Python™

More than 90% of piezoelectric actuator failures are due to cracks. Micro cracks are often created in the piezoelectric wafer during the manufacturing process already. Under normal operation (deflection) the inertial force experienced by the piezo actuator (particularly when driven by DC and “rectangular” signal pulses) can induce high cyclic stress (tensile/compression) onto the piezo layer. As the layer consists of a brittle, ceramic material the micro cracks begin to grow under this cyclic stress, causing a reduction in performance of the piezo actuator. These cracks eventually propagate throughout the entire wafer, resulting in total failure. Cracks may also be caused by thermal shocks associated with soldering, handling and other mechanical impact.

Most metals used in piezo electrodes (such as Ni or Ag) can migrate along micro cracks and grain boundaries of the piezo material under conditions of high humidity and applied voltage. This electrochemical migration results in dendrite growths and eventually leads to short circuiting within the piezo layer.

With Python™ technology the piezo layer is hermetically sandwiched between two durable epoxy resin layers. This sandwich-like structure is carefully engineered to give the piezo layer significant negative pre-stress. This compressive stress helps hinder the development of micro cracks and further prevents propagation of cracks during operation. The epoxy resin layers also protect the piezo actuator from handling stress and accidental drops or impact. The Python™ bending actuator is further conformally coated with two thin silicone rubber layers to protect the piezo layer from humidity and condensation.

• Isolated electrical connection: Soldering directly onto the piezo layer can cause cracks and failure due to thermal shock. That is why the Python™ bending actuator comes with thermally isolated solder pads to prevent this risk.

• Convenient mechanical integration: Similarly, clamping directly onto the brittle piezo layers may also cause cracks and failures. Therefore the Python™ piezo actuator includes passive extended portions at both ends. These parts can be drilled, machined, and clamped for mechanical integration without any impact on the actual piezo layers.

The Python™ Unimorph consists of one piezo layer attached to a substrate, and encapsulated by protective layers. When positive voltage is applied, the piezo layer contracts in a horizontal direction inducing upward bending deflection. Negative voltage with lower amps can also be applied to induce downward bending deflection. The Python™ Unimorph has a dome shape allowing its piezo layer to take on additional compressive pre-stress, resulting in superior reliability.

The Python™ Bimorph consists of two piezo layers attached to the top and bottom surface of a substrate, and encapsulated by protective layers. When positive voltage is applied to the top piezo layer, it contracts in an in-plane direction inducing upward bending deflection. Similarly, when positive voltage is applied to the bottom piezo layer, the layer contracts in an in-plane direction inducing downward bending deflection. Negative voltage with lower Amps can also be applied to induce downward bending deflection. The deflection behavior is bi-directional and symmetrical. Negative voltage can also be used along with positive to achieve additional bending deflection.

Unlike conventional piezo energy harvesters which usually include one or two bare piezo layers attached to a substrate, the Python™ Energy Harvester includes a protective layer with a non-uniform thickness. The varying thickness of this layer is optimized so that the Python™ Energy Harvester receives more uniform stress and strain distributions on the piezo layer under bending conditions. This allows the Python™ Energy Harvester to harvest up to 3x more electrical energy through bending / vibration, while maintaining lower stress levels and therefore higher reliability when compared to conventional harvesters.