- THE MAGAZINE
PIA devices can be used in acceleration or impact experiments and offer some benefits over other traditional testing systems. For example, a PIA device can produce a rapid steep pulse (an almost square wave) rather than the oscillation offered by shaking systems. In addition, a PIA system's maximum accelerations and largest magnitude pulse powers can be realized independently of the repetition rate.
HILTI AG, a manufacturer of electro-pneumatic chiseling and demolition products, uses PIA systems in research and development programs because they allow significant reduction of the trial phase on the components. It is the reproducible impact parameters, high power levels and high repetition rates that make PIA devices ideal in such cases. As no broad basis for PIA best practices exists, each application is treated as a custom case. Accordingly, no standardized specification literature is available.
Due to the higher pulse power, a PIA system requires an electrical and mechanical system structure that is different than that of a shaking system. These fundamental differences allow acceleration or impact to be maximized for every pulse because they are not coupled to the cycle rate of the system.
PIA devices can be produced in a range of sizes from small-sized, mini-devices up to high-power piezo-hammers. In addition, PIA devices can be combined in phased arrays to take advantage of synchronized multiple course impact generation.
Acceleration-Based TestingIn the case of acceleration-based testing, the moving end of the PIA device is allowed to move freely. This unhindered motion can result in rapid acceleration rates up to 100,000 m/sec² (10,000 g), which can be accomplished using a piezo stack with a 20 µm stroke and a 20 µs rise time. The variation in the acceleration rate is a function of the electrical input parameters. In a typical setup, the test specimen is attached to the moving end of the PIA device.
Impact-Based TestingTraditional impact testing involves a collision between a moving striker and a contact surface that is at rest. Impact testing using a PIA device is conducted with both partners at rest and in direct contact. Activating the piezo stack with a high-voltage current pulse, which causes the internal stress to jump almost instantaneously to a high level, creates the shockwave.
The result of this internal stress is a forward-moving impulse that is directed into the impact partner at the contact point and a recoil impulse that moves the opposite direction. The shape of the collision partners and the material types involved directly affect the complexity of the resulting shockwave propagation. It is not uncommon for such testing to be carried out using metal bars that allow straightforward computations according to the mechanical theory of thin bars.
In the case of impact-based testing, two configurations of PIA devices should be considered: a PIA without a seismic mass, and a PIA device that has a seismic mass added to the bottom of the apparatus. Adding the seismic mass at the non-contact end of the PIA device causes the recoil impulse to be reflected back toward the impact partner, thus increasing the total impulse output duration. As a result of the reflection, the energy content of the impact is almost double that of a PIA device that does not have a seismic mass (see Figure 1). The resulting pulse is effectively a double pulse with the time between peaks being equivalent to the propagation time of the reflected recoil pulse through the piezo stack.
The operating mode of the PIA should be considered when seeking to maximize the impulse energy. As with standard piezo actuators, there are two operating modes: unipolar and semi-bipolar. If an actuator is operated in unipolar mode, the initiating electrical pulse is applied to a device that is discharged; that is, the pre-pulse voltage is 0.
A semi-bipolar operating mode involves pre-conditioning the PIA with a negative voltage, such as -100 V, which then increases the size of the voltage step when the shockwave-initiating electrical pulse is supplied. It should also be noted that, when using a PIA device, necessary pulse powers are dependent on the active size of the device and can easily reach a multi-kilowatt range when pursuing a 100 µs duration.
Phased ArraysAs PIA systems possess precise timing characteristics, the exact synchronization of multiple devices is possible and can be used to produce coherent shockwave fronts in extended structures. Different shockwave patterns with variable directions of propagation can be created-without changing the hardware-when a multi-channel, high-powered pulse generator is used to drive each PIA unit separately. Although no practical experiences utilizing PIAs in phased arrays can currently be cited, the outlook for such applications and the suitability of the hardware are positive.
As piezo stacks are durable devices, integrating such PIA devices into extended mechanical structures or systems in a permanent manner for the purpose of health monitoring is possible. PIA systems can also act as a shock absorber when used in an inverted operation mode.
In all cases, PIA systems are a custom application of piezo stack technology that require a commitment to collaboration among the end user, the supplier and/or the system designer. Accordingly, costs associated with such systems can vary tremendously. The majority of PIA systems used today are implemented in research and development or product testing settings.
This PIA summary is supported by research and development information supplied by Lutz Pickelmann, Ph.D., Piezomechanik GmbH. For additional information, contact APC International, Ltd. at P.O. Box 180, Duck Run, Mackeyville, PA 17750; (570) 726-6961; fax (570) 726-7466; e-mail firstname.lastname@example.org; or visit www.americanpiezo.com.