piezoelectric materials generate less heat, but have higher maximum vibration velocities even with lower input powers. Reduced input-power requirements bear fewer loads on the drive circuit and power sources, which eventually aids the miniaturisation of the total system. Motors utilising these materials can be built in miniature while sustaining their superior output capabilities, including:

  • efficient operation with more input-power density converted into mechanical energy
  • less electrical power consumption and longer battery lifetime
  • powerful devices with high-power piezoelectric materials
  • scaling down the devices owing to the high power-density materials.

Developing USMs that use lead-free piezoelectric materials also eliminates hazardous and toxic components, improving safety for PMD users.

2. A better understanding of loss mechanisms in piezoelectric materials and developing novel drive techniques

The latest research on loss mechanisms not only permits the development of advanced materials but also enables the command and control of heat accumulation during high-power operation. Further benefits include:

  • safer operation with controlled heat dissipation
  • miniaturisation through eliminating the bulky heat sinks and by using smarter device packages
  • reduced cost with increased device life.

With better understanding of loss mechanisms and the creation of new techniques in the high-power characterisation of piezoelectric materials, the research and development of new excitation schemes for USMs is possible. By comparing with the conventional ‘resonance’ drive of a USM, it has been demonstrated that ‘anti-resonance’ can substantially reduce input-power requirements. In addition, we have also introduced smooth impact drive motors working at the resonance mode (see the technical phrases panel on the previous page for an explanation of terms). These new techniques enhance efficiency and ease the drive-circuit requirements with fewer loads on both the battery and driving circuits.

Simpler drive circuits, with lower voltage and current requirements, can be built on miniature chips, aiding miniaturisation, saving space for other components and reducing the overall cost of the device. Decreased electric power usage also enhances the safety of the PMD’s in-vivo operations.

3. Advances in multilayer technology

Developments in electrode materials and multilayer manufacturing techniques help to decrease the unit cost of multilayer devices and reduce the ohmic losses within these devices.

Besides the improved efficiency, which leads to longer battery life, and higher level of power densities that enable more powerful and effective PMDs to be built, the reduced losses help to eliminate temperature-related stresses on materials, which leads to more durable devices that are more affordable for patients.

Today, layer thicknesses are reduced down to 30┬Ám – thinner multilayers mean lower input voltages, which reduces the risk of electric and thermal shock – and the number of layers in a multilayer device can reach above 100.

USMs utilising these multilayer piezoelectric materials can be driven at lower voltages producing larger strokes and higher torques. A further benefit is that it simplifies the drive circuitry, aiding miniaturisation.

As the medical application regulates the usable drive voltage, multilayer technology is a ‘must adopt’.

4. Advances in microelectronics

Advances in microelectronic areas such as micro-electromechanical systems (MEMS) and digital signal processing (DSP) bring to the table all-in-one chips, which include all the driving elements on one chip and aid further miniaturisation.

Utilising these miniature chips has the effect of promoting overall efficiency, for example by alleviating the load on power sources that leads to effective devices with longer battery life, and it also saves space for the battery.

5. Multi-functional motor designs

Multi-functional USMs are becoming a popular alternative method for further miniaturisation, and they can replace multiple USMs that have unidirectional output capability.

Using fewer actuators in the PMD saves space for the battery and other components. Thi s affects the battery life, effectiveness and the footprint of the PMD, while a simplified drive unit also decreases the cost.

Conclusion

USMs that utilise key technological advancements provide better options for the manufacturers and users of low-power PMDs. USMs are the melting pot of many technological developments.

Thanks to strong, efficient, miniature, safer and cost-effective USMs, patients will enjoy their compact, wearable and comfortable, inexpensive, effective and powerful devices more without the hassle of frequent charging or battery replacement.