TechArchive

Original article date: October 1995

The reverse of the piezoelectric effect is used in micropositioning. KEVIN GRIMLEY of Lambda Photometrics explains the basics.

The piezoelectric effect – where mechanical energy is converted into electrical energy – is well known. A common use is in a cigarette or gas grill lighter, where a lever applies a pressure to a piezo, inducing an electric field which is strong enough to create a spark to ignite the fuel.

The reverse effect is used in micropositioning. Here, an electric field applied to a piezo ceramic is used to produce precise motion. The most common form of piezo ceramic used today is a lead zirconate titanate based material. Modifications to the material produce two main types of piezo actuator: low voltage devices requiring around 100V for full expansion and high voltage devices which need 1000V to reach maximum extension.

PZT devices have traditionally had a high price tag associated with them. This is now changing as volume increases and historical knowledge is used in product design. Standard products in general will provide the lowest cost solution, but even special designs can have prices substantially reduced if volume batches are manufactured.

Examples of application areas are: fibre optic alignment, machine tool alignment, active damping, hydraulic servo valves, shock wave generation, image enhancement through mirror tilting, wafer mask alignment, adaptive mirrors for carbon dioxide laser welding and an auto focus for surface profiling. In many of these, standard products can be used.

Future application areas include the use of PZTs in the implementation of smart structure technology and the development of PZT based drive systems. Already available are rotary PZT motors with an outside diameter of just 3mm and a linear PZT motor with millimetres of high resolution travel. Such developments will have impact in a wide range of industries right from medical products through to precision tooling machines.

Piezoelectric actuators (PZTs) can offer several benefits and advantages over other positioning methods:

• PZTs are solid state, the motion being based on crystal orientation, so no maintenance is required;
• Different PZT designs can provide performance such as carrying loads in excess of three tonnes or operating dynamically at frequencies above 70kHz;
• PZTs act as capacitive loads and require very little power, thus simplifying electrical needs;
• As PZT motion derives from crystal effects, and there are no moving parts, there is no stick slip effect, so nanometer step movements and resolutions are readily attainable;
• Piezo materials are capable of providing positioning in ultra high vacuum and very low temperature (4K) environments.
• Most PZTs are constructed from thin wafers of electroactive ceramic material stacked in series but connected electrically in parallel. The configuration is shown in Fig 1. The thickness of the wafers determines the maximum operating voltage of the actuator.

High voltage PZTs are constructed from wafers ranging from 0.5-1.0mm thick, whereas low voltage stacks use approximately 0.1mm thick ceramic. When an electric field is applied to a piezoelectric material, dipoles in the crystalline lattice orient themselves to produce expansion in the direction of the field. A strain of up to 0.13% can be achieved. This means that a 100mm long PZT can expand by 130um if the maximum field strength is applied.

The amount of expansion available is the same for both high and low voltage ceramic material. The materials do, however, exhibit different temperature characteristics. High voltage ceramic can be used at up to 150degC, whereas low voltage material is limited to a maximum of 80degC. High voltage PZTs have the added advantage of being available in large cross-sectional areas, which allows them to be used in high force applications. Many users, however, prefer to work with low voltage ceramics, because the electronics tend to be simpler and lower in cost.

Mechanically speaking, a PZT can be regarded as a spring-mass system. The stiffness – or spring constant – is a function of the Young’s Modulus of the ceramic, the cross-section of the active material and the compound effects of the wafer stack. The Young’s Modulus for a pieze ceramic is about one quarter that of steel. Stiffness losses at the wafer interfaces, however, mean that in practice, this figure is generally 60-90% of the calculated value for an actuator.

Piezo material (like all ceramics) is brittle and so care needs to be taken during use to prevent breakage. High pulling or shear forces must be avoided and this is usually achieved through the design of the actuator and associated mechanical system. A common solution is to integrate a spring preload into the PZT mechanism which applies a compressive force to the ceramic material. With large cross-sectional area ceramics, spring preloads of up to 3.5kN can be applied. In dynamic applications, a number of factors have to be considered before the correct selection of a PZT device can be made. These factors are:- travel range and dynamic frequency, force generation, power available and heating effects.

Travel range and frequency are generally considered together, for example 100um at 100Hz is possible, but 100um at 1kHz is not. Any load on a PZT reduces the resonant frequency and hence the maximum theoretical operating frequency. The acceleration forces generated in a PZT can be high, especially if an electrical pulse with a short risetime is sent to the device. Therefore, particularly in applications where an extremely fast movement is required, the voltage pulse applied to the PZT should not have a risetime which is faster than the device can handle mechanically. If only a limited power is available from the amplifier being used to drive the PZT then this may determine the maximum operating frequency. At high frequencies, a lot of power is dissipated as heat into a PZT stack. It is important to ensure that the temperature rise is kept to below the maximum operating figure for the PZT being used. If this is a potential problem then appropriate cooling measures can be taken.

All PZT actuators exhibit hysteresis and drift. In some applications this is not a problem, but where accuracy and drift-free operation are required, a position feedback system must be used. A number of different methods can be used to provide feedback information:

Strain Gauge Sensor:-usually fitted to the piezo stack inside the casing this a compact means of providing positional feedback. Resolution and repeatability better than 0.2%.

Inductive (LVDT) Sensor:-usually installed at the active surface of a positioning device, this provides a direct measurement method, also with a resolution and repeatability of better than 0.2%.

Capacitive Sensor:-an extremely accurate measuring system that is also installed on the active surface of a component. Resolution and repeatability is up to 0.001% giving, for example, a resolution of 1nm in a 100um PZT.

System Signal:-any output from a detector or monitoring system (eg a photodetector) can be used to provide a position feedback signal. The accuracy would then be determined by the quality of both the feedback signal and the processing electronics. Typically the electronics provide three means of controlling PZT movement: manual adjustment from a potentiometer; remote control via a 0-10V analogue signal; and computer control via RS232 and IEEE 488 interfaces.

• Lambda Photometrics
• Tel: 01582 764334
• Fax: 01582 712084

October 1995