Ringactuators are piezoceramic stacks with a center bore. This design allows additional features in mechanical, optical and electronic properties compared to bulk stacks.
Piezo-ringactuators are available in available in monolithic low voltage design and discretely manufactured high voltage versions. The actuators are offered with preloaded stainless steel casing ("VS"-label) as well as in "bare" versions without casing. Mechanically preloaded actuators are preferentially used when
Shear forces (e.g. horizontally mounted actuator with additional masses on moving end)
Acceleration forces during dynamic operation are applied to the actuator.
Electronic Supplies, Power Requirements
Piezoelectric stacks show a distinct electrical poling. The stated maximum voltage has only to be applied with the right electrical polarity. The "bare" stacks without casing are completely electrically insulated ("potential free") so both positive and negative supplies can be used by proper choosing the ground for one of the leads. For ringactuators with casing VS, the electrical polarity is fixed and is positive within the standard product range of actuators and power supplies/amplifiers. So a free combination of actuators and supplies is achieved to optimize the power requirements. Ringactuators with casing VS can be set to negative polarity on request (without additional charge).
N.B.: piezoelectric stacks expand, when the operating voltage of correct polarity is increased. A voltage with opposing polarity can be applied to some extent (approx. up to 10% of max. voltage rating). Stackactuators show then a contraction. This effect can be used to increase the total displacement range of the actuators or to generate an oscillation by a bipolar signal generator.
Electrical Capacitance: Piezoelectric actuators behave like an electrical capacitor for driving frequencies well below their resonant frequencies. The stated capacitances in the data sheet are valid for room temperature. Due to variations in materials composition, the individual capacitances may vary by +15%. The dielectric constant of the PZT materials varies with temperature and increases up to 50% for a temperature rise up to 80oC.
Optimizing the Power Requirements
For the operation of piezoelectric actuators, electrical power (current) is only needed for a change of position or force. For highly dynamic applications, the power requirements can be rather high resulting in high efforts and costs for the supplies. Further on, the actuators warm up by dynamic operation due to dissipative processes, converting some percent of the energy input to heat. These aspects show clearly the need for power reduction/optimization techniques for dynamic operation of actuators.
Power Matching Within An Piezo-Actuated System
Reduction of Strain - Usually, one tries to maximize strain by applying the maximum operating voltage to the actuator, to get maximum effect from an actuator with a distinct length. A short look shows, that this is preferentially a good strategy in quasistatic applications with no or low dynamics (low operating frequencies and/or stoke) and low power levels. For highly dynamic applications, the situation changes. A high strain means nothing else as a high energy content which has to be supplied by the electronics and is stored in most cases as reactive power. Then it might be reasonable to use a longer actuator with a voltage below the maximum voltage rating, leading to a reduced strain for the same stroke compared to a short stack.
A ringactuator CTC-HPSt500/15-8/15 (max. stroke 15 mm at 500 V) is used to produce a 5mm-oscillation together with a 150 V supply. This arrangement needs only 1/3 of the power compared to a CTC-HPSt500/15-8/5 actuator (stroke 5mm at 500 V) together with a 500 V supply for the same operating conditions.
Bipolar Operation - A further reduction of power requirements is given for a bipolar operation of an actuator around zero position compared to a one-side operation due to an unipolar driving signal for the same motion characteristics. For this strategy attention has to be paid to depolarizing effects due to countervoltage (for common actuators max. 10% counter voltage, higher ratings for hard piezoceramics).
Resonant Systems - Resonant actuators systems show the highest power efficiencies in converting electrical to mechanical energy compared to broadband application under the conditions of a forced motion. The reason is, that electronics has only to supply the effective power to the system and not the potentially large reactive power resulting in a minimized selfheating. The efficiency in producing a distinct oscillation is defined by the quality factor of the resonant systems. By definition, these resonant systems are working on discrete frequencies and are not broadband.
Piezomaterials Composition - There exist a variety of formulations of PZTs with differing properties such as strain, power dissipation, quality factor, cohercive field strength (stability against electrical depolarization). Depending on the individual applications, power matching can be optimized by the proper choice of the PZT-material. Please contact us for detailed consulting.
