This article discusses the recent demand for silent alarms. If the device is kept close to a user’s body, such as on her\his wrist or in a back pocket, vibration is well suited for this type of alarm. The technology has been used for sometime in pagers, for example, and the field could someday be expanded to watches and a variety of medical equipment. The most common form of vibrating alert involves a motor that rotates an eccentric weight in an electromagnetic field. However, the smallest of these motors is still too large to fit in watches and other consumer products. Piezoceramic materials can produce motion by receiving electric potential across their polarized surfaces. Using finite-element analysis and optimization, engineers at Philips Corp. in Sunnyvale, Calif., have developed a miniature vibrator/piezo-bender that consumes only 8 milliamperes. In addition to optimization and actual shake tests, ergonomic research has been done to find the best way of improving the vibration performance of these silent alarms. Although the piezo-vibrator device shook strongly enough to get the wearer's attention in most environmental circumstances, that awareness improved greatly when vibrations were pulsed or interrupted.
Alarms are everywhere--from fire and car alarms to alarm clocks. Currently on the rise, however, is the demand for silent alarms. If the device is kept close to a user’s body, such as on his or her wrist or in a back pocket, vibration is well suited for this type of alarm. The technology has been used for some time in pagers, for example, and the field could someday be expanded to watches and a variety of medical equipment.
The most common form of vibrating alert involves a motor that rotates an eccentric weight in an electromagnetic field. However, the smallest of these motors is still too large to fit in watches and other consumer products. Most of these vibrators have an armature resistance of at least 10 ohms and a terminal voltage of 1.3 volts. Such motors draw between 40 and 160 milliampères of current from the power source, which is far too demanding for the normal lithium or zinc-air batteries found in watches.
Piezoceramic materials can produce motion by receiving electric potential across their polarized surfaces. Using finite-element analysis and optimization, engineers at Philips Corp. in Sunnyvale, Calif., have developed a miniature vibrator/piezo-bender that consumes only 8 milliampères. Small enough to fit in a wristwatch, the device can still produce enough vibration to get the user’s attention.
A piezoelectric crystal vibrates when an alternating voltage is applied to it, and almost all of the energy given to the material is converted into mechanical motion. Piezoceramic materials, commonly considered to be much more efficient than electromagnetic instruments, make very good candidates in a low-power-consumption design for vibrators oscillating in their fundamental modes. The problem is whether that tiny piece of piezoceramic can shake enough to attract someone’s attention.
When it is excited at low frequency, a piezoceramic material vibrates; at high frequencies it also produces sound, as a transducer does. The resonant frequency of the ceramic is too high to produce an audible tone by itself, so a metal plate must be attached that vibrates with the contraction and expansion of the piezoceramic. Therefore, both audible and silent alerts can be generated from the same source by exciting it with two different frequencies.
PZT-5H, the piezoceramic material chosen for the silent alert, is used in applications requiring fine movement control such as in ink-jet printers. The material provides extremely high permissivity, coupling, and piezoelectric constant. It has the lowest Curie point (the temperature at which a material’s magnetic or ferroelectric properties change) of the PZT-5 family of “soft” materials, restricting its operating temperature range, and it has a lower time stability than most piezo materials.
One possible design for the silent alert includes a 15.8- by 3.8- by 0.4-millimeter cantilever beam, which is referred to as the bender. This design has been analyzed with ANSYS FEA software from ANSYS Inc. in Canonsburg, Pa., and is used both for its piezoelectric analysis capabilities and for its integrated pre- and postprocessing features. The program changes the dimensions of the metal plate, the piezo film, and the tungsten weight to produce the maximum shake. Use of the multifield elements is not very different from the use of standard elements. The multifield elements are independent of analysis types in the program, so they can be used for general coupled-field static analysis in harmonic analysis. Similarly, transient coupled-field analyses can be executed in harmonic or modal analysis.
With more degrees of freedom, the multifield elements require larger matrices that need more memory and longer solution times. ANSYS Solid 5 elements were used to formulate the structural analysis. Once the shape was optimized, the program output frequency and stress for the data of a typical run; this output was produced by applying only voltage on one surface of the piezoceramic and not through an induced force or deflection to the beam (the other surface is assumed to have zero voltage by default).
