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FAQ

Components

The coupling that I measure on my parts is different from the catalog value for the material. What is the reason?
Catalog values are defined according to the EN-50324 standard. They rely on some specific geometry (flat disc, thin bar etc.). In most cases for actual products, geometry is not ideal, so the effective coupling k_eff differs from catalog values.

Actuators - linear

I want to apply coating on my actuator; is that going to reduce motion?
Any additional material in parallel with the piezo element will result in increased load on the piezo element, thereby reducing performance. However in most cases the coating has a low stiffness compared to the piezo element so the impact will be minor (<5%).

Can I use oil for cooling a piezo element?
Yes. For applications where self-heating is significant, it is possible to use air flow or oil flow for cooling purposes. The oil needs to be clean dielectric oil (transformer oil) as any impurities can provoke electro-chemical reactions ultimately leading to a failure of the actuator. Be aware also that for large cross-sections, the thermal gradient in the actuator can be significant so maintaining an acceptable surface temperature might not be sufficient.

Can apply a torque on my piezo stack?
It is not recommended to apply torque on a piezo stack. In the case where a stack is preloaded using a nut, it is important to make sure that the tightening torque is not transmitted to the stack. Shear stress above 1MPa can lead to failure of the adhesive layer between the ceramic elements.

How much preload can I apply on my piezo stack?
We recommend that piezo stacks operate in the range 0 to 80 MPa. Preload must be designed to ensure that stress remains in this range in all conditions. Usual values are 10 to 20 MPa.

Can I apply a negative voltage to a multilayer piezo element?
Yes, it is possible to increase the quasi-static displacement of a piezo element by applying a negative voltage. However in order to avoid the risk of de-poling, the electrical field should be limited: at room temperature and without load, -0,7kV/mm for NCE51 and NCE57, -0,25kV/mm for NCE59, -1,5kV/mm for NCE46. These recommendations have to be decreased at high temperature and high load. 

Actuators - bending

I cannot measure the resonance frequency stated on the datasheet of my bending actuator. What could be the reason?
The specified resonance frequency is defined in clamped-free (cantilever) configuration, without added mass. First resonance will be higher in free-free configuration, so clamping has an impact. Similarly, adding a mass (mirror...) at the tip of the actuator will lower the first resonance. Please contact us so we can evaluate your requirements.

Is it possible to customize an actuator through machining of the sintered ceramic?
It is possible to machine sintered ceramic using diamond tools. However it is easier to machine ceramic in green state (before sintering), so please contact us so we can do the machining within our process. In the case of multilayer products, if the internal electrodes are exposed, the maximum applied voltage needs to be decreased to avoid arcing. Alternatively, the exposed area will have to be coated.

Actuators - shear

The specification for shear plates does not mention blocking force. What is the value?
Blocking force is very difficult to measure for shear plates due to their very high stiffness. This is why we don't mention it on the specifications. It is possible to estimate blocking force by multiplying free displacement with stiffness, obtained using a catalog value for s55E of 4,3E-11 m^2/N.

Can shear elements be used at high frequency?
Thanks to their high stiffness and high resonance, shear elements can be used for rapid operation (fast response). However continuous operation at high frequency can be an issue. This is because the non-linearity implies high losses (higher than for d33 actuator), so the element will heat-up. Typically, a few minutes at 2 kHz is not a problem. If higher frequency or longer duration is required, the operating voltage has to be reduced otherwise the actuator will slowly degrade (loss of poling).

Is there a recommended preload for shear actuators?
Usual d33 stack actuators require a preload for high frequency operation because the inertial force can become substantial, creating tensile stresses in the ceramic. For shear actuators, the inertial force is a shear force, so an axial preload will have limited effect. It can however be beneficial in some cases in order to improve stiffness (by compressing the glue lines) or in order to avoid bending (if center of gravity is far from the fixed end of the stack).

What is the maximum axial load that can be applied on shear elements?
Our shear plates are usually tested under 3,5 MPa. Piezo elements can withstand high compressive pressures (>50 MPa) however in practice, we recommend not to apply more than 5 MPa axial load on shear elements. This is because small defects in the contact surfaces can lead to stress concentrations and break the element. If high pressure is required, we recommend to use contact surfaces (both on the shear plate and on the structure) with superior flatness.