Positioning Sensitivity of Piezoelectric actuators - The piezoelectric effect is unlimited in positioning sensitivity, there exists no principle "smallest step". Piezocontrolled Scanning Tunnel microscopes (STM) or Atomic Force Microscopes (AFM) are dealing with structures in Angstrom and Sub-Angstrom-range. In practice, the positioning sensitivity is only limited by the stability and noise of the electronic supplies.
Notice: Piezoceramic material and stacks can be damaged by tensile forces.
The design of piezoactuated mechanical structures should avoid tensile stress for the actuators. Actuators should be preferentially operated under "preload conditions" (applying a permanent compressive load). Tensile forces are not only produced externally, but can be generated inside as acceleration forces due to dynamic operation (e.g. together with powerful supplies or electronic switches). Use preferentially preloaded actuators for highly dynamic applications.
Do not discharge a piezoelectric actuator by simple short-circuiting.
Stiffness: (inverse compliance, spring constant of the actuator) The stated data for the actuators stiffness are defined for a load of approx. 10% of the max. load.
Mechanical Resonance Frequencies
Like any other mechanical structure, a piezoelectric actuator shows characteristic resonance's depending on actuators size, materials, manufacturing techniques, mounting conditions, and externally coupled masses etc. Resonance's are not only observed for the axial movement of the actuator, e.g. there exist radial modes for diameter oscillations of the stacks. In positioning applications, such resonances may cause problems e.g. for a feedback controlled system, if a resonance is within the operating frequency band, it is not possible to get a stable position. So it is a main task in designing such positioning arrangements, to ensure a sufficient high frequency of the basic modes of the structure so not to interfere with the operating frequencies. Normally, the lowest resonances of an actuator are axial modes, special cases with lower radial modes are indicated. The datasheet states the lowest axial mode without externally coupled masses and the actuator onside fixed on one side to a support, the other side moving freely. The coupling of additional masses leads to a decrease of the resonant frequencies of the system, compared to the unloaded actuator. An estimate for the resulting resonant frequency of a mass loaded actuator is given by the simple spring-mass model, where the actuators stiffness is set as spring constant.
Thermal Properties of Piezo-Ringactuators
Operating temperature range: -40oC thru +80oC
Actuators for wider temperature range (e.g. cryogenic temperatures ) on request.
for the ceramic actuator body, the following expansion coefficients are valid
Low voltage actuators: a » -3x10-6
High voltage actuators: a » 8x10-6 near room temperature.
Selfheating of Actuators
Due to internal dissipative effects of the ceramic material, some percentage of the supplied (active or reactive) electrical power will be converted to heat. For dynamic operation e.g. of some hundreds of Hertz with maximum stroke, the upper temperature limit of common actuators can be reached. Still higher dynamics then require power matching e.g. by reducing strain according the statements on page 6.
Acuators without casing:
"Bare" ringactuators have to be mounted purely by the front faces and never by the circumference. Radial clamping damages the ceramic body resp. may cause improper function due to the not well defined mounting reference. Mounting of "bare" ringactuators by the front faces can be done by clamping, optionally available threaded endpieces HAg or by using adhesives like epoxies or cyanoacrylates.
Actuators with stainless steel casing
The ringactuators CTC-HPSt are offered additionally with mechanically preloaded stainless steel casings VS. Now clamping at the circumference is possible as well as by mounting by the threaded endpieces. Additional screw caps are provided for easy mounting of mechanical and optical components.
When designing a piezoactuated system, please pay attention to the potential high
voltages and currents within such an arrangement, which may cause danger to life and
health, when handled improperly. Only authorized personnel should deal with high power
setups. Piezoelectric actuators as well as eletrostrictive actuators are electrical
capacitors, which can store electrical charge and energy for a long period. For bigger
actuators, the amount of stored energy can be large. Prevent damaging peripheral
electronics by erroneously connecting charged actuators.
Notice, that piezoelectric actuators can be electrically charged even by changing the mechanical load of an actuator or its temperature. Before starting any manipulation of an actuator, switch off all supplies and discharge the actuator by using a resistor.
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