Piezoelectric Tube Scanners

Piezoelectric Tube Scanners

Piezoelectric tube scanners are thin cylinders of radially poled piezoelectric material with four external electrodes and a continuous internal electrode. When a voltage is applied to one of the external electrodes, the actuator wall expands which causes a vertical contraction and a large lateral deflection of the tube tip. A circumferential electrode can be used for vertical or radial extension and contraction.

Piezoelectric tube scanners are used extensively in scanning probe microscopes and applications such as fibre stretching and beam scanning. Customised dimensions and/or specifications are available on request.

Specifications:

NOTES:

           1. First bending mode resonance frequency with a fixed base and free end.

Features

When the base of the tube is fixed, the tip translations Δx and Δy are approximately

where Δx and Δy are the x and y axis deflection, d₃₁ is the piezoelectric strain constant, L is the length of the tube, D is the outside diameter, h is the tube thickness, and V_x and V_y are the electrode voltages which are applied oppositely to either side of the tube.

Vertical elongation due to a voltage applied on all four quadrants or the internal electrode is approximately

The diameter expansion due to a voltage applied on all four quadrants or the internal electrode is approximately

The expansion range in the vertical and radial directions can be doubled by driving the internal and external electrodes with opposite voltages.

Driving Piezoelectric Tubes with the TD250 Amplifier

The TD250 is an ultra-low noise, six-channel 250V amplifier optimized for driving piezoelectric tube scanners. Although many configurations are possible, the driven internal electrode configuration shown below provides the maximum X, Y and Z travel range [1]. This configuration is also referred to as the “Full Length” configuration [1]. The performance of this method is compared to other configurations in reference [1].

In the driven internal electrode configuration, the X and Y electrodes are driven in the standard way with equal and opposite voltages, as shown below. By applying a negative voltage to the internal electrode, a contraction equal to half the vertical scan range can be obtained. That is, the vertical deflection is

where V_z is a strictly negative voltage applied to the internal electrode.

This method exploits the higher positive electric field strength of the piezoelectric material, which is usually five times the negative electric field strength. Care must be taken not to apply positive voltages to the internal electrode since this can lead to depolarisation. For example, when using a non-inverting channel, the input is enforced to be negative as shown below. Alternatively, an inverting channel can be used and the input is enforced to be positive.

Driving piezoelectric tubes with the TD250-INV

Deflection and Resonance Frequency Calculator

The following calculator estimates the lateral and vertical travel range of a piezoelectric tube, with the conditions:

• The driven internal electrode configuration is assumed. That is, only negative voltages are applied to the internal electrode. If the tube is only used for elongation, the reported vertical travel range can be doubled.
• The “passive length” is an additional structure added to increase the travel range. This length is not included in the calculation of resonance frequency.

Deflection and Resonance Frequency Calculator

Mounting

The most common mounting configuration is the cantilever arrangement with a fixed base and free end. The base can be bonded directly to an insulating surface with a two part epoxy such as Araldite, or a high viscosity Cyanoacrylate such as Loctite Super Glue Gel. Piezoelectric Tubes can also be bonded to a conductive surface by removing a small amount of electrode as described in “Electrodes” below.

Electrodes

The tubes are supplied with a Nickel thin film electrode. The internal electrode is continuous and the external electrodes are quartered. Electrode area can be removed by etching with dilute Nitric Acid. Custom electrode configurations are available on request.

The electrode clearance specification is the length of external electrode removed from both ends, which provides electrical clearance to a mounting surface. The internal electrode is continuous.

In applications that require high magnetic fields, the Nickel electrodes can be replaced with Copper or Gold. Copper is an economical choice but Gold provides excellent corrosion resistance and electrical conductivity.

Electrical Current Requirements

The required current is I=C dV/dt where I is the current, C is the effective capacitance, and dV/dt is the voltage rate of change. For a sine-wave, the required peak current is equal to: Ip=2(pi)fV_p-p where V_p-p is the peak-to-peak voltage. For a triangle wave, the required peak current is equal to: Ip=2CfVp-p

Wiring Service

Piezoelectric tubes can be supplied with wires attached to all external and internal electrodes. The standard options and order code suffixes are:

Soldering Instructions

Wires can be attached using conductive epoxy (Circuitworks CW2400) or solder and Rosin flux.

Conductive epoxy has the advantages of being flexible and avoids the requirement for heat so there is no risk of thermal depolarization; however, it is time consuming to apply and requires some care to avoid short-circuits due to spillage. Epoxy is recommended for applications that involve continuous operation with full-range cycling of deflection.

Soldering is quicker and easier than conductive epoxy but requires some care to avoid overheating the tube. The required materials and tools include:

• Gloves, safety goggles
• Piezotube with Nickel electrodes
• Insulated wire, e.g. AWG30 Kynar insulated wire
• Superior Flux 67, or rosin flux, e.g. Chemtronics CW8200
• Cotton buds
• 60/40 Sn/Pb 0.5mm Solder wire, e.g. Multicore 3096525-M
• Temperature controlled soldering iron
• Isopropanol or Acetone

The recommended steps are:

1. Strip the end of the wire by 3-5 mm, then tin the wire with solder. Finally, trim the tinned wire to 2mm.
2. Strip the end of the wire by 3-5 mm, then tin the wire with solder. Finally, trim the tinned wire to 2mm.
3. Set the soldering iron temperature to 300C, and preferably check with a temperature sensor.
4. Deposit a 2mm diameter spot of solder on the tube using a brief contact of the hot tip and solder.
5. Attach the tinned wire to the solder spot using a brief contact.
6. Clean off the flux with isopropanol or acetone. An isopropanol bath is recommended for 10 minutes to assist with removing all residues.
7. Handling the tube with gloves will avoid finger marks on the tube.

Vacuum Compatibility

Piezoelectric tubes do not contain any outgassing materials and are fully vacuum compatible.

Cryogenic Compatibility

The material PZT-5H works well at cryogenic temperatures. As a guide, the displacement sensitivity is reduced by a factor of 5. However, at cryogenic temperatures, the applied voltage can be increased from +/-250 V to +/-1000 V, which can regain the majority of room temperature deflection but requires a high voltage.

For example, the predicted deflection of the TB5509 tube is 66 with an applied voltage of +/-250 V. At cryogenic temperatures, the displacement will reduce to 66 μm x 0.2 = 13.2 μm .

However, if the voltage is increased to +/-500 V, the deflection will be approximately 66μm x 0.2 x 500/250 = 26.4 μm .

If the voltage is increased to +/-1000 V, the displacement will be approximately 66 μm x 0.2 x 1000/250 = 52.8 μm

Options/OEM Customization

• Custom dimensions and thickness
• Custom electrode configurations
• Custom wiring arrangements / connectors
• Mounting platform design and fabrication

Piezoelectric Properties

The piezoelectric material is similar to PZT-5H and Navy Type VI.