What is the maximum shear load that can be applied on shear elements?
The ceramic can withstand very high loads. In practice it is the interface between the ceramic and the substrate that defines the maximum shear load.

If the movement is transmitted by friction, the maximum load will depend on the pressing force and the coefficient of static friction at the interface. For example 2 MPa pressing force * 0,2 friction coefficient = 0,4 MPa, i.e. about 10 N for the 5x5 mm shear plate.

If the element is bonded using epoxy, the maximum load will depend on the strength of the epoxy that is used. Typically epoxies can be used safely at 5 MPa in shear stress, i.e. about 125 N on a 5x5 mm shear plate.

Amplified actuator

What materials are used for NAC26xx products?
The piezoelectric material is NCE51F and mechanical parts are stainless steel (equivalent AISI303). Standard components (preload screw…) are steel but can be changed for low-magnetic applications.

Why is there a force limit on NAC26xx products?
A high preload on the piezoelectric stacks keeps the components in place through friction. If the external load reaches the point where it overcomes the preload, the stacks may leave their positions. Note that maximum recommended load is specified in "active" conditions (bias voltage applied). It is possible to increase the preload or use positive locking of the stacks for specific applications; please contact us.

Motor

Is the PAD UHV-compatible?
Our standard PAD7220 is not UHV compatible due to the use of lubricants and polymer parts. However other PAD designs can be UHV compatible. Please contact us for further information.

What is the impact of torque on repeatability of the system?
A changing torque will have some impact on precision. The motor as well as other parts in the system (mounting bracket, shaft etc.) have a certain stiffness, so the absolute position will not be the same if torque varies. If extreme precision is required at the point of load, it is necessary to add a position sensor at that location and implement closed-loop operation.

Does the PAD have a holding torque when powered off?
Yes, when powered off, a PAD exerts a holding torque which depends on the motor's design. In some cases when power is off the motor can exhibit some backlash. This is because the piezo elements do not generate the force required to press the internal gear together.

The torque feedback value doesn't react to an applied torque on the output shaft. What could be the reason?
The PAD system calculates a torque feedback value for every cycle. This means that the value is only updated when the motor is turning and the update rate (therefore the bandwidth) depends on the rotational speed.

Drivers

Are NDR62xx drivers available as OEM modules and can they be racked?
Our NDR62xx drivers are primarily meant for laboratory use. Thermal management is specifically adapted to their enclosure so we do not offer an OEM version. However if your application requires it, we can supply rackable units containing several channels.

Materials

How is Qm (mechanical quality factor) measured?
Qm and other standard material properties are measured in accordance with EN50324. Unless otherwise specified, measurements are performed according to "part 2" of the standard. Measurements are performed on monolayer samples of standard geometry (typically Ø20*1 mm).

How do the piezoceramic circuit parameters vary with temperature?
It is easier to refer to the “physical” parameters than to the equivalent circuit parameters (R, L, C, C0). Generally speaking:

  • The static capacitance increases with temperature. Read more in the section Thermal properties of piezoceramic material.
  • Resonance frequency (related to L and C) depends on the speed of sound in the material so it will not change much (only a few % up or down depending on material and vibration mode).
  • Coupling (related to C and C0) will remain relatively stable.
  • The losses (related to R) will be more sensitive to temperature. Depending on the material, the losses can be multiplied by 5.
  • You can find more information under Temperature dependence for NCE40, NCE41, NCE51, NCE80 and NCE81.

What are the thermal properties of the piezoceramic material?
The traditional piezoceramic family has relatively similar material compositions and structures. The following general values can therefore be used for all our products:

  • Thermal conductivity: 1,5 W/m K
  • Specific heat Capacity: 420 J/kg K
  • Conductivity is rather low (similar to many types of glass) so it can sometimes be difficult to extract heat from large cross-section stacks.

My actuator is drawing more current than expected. What is the reason?
Due to the hysteresis phenomenon, the capacitance of a piezo element will change as it is charged. For large variations in voltage, the apparent capacitance can be up to 40 % larger than for small variations. The value stated in the datasheet is for small signal excitation.

Is the operation of a piezo element affected by magnetic and electric fields?
Our piezo elements are completely non-mangetic. They are not affected by magnetic fields. The ceramic is sensitive to high electric fields (up to 3 kV/mm) so electromagnetic interference is usually negligible.

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Performance Dimensions
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