The computed output is remarkably close to the experimental result. The natural frequency of the beam described earlier is computed to be 173.53 hertz, whereas the actual measurement is approximately 175 hertz—a difference of less than 1 percent.
With a sample bender tuned to resonate at 170.25 hertz and placed on the back of a watch case, the performance peak is located at that same frequency. A relatively narrow bandwidth of the frequency-response curve results, which means that a mismatch of 0.5 hertz or more in drive frequency or resonance frequency would downgrade the shake performance to unacceptable levels.
The most common vibrating alert is still too large to fit in watches and other consumer products.
The average total tuning range of the bender mechanism is only about 9 hertz. Differences between assemblies resulted in case-back resonant frequencies varying by 23 hertz; most of that is related to the thickness of the piezo bender, which greatly affects spring stiffness. The temperature extremes of — 20°C and 60°C encountered in temperature-cycling and temperature-shock testing increase the natural frequency of the assembled devices by as much as 2 to 10 hertz. Such variations highlight the importance of the control system.
In the design under consideration, a 60-percent-efficient driving circuit uses a converter to progressively charge a capacitive piezo with a predetermined number of successive current pulses. An optimization circuit determines the peak current after the coil to discharge to the capacitive piezo. Thus, the number of successive current pulses used to charge the piezo is a factor of the coil and voltage of the battery at the time, and is not constant. The drive circuit is designed to energize a piezoceramic from a single-cell battery to produce a charge of approximately 50 volts, which is converted to motion by the piezo material. The battery is approximately 1.5 volts. The charge built across the piezo is a function of its capacitance, which in turn is a function of the piezo material’s area and thickness. The converter consumes little power but provides a high step-up ratio.
For positive piezo polarity, the drive circuit provides several successive current pulses that charge the piezo. The piezo then discharges when its polarity becomes negative and before the converter provides the next pulse of current. While the piezo is at negative polarity, the converter provides several current pulses in succession that charge driving circuit the piezo in the same direction again. The piezo is then discharged when its polarity becomes positive and before the next current pulse occurs. The piezo is continuously charged and discharged in this manner. The number of current pulses required to charge the piezo might vary due to the characteristics of the piezo used.
The design variables are independent and have been dictated by the geometric constraints. The objective variable (the function to be minimized) and the state variable are both dependent. In the ANSYS analysis, the objective function is to maximize the reaction force on support. The equivalent stiffness of the beam is a state variable in this analysis, because it is a function of the thickness of back plate and piezo. The voltage applied to the piezo is also a state variable, a function of the piezoceramic’s frequency and capacitance. To avoid depoling, an upper limit of 50 volts has been assigned to the element.
Because the cantilever is equivalent to the classic spring- mass system of force being the product of mass, deflection, and frequency, the operating frequency should be raised to increase the shake. However, if the frequency of the part increases (and therefore the period decreases), the needs to discharge the potential sooner, resulting in lower voltage built up across the piezo. Lower voltage means less power transmitted to the system.
Equations to determine the piezoceramic element’s capacitance have been placed inside the optimization program within ANSYS. After the harmonic analysis is done, the optimization program recalculates the capacitance and voltage. It then applies the new voltage to the master-degrees-of-freedom nodes chosen on the surface of the piezo. The modal analysis next computes the stress based on the new voltage.
Pulsing the Shake
In addition to optimization and actual shake tests, ergonomic research has been done to find the best way of improving the vibration performance of these silent alarms. Although the piezo-vibrator device shook strongly enough to get the wearer’s attention in most environmental circumstances, that awareness improved greatly when vibrations were pulsed or interrupted.
The most effective duty cycle was an “on” period of 0.6 second followed by 0.4 second of “off.” The duty cycle could be either preprogrammed or selected by the user in the finished product. As an audio alert, the product at 3,200 hertz demonstrated a sound pressure level of 75 decibels at 10 centimeters, which is better than the specifications for a normal wristwatch alarm. The result is a compact, external, integrated transducer-vibrator that will fit in a wristwatch and draw only 8 milliampères of